Download Measurements on laser produced plasma using Faraday

Calhoun: The NPS Institutional Archive DSpace Repository Theses and Dissertations Thesis and Dissertation Collection 1989 Measurements on laser produced plasma using Faraday-cups. Youn, Duck-Sang. Monterey, California. Naval Postgraduate School http://hdl.handle.net/10945/26060 Downloaded from NPS Archive: Calhoun NAVAL POSTGRADUATE SCHOOL Monterey, California b^ v^A Y MEASUREMENTS ON LASER PRODUCED PLASMA USING FARADAY-CUPS by Youn , Duck-Sang December 1989 Thesis Advisor Approved Fred for public release; distribution R is . Schwirzke unlimited. T 249593 ) Inclass.ified • Daae REPORT !)()( I MEM A HON PAGE lb Restrictive Markings Report Security Classification Unclassified a Jecuritv Classification -\uthontv Lg 3 Approved Organization Report Number(s) Name of Performing Organization Naval Pom graduate School I Klonterev. city, srare. CA 5 6b Office Symbol la pc Address ( (city, state, 8b Office Symbol c 17 From . . Procurement Instrument Identification 10 Source of Funding Numbers Program Element No [ Project No To is?.! i 19 Absu&cl-fcontinue Subgroup on reverse Task No Work 14 Date of Report (year, month, day) 15 93 . j Number December 1989 tary Notation The views expressed in this thesis arc those of the author and do not of the Department of Defense or the U.S. Government. - i and ZIP code) 93943-5000 Unit Accession No MEASUREM EMS ON LASER PRODUCED PLASMA USING FARADAY-CUPS 13b Time Covered 3a Type of Report Master's Thesis Sur 9 CA Duck-San2 . p sition (city, state, applicable and ZIP code) Youn unlimited of Monitoring Organization Monterev. urity classification) Authorise Name 7b Address if is Naval Postgraduate School 61 applicable) and ZIP code) ( 16 if for public release; distribution Monitoring Organization Report Number(s) 7a 93943-5000 Funding Sponsoring Organization Address Distribution Availability of Report Downgrading Schedule ion if Page Count reflect the official policy or po- 18 Subject Terms (continue on reverse if necessary and identify by block number) Laser produced plasma. Velocities. Temperature, Faraday Cup measurements necessary and identify by block number) Experiments were performed on laser produced plrsma from Lexan targets in a vacuum chamber at pressures of a few times 10 _i Torr. Plasma diagnostics was performed using Faraday-cups biased at -50 volts to + 105 volts DC. In analyzing 460 oscilloscope trace pictures of Faraday-cup signals, the plasma expansion velocities measured was determined to be 1.25 x 10'anj sec. r\t a distance of 10 cm from the laser-target impact point, the plasma electron temperature was determined from Faraday cup measurement to be about 2.4 ev. The plasma density was approximately 10 u cm' 3 . 20 Distribution Availability of Abstract IX unclassified unlimited 22a Name Fred R D same 21 Abstract Security Classification as report D DTIC Unclassified users of Responsible Individual . Schwirzke DD FORM 1473.84 MAR 83 APR edition may 22b Telephone (include Area code) 22c Office Symbol (408) 646-2635 61Sw be used until exhausted security classification of this page All other editions are obsolete Unclassified Approved for public release; distribution is unlimited. Measurements on Laser Produced Plasma Using Faraday-Cups by Youn Lieutenant B.S., Duck-Sang , Commander , Republic of Korea Korean Naval Academv Submitted , Navy 1979 in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN PHYSICS from the NAVAL POSTGRADUATE SCHOOL December 1989 Karlheinz E . Woehler, Chairman. Department of Physics ABSTRACT Experiments were performed on a vacuum chamber performed using at pressures of a At a Faraday-cups biased at -50 volts to measured was determined distance of 10 cm from in few times 10~ 5 Torr. Plasma diagnostics was lyzing 460 oscilloscope trace pictures of sion velocities produced plasma from Lexan targets laser Faraday-cup +105 volts signals, the DC. In ana- plasma expan- to be 1.25 x 10'cm/ sec. the laser-target impact point, the plasma electron temperature was determined from Faraday cup measurement to be about 2.4 The plasma density was approximately 10 13 in cm~ l . ev. . TABLE OF CONTENTS INTRODUCTION I. BACKGROUND AND THEORY II. A. B. C. INTRODUCTION LASER TARGET INTERACTIONS PLASMA SURFACE INTERACTIONS 3 3 - 4 - Debye Shielding Length 4 2. Sheaths Effect 6 3. Child 1 D. 3 - Langmuir Law 9 ELECTROSTATIC PROBE AND FARADAY CUP MEAS- UREMENTS III. A. B. C. IV. 10 EXPERIMANTAL EQUIPMENT AND PROCEDURES INTRODUCTION EXPERIMENTAL APPARATUS 14 14 14 1. Laser 14 2. Vacuum System 14 3. Optics 16 4. Energy Meter 17 5. Oscilloscope 17 6. Detector Assembly 17 7. Experimental Set - Up 19 EXPERIMENTAL PROCEDURE and Preparation 25 1. Target 2. Energy Measurements 25 3. Faraday Cup Signal Measurements 25 Description EXPERIMENTAL RESULTS 25 27 IV A. B. V. MEASUREMENT ON LASER PRODUCED PLASMA and Sparking An Ionization 2. Response Signal as Function of Distances 3. Measurements 4. Difference of Signals in Electric and Perpendicular Parallel 27 Field 1. to 30 Streaming Plasma 32 between Various Detectors EVALUATION OF PLASMA PARAMETERS 1. Plasma Expansion Velocity 42 2. Measurement of Electron Temperature 44 3. Plasma 49 Density 57 57 DISCUSSION OF RESULTS 57 CONCLUSION AND RECOMMENDATIONS VI. APPENDIX A. LUMONICS TE-822 60 HP CO, LASER OPERATING PROCEDURE A. B. 40 42 SUMMARY AND DISCUSSION OF RESULTS A. SUMMARY OF RESULTS B. 27 61 1. Laser Enclosure Cover 2. Laser Output Port Protective Cover Interlock 61 3. Cooling Water Flow 62 4. Laser Power 5. Gas on 6. Plasma Laboratory Door , Interlocks (2) 61 Key 62 off Switch LASER SYSTEM START-UP LASER SYSTEM SHUT DOWN PROCEDURES 62 62 62 : APPENDIX B. VEECO 400 VACUUM SYSTEM OPERATING PROCEDURES A. VACUUM SYSTEM START-UP B. HIGH VACUUM CHAMBER OPERATION 67 68 68 70 C. D. E. OPENING THE CHAMBER SYSTEM IDLING CONDITION SHUT DOWN THE SYSTEM COMPLETELY APPENDIX LIST C. SINGLE LANGMUIR PROBES OF REFERENCES 70 71 71 73 77 INITIAL DISTRIBUTION LIST 79 VI LIST OF Table VOLTAGE CURRENT CURRENT DENSITY MEASURED WITH FARADAY CUP WITH ONE ENTRANCE HOLE D = 10CM 2. VOLTAGE, CURRENT, CURRENT DENSITY MEASURED WITH FARADAY CUP WITH 4 SMALL ENTRANCE HOLES 1. . , ) ( Table TABLES (D= 10CM 55 56 ) vu LIST OF FIGURES Figure 1. Plasma Figure 2. Lumonics TE-822 Figure 3. VEECO Figure 4. RJ-7000 Energy Meter and RJP-700 Probe 18 Figure 5. Faraday Cup Schematic 21 Figure 6. Circuit Figure 7. Faraday Cup Cover Hole, Schematic Figure 8. Comparison of Current Signals of Two Faraday Cups FC1 with 0.3 Wall Interaction - mm HP CO^ 8 Laser 15 400 Vacuum System 16 Schematic 21 Entrance Hole and FC3 with 0.5 22 mm Hole. Bias V = -25 22 Volt Figure 9. Figure 10. Figure 1 Figure 12. 1. Two Kinds of Detector Rim, Schematic Different 23 Methods of Connection of Detector to BNC Cable . . Configuration of Equipment Sparking Potential (V) as Distance Pd in a 24 Function of The Reduced Electrode Several Gases [Ref. 14] Figure 13. Distance 10 cm, Potential -20 V in 28 Thin and Round Edge Faraday Cup Figure 14. 29 Distance 10 cm, Potential -20 V in Thick and Sharp Edge Faraday Cup 29 Figure 15. Peak Voltage vs Bias Voltage at Variable Distance ( Normal Faraday Cup) 31 Figure 16. Parallel Orientation of Faraday Figure 17. Peak Voltage vs Time at ±5V Cup 32 Probe Voltage in Parallel Orien- tation (1-Electron Current, 2-Ion Current) Figure 18. Peak Voltage vs Time at ±10V Probe Voltage 33 in Parallel Orien- tation (1-Electron Current, 2-Ion Current) Figure 19. Peak Voltage vs Time tation 23 (1 at ±20V Probe Voltage -Electron Current, 2-Ion Current) VUl 33 in Parallel Orien- 34 Time Figure 20. Peak Voltage vs 30V Probe Voltage at in Parallel Orien34 tation (1-Elcctron Current, 2-Ion Current) 40V Probe Voltage Figure 21. Peak Voltage vs Time at in Parallel Orien- 35 tation (1-Electron Current, 2-lon Current) Figure 22. Perpendicular Orientation of Faraday Cup 5V Probe Voltage Figure 23. Peak Voltage vs Time at 36 Perpendicular in 37 Orientation (1-Electron Current, 2-Ion Current) Figure 24. Peak Voltage vs Time 10V Probe Voltage at Perpendicular in 38 Orientation (1-Electron Current, 2-lon Current) Figure 25. Peak Voltage vs Time 20V Probe Voltage at Perpendicular in Orientation (1-Electron Current, 2-Ion Current) Figure 26. Peak Voltage vs Time 30V Probe Voltage at 38 Perpendicular in Orientation (1-Electron Current, 2-Ion Current) Figure 27. Peak Voltage vs Time 40V Probe Voltage at 39 in Perpendicular- 39 Orientation (1-Electron Current, 2-Ion Current) Figure 28. Peak Voltage vs Bias Voltage at Variable Distance Faraday Cup Figure 29. Distance tation 41 ) 5 cm, Probe Bias Voltage -20 V, in Parallel Orien- (Faraday Cupl-lOcm from Target, FC2-5cm from Target) 45 Figure 30. Distance tation Gap (Soldered Gap 10 cm, Probe Bias Voltage -20 V, in Parallel Orien- (FCl-15cm, FC2-5cm) Figure 31. Distance Gap 5 cm, Probe Bias Voltage +20 V, entation (FCl-lOcm, Figure 32. Distance Gap Gap 5 in Parallel Ori- FC2-5cm) 10 cm, Probe Bias Voltage entation (FCl-15cm, Figure 33. Distance 45 46 +20 V, in Parallel Ori- FC2-5cm) 46 cm, Probe Bias Voltage +20 V, in Perpendicular Orientation (FCl-lOcm, FC2-5cm) Figure 34. Distance Gap 10 cm, Probe Bias Voltage Orientation (FCl-15cm, FC2-5cm) Figure 35. Ln Ie vs Probe Bias Voltage IX 47 +20 V, in Perpendicular 47 48 Figure 36. Peak Voltage vs Bias Voltage (-20 d V + 10 V ), Distance = 10cm 51 Figure 37. Current vs Bias Voltage (-10 V to +7 V Figure 38. Peak Voltage vs Bias Voltage (0 v to Figure 39. Current vs Bias Voltage (0 Figure 40. to V VEECO Vacuum Chamber to + + 105 ), = 10cm .. 52 Distance d = 10cm 53 Distance d 105 V V Distance d ), Controls Figure 41. V-I Characteristics of Single Langmuir Probe ), = 10cm . 54 72 74 ACKNOWLEDGEMENTS I would express like to my gratitude and appreciation to professor Fred R. Schwirzke and Richard C. Olscn for their friendly advice and guidance phases of I wish to thank Mr. Robert Sanders for his assistance in To my offer experiment and Finally. I my am thanks this research throughout. thanks for their reliable assistance and friendship during the this research. grateful to support throughout And operation of the and colleagues Lieutenant Michael V. Hensen, and Richard friends I in the Plasma Physics Laboratory, and the craftsman Mr. George the Jaksha who supported Down. all this research. equipment K. in to two and half years this my in my wife, Youn-Young, work and my completion of son. Hyun-sung for love Monterey. California. XI for her understanding and studies at the NPS. and being healthy and patient for I. INTRODUCTION Laser plasma production has been of interest to researchers since the early 1970s. The use of the laser as a scientific instrument quickly drew the attention of the academic community. considerable research action with matter include (1) heating solid materials. (4) emission of plasma production. laser interaction, and and Industrial effort. The dominated technology are considerable. (6) became Laser-target interaction soon [ Ref. military applications power effort of high 1] charged particles, (2) melting, (3) (5) a topic of of laser- laser inter- vaporization of emission of neutral particles discharge in gases produced by high energy (7) electrical (8) application of these effects to material processing [ Ref. 2]. now understood It is of the light energy is that when solid targets are struck absorbed and a part by reflected, the is a laser amount of being dependent on the target material and surface conditions The amount of radiant energy and beam, some Ref. [ reflection 1]. the time-length of the laser pulse striking the target are the controlling parameters which determine the results of the interaction. plasma If a collisions to electrons Initially, the and is formed, the absorbed energy ions, and part of absorbed photon energy transferred through heat the material. it is, is for all practical considerations, in- stantly turned to heat through particle collisions, rapidly heating a small quantity of surface material to very high temperatures. will rapidly produce a Different types of vestigate plasmas. A energy laser pulse of sufficient plasma, a dense cloud of ionized particles. Langmuir probes are widely used as diagnostic tools to in- Probe theories, depending upon the nature and parameters of the plasma, relate the measured value of the probe current ( or voltage ) plasma density, electron temperature, floating and plasma potentials and lations [ Ref. 3]. The shape and size to the oscil- of the selected probe depend upon the plasma parameters and the quantity be determined. to Naval Postgraduate School (NPS) on laser Previous research at the produced plasma, particle velocity, and density was conducted using various diagnostic techniques. Brooks used floating double probes, magnetic probes and quartz pressure Peak plasma density measured probe. at cm and distance of 2.4 the in the main plasma was the expansion velocity was -3 2.5 x 10 l4 (cm ) 1.1 x 10 7 cm/sec [Ref.4]. Callahan measured the plasma ion velocity of aluminum ions with a floating double electrostatic probe. The results were plasma velocities of 5.6 x 10 6 cm/sec and plasma ion density varied from 10 n (rm -3 ) plasma [Ref. 10 13 (ca?2~ 3 ) for the streaming 5]. The Faraday cup is to is a metallic electrode (usually of cylindrical shape inserted into the plasma. From the current collected by the ), which Faraday cup as a function of probe bias voltage, information can be obtained about the plasma density and temperature. Interpretation o^ the Faraday cup signals is usually complicated, however, because the current-voltage characteristic curve depends on the details o\~ the plasma Faraday cup interaction, for example, the emission of secondary electrons. A dense plasma is produced by focusing the pulsed CO, vacuum chamber. surface of a target suspended inside a and the thermal energy of the plasma tion. Finally, the lifetime of such a The plasma terminates plasma is Faraday cups. on the The plasma expands rapidly converted into directed ion at the wall of the mo- vacuum chamber. The quite short. objective of this experiment eters using is laser light is to measure laser produced plasma param- II. BACKGROUND AND THEORY INTRODUCTION A. The irradiation of solid targets by high power lasers has numerous research papers and reports which describe and is electrical effects of laser target interactions. When focused onto a solid surface, a high density plasma experimental and theoretical work done on to this the thermal, mechanical, a high is dominated and when the when initially the plasma power laser pulse Much formed. phenomenon has been understanding the laser heating and plasma dynamics shortly after the laser pulse, been the subject of is in the of the directed time during and expected to be collisionally high temperature of the ions and electrons is being converted to the ordered kinetic energy of expansion. A and basic understanding of each of these processes relative importance is a and their interrelationships necessary prerequisite before conducting experimental research. LASER -TARGET INTERACTIONS B. Laser radiation absorption by a solid metal target causes heat to be deposited in a surface layer of the target material. The surface temperature heat waves are propagated into the target. absorbing target layer as fast as it is The heat cannot increases and diffuse out of the being injected so the temperature continues to increase until the target material begins to vaporize. At will the time of vaporization, the surface temperature of the target material begin to depend on the rate of vaporization; that anism. The thermal the evaporation conductivity, heat capacity, and density are dependent. The reflectivity At is, sufficiently high laser is all mech- temperature temperature and thermodynamic phase dependent. power density the blow off will be in the plasma state. In addition to the up on the The target thermal change to the target, there due to recoil from the vapor blow transition of a gas into the plasma off. state involves various particle inter- among themselves and action processes, collisions of the particles with radiation encompass most of the energy transfer stripping of electrons from atoms and molecules, hot ( ) electrons colliding with called recombination. is body recombination, in a strong pressure built is is [ Ref. Ionization, the accomplished by the energetic atoms and molecules. The inverse of The atom or molecule has which two electrons this process Three to rid itself of energy. collide simultaneously with one carrying away the excess energy and the other attaching a 6]. interactions itself, an ion, takes place in dense plasma. There are three basic methods of radiation emission from in particle interactions plasma; discrete radiation, recombination radiation, and bremsstrahlung. a Discrete radiation arises from electron transitions from one energy level to an- other in the same atom. Discrete radiation picture of this form of radiation consists of Recombination radiation ion. A lower energy state the form of a photon. free electron This C. is and an ion is is emitted of a single numerous when wave is A length. complete spectral lines. a free electron is captured by an assumed by the electron and the energy Bremsstrahlung in is is emitted in radiation from interaction between a which the electron is only decelerated, not captured. referred to as free-free radiation. PLASMA SURFACE INTERACTIONS - 1. Debye Shielding Length When a plasma is subjected to an external electric field the electrons and ions will drift in opposite directions. separation and an internal electric As they field, drift apart, they produce a charge which opposes the external field. The width of the region, over which the charge separation can occur to balance the external electric field, cles. If is proportional to the thermal energy of the plasma parti- plasma the dimension of the bulk greater than the dimension of the is region over which this charge separation occurs, the interior of the plasma will be shielded from the external The width of field. the charge separation can be determined, approximately, by considering a plasma where the ions are fixed in space, over the time frame of interest, and Boltzmann the obey electrons relation applies. Maxwellian a distribution Using Poisson's Equation in such the that one dimension gives 2 d V dx where V is e and the electric potential ("/-««) £ 2 e is (2-1) the electric charge. Since the ions are considered fixed, the density of ions will be constant and equal to the plasma density n . The Boltzmann relation then gives the electron distribution in the region of interest as ne = n exp( -p— ATe (2.2) ) Substituting equation 2.2 into equation 2.1 gives .") en d'V To e y expanded solve equation 2.3 the exponential can be in a Taylor series, and neglecting terms of second order and higher gives d i dx The solution of equation 2.4 which differential equation is 2 = e "oU I /-, ,t *oKT. ,. ,> (2-4) e is a homogenous second order linear ordinary given by V=V exp(--^-) Ad (2.5) where, / F.nk'T. (2.6) This quantity is known as the Debyc length and is a measure of the thickness of the sheath that screens the bulk plasma from the effect of external fields. screening applies as long as the dimensions of the bulk plasma Dcbye greater than the length. Sheath Effects 2. When plasma comes a wall will recombine. hit the locity is This in contact with a wall, the electrons and ions that But since the electrons are moving than the ions, the electrons plasma and an Figure be than the ions resulting in a will then be at a lower potential than lost faster The wall net positive charge of the plasma. the bulk will electric field will exist. illustrates the potential variation in a 1 at a higher ve- plasma which is tact with a wall. Due sheath, will exist. This layer will isolate the bulk plasma from the wall. fect of the sheath is Dcbye to only those electrons enough ergetic shielding a layer of charge separation, calledthe to accelerate the ions ter the region until the flux in the to cross a in con- of ions is and The ef- to decelerate the electrons that en- balanced by the flux of electrons. Therefore, high energy tail of the velocity distribution will be en- the sheath and others will be repelled back into the plasma. The ered. 1. variation of the potential in the plasma sheath will In order to simplify the The now problem, the following assumptions ions enter the sheath region with a drift velocity u , will be consid- be made: given by the Bohm criterion, 2. The 3. The sheath 4. The ions have boundary. T. = region so that is all ions have a velocity u collisionless and in steady state, potential decreases monotonically with x, at the plasma-sheath 5. The electrons follow the Boltzmann Applying conservation of energy i- where r(.v) < 0. relation ( to the ions in the equation 2.2 ). sheath region gives Mu 2 - -|- Mu 2 - eK(x) (2.7) Solving equation 2.7 for the ion velocity gives u=^(u -ljf) 2 The ion density within the sheath nuity which is is (2.8) determined from the ion equation of conti- given by n {x)u(x) t Using equation 2.8 =n u (2.9) # for the ion velocity, equation 2.9 can be solved for the ion density which gives ni(x) = n {\-lf-)- 112 (2.10) Substituting equation 2.2 and 2.10 into equation 2.1 yields dx This first is 2 Lu [ —— ex P( _FF-)-( 1 ; KTe the non-linear planar sheath equation. u^M ) ] < By multiplying each side 2 - n ) by the derivative of the potential, the equation can be integrated once to give ( ^^) = i^{[exp(^f)-l] + ^[(l-^-) 1 '2 -l]}(2.12) Further solution of equation 2.12 would require numerical methods. the electric field inside the plasma inside the plasma must is zero, then the first derivative of the potential also be zero. therefore greater than zero , If The left hand side of equation 2.12 and the following inequality results is — - ; V= © / ©... n '///> I///////////, / \ ' Vw mm ,;, % % -d Figure 1. Plasma exp(( - Wall Interaction eV ,4i v T77 } a: r. [(1 i/2_ _^£li ) 1]>0 A/«g This inequality can be greatly simplified by expanding the Taylor quality series is and neglecting terms of known as the Bohm third order and higher. sheath criterion and u > (2.13) is left The hand term resulting ine- given by KT„ (2.14) M This requires that the ions must enter the sheath region with greater than the ion acoustic velocity of the plasma. there must be a finite electric field within the plasma. that the potential is and the only an approximation [ 7]. a velocity In order for this to occur, Therefore, the assumption electric field are zero at the Rcf. in plasma-sheath boundary Child 3. - Langmuir Law The current flow between the cathode and anode of by the space charge of the electrons that limited exist electrons distort the external field and thereby reduce at the a vacuum diode is The between them. and even reverse the field cathode surface. As an example, d apart. Assume consider a diode consisting of infinite that there cathode, and that the compared with the is flat plates a distance an unlimited supply of electrons available from the of the electrons after emission initial velocity is negligible velocity that they will gain while crossing the electrode gap. The kinetic energy of the electrons crossing the ried by the electrons is gap and the current density car- given by -jmv 2 = eV (2.15) and j = env (2.16) f Using Poisson's equation, in one dimension, the current-voltage relationship can be derived as follows: > d V(x) en(x) ~TJ - = dx' The electron velocity is (2.17) determined from equation 2.15 to be » The electron density is determined using equation 2.16 and 2.18. and after substituting into equation 2.17 gives it dx I' 2 _ J_ £ ° I ^ m I/-1/2 V~^ 2e (2.19) equation 2.19 both sides are multiplied by the In order to solve first derivative of the potential and reduced to the following form d _(M-\L dx Equation 2.20 is y dx ' lm = JEoV « r/-W2 dV (7?()) K dx ' then integrated over the gap using the boundary conditions that the electric field and the potential are zero at the cathode. The result of this integration gives boundary conditions, and Integrating once again over the gap, applying the re- arranging the desired current-voltage relationship yields ; £n = 4 hr- _JL FT /_£ y Equation 2.22 rent flow [ is Ref. known 8]. as the Although V3 ' 2 X Child-Langmuir law it (2 2°) ! for space charge limited cur- has been derived here using simple assumptions, the proportionality of the current density to the 3/2 ference remains for It difficult dif- geometries under non-relativistic conditions. should also be pointed out that the Child-Langmuir law applies to both positive D. more power of the potential and negative charge carriers. ELECTROSTATIC PROBE AND FARADAY CUP - MEASUREMENTS The electrostatic probe has been used as measuring plasma characteristics. Langmuir (1924 - 1929 filament ones are called ). The a fundamental diagnostic original use tool for and theory was done by Hence, the electrostatic probes, particularly the single Langmuir probes. 10 The probe basically consists of one or more small, metallic electrostatic electrodes that are inserted into the plasma to be characterized. may be cylindrical or spherical in may shape, or The electrodes some other just be a plate or regular or irregular geometric shape which suits the probe surface area calcu- model and does not lation The Schott 1. interfere with the plasma flow. basic assumptions associated with electrostatic probe use arc listed Rcf. [ by 9]: The plasma to be infinite, homogeneous and quasincutral in the absence of the probe. 2. Electrons and ions have Maxwellian velocity distributions with temperatures 7~ e 3. and respectively, with T, Electron and ion mean T $>Tr e much free paths are greater than the electrostatic probe radius for a cylindrical or spherically shaped probe. 4. Each charged particle hitting the probe to be absorbed and not to react with the probe material. 5. The region in front of the probe surface, where the plasma parameters devifrom their values in the undisturbed plasma to be confined to a space charge sheath with a well defined boundary. Outside of this boundary the ate space potential 6. is assumed The sheath thickness d to be constant. to be small compared to the lateral dimensions of the plane probe. probe configuration, the probe operates by a single electrode In the single serted into the plasma and attached to a variable power supply. in- The power supply can be biased at various potentials, positive or negative with respect to the plasma for optimal probe signal response. a function of the of a metallic The plate confinement is The other probe potential. vacuum chamber) normally fixed vessel. is Current at the probe a at a ground is measured as electrode (using a plate or the wall for the plasma. conducting section of the wall of the plasma In electron discharge tubes the customary experimental rangement uses the anode of the tubes as the reference electrode. 11 ar- The theory of electrostatic probes is complicated because the probe electrodes At and near arc themselves plasma boundaries. the plasma boundaries the plasma characteristics are changing, usually very markedly. A thin layer called the and ion number density Debye sheath differ boundary where electron exists at the from the values within the plasma and electric fields are present. The characteristic of and the positive or negative current Faraday cup signal was used plasma pulse arrived to the Faraday cup at the to a basically similar to a single is is done by collected in a static electric Faraday cup. The determine the time at which the laser produced Faraday cup location. When a DC voltage is applied plasma, the ions are attracted by the negative electrode in a and the electrons by the positive one. havior of a plasma is Separation of ions and electrons probe characteristic. fields Faraday-cup response A fundamental characteristic of the be- ability to shield itself is its from the electric field by forming Debye sheath. A Faraday cup's detector schematic diagram detailed Experimental Apparatus is shown in Figure 5 (see ). The Faraday-cup was used to measure the ion density, n„ using equation (2.16): ] where , j = A = v, d t = = current density Faraday - ion velocity = qn?i (2.16) = I/A cup hole area = d/t distance from the target to the Faraday = The time - difference between the laser shot received signal from the target (i.e v the time to the it cup and the takes the ions to travel Faraday-cup 12 ) One problem in this experiment is that the laser pulse produced during the pulse duration. 13 is long, i.e. plasma is EXPERIMANTAL EQUIPMENT AND PROCEDURES III. INTRODUCTION A. This experiment was designed to determine the velocity and density of a laser the produced plasma vacuum chamber in a via a vacuum chamber. The beam splitter, mirrors, quired a variety of equipment which is laser energy and a was directed 1 into NaCl window. This re- described in the following sections. EXPERIMENTAL APPARATUS B. 1. Laser The Lumonics TE-822 page 15) laser was HP C0 2 TEA high energy pulsed (Figure 2 on This laser's active utilized to irradiate all targets. CG r A 2 and consists of a continually flowing gas mixture of He, pulses mode, it is capable of delivering a maximum 2 tiple pulse pulse. The mode, the laser's laser is medium In the single . of 20 joules of output with an adjustable pulse width of 0.05 microseconds to 5.0 microseconds. maximum capable of delivering a In the nonregulated high voltage power supplies were cooled by an formal laser operating procedure 2. The is documented in a manual VEECO conjunction with the 400 vacuum system C0 2 ( Ref. 10]. Figure 3 on page 16) laser for research of pump, a water cooled diffusion pressure range of the chamber is pump, and from 1.0 The mechanical pump was used a liquid utilized in A consists of a at the mechan- r 2 cooled cold trap. atmosphere down to reduce the 14 is plasma surface interactions Naval Postgraduate School. The vacuum pumping system ). [ Vacuum System The (atm mul- of 8 joules per external water refrigeration unit and controlled by a voltage regulator. ical C0 to 10 -9 The atmospheres background pressure to 1.0 Figure 2. Lumonics TE-822 HP CO^ Laser mTorr. the background pressure was further reduced by the diffusion pump. Three different gauges are required to determine the pressure in different ranges. Pressure between 760 Torr and 10 Torr were monitored on the Matheson pressure gauge (model 63-5601 between 10 Torr and 1.0 ) mounted on top of the mTorr were monitored on chamber above the diffusion pump. Ref. is system. 15 a fixed 1]. Pressure Pressures below 1.0 ionization gauge located below the This gauge 1 the Granville- Phillips service 275 pressure gauge also located on top of the chamber. mTorr were obtained from an [ component of the chamber just VEECO 400 Figure VEECO 3. 400 Vacuum System Optics 3. The window for allowing the laser characteristics of each * One beam optics consisted of a 7.62 cm beam component focusing lens, and an optical splitter, to enter the vacuum chamber. The are listed below diameter ZnSe beam splitter specific : with 99.38 % reflectance, and 0.13% transmitance. * One 7.62 cm diameter ZnSe and 38 cm * One 7.62 focusing lens with 98.5 transmitance, focal length. cm diameter NaCl window This combination provided a 90.1 ergy into the % % with 92 % transmittance. transmission factor of the total vacuum chamber. 16 beam en- Energy Meter The energy output of the laser was measured using the laser precision corporation model Rj-7100 energy meter with the model Rjp-736 energy probe which designed to detect laser pulses of wave lengths betw en 0.35 and 11.0 T is microns ( Figure 4 on page of a laser pulse of /iJ to 10 J 1 Rcf. 12]. [ nsec to 18). The meter capable of measuring the energy is msec duration, and 1 The energy detected at the total probe is energy ranging from 10 only a small percentage (0.13 % lated from the reflectance and transmittance of the ZnSe beam ) of the total energy of the laser pulse, whose flux on the target as the transmittances of the ZnSe focusing lens is calcu- splitter, as well and NaCl window. Oscilloscope 5. Faraday-cup signals were recorded using the Tektronix 7844 dual beam oscilloscope and C-50 C-70 series cameras. The laser " sync-out throughout the experiment as the oscilloscope triggering input. camera was operated manually to ensure the single " was used The C-50 C-70 sweep traces were recorded. This also afforded the opportunity to use double exposures to record different signal inputs. An attempt was also made to use the Tektronix 7844 dual beam oscilloscope to record two different Faraday-cup signals. Detector Assembly 6. Figures throughout The cup this 5 and 6 depict the Faraday cup detector and experiment. detector circuit used The Faraday cup cover was made of brass tube. was connected with a 90 degree bend to the aperture smaller than the detector diameter diameter. The cup was detector. The charge is electrically insulated collector cup BNC. An entrance used to define the entrance plasma with nylon strips at the end of the registers time resolved signals of ion 17 and Figure RJ-7000 Energy Meter and RJP-700 Probe 4. electron arrivals. The time circuit response time. rent limiting 50 The scale for such collection voltage to the Faraday cup necting methods were used. One mm Figure 7. and an area of 0.33mm 2 while the other has , mm each and a total area of 0.39mm 2 The cover with four small plasma entering that cup than the cup with one conducted to supplied through a cur- , Both different Faraday cup covers were used to determine the parameters of the plasma flow. less detector cover has one hole drilled at the center of the four small holes with a diameter of 0.353 in 2/is two kinds of Faraday cup covers, detectors and con- cover with a diameter of 0.651 shown is than the Q resistor. In this experiment, as is less hole. holes resulted in This experiment was study the change of the I-V characteristic as a function of entrance 18 If a relatively large area. plasma stream enters through the hole in the cover and into the cup. the I-V characteristic will be similar to the one of double probe. In previous experiments by F. Schwirzkc at the Kernforschungszentrum, Karlsruhe. Germany, using Faraday cups to diagnose a streaming carbon plasma produced by plasma guns was observed compares two Faraday cup Figure 8 on channel 1. it 2, CHI, has CH2, has a 0.5 a smaller hole mm that the current showed sharp spikes. FC signals at a bias voltage of -25 V. FC diameter entrance hole while diameter of 0.3 mm. 1 3 on Channel These current spikes can be explained by a spark discharge inside the Faraday cup between the cup and the cover. The electric field inside the Faraday cup depends on the the separation distance between cup and cover. bias voltage Sharp corner of the cup and will en- hance spark formation. To observe how sparking depends on ferently shaped detectors were used. One this dif- with thick walled and with sharp corners and the other one thin and with round corners as During the course of two the pressure and distance, experiment it cup with different connections of the detector shown was observed to the BNC Figure in that two 9. Faraday cable gave different Figure 10 shows that one detector was mechanically joined to the inner results. hollow conductor o^ the coax cable (called normal) while the other one was soldered to the cable. Experimental Set 7. The arrangement tical in 13], is all Up of the equipment for this experiment respects, to the previous experiments except for the use o^ Faraday cups. directed onto the Figure the - 1 1. The DiMSL IR ZnSe beam performed by Henson The output beam of splitter located in the radiation transmitted through the detector. 19 was almost beam the [ Ref. Lumonics Laser beam path splitter iden- is as shown in incident on the The window reflected into the beam line The primary target on a line 26 + locations, L,, were normal. A in figure for evaluating the effects of was held 1 1 . Inside the NaCl chamber, the during various phases of the exper- at distances of 5 cm, 10 cm, 15 cm, on a second location, 3 degrees off the target, from the target as shown target the target. in several locations 30 degrees off the target The passed through the ZnSe focusing lens and vacuum chamber, and onto Faraday cup was located iment. is and L 2 , was opposite at a distance of 18cm «* the 20cm These different locations provided a means plasma expansion on the signals detected. in a fixture inside of 13.6" from the lens. 20 the vacuum chamber at a distance Cover ->n -12.1 mm yg-4 1& 5mm fffl —A / i I i V Inlet Detector hole Screw Nylon strip ^ Figure 5. BNC Faraday Cup Schematic osc 50 SI lOOnF Faraday-cup 100k 51 Battery (-50V to Potentio meter Ground mMMMMMZMMMZMMMM Figure 6. Circuit Schematic 21 +105V) m X Q 04' m c -i ... j r-r < / Q 47 mm , mm 0'ie o <x o O I" j e I > 1 5-t 0.353 mm / < 4 small hoi*? Figure 7. Faraday Cup Cover Hole, Schematic Sp<xr k Figure 8. Comparison of Current Signals of Two Faraday Cups FC1 with 0.3 Entrance Hole and FC3 with 0.5 22 mm Hole. Bias V = -25 Volt mm > 0.51 0.26 mm mm — <\V- ""fc— 1 1 V v._.-- N --_ BNC 9. Two BNC (Thin and round corner) (Thick and sharp corner) Figure -v Kinds of Detector Rim, Schematic d = 0.85 mm d ^ 0.85 mm X hole M solder BNC iA (Normal Figure 10. Different BNC (Solder) Methods of Connection of Detector to 23 BNC Cable Q C_3 En ergy meter probe Vaccum chamber Nacl window / \ S" \ Beam \ / _—^1 UN n. splitter,' L2 FocusingLens i w e R w £-^ Ai r' Lumonics Co 2 Laser \ J Cooler Figure 11. Configuration of Equipment 24 / Target holder . \ C. EXPERIMENTAL PROCEDURE Target 1. Description and Preparation Lexan, a polycarbonate manufactured by General Electric, was used as due the target material to prior to mounting in ractangular plates of the 3mm it vacuum chamber. These rectangular plates were placed rotated between shots cm with reagent grade ethyl alcohol, and blow drying it in the Lexan target prepara- ease of preparation for use. approximately 6.87 tion consisted of cutting thick material, rinsing its order to provide a flat new against the target holder, and target area clear of damage from the previous laser shot. Energy Measurements Laser energy readings were recorded for shots a. the 25 KV KV potential shots. This included 480 Over the course of the experiment, the capacitor potential. average energy at the 25 all was founded to be 1 1.82 ± 0.84 J. During any run. the energy readings never varied by more that 10 %, and were generally less than 3. 5 %. Faraday Cup Signal Measurements Electron and ion current signals were detected by the Faraday cup and displayed on the Tektronix 7844 dual beam oscilloscope. Signal traces were recorded on polaroid 47 and 667 film for evaluation and comparison. Variables in the investigation included detector location and cup bias voltage. Once mounted on the target wheel, the targets were placed in the chamber, the chamber sealed and evacuated to 10~ 5 Torr. justed by placing a plexiglass stopper before the 25 vacuum The energy was ad- vacuum chamber and firing the laser while adjusting the capacitor voltage. the stopper was removed, Once the beam energy was the lab darkened, and the laser shot made. Plasma formation was determined by observing the action with a polaroid camera. If there was light laser-target inter- emitted from the surface, plasma was formed. Most of the data were recorded using type 47 camera shutter was held open by ond before and a its rol film. The remote cable release for approximately 3 sec- after the shot. The observed Faraday cup and adjusted, signal was caused by generation of the plasma expansion away from the target surface. Therefore, a series of experimental shots were performed to determine to what extent the cup bias voltage influenced the current reading. L 2 for different Signals were recorded at select locations L, and cup bias voltages. 26 EXPERIMENTAL RESULTS IV. The experimental sented in this section. results of the research Each aspect of the ol' results has The data subsections for easy identification. consisted exclusively completed for this thesis arc pre- been subdivided into several collected during this experiment Determination of signal oscilloscope trace photographs. amplitude and time are derived directly from these traces. MEASUREMENT ON LASER PRODUCED PLASMA A. When form negative provided an and Sparking Ionization 1. its an electron or ions, it is 2 in An Electric positive ion Field moves through likely to excite or ionize energy exceeds the coresponding atoms or molecules by critical values. found throughout the gas together with the primary tributed over a region which If sufficient gas the voltage is which does not collisions In the presence of however, excited atoms as well as new electrons and ions are electric field, the space. a gas trapped is in is bounded by particle; they the electrodes may be dis- and the walls confining high enough between the cover and the detector and the cup then breakdown can occur within the Faraday cup. In order to find the breakdown field or sparking voltage gas pressure, and the electrode distance, equation (4.1) V p d : : : Sparking Potential terms of the Ref. 14] can be used E V pd where [ in P (volts) pressure discharge gap with two plane electrodes. 27 (4..) Normally, from the Paschen curve the 1^ = 330 (volts) at pd mmHg as shown in Figure air is = 12 0.57 [ minimum sparking potential for Therefore, V/pd = 579 V/cm mmllg cm. Ref. 14]. In this experiment, the calculated Faraday cup sparking potential becomes, using equation cm = 40 V/cm. = 0.5 x 10- 5 If mmHg cm ever, the pressure cause slowly its the may vacuum is 5 approximatly 10 mmHg, and the breakdown voltage would be very increase in the then Pd large. the small hole in the cover of the quired pressure for the Paschen a value Two which can minimum would exist inside due be P= is only Faraday cup. The re40 = V 0.07 579 ^579) to desorption. kinds of Faraday cup's rim were used. than the other. How- Faraday cup when the plasma enters be- contact with the inside surface leads to desorption of gas which pumped through mmHg, pressure V/d = 20 V/0.5 (4.1), One is sharper and thicker Figures 13 and 14 shows the Faraday cup current as function of time, at a distance of 10 cm from target, bias potential -20V, and shape of cup rim. >_ h Figure 12. I * »* mm Jig cm Sparking Potential (V) as a Function of The Reduced Electrode Distance Pd in Several Gases [Ref. 14] 28 Figure 13. Distance 10 cm, Potential -20 V in Thin and Round Edge Faraday Cup Figure 14. Distance 10 cm, Potential -20 V in Thick and Sharp Edge Faraday Cup 29 2. Response Signal as Function of Distances By varying we observed the distance to 5 cm, 10 cm, 15 cm, 20 cm, from the target that close to the target the arly with positive or negative similiar to a cup bias voltage double probe characteristic. diameter of cover hole, then , and ion current increased electron If This result (see figure 15). Debye length the an equal electron and ion current would be drawn At cup and cover target and thus lower plasma density, the ion current curve tends a bias voltage a single probe characteristic. a applied. is The reason for this is a larger distance positively biased cup. One 20 cm. This effect is shown in Figure will using the four small hole cover, a single probe characteristic. (i.e it be turned away same distance of resulted in a curve approuaching y small ion saturation current) 30 to saturate like 15. hole and four small hole covers were tested at the When from the that, if the cover hole radius becomes smaller then the Debye length, then the electrons by the is larger than the is to the if line- x o v o in D= 2QC Ml4 HO L ET D=10CM(PERPENT __ D=10CM(4H0LE PERPEN) o 50 -40 -30 -20 -10 VOLTAGE Figure 15. Peak Voltage 31 30 •10 (V) vs Bias Voltage at Variable Distance Cup) 20 10 ( Normal Faraday 3. Measurements The transition Parallel and from double Perpendicular to Streaming to single probe characteristic was very pro- nounced when the Faraday cup was aligned perpendicular The parallel orientation of Faraday cup is Plasma shown in to the plasma flow. Figure 16 on page 32. The directed ion flow (and electrons) enter into the detector with equal flow rates. The in ion and electron branch of the characteristic are almost the same, as shown Figures 17 through 21 on page 33-35. This is similiar to a double probe characteristic. Vaccum chamber 'v-'v Target *v Laser is" 1 Faraday cup Figure 16. Parallel Orientation of Faraday 32 Cup y y z»v 20*V xvs WMiWP^WiHW^f' 1 Figure 17. Peak Voltage vs Time at ±5V Probe Voltage in Parallel Orientation (1-Electron Current, 2-Ion Current) 20kV 20*V / mmmmmm Figure 18. Peak Voltage vs Time at ± 10V Probe Voltage (1-Electron Current. 2-Ion Current) 33 in Parallel Orientation Figure 19. Peak Voltage vs Time at 120V Probe Voltage in Parallel Orientation in Parallel Orientation (1-Electron Current, 2-lon Current) Figure 20. Peak Voltage vs Time at ± 30V Probe Voltage (1-Electron Current, 2-lon Current) 34 Figure 21. Peak Voltage vs Time at ± 40V Probe (1-Electron Current, 2-Ion Current) 35 Voltage in Parallel Orientation The second case was shown in the perpendicular orientation of the Figure 22 on page 36. Electron current amplitude the parallel position, but the ion current is is reduced Faraday cup was the same as as in amplitude. The ion current in given by the ion flux perpendicular to the direction of flow, Figures 23 through 27 on pages 37-39. j x —K ,** \ ( i .aser Vaccum chamber ^---'"" 1 1 \ / / \ \ Figure 22. X Faraday ctfp Perpendicular Orientation of Faraday 36 i t \ f ./ Cup — Target 1 Figure 23. Peak Voltage vs Time at ± 5V Probe Voltage tation (1-Electron Current, 2-Ion Current) 37 in Perpendicular Orien- Figure 24. Peak Voltage vs Time at ±10V Probe Voltage in Perpendicular Orien- tation (1-Electron Current, 2-Ion Current) Figure 25. Peak Voltage vs Time at ±20V Probe Voltage tation (1 -Electron Current, 2-Ion Current) 3S in Perpendicular Orien- Figure 26. Peak Voltage vs Time at ±30V Probe Voltage in Perpendicular Orien- tation (1-Electron Current, 2-Ion Current) Figure 27. Peak Voltage vs Time at ± 40V Probe Voltage entation (1-Electron Current, 2-Ion Current) 39 in Perpendicular Ori- Various Detectors Difference of Signals between •6 4. In Figure 15 and 28, the ion and electron currents are plotted different Faraday cup not vary much in detectors. The soldered and normal connected cups the electron current. measurement or if two did However, the soldered detector showed a reduced ion current compared to the normal detector. a fluctuation in the for It is unknown if this is just the soldered surface has an influence on the ion measurement, for example by changing the secondary electron emission coefficient. 40 o o CO 1 o in CM 1 - O o CM ! 1 o : . •' ./ . 1 ; »£ o o • y if x/ ' Jr ^ / 1 o W o a o > 1 A o * / *&:...y. ID w ' o l r i-H y^ o t D=5CM D=10CM a D=15CM "+"D=20CM o '• \ o i?ri?Mn CM - O CO o o CO 30 i i i -40 20 -10 10 20 30 VOLTAGE(V) Figure 28. Peak Voltage vs Bias Voltage at Variable Distance Cup) 41 (Soldered Faraday 4 EVALUATION OF PLASMA PARAMETERS B. The numeric values calculated throughout the figures 1. text, but arc presented in this chapter for ease of reference to the from which they are derived. Plasma Expansion Velocity The expansion was the following paragraphs arc referred to in The investigated. of 4.8 x 10 velocity of a laser-produced plasma from an Lexan target vacuum chamber at a pressure target -5 to 2.2 x 10 5 was suspended Torr. A ruler is in a used to measure the distance of the probe from the surface of the target. 2 Faraday cups were placed along the same line of plasma expansion with a distance gap of To be it probe f , had was desirable A bias. 10cm between them. compare to the signal peak values to travel and t is assumption, i.e. the time of the d It reference, a was determined for different values of was assumed and calculations linear time to distance relationship this same time on each photograph. to be displayed were made on the basis of plasma had or able to consistently record the signals in the zero time mark, that 5cm = vt, plasma where d is the distance the arrival at the cup. Error analysis of the calculations of velocity and distance was also necessary to insure that the measured quantities were dependable. for error analysis for distance d In this found were d in =± reference was used [ Ref. 15]. A simple scheme The measurement 0.05cm. experiment the velocity was calculated from two measurements, one with parallel orientation, the other with perpendicular orientation Faraday cup. For parallel orientation, the ion velocity ures 29, 30 on pages 45. 32. time. From maximum v of the was calculated from Fig- Electron velocities were calculated from Figures 31 and these calculations follow that, ions For the errors and electrons arrive of the signal the velocity = 10 0.8 x 10 -6 = is 7 1.25 x 10' 42 cm/ sec at the same The velocity Faraday cup. 47. The was also calculated with the perpendicular orientation of the Electron velocity was calculated from Figures 33 and 34 on page figures show, an electron arrival time of t= 0.8 orientation, therefore, v = 0" 1.25 x 1 cm / sec 43 . /usee as in the parallel 2. Measurement of Electron Temperature Electron electrostatic plasma temperature can found from probe characteristics, using the following equation d In L electrostatic the slope of the (4.1) P (4.1) KTe dVp For be probe diagnostics see Chen [ Ref. 16] The measured value of d In L dVp was derived by extrapolation of small hole curves. a distance 10 The plasma cm from i , {Volty 1 2.4 the graph in Figure 35 from the normal electron temperature the target surface. 44 is and 4 approximately 2.4 eV at Figure 29. Distance Gap 5 cm, Probe Bias Voltage -20 V, in Parallel Orientation (Faraday Cupl-lOcm from Target, FC2-5cm from Target) Figure 30. Distance Gap 10 cm. Probe Bias Voltage -20 V. in Parallel Orientation (FC 1-1 5cm. FC2-5cm) 45 Figure 31. Distance Gap 5 cm, Probe Bias Voltage +20 V, in Parallel Orientation + 20 V, in Parallel Orientation (FCl-lOcm, FC2-5cm) Figure 32. Distance Gap 10 cm, Probe Bias Voltage (FC 1-1 5cm, FC2-5cm) 46 Figure 33. Distance Gap 5 cm, entation (FCl-lOcm, Probe Bias Voltage + 20 V, in Perpendicular Ori- FC2-5cm) -<-...' • -, ."." . Figure 34. Distance Gap 10 cm, Probe Bias Voltage entation (FCl-15cm, FC2-5cm) 47 +20 V, in Perpendicular Ori- T -6 -3 -1 VOLTAGE(V) Figure 35. Ln Ie vs Probe Bias Voltage 48 Plasma Density 3. Several approaches were taken to derive the plasma density. Faraday cups were located 10 cm from the of experiments. Faraday cup perpendicular orientation the to direction In this series target with of plasma the flow. Current-voltage characteristic curves were obtained by recording current versus time for a series of Faraday cup voltages. Then, for a given time after the firing of the laser, curves of currents as a function of Faraday cup potentials were plotted. The ion plasma density. «,(cm -3 ), ;• where From Using v,_ = 2 Bohm x 10 4 cm sec, from /;, figure 35 37. The the mass proton (Lexan), ion saturation current density one hole Faraday cup. = -p— = and then and the ion saturation current density was calculated from and Figure 2 for the Therefore. (2.23) envi criterion for the formation of an ion sheath follows e 1. derived using the equation (2.23) j)= ion saturation current density. T = 2A cV 1.5 Tables A/cm the = is and/ = 0.03 A/cm 4.54 x 10 13 (cm -3 ) in the first case, 2 was j — 0.11 t for the 4 small holes. and 1.24 x 10 13 (cm~ 3 ) in the latter. Using the Bohm approximate formula (4.2) for the ion saturation current I^QAenll-jj-A i where, A = Faraday cup hole area [ one hole = 3.33 x 10~ 3 (cm 2 )] [ 4 small holes = 3.91 x \Q-\cm 2 )] 49 (4.2) The Il0 = measured and 3.6 x \0~*(A), //0 = 4 1.0 x 10~ (.4) Therefore, rt,%7.88 x 10 13 cm -3 (one hole for and one 4 is was Figure 37, holes respectively. n,^\.S6 x 10 13 cy??- 3 (4 small holes ), plasma density, n e {cm-^) Electron from current saturation ion ). derived from the electron saturation current using the equation (4.3) Ieo where, Ieo = = ^j-n e v e Ae (4.3) electron saturation current 7KT e (8 A = The hole data ne = ), Ieo Faraday cup hole put 6.95 x into 10'Vm- was estimated x 10~ 3 A (4 small holes equation 3 (4.2), then (4 small holes = 1 .04 x \0 6 cm/ sec area. electron saturation current = —11.3 ) ) from Figure calculated ). 50 Ieo ne = = — 13.3x 39. 10 _3 A Therefore, 9.6 x 10 13 ca?7~ 3 (one (one all the hole) '1 t U 1 i 1 1 ' 1 ••/* CO / / / 1 j I CD - LEGEND : I > o to tO w PL, 1 1 ONE HOLE(PERPEN) 4 SMALL IIOLEiPEI^PEN] j I 1 1 /y : / ! I ' J lO / /' /' /' I • • • •J j 1 ; •1 A lO CM <: 1 j j-y • w o < o > 1 j j / / I / / / / i to ji I ji to I // to - I „,--< QZ.^' lO PI 1_l lO CM -12 i -25 -22 -19 -16 -13 -10 -4 -7 5 8 VOLTAGE(V) Figure 36. Peak Voltage vs Bias Voltage (-20 51 V to + 10 V ), Distance d= 10cm 11 : 11 A d, 1 / 1-- / i A i ,A J: / / / 4 LEGEND ONE HOLE 4 SMALL HOLE * co c(i o / / ' / / 1 / < o / A / / / j/ A 1 , / 1 / CO / 1 / 1 i CO 1 u i / • ' 1 / 1 / ; '/ CO rA / / ' I M 1 C\} Ti 1 \ II ; 1 v H -f \ -w~ L J _ ' _ - - -c ^.- ; i -10 -8 .... -6-4-2 i i i i 1 2 1 4 VOLTAGE(V) Figure 37. Current vs Bias Voltage (-10 V 52 to +7 V ), Distance d= 10cm 1 — ' o o —— i Cr> i i O I \a co i o co o I o o ^ o m CD I U--Q / * r. .jt*. r /* I > A o ^ mO v >> I r , O > <c w // 7/ // // // o S ° t m co /// / / o , ' '• • • ! ! CO o I /<2 i / in CM o / / // O o I v CM / / o I J / in 7 / •r-l CD O / LEGEND ONE HOLE 4 SMALL HOLE / ' '. • r ' 10 1 20 30 i i 1 40 50 60 70 VOLTAGE Figure 38. Peak Voltage vs Bias Voltage (0 v to 53 00 90 100 (V) +105 V ), Distance d= 10cm 110 i ; i ; CO CO • • ,-Q „-" »/v" / • C\2 / i / ,' n H W K • / / / ® ' i / / o ' ! / • / / / / ) u A y / / / / CD - / / a. - (M LEGEND ONE HOLE 4 SMALL HOLE ' -v 3 / o / , -v / / // // // // / A A / / / / I 1 1 1 10 20 30 60 50 40 70 00 90 VOLTAGE(V) Figure 39. Current vs Bias Voltage (0 V to 54 + 105 V ), Distance d= 10cm 100 110 Table 1. VOLTAGE CURRENT CURRENT DENSITY MEASURED WITH FARADAY CUP WITH ONE ENTRANCE HOLE(D=I0CM) probe voltage(V) , peak. , voltagc(mV) current(10- 3 A) density current (A/cm -20 20 0.4 0.12 -10 18 0.36 0.11 -7 17 0.34 0.1 -4 12 0.24 0.07 -15 0.3 0.09 10 -100 2 0.6 20 -200 4 1.2 30 -300 6 1.8 40 -380 7.6 2 28 50 -430 8.6 2.58 60 -550 11 3.3 70 -620 12.4 3.72 80 -720 14.4 4.32 90 -800 16 4.8 100 -890 17.8 5.35 105 -900 18 5.4 55 : ) Table 2. VOLTAGE, CURRENT, CURRENT DENSITY MEASURED WITH FARADAY CUP WITH 4 SMALL ENTRANCE HOLES( D= 10CM ) probe current density peak voltage(mV) current(10" 3 A) -20 6 0.12 0.03 -10 5 0.1 0.025 -7 5 0.1 0.023 -4 3 0.06 0.015 -12 0.24 0.06 10 -96 1.92 0.49 20 -185 3.7 0.9 30 -250 5 1.28 40 -340 6.8 1.74 50 -440 8.8 2.25 60 -520 10.4 2.66 70 -590 11.8 3.02 80 -680 13.6 3.48 90 -700 14 3.58 100 -720 14.4 3.68 105 -760 15.2 3.89 voltage(V) 56 (A /cm 2 ) SUMMARY AND DISCUSSION OF RESULTS V. The objective of this experiment produced plasma using Faraday cups. was to perform measurements on laser Current signals were measured at different distance from the target and at variable probe bias voltage. SUMMARY OF RESULTS A. support the analysis of the data In order to results follows 1. The to be presented, a summary of the : shielding distance can be computed from the Debye length with the electrostatic potential replacing the usual thermal energy. 2. The Faraday cup detectors with electrostatic separation of ion and component of an expanding laser produced plasma offers some ininto the dynamics of the plasma expansion. use of the electron sight 3. The plasma expansion cm 4. velocities measured was determined to be 1.25 x 10" sec. The electron temperature the electron repelling T = Figure 35, that e was evaluated from the semi-logarithmic plot of was found from part of the characteristics. It 2.4el' , 10 cm from the target surface. 5. Electron saturation current was almost 10^(A), ion saturation current was nearly 10~ 4 (/i), as shown Figures 37 and 39. 6. The plasma and electron density was approximatly lO'^m -3 at the distance of 10 cm. since the plasma was quasineutral the density of ions, n, and ion the density of electrons. n e plasma density B. . might be approximately equal, therefore, the is DISCUSSION OF RESULTS The plasma potential may not be defined experimentally by a clear change in the behavior of the current characteristic, and that the current to the probe at plasma potential will be greater than the thermal current to the probe 57 in the sta- tionary plasma case [Ref. the area of the The 17]. ion current is Faraday cup projected normal given by the ion flux intersecting to the direction of flow, proximation which should be very good for the energetic ions found an ap- in a laser produced plasma. The sparking ( where, P potential V = pressure, d= distance between molecules in the gap collide with gas molecules cup and few ionizations take ever, / is to ionize. In to occur, cover and detector propotional to (Pd). At low P, / is and few electrons can breakdown against Pd correlate for the following reasons: V place. In order to ; ( mean ) free similiar large At large P, mean free howpath electronic or molecular excitation. order to produce enough ionization in the gap, A ) is have a number large enough for small and few electrons acquire sufficient energy over a for larger P. path most of them impinge on the has to be larger as P becomes smaller. Hence most of the electrons produce The number of argument would apply V must be large and for a variation of d. is higher In this experiment, sharp and thick rim of Faraday cup detector shows larger peak voltage than detector with thin and round rim larger amplitude (i.e. ). Current- Voltage characteristic curves were obtained by recording current versus time for a series of probe voltages ; then, for given times after the Firing of the laser, currents were plotted as a function of probe potentials. The saturation of ion current for negative voltages floating potential, for which I = 0. is clearly evident. The occurs at VzzO, as described in the previous chapter of this thesis. In the vicinity of the floating potential, the current appears to increase expo- nentially with voltage relative to the saturation ion current. few volts negative relative ture and appears to At a potential of a ground, the characteristic curve changes to level off to the ion saturation current. At curva- positive potentials, however, the current increases approximately linearly with voltage. 58 its In the parallel orientation the probe, if the hole diameter dition equal electron Faraday cup eharacristic corresponds was greater than the Debye and ion current would flow 59 length. into the Under Faraday cup. to double this con- VI. CONCLUSION AND RECOMMENDATIONS In this investigation the focused beam used to irradiate Lexan targets within a placed inside the of single pulses from a signals received Targets were irradiated at a pressure of 10~ 5 Torr. The was to evaluate laser plasma offers some velocity, the density and the electron temperature of the and electron components of an expanding insight into the Faraday cups transverse nostic tools in expanding laser plasmas laser produced and plasma flows, may be used as diag- similiar environments. In order to pursue the results of this research, the following 1. electrostatic dynamics of the plasma expansion. to high velocity for future research are presented was produced plasma using plasma were determined. The use of the Faraday cup detectors with separation of ion laser were displayed on an oscilloscope. Faraday cups. The flow 2 vacuum chamber. The Faraday cup was vacuum chamber, and objective of this experiment C0 recommendations : plasma velocities of laser-target interaction using Faraday cup, but with different target materials (metals increase in plasma ion mass should markedly reduce Investigate the ). An their velocities. 2. A concurrent project for further research is to use alternate for examination of the laser produced plasma diagnostic techniques plasma characteristics. 3. An investigation of plasma expansion velocity and plasma density should be conducted for varing background pressure and irradiances. It is into the hoped that this study measurement on the will help to stimulate further interest laser and research produced plasma using Faraday cups. 60 APPENDIX The CO : LUMONICS TE-822 HP CO^ LASER OPERATING PROCEDURE A. high energy pulsed laser study of plasma surface interactions be operated in strict the primary research instrument for the is at the Naval Postgraduate School. It must accordance with the operating procedures and safety prec- autions as established by prior research and updated in this appendix [ A. B ]. Prior to operating the laser system, an individual should complete a retina scan eye examination, receive an orientation briefing from the Physics Depart- ment's Lab technician, and become thoroughly familiar with all proccdual and safety aspects of the laser system. During the orientation hazards and safety requirements briefing, the potential The most detrimental hazard associated with the laser system should be stressed. is the invisible C0 beam 2 (10.6 microns ) which is ourside the visible range. advertent exposure of the eyes and other body parts could result therefore, eye protection should be confinement facility worn by all fired. is injury in all interlocks sho should be UNDER NO CIRCUMSTANCE should an interlock be overridden unless the Physic Department's Lab technician present or notified. The electrical interlocks, pulse initiation circuit, include 1. which are contained in is the laser : Laser Enclosure Cover Interlocks Enrure that the ; personnel, the target container should be completely closed, and operational before the laser In- (2) laser cabinet covers are in place to prevent electrical shock from the high voltage power supplies and other interior electrical components. 2. Laser Output Ensures that the laser Port is Protective Cover Interlock not inadvertently pulsed with the output port protective cover in place causing reflection back into the internal optics of the laser. 61 Cooling Water Flow 3. Ensures that proper cooling water flow and presure arc maintained laser in the system so that the temperature sentitive high voltage power supplies do not overheat and fail Thermal on thermal overload. interlocks associated with the high voltage power supplies are designed to trip on temperatures in excess of 125 degrees Fahrenheit. Laser Power Key 4. Ensures that power not available to the laser system until consciously is applied by the operator. Gas on 5. / off Switch Ensures that high voltage How has been properly established Ensures that the laboratory door o\ not applied to the firing circuit unless gas the laser. in Plasma Laboratory Door 6. operators is this is lasei firing circuit will opened during laser be temporarily disabled system operation. An problem. interlochs can never replace the requirement for an alert It is the audible alarm alerts Although the interlock system does afford considerable ator. if with this in mind TE-822 within the Lumonics safety, electrical and conscientious oper- that that following operational procedure contained HP instruction manual [Ref.10]. LASER SYSTEM START-UP A. 1. Turn on the external voltage regulator and adjust its output for 1 19 volts. NOTE The high voltage power supplies inside the laser are necessary to regulate the input voltage fore, it laser output and performance characteristics. 2. is regulated ; there- order to acquire consistent Activate the laboratory door interlock by placing the toggle switch on the control box to the 3. in NOT left Initiate cooling of the door to the on position. water flow and set the thermostat on the cooling unit to 15 degrees Celsius. 4. Set the mode select switch to single 62 and the MULTIPLIER setting to X10. NOTE MULTIPLIER The and X10. XI, XI, control setting has three positions which are Three setting are used in conjunction with the INTERNAL RATE potentiometer and apply their stated multiplication factors to establish a desire IN pulse repetition frequency. the XI and X10 positions the capacitors in the and the front panel voltmeter con- laser firing circuit are continuously charged, tinuously registers the high voltage power supply voltage mode, repetitive pulsing not po possile but the is level. IN SINGLE X10 MULTIPLIER shot setting is used so that the high voltage power supply voltage can be monitored continuously during the conduct of the laser start up procedure. 5. Open the Helium, Carbon Dioxide and Nitrogen Cylinder valves. NOTE The pressure regulators on each bottle are not adjustable until the gas is flowing through the laser system. 6. Turn the LASER POWER KEY to ON GAS OFF inWHITE and the and note that the INTERLOCKS OPEN indicator is WARM UP INCOMPLETE indicator is YELLOW. NOTE The WARM UP INCOMPLETE indicator will extinguish after approximately minute after the LASER POWER KEY is turned on. 7. Slowly open the HEAD EXHAUST VALVE by placing the valve operadicator is GREEN, the 1 tor in the vertical position. The valve is located in the lower right hand corner of the control panel. CAUTION Failure to open the quickly become HEAD EXHAUST VALVE The overpressurized. NON-RESETTABLE personnel to replace. 5 psig pressure relief valve if equipped is with to a which requires maintenance Failure to open this relief valve will place the laser system out of commission until a replacement valve of gas or laser can cause the head the laser energy is is installed. If there is excessive use extremely erratic, notify the Physics Department 63 Lab Tcchnican, open cator GAS ON Depress the is lit 9. for gas leaks these are the ; push button and observe the GREEN GAS OFF while the indicator is RED GAS ON indi- extinguished. After 15 seconds, adjust the pressure regulators to 10 psig. Adjust the 10. six three on the rear panel 6 and check blown head gasket. signs of a 8. the laser cabinet, SCFH Brooks flowmeters (three on the front control panel and to the following reading ) : 8 SCFH for A'2 and C0 2 , and for He. NOTE The gas flow rates have been established for plasma research Postgraduate School. These flow rates will produce single shot energies up joules with pulse widths of approximately 5 microseconds. be changed in at the Naval to 15 These flow rates can accordance with the Lumonics laser instruction manual. NOTE Continually monitor the pressure regulators and Brooks flowmeters throughout the operation of the laser to insure that the pulse width and energy output of the laser do not change. Fluctuation in the gas flow rate can change the pulse width and laser output significantly. 11. LASER OUTPUT PORT PROTECTIVE COVER. CAUTION remove the LASER OUTPUT PORT PROTECTIVE COVER Remove Failure to could result in the damage to the internal optics of the laser. There an is interlock to prevent the firing of the laser with the cover in place ; electrical however, it should be physically verified that the cover or alignment mirror has been removed before firing. 12. Allow the gas to flow through the laser cabinet for 30 minutes before firing the laser. NOTE Do not stop the gas flow once than 30 minutes will occur. The has be initiated except when a delay of more it C0 2 Will diffuse out of the molecular sieve inside the laser cavity thereby producing erratic shots. 64 Open 13. the air cylinder and sot the pressure regulator to 18 psig tablish a flow rate of 4 SCFH After the 30 minute 14. checks of the and es- by adjusting the 6 flowmeters on the rear panel. warm up time is complete, prepare for an alignment laser. NOTE Before every firing period, and alignment are verified. laser and earth quakes can covers, 15. it strongly is recommended that the burn pattern Temperature changes, removal of shift the alignment of the laser cabinet laser. Put a piece of black weighing paper on a brick, and place the brick at the alignment verification spot located inside the target containment area. that the laser beam path is clear of all Insure obstacles except the bricks. CAUTION The target containment facility should this point. NO ONE SHOULD BE FACILITY. have the door and windows closed INSIDE at THE TARGET CONTAINMENT All personnel should always put eye protection on anytime the high voltage power supply is going to be activated. HV CONTROL KNOB fully counterclockwise to its MINIMUM setting and depress the RED HIGH VOLTAGE ON push button. 17. Turn the HV CONTROL KNOB clockwise until the volumeter indicates 16. Set the 25 HV. CAUTION NEVER HV at a signer allow the high voltage to exceed 40 HV. the laser can operate at 40 slow single pulse rate of one shot every minute recommends using the laser at settings of 36 HV however, the laser de- ; and below to avoid damage to the high voltage power supplies and internal optics. 18. pressed. The laser will Check now fire to insure the protection, and press the 19. Press the each time the chamber SINGLE fire is clear SINGLE and all fire push button 65 de- personnel are wearing eye push button. GREEN HIGH VLOTAGE OFF CAUTION is push button. GREEN HIGH VOLTAGE OFF The push button should be illuminated before entering the target containment area to prevent accidental of the laser. NOTE The burn pattern 30 mm by 33 mm. If for this laser approximately a rectangle with dimensions is the burn pattern is not uniform, a cavity realignment be necessary as described by the Lumonics laser instruction manual. setting 21 or below, is and in fact on the laser output port, turn on the He Ne verify that the center of the damaged correspond to the alignment spot alignment is HV the burn pattern will be nonuniform. 20. Place the alignment mirror laser, If the will correct, the laser is in the target containment prepared for research. not correspond to the center of the scribed in the Laser Instruction area on the wieghing paper does damaged Manual or If facility. the If the alignment spot does area, either realign the laser as de- mark a new spot if the alignmnet is close. 21. While using the He Ne laser tem, align the beam splitter flection line of site as a to align the optical components of the and the energy meter probe. Place a brick dump for the C0 2 in sys- the re- laser. WARNING All optics and detector surfaces should be free of dust. Use canned gas to remove the dust. 22. Remove the alignment mirror, put on eye protection. Push the the SINGLE 23. sire fire close the target cotainment facility, RED HIGH VOLTAGE ON and push button, push push button, and observe the energy meter reading. Adjust the HV CONTROL KNOB setting on the front panel to the de- energy otuput. NOTE Verify the energy output of the laser before irradiating targets minutes has elapsed since the previous amount of fluctuation inevitable with a firing. C0 2 66 if more than 5 This verification will reduce the laser. LASER SYSTEM SHIT B. 1. Insure that the DOWN PROCEDURES GREEN HIGH VOLTAGE OFF : push button is illuminated 2. Close 3. Wait all of the gas tanks until all SCFH meters arc reading zero, and then push the GAS OFF push button. HEAD EXHAUST VALVE. 5. Turn the LASER POWER KEY to OFF WARNING Before turing off the cooling unit, the HEAT 4. Close the light must be illuminated. necessary increase the temperature of the cooling unit the HEAT If light illumi- nates. of 6. Turn 7. Cover the 8. Turn 9. Cover the off the cooling unit. LASER OUTPUT PORT. off the door interlock switch and the audible alarm switch. laser with the electric blanket and turn the blanket on the setting 6. NOTE It is only necessary to turn on the electric blanket to maintain the optics of the laser at a constant temperature. If the laser veral days, then temperature control of the optics 10. If the laser is is is not going to be in use for se- not required. not going to be utilized for several days, turn off the external voltage regulator. 67 . APPENDIX VEECO B. VACUUM SYSTEM OPERATING 400 PROCEDURES The VEECO for research of CQ 2 laser in 400 Vacuum System in strict in this Prior to operating the C0 2 laser Targets can be irradiated with the reduced presure conditions ranging from 760 torr dures as established to 10~ 6 Torr. This accordance with the updated operating proce- appendix [ Ref. 11 vacuum system, an ]. individual should receive an orien- from the Physics Department's Lab technician and bocome briefing throughly familiar with all During the orientation associated with the procedure and safety aspects of the vacuu system. briefing, the potential vacuum system should be these hazards are the exhaust and the utilized in conjunction with the plasma surface interactions. system must be operated tation is electrical hazards and safety requirements stressed. fumes that can develop danger created if if The most significant of the exhaust system fails Upon the cooling hose breaks. detection of any unusual odors, leaks or sounds, the Physics Department's Lab technician should be immediately notified. tional procedure is It is with this in mind that the following opera- provided. VACUUM SYSTEM START-UP A. depicts the position of the controls on the Figure VEECO vacuum system referenced in the following instructions in Figure 40 on page 72. NOTE CLOCKWISE rotation of the vents or COUNTER-CLOCKWISE rotation opens the valves. valves CLOSES control counter-clockwise them. WARNING When SLOWLY 1 2. ever any valve to avoid Close all is opened, rotate the damaging the vacuum system. valves and vents. Set the pressure MULTIPLIER KNOB 68 to 10 4 position. Turn on 3. chanical MECHANICAL PUMP ON OFF SWITCH. the pump run for approximately 30 minutes to outgas the oil Let the mereservoir. NOTE The time outgas the to oil depend on how long the reservoir pump has been off. Open 4. the FORELINE VALVE to allow the diffusion pump to oil be outgassed. • Turn on 5. the vacuum gauge POWER ON BLUB Set the 6. using the POWER ON OFF SWITCH. The should illuminate. TC-1 TC-2 THERMOCOUPLE SWITCH to the TC-1 position. NOTE TC-1 allows observation of uum TC-2 system. 7. Turn on the pressure in the foreline subsystem of the vac- allows observation of the pressure of the chamber. the cooling tap water. WANING if the diffusion pump water, the diffusion oil on off switch is turned without the flow of the cooling overheats and evaporates causing the he heating coil to burn out. 8. When pump the thermocouple gauge gets below 20 microns, turn on the diffusion on oil switch. NOTE As the diffusion oil heats, the pressure on the thermocouple gauge crease initially, and then begin decreasing again. a flow swich on the cooling water line. pump 9. if there Add 10. is a loss of cooling The diffusion The flow switch pump is will in- wired to will turn off the diffusion water flow. liquid nitrogen to the cold trap. After 20 minutes, turn on the ion gauge by pressing the CURRENT ON push button. The FILAMENT ON BLUB 69 FILAMENT should illuminate. HIGH VACUUM CHAMBER OPERATION 1. Close the FORELINE VALVE. 2. Switch the TC-l/TC-2 THERMOCOUPLE SWITCH B. TC-2 to the position. 3. Open the ROUGHING VALVE. WANING In the following step, be sure that the IONIZATION GAUGE does not go off scale. THERMOCOUPLE GAUGE reads below 50 microns, CLOSE ROUGHING VALVE, and OPEN the FORELINE VALVE. SLOWLY 4. the When the HIGH VACUUM VALVE. 5. When the IONIZATION GAUGE PRESSURE MULTIPLIER KNOB to 10 open the 6. it 5 below 0.2 x 10 4 using the switch the torr, Torr. READ CUR- After the pressure gets below 5 x 10 -5 Torr, Switch up the RENT SWITCH just gets to read the emission current, CURRENT ADJUST KNOB it should read 10 and ma ; if not, ad- CURRENT SET KNOB. NOTE It in will probably be necessary to outgas the ion for approximately 15 minutes order to read higher vacuum. DEGAS ON OFF SWITCH. FILAMENT ON BLUB may extinguish. Wait MENT CURRENT ON push button again. If 7.Turn on the again, wait five minutes and When the 6 C. 1 Slowly open the into the chamber, it and the minute and push the FILA- the filament bulb extinguishes reaches 0.2 x 10" 5 torr, switch the Torr. OPENING THE CHAMBER 1. Set the PRESSURE MULTIPLIER KNOB 2. Close the HIGH VACCUM VALVE. 3. trip, try again. IONIZATION GAUGE PRESSURE MULTIPLIER KNOB to 10" 8. The filament may CHAMBER VENT. can be opened. 70 4 to 10~ setting. After the air has stopped flowing D. SYSTEM IDLING CONDITION 1. Complete the HIGH VACCUM CHAMBER OPERATION to evacuated the chamber. 2. Colse the HIGH VACCUM VALVE. 3. Press the FILAMENT CURRENT OFF push button. NOTE The vaccum system can operate next experiment E. is in this configuration for several days until the conducted. SHUT DOWN THE SYSTEM COMPLETELY Press the FILAMENT CURRENT OFF push button. 2. Close the HIGH VACCUM VALVE and the ROUGHING VALVE. 3. Turn off the DIFFUSION PUMP SWITCH. 1. WANING Let the diffusion the pump pump cool down for at least 30 minutes to avoid damping before pressing with these procedures. FORELINE VALVE. 4. Close the 5. Turn 6. The mechanical pump can be off the cooling water. secure the mechanical and open the minutes. W hen r the air running in this position indefinitely. To MECHNICAL PUMP ON OFF MECHANICAL PUMP VENT for approximately 5 flow has stopped, close the MECHANICAL PUMP pump, SWITCH left turn off the VENT. 71 CM 30 * c F 6 G H o o (T) (?) y r A - B - C D E F - G H - I - .1 - K L - M N - P - Q - R - S - T U - V - Figure 40. I L M (?) r o ROUGHING VALVE MECHANICAL PUMP VENT FORELINE VALVE Chamber Controls 72 O » MECHANICAL PUMP ON/OFF SWITCH DIFFUSION PUMP ON/OFF SWITCH TC-l/TC-2 THERMOCOUPLE SWITCH THERMOCOUPLE GAUGE IONIZATION GAUGE ZERO ADJUSTMENT POWER ON BULB DEGAS ON BULB FILAMENT ON BULB POWER ON/OFF SWITCH DEGAS ON/OFF SWITCH CURRENT SET KNOB CURRENT ADJUST KNOB READ CURRENT SWITCH FILAMENT CURRENT OFF PUSH BUTTON FILAMENT CURRENT ON PUSH BUTTON PRESURE MULTIPLIER VALVE HIGH VACUUM VALVE CHAMBER VENT VEECO Vacuum u o p q w. 1 S j V n APPENDIX One SINGLE LANGMUIR PROBES C. of the most venerable of plasma diagnostic techniques involves the current-voltage characteristics of probes immersed in the plasma. acteristics for a single probe, and ion-free paths, are shown Typical char- whose dimensions are small compared for a plasma, in the absence of to electron magnetic field, in Figure 41 on page 74. The probe potential point, such as the is measured with respect anode or walls of a to some convenient, fixed-potential discharge tube or a "floating probe". The requirement that the potential difference between the plasma and the reference point remain fixed often excludes the use of the anode for this purpose. probe potential can be specified in respect to the The plasma potential denoted by V (potential). When V made is very negative The random collected. all ion current passing through an area related to the ion densitv and the velocity 7 ./, /, = = the the random random n ion current, i the area of the probe, m.= the ion A in the plasma is / 2AT , n Amp ion current density, Ap = and only ions are : v < i where, electrons are repelled Amp/cm 2 mass n,= th e ion d ensitv. v Equation 1 = / 2KT —— = mean /— would be • kinetic ion velocity. valid also for ion current collected of the probe caused no perturbation in the surrounding The probe does perturb the plasma, however, the pies provides an energy sink for all particles 73 which by a probe if the presence random plasma currents. volume which the probe occustrike it and the fringing fields I (probe current) III eo II a ® y (probe potent. Lai) iO J (current density) _-——TT i II J\ Figure 41. V-I Characteristics of Single Langmuir Probe 74 extend for a considerable distance into the surroundings. lected ion current density seems than of the ion temperature. to be more This effect As a result, the col- a function of the electron temperature is due to the formation of a positive sheath around the probe, caused by electrons repelled by the probe's electric The extent of the sheath's influence For T, < T e the ion current density and equation can be modified 1 is determined by the electron temperature. nearly independent of the ion temperature, is to : J^OAn^-jzf- {Amp/cm When the probe potential, V, made is less 2 (2) ) negative a few of the high energy As electrons are collected, partially cancelling the positive ion current. tential changed further is in currents collected just cancel. the positive direction the random This probe-plasma potential. "floating potential", the value at field. which a floating Vf 'm ion the po- and electron Figure 41 is the probe would ride when placed into the plasma. V Increasing results in a steep rise in electron current, in region This current eventually saturates at the "space potential" value, (Figure 41). due beyond Vf to space-charge limitation in current collection. of V, we have = I] rithmic dependence 0. In region A , = The Jo =J + e = total current, since we write our definition = F )V + {-jf In AJ (3) the area of the probe sheath *zA p \Jf ; \ the electron charge. probe current it s, the probe electron current follows a loga- J = the random electron current density s to V : In Ie where II According II is is the difference between the electron current and ion the probe current, I, that : 75 we measure, to find Ie in equation 3 W+l/,1 (4) and Equation 4 implies that the probe sheath A s faAp so that ness is curve slope constant function a When we this is in plot the logarithm of the region VmO [f . s, to probe dimensions, to not strictly true, since the sheath thick- T t I,, vs probe voltage V, we obtain the slope of the electrons have a Maxwellian distribution the : </ln/. (/In/. o dV dVp KTe Increasing the probe \oltage beyond J due is eompared thin the potential applied. o\~ constant and yields is This in size. is V i v does not cause space charge. The saturation current is ene A s ' much higher than and the ion density J to rise : 2KTe ///,. Comparing equation (6) with equation (2) we note that /, Ji The V 2/72, electron density can be determined from Equation (6) from Equation (2). In a plasma the two are essentially the same, giving us check on the measurement o( plasma due to the n. a cross In curve-region III, the stray electric field in the probe max be very large and considerable perturbation occurs. > REFERENCES LIST OF 1. Ready, of high power laser radiation F., Effects J. , pp.433, Academic Press, 1971. 2. Bykovskii . A yu. Neutral particles from the interaction of laser ra- et al., ., diation with a solid target , Soviet Physics-Technical Physics, V.19, pp.1635, June 1975. 3. Chang. C. Hashmi, M., and Pant, H. C, Study of a T., by Langmuir probes 4. Brooks with K , M . .. laser produced plasma Plasma Physics, V.19, pp.1 129-1 138, 1977. , An investigation laser-produced plasmas , of early disturbance found in association M.S. Thesis. Naval Postgraduate School, Monterey. California, December 1973. 5. Callahan D , . J Laser plasma particle velocities ., , M.S. Thesis. Naval Postgraduate School, Monterey, California, June 1976. 6. Frank- Kamenetskii, D. A., Plasma, the fourth state of material Plenum 7. Press, 1972 Chen. F. V.l, pp.290, 8. Langmuir, Plenum I.. The currents in high 9. Lochte, W., pp.159, . Introduction to plasma physics F., , and controlled fusion, 2nd ed , Press, 1984. effect vacuum , of space charge and residual gases on thermionic Physical Review. V. 2, pp.450-486, 1913. Holtgrevcn ., Electrical probes, pp.668-731. North Holland, Amsterdam, 1968. 77 in plasma diagnostics , 10. Lumonics Inc ., Instruction manual, lumonics TE 820 HP series co 2 lasers , 1976. 1 1. Matheson Gas Products, Inc Matheson gas products catalog 88 ., , pp.180, 1988. 12. Laser Precision Corp instruction 13. manual , ., Rj-7000 series radiometer and Rjp-700 series probes February 1984. Henson, M. V., Generation of electromagnetic radiation due breakdown and unipolar arcing , to laser induced M.S. Thesis, Naval Postgraduate School, Monterey. California. September 1989. 14. Engel 15. Baird. D. . A. V., Ionized gases Huddlestone pp.1 13. 17. , , : An introduction of measurement theory and pp.198, Prentice-Hall, Inc., 1962. R. H., Leonard Academic Segall. S. B., to pp.147, Oxford, Claren-Don Press, 1965. C Experimentation experiment design 16. , , S. L., eds. Plasma diagnostics techniques , Press, 1965. Koopmann, D. W., streaming plasma dignostics , Application of cylindrical Langmuir probes Physics of Fluids, V.16, pp.1 149, July 1973. 78 INITIAL DISTRIBUTION LIST No. Copies 1. Defense Technical Information Center Cameron 2 Station Alexandria. VA 22304-6145 2. Library. Code 0142 Naval Postgraduate School Monterey. CA 93943-5002 3. Prof. K. E. 4. Prof. F. R. Schwirzke. 5. Prof. R. C. Olsen, 2 Woehler, Code 61Wh Department of Physics Naval Postgraduate School Monterey. CA 93943-5000 Code Department of Physics Naval Postgraduate School Monterey. CA 93943-5000 Code 61 1 Sw 2 61 Os 2 Department of Physics Naval Postgraduate School Monterey, CA 93943-5000 6. LCDR. Youn. Duck Sang Postal In 2 Code 560-231 Hu Dong 1 ga 171-41. Duck Jin Gu Jeon-Ju CityRepublic of Korea 7. LT. Michael V.Henscn Surface Warfare Officers School 1 Command Newport, RI 02841-5012 8. Naval Academy Librarv Code 602 Hae city Gyung 1 Postal Jin , Nam Republic of Korea 79 Naval War College Library Code 602 Hac City, Gyung Postal Jin Nam Republic of Korea 80 Y635 *oun CI Heasi Parad 3 1 4UC P a S™a V-cu ps . o.i 2ia 5 if /635 c.l Youn Measurements on laser produced plasma using Faraday-cups. u «n«

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