Download Estimation of electron temperature and radiation emission of a low

Indian J Phys (January 2014) 88(1):97–102
DOI 10.1007/s12648-013-0373-6
ORIGINAL PAPER
Estimation of electron temperature and radiation emission of a low
energy (2.2 kJ) plasma focus device
M Z Khan1,2*, S L Yap1 and C S Wong1
1
Plasma Technology Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
2
Department of Physics, Federal Urdu University of Arts, Science and Technology, Islamabad 45320, Pakistan
Received: 13 May 2013 / Accepted: 29 July 2013 / Published online: 14 August 2013
Abstract: Radiation emission in a 2.2 kJ Mather-type plasma focus device is investigated using a five channel BPX65
PIN diode spectrometer. At optimum condition, radiation emission from the system is found to be strongly influenced in
hollow anode and filling gas pressure. Maximum X-ray yield in 4p sr has been obtained in case of hollow anode in argon
gas medium due to interaction of electron beam. Results indicate that an appropriate design of anode can enhance radiation
emission by more intense interaction of expected electron beam with hollow anode. The outcome is helpful to design a
plasma focus with enhanced X-ray generation with improved shot-to-shot reproducibility in plasma focus device.
Keywords:
Plasma focus; Radiation emission; Argon plasma; Electron temperature; Electron beam
PACS Nos.: 52.58.Lq; 52.70.La
1. Introduction
There are number of ways to study electron/ion temperature as well as X-ray yield within different kind of devices
such as Mather/Filippov type dense plasma focus [1, 2], rf
plasma [3], plasma blob [4], dusty plasma [5, 6] etc. In
Mather-type plasma focus device, the plasma pinching
phenomena has been developed as a plasma fusion device
with an emphasis to generate neutrons. Due to the failure of
these attempts for many reasons, scientists have explored
other use of such device. Application prospective of the
dense plasma focus as an intense X-ray source has led to
influential studies of such device [7–9]. Plasma focus [7,
10], laser plasma [11], Z-pinch [12] appear more favorable
single beam point source of various types. Plasma focus
(PF) have advantages due to simpler in design, low cost
and compact. Uses of PF device have already been
revealed in the field of X-ray lithography [7, 13], X-ray
microscopy [14] and X-ray radiography [15] etc.
*Corresponding author, E-mail: [email protected]
In PF devices, several determinations have been made
to study X-ray emission. Kato and Be [7] have studied Xray emission from a low inductance 2.8 kJ (\924 kA)
plasma focus device using an admixture of gases and
obtained the X-ray yield of 112 J into 4p sr. Burkhalter
et al. [9] have calculated X-ray emission from Neon
plasma by varying the bank energy from 1 to 4 kJ (340–
370 kA) and by changing diameter of anode and cathode.
They have estimated X-ray yield up to 14 J into 4p sr.
They have reported the improvement in X-ray yield by
increasing bank energy and by reducing diameter of anode.
X-ray production and pinched features are meaningfully
modified when PF is operated with an admixture of gases
[16]. Gurei et al. [17] have described that a PF device
(4 kJ, 400 kA) depends on the dynamics of plasma,
emissions of neutron and X-rays.
It is obvious that X-ray yield in a PF depends on different experimental factors like circuit inductance, bank
energy, anode geometry and operating medium. Purpose of
the present work is to investigate X-ray emission in a low
energy 2.2 kJ Mather-type plasma focus device. The estimated electron temperature and maximum X-ray yield has
been obtained with shape of hollow anode with argon gas
at optimized pressure. The outcome is helpful to enhance
X-ray generation with better shot-to-shot reproducibility in
dense plasma focus.
Ó 2013 IACS
98
M Z Khan et al.
Fig. 1 Schematics of UM plasma focus device
2. Experimental details
Mather-type PF device was energized by a single of 30 lF
Maxwell capacitor charged up to 12 kV in present experiment. Actual depth of hollow anode was 27 cm up to
closing end of the system but effective length of hollow
anode was 18 cm as shown in Fig. 1. The calculated total
external inductance was found to be 165 nH. The discharge
tube consisted of an inner electrode, which was made of
hollow copper pipe (of 1.9 cm diameter/18 cm length).
The photo of plasma focus device and outer electrode was
seen as a group of six copper rods which formed shape of a
squirrel cage with an inner diameter of 3.2 cm as shown in
Fig. 2. Hollow anode and cathode were separated by a
standard Pyrex glass insulator. Component specification of
plasma focus device was listed as per Table 1. A rotary van
pump was used to evacuate the chamber to lower than
10-2 mbar, which was sufficient vacuum for the present
experiment. The chamber was refreshed after every shot
and refilled argon gas to reduce gas contamination having
considerable effect on output radiation.
Identical coaxial cables were used for BPX65 PIN diode
spectrometer detector. Rogowski coil and high voltage
probe were used to identify signals at oscilloscope. All
coaxial cables were shielded with aluminum foils during
experiment to reduce the effects of electromagnetic (EM)
noise on data signals. All electrical signals were recorded
by DPO 4043 digital storage oscilloscope. Digital oscilloscope was triggered simultaneously for all signals. BPX-65
PIN diode was housed at 43.50 cm far from the tip of
hollow anode to measure radiation emission from focused
Fig. 2 Inner hollow anode and cathode arrangement of UM-PF
system
Table 1 The components with applied specification of UM-PF
device
Components
Diameter (cm)
Length
(cm)
Material
Vacuum
chamber
14.25/14.50 (O.D/I.D)
61.50
Stainless steel
Hollow anode
1.90/1.60 (O.D/I.D)
18.00
Copper
Cathode rod
0.95
27.20
Insulator sleeve
2.00
5.00
Copper
Pyrex
Table 2 A selection of five PIN diodes covered with Al
foils ? Aluminized Mylar (lm)
No. of PIN diode
Foils
Thickness (lm)
1
Aluminized Mylar
23
2
Aluminized Mylar ? Al
23 ? 20
3
Aluminized Mylar ? Al
23 ? 30
4
Aluminized Mylar ? Al
23 ? 40
5
Aluminized Mylar ? Al
23 ? 100
plasma. The glass windows of PIN diodes were detached
from X-ray detection. The windows were covered with
different thickness of Al foils as given in Table 2.
The fundamental circuit of BPX65 diode was designed
as given in Fig. 3 with reverse bias at -45 V. The transmission curves of BPX65 PIN diode were attached with
Estimation of electron temperature and radiation emission
99
absorption filters, as presented in Fig. 4. Al foils masking
the PIN diode X-ray spectrometer may help to estimate Xray yield in 4p-geometry and the system efficiency for Xray generation. Method of determination of radiation
emission from the device was described elsewhere in detail
[18, 19].
3. Result and discussion
PF device is a magneto-hydro-dynamic coaxial plasma
accelerator [20], where magnetic energy is stored behind
the moving current sheath. A part of this energy is transformed into plasma energy during rapid collapse of current
sheath towards the axis beyond end of central electrode.
Due to growth of sausage instabilities, pinched plasma gets
disrupted within a few tens of nanosecond. Most of the
radiation lies mainly on SXR region (wavelength range 1–
50 Å) [21].
Figure 5 shows schematic of UM plasma focus device
corresponding to their typical signals of Rogowski coil,
high voltage probe and two X-ray with specific Al foil
thickness. The typical signal of X-ray signal using Al foil
(20 lm, 30 lm) with 23 lm Aluminized Mylar is shown in
Fig. 6. Strong focus gives information of a signal pulse,
which starts from 4.65 ls and ends at 4.67 ls with the peak
value at 4.66 ls. A signal of X-ray pulse appears with
maximum peak value 4.66 ls (starts from 4.65 ls to end at
4.67 ls), which is correlated with voltage spike. Delay
period of compression may be the position of hollow anode
top about 9 cm below the tops of cathode rodes. X-ray
pulse peak is synchronized with the peak of Rogowski
signal and high voltage signal from both PIN diode covered
Fig. 3 The fundamental circuit of BPX65 PIN photodiode
Fig. 4 Transmission curves of Aluminized Mylar (23 lm), [Aluminized Mylar (23 lm) ? Al foil (20 lm, 30 lm, 40 lm, 100 lm)]
with Al foil (20 lm and 30 lm) is observed in Fig. 7.
Therefore, radiation emission is expected from the focused
region with small contribution of electron beam hitting
with the edge-surface of hollow anode. It may be assumed
that X-ray pulse could be due to strong interaction of
electron beam with edge-surface of hollow copper anode.
The sharp uniqueness in Rogowski signal and in voltage
signal has directed the development of strong plasma focus
[16]. The operational pressure regime for radiation emission has been obtained with hollow anode by varying argon
gas filling pressure. The optimum pressure of 1.70 mbar of
argon gas has been ascertained by observing a maximum
dip in Rogowski coil signal and maximum spike in voltage
signals along with BPX65 PIN diode signal. The peak/
peaks in BPX65 PIN diode signal have also been observed
from experiments. It has been observed that X-ray emission
Fig. 5 Schematic of typical signal photo of oscilloscope for plasma
focus electrode system
100
M Z Khan et al.
Fig. 6 Typical signal of X-ray
with Al foil (20 lm and 30 lm)
Fig. 7 Typical signal of
Rogowski coil, high voltage, Xray with (20 lm and 30 lm) Al
foil
occurs around 20–30 ns duration in brief pulse/pulses
coincident with Rogowski signal. Johnson [8] and Zakaullah et al. [22] have reported SXR pulse of duration
*125 ns, which is inconsistent with observed pulse duration here. On the contrary, pulse duration of 10–20 ns has
been reported [9, 23] such irregularity might be due to
differences in device parameters.
BPX65 PIN diode signal is clearly depicted by a single
peak at optimized argon gas filling pressure within copper
hollow anode. The peak is generally broader during number of reproducible shots. Figure 8 shows the variation of
X-ray yield in 4p sr and system efficiency versus argon gas
pressure having constant voltage of 12 kV. The maximum
X-ray yield is 2.53 mJ in 4p sr at optimized pressure of
1.70 mbar of argon gas.
X-ray yield is not much notable as compared with these
reported [7, 9] but estimation of electron temperature is a
remarkable result with argon plasma at optimized pressure
of 1.70 mbar with hollow copper anode. X-ray yield is
small due to high inductance. It could be enhanced by
reducing the system inductance, geometry size of system
and other factors.
The ratio of X-ray signal R = I/I0 (where I and Io are
intensities) for different Al foil thicknesses, represents the
Estimation of electron temperature and radiation emission
101
temperature is 7 keV at optimum pressure of 1.70 mbar of
working argon gas with a constant voltage of 12 kV. For
determination of electron temperature, highest electron
temperature is 7 keV at 1.70 mbar pressure argon gas. This
is a very remarkable result obtained using hollow anode in
UM plasma focus device. Limitation of the system is that
we are not able to see photograph of focus but oscilloscope
photo is shown to represent the strong focus.
4. Conclusions
Fig. 8 Variation of total X-ray yield in 4p-geometry and system
efficiency versus argon gas pressure having constant voltage 12 kV
range of electron temperature from 3 up to 7 keV with
pressure range of 1.0–2.5 mbar. The estimated electron
temperature from ratio method is found to be around 7 keV
with optimized argon gas pressure of 1.70 mbar as show in
Fig. 9. It has extensive evidences in plasma focus device
for a particular gas at its optimum pressure operation and
the instability of accelerated electron beam is more energetic. Thus the electron temperature is expected to be
highest at optimum pressure. Even though plasma density
is high, a consequence at presence higher than optimum
pressure is observed which tends to suppress m = 0 sausage instability. Electron temperature is expected to be low
because a substantial factor of the pinch current begins to
flow through rarefied pinch plasma as an effect of dampening in Rayleigh–Taylor (RT) instability creation [24, 25].
In our experimental findings, measured at higher electron
Fig. 9 Calculated absorption curves of Al foils for X-rays from
copper plasma at various temperature and X-ray with argon gas
pressure 1.70 mbar, estimated electron plasma temperature 7 keV
Radiation emission is presented in 2.2 kJ small plasma
focus device using hollow anode in argon gas filling
pressure. Five-channel diode soft X-ray spectrometer is
deployed to study radiation emission. In 4p sr, estimated
electron temperature and maximum X-ray yield are 7 keV
and 2.53 mJ respectively at the optimized pressure of
1.70 mbar for working gas argon with constant voltage
12 kV. It is very significant result using copper hollow
anode. The outcomes are helpful in designing a plasma
focus with enhanced X-ray generation with improved shotto-shot reproducibility.
Acknowledgments Authors acknowledge Mr. Jasbir and Mr. Lim
for technical support. They acknowledge University of Malaya (UM)
Kuala Lumpur, Malaysia and Federal Urdu University of Arts, Science and Technology (FUUAST) Islamabad, Pakistan for funding of
current study. The project is supported by Grant Number RG10210AFR.
References
[1] J W Mather Phys. Fluids Suppl. 7 5 (1960)
[2] N V Filippov, T I Filippova and V P Vinogradov Nucl. Fus.
Suppl. 2 577 (1962)
[3] B T Goh, S K Ngoi, S L Yap, C S Wong, C F Dee and S A
Rahman J. Non-Cryst. Solids 363 13 (2013)
[4] N Sasini, R Paikaray, G Sahoo, D C Patra, J Ghosh and A
Sanyasi Indian J. Phys. 86 151 (2012)
[5] P Chatterjee, B Das and C S Wong Indian J. Phys. 86 529
(2012)
[6] V Prakash, Vijayshri, S C Sharma and P Gupta Phys. Plasmas
20 053701 (2013)
[7] Y Kato and S H Be Appl. Phys. Lett. 48 686 (1986)
[8] D J Johnson J. Appl. Phys. 45 1147 (1974)
[9] P G Burkhalter, G Mehlman, D A Newman, M Krishnan and R
R Prasad Rev. Sci. Instrum. 63 5052 (1992)
[10] S Lee et al. IEEE Trans. Plasma Sci. 26 1119 (1998)
[11] D J Nagel et al. Appl. Opt. 23 1428 (1984)
[12] I N Weinberg and A Fisher Methods Phys. Res. A 242 535
(1986)
[13] W Neff, J Eberle, R Holz, R Lebert and F Richter SPIE X-ray
Instrum. 1130 12 (1989)
[14] R Feder, J S Perlman, J C Riordan and J L Costa J. Microsc. 125
347 (1984)
[15] C H Moreno, A Clausse, J F Martinez, R Llovera and A
Tartaglione Proc. Nukleonika 46 S33 (2001)
102
[16] F N Beg, I Ross, A Lorenz, J F Worley, A E Dangor and M G
Haines J. Appl. Phys. 88 3225 (2000)
[17] A E Gurei et al. Proc. Inter. Symp. Plasma Poland: Warsaw
(2001)
[18] M Z Khan, S L Yap, M A Khan, A U Rehman and M Zakaullah
J. Fusion Energy 32 34 (2013)
[19] M Z Khan, S Ahmad, M Zakaullah, A Waheed, R Ahmad and G
Murtaza J. Fusion Energy 21 211 (2003)
[20] J W Mather Phys. Fluids 8 366 (1965)
[21] K Hirano, N Hisatome, T Yamamoto and K Shimoda Rev. Sci.
Instrum. 65 3761 (1994)
M Z Khan et al.
[22] M Zakaullah, I Akhtar, A Waheed, K Alamgir, A Z Shah and G
Murtaza Plasma Sources Sci. Technol. 7 206 (1998)
[23] M Sadowski, H Herold, H Schmidt and M Shakhatre Phys. Lett.
A 105 117 (1984)
[24] N J Peacock, M G Hobby and P D Morgan Fifth IAEA Conf.
Plasma Phys. Contr. Nucl. Fus. Res. (IAEA-CN-28/D-3 Tokyo)
(1972)
[25] A Jeffery and T Taniuti Magnetohydrodynamic Stability and
Thermonuclear Confinement (New York: Academic Press)
(1966)