Download Ion Velocity Measurements and Characterization of a

Ion Velocity Measurements and Characterization of a
Laser Induced Plasma
J. Pilgram, C. G.Constantin, P. V. Heuer, and C. Niemann
Univserity of California, Los Angeles
Ions emitted by a laser produced plasma were detected with a Faraday cup. It was determined that
the majority of the ion velocities fell within the range of 50 to 250 km/s. A beam of ions was created
by passing the plasma through a 2mm iris to create a beam with a width was about 1cm. To determine
the ion species present in the plasma and the velocity distribution, the ions were passed through electric
and magnetic fields then detected in a 2D plane by a Faraday cup. A rough Thomson parabola shape was
detected, however, the field of view, resolution, and sensitivity of the detector were not sufficient enough
to distinguish between the ion species. The strength of the fields as well as the sensitivity of the detector
needs to be adjusted in order to draw conclusions about the ion composition of the laser produced plasma.
Introduction
on a screen or plate placed at some distance from
electric and magnetic field plates. The result is mulPhenomena occurring in space such as supernovae tiple parabola patterns in which each independent
and coronal mass ejections, are the result of one parabola trace represents a different charge state.
plasma exploding into another. The accelerated The deflection patterns of an ion with a charge to
plasma from the explosion flows into the ambient mass ratio of Z/A and velocity v is described by the
background plasma at a speed higher than the lo- following equations from R. Weber et. al [2]:
cal sound speed, causing compressions through elecZeE
L
tromagnetic forces [1]. These compressions lead to
z=
(1)
L(D + )
2
Amp v
2
collisionless shockwaves in the plasma, which are believed to be one source of cosmic ray particle accelZeB
L
x=
L(D + )
(2)
eration. The High Energy Density Physics (HEDP)
Amp v
2
group at the University of California, Los Angeles
(UCLA) aims to detect and characterize these types Where e is the charge of an electron, mp is the mass
of shock waves and possibly detect the accelerated of a proton, x is the horizonal axis of the detection
particles. To do this, they are conducting experi- screen, y is the veriticle axis of the detection screen,
ments in the Large Active Plasma Device (LAPD) at L is the length of the electric or magnetic plates, D
UCLA in which a, high desnity polyelthylene, C2 H4 , is the distace from the plates to the detection screen,
target is shot with a high powered laser, causing a and E and B are the relative strengths of the fields.
plasma bubble to form and propagate through the
In this paper, I will describe how the velocities as
length of the LAPD [1]. To understand the physics
well as a beam of laser produced plasma ions were
behind how the shockwaves form and create accurate
measured and characterized. I will also describe a
simulations of how the shockwaves will propagate, the
way in which a Faraday cup can be used instead of
ion distribtution of the laser produced plasma needs
a detection screen or plate to obtain the data needed
to be characterized.
to create a Thomson parabola.
One method used to distinguish between the ions
in a plasma is by creating a Thomson parabola. This
is done by passing the ions through an iris, which Apparatus
acts as a large pinhole, into electric and magnetic
fields which bend the ions based on both charge to The experimental set up was placed inside of a 0.4
mass ratio and velocity. These ions are then detected m by 0.8 m diameter cylindrical vacuum chamber
1
that was pumped to an average pressure of 9.2e-3
Torr. Magnetic field coils of the same diameter were
present at the top and bottom of the chamber. Inside
the chamber there were three Velmex X-slide motor
drives. These motor drives were used to mount and
move the Faraday cup through the chamber in order
to detect a large range of the ions located throughout the chamber. Figure 1 shows a basic layout of the
probe drives with respect to the position of the laser
path and a high density polyethylene target used to
create the plasma.
gitudinal mode. It is capable of a variable output
energy per pulse of 1 to 20 J built up in an 8-pass
circuit at a repetition rate of 1-6 Hz. The laser can
run at a repetion rate below 1 Hz if desired. The
laser’s pulse width (Full Width at Half Maximum) is
15ns [4]. The laser energy used while acquiring data
was 1J at a repitition rate of 0.5 Hz.
Figure 2: Experimental set up. The red line represents the path of the laser beam. The black beams on
the bottom chamber floor are the motor drives. The
blocking plate around the iris is not shown.
Figure 1: Layout of the motor drives with respect
to the perimeter of the vacuum chamber. The arrows
indicate the direction of positive movement according
to the motor drive software.
Ion Cloud and Velcocity Measurements
In addition to the motor drives and target, the
experimental set up also consisted of a Farday cup
with a 6mm diameter apperture mounted on the zaxis motor drive. An iris was placed at a distance
of 5.08 cm from the target with a blocking plate surrounding it to block additional plasma. Electric field
plates, 17 cm wide and separated by a distance of
12.7 cm, were placed between the iris and the y-axis
motor drive. The experimental set up inside of the
chamber is shown in Figure 2
Data for the general plasma was taken without
the electric field plates in place in order to have the
freedom to move throughout the chamber. When the
Faraday cup was working correctly, and the ion beam
had been characterized, the plates were inserted and
the distance from the target to the Farday cup was
41.402 cm for all of the Thomson parabola measurements.
The laser used in this experiment was the high
repetition rate Peening Laser from Lawrence Livermore National Laboratory. The use of this high repetition rate laser enabled fast and detailed 2D data
acquisition. The Peening laser consists of a Nd:glass
system with a wavelength of 1053nm in a single lon-
In order for the Faraday cup to detect a real signal,
it was determined that a negative bias had to be applied to the cup in order to seperate charge neutralizing electrons from the ions. Thus, a bias box was
made using the circuit from S. Neff et. al [3].
Figure 3: Schematic of the Faraday cup and bias
box circuit. V = 75 V, Rosc = 50Ω, Rbias = 10kΩ,
Cbias = 22nf
2
Thus by multiplying equation 3 with equation 5 and
substituting in energy as a function of velocity:
After the bias was added to the circuit, the Faraday cup signal began to detect signals that resembled
other ion spectra from a similar set up [3]. Scans from
the y = 0 to the y =234 motor positions were taken
without the electric field plates or iris present in order
to verify that the detector was functioning properly,
as well as find a general shape of the plasma cloud and
to estimate the amount of ions per laser shot. The
Farday cup signals, as can be seen in Figure 4, both
increase in amplitude and arrival time as distances to
the target decrease, as would be expected.
dN
x Uosc
= 2
dv
v Rosc e
(6)
Where x is the distance of the Faraday cup from the
target.
The voltage of the Faraday cup signal was converted into dN/dv using equation 6 and plotted
against velocity to give a spectrum of the number of
ions per velocity. From this, it was determined that
most of the ions fell with in the expected velocity
range of 50 km/s to 200 km/s. Data was also taken
at a single position in the chamber with various laser
energies. It was determined that higher laser energies
led to more ions at higher velocities, as well as more
ions overall.
Figure 4: Voltage vs arrival time for Faraday Cup
Signal of a scan for motor positions y = 0 mm to y
= 234 mm at x motor position of 0 mm and z motor
position of 32 mm. The very first peak occurring very
close to zero in the plot is a photoelectron peak and
does not represent any of the plasma ions.
In order to determine the ion velocity spectrum,
the voltage signal from the Faraday cup needed to be
converted into number of ions per velocity. From S.
Neff et al. [3] it was shown that:
1/2
dN
mp x
=
Uosc
dE
(2E)3/2 Rosc e
(3)
where Uosc is the Faraday cup voltage, Rocs is the
resistance of the scope and E is the kinetic energy of
the ion.
Figure 5: Top: dN/dv for a typical shot on the blow
off axis at a distance of 0.3983 m from the target.
Therefore, to get the signal into the form of num- Bottom: dN/dv for laser energies of 1 J, 2.5 J and
ber of ions per velocity the signal must be converted 4 J. Data was taken on what was believe to be the
blow off axis at a distance of 0.2738 m.
in the following way:
dN dE
dN
=
dE dv
dv
dE
= mp v
dv
By integrating the dN/dv curve, the number of
(4) ions detected in a single shot could be determined.
From this, an estimate for the total number of ions
per laser shot was estimated in two ways. When a
(5) plasma is created by the laser, the ions spread out
3
over a hemisphere. If it is assumed that the ions
spread throughout the chamber with equal probability, the total number of ions produced per laser shot
can be determined by integrating the dN/dv curve,
and multiplying this by the area of a hemisphere,
where the radius is the distance from the Faraday
cup to the target. However, data from only one position of the Faraday cup was used and therefore only
a small area of the total hemisphere was detected. To
correct for this, the number was scaled by the area of
the Faraday cup apperture.
z motor positions at multiple distances from the target were taken around what was thought to be the
center of the beam. These were used to find a rough
size of the ion beam created by the iris. The maxima
of the ion signals for each motor position of the scan
were plotted against the motor position at which each
maximum value occurred. This was done for both the
photoelectron peaks occcuring at the beginning of the
signal as well as the ions signals. From this, the average full width half maximum and thus the average
beam size was determined to be around 1cm. The
beam size for both the photoelectron and ion signals
2
2πR
(7)
Ntot = N
were compared to the theoretical beam size at each
πr2
distance. It was determined that both the photoWhere N is the number of ions per shot, R is the diselectrond and ions followed the general trend of the
tance from the Faraday cup to the target and r is the
theoretical beam size.
radius of the Faraday cup apperture.
Using equation 7 , the total number of ions per
shot was estimated to be 8.9 ∗ 1014 ions. Another
way to estimate the number of ions per laser shot is
to use the theoretical shape of a plasma bubble which
follows a distribution of cos4 (θ). Thus the number of
ions per laser shot for a plasma bubble can be determined by integrating over the total area where the
plasma will be present and scaling by the apperture
of the Faraday cup.
Z π2 Z R
1
Ntot = 2
N cos4 (θ)RdRdθ
(8)
πr − π2 0
By integrating equation 8 it can be shown that
N R2 3π
(9)
2πr2 8
Using equation 9 the number of ions per laser shot
for the plasma bubble was estimated to be 8.4 ∗ 1013
ions. Both estimations fell within the expected range
of ions per laser shot, however, the calculations were
done assuming that there were only singly ionized
ions. The actual number of ions per shot was smaller
depending on which ionation states were present in
the plasma. There is also an order of magnitude difference between the two methods. The reason for
this is believed to be the fact that a plasma bubble
was not actually formed. For a plasma bubble following a cos4 (θ) distribution to be formed, a magnetic
field needs to be present, however there was no field.
Thus the estimate using the plasma bubble distribution will underestimate the number of ions leading to
the large discrepancy between the two methods.
Ntot =
Figure 6: Top: plot of the maximum of the ion signals vs motor position in an x motor scan at a distance of 0.2438 m from the target. This shows the
size of the ion beam through the iris. Bottom: plot
of the theoretical beam size vs distance from target
vs the beam size for both the ion and photoelectron
peaks at multiple distances.
Ion Beam Characterization
It is believed that the ion peak beam is larger
than the theoretical beam size due to the geometry
of the system. The iris has an apperature size of 2
mm and the Faraday cup has an apperature size of 6
The iris was added to the experimental set up to create an ion beam. Scans over a large range of x and
4
mm, which could be causing the signals from multiple
motor postitions to overlap.
Electric Field Breakdown
When determining the field strengths needed to create a Thomson parabola with the conditions of the experimental apparatus, an error of a factor of ten was
made, leading to the belief that electric and magnetic
field strengths needed to be very large. This error
was not known at the time and thus it was believed
that an electric field strength of about 11,000 V/m
was needed. When attempting to create this electric
field, it was discovered that the field was breaking
down and arcing to both the ground plate and other
grounded metals in the chamber. This occurred because the conditions of the experimental apparatus
were perfect for breakdown at the desired voltage.
Figure 8: Paschen Curve for air.
The pressure of the vacuum chamber happened to
be right around the minimum of the Paschen curve
for air, around 4e-2 Torr, resulting in the necessity
for small electric fields. In an attempt to fix this issue of electric field breakdown, plexiglass plates were
added to both sides of the top (charged) plate and
kapton tape was placed on all edges of exposed metal
on the plate. Kapton tape was also placed on the
metal present on the z-axis motor drive and the metal
posts holding the target clamp in place were replaced
with plastic. This was done because arching was observed between the plate and the target clamp, thus
the plastic posts would make the target clamp a floating metal instead of a ground source. The chamber
was also allowed to pump over a four-day period in
hopes of achieving a lower pressure.
When testing the effectiveness of the plexiglass
and kapton tape, the pressure of the chamber was
9.1e-3 Torr. It was discovered that the lower pressure,
plexiglass and kapton tape allowed for higher voltages without breakdown. When tested with the lower
pressure and plexiglass plates, it was found that the
breakdown voltage increased from 660 V to 2700 V
leading to a possible increase of electric field strength
of 16000 V/m. Beyond this voltage, breakdown began occurring at the high voltage connection flange.
No attempt was made to fix this problem due to the
fact that the field was sufficient with the current voltage. When the laser was fired, the breakdown voltage
decreased due to the interactions of the field with the
laser and produced plasma, however the field was still
within range of what was believed to be necessary.
When using the laser, the maximum possible voltage
without a breakdown was 1400 V corresponding to
an electric field strength of 11,000 V/m.
Figure 7: Image of the electric field break down. The
pressure of the chamber was around 4e-2 Torr with
a voltage of 660 V which corresponds to an electric
field strength of about 5,200 V/m.
For a gas at any pressure there is a voltage at
which an electric field will break down. This occurs
because the mean free path of the electrons in the
gas is larger than the distance between the molecules.
When a voltage is applied at the breakdown voltage
of the system, the electrons collide with each other,
accelerating them and thus, causing the electrons to
have enough energy to ionize the gas, create a plasma.
This plasma arcs to the grounded materials system to
discharge the energy, breaking down the field through
the flow of current in the plasma. The voltage and
pressure at which an electric field will break down in
a gas can be described by what is called a Paschen
curve.
If voltages of these strengths are desired for future
testing, either shielding would need to be added to the
system’s high voltage connection ports, or a second
vacuum pump would need to be added to achieve a
lower pressure.
5
Thomson Parabola
rated the amplifier leading to ringing in the signal
and thus was not used. From the contour data, it
was determined that this described set up will work
to create a Thomson parabola pattern of the ions,
but smaller field strengths and higher resolution are
needed to characterize the composition of the laser
induced plasma.
The source of the dark region occurring near the
top left and top center of Figure 9 is thought to be due
to the fact that the electric field occasionally broke
down when the plasma was produced and thus the
ions were not actually defected in these regions. To
determine the exact source of these higher density
regions, more data would need to be taken with and
without fields present. There was also no distinct center of the ion beam with no fields present and an iris
size of 1.5 mm. This is also believed to be a possible
explanation of the higher density areas.
From the corrected simulation to find the parameters for the Thomson parabola, it was determined
that the following parameters should be used: electric
field strength of 2500 V/m, magnetic field strength
of 100 G (0.01 T), and x-axis span of 10 cm, and zaxis span of 5 cm from the bean center with no fields
present. The iris should also be adjusted to achieve
a more distinct ion beam and the resolution of the
Faraday cup needs to be increased in order to achieve
a Thomson parabola that can be used to determine
the composition of the plasma.
The factor of ten error was unknown at the time of
data acquision for the Thomson parabola, and thus
the following parameters were used. Electric field
strength of 11,000 V/m, magnetic field strength of
0.024 T (240 G), and an iris size of 1.5 mm. Data
was taken using the Faraday cup at a distance of 41.4
cm from the target. The Faraday cup was placed at a
fixed z-motor position and the x-motor was moved in
10 mm steps and data was taken at each x position.
This was done for six z-motor positions each 10mm
apart. The data was then pieced together to give a
2D picture and a contour plot of the maximums of
each Faraday cup signal excluding the photoelectron
peaks vs the distance from the expected ion beam
center with no field present. This can be seen in Figure 9.
Figure 9: Contour plot of the 2D scan data. Zero
denotes the expected position of the center of the ion
beam with no fields present. The parabola traces are
correct the expected ion positions based on equations
1 and 2. The z-axis is negative due to the orientation
of the motor drives (See Figure 1)
The calculation error was found and it was discovered that the ions were deflected much more than
desired. Unfortunately, there was no available time to
take data with corrected fields. However, the fastest
of the present ions were still within the field of view
with the used parameters. It can be seen in Figure
9 that the bottom right of the contour plot generally follows the expected shape of the ions in the field
of view, however the signal was weak and noisy and
there was not enough resolution to distinguish between the ion species in this region. An amplifier
was used to take another sample of data to increase
the signal strength but the photoelectron peak, which
is much higher intensity than the ion peaks, satu-
Figure 10: Plot of the theoretical Thomson parabola
for the fixed parameters. 0 indicates the expected
center of the ion beam with no fields present. The
z-axis is negative due to the orientaion of the motor
drives (See Figure 1)
In the future, a biased grid should be added to
the Faraday cup to eliminate the photoelectron peak,
which would allow for amplification of the ion signal.
Using an amplifier will also allow for a decrease of the
the Faraday cup apperture size, as well as a decrease
6
of the iris size, leading to better resolution. With Acknowledgments
a smaller aperature size, smaller steps can also be
taken between data points with the result of a higher The author would like to thank Cristoph Niemann,
Carmen Constantin, Peter Heuer and the rest of the
resolution Thomson parabola pattern.
HEDP research group for welcoming her into their
goup and contributing to and helping her with the
research that was conducted. She would also like
to thank Francoise Queval for organizing the 2016
UCLA Research Experinece for Undergraduates program, giving the students in the program the opproConclusion
tunity to learn and grow in their academic and scientific careers. This research was funded and supported
by the National Science Foundation (Gran No. REU
A Faraday cup biased at -75 V was used to detect PHY-1460055).
ions from a laser produced plasma. The data taken
by the Faraday cup was used to determine the velocity spectrum of the ions, in which the velocities of References
the ions fell between 50 and 250 km/s, and the number of ions per shot was determined to be around [1] D.B. Schaeffer, E.T. Everson, A.S. Bondarenko,
S.E. Clark, C.G. Constantin, S. Vincena, B. Van
8.9 ∗ 1014 ions. Using the described experimental set
Compernolle, S.K.P. Tripathi, D. Winske, W.
up, a rough Thomson parabola pattern was detected
Gekelman, and C. Niemann, Physics of Plasmas,
despite a calculation error leading to incorrect elecvol. 21 (5) (2014).
tric and magnetic field strengths. The strengths of
both the electric and magnetic fields need to be lowered, leading to less deflection and thus a better field [2] R. Webber, J. E. Balmer, and P. Ladrach, Rev.
Sci. Instrum., vol. 57 (7) (1986).
of view of the ions. The photoelectron peak needs to
be eliminated with the use of a bias grid to allow for [3] S. Neff, S. Wright, C. Plechaty, J. Ford, R. Royle,
amplification of the signal and better resolution needs
and R. Presura, 16th IEEE International Pulsed
to be achieved through reduction of both the Faraday
Power Conference, vol 1-4 (2007).
cup and iris apperture sizes. With these corrections,
detection of a Thomson parabola pattern that can be [4] C.B. Dane and L.A. Hackel, J. Daly, J. Harrisused to determine the composition of a laser induced
son, Materials and Manufacturing Processes, vol.
plasma should be possible.
15 (1) (2000).
7