Velocity Measurements and Characterization of a Laser Induced

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 cup. It was determined that the majority of the ion velocities fell within the range of 50 to 250 km/s. A beam of 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 mul- Phenomena occurring in space such as supernovae tiple parabola patterns in which each independent and coronal 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 elec- ZeE L tromagnetic forces [1]. These compressions lead to z = L(D + ) (1) Am v2 2 collisionless shockwaves in the plasma, which are be- p lieved to be one source of cosmic ray particle accel- ZeB L eration. The High Energy Density Physics (HEDP) x = L(D + ) (2) Ampv 2 group at the University of California, Los Angeles (UCLA) aims to detect and characterize these types Where e is the charge of an , 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 is the distace from the plates to the detection screen, UCLA in which a, high desnity polyelthylene, C2H4, 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 gitudinal mode. It is capable of a variable output Torr. Magnetic field coils of the same diameter were energy per pulse of 1 to 20 J built up in an 8-pass present at the top and bottom of the chamber. Inside circuit at a repetition rate of 1-6 Hz. The laser can the chamber there were three Velmex X-slide motor run at a repetion rate below 1 Hz if desired. The drives. These motor drives were used to mount and laser’s pulse width (Full Width at Half Maximum) is move the Faraday cup through the chamber in order 15ns [4]. The laser energy used while acquiring data to detect a large range of the ions located through- was 1J at a repitition rate of 0.5 Hz. out 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.

Figure 2: Experimental set up. The red line repre- sents the path of the laser beam. The black beams on the bottom chamber floor are the motor drives. The Figure 1: Layout of the motor drives with respect blocking plate around the iris is not shown. 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 Mea- In addition to the motor drives and target, the surements experimental set up also consisted of a Farday cup with a 6mm diameter apperture mounted on the z- In order for the Faraday cup to detect a real signal, axis motor drive. An iris was placed at a distance it was determined that a negative bias had to be ap- of 5.08 cm from the target with a blocking plate sur- plied to the cup in order to seperate charge neutral- rounding it to block additional plasma. Electric field izing from the ions. Thus, a bias box was plates, 17 cm wide and separated by a distance of made using the circuit from S. Neff et. al [3]. 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 measure- ments. The laser used in this experiment was the high repetition rate Peening Laser from Lawrence Liver- more National Laboratory. The use of this high rep- etition rate laser enabled fast and detailed 2D data Figure 3: Schematic of the Faraday cup and bias acquisition. The Peening laser consists of a Nd:glass box circuit. V = 75 V, Rosc = 50Ω, Rbias = 10kΩ, system with a wavelength of 1053nm in a single lon- Cbias = 22nf

2 After the bias was added to the circuit, the Fara- Thus by multiplying equation 3 with equation 5 and day cup signal began to detect signals that resembled substituting in energy as a function of velocity: other ion spectra from a similar set up [3]. Scans from dN x U the y = 0 to the y =234 motor positions were taken osc = 2 (6) without the electric field plates or iris present in order dv v Rosce to verify that the detector was functioning properly, Where x is the distance of the Faraday cup from the as well as find a general shape of the plasma cloud and target. to estimate the amount of ions per laser shot. The Farday cup signals, as can be seen in Figure 4, both The voltage of the Faraday cup signal was con- increase in amplitude and arrival time as distances to verted into dN/dv using equation 6 and plotted the target decrease, as would be expected. 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:

dN m1/2x = p U (3) 3/2 osc dE (2E) Rosce where Uosc is the Faraday cup voltage, Rocs is the resistance of the scope and E is the kinetic energy of Figure 5: Top: dN/dv for a typical shot on the blow the ion. 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 in the following way: blow off axis at a distance of 0.2738 m.

dN dE dN By integrating the dN/dv curve, the number of = (4) dE dv dv ions detected in a single shot could be determined. From this, an estimate for the total number of ions dE per laser shot was estimated in two ways. When a = m v (5) dv p plasma is created by the laser, the ions spread out

3 over a hemisphere. If it is assumed that the ions z motor positions at multiple distances from the tar- spread throughout the chamber with equal probabil- get were taken around what was thought to be the ity, the total number of ions produced per laser shot center of the beam. These were used to find a rough can be determined by integrating the dN/dv curve, size of the ion beam created by the iris. The maxima and multiplying this by the area of a hemisphere, of the ion signals for each motor position of the scan where the radius is the distance from the Faraday were plotted against the motor position at which each cup to the target. However, data from only one posi- maximum value occurred. This was done for both the tion of the Faraday cup was used and therefore only photoelectron peaks occcuring at the beginning of the a small area of the total hemisphere was detected. To signal as well as the ions signals. From this, the av- correct for this, the number was scaled by the area of erage full width half maximum and thus the average the Faraday cup apperture. beam size was determined to be around 1cm. The 2πR2 beam size for both the photoelectron and ion signals Ntot = N (7) were compared to the theoretical beam size at each πr2 distance. It was determined that both the photo- Where N is the number of ions per shot, R is the dis- electrond 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 deter- mined by integrating over the total area where the plasma will be present and scaling by the apperture of the Faraday cup. π Z 2 Z R 1 4 Ntot = 2 Ncos (θ)RdRdθ (8) πr π − 2 0 By integrating equation 8 it can be shown that NR2 3π N = (9) tot 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 dif- ference between the two methods. The reason for this is believed to be the fact that a plasma bubble Figure 6: Top: plot of the maximum of the ion sig- was not actually formed. For a plasma bubble follow- nals vs motor position in an x motor scan at a dis- ing a cos4(θ) distribution to be formed, a magnetic tance of 0.2438 m from the target. This shows the field needs to be present, however there was no field. size of the ion beam through the iris. Bottom: plot Thus the estimate using the plasma bubble distribu- of the theoretical beam size vs distance from target tion will underestimate the number of ions leading to vs the beam size for both the ion and photoelectron the large discrepancy between the two methods. 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 The iris was added to the experimental set up to cre- of the system. The iris has an apperature size of 2 ate an ion beam. Scans over a large range of x and mm and the Faraday cup has an apperature size of 6

4 mm, which could be causing the signals from multiple motor postitions to overlap.

Electric Field Breakdown

When determining the field strengths needed to cre- ate a Thomson parabola with the conditions of the ex- perimental 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 Figure 8: Paschen Curve for air. 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 The pressure of the vacuum chamber happened to field, it was discovered that the field was breaking be right around the minimum of the Paschen curve down and arcing to both the ground plate and other for air, around 4e-2 Torr, resulting in the necessity grounded metals in the chamber. This occurred be- for small electric fields. In an attempt to fix this is- cause the conditions of the experimental apparatus sue of electric field breakdown, plexiglass plates were were perfect for breakdown at the desired voltage. 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 ob- served between the plate and the target clamp, thus the plastic posts would make the target clamp a float- ing 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 volt- ages without breakdown. When tested with the lower Figure 7: Image of the electric field break down. The pressure and plexiglass plates, it was found that the pressure of the chamber was around 4e-2 Torr with breakdown voltage increased from 660 V to 2700 V a voltage of 660 V which corresponds to an electric leading to a possible increase of electric field strength field strength of about 5,200 V/m. of 16000 V/m. Beyond this voltage, breakdown be- gan occurring at the high voltage connection flange. For a gas at any pressure there is a voltage at No attempt was made to fix this problem due to the which an electric field will break down. This occurs fact that the field was sufficient with the current volt- because the mean free path of the electrons in the age. When the laser was fired, the breakdown voltage gas is larger than the distance between the molecules. decreased due to the interactions of the field with the When a voltage is applied at the breakdown voltage laser and produced plasma, however the field was still of the system, the electrons collide with each other, within range of what was believed to be necessary. accelerating them and thus, causing the electrons to When using the laser, the maximum possible voltage have enough energy to ionize the gas, create a plasma. without a breakdown was 1400 V corresponding to This plasma arcs to the grounded materials system to an electric field strength of 11,000 V/m. discharge the energy, breaking down the field through If voltages of these strengths are desired for future the flow of current in the plasma. The voltage and testing, either shielding would need to be added to the pressure at which an electric field will break down in system’s high voltage connection ports, or a second a gas can be described by what is called a Paschen vacuum pump would need to be added to achieve a curve. lower pressure.

5 Thomson Parabola rated the amplifier leading to ringing in the signal and thus was not used. From the contour data, it The factor of ten error was unknown at the time of was determined that this described set up will work data acquision for the Thomson parabola, and thus to create a Thomson parabola pattern of the ions, the following parameters were used. Electric field but smaller field strengths and higher resolution are strength of 11,000 V/m, magnetic field strength of needed to characterize the composition of the laser 0.024 T (240 G), and an iris size of 1.5 mm. Data induced plasma. was taken using the Faraday cup at a distance of 41.4 The source of the dark region occurring near the cm from the target. The Faraday cup was placed at a top left and top center of Figure 9 is thought to be due fixed z-motor position and the x-motor was moved in to the fact that the electric field occasionally broke 10 mm steps and data was taken at each x position. down when the plasma was produced and thus the This was done for six z-motor positions each 10mm ions were not actually defected in these regions. To apart. The data was then pieced together to give a determine the exact source of these higher density 2D picture and a contour plot of the maximums of regions, more data would need to be taken with and each Faraday cup signal excluding the photoelectron without fields present. There was also no distinct cen- peaks vs the distance from the expected ion beam ter of the ion beam with no fields present and an iris center with no field present. This can be seen in Fig- size of 1.5 mm. This is also believed to be a possible ure 9. explanation of the higher density areas. From the corrected simulation to find the param- eters 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 z- axis 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.

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 dis- covered that the ions were deflected much more than desired. Unfortunately, there was no available time to take data with corrected fields. However, the fastest Figure 10: Plot of the theoretical Thomson parabola of the present ions were still within the field of view for the fixed parameters. 0 indicates the expected with the used parameters. It can be seen in Figure center of the ion beam with no fields present. The 9 that the bottom right of the contour plot gener- z-axis is negative due to the orientaion of the motor ally follows the expected shape of the ions in the field drives (See Figure 1) of view, however the signal was weak and noisy and there was not enough resolution to distinguish be- In the future, a biased grid should be added to tween the ion species in this region. An amplifier the Faraday cup to eliminate the photoelectron peak, was used to take another sample of data to increase which would allow for amplification of the ion signal. the signal strength but the photoelectron peak, which Using an amplifier will also allow for a decrease of the is much higher intensity than the ion peaks, satu- 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, resolution Thomson parabola pattern. Carmen Constantin, Peter Heuer and the rest of the 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 pro- Conclusion gram, giving the students in the program the oppro- tunity to learn and grow in their academic and scien- tific 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 veloc- ity spectrum of the ions, in which the velocities of References the ions fell between 50 and 250 km/s, and the num- ber of ions per shot was determined to be around [1] D.B. Schaeffer, E.T. Everson, A.S. Bondarenko, 8.9 ∗ 1014 ions. Using the described experimental set S.E. Clark, C.G. Constantin, S. Vincena, B. Van up, a rough Thomson parabola pattern was detected Compernolle, S.K.P. Tripathi, D. Winske, W. despite a calculation error leading to incorrect elec- Gekelman, and C. Niemann, Physics of Plasmas, tric and magnetic field strengths. The strengths of vol. 21 (5) (2014). both the electric and magnetic fields need to be low- ered, leading to less deflection and thus a better field [2] R. Webber, J. E. Balmer, and P. Ladrach, Rev. of view of the ions. The photoelectron peak needs to Sci. Instrum., vol. 57 (7) (1986). 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. Harris- used to determine the composition of a laser induced son, Materials and Manufacturing Processes, vol. plasma should be possible. 15 (1) (2000).

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