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Ion Velocity Measurements and Characterization of a Laser Induced Plasma

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Ion Velocity Measurements and Characterization of a Laser Induced Plasma 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 mul- Phenomena 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 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 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 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 electrons 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.
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