Development of high energy pulsed plasma simulator for plasma-lithium trench experiment

To simulate detrimental events in a tokamak and provide a test-stand for a liquid lithium infused trench (LiMIT) device, a pulsed plasma source utilizing a theta pinch in conjunction with a coaxial plasma accelerator has been developed. An overall objective of the project is to develop a compact device that can produce 100 MW/m2 to 1 GW/m2 of plasma heat flux (a typical heat flux level in a major fusion device) in ~ 100 us (< 0.1 MJ/m2) for a liquid lithium plasma facing component research. The existing theta pinch device, DEVeX, was built and operated for study on lithium vapor shielding effect. However, a typical plasma energy of 3 - 4 kJ/m2 is too low to study an interaction of plasma and plasma facing components in fusion devices. No or little preionized plasma, ringing of magnetic field, collisions of high energy particles with background gas have been reported as the main issues. Therefore, DEVeX is reconfigured to mitigate these issues. The new device is mainly composed of a plasma gun for a preionization source, a theta pinch for heating, and guiding magnets for a better plasma transportation. Each component will be driven by capacitor banks and controlled by high voltage / current switches. Several diagnostics including triple Langmuir probe, calorimeter, optical emission measurement, Rogowski coil, flux loop, and fast ionization gauge are used to characterize the new device. A coaxial plasma gun is manufactured and installed in the previous theta pinch chamber. The plasma gun is equipped with 500 uF capacitor and a gas puff valve. Plasma produced in the chamber by the plasma gun has ne ~ 10^21 m-3, Te ~ 10 - 20 eV that lasts for 150 us. The velocity of the plasma ranges from 2.5 x 10^4 to 4 x 10^4 m/s. The increase of the plasma velocity with the plasma gun capacitor voltage is consistent with the theoretical predictions and the velocity is located between the snowplow model and the weak - coupling limit. Plasma energies measured with the calorimeter ranges from 0.02 - 0.065 MJ/m2 and increases with the voltage at the capacitor bank. A cross - check between the plasma energy measured with the calorimeter and the triple probe / optics shows that the plasma energies are in agreement with each other. The effect of theta pinch on preionized plasma has been investigated when operated in conjunction with the coaxial plasma gun. The previous theta coil (1 turn, 40 nH) is connected with 72 uF capacitor bank to handle more energy. The theta coil is reconfigured as a two - turn coil (160 nH) to facilitate the operation of a crowbar. The two - turn coil achieves a maximum current of 300 kA (= 1.2 T) at 20 kV of the main capacitor bank voltage and the operation of the crowbar allows for a monotonically decreasing current. With the 2 - turn theta coil, a maximum plasma energy of ~ 0.08 MJ/m2 is achieved with 6 kV at the plasma gun and 20 kV at the theta pinch. Plasma velocities of 34 - 74 km/s are observed at the first few peaks of theta pinch current. A problem of plasma transport with short delay times is observed. The effects of guiding magnetic field and crowbar on plasma transport has been studied. Guiding magnets (0.3 T and 0.15 T, respectively) are equipped with SCR switches and driver circuits, capacitor banks (8 mF and 4 mF, respectively), and a magnetic flux excluder. Optical emission and calorimeter measurements for plasma gun and guiding magnet experiments show that the guiding magnetic field form a magnetic pressure that impedes plasma from transport to the target chamber. Two effects are observed in the theta pinch and the guiding magnetic field experiment. 1) A less delay time results in a less decrease in plasma energy. 2) When theta pinch magnetic field is reversed, the plasma produces a less emission. A crowbar experiment with the guiding magnetic field shows that a long lasting unipolar magnetic field at the theta pinch and the guiding magnet field give an enhanced plasma transport and a plasma heat flux of 0.42 GW/m2. Ideal MHD simulations are carried out to verify the experimental results on plasma transport. Ideal MHD and Athena MHD simulation code are found to be reasonable approximations to simulate behaviors of the plasma in the experimental condition. Simulation results for the effect of guiding magnetic field on plasma gun show that plasma is accumulated at the guiding magnetic field due to magnetic pressure and it is in agreement with the experimental results. Simulations also show the divergence of plasma when the magnetic field from the guiding magnets is in the opposite direction to the theta pinch magnetic field. Lithium vapor shielding phenomenon is studied in the newly developed machine. A theoretical model for plasma energy absorbed in lithium vapor cloud is derived based on corona equilibrium Experiments in a prototype theta pinch device shows a reduction in plasma energy on a target of 2.7 +- 1.7 J (29 +- 19 %) at 1.6 mTorr and approximately 0 J at 9.5 mTorr. The energy reduction is in agreement with a prediction the theoretical model. Experiments conducted in a new plasma gun do not show much of visible difference in terms of target temperature. The experiments with the plasma gun with the theta pinch, guiding magnetic fields, and the crowbar shows approximately 40 +- 23 J (26 +- 15 %) of energy reduction. The model predicts a similar energy level for the cases and finds that the main differences of plasma parameters in the theta pinch device and in the plasma gun is a larger sputtering with crowbar due to faster plasma velocity and presumably hotter ions / electrons. Finally, the dissertation concludes with a few ways to further improve the device and increase the plasma heat flux. A change in the system design as well as a compact toroid generation are proposed and preliminary results are presented. The dissertation also suggests hardware upgrades which include an increase in the energy at the plasma gun / the theta pinch capacitor banks. At the same time, additional diagnostics will allow to further investigate the effect of pinching on the plasma from the plasma gun as well as determine the overall effect of the guiding magnetic field