Characterization of ion properties in a linear pulsed plasma-material interaction test stand

In the edge and divertor regions of the magnetically confined plasma, tokamaks experience off-normal events in which high intensities of heat flux incident on wall component surfaces result in intolerable levels of damage. Proposed solid divertors will not be able to withstand these fluxes, especially in larger toroidal machines such as ITER and DEMO. In addition to this detrimental effect on solid wall materials, demonstrations have shown that the erosion of these materials can cause impurity generation and transport within the bulk plasma, leading to high radiative losses. To avoid these and other major issues, liquid metal divertor and wall schemes have been proposed and studies have been done to understand their effect on bulk tokamak plasmas. To simulate extreme events in the tokamak boundary and provide a test stand for liquid-metal plasma-facing components, a pulsed plasma source utilizing a theta pinch in conjunction with a coaxial plasma accelerator has been developed[1-3]. The ThermoElectric-driven Liquid-metal plasma-facing Structures (TELS) device will provide fusion-relevant plasma flux incident on structures with flowing liquid metal surfaces. In order to accurately quantify the ability of TELS to provide a simulated disruption or edge localized mode (ELM) plasma, a suite of diagnostics was used to measure a variety of plasma parameters. The objective of this thesis was to develop an electrostatic analyzer to measure the ion information in TELS and use the results to understand particle energy distribution and loss as a function of distance from the plasma source. It has been previously observed that TELS plasmas can bombard a target with an electron density of 3 x 1021 m-3, an electron temperature of 20 to 30 eV, and a peak energy flux of 0.08 MJ/m2 over the pulse length of 100 to 200 µs[2]. To validate that the experimentally observed heat flux delivered to a target corresponds to the isotropic magnetohydrodynamic (MHD) predictions based on electron and ion temperatures, it became necessary to evaluate the ion temperature and subsequent energy distribution. In addition to these theoretical comparisons, the ion temperature and transport of ions down the length of the chamber is important in order to fully characterize a system that preferentially heats and accelerates ions as a function of the energy coupling in the theta pinch. Two major conditions were imposed to observe how ions behave as a function of the effect of the compression with the theta pinch. First, the analyzer system comprised of an analyzer open to the plasma and an analyzer closed to the plasma was inserted into the target chamber, at a distance of 16 inches (40.6 cm) downstream of the theta pinch. The closed analyzer was used as an experimental control, so that the effects of electromagnetic and circuit noise were eliminated from the open analyzer signal. The ion current showed two prominent features that followed in suit with the way the PiP discharges, and the time-averaged temperatures were taken with respect to the duration of these features during the pulse. Without guiding magnetic fields used to prevent ion diffusive losses, the measured ion signal showed an ion temperature of 22.83 ± 7.43 eV for the first peak and 17.59 ± 11.53 eV for the second peak. This measurement was used as a basis against which to compare ion temperatures subject to different pulse conditions. Secondly, the analyzer system measured the ion information from the use of only the coaxial plasma accelerator as a comparison by which the theta pinch can be proven effective or not. Without the use of guiding fields, the measured signal showed an ion temperature of 10.40 ± 6.62 eV for the first peak and 7.70 ± 3.57 eV for the second peak. The effects of these results from a plasma transport and a plasma-material interaction basis will be discussed