Determining the ion temperature and energy distribution in a lithium-plasma interaction test stand with a retarding field energy analyzer

Abstract

The ThermoElectric-driven Liquid-metal plasma-facing Structures (TELS) experiment at the University of Illinois is a gas-puff driven, theta-pinch plasma source that is used as a test stand for off-normal plasma events incident on materials in the edge and divertor regions of a tokamak. The ion temperatures and resulting energy distributions are crucial for understanding how well a TELS pulse can simulate an extreme event in a larger, magnetic confinement device. A retarding field energy analyzer (RFEA) has been constructed for use with such a transient plasma due to its inexpensive and robust nature. The innovation surrounding the use of a control analyzer in conjunction with an actively sampling analyzer is presented and the conditions of RFEA operation are discussed, with results presented demonstrating successful performance under extreme conditions. Such extreme conditions are defined by heat fluxes on the order of 0.8 GW m and on time scales of nearly 200 μs. Measurements from the RFEA indicate two primary features for a typical TELS discharge, following closely with the pre-ionizing coaxial gun discharge characteristics. For the case using the pre-ionization pulse (PiP) and the theta pinch, the measured ion signal showed an ion temperature of 23.3 ± 6.6 eV for the first peak and 17.6 ± 1.9 eV for the second peak. For the case using only the PiP, the measured signal showed an ion temperature of 7.9 ± 1.1 eV for the first peak and 6.6 ± 0.8 eV for the second peak. These differences illustrate the effectiveness of the theta pinch for imparting energy on the ions. This information also highlights the importance of TELS as being one of the few linear pulsed plasma sources whereby moderately energetic ions will strike targets without the need for sample biasing.

Free surface stability of liquid metal plasma facing components

“…Diagnosis with calorimetry has shown energy densities of 0.15 MJ m −2 deposition on target (roughly equivalent to an ASDEX Type I ELM [9]) in approximately 150 µs. Characterization of the plasma with triple Langmuir probes and RFEAs has yielded an ion temperature of approximately 20 eV after use of the theta pinch [10], and use of photodiodes has shown an approximate plasma velocity of 65 km s −1 [11]. Subsequently, a fast frame camera (Vision Research Phantom Miro eX4 Color Camera) is used to characterize lithium ejection during shots.…”

“…By employing (10) and the two no normal flow boundary conditions (6), the shallow water equation for continuity, (9), becomes:…”