Abstract
The ELMO Bumpy Torus (EBT) confines a low-density, steady-state, microwave-driven plasma. On the basis of favorable initial diagnostic results, major machine upgrades—increases in microwave power and magnetic field—were initiated, and the feasibility of Thomson scattering was considered. From the time of the first laser measurements, in which a two-point spectrum was integrated during 30 or more Q-switched pulses, the Thomson scattering system has evolved into a reliable diagnostic tool that is able to obtain scans along 20 cm of the plasma diameter and to determine the central electron temperature at densities as low as 2×1011 cm−3. Based on a 25-J ruby laser and a seven-channel polychrometer, the present system (1) reveals a bulk plasma with Te=20–100 eV and ne=5–10×1011 cm−3 and (2) indicates a suprathermal tail consistent with x-ray measurements. With up to 200 kW of microwave power inside the machine, major problems to overcome were radiation damage to optical materials (≤3000 R/h) and plasma coating of windows and dumps.
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