Combined neutral beam probe and laser-induced fluorescence for plasma diagnostics

Abstract

The combined use of neutral beam probe and laser-induced fluorescence techniques is promising in order to diagnose edge plasmas instead of electrical probes. Electrical probes have been used so far in edge plasmas in tokamaks. In the plasma layer near the limiter, however, the thermal burden on the probes could cause a mechanical problem. Also, the strong magnetic field would make the measurement with electrical probes rather difficult to interpret. In this paper, the feasibility of measuring the electron density profile with a Li⁰-beam probe is demonstrated in this application to ECR test plasmas whose plasma parameters are similar to those of edge plasmas. 

Two kinds of Li⁰-beams of thermal and several eV energies are used for the demonstration. The latter beams are produced by laser-induced evaporation of thin Li-films. The injected Li-atoms emit photons by impact excitation of plasma electrons. The local electron density can be obtained from the measurement of the locally emitted photon flux of the resonance line (2²S-2²P, 670.8 nm), as the local Li⁰-beam density is measured by means of laser-induced fluorescence at the resonance line with a flashlamp-pumped dye laser shot along the Li⁰-beam axis. The electron density is determined from the relative intensity ratio of the emission by impact excitation to the fluorescence.