Neutral beam probe for edge plasma analysis in tokamaks (abstract)

Abstract

A neutral beam diagnostic system has been designed to do simultaneous space potential and density measurements in the tokamak plasma edge to complement existing probe diagnostics. Rapid radial profiles are possible by computer control of the energy and the injection angle of the neutral beam. A particular application under consideration is the Tokamak de Varennes, which is presently being upgraded to operate either as a limiter or as a divertor machine. In this application, an ion gun is used to produce a Tl+1 beam that can be steered radially and toroidally to scan the plasma edge. The ion beam is neutralized downstream in a charge-exchange cell and the resulting Tl neutral beam is injected through a side port into the plasma. Singly charged Tl ions produced in the plasma exit through a top port of the tokamak and are collected by an electrostatic energy analyzer. Numerical simulations of the trajectories show that beam energies between 40 and 55 keV are sufficient to probe the plasma edge radially from 23 to 29 cm with a few millimeters resolution. The use of a neutral beam allows measurements in a tokamak edge plasma with BT∼1.0 T with a reduction in beam energy by more than a factor of 3 compared to the use of ion beams. Details of the beam probe hardware and the Tokamak de Varennes application are presented.