A new method of radial potential profile measurement utilizing magnetic field gradient near a mirror throat

Abstract

A gold neutral beam is perpendicularly injected into a plasma column along a main chord near an inner mirror throat of a plug/barrier cell in the tandem mirror GAMMA 10, in order to directly measure time evolution of the electrostatic radial potential profile during one shot. The new method pays attention to the fact that trajectories of the ions, which are ionized at each point on the path of the neutral beam in the plasma column, are shifted due to the magnetic field gradient and the electrostatic potential. After traveling about a half period of Larmor motion, ions are detected by a plate type of ion detector. The radial profiles of the plasma potential are determined analyzing positions of the ion beam spots on the detector plane. The main advantage of this method is that radial potential profiles in a strong magnetic field region can be measured as a function of time during one shot without sweeping incident beam angles. © 1996 American Institute of Physics.