Feedback-controlled steady-state plasma

Abstract

A current-free magnetized plasma with independently variable density and electron temperature has been produced and studied. The plasma, which may be either steady state or pulsed, is generated at one end of a 1.5-m-long vacuum system by a 10 GHz microwave discharge at electron gyroresonance in a d.c. magnetic field. In the main body of the plasma the magnetic field can be varied from 1 to 10 kG and plasma densities 5*108<or approximately=ne<or approximately=5*1011cm-3 and temperatures 2<or approximately=Ti<or approximately=20 eV, Ti<or approximately=1 eV can be achieved. All the noble gases as well as hydrogen have been used at pressures ranging from 2*10-5 to 2*10-3 torr. Electron plasma waves and ion acoustic waves have both been used for diagnostic purposes. Unless special precautions are taken, the plasma has density fluctuations of 5-10 per cent. A feedback control system which uses a rapidly responding adjustment of discharge power has been used to significantly reduce these fluctuations. This control will also keep the plasma density steady even in the presence of fluctuations in line voltage or gas feed.