Damping and Plasma Heating with the Fast Wave.

Abstract

Wave damping and plasma heating have been observed for the fast (right-hand polarized) wave at a frequency equal to 1,6 times the ion cyclotron frequency. The waves were launched by a one turn loop around the cylindrical plasma column and damped in a magnetic beach. The current in the one turn loop came from the ringing discharge (Q > 5, f = 5 MHz) of an initially charged capacitor (0.01 yf, 32 kV, 5 J) into the loop. This current produced a peak, on-axis, vacuum magnetic field of about 0.04 T, which is approximately 20% of the background magnetic field (B ). The plasma was produced from a 10 mTorr initial filling of hydrogen gas by an axial current from a ringing capacitive discharge (Q - 5, f = 50 kHz), At the end of this axial discharge, when the waves were launched, the fully ionized plasma had a temperature of about 2 eV and an electron density of about 3 x 10 20/ m3 , Wave damping measurements are reported in a series of graphs of wave magnitude (b ) versus Bo for two fixed distances from the one turn loop^ and b z versus z for two fixed values of Bo , From these curves, the wave damping is seen to be very strong and very sharp. The wave damping curves correlate well with the plasma heating as shown by graphs of the signal from an unintegrated diamagnetic loop versus B for two fixed positions. The degree of plasma heating as determined by a diamagnetic loop and spectroscopy also agrees with the calculated energy content of the fast wave prior to damping. This damping and heating is believed to be the result of a magnetosonic column resonance. The fast wave is also observed to damp at the second harmonic of the ion cyclotron frequency.