CYCLOTRON-RESONANCE PHENOMENA IN A PURE ELECTRON-PLASMA

An experiment designed to elucidate the features of cyclotron resonance in a rotating cylindrical pure electron plasma column is described. The density is well below the Brillouin limit and varies with radius, as does the rotational angular velocity. Thus the steady state is not the rigid-rotor equilibrium which is frequently studied theoretically. The readily observed modes are found to have k(z) almost-equal-to 0 and m = 1,2,3,4... (e(ikzz+im-theta) with frequencies which are within a few percent of the cyclotron frequency (140 MHz). A single m = 1 mode is found that is downshifted from the cyclotron frequency by an amount equal to the m = 1 diocotron frequency. For each of the higher m values, bands of closely spaced discrete modes are found which are upshifted from the cyclotron frequency by the Doppler effect of rotation. The bands of discrete modes are explained as radially trapped azimuthally propagating Bernstein modes in a rotating plasma and approximate theories for these modes are outlined. By comparing the results with experiments, plasma parameters such as the ratio of Larmor radius to plasma scale length, the central rotation frequency, and the ratio of peak to average density can be inferred.