Measurements of reduced parallel electron distributions using whistler wave absorption

We present the first results of a diagnostic designed to measure the reduced parallel electron distribution using resonant absorption of whistler waves at the electron cyclotron frequency. According to warm-plasma theory, a whistler wave that is swept in frequency is Doppler-shifted into resonance with those parts of the electron phase space density with a velocity component parallel to $B_0$. In our experiments, whistler waves were launched and received by a pair of dipole antennas immersed in a cylindrical discharge plasma at two positions along the axial background magnetic field. The whistler wave frequency was swept from somewhat below and up to $|\omega_{ce}|$. The measured wave absorption through the plasma was proportional to the reduced parallel electron distribution function. The background theory and initial results from this diagnostic are presented here. These first results show that this diagnostic succeeds in measuring changes to the distribution function due to cooling of the plasma during the transition from the discharge to the afterglow.