Plasma Dispersion Relation Measurements through Active Injection of Wave Packets

A probe-based diagnostic is developed to emit and receive harmonically-rich wave packets in a plasma for measuring the dispersion relation simultaneously at multiple frequencies. The underlying methodology allows for expedited data acquisition and gives direct control over the signal-to-noise ratio of the measurements. The dispersion relation is measured by conducting a frequency-domain analysis of ion-saturation-current probe time-traces that uses coherence metrics to identify wave propagation and interferometry to calculate wavenumber as a function of frequency. The diagnostic is implemented in a 250 W magnetized RF argon plasma source to measure the dispersion relation of electrostatic ion-cyclotron waves perpendicular to the background magnetic field. A comb generating circuit excites harmonically-rich wave packets by generating tens of harmonics of an input square wave’s fundamental frequency. The nearly three dozen simultaneously measured wavenumbers agree with the prediction of a fluid plasma wave model for frequencies spanning 6 harmonics of the ion-cyclotron frequency.