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Excitation and characteristics of electrostatic ion-cyclotron waves launched by a grid

Previous experimental studies on electrostatic ion-cyclotron waves (EICWs) and their instability were often performed in a narrow plasma column in Q-machines, in which the wave was conventionally generated via the instability driven by an electron current parallel to a magnetic field. The propagation characteristics of wave patterns have seldom been studied, especially the different characteristics in pulse and continuous wave patterns. In this paper, we present the experimental launching of the EICW via a grid applied with either a pulsed or a sinusoidal external excitation voltage in a magnetized plasma device. Either pulsed or continuous EICWs were excited, and the wave pattern trajectories were observed. Using the time-of-flight method, both phase and pulse (group) velocities of the wave signals were directly measured. It was shown that for the continuous EICW excitation, the measured dispersion relation is in agreement with the result from the kinetic theory in some range of parameters in the fundamental and harmonic frequency bands. The dependence of the measured pulse and phase velocities on the magnetic field shows agreement with the fluid theory only in the low field strength case but is closer to the result from the kinetic theory in the high field strength case.