First exploration of bump-on-tail instability excited by energetic electrons in helicon plasma source

Energetic electrons significantly influence power coupling and transport in helicon plasmas. Although energetic electrons and Landau damping processes have been reported in previous helicon studies, their nonlinear interaction with waves and the resulting saturation dynamics have not been quantified. Here, we present the first self-consistent analysis of the bump-on-tail (BOT) instability in helicon discharges using the BOT model adapted to helicon-relevant parameters. Under experimental conditions, the instability exhibits weakly nonlinear growth and saturates without frequency chirping, resulting from the balance between energetic-electron drive and Krook-type collisional relaxation. Parameter scans show that collisionality plays a dual role—damping the wave while continually repopulating the resonant velocity region—leading to enhanced saturation amplitude and stronger modifications to the electron distribution. Increasing the energetic drive further intensifies the nonlinear wave–particle coupling and broadens the plateau in velocity space. By coupling the BOT-induced electric field to fluid and Poisson equations, we show that the resulting density and flux perturbations can be an order of magnitude larger in rotating plasmas than in stationary ones. The model thus reveals a kinetic-to-fluid coupling pathway, showing that resonant electrons can modify macroscopic transport and power deposition. These results advance the understanding of wave–particle interaction in high-density radio frequency (RF) plasmas and provide a physics basis for optimizing power coupling and stability in next-generation helicon devices.