Numerical simulation of waves driven by plasma currents generated by low‐frequency Alfvén waves in a multi‐ion plasma

Abstract

When multi‐ion plasma consisting of heavy and light ions is permeated by a low‐frequency Alfvén (LFA) wave, the crossed‐electric‐and‐magnetic field (E × B), and the polarization drifts of the different ion species and the electrons could be quite different. The relative drifts between the charged‐particle species drive waves, which energize the plasma. Using 2.5‐dimensional (2.5‐D) particle‐in‐cell simulations, we study this process of wave generation and its nonlinear consequences in terms of acceleration and heating plasma. Specifically, we study the situation for LFA wave frequency being lower than the heavy‐ion cyclotron frequency in a multi‐ion plasma. We impose such a wave to the plasma assuming that its wavelength is much larger than that of the waves generated by the relative drifts. For better understanding, the LFA‐wave driven simulations are augmented by those driven by initialized ion beams. The driven high‐frequency (HF) wave modes critically depend on the heavy ion density nh; for small values of nh, the lower hybrid (LH) waves dominate. On the other hand, for large nh a significantly enhanced level of waves occurs over a much broader frequency spectrum below the LH frequency and such waves are interpreted here as the ion Bernstein (IB) mode near the light ion cyclotron harmonics. Irrespective of the driven wave modes, both the light and heavy ions undergo significant transverse acceleration, but for the large heavy‐ion densities, even the electrons are significantly accelerated in the parallel direction by the waves below the LH frequency. Even when the LFA wave drive is maintained, the ion heating leads to the cessation of HF wave excitation just after a few cycles of the former wave. On the basis of marginal stability seen in the simulations, an empirical relation for LFA wave amplitude, frequency and ion temperature is given.