Stability Analysis of Plasma Waves Driven by Runaway Electrons in Tokamak Hot Plasmas

The local stability analysis of plasma waves driven by runaway electrons (RE) is performed considering hot plasma Maxwellian background. This allows including hot plasma waves, such as the electron plasma waves (EPW) and the ion Bernstein waves (IBW), which are excluded by cold background plasma models. In addition, a new analytic model of RE distribution is proposed, based on the skew normal distribution. It seems appropriate to describe RE distribution with electrons that tend to accumulate around a peak in the momentum space. It is like distribution functions obtained by numerical solutions of the RE kinetic equation. Based on the perturbation theory, the wave equation and, for normal plasma modes, the growth rates or damping rates are derived. To this end, the contribution to the anti-Hermitian dielectric tensor due to the RE has been calculated for different RE distribution functions by the numerical code REDHPW (Runaway Electron Driven Hot Plasma Waves). It has been developed to analyse the local stability of the normal plasma modes. This analysis has been performed for typical plasma and RE parameters during the current ramp-up phase of FTU discharges. Lower hybrid waves (LH), already identified in FTU by experimental analysis, are shown as the leading unstable waves, with much larger growth rates than the whistler waves (WW). EPW and IBW are also found unstable. At the EPW-IBW confluence RF emissions near the integer multiples n of the ion cyclotron frequency fci, are expected. Nonlinear interactions, due to the large wave electric field expected at the confluence of the modes, might excite sub-harmonic oscillations at nfci/3 frequencies. Experimental data are discussed, suggesting the excitation of such hot waves during the ramp-up phase of plasma discharges in FTU.