Whistlers: a probable mechanism to mitigate runaway electrons in Tokamaks

Whistler waves are commonly identified in ionospheric plasma with free energy attributed to energetic electrons, temperature anisotropy, and loss-cone distribution etc. Recently Spong et al., has shown that whistler activity can have significant role to play in runaways mitigation. They reported that whistler is observed in tokamak plasmas excited by runaway electrons and have correlation to the energy of runaways. Their measurements are mostly based on electron cyclotron synchrotron radiation and hard X-ray diagnostics. The identification of the mode is purely based on frequency spectrum. In tokamaks, these runaway electrons can be mitigated either by enhanced collisional drag force or synchrotron radiation damping or by the excited instabilities. Whistler diagnostics is currently seen as an effective tool to understand the runaway physics and hence opens up possibility of its adaption as runaway mitigation mechanism. In Large Volume Plasma Device (LVPD), oblique whistlers are excited by the reflected particles in presence of energetic electrons. These whistlers are investigated for the frequency - wavenumber spectrum and normalised power spectrum for varying energy of energetic electrons. An inverse correlation between the wave amplitude and the energy of energetic electrons is observed. Also, its parallel wave number starts dominating over perpendicular component with the energy increase of energetic electrons. Despite insignificant energy variation of energetic electrons, appreciable mode damping is observed. These observations are well supported by the theory of whistler growth driven by reflected electrons in a loss-cone in presence of energetic electrons. These concerns to the problem of wave particle interaction and scattering of whistler wave by energetic electrons and vice-versa. Observations made in LVPD can be useful towards developing an understanding on larger prospective of runaway mitigation in tokamaks by (externally injected) whistlers as they can lead to pitch angle scattering of energetic electrons. A comparison on the observation of the whistler mode growth in DIII-D tokamak with LVPD data will be presented. (author)