Whistler instability driven by relativistic electron tail in tokamaks

A new model of relaxation oscillations in runaway tokamak discharges is presented. It is shown that the relaxations produced in runaway regimes may be explained via the whistler wave instability excited by the anomalous Doppler effect of the relativistic electron tail, with perpendicular energy much smaller than the parallel one. Using a nonlinear set of equations for the electron energy balance, unstable wave energy, and plasma ionization-recombination, we obtain the relaxation oscillations in 'cold' plasma runaway discharges. It is found that the whistler wave instability is limited by growth of the plasma density due to ionization of neutrals during strong gas puffing. Then, the electron temperature drops, diminishing the plasma density, and the instability is renewed.