(TH/P6-2) Destabilization of magnetosonic-whistler waves by a relativistic runaway beam
G. Pokol1),
T. Fulop2),
P. Helander3),
M. Lisak2)
1) Department of Nuclear Techniques, Budapest University of Technology and Economics, Association EURATOM, Budapest, Hungary
2) Department of Radio and Space Science, Chalmers University of Technology, Goteborg, Sweden
3) EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, U.K.
Abstract. Magnetosonic-whistler waves may be destabilized by runaway electrons with strongly anisotropic velocity distribution. The unstable wave frequency is well below the non-relativistic electron cyclotron frequency but above the ion cyclotron frequency. The linear instability growth rate of the magnetosonic-whistler wave destabilized by an avalanche of relativistic runaway electrons through the anomalous Doppler-resonance is calculated in a local analysis using the homogenous plasma approximation. The perturbative stability analysis is complemented by numerical solution of the dispersion equation including the full hot plasma dielectric tensor. In the parameter range relevant to disruptions in large tokamaks, the growth rate is largest for nearly perpendicular propagation. By assuming that the dominant damping mechanism in the cold post-disruption plasmas is due to collisions, the local threshold of the instability can be shown to depend on the fraction of runaway electrons, the magnetic field and the temperature of the background plasma. The dependence on the magnetic field is consistent with the experimental observations suggesting that there is a critical toroidal magnetic field below which there is no runaway current after the disruption. One reason for the absence of the runaways may be that the instability scatters the runaways in pitch-angle and prevents the beam from forming. Indeed, the quasilinear analysis shows that the main result of the instability is pitch-angle scattering of the runaway electrons on a typical time scale of a microsecond.
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