Control of runaway electron energy in tokamak

One way of mitigating runaway electron (RE) damage of the plasma-facing components is by limiting the RE energy under a few MeV, while not reducing the runaway current appreciably. Here we report a physics mechanism by which such momentum-space engineering can be facilitated by externally injected whistler (helicon) waves. By introducing a wave that resonantly interacts with runaways at a chosen range of energy, the enhanced scattering would reshape the vortex by cutting off the highly relativistic part. The power requirement from external wave injection is estimated to be practical. The investigation has been extended to the tokamak geometry using a bounce-averaged formulation. It is found that the magnetic trapping effect reduces the volume of runaway vortex as the momentum-space fluxes are strongly modified in the trapped-region. As a result, the avalanche growth rate is reduced at off-axis locations. The reduction can be effectively enhanced by injected whistler waves. The induced pitch-angle scattering pushes the runaway vortex into trap-region where electrons are no longer accelerated. So the wave injection scheme becomes more efficient in large tokamaks and can even be designed to substantially increase the avalanche threshold electric field.