Magnetic field threshold for runaway generation in tokamak disruptions

Paper in proceeding, 2008

Experimental observations on large tokamaks show that the number of runaway electrons produced in disruptions depends sensitively on the magnetic field strength. The presence of a whistler wave instability (WWI) excited by runaway electrons may be the reason for this observation since the linear growth rates of these waves are such that they are stable for high magnetic field (so the runaway beam can form) but unstable for low magnetic field. The quasi-linear diffusion process due to the WWI represents a very efficient pitch-angle scattering mechanism for runaways and consequently may stop runaway beam formation. In this work, the criterion for runaway suppression by WWI is compared with a criterion for substantial runaway production obtained by calculating how many runaway electrons can be produced before the induced toroidal electric field diffuses out of the plasma.