Suppression of runaway electron avalanches by radial diffusion

The kinetic theory of runaway electron avalanches caused by close Coulomb c ollisions is extended to account for radial diffusion. This is found to slo w down the growth of avalanches. An approximate analytical formula for the growth rate is derived and is verified by a three-dimensional Monte Carlo c ode constructed for this purpose. As the poloidal magnetic flux that is ava ilable to induce an electric field in a tokamak is limited, avalanches can be prevented altogether by sufficiently strong radial diffusion. The requis ite magnetic fluctuation level is sensitive to the mode structure and the s peed of the disruption. It is estimated to be delta B/B similar to 10(-3) f or parameters typical of large tokamaks. [S1070-664X(00)03010-X].