Loss of relativistic electrons when magnetic surfaces are broken

Abstract

Relativistic runaway electrons in ITER could be confined in a broad annulus of stochastic magnetic field lines bounded by an annulus of magnetic surfaces. The outer confining annulus can be broken by either an evolution of the magnetic field or by the drift of the plasma into the walls. Both possibilities are studied, and in both cases, the relativistic electrons in the stochastic region are lost to the walls in a short pulse of length τℓ along a narrow tube, which carries a flux fψst, where f∼10−3 and ψst is the toroidal flux in stochastic annulus. Both τℓ and f are determined by two parameters: the time it takes for a toroidal transit of a relativistic electron τt=2πR/c, which is approximately 0.1 μs in ITER, and the evolution time τev, which is of order 100 ms in most conditions of interest for ITER. The concept of turnstiles in Hamiltonian mechanics is used to obtain the relation between τℓ and f and τt and τev. The turnstile concept is also important in the theory of divertors for stellarators.