The divertor program in stellarators

Abstract

Two significant problems that need to be solved for any future fusion device are heat removal and particle control. A very promising method to attack these problems in tokamaks and helical devices is the use of a divertor, providing a controlled interaction zone between plasma and wall. By carefully designing a divertor, conditions can be created in front of the divertor targets, which lead to a sufficient reduction of the power load on the targets by strong radiation redistribution. Any solution of course needs to allow for an energy confinement which is at least sufficient for the realization of a fusion reactor. Since energy confinement has been found to be strongly related to edge anomalous transport and edge plasma profiles, the ultimate aim is to find an integral solution which is optimum with respect to exhaust, heat load and energy confinement.Two different types of divertors are presently being investigated in helical devices: the 'helical divertor' and the 'island divertor'. So far divertor concepts have been investigated only in a few helical devices. Theoretical and experimental efforts have mainly concentrated on the suitability of divertor magnetic field structures, while detailed studies of the divertor plasma properties for the two types of divertor configurations have only recently begun. In the course of this exploration, a promising new high-density H-mode (HDH) plasma operational regime has been discovered on the Wendelstein stellarator W7-AS. It benefits from high-energy (up to twice the value of the International Stellarator Scaling ISS95) and low impurity confinement times, complemented by edge radiated power fractions of up to 90% in detached regimes. This allowed quasisteady-state operation for up to 50 energy confinement times and so far was only constrained by machine operability.

ECRH and ECCD scenarios for W7-X

“…, which may exceed the allowed range for a proper island divertor operation without external field compensation [10]. The acceptable shift of the X-point, ∆Z[cm] 0.3 I p [kA], is only a few centimeters, and the net plasma current is limited by |I p | < 10 kA [11].…”

Physics of island divertors as highlighted by the example of W7-AS

“…An overview of the divertor activities in stellarators is given in [4]. Both theory and experiment are at the very beginning.…”

The DED at TEXTOR: Transport and Topological Properties of a Helical Divertor

“…The DED is a helical type of divertor with an open structure, comparable to the ergodic divertor in ToreSupra [1] . Such a divertor has topological similarities to helical or island divertors in heliotrons [2] or stellarators [3]. The ergodic divertor, existing of 16 helical coils at the HFS of TEXTOR, generates a resonant magnetic perturbation which focuses the particle and heat flux onto divertor target plates.…”