Designing Non-Resonant Divertors

Non-resonant divertors for stellarators do not rely on the presence of edge islands and are therefore a possible divertor solution for configurations that have substantial bootstrap current. Previous work has examined simple wall structures by uniformly expanding the plasma last closed flux surface. It was shown that the strike line positions at the wall is resilient to changes in the plasma boundary shape on the order expected from bootstrap current evolution [1]. In this work we show how it is possible to increase the distance between the divertor plates and the plasma. Increasing the distance is desired to improve divertor closure and to increase neutral pumping efficiency. We determine the limits both of the divertor/plasma distance, and the limits of divertor closure. We provide a simple agorithm for a non-uniform expanded divertor, based off of the flux surface expansion at the plasma boundary. Using this algoritm and results from simple field-line following with field-line diffusion, we show the maximum allowable expansion for various configurations. Once the maximum expansion is determined, resiliency can be checked by perturbing the boundary. We use EMC3-EIRENE simulations to calculate the neutral pressure.

[1] Bader, A., et al. "HSX as an example of a resilient non-resonant divertor." Physics of Plasmas 24.3 (2017): 032506.