Towards understanding the relative role of divertor geometry and magnetic topology on detachment

Towards understanding the relative role of divertor geometry and magnetic topology on detachment

Towards understanding the relative role of divertor geometry and magnetic topology on detachment 150 150 amit
UKAEA-CCFE-CP(19)42

Towards understanding the relative role of divertor geometry and magnetic topology on detachment

Plasma detachment needs to be achieved in ITER [1] and future devices such as DEMO to dissipate most of the power in the Scrape-Off-Layer (SOL) and reduce the particle flux reaching the divertor targets. In order to enhance our capability to improve current, and design future tokamaks, we must improve our understanding of the relative effect on detachment of physical (baffles, strike point angle) and magnetic geometry (conventional vs more advanced topologies). For example, it is predicted by analytic calculations and modeling that the detachment threshold is reduced with increasing total flux expansion [2] [3] [4] (i.e. low Bt/high Rt at the target); how is that effect modified by varying the angle between the strike point and the target or divertor closure? In this SOLPS-ITER modeling study of density-ramp discharges for TCV and MAST-U, the magnetic topologies are varied from a conventional divertor to the placement of the strike point at progressively larger Rt; divertor baffling and poloidal strike point angle are varied as well. The density ramps are modelled by performing multiple steady-state simulations corresponding to increasing gas puffing rates, realistic pumping rates and sputtering (physical and chemical) of carbon from the walls. From those scans we abstract out the sensitivity of the detachment threshold and window [2] to variations of total flux expansion and neutral pathways, as a measure of what differences are created through various magnetic and physical divertor changes. The divertor ionization source is found, as in previous experiment and modeling [5] [6], to be the primary determinant of the target ion current during detachment, at least through the roll-over of the target ion current. The ionization source roll-over is due to limits in power available for ionization. In the TCV equilibria that are considered in this study, total flux expansion is expected to lead to a ratio of detachment thresholds of low-Rt to large-Rt equilibria, Rthres, of 1.32. Instead, both recent TCV experiments [7] and modelling lead to a Rthres of 0.75. The SOLPS-ITER modelling demonstrates that the low value of Rthres is due to enhanced neutral trapping in the low-Rt configuration compared to that in the high-Rt configuration, an effect similar to that observed in DIII-D [4]. By making magnetic and physical divertor configuration changes we much more closely equalize the neutral trapping as a function of Rt and we manage to get Rthres closer to the predicted scaling. In MAST-U, the difference of neutral trapping between configurations is less important because of the divertor design providing strong baffling. The implication of this work is that with careful choice of baffling, strike point angle and Rt one can achieve even lower detachment thresholds and better detachment control than in current designs.

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46th European Physical Society Conference on Plasma Physics (EPS), Milan, 8-12 July 2019
Published date:
07/12/2019