The role of B T -dependent flows on W accumulation at the edge of the confined plasma

Nuclear Fushion

article

S.A. Zamperini¹  J.H. Nichols³  P.C. Stangeby⁴  D.C. Donovan¹  J.D. Duran¹  J.D. Elder⁴  E.A. Unterberg³  D.L. Rudakov⁵ 

[1] University of Tennessee;General Atomics;Oak Ridge National Laboratory;University of Toronto;University of California

关键词: impurity transport;    collector probes;    scrape-off layer;    DIII-D;    metal ringscampaign;   

DOI  :  10.1088/1741-4326/ac3fe7

来源: Institute of Physics Publishing Ltd.

【 摘 要 】

Near-separatrix impurity accumulation between the crown and the outer midplane of tokamaks is a common feature in results from codes such as SOLPS-ITER and DIVIMP; however, experimental evidence of accumulation has only recently been obtained and is reported here. The codes find that the poloidal distribution of impurity ions in the scrape-off layer (SOL) depends primarily on toroidal field ( B _T)-dependent parallel flow patterns of the background plasma and the parallel ion temperature gradient (∇_|| T _ion) force. Experimentally, Mach probes used in L-mode plasmas with favorable (for H-mode access)B _T measure fast ( M∼ 0.3–0.5) inner-target-directed (ITD) background plasma flows at the crown of single-null discharges. This study reports a set of DIVIMP simulations for two similar H-mode discharges from the DIII-D W metal rings campaign differing primarily inB _T-direction to assess the effect that fast ITD flows have on the distribution of W ions in the SOL. It is found that for imposed ITD flows ofM= 0.3, W ions that otherwise accumulate due to the ∇_|| T _ion-force are largely flushed out. It is also found that doubling the radial diffusion coefficient from 0.3 to 0.6 m² s⁻¹ prevents accumulation due to rapid cross-field transport into the far-SOL, where background plasma flows drain W ions to the divertors. Far-SOL W distributions from DIVIMP are then used to specify input to the impurity transport code 3DLIM, which is used to interpretively model collector probe (CP) deposition patterns measured in the 'wall-SOL'. It is demonstrated that the deposition patterns are consistent with the DIVIMP predictions of near-SOL accumulation for the unfavorable- B _T direction, and little/no accumulation for the favorable- B _T direction. The wall-SOL CPs have thus provided the first experimental evidence, albeit indirect, of near-SOL W accumulation—finding it occurs for the unfavorable- B _T direction only. For the favorable- B _T direction, fast flows can largely prevent accumulation from occurring.

【 授权许可】

Unknown