Impurity behaviour under puff-and-pump radiating divertor conditions

• Published in: Nuclear fusion 2009-06, Vol.49 (6), p.065013-065013 (8)
• Main Authors: Petrie, T.W, Porter, G.D, Brooks, N.H, Fenstermacher, M.E, Ferron, J.R, Groth, M, Hyatt, A.W, La Haye, R.J, Lasnier, C.J, Leonard, A.W, Luce, T.C, Politzer, P.A, Rensink, M.E, Schaffer, M.J, Wade, M.R, Watkins, J.G, West, W.P
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Magnetic confinement and equilibrium; Physics; Physics of gases, plasmas and electric discharges; Physics of plasmas and electric discharges; Plasma dynamics and flow; Plasma properties; Power exhaust; divertors; Transport properties
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/0029-5515/49/6/065013

The effectiveness of the puff-and-pump technique to enrich a seeded impurity in the divertor relative to the core and, thereby, to maximize radiation in the divertor depends sensitively on both the magnetic geometry and the ion B × ∇ B drift direction. In the puff-and-pump scenario used here, argon impurities injected into the private flux region are inhibited from accumulation in the core plasma by enhanced plasma flows to the divertor created by a combination of deuterium gas puffing upstream of the divertor targets and particle pumping near the divertor targets. Modelling of single-null, H-mode plasmas with the UEDGE fluid transport code indicates that particle drifts in the scrape-off layer and divertor strongly affect the locations where the argon seed impurity accumulates. It is also found in double-null cases that argon always shows a larger accumulation in the divertor out of which the ion B × ∇ B drift is directed, regardless of the divertor into which the argon is injected. Experiments have shown that the degree to which the deuterium gas-puffing rate inhibits the escape of the seed impurity from the divertor(s) depends critically on the direction of the ion B × ∇ B drift and on whether the plasma is single-null or double-null. The transition in behaviour from double-null to single-null character during puff-and-pump occurs for |d R sep | = 0.4 cm when the ion B × ∇ B drift was pointing away from the dominant divertor. The lowest argon density buildup in the main plasma of any of the configurations studied during puff-and-pump was achieved in single-null plasmas with the ion B × ∇ B drift direction away from the divertor.