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Helium exhaust experiments on JET with Type I ELMs in H-mode and with Type III ELMs in ITB discharges

Zastrow, K.D. and Cox, S.J. and von Hellermann, M.G. and O'Mullane, M.G. and Stork, D. and Brix, M. and Challis, C.D. and Coffey, I.H. and Contributors, JET EFDA (2005) Helium exhaust experiments on JET with Type I ELMs in H-mode and with Type III ELMs in ITB discharges. Nuclear Fusion, 45 (3). pp. 163-175. ISSN 0029-5515

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An analysis of helium exhaust experiments on JET in the MkII-GB divertor configuration is presented. Helium is pumped by applying an argon frost layer on the divertor cryo pump. Measurement of the helium retention time, tau(He)(*),, is performed in two ways: by the introduction of helium in gas puffs and measurement of the subsequent decay time constant of the helium content, tau(He)(d*); and by helium beam injection and measurement of the helium replacement time, tau(He)(r*). In ELMy H-mode, with plasma configuration optimized for pumping, tau(He)(d*) approximate to 7.2 x tau(E)(th) is achieved, where tau(E)(th) is the thermal energy replacement time. For quasi-steady internal transport barrier (ITB) discharges, the achieved tau(He)(r*) approximate to 4.1 x tau(E)(th) is significantly lower. The achieved helium recycling coefficient, confirmed by an independent measurement to be R-eff approximate to 0.91, is the same in both scenarios. None of the discharges are dominated by core confinement. The difference in tau(He)(*)/tau(E)(th) is instead due to the confinement properties of the edge plasma, which is characterized by Type I ELMs for the H-mode discharges studied, and Type III ELMs for the quasi-steady ITB discharges. This difference is quantified by an independent measurement of the ratio of the helium replacement time with a helium edge source to the energy confinement time.