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Improved performance of ELMy H-modes at high density by plasma shaping in JET

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Parail,  V.
Tokamak Theory (TOK), Max Planck Institute for Plasma Physics, Max Planck Society;

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Zastrow,  K. D.
Experimental Plasma Physics 4 (E4), Max Planck Institute for Plasma Physics, Max Planck Society;

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Conway,  G. D.
Experimental Plasma Physics 1 (E1), Max Planck Institute for Plasma Physics, Max Planck Society;

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Stober,  J.
Experimental Plasma Physics 1 (E1), Max Planck Institute for Plasma Physics, Max Planck Society;

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Suttrop,  W.
Experimental Plasma Physics 2 (E2), Max Planck Institute for Plasma Physics, Max Planck Society;

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Kallenbach,  A.
Experimental Plasma Physics 4 (E4), Max Planck Institute for Plasma Physics, Max Planck Society;

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Citation

Saibene, G., Sartori, R., Loarte, A., Campbell, D. J., Lomas, P. J., Parail, V., et al. (2002). Improved performance of ELMy H-modes at high density by plasma shaping in JET. Plasma Physics and Controlled Fusion, 44(9), 1769-1799.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0027-409E-C
Abstract
We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged,for the standard,Q = 10 operation in ITER. The experiments described are single lower null plasmas,with standard q profile, neutral beam heating and. gas fuelling, with average plasma triangularity delta calculated at the separatrix similar to0.45-0.5 and elongation kappa similar to 1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the and the maximum density achievable in thermal energy confinement time steady state for a given confinement enhancement factor increase with delta. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density n(e) similar to 1.1 n(GR) for H-98 similar to 0.96. In this plasma configuration, at 2.5 MA/2.7 T (q(95) similar to 2.8), a line average density similar to95% n(GR) with H-98 = 1 and beta(N) similar to 2 are obtained, with plasma thermal stored energy content W-th being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to n(ped) similar to n(GR) (and n(e) similar to 1.1n(GR)). A change in the Type I ELMs behaviour is observed for pedestal densities n(ped) greater than or similar to 70% n(GR), with their frequency decreasing with density (at constant P-sep), enhanced divertor D-alpha emission and increased inter-ELM losses. We show that this change in the ELM character at high pedestal density is due to a change in transport and/or stability in the pedestal region, with the ELMS changing from Type I to mixed Type I and Type II. The similarity of these observations with those in the Type II ELM regime in ASDEX Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed. Finally, we present the first results of experiments by studying in more detail the effects of the plasma boundary geometry, in particular by investigating separately the effect of the upper and lower triangularity, at high average delta. We show that the changes to the lower delta (or of the radial position of the x-point) affect the pedestal parameters, the size of ELM energy losses as well as the global energy confinement of the plasma.