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

Abstract

We present the results of experiments in JET to study the effect of plasmashape on high density ELMy H-modes, with geometry of the magnetic boundarysimilar to that envisaged for the standard Q = 10 operation in ITER. Theexperiments described are single lower null plasmas, with standard qprofile, neutral beam heating and gas fuelling, with average plasmatriangularity δ calculated at the separatrix ~0.45-0.5and elongation κ~1.75. In agreement with the previous resultsobtained in JET and other divertor Tokamaks, the thermal energyconfinement time and the maximum density achievable in steady state for agiven confinement enhancement factor increase with δ. The newexperiments have confirmed and extended the earlier results, achieving amaximum line average density ne~1.1nGR for H98~0.96. Inthis plasma configuration, at 2.5 MA/2.7 T (q95~2.8), a lineaverage density ~95% nGR with H98 = 1 and βN~2are obtained, with plasma thermal stored energy content Wth beingapproximately constant with increasing density, as long as the dischargemaintains Type I ELMs, up to nped~nGR (and ne~1.1nGR).A change in the Type I ELMs behaviour is observed for pedestal densities nped≳70% nGR, with their frequency decreasing with density(at constant Psep), enhanced divertor Dα emission and increasedinter-ELM losses. We show that this change in the ELM character at highpedestal density is due to a change in transport and/or stability in thepedestal region, with the ELMs changing from Type I to mixed Type I and TypeII. The similarity of these observations with those in the Type II ELMregime 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 detailthe effects of the plasma boundary geometry, in particular by investigatingseparately the effect of the upper and lower triangularity, at high average δ. We show that the changes to the lower δ (or of the radialposition of the x-point) affect the pedestal parameters, the size of ELMenergy losses as well as the global energy confinement of the plasma.