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

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 δ calculated at the separatrix ~0.45-0.5 and elongation κ~1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with δ. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density n_e~1.1n_GR for H₉₈~0.96. In this plasma configuration, at 2.5 MA/2.7 T (q₉₅~2.8), a line average density ~95% n_GR with H₉₈ = 1 and β_N~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~n_GR (and n_e~1.1n_GR).

A change in the Type I ELMs behaviour is observed for pedestal densities n_ped≳70% n_GR, with their frequency decreasing with density (at constant P_sep), enhanced divertor D_α 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 δ. We show that the changes to the lower δ (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.