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 ne~1.1nGR for H98~0.96. In
this plasma configuration, at 2.5 MA/2.7 T (q95~2.8), a line
average density ~95% nGR with H98 = 1 and βN~2
are obtained, with plasma thermal stored energy content Wth being
approximately constant with increasing density, as long as the discharge
maintains 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 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.