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 H 98 ∼ 0.96. In this plasma configuration, at 2.5 MA/2.7 T (q 95 ∼ 2.8), a line average density ∼95% n GR with H 98 = 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

A neutral particle analyser was deployed on JET for a measurement of the distribution function of megaelectronvolt energy protons driven by the high power ion cyclotron resonance frequency heating of deuterium plasmas in the hydrogen minority scheme. Unexpectedly, efficient neutralization of megaelectronvolt energy protons was observed in the plasma centre without recourse to injection of a beam of atoms to provide electron donors for charge exchange neutralization reactions. A model is presented that elucidates the role of carbon and beryllium, the main intrinsic impurities in JET plasmas, in this process and establishes charge exchange between hydrogen-like ions of the impurities and protons as the main neutralization process. A model calculation for deducing the proton energy distribution function f(Ep) from the measured hydrogen flux is described. The effects of uncertainties in neutralization cross-sections on the inferred f(Ep) are examined. The validity of this model of impurity induced neutralization (IIN) is tested by using it to describe the measured hydrogen flux in different conditions of plasma heating and fuelling. A crucial point in making these tests was to use measured local values of bare impurity ion densities; the required measurements were realized using charge exchange spectroscopy. Using IIN modelling and a procedure in which a known change in the density of deuterium atoms at the plasma centre was made by applying neutral beam injection, we have deduced the background thermal deuterium atom density at the plasma centre, which is an important new diagnostic result. Concerning future experiments, the model predicts that carbon and beryllium impurities will be major contributors to neutralization of hydrogenic ions (protons, deuterons and tritons) in ITER plasmas, for ion energies E

This paper describes experiments with highly shaped JET H-mode plasmas, which were directed to developing regimes where Type I ELMs are replaced by other edge relaxations, while maintaining the pedestal pressure of Type I ELMy H-modes. It was found that Type II ELMs coexisted with Type I, up to densities of the order of the Greenwald limit, where Type III ELMs appear, and the good confinement was lost. Only at the highest edge collisionality was it observed that Type II ELMs completely replace Type I. At high β p and q 95 , 'grassy' ELMs replace Type I completely. The MHD spectra characteristics for grassy ELMs are significantly different from those of Type II ELMs. This paper details the experiments, briefly compares the results to those obtained elsewhere and suggests open lines of investigations for the assessment of the potential of grassy ELM regimes as an ELM mitigation technique.