Type-I edge-localized modes (ELMs) have been mitigated at the JET tokamak using a static external $n=1$ perturbation field generated by four error field correction coils located far from the plasma. During the application of the $n=1$ field the ELM frequency increased by a factor of 4 and the amplitude of the ${\mathrm{D}}_{\alpha }$ signal decreased. The energy loss per ELM normalized to the total stored energy, $\Delta W/W$, dropped to values below 2%. Transport analyses shows no or only a moderate (up to 20%) degradation of energy confinement time during the ELM mitigation phase.

A survey has been carried out into the causes of all 2309 disruptions over the last decade of JET operations. The aim of this survey was to obtain a complete picture of all possible disruption causes, in order to devise better strategies to prevent or mitigate their impact. The analysis allows the effort to avoid or prevent JET disruptions to be more efficient and effective. As expected, a highly complex pattern of chain of events that led to disruptions emerged. It was found that the majority of disruptions had a technical root cause, for example due to control errors, or operator mistakes. These bring a random, non-physics, factor into the occurrence of disruptions and the disruption rate or disruptivity of a scenario may depend more on technical performance than on physics stability issues. The main root cause of JET disruptions was nevertheless due to neo-classical tearing modes that locked, closely followed in second place by disruptions due to human error. The development of more robust operational scenarios has reduced the JET disruption rate over the last decade from about 15% to below 4%. A fraction of all disruptions was caused by very fast, precursorless unpredictable events. The occurrence of these disruptions may set a lower limit of 0.4% to the disruption rate of JET. If one considers on top of that human error and all unforeseen failures of heating or control systems this lower limit may rise to 1.0% or 1.6%, respectively.

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_ped70% 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.