In JET, both high density and low-q operation are limited by disruptions. The density limit disruptions are caused initially by impurity radiation. This causes a contraction of the plasma temperature profile and leads to an MHD unstable configuration. There is evidence of magnetic island formation resulting in minor disruptions. After several minor disruptions, a major disruption with a rapid energy quench occurs. This event takes place in two stages. In the first stage there is a loss of energy from the central region. In the second stage there is a more rapid drop to a very low temperature, apparently due to a dramatic increase in impurity radiation. The final current decay takes place in the resulting cold plasma. During the growth of the MHD instability the initially rotating mode is brought to rest. This mode locking is believed to be due to an electromagnetic interaction with the vacuum vessel and external magnetic field asymmetries. The low-q disruptions are remarkable for the precision with which they occur at qψ = 2. These disruptions do not have extended precursors or minor disruptions. The instability grows and locks rapidly. The energy quench and current decay are generally similar to those of the density limit.

New experiments on COMPASS-D, DIII-D and JET have identified the critical scalings of error field sensitivity and harmonic content effects, enabling predictions of the requirements for larger devices such as ITER. Thresholds are lowest at low density, a regime proposed for H mode access on ITER. Results suggest a moderate error field sensitivity (δB/B~10-4) for ITER, comparable with the size of its intrinsic error, although there are uncertainties in scaling behaviour. Other studies on COMPASS-D and DIII-D show that sideband harmonics to the (2,1) component play an important role. Thus a correction system for ITER will be important, with flexibility to correct sidebands desirable, possibly assisted by beam rotation. Such a system has been designed and is capable of reducing multiple harmonic error levels to ~2×10-5 .

Error field induced modes have been a significant concern for next step devices. Experiments on JET, using the lower saddle coils to simulate such fields, have determined the critical scaling dependences of error field sensitivity in terms of global plasma parameters, under the conditions most relevant to next step operation. These experiments indicate relatively weak toroidal field scaling for the error field strength required to induce a locked mode, while confirming the linear density scaling observed on other devices. Correction of intrinsic errors improves low density access and assists end-of-pulse disruption avoidance. Additional heating studies have shown a strong effect of q = 2 rotation on the threshold, but a much weaker dependence on β and the H mode transition. Error field modes have also led to neoclassical tearing modes at appropriate collisionality and β. Other parameter dependences have been examined: plasma inductance has a strong effect, but plasma configuration (limiter or divertor) is less important. Application of a dimensional scaling constraint enables determination of machine size scaling, which is found to be weak. This leads to the relatively tolerable error field sensitivity expected for devices such as ITER FDR. However, there remain some important questions to be answered about the effects of rotation on error field sensitivity, and for ITER FDR some means of correction remains prudent.

Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

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