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.

In the Joint European Torus the ablation of injected pellets produces a striking resonance effect when the pellets reach surfaces with q values 1 and j. Subsequently, structures with mode numbers m = l,/i = 1 and m =3,« = 2 are observed with the soft-x-ray cameras for up to 2 s as compact snakelike perturbations. These structures, which persist through several sawtooth collapses, give information on the radii of the q = 1 and q = j surfaces and the ^-profile evolution. The observations can be explained by the formation of magnetic islands.

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The soft X ray diagnostic at JET views the plasma from six directions with a total of 215 lines of sight. The good coverage of the plasma makes it possible to make detailed tomographic reconstructions of the soft X ray emission during various conditions. One of the tomography methods applied at JET is discussed: a grid based constrained optimization method that uses anisotropic smoothness on flux surfaces as regularization. This method has made it possible to study in detail the transport of heavy trace impurities injected into the plasma by laser blow-off. Impurity injection experiments in hot ion H mode and optimized shear plasmas are presented and discussed. The addition of a number of features to the algorithm, notably a non-negativity constraint, has made it possible to reconstruct very localized soft X ray emission from the wall during edge localized modes (ELMs). The detectors suffer damage from the neutrons produced in deuterium-deuterium (DD) fusion reactions. This damage influences the sensitivity of the detectors, which makes it necessary to cross-calibrate the cameras. A method based on tomographic reconstructions has been developed to achieve the cross-calibration.