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.

Steady state high performance with improved core confinement and sustainable plasma edge conditions has been approached on JET in a double barrier (DB) mode. The DB mode combines an internal transport barrier of the optimized shear regime with an edge transport barrier of an ELMy H mode regime. Improved confinement with an H factor HITER-89 ≈ 2 has been maintained for four energy confinement times. Ion and electron temperature profiles remain peaked in the DB mode, while the density profile is broad and similar in shape to the conventional ELMy H mode profile. The energy confinement improves across the whole plasma cross-section, and the ion heat conductivity falls to the neoclassical level in the core. Particle transport studies show that impurity accumulation can be avoided in the DB mode. In DT discharges the DB mode has attained a fusion gain of Q ≈ 0.4, producing 6.8 MW of fusion power, compared with Q ≈ 0.2 in the conventional sawtoothing ELMy H mode. The ELMs are more benign, with an amplitude an order of magnitude smaller in the DB mode. The DB mode has a high potential to improve performance in reactor relevant conditions.

Results of particle transport analysis are presented for both low and high charge impurity species in JET optimized shear plasmas. The impurity content in the plasma centre is sensitive to the strength of the internal transport barrier. In a strong transport barrier plasma, the diffusion coefficient in the centre is about an order of magnitude lower than that in plasmas with weak transport barriers. Impurity transport is close to the neoclassical transport prediction in the centre of the plasma, whereas it is much higher than the neoclassical theoretical value in the plasma edge. A convection term in the vicinity of the transport barrier was required in order to account for the measurements. These data support theories of turbulence suppression by the E × B shearing rate.