Edge Te and ne Profiles during Type-I ELM mitigation Experiments with Perturbation Fields on JET.

ELM mitigation experiments have been carried out at JET in both Type-I and compound ELMy H-mode plasmas. This mitigation was achieved through edge ergodisation induced by the ex-vessel Error Field Correction Coil (EFCC) system and was found to be independent of the distance of the plasma edge to the JET wall. In all cases the detrimental effect of the ELM on pedestal confinement was greatly reduced during the ergodisation phase. The new High Resolution Thomson Scattering (HRTS) diagnostic, which has recently become operational at JET, is used to study the behavior of electron temperature, density and pressure pedestal profiles during the mitigation phase. The pedestal properties are monitored in a consistent way by applying a tangent hyperbolic fit to the HRTS measurements. Stability modelling with the ELITE code using the fits to the spatially resolved pedestal profile measurements suggests that the observed ELM mitigation can be understood in terms of the peeling-ballooning model. The mitigation correlates with a reduction of the edge pressuregradient (due to both a reduced height and an increased width of the edge pressure transport barrier) which is consistent with the linear ELM stability theory. It is observed that in case of an n = 1 external field, the barrier position and height are correlated to the toroidal direction of the applied perturbation. Agreement between the barrier position measured by HRTS and obtained from plasma equilibrium reconstruction is found: a main toroidally symmetric and a secondary toroidally asymmetric deformation of the plasma column are induced by the external perturbation.
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EFDP07059 1.85 Mb