Effect of ELM Mitigation on Confinement and Divertor Heat Loads on JET

Full text: In JET several techniques have successfully demonstrated their capability for ELM amelioration (reduction of ELM losses and increase of ELM frequency), including resonant perturbations of the edge magnetic field by using the error field correction coils (EFCCs) and ELM magnetic triggering by fast vertical movements of the plasma column (vertical kicks). In this paper we present a summary of recent dedicated experiments in JET focused on integrating kicks and EFCCs into similar plasma scenario and scans in order to compare their performance. The impact of each control method on the plasma confinement and the effect of the reduction in ELM size on the divertor heat loads have been studied. In addition, the potential of using EFCCs for ELM mitigation in He4 plasmas has also been explored in JET, and a comparison of those results with the effects observed in D plasmas is included in this paper. The plasma response to the application of these two ELM control methods in D plasmas shares common features: the reduction in ELM size is accompanied by a reduction in pedestal pressure (mainly due to a loss in density), resulting in a 10% reduction of the thermal stored energy. A key ingredient in the ELM mitigation experiments in JET is the diagnosis of the ELM-resolved divertor heat load profiles on the outer target by a fast high resolution infrared camera. It was found that the averaged ELM peak-power decreases almost linearly with the ELM size, but a smaller reduction is observed on the peak heat flux (40% for EFCCs when the reduction in peak power is ∼ 50%). This difference is related to a reduction in the width of the ELM heat flux profile. This observation is common to any small ELM regime, independently of the method employed to reduce the ELMs size (gas fuelling, kicks or EFCCs and also enhanced toroidal ripple). Interestingly, in the case of kicks or EFCCs the reduction in ELM losses is accompanied by some pedestal density loss, resulting in low pedestal collisionality. In contrast, a stronger reduction in the peak heat flux is observed in gas fuelled plasmas, where the reduction in ELM losses is correlated with an increase in collisionality. A detailed analysis of the divertor heat loads for mitigated ELMs will be compared to the analysis done for the database of spontaneous ELMs in JET. The implications of these new data for ITER will be discussed. (author)