Edge T_e and n_e profiles during type-I ELM mitigation experiments with perturbation fields on JET

Abstract

Edge localized mode (ELM) mitigation experiments have been carried out at JET in type-I ELMy H-mode plasmas. ELM mitigation was achieved through a stochastic edge induced by the ex-vessel error field correction coils 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 considerably reduced during the stochastization phase. The new high resolution Thomson scattering (HRTS) diagnostic, which has recently become operational at JET, is used to study the behaviour of electron temperature, density and pressure pedestal profiles during the mitigation phase. The pedestal properties are monitored in a consistent way by applying a hyperbolic tangent 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 pressure gradient (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 the case of an n = 1 external field, the barrier position and height are correlated with the toroidal direction of the applied perturbation. Agreement is found between the barrier position measured by HRTS and obtained from plasma equilibrium reconstruction. The external perturbation induces a deformation of the plasma column that is primarily toroidally symmetric with a secondary component that is toroidally asymmetric

Overview of ELM Control by Low n Magnetic Perturbations on JET

“…Figure 2 shows comparison of the edge pedestal recovery after a normal type-I ELM crash and a mitigated ELM crash with n = 1 field. Here, the pedestal electron density, temperature and pressure are measured by the high resolution Thomson scatting and averaged over several ELM cycles [5]. With n = 1 field, the ELM frequency increased by a factor of ∼ 2 (from ∼ 15 Hz to ∼ 30 Hz), and the pedestal density recovered only up to ∼ 80 % of that in the case without n = 1 field.…”

“…Transport analysis using the TRANSP code shows up to 20 % reduction of the thermal energy confinement time because of density pump-out, but when normalised to the IPB98(y, 2) scaling the confinement time shows almost no reduction. Stability analysis of the controlled ELMs suggests that the operational point with n = 1 field moves from the intermediate-n peeling-ballooning boundary to the low-n peeling boundary, and the radial width of the most unstable mode reduced from ∼ 3 % down to ∼ 1 % of the normalised minor radius [5,6]. The results of ELM control with the n = 2 fields on JET demonstrate that the frequency of ELMs can be increased by a factor of 3.5 with the present capability of the EFCC power supply.…”

Experience With High-$Z$ Plasma-Facing Materials and Extrapolation to Future Devices

“…However, it has to be stated, that the scaling of the 'flushing' effect is not yet investigated. Suppression of ELMs by increasing the overall edge transport with edge resonant magnetic field perturbations as pioneered by DIII-D [65,66] and JET [67,68], may also be a solution. However, common to these experiments is the tendency that the separatrix temperature increases during the ELM suppression phases, which is consistent with an increase of Z ef f compared to similar discharges without edge perturbation as reported by [69] and which would be counter productive in the case of high-Z PFCs.…”

Benefits and Challenges of the Use of High-Z Plasma Facing Materials in Fusion Devices

“…However, it has to be stated, that the scaling of the 'flushing' effect is not yet investigated. Suppression of ELMs by increasing the overall edge transport with edge resonant magnetic field perturbations as pioneered by DIII-D [84,85] and JET [86,87], may also be a solution. However, common to these experiments is the tendency that the separatrix temperature increases during the ELM suppression phases, which is consistent with an increase of Z e f f compared to similar discharges without edge perturbation as reported by [88] and which would be counter productive in the case of high-Z PFCs.…”