J. Pearson

Mitigation of type-I ELMs with n?=?2 fields on JET with ITER-like wall

Recently, strong mitigation of Type-I Edge Localized Modes (ELMs) has been observed with application of the n = 2 field in high collisionality (nu^*_e=2.0) H-mode plasma on JET tokamak with ITER-like wall. In this experiment, the EFCC power supply system has been enhanced with a coil current up to 88kAt (twice than before). With an n = 2 field, the large type-I ELMs with frequency of ~ 45 Hz was replaced by the high frequency (few hundreds Hz) small ELMs. No density pump-out was observed during an application of the n = 2 field. The influence of the n = 2 field on the core and the pedestal electron pressure profiles is within the error bar and it can be neglected. During the normal type-I ELM H-mode phase, the maximal surface temperature (Tmax) on the outer divertor plate was overall increasing and associated with large periodical variation due to the type-I ELMs. However, during an application of the n = 2 field, Tmax was saturated and has only small variation in few degrees due to the small mitigated ELMs. Splitting of the outer strike point has been observed during the strong mitigation of the type-I ELMs.

Experimental observations of plasma edge magnetic field response to resonant magnetic perturbation on the TEXTOR Tokamak

The plasma response to a resonant magnetic perturbation (RMP) is experimentally measured on the TEXTOR tokamak using a fast movable magnetic probe. It is shown that, due to the plasma response, the magnetic field significantly differs from the vacuum field. Clear linear and non-linear responses to RMP are observed by varying the intensity of these perturbations or the plasma edge safety factor. Both the radial profile evolution and the dependence of plasma response on the edge safety factor show a stronger resonant effect when the RMP rotates at 5 kHz in the counter-current direction.

Observations of multi-resonance effect in ELM control with magnetic perturbation fields on the JET tokamak

Multiple resonances of the edge-localized mode (ELM) frequency caused by the application of low n (=1 or 2) magnetic perturbations for ELM control have been observed on JET. With a low n field applied, a strong increase in ELM frequency, fELM, by a factor of ~4.5 was found in many separated narrow windows of q95 (resonant q95), while the fELM increased only by a factor of ~2 for non-resonant q95 values. The fractions of increase in fELM with different resonant q95 values are not the same. An analysis of ideal external peeling modes shows that both the dominant unstable peeling mode number and fELM depend on the amplitude of the normalized edge current density as well as the edge safety factor, qa.

On the effects of magnetic perturbations on fast ion losses studied at TEXTOR

One criterion for the ignition of a fusion plasma is sufficient fast ion confinement. A key aspect in that context is the maintainability of good fast ion confinement in the presence of non-axisymmetric fields, such as those found in stellarators and during the application of resonant magnetic perturbations (RMPs) in tokamaks. This paper focuses on the influence of RMPs on the fast ion losses, studied at the Tokamak experiment for technology-oriented research (TEXTOR), a medium-sized device. TEXTOR is equipped with a flexible perturbation coil system, the dynamic ergodic divertor, allowing for the application of static or rotating perturbation fields. A rotating directional probe is used for the radially resolved fast ion loss measurements. The results achieved are presented and discussed for the poloidal/toroidal perturbation modes m/n = 3/1 and 6/2, with static and rotating fields.

Modified heat load deposition of the ELM crash due to n=2 perturbation fields at JET

Significant changes in the edge localized mode (ELM) crash heat load deposition patterns compared to typical ELMs are seen via infra-red observations during resonant magnetic perturbation experiments at the Joint European Torus (JET). These modifications result from the changed magnetic topology of the plasma, caused by the perturbations. Dependences on the perturbation strength and the edge safety factor are analysed and discussed. A thermoelectric current model shows that current filaments in the plasma edge could explain the observations. This study gives an insight into how the changed magnetic topology affects the peak heat fluxes of ELMs which is crucial for understanding ELM control.

Local measurements of screening currents driven by applied RMPs on TEXTOR

In order to understand the mechanism by which resonant magnetic perturbations (RMPs) mitigate or suppress edge-localized modes, it is necessary to understand the plasma response to the application of RMPs. TEXTORs fast movable Mirnov probe can provide direct measurements of the plasma response to RMPs applied using the dynamic ergodic divertor. The effect of toroidal plasma rotation is investigated, and a change in the phase of the plasma response at certain values of rotation is found. Jumps in the phase of the magnetic field are found to occur on resonant surfaces, indicating the formation of screening currents on these surfaces. The first observations of screening currents on multiple surfaces are presented, and the transition from screening to field penetration with increasing strength of the applied RMP field is observed.

Modelling of edge localized modes with a current relaxation model on JET and TEXTOR

The mitigation of edge localized modes (ELMs) has been investigated using external resonant magnetic perturbations (RMPs) on JET. An interesting observation encountered is the formation of resonant structures in the dependence of ELM frequency on the edge safety factor. Using a theory based on an extended Taylor relaxation process, initialized by an external peeling mode, this behaviour is modelled and analysed leading to a prediction of a condition that a low edge current density is required for the resonant nature to be observed. This model is outlined and the results compared with recent experimental observations from JET.The TEXTOR tokamak already has a low edge current density due to its limiter configuration and so is predicted to show the resonant behaviour even without the RMP fields being applied. Similar experiments investigating the effect of the edge safety factor on the ELM frequency are conducted on TEXTOR to validate this prediction. It was found from the model that a case with a low normalized edge current density of 0.01 shows good agreement with observations. For such a case the multi-resonance effect is less well defined than observed on JET due to the appearance of highly detailed structure in the model predictions.

Findings of pre-ELM structures through the observation of divertor heat load patterns at JET with applied n = 2 perturbation fields

Resonant magnetic perturbation experiments at JET with the ITER-like wall have shown the formation of radially propagating pre-ELM structures in the heat flux profile on the outer divertor. These appear a few milliseconds before the major divertor heat load, caused by type-I edge-localized modes (ELMs). The formation of the pre-ELM structures is accompanied by an increase in the Dα emission. For some pronounced examples, the propagation appears to end at the positions where an increased heat load is seen during the ELM crash a few milliseconds later. These observations are presented and discussed along with a comparison of a thermoelectric edge currents model.