M. Rack

Power load studies in JET and ASDEX-Upgrade with full-W divertors

For the design and operation of large fusion devices, a detailed understanding of the power exhaust processes is necessary. This paper will give an overview of the current research on divertor power load mechanisms. The results shown are obtained in JET with the ITER-like wall (ILW)and ASDEX-Upgrade with tungsten coated plasma-facing components (PFCs). The challenges of infrared thermography on an ITER-like bulk tungsten divertor are presented. For the steady-state heat load, the power fall-off length ?q in JET-ILW is compared to an empirical scaling found in JET and the ASDEX-Upgrade with carbon PFCs. A first attempt to scale the divertor broadening S in the ASDEX-Upgrade with tungsten PFCs is shown. The edge localized mode (ELM) duration tELM in JET-C and JET-ILW is compared. For similar pedestal conditions (Te,ped and ne,ped), similar ELM durations are found in JET-C and JET-ILW. For higher ne,ped at the same pedestal pressure pe,ped, longer ELM durations are found in JET-ILW. The pedestal pressure pe,ped is found to be a good qualifier for the ELM energy fluency in both JET-C and JET-ILW. Improved diagnostic capabilities reveal ELM substructures on the divertor target occurring a few milliseconds before the ELM crash.

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.

ELM-induced transient tungsten melting in the JET divertor

The original goals of the JET ITER-like wall included the study of the impact of an all W divertor on plasma operation (Coenen et al 2013 Nucl. Fusion 53 073043) and fuel retention (Brezinsek et al 2013 Nucl. Fusion 53 083023). ITER has recently decided to install a full-tungsten (W) divertor from the start of operations. One of the key inputs required in support of this decision was the study of the possibility of W melting and melt splashing during transients. Damage of this type can lead to modifications of surface topology which could lead to higher disruption frequency or compromise subsequent plasma operation. Although every effort will be made to avoid leading edges, ITER plasma stored energies are sufficient that transients can drive shallow melting on the top surfaces of components. JET is able to produce ELMs large enough to allow access to transient melting in a regime of relevance to ITER. Transient W melt experiments were performed in JET using a dedicated divertor module and a sequence of I-P = 3.0 MA/B-T = 2.9 T H-mode pulses with an input power of P-IN = 23 MW, a stored energy of similar to 6 MJ and regular type I ELMs at Delta W-ELM = 0.3 MJ and f(ELM) similar to 30 Hz. By moving the outer strike point onto a dedicated leading edge in the W divertor the base temperature was raised within similar to 1 s to a level allowing transient, ELM-driven melting during the subsequent 0.5 s. Such ELMs (delta W similar to 300 kJ per ELM) are comparable to mitigated ELMs expected in ITER (Pitts et al 2011 J. Nucl. Mater. 415 (Suppl.) S957-64). Although significant material losses in terms of ejections into the plasma were not observed, there is indirect evidence that some small droplets (similar to 80 mu m) were released. Almost 1 mm (similar to 6 mm(3)) of W was moved by similar to 150 ELMs within 7 subsequent discharges. The impact on the main plasma parameters was minor and no disruptions occurred. The W-melt gradually moved along the leading edge towards the high-field side, driven by j x B forces. The evaporation rate determined from spectroscopy is 100 times less than expected from steady state melting and is thus consistent only with transient melting during the individual ELMs. Analysis of IR data and spectroscopy together with modelling using the MEMOS code Bazylev et al 2009 J. Nucl. Mater. 390-391 810-13 point to transient melting as the main process. 3D MEMOS simulations on the consequences of multiple ELMs on damage of tungsten castellated armour have been performed. These experiments provide the first experimental evidence for the absence of significant melt splashing at transient events resembling mitigated ELMs on ITER and establish a key experimental benchmark for the MEMOS code.

Characterisation of the deuterium recycling at the W divertor target plates in JET during steady-state plasma conditions and ELMs

Experiments in the JET tokamak equipped with the ITER-like wall (ILW) revealed that the inner and outer target plate at the location of the strike points represent after one year of operation intac ...

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.

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.

Characteristics of pre-ELM structures during ELM control experiment on JET with n=2 magnetic perturbations

Radially propagating pre-ELM (edge localized mode) structures in the heat flux profile on the outer divertor have been observed both with and without magnetic perturbations on Joint European Torus. ...