N. Ben Ayed

Filament structures at the plasma edge on MAST

The boundary of the tokamak core plasma, or scrape-off layer, is normally characterized in terms of average parameters such as density, temperature and e-folding lengths suggesting diffusive losses. However, as is shown in this paper, localized filamentary structures play an important role in determining the radial efflux in both L mode and during edge localized modes (ELMs) on MAST. Understanding the size, poloidal and toroidal localization and the outward radial extent of these filaments is crucial in order to calculate their effect on power loading both on the first wall and the divertor target plates in future devices. The spatial and temporal evolution of filaments observed on MAST in L-mode and ELMs have been compared and contrasted in order to confront the predictions of various models that have been proposed to predict filament propagation and in particular ELM energy losses.

Inter-ELM filaments and turbulent transport in the Mega-Amp Spherical Tokamak

Results on edge turbulence in periods separating edge localised modes (ELMs), i.e. inter-ELM periods, in Mega-Amp Spherical Tokamak (MAST) are presented. It is shown through combined measurements of fast camera images and reciprocating Langmuir probes that filamentary structures contribute to transport during these periods. Analysis of Dα light emission reveals that inter-ELM filaments are the lowest amplitude fluctuations in the MAST scrape-off layer (SOL) relative to L-mode and ELM filaments. Physical properties such as size, density and mode numbers have also been characterized, along with measurements of the spatio-temporal evolution: inter-ELM filaments are found to rotate in the vicinity of the last closed flux surface and propagate radially outwards. Motion of these filaments is found to depend strongly on plasma density such that with increasing density, there is an enhancement of the radial transport manifested by an increased number of filaments which leave the edge and travel faster into the SOL. Camera images show that intermittent fluctuations in ion saturation current signals correspond to inter-ELM filaments passing the probe. Measured radial e-folding lengths indicate larger decay lengths at higher densities. Similar trends are also obtained in simulations of a filament propagating radially and losing particles on ion parallel loss timescales. Finally, a discussion is presented on how the radial velocity and Isat measurements reported in this paper are used to test the velocity scalings predicted by different theories.

Experiments and simulation of edge turbulence and filaments in MAST

Experimental and simulation results on filamentary structures observed in the Mega-Amp Spherical Tokamak (MAST) are presented and discussed. Fast camera data have been used to determine the mode number, toroidal and radial sizes and velocities of the filaments observed in L-mode, inter-edge localized mode (ELM) periods and ELMs which are summarized. Automated methods are applied to the analysis of L-mode image data in order to measure dependence with plasma parameters. This indicates that the mode number of L-mode edge turbulence increases with density and decreases with q95, while filament width has the opposite dependence.Simulations of L-mode discharges using the 3D, 2-fluid BOUT code produce similar sizes and radial velocities to the observations, and indicate that the source of these filaments is within a region ~2 cm from the plasma edge in a spontaneously formed E × B shear layer. Ion temperature fluctuations in these filaments are found to be approximately double the magnitude of electron temperature fluctuations, probably due to fast parallel electron heat transport.

Experimental investigation into ELM filament formation on MAST

Radial profiles of electron temperature and density through type I ELM filaments have been obtained from a new edge Thomson scattering diagnostic at MAST. The lasers were fired in burst mode, 5 µs apart, to study profile evolution of a single filament as it moves toroidally past the laser beams. The plasma particle and energy loss due to each filament can be deduced from these profiles. As the filaments move out of the plasma, the ne pedestal is seen to collapse locally inwards by as much as 7.5% of the plasma minor radius. Insight into the toroidal structure of the perturbation has been obtained from high time resolution interferometry data. The interferometer data show excursions in line integral density through the midplane of the plasma occurring from 150 µs before the onset of the ELM particle loss. These excursions are due to the evolution of the spatial structure of the plasma during the ELM and indicate that the filaments may develop from broader structures. By combining the toroidal structure information from the interferometer and the radial structure information from the TS system with other diagnostic data on MAST, a two dimensional picture of the ELM phenomenon is obtained.

The effect of the plasma position control system on the three-dimensional distortion of the plasma boundary when magnetic perturbations are applied in MAST

When resonant magnetic perturbations are applied in MAST, the plasma edge boundary experiences a three-dimensional (3D) distortion, which can be a few percent of the minor radius in amplitude, in good agreement with ideal 3D equilibrium modelling. This displacement occurs in plasmas both with radial position feedback control applied, and without feedback. When position feedback control is employed, an applied non-axisymmetric field can lead to an exacerbated edge displacement due to an ancillary axisymmetric position correction, with the direction of the correction dependent upon the phase of the applied field with respect to the toroidal position of the sensors used in the controller. This suggests that future machines reliant upon resonant magnetic perturbations for controlling ELMs should consider using a plasma control system capable of applying a position correction which accounts for the non-axisymmetry of applied magnetic perturbations.

Determining advection mechanism of plasma filaments in the scrape-off layer of MAST

The scrape-off layer (SOL) of fusion devices is typically composed of filamentary structures that propagate with a high radial velocity away from the bulk plasma. When radial and parallel transport times are comparable, these coherent structures constitute an intermittent heat and particle flux which can reach the material wall; in time causing wear to plasma facing components. Qualitative models predict that the parallel currents, driven by the divertor sheath, have a direct impact on this radial velocity. In this work, the predictions for radial velocity of plasma filaments in the SOL from models are tested against data from the MAST tokamak and simulation. We apply a statistical method of window averaging to MAST Langmuir probe data in order to examine the scaling of the radial velocity of filaments with the plasma density inside the filaments. Our analysis strongly suggests that the radial dynamics emerge from the competition of multiple mechanisms and not from a single process. At intermediate distances from the bulk plasma, a new model proposed here, in which the parallel current depends on a constant target density appears to be the most relevant for the MAST plasma. This is confirmed using a TOKAM2D simulation with a modified parallel current term.

Alfvén eigenmodes in magnetic X-point configurations with strong longitudinal fields

The complex magnetohydrodynamic (MHD) properties of magnetic X-points have attracted much attention, due to the fact that configurations with this type of topology are common to many laboratory and natural plasma environments, and play a key role in processes such as magnetic reconnection and mode conversion. One such environment is the tokamak plasma in which divertor X-points are produced either by currents in the external coils, or in the plasma itself in the form of magnetic islands; in both cases, the effects of the X-point on nearby propagating Alfven waves is still unknown. The fast magnetoacoustic wave is of particular importance in this context since, unlike the shear Alfven wave, it can carry free magnetic energy into the X-point itself where it can be efficiently transformed into heat, mass flows or energetic particles (1,2). The shear wave is also of interest, however, since, except for the special case in which there is no longitudinal field Bk and no variations in that direction, the presence of an X-point causes the shear and fast waves to be coupled (3). This coupling was studied analytically in (4) for the case of an X-point with zero equilibrium current but finite Bk , variations in the longitudinal direction being neglected. They showed that the coupling is associated with the formation of singular structures in the current density near the X-point separatrix. McClements and co-workers (5) solved the initial value problem of a fast wave being driven up by a shear wave for the case in which Bk is smaller than B⊥, the field in the X-point plane, taking into account resistivity and electron inertial effects; again, variations in the longitudinal direction were neglected. In this contribution, a three dimensional perturba- tive analysis is presented of Alfven waves in a magnetic X-point configuration with a strong longitudinal guide field. 2. Equilibrium field