[en] A fast reciprocating probe has recently been installed on MAST. It has been used to measure the outboard, mid-plane scrape off layer (SOL) of L-mode plasmas, and to study the intermittent fluctuations in the SOL in L-mode and ELMy H-mode discharges. In this paper, the system and the experiments are introduced

[en] Initial target plasma parameters have been obtained using Langmuir probe arrays in MAST DND plasmas. A clear in-out asymmetry was observed in the target parameters. All plasma parameter scale lengths were significantly elongated on the outboard side due to strong poloidal flux expansion. The electron temperature scale length λ_Te is typically 5-8 times broader than the density scale length λ_ne, suggesting a significant difference between heat and particle diffusivities in the SOL. A rough estimation of the SOL collisionality indicates that the inner SOL may be collisional (ν^*∼50) whereas the outer SOL is marginally collisional (ν^*∼7). The peak power density is about 4-5 times higher (∼1 MW m^-2) on the inboard side, which has a SOL power density scale length ∼10 times narrower (∼0.5 cm) than the outboard. The total power flow was approximately up-down symmetric, whilst the in-out power asymmetry was observed to be P_in:P_out∼1:6.5, exceeding the ratio of ∼1:3 for the inboard and outboard separatrix surface areas. A total of about 2/3 of the power entering the SOL reached the strike points (300 kW out of P_SOL∼450 kW)

[en] Two divertor tiles from the Mega-Ampere Spherical Tokamak (MAST) were analysed using interference fringe analysis, nuclear reaction analysis and Rutherford backscattering analysis. The analysis allowed the quantification of the co-deposited layers thickness of the deuterium and carbon mixture and the detection of the presence of other impurities such as oxygen and iron. A layer thickness varying from 10 to 270 nm was measured with a typical D/(D+C) ratio of ∼0.36. During the same campaign in which the analysed divertor tiles were exposed to the plasma, the ion flux to the divertor plates has been measured by a set of Langmuir probes mounted on the divertor ribs. This paper shows that the sputtering co-efficient that can be inferred from the ion fluences measured by the probes and by the redeposition of C in those amorphous layers is consistent with physical and chemical sputtering yields in conventional tokamaks. This paper reports the beginning and the intended developments of a project undertaken in MAST to study and quantify the phenomena of plasma surface interaction both at the walls and at the divertor of the machine in order to contribute to the material choice and design of the plasma facing components of a next step device such as ITER

[en] Full text: Activities on the MAST tokamak are focused on the core goals of improving first principles understanding of transport, stability and edge physics and investigating technological solutions to heating, current drive and plasma exhaust, both for ITER and future spherical tokamak (ST) devices. The study of transients is a key topic for MAST. Observations during the early phase of ELMs are consistent with filamentary structures expanding beyond the LFS separatrix at a radial velocity of 1 km/s. Observations during the thermal quench in locked mode disruptions reveal a phase where the divertor target heat flux width is significantly broadened and another where up to half the core thermal energy is released with no broadening. Efforts to dynamically control both static and transient power loads in MAST are being tested. In particular, toroidally asymmetric active (external power supply) and passive (plasma induced) electrical biasing of divertor components are being employed to induce broadening of the divertor wetted area and rapid sweeping of the strike-points. Adding confinement data from quasi-steady state MAST H-mode plasmas to the international confinement database suggests a stronger aspect ratio dependence than indicated by the IPBH98y2 scaling and provides support for a collisionality dependence absent from this scaling. Edge density profiles at the high and low field side provide support for an analytical model of the density pedestal width dependent on the neutral penetration depth. Regimes with strong ITBs are now also routinely accessed in MAST. An ion ITB is typically formed with co-NBI, whilst an electron ITB is formed with counter-NBI. A strongly peaked density profile and high toroidal velocity accompanies the electron ITB. Transport modelling of core and edge plasmas is undertaken for all plasma regimes. B2SOLPS5.0 L-mode modelling suggests that the neo-classical ion heat flux at the edge is comparable with the anomalous radial ion heat flux, while neo-classical contributions to the particle and electron heat fluxes are negligible. TRANSP analysis indicates that heat diffusivities are close to the ion neo-classical level over a significant region of the plasma in NBI H-mode plasmas and in the region of both electron and ion ITBs. This work was jointly funded by the UK Engineering and Physical Sciences Research Council and EURATOM. (author)

[en] Full text: Regimes with an L-mode edge remain an important candidate operating scenario for fusion power plants due to the absence of ELM power loads. Edge fluid codes have had success in modelling many features of phenomena observed in the SOL and boundary of these plasmas but recent observations on a number of devices have been difficult to reproduce, including significant recycling at surfaces far from the plasma boundary. One possible explanation is that radially propagating plasma filaments play a key role in cross-field transport, modifying or even dominating the diffusive-like processes usually represented in the codes by poloidally and radially constant perpendicular diffusivities. Images from a wide-angle camera operating at >20,000 fps on MAST clearly show the evolution of field-aligned, filamentary structures at the edge of L-mode plasmas and density and temperature profiles from an edge Thomson scattering system with sub-microsecond time resolution and 1 cm spatial resolution show spatially isolated peaks. These observations are complemented by data from a reciprocating Gundestrup probe with integral triple probe, giving 1 microsecond time resolution of particle flux; density; temperature and plasma flow across the LFS mid-plane SOL. These data reveal a highly intermittent characteristic, with the fluctuating component dominating the time averaged value even close to the separatrix. Initial analysis confirms that the intermittent bursts are correlated with the filamentary structures. Experiments have been conducted to estimate the total radial particle flux due to the filaments from an analytic particle balance model that utilises reciprocating probe and mid-plane D_α measurements as input. These analyses are allowing a picture to be constructed of intermittent L-mode transport in MAST in three dimensions. The size of the filaments; their mode number; frequency; temperature and density evolution; radial and toroidal velocity have all been estimated. Simulations of these plasmas using the BOUT turbulence code show characteristics of intermittent transport similar to those observed experimentally. Quantitative, model independent information on the extent of agreement between the experimental data set and BOUT simulations has been obtained using an array of powerful statistical techniques, including differencing and rescaling. (author)

[en] ELM characteristics in a large spherical tokamak with significant auxiliary heating are explored. Access to H-mode is significantly easier when the plasma magnetic configuration is near to a connected double null. The change in ELM frequency with core density and/or auxiliary heating power is broadly similar to conventional devices of a similar size in the type III regime. Power at the targets during an ELM arrives predominantly at the outboard side, irrespective of ELM frequency. The data are consistent with transport into the scrape-off layer being driven predominantly by diffusive processes in quiescent periods between ELMs, and thus being distributed roughly according to the 4:1 ratio of outboard to inboard separatrix surface areas. In L-mode and during ELMs themselves, however, the transport is augmented by turbulent processes, which are more prominent at the outboard side, where the field is very low and where the magnetic surfaces have bad curvature

[en] Long pulse lengths, and high power and particle fluxes ensure that the biggest incremental step in a next-step fusion device will be the understanding and control of the plasma-wall interaction region. High confidence, predictive models are needed to bridge the considerable gap from existing devices. Modelling the strong, non-linear feed-back between the plasma and surface physics in this region will require significant improvements in the range and quality of diagnostic information as well as enhanced data on atomic and molecular processes and material properties. This paper highlights the key data required, discusses the need for improved diagnostics to generate these data and examines the case for future, dedicated experiments for studying plasma-wall interactions

[en] In response to continuing concerns over the use of carbon containing materials in the DT phase of a next-step fusion device, a range of techniques for removing accumulated dust, flakes and films from the vessel as well as diagnosing the total inventory without vessel intervention have been evaluated. These techniques are described and the results of initial proof-of-principal experiments are discussed. (orig.)

[en] The DTOKS code, which models dust transport through tokamak plasmas, is described. The floating potential and charge of a dust grain in a plasma and the fluxes of energy to and from it are calculated. From this model, the temperature of the dust grain can be estimated. A plasma background is supplied by a standard tokamak edge modelling code (B2SOLPS5.0), and dust transport through MAST (the Mega-Amp Spherical Tokamak) and ITER plasmas is presented. We conclude that micron-radius tungsten dust can reach the separatrix in ITER. (authors)

[en] The MAST (Mega-Ampere Spherical Tokamak) and START (Small Tight Aspect Ratio) tokamaks have an unusually large ratio of vessel volume to plasma volume (∝10:1 and ∝20:1 respectively). The vessel then acts as a large reservoir of fuel gas, accumulated from pre-fill, gas-puffing and recycling. 0D and 1D modelling of a plasma/gas envelope/wall system indicate the importance of surface conditioning to achieve strong wall pumping thereby reducing the density of the gas envelope, achieving good density control and allowing good neutral beam injection transmission. Comments are made on the role of neutral deuterium/hydrogen in the plasma edge, in light of these theoretical results and data from the first MAST experimental campaign. (Abstract Copyright [2002], Wiley Periodicals, Inc.)