Information on MAST from EURATOM/UKAEA Annual Report 2006/07

The text below is an extract from the Executive Summary of the 2006/07 EURATOM/UKAEA Annual Report, and covers MAST activities from April 2006 to March 2007. The MAST chapter from the full report can be downloaded as a pdf file.

The Association’s experiment, MAST, and a complementary experiment at Princeton (NSTX) are the world’s two leading spherical tokamaks.  MAST plays a powerful role in the fusion programme as it simultaneously contributes to the exploration of improved fusion power plant concepts, and provides key input to important ITER physics issues. Furthermore, the spherical tokamak may form the basis of a fusion Component Test facility (CTF), which could potentially accelerate the development of a commercial fusion power station. The main aims of the MAST programme are thus twofold: (a) to address key physics issues for ITER, complementing and extending data from conventional tokamaks, contributing data on many topics to the International Tokamak Physics Activities (ITPA), and (b) to explore the potential of the spherical tokamak as a basis for a future fusion power plant after ITER and/or a CTF.

Good progress was made in several key areas in 2006/07, namely: energy confinement scaling; transport studies; plasma fuelling by pellet injection; pedestal physics and ELM (Edge Localised Mode) dynamics; edge turbulence and filamentary structures; scrape-off layer (SOL) physics (e.g. SOL flows); disruption studies; fast particle instabilities; and EBW-assisted (Electron Bernstein Wave) plasma start-up using high power microwaves at 28GHz.  The results of all these experiments are reported in Chapter 5; two highlights are now summarised.

MAST has very good access for diagnostics and camera views, and this has been particularly valuable in studying phenomena at the edge of the plasma, in particular ELMs. Studies in 2006/07 have covered a range of ELM physics including the energy lost in an ELM and how the temperature and density at the edge of the plasma respond. An aspect that has received particular attention is the filaments that leave the main plasma and the energy that they carry (filaments also occur in other plasma regimes). Figure 1 shows fast camera images of filamentary structure during two ELMs, revealing a wealth of structure and complicated dynamics, with different filaments leaving at different times; this is the first time that this detail has been observed. Information from these and other measurements has been used to show that an appreciable fraction of the energy lost during an ELM is via these filaments.  Work on MAST on ELMs is improving understanding of these instabilities, which are of particular concern for ITER.

Fig 1. Visible images obtained during two ELMs in different MAST plasmas

Tokamak plasmas are usually fuelled by gas puffing at the edge but it is recognised that more central fuelling will be needed in ITER and power station plasmas, and so fuelling using injection of fast pellets is studied. MAST is well equipped to study the physics of pellet deposition; the frozen pellets can be launched from the outboard mid-plane and also from the top (Figure 2a), which is closer to the geometry anticipated in ITER. Figure 2b shows an image of the evaporation trajectory of a pellet in the visible bremsstrahlung spectrum. The insert graph reveals the fine structure in the emission intensity during the pellet evaporation process. Both absolute intensity and the structure of the emission can be used to improve pellet evaporation models used for extrapolation of pellet penetration depth to ITER and other devices.

Fig 2(a) Geometry of pellet fuelling on MAST. (b) Narrow band bremsstrahlung imaging of a pellet during the evaporation process (right)

There were also many successful technical developments in 2006/07. The first JET-style injector for the MAST neutral beam heating system was operated reliably into plasma at powers approaching 2MW. Several important diagnostic advances were made including operation of a prototype Motional Stark Effect system for plasma current profile measurements (developed with VR Sweden) and first fluctuation measurements from a beam emission system (with HAS Hungary, Figure 3). New edge probes for plasma scrape-off layer flow measurements and impurity transport studies have been constructed (separate projects with Imperial College, London and the University of Strathclyde – Figure 4). A set of coils for controlled excitation of Toroidal Alfvén Eigenmodes (TAEs) has been installed and first results are expected soon. Finally work has started on the design of a major upgrade to MAST’s already world leading Thomson Scattering System, which will be partly funded by the University of York as part of a collaboration in which UKAEA is funding a new readership at York in experimental fusion physics.

Fig 4. Alignment of the Divertor Impurity Probe, following its installation on top of MAST. Samples can be exposed to plasma and observed with visible light spectrometers viewing through gaps between tiles and then removed for analysis. These studies are part of a PhD project with Strathclyde University, aimed at improving understanding of erosion and transport mechanisms for heavy impurity ions. This is an important issue for ITER where the divertor targets will be a mix of heavy and light materials (tungsten and carbon)

For further information please see Chapter 5 of the 2006/07 Annual Report (PDF file)

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