A major machine upgrade, with around £20M of procurement contracts, has recently been approved for the Mega Amp Spherical Tokamak (MAST) at Culham Centre for Fusion Energy in the UK. This will enhance MAST’s role in international research and offer EFDA Associations significant opportunities for collaboration.

Spherical tokamaks like MAST confine the plasma in a tighter magnetic configuration than the conventional JET or ITER-style design, resulting in a cored apple-shaped plasma rather than the usual ring doughnut shape. This offers the potential for more compact and economical fusion operation. Since being commissioned, in 2000, MAST has made significant contributions to fusion physics, particularly in understanding the instabilities that form at the edges of the plasma.

The Super-X divertor increases the distance that exhausted plasma travels before hitting the target by steering in a particular way along magnetic fields. (Picture: CCFE)

MAST Upgrade (MAST-U) has three core objectives that are central to the drive towards commercial fusion power:

  • Exploring the suitability of the spherical tokamak (ST) as a candidate for a Component Test Facility (CTF), as well as the viability of an ST as a compact design for fusion power plants.
  • Adding to the knowledge base for ITER by simulating ITER scenarios in an ST configuration in order to assess how the alternative shape affects plasma parameters.
  • Evaluating the Super-X divertor design (see below) that, if successful, could be adopted by future fusion devices, including DEMO. MAST-U will also test steady state tokamak operation with current driven by neutral beams.

According to these objectives, MAST Upgrade will provide high performance plasmas with pulse lengths of five seconds (up to ten times longer than the present duration), allowing the study of stable operating regimes that could be used for the design of future fusion machines. Improved parameters approaching fusion conditions will enable highly accurate scaling of plasma confinement models to DEMO, ITER and a CTF. Target parameter deliverables – including temperature up to 50 million °C and density over twice that of the existing machine – will allow experiments at significantly higher plasma pressure. MAST-U will be the first machine to include the high power exhaust system Super-X divertor. This design steers the exhausted plasma along magnetic field lines in such a way that it travels an increased distance before interacting with the targets. The particles are thus radiatively cooled and spread over a larger area, which will significantly reduce the power loads on the targets. With this configuration, MAST-U will be able to study the reduction in power loadings that will be required to handle the plasma exhaust in a CTF and future reactors. MAST-U will also be equipped with a ‘conventional’ divertor to compare the two systems.

A Component Test Facility (CTF) would test complete components like blanket modules or first wall structures under fusion power plant conditions and thus complement the qualification of materials by IFMIF. The EFDA facilities review 2008 lists a CTF as desirable in order to reduce those risks for DEMO, which are associated with the qualification of nuclear technology components. CTFs would have to be driven in steady state mode for some days or weeks and produce enough neutron, particle and heat flux to resemble fusion reactor conditions. Spherical Tokamaks offer attractive features for such a facility, in that they are, for example, compact assemblies and, due to their relatively small plasma, can produce sufficient neutron flux while consuming low amounts of tritium.

MAST Upgrade plan

The upgrade to MAST will be implemented in two stages. The first stage will be ready for plasma operations in 2015, with the second phase following two to three years later (subject to funding). Stage 1 will address scenarios and issues for plasmas that have reached steady conditions. It will enable normalised plasma pressures to be reached in excess of those required for CTF and DEMO. Stage 1 will include the following machine improvements: A new central solenoid with improved insulation and cooling, allowing extended pulse lengths; Increased neutral beam power (from 5 MW to 7.5 MW), including onand off-axis beams allowing heating in different configurations; An upgraded main toroidal field (from 0.55 T to 0.84 T); A complete set of new divertor coils to fully control the plasma in both a ‘standard’ and a ‘Super-X’ divertor configuration; Completely new carbon plasma-facing armour. New diagnostic systems, particularly divertor diagnostics, will also be installed. In stage 1, MAST-U is expected to prove the physics concept of high heat load divertors, to achieve sustained stable operation at high plasma pressure with high fast particle pressure, and to prove the use of current profile control for enabling stable high performance operation (including definition of the neutral beam current drive efficiency). Stage 2 upgrade, if funded, would include improved density profile and ELM control, further enhanced neutral beam heating power, a microwave system to test plasma current drive and enhanced diagnostics for fast particle studies. Once the Stage 2 programme is completed, in around 2020, it would be possible to install tungsten plasmafacing components and divertor targets to test aspects of tungsten operation in a CTF or DEMO. The key parameters of the current MAST machine and how they will improve following the two stages outlined above can be found on the MAST-U website: http://www.ccfe.ac.u/MAST_upgrade.aspx

Opportunities for collaboration

MAST-U will offer many possibilities for researchers from other Associations to work with CCFE on areas of mutual interest, both in physics and in tokamak engineering. The MAST programme already involves 14 European Fusion Associations, five laboratories from the US and Russia, and nine UK and overseas Universities. With the upgrade, CCFE will be looking to expand these links and open the new MAST programme up to collaborators as a ‘user facility’. MAST has perhaps the best set of diagnostics of any tokamak now operating, for example its recently-upgraded Thomson scattering system (FN May 2010). Many of these have been implemented with collaborators (see May 2010 and this issue). CCFE will seek similar partnerships to design and build diagnostics and other components for MAST-U. Associations will be able to use MAST-U as a platform for developing new diagnostics and CCFE is inviting proposals on systems for the project. When completed, MAST-U will enable increased input from collaborators on experimental campaigns. Scientists using MAST-U will benefit from the expertise of Culham’s renowned plasma theory programme, which will be fully involved in the project. In addition, facilities for remote collaboration are now available at MAST – the University of York has already set up a ‘virtual’ control room at its campus, allowing researchers to run experiments and analyse data.

To discuss collaboration opportunities on MAST Upgrade, please contact

Dr. Derek Stork
Telephone: +44 (0)1235 466582
email derek.stork@ccfe.ac.uk
Nick Holloway, CCFE