MAST Upgrade Divertor Facility: A Test Bed for Novel Divertor Solutions

Abstract

The challenge of integrated exhaust consistent with the other requirements in DEMO and power plant class tokamaks (ITER-like and alternative DEMOs, FNSF approaches) is wellknown and the exhaust solution is likely to be fundamental to the design and operating scenarios chosen. Strategies have been proposed such as high main plasma radiation (e.g. [1]), but improved solutions are sought and will require revised research methodologies. While no facility can address all the challenges, the new MAST Upgrade tokamak enables exploration of a wide range of divertor plasma aspects in a single device and their relation with the core plasma (e.g. access to H-mode), in particular the development of fundamental understanding and new ideas. It has a unique combination of closed divertor, capability of a wide range of configurations from conventional to long-leg (including Super-X), and fully symmetric double null (plasma and divertor structures). To extrapolate to DEMO and power plant scale devices where full integrated tests in advance are not feasible yet different physics mechanisms may dominate, theory-based models are likely to be essential, for confident performance prediction, optimisation, and a "qualification" of the concept. Development and validation of such models is at the heart of the programme around MAST Upgrade. Amongst the many areas to be explored, there will be a strong focus on the closely coupled topics of plasma detachment and cross-field transport mechanisms (e.g. plasma filaments), key ingredients of effective and reliable protection of the plasma facing components at DEMO-scale. Index Terms-Fusion reactor design, divertor, plasma exhaust, plasma filaments, super-X, tokamak devices. I. INTRODUCTION-ALTERNATIVE EXHAUST APPROACHES Tokamaks at reactor scale need effective and practical exhaust systems. An integrated exhaust solution accommodates both a high performance plasma and the engineering and materials requirements of reliably protected long-lifetime plasma-facing components (PFCs). Its many challenges are well documented-it involves far more than the divertor configuration. The fastest path would be to use the single-null divertor configuration (e.g. as implemented on ITER) accompanied by a highly radiating main plasma and a fully detached divertor [1]. However, while there is some basis for optimism, it is not yet clear whether such a constraint on the

Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

“…The Mega Amp Spherical Tokamak -Upgrade (MAST-U) is a new spherical tokamak at the Culham Centre for Fusion Energy (CCFE) in the United Kingdom [10,11,12]. MAST-U was built specifically for investigating ADCs to 1) determine their feasibility and divertor performance; and 2) use magnetic shaping to improve our understanding of plasma-edge physics.…”

UKAEA capabilities to address the challenges on the path to delivering fusion power

“…It is important to emphasize that MAST Upgrade is primarily intended to be a very flexible test-bed to study exhaust physics in many divertor configurations, from conventional divertor (as in JET and ITER) to X-divertor, snowflake, super-X and even extended inner-leg configurations, in single and double null versions [23]. The design of exhaust solutions for DEMO will rely heavily on theory-based models, given the change in physics parameters from existing or planned devices, so MAST Upgrade will be used to confront and thus improve the models that allow the next steps to be taken, rather than being a prototype [7].…”

“…The UK Atomic Energy Authority (UKAEA) (https://www.gov.uk/government/organisations/ukatomic-energy-authority) has a mission to make major contributions to the development of fusion power as a large-scale carbon-free commercial energy source. It is and will continue to be a major player in the global fusion enterprise, building on its long involvement in plasma research together with experience of operating JET, constructing the new MAST Upgrade device [7] (http://www.ccfe.ac.uk/mast_upgrade_project.aspx, http://www.ccfe.ac.uk/assets/ documents/other/MAST-U_RP_v4.0.pdf) and the more recent expansion into materials science and now wider fusion technology. UKAEA contributes in many areas of science and technology, has growing ties with many universities and increasingly acts as a link to industry, which will be a major contributor and stakeholder in the future.…”

2D measurements of plasma electron density using coherence imaging with a pixelated phase mask

“…Solutions to this problem will likely include maintaining the divertor in a detached state and with an optimized geometry for both the machine and the magnetic field. When it begins operations, the MAST Upgrade spherical tokamak will test a range of novel divertor configurations, aided by a suite of diagnostics "designed for as high space and time resolution as is currently feasible" 2 .…”