MAST Upgrade

Overview

The Mega Ampere Spherical Tokamak Upgrade (MAST-U) is a fusion energy experiment located at the Culham Centre for Fusion Energy (CCFE) in the United Kingdom. As a spherical tokamak, it features a very low aspect ratio (the ratio of the major radius to the minor radius of the plasma), giving it a nearly spherical plasma shape. This geometry allows for potentially more efficient plasma confinement, enabling higher plasma pressure for a given magnetic field strength compared to conventional tokamaks. The primary mission of MAST-U is to address critical challenges for the design of future fusion power plants, particularly the problem of power exhaust. Its defining feature is the Super-X divertor, a novel magnetic configuration designed to reduce the intense heat and particle fluxes that strike the machine's walls. Data from MAST-U is intended to inform the design of compact, economically viable fusion reactors, such as the UK's Spherical Tokamak for Energy Production (STEP) program.

Physics / Mechanism

MAST-U operates on the principles of magnetic confinement fusion, using a system of powerful electromagnets to contain a high-temperature plasma in a toroidal (doughnut-shaped) vacuum vessel. Like all tokamaks, it uses a combination of a strong toroidal magnetic field, generated by external coils, and a poloidal field, generated by a large current flowing through the plasma itself, to create helical magnetic field lines that confine plasma particles.

The key innovation of MAST-U is its divertor system. A divertor is a component designed to magnetically channel particles and heat from the edge of the main plasma to a separate target area where they can be neutralized and pumped away. This prevents the plasma from directly impinging on the main chamber wall, which would cause damage and release impurities into the plasma. Standard divertors in machines like ITER face extreme heat loads, potentially exceeding 10 MW/m², which is a major engineering challenge.

MAST-U is the first device to test the Super-X divertor (SXD) concept at scale. The SXD increases the distance the plasma must travel along the magnetic field lines from the X-point (where the magnetic field lines diverge) to the target plates. By expanding the magnetic flux tube, it significantly increases the wetted area over which the heat is deposited, thereby reducing the peak heat flux. This is achieved by using additional poloidal field coils located far from the plasma. The longer connection length also allows more time for the plasma to cool through radiation and atomic processes before it reaches the target. Initial experiments on MAST-U have demonstrated a reduction in peak heat flux at the target plates by a factor of 10 or more compared to a conventional divertor configuration in the same device [1, 2]. This reduction is critical for the material survivability and lifetime of components in a commercial fusion power plant.

Beyond the divertor, MAST-U's spherical geometry leads to high beta (the ratio of plasma pressure to magnetic pressure), a measure of confinement efficiency. It also explores non-inductive current drive methods, which are necessary for achieving the steady-state operation required for a power plant.

Historical Development

MAST-U is the successor to the original Mega Ampere Spherical Tokamak (MAST), which operated at Culham from 1999 to 2013. MAST was a highly successful experiment that made significant contributions to understanding spherical tokamak physics, particularly in the areas of plasma stability, H-mode (high-confinement mode) access, and the behavior of energetic particles.

By the late 2000s, it became clear that the challenge of power exhaust was a critical issue for all magnetic confinement concepts, including spherical tokamaks. The conventional divertor design was projected to be inadequate for the heat loads expected in a compact, high-power spherical tokamak reactor. In response, the Super-X divertor concept was proposed by researchers at CCFE. The concept promised a viable solution by extending the divertor leg to a larger radius where the magnetic field is weaker, allowing for significant flux expansion.

The decision to upgrade MAST was made to provide a dedicated facility to test this novel divertor concept. The upgrade project began in 2013 following the shutdown of MAST. The extensive rebuild involved a complete redesign of the machine's internal components, including new divertor coils, a new central solenoid, a more powerful vacuum pumping system, and upgraded plasma heating systems. The construction and commissioning phase was a multi-year effort, culminating in the achievement of first plasma on October 29, 2020 [3]. The initial experimental campaigns began in 2021, focusing on commissioning the new systems and conducting the first tests of the Super-X divertor.

Current Status

As of early 2026, MAST-U is in full scientific operation, conducting its fourth and fifth experimental campaigns (C4 and C5). The primary focus remains the comprehensive characterization of the Super-X divertor across a wide range of plasma conditions. Researchers are systematically studying the reduction in heat flux as a function of divertor leg length, plasma density, and heating power. Results have consistently validated the core principles of the SXD, showing significant reductions in target temperature and heat flux compared to conventional divertor configurations [4].

The operational parameters of the device have been progressively increased. Plasma currents have reached over 1 MA, and the neutral beam injection (NBI) heating systems provide up to 5 MW of power, enabling access to H-mode plasmas. A suite of advanced plasma diagnostics, many of them new or significantly upgraded from MAST, provides high-resolution data on plasma temperature, density, impurities, and heat flux profiles at the divertor targets.

Ongoing experiments are also exploring synergies between the Super-X divertor and advanced confinement scenarios. This includes investigating the compatibility of the SXD with high-beta plasmas and developing techniques for controlling plasma-wall interactions and impurity transport in this novel magnetic geometry. The experimental results are being used to benchmark and validate sophisticated plasma physics models, such as SOLPS-ITER, which are crucial for extrapolating these findings to future devices like STEP.

Notable Implementations

MAST-U is a unique device, and its implementation is centered at the UK Atomic Energy Authority's Culham campus. Its key subsystems and features represent the state of the art for spherical tokamaks:

• Super-X Divertor Coils: The core of the upgrade. A set of new poloidal field coils installed in the vacuum vessel allows for the creation of a wide range of magnetic divertor configurations, from a conventional shape to a long-legged Super-X geometry. This flexibility is essential for systematically studying the physics of heat flux reduction.
• Central Solenoid: A new, more robust central solenoid (or central column) was constructed. This component is critical for inducing the plasma current and is a major engineering challenge in compact spherical tokamaks due to the limited space.
• Plasma Heating Systems: MAST-U is equipped with two neutral beam injectors, each capable of delivering up to 2.5 MW of heating power. These systems are essential for reaching the high plasma temperatures and pressures needed for fusion-relevant studies.
• Cryogenic Pumping System: A powerful cryopump system was installed within the divertor region. This system is designed to pump away the neutralized particles (deuterium) and impurities from the divertor chamber, which is crucial for maintaining plasma purity and controlling plasma density.
• Advanced Diagnostics: The device features a comprehensive suite of over 70 diagnostic systems. These include Thomson scattering for measuring electron temperature and density, charge exchange recombination spectroscopy for ion temperature, and a new suite of infrared cameras and Langmuir probes specifically designed to measure conditions at the divertor target plates with high precision [5].

Open Challenges

Despite its early successes, MAST-U faces several scientific and engineering challenges that are the focus of its research program:

1. Detachment Control in the SXD: Achieving and controlling a state of 'detachment'—where the plasma pressure and temperature drop dramatically just before hitting the target plates—is a key goal. While the SXD facilitates detachment, robustly controlling it across various operating scenarios remains an active area of research. Uncontrolled detachment can lead to instabilities that disrupt the plasma.
2. Impurity Management: The large, cold volume of plasma in the long divertor leg could potentially trap impurities. Understanding how impurities are transported and accumulate in the Super-X configuration is critical, as excessive impurities can radiate energy from the core plasma and degrade its performance.
3. Integration with Core Plasma Performance: A successful divertor solution must be compatible with high-performance core plasma scenarios (e.g., H-mode). Researchers are investigating the complex interplay between the plasma edge conditions created by the SXD and the confinement properties of the main plasma. The goal is to find an operating window that simultaneously provides good core confinement and manageable exhaust conditions.
4. Steady-State Operation: Like its predecessor, MAST-U operates in pulsed mode. Extrapolating its findings to a steady-state power plant requires further development of non-inductive current drive techniques that are efficient in the high-beta, low-aspect-ratio environment of a spherical tokamak. The Lawson criterion for net energy gain must be met under continuous, not just pulsed, conditions.

Outlook

The 5-15 year trajectory for MAST-U is focused on solidifying the physics basis for a compact fusion power plant. In the near term (5 years), the experiment will aim to complete its characterization of the Super-X divertor, testing its performance at the machine's full design parameters of 2 MA plasma current and 5 MW of heating power. This will involve pushing to longer pulse durations and exploring detachment in deuterium plasmas relevant to a reactor.

Over the medium term (5-10 years), the research program will likely shift towards integrating the divertor solution with advanced core plasma scenarios. This includes developing robust control schemes and exploring methods to optimize both energy confinement and power exhaust simultaneously. The data gathered will be a primary input for the engineering design of the UK's STEP prototype power plant, which aims to begin construction in the early 2030s [6].

In the longer term (10-15 years), MAST-U could be further upgraded to test other technologies relevant to a power plant, such as novel plasma-facing materials or alternative heating and current drive systems. It will continue to serve as a crucial facility for training the next generation of fusion scientists and engineers. The success of the MAST-U program and its Super-X divertor concept is considered a key step on the critical path toward realizing a commercially viable spherical tokamak fusion power plant.

References

1. First results from the Super-X divertor in MAST Upgrade — Nuclear Fusion (2021)
2. MAST Upgrade: a new spherical tokamak to address the key challenges of fusion power plants — Philosophical Transactions of the Royal Society A (2019)
3. UK's MAST Upgrade fusion experiment achieves first plasma — UK Atomic Energy Authority (2020)
4. Overview of the MAST Upgrade experimental programme — Nuclear Fusion (2024)
5. The diagnostics of the MAST Upgrade Super-X divertor — Review of Scientific Instruments (2018)
6. Spherical Tokamak for Energy Production (STEP) conceptual design overview — Fusion Engineering and Design (2024)