An overview of shared technical challenges for magnetic and inertial fusion power plant development.

Fusion energy is an area of active development and innovation worldwide, with many design concepts studied, each exhibiting a range of technical challenges. A significant portion of technical challenges will be unique for a given design concept; however, there are several overarching challenges that any design must address to some degree. These include tritium handling and the tritium cycle; materials and their survivability in the high-energy neutron environment of D-T fusion; neutronics and the validation of nuclear data; remote handling and maintenance activities; and integrated holistic approaches to fusion plant design. This paper provides an overview of these aspects for magnetic and inertial fusion approaches with a view to highlighting commonality and the benefits of shared knowledge that this may bring. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

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

Fusion power could be one of very few sustainable options to replace fossil fuels as the world's primary energy source. Fusion offers the potential of predictable, safe power with no carbon emissions and fuel sources lasting for millions of years. However, it is notoriously difficult to achieve in a controlled, steady-state fashion. The most promising path is via magnetic confinement in a device called a tokamak. A magnetic confinement fusion (MCF) power plant requires many different science, technology and engineering challenges to be met simultaneously. This requires an integrated approach from the outset; advances are needed in individual areas but these only bring fusion electricity closer if the other challenges are resolved in harmony. The UK Atomic Energy Authority (UKAEA) has developed a wide range of skills to address many of the challenges and hosts the JET device, presently the only MCF facility capable of operating with both the fusion fuels, deuterium and tritium. Recently, several major new UKAEA facilities have been funded and some have started operation, notably a new spherical tokamak (MAST Upgrade), a major robotics facility (RACE), and a materials research facility (MRF). Most recently, work has started on Hydrogen-3 Advanced Technology (H3AT) for tritium technology and a group of Fusion Technology Facilities. This article is part of a discussion meeting issue 'Fusion energy using tokamaks: can development be accelerated?'

Observation of lobes near the X point in resonant magnetic perturbation experiments on MAST.

The application of nonaxisymmetric resonant magnetic perturbations (RMPs) with a toroidal mode number n = 6 in the MAST tokamak produces a significant reduction in plasma energy loss associated with type-I edge localized modes (ELMs), the first such observation with n > 3. During the ELM mitigated stage clear lobe structures are observed in visible-light imaging of the X-point region. These lobes or manifold structures, that were predicted previously, have been observed for the first time in a range of discharges and their appearance is correlated with the effect of RMPs on the plasma; i.e., they only appear above a threshold when a density pump out is observed or when the ELM frequency is increased. They appear to be correlated with the RMPs penetrating the plasma and may be important in explaining why the ELM frequency increases. The number and location of the structures observed can be well described using vacuum modeling. Differences in radial extent and poloidal width from vacuum modeling are likely to be due to a combination of transport effects and plasma screening.

Magnetic reconnection triggering magnetohydrodynamic instabilities during a sawtooth crash in a Tokamak plasma.

Thomson scattering measurements with subcentimeter spatial resolution have been made during a sawtooth crash in a Mega Ampere Spherical Tokamak fusion plasma. The unparalleled resolution of the temperature profile has shed new light on the mechanisms that underlie the sawtooth. As magnetic reconnection occurs, the temperature gradient at the island boundary increases. The increased local temperature gradient is sufficient to make the helical core unstable to ideal magnetohydrodynamic instabilities, thought to be responsible for the rapidity of the collapse.

Control of magnetohydrodynamic stability by phase space engineering of energetic ions in tokamak plasmas.

Virtually collisionless magnetic mirror-trapped energetic ion populations often partially stabilize internally driven magnetohydrodynamic disturbances in the magnetosphere and in toroidal laboratory plasma devices such as the tokamak. This results in less frequent but dangerously enlarged plasma reorganization. Unique to the toroidal magnetic configuration are confined 'circulating' energetic particles that are not mirror trapped. Here we show that a newly discovered effect from hybrid kinetic-magnetohydrodynamic theory has been exploited in sophisticated phase space engineering techniques for controlling stability in the tokamak. These theoretical predictions have been confirmed, and the technique successfully applied in the Joint European Torus. Manipulation of auxiliary ion heating systems can create an asymmetry in the distribution of energetic circulating ions in the velocity orientated along magnetic field lines. We show the first experiments in which large sawtooth collapses have been controlled by this technique, and neoclassical tearing modes avoided, in high-performance reactor-relevant plasmas.