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Significant progress has been made on the Mega Ampere Spherical Tokamak (MAST) towards a fundamental understanding of transport, stability and edge physics and addressing technological issues for future large devices. Collaborative studies of the L–H transition with NSTX and ASDEX Upgrade confirm that operation in a connected double-null configuration significantly reduces the threshold power, Pthr. The MAST data provide support for a theory for the transition based on finite β drift wave turbulence suppression by self-generated zonal flows. Analysis of low and high field side density gradients in the H-mode pedestal provides support for an analytical model of the density pedestal width dependent on the neutral penetration depth. Adding MAST data to international confinement databases has enhanced confidence in scalings for ITER by significantly expanding the range of β and ɛ explored and indicates a slightly stronger ɛ dependence than in current scalings. Studies of core transport have been conducted for well-diagnosed L-mode, H-mode and internal transport barrier (ITB) discharges using TRANSP, and microstability and turbulence studies have been carried out using GS2. Linear micro-stability analysis indicates that ITG modes are typically unstable on all flux surfaces with growth rates that are comparable to the equilibrium E × B flow shearing rate. Mixing length estimates of transport coefficients from ITG (neglecting flow shear) give diffusion coefficients that are broadly comparable with observed thermal diffusivities. Non-linear, collisionless ETG calculations have been performed and suggest radially extended electrostatic streamers up to 100ρe across in radius. Transport from ITG could easily be suppressed in regions where the E × B shear flow rate, ωSE, exceeds the ITG growth rate, possibly contributing to ITBs. Toroidal rotation, driven by neutral beam torque, is the dominant contribution to ωSE via the vφBθ term in the radial electric field. Early edge localized mode activity on MAST is associated with the formation of narrow filamentary structures following field lines in the edge. These filaments rotate toroidally with the edge plasma and, away from the X-points, accelerate radially outwards from the edge up to 20 cm. Studies of disruptions on MAST demonstrate a complex evolution of core energy loss and resultant divertor power loads, including phases where the target heat flux width is broadened by a factor of 8. Observations of energetic particle modes driven by super-Alfvénic beam ions provide support for a model for the non-linear evolution of toroidal Alfvén eigenmodes (AEs) forming Bernstein–Green–Krushal waves. The AE activity reduces to low levels with increasing β. Plasma start-up without a central solenoid and in a manner compatible with future large spherical tokamak (ST) devices has been demonstrated using breakdown at a quadrupole magnetic null. Closed flux surface plasmas with peak plasma currents up to 370 kA have been generated and sustained for 0.3 s. New error field correction coils have extended the operational space for low density plasmas and enabled scaling studies of error field induced locked mode formation in the ST.

The low aspect ratio of the mega amp spherical tokamak (MAST) allows differentiation between different forms of the H-mode threshold scaling. With optimized fuelling using inboard puffing, and a connected double null divertor (DND) magnetic configuration, the H-mode power threshold data lie about 1.7 times higher than recent scaling laws. Slight magnetic configuration changes, of the order of the ion Larmor radius, around a connected DND significantly influence H-mode access. H-mode confinement in discharges with low frequency edge localized modes (ELMs) is generally consistent with international scaling laws, e.g. IPB98(y,2). Strong indications of both particle and energy internal transport barriers have been seen. Normalized beta values β_N > 5 have been obtained, approaching the ideal n = 1 no wall external kink stability limit. Sawtooth triggered neo-classical tearing modes have been observed; numerical modelling of the island evolution reproduces mode behaviour well and confirms the significance of stabilizing field curvature effects. Divertor power loading studies, including transient effects due to ELMs, show a strong bias of power efflux to the outboard targets, where it is more easily handled. ELM energy losses, Δ W_ELM, are less than 4% of the stored energy in all regimes explored so far, but ELM effluxes extending 30 cm outside the outboard separatrix have been measured. Toroidally asymmetric divertor biasing resulted in significant broadening of the D_α profile on the biased components and a reduction in the total power to the unbiased components. Halo current magnitudes and asymmetries are generally small compared with conventional tokamaks; recent measurements show that the plasma behaves more as a voltage source than a current source. Initial neutral beam current drive experiments indicate non-inductively driven current values (I_NBI ~ 0.3I_p) comparable with code predictions.

A new scenario to delay or prevent neoclassical tearing mode (NTM) onset is presented. By active sawtooth destabilization, short period and low amplitude sawteeth are generated, such that the sawtooth produced NTM seed island is reduced and the threshold normalized plasma pressure for triggering of NTMs, β_Nonset, is increased. The scenario has been explored experimentally in the Joint European Torus (JET). Ion cyclotron resonance frequency (ICRF) waves tuned to the 2nd harmonic H-minority resonance have been used for sawtooth control. Whereas ICRF waves generally induce sawtooth stabilization, favouring the triggering of NTMs and reducing β_Nonset, the present experiments show that by toroidally directed waves, ion cyclotron current drive is produced, and that sawteeth can be destabilized by careful positioning of the 2nd harmonic H resonance layer with respect to the sawtooth inversion radius. As a result, NTM onset is delayed and β_Nonset is increased above its value obtained in discharges with additional heating from neutral beam injection alone.

Magnetohydrodynamic (MHD) mode activity in the Alfvén frequency range has been detected in the absence of energetic ions during discharges in several conventional tokamaks and spherical tokamaks, including the Tokamak Fusion Test Reactor (TFTR) and the Mega-Amp Spherical Tokamak (MAST). In TFTR the dominant toroidal mode number n was found to be zero; this is also the case in MAST discharges for which mode number information is available. The observed properties of these modes are shown to be consistent with global Alfvén eigenmodes (GAEs). Although they appear to have little or no effect on plasma performance in present-day devices, the fact that they are frequently observed in MAST Ohmic discharges suggests that they could be used as a diagnostic of plasma equilibrium parameters. In principle, they could also provide the basis for a plasma heating scheme. A possible mechanism for the excitation of the Alfvén eigenmodes in the absence of fast ions is suggested by two-fluid simulations of various tokamaks, in which high-frequency mode activity is found to be correlated with relatively long-timescale MHD events in the plasma, such as internal reconnection events (IREs) or edge localized modes (ELMs). A simple analytical model describing the excitation of Alfvénic modes by either IREs or ELMs is proposed. The coupling of low- and high-frequency MHD is predicted to be strongest for radially-extended modes: this is consistent with the low mode numbers of the activity observed in TFTR and MAST.

The large physical size of the JET tokamak, its heating systems and diagnostics, and its capability to operate with full deuterium-tritium (D-T) plasmas, including high-power tritium neutral beam injection (NBI), give it unique possibilities in fast particle research in fusion plasmas. These have already been used to generate significant (2-3 MW level) power in fusion α-particles in the 1997 D-T campaign. Recent JET experiments have concentrated on two important scenarios of relevance to next-step tokamak devices: the ELMy H-mode plasmas and plasmas with strong internal transport barriers (ITBs). The achieved progress will help in preparation for a possible second D-T experiment on JET. Fast particle studies have also been carried out recently using ion cyclotron resonance heating (ICRH)-accelerated particles and external-excitation methods to study Alfvén eigenmodes (AEs). Looking towards the future, the capability of JET will be enhanced by upgrades to the NBI system, ICRH system and various diagnostics. Results of the first JET D-T experiment (DTE1) form a basis on which to elaborate a second D-T experiment (DTE2) which could be proposed after these enhancements. The α-physics part of this programme would be divided between the investigation of α-particle confinement, heating and loss processes in the `integrated scenarios' (where the discharge is as close as possible to an ITER-relevant scenario), and dedicated `α-physics' experiments, with specially prepared plasmas. In ELMy H-mode plasmas the fusion performance could reach Q( = P_fusion/P_input) of ~0.33 at the highest combined heating powers, corresponding to ⟨β_α⟩~6×10^-4, allowing a test of the margins of TAE stability in quasi-steady-state conditions. The integrated-scenario fast particle programme could concentrate on the instabilities and heating in plasma regimes with strong steady-state ITBs, with expected Q values ~0.58 and ⟨β_α⟩~2×10^-3, demonstrating the compatibility of these operating scenarios with α-effects. Excitation of TAEs by α-particles in the plasma core could also be studied in such integrated scenarios. An issue which will receive attention is the confinement of MeV energy ions in the centre of ITB plasmas with strongly reversed shear, where the low current density in the centre may lead to the α-particles entering loss orbits. In preparation for a D-T campaign, studies of triton burn-up in deuterium ITB plasmas will begin in the 2002 experimental campaigns. Special `afterglow' experiments to measure TAEs after the termination of the (stabilizing) NBI have already been explored in JET deuterium ITB scenarios and would be planned for DTE2. It is intended to develop special versions of ITB plasmas with dominant ion heating which would maximize the sensitivity to degradation of α-heating effects.

Magnetic confinement fusion science leads many other branches of plasma physics in its capacity to predict, interpret and understand the behaviour of energetic particle populations. The range of applications of this capability should be extended, for the mutual benefit of fusion research and of other branches of science. In this paper we review progress in applying fusion-derived techniques to one of the central questions of astrophysics: the origin of the cosmic ray population that is magnetically confined within our Galaxy. While it is widely believed that supernova remnant shocks provide the main acceleration sites for cosmic ray electrons and protons, the fundamental `injection' problem remains. Namely, how particles are initially accelerated from ambient thermal to mildly relativistic energies, beyond which Fermi-type processes take over. The cosmic ray injection environment is magnetized and has many other physical resemblances to beam-heated and deuterium-tritium tokamak plasmas; in consequence, many of the same physical processes come into play. These include, for example, collective beam-plasma instability, resonant wave-particle coupling, and the stochasticization of particle orbits. A broad range of analytical and numerical techniques familiar in the fusion context has been successfully applied to the injection problem (see, for example, Dieckmann M.E. et al 2000 Astron. Astrophys. 356 377). Ideas from magnetic fusion have also been used to help design and interpret recent magnetized plasma experiments (Woolsey N.C. et al 2001 Phys. Plasmas 8 2439) using the high-power VULCAN laser, which address the cosmic ray injection problem from a new perspective.

JET experiments using deuterium and tritium have made it possible to study sawtooth stability in plasmas approaching thermonuclear conditions. Record fusion yields were obtained in discharges where the sawtooth was delayed. In a sequence of discharges designed to study alpha heating, a new phenomenon was observed: the sawtooth period increases with tritium concentration. The internal kink stability for both high performance and alpha heating experiment plasmas was studied. Calculations for the record fusion discharge at the time of maximum fusion power (P_fus = 16 MW) showed that alpha particles make a significant stabilizing contribution to the potential energy of the m = 1 internal kink instability. The scaling of sawtooth period with tritium concentration implies a dependence on mean ion mass: possible reasons for such a dependence are considered. Calculations of the kinetic fast particle contribution to the kink potential energy indicate that the rise in sawtooth period with tritium concentration is likely to have arisen from two effects: an increase in the slowing down time of the beam ions, which in these plasmas is proportional to the mean ion mass, and an increase in the proportion of beam ions at the full injection energy.

Neutral particle analyser (NPA) measurements of emitted hydrogen flux, integrated along a chord through the plasma centre, have been used to infer a perpendicular tail temperature T_⊥ of minority ion cyclotron range of frequencies (ICRF) heated protons in the Joint European Torus (JET). A relation is established between T_⊥ and the perpendicular proton temperature T_⊥(0) at the position of maximum power deposition. At energies E >> T_⊥(0), the value of T_⊥ inferred from the NPA is nearly equal to T_⊥(0). In the opposite limit E << T_⊥(0), it is shown that T_⊥ << T_⊥(0). Applying this model to NPA measurements from a JET pulse with varying ICRF power P_RF, it is found that the scaling of T_⊥(0) with P_RF in the 2 to 10 MW range is almost linear, as expected. Moreover, calculated values of the ICRF heated proton energy content, based on deduced values of T_⊥(0), agree with diamagnetic measurements. The NPA measurements thus give access to a fundamental property of the heated proton energy distribution, which plays a crucial role in sawtooth stabilization