MAST is one of the new generation of large, purpose-built spherical tokamaks (STs) now becoming operational, designed to investigate the properties of the ST in large, collisionless plasmas. The first six months of MAST operations have been remarkably successful. Operationally, both merging-compression and the more usual solenoid induction schemes have been demonstrated, the former providing over 400 kA of plasma current with no demand on solenoid flux. Good vacuum conditions and operational conditions, particularly after boronization in trimethylated boron, have provided plasma current of over 1 MA with central plasma temperatures (ohmic) of order 1 keV. The Hugill and Greenwald limits can be exceeded and H mode achieved at modest additional NBI power. Moreover, particle and energy confinement show an immediate increase at the L-H transition, unlike the case of START, where this became apparent only at the highest plasma currents. Halo currents are small, with low toroidal peaking factors, in accordance with theoretical predictions, and there is evidence of a resilience to the major disruption.

Analysis of experiments with electron cyclotron resonance heating (ECRH) requires a good knowledge of the ECRH power profile. This profile is reconstructed by analysis of the transient process after on-axis ECRH switching on in special experiments with suppressed sawtooth oscillations in the T-10 tokamak. The calculations show that the absorbed ECRH power, P T EC , determined by the change in time derivative of the electron temperature at the region of ECRH power input, and the absorbed ECRH power, P β EC , determined by the magnetic measurements, are several times different. Depending on the plasma density and plasma current, their relation, γ EC = P T EC /P β EC , changes from 0.2 to 0.4. Analysis of different explanations for this effect shows that adequate description of the transient process demands introduction of a ballistic jump in the total heat flux just after on-axis ECRH switching on. The effective heat diffusivity increases up to values of 10-15 m 2 s −1 in the first 100-200 µs and decreases down to values of 1.5-2.0 m 2 s −1 during the following 1-2 ms. Note that such a non-monotone dependence of the effective heat diffusivity cannot be described by the modern critical gradient models. It seems that plasma reacts directly to the deposited power but not to the corresponding consequences (the increase in temperature or gradients). Different physical mechanisms could be proposed for this process (partial destruction of magnetic surfaces, fast transition of information through the turbulent cell connections), but each of them needs further confirmation.

The optical thickness of the plasma is often insufficient to fully absorb the microwaves during heating at the second harmonic of the electron cyclotron frequency. An analysis of the experiments to the T-10 tokamak allows us to find the criteria for the full absorption, and to construct a canonical profile transport model for the full and partial absorptions of microwaves. The conditions to the equivalence of discharges in different tokamaks, and in a pair of tokamaks with the optimized W7-X stellarator are formulated. For equivalent discharges, calculations to the T-15MD tokamak under construction with the obtained model coincide with measurements of electron and ion temperatures in the W7-X over a wide range of plasma densities. The validated model is used to analyze future shots of T-15MD.

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Transition to an all-metal wall was realized on T-10 by replacement of graphite limiters with tungsten ones. Light impurity levels remained high and W accumulation in the plasma core was revealed. A movable lithium (Li) limiter was added to investigate the possibility of the limitation of tungsten and light impurity levels in plasma. For the first time, tokamak results on tungsten protection with Li were obtained in OH and ECRH regimes. After lithization the tungsten density in the core dropped more than an order of magnitude, while W influx into plasma decreased 2–4 times. Drastic drops of light impurities in plasma were observed together with improvement of energy confinement time and density limit values. Nevertheless, Li levels in plasma remained low in both OH and ECRH regimes. Li density in the core as low as 0.5% of ne was obtained. Tungsten transport in T-10 plasma was investigated and results on prevention of W accumulation with central ECRH were obtained. Effects of plasma exposure on ITER-grade tungsten plates from limiters were studied. Investigations of density fluctuation with correlation reflectometry confirmed a decrease of fluctuation amplitude on high field side. Modeling showed that this effect can be, to a great extent, explained by the non-locality of reflectometry. Toroidal correlations at a distance of 2.5 m along field lines were studied. Three-wave interaction between geodesic acoustic modes and broad-band turbulence was found by analysis of heavy ion beam probe diagnostics data. The possibility of plasma current control and the prevention of non-thermal electron beams formation at density limit disruption by means of ECR heating and the controlled operation of OH power supply system has been demonstrated. The study of plasma density decay after gas puff switch off during density ramp-up phase in OH regimes and the effect of ‘density pump out’ during ECRH showed that both effects can be explained by the assumption regarding electron transport growth upon reaching marginal pressure profile. A new type of internal transport barrier in the narrow zone near q  =  1 surface has been found.