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The work presents the results of the energy confinement study carried out on the compact spherical tokamak Globus-M2 with a toroidal magnetic field as high as 0.8 T. A reproducible and stable discharge was obtained with the average plasma density (5–10) × 1019 m−3. Despite the increase in the magnetic field, the neutral beam injection (NBI) led to clear and reproducible transition to the H-mode accompanied by a decrease in the turbulence level at the plasma edge. NBI allowed effectively heat the plasma: electron and ion temperatures in the plasma core exceeded 1 keV. Compared to the previous experiments carried out with a toroidal magnetic field as high as 0.4 T plasma total stored energy was increased by a factor of 4. The main reason or this phenomenon is a strong dependence of the energy confinement time on the toroidal magnetic field in the spherical tokamak. It was experimentally confirmed that such kind of dependence is valid for ST with magnetic field up to 0.8 T. It has also been shown that the enhancement of the energy confinement in Globus-M2 with collisionality decrease is associated with an improvement of both electron and ion thermal insulation.

The current work reports on the significant rise of the fusion triple product in experiments carried out on the compact spherical tokamak (ST) Globus-M2 with a twofold increase in the toroidal magnetic field. A tenfold rise in the n . T . τ E product was recorded during an increase in the magnetic field from 0.4 to 0.8 T and the plasma current from 0.25 to 0.4 MA at an unchanged auxiliary heating power value. Limited reasons may affect this positive trend, among which are energy confinement improvement and an increase in the efficiency of neutral beam heating. Despite the increase in the magnetic field, the neutral beam injection (NBI) led to clear and reproducible transition to the H-mode accompanied by a decrease in the turbulence level at the plasma edge. It was experimentally confirmed that strong dependence of the energy confinement time on the magnetic field value is conserved at a higher magnetic field approaching 0.8 T. Enhancement of energy confinement is connected to a collisionality (ν *) decrease. While for conventional tokamaks the confinement dependence on collisionality becomes weaker with decreasing ν * dependence, in the ST, in contrast, we observe its strengthening.

In the final Globus-M experimental campaign B tor and I p were raised by 25% up to 0.5 T and 250 kA, respectively, expanding available range of experimental parameters. In these experiments an overall improvement in plasma performance was observed, plasma total stored energy and neutron rate increased significantly. The energy confinement time dependence on B tor and n e was close to linear. L-H transition threshold was 5–10 times higher than threshold predicted by the scaling. Despite the small plasma-wall distance, the level of radiation losses in the main plasma volume was relatively small and decreased as B tor rised. An increase in B tor and I p led to a decrease in the sawtooth-induced losses. While a rise in the plasma current led to a decrease of fast ion losses per TAE burst at the same amplitude; frequency of bursts and their amplitude increased. Doppler backscattering, applied to study TAE localization for the first time, revealed that TAE fluctuations are localized in the region of normalized minor radii ρ = 0.5–0.75. A new movable 9-pin head probe provided data, necessary for power decay length scaling investigation. Significant power decay length contraction, stronger than predicted by Eich’s scaling, was observed.

An on-line plasma shape reconstruction algorithm is necessary to design the plasma position and shape control system in modern tokamaks. An algorithm aimed at solving this problem is proposed. A description of the mathematical procedure is provided and experimental data incorporation is discussed. An example of an application of this algorithm is demonstrated using experimental data from Globus-M discharge #10292.

The results of the experimental campaign on Globus-M (R = 0.36 m, a = 24 m) devoted to investigating density limits and density control are reported. The experiments were performed at Btor = 0.4 T, Ip = 0.18–0.25 MA, q95 = 3.5–5 and plasma vertical elongation, κ ~ 1.5–1.7. The density limits achieved with the gas puffing method of density control in the previous periods in ohmic heating (OH) regime are discussed. The progress made in OH scenario optimization helped the density to approach the Greenwald limit. Co-current neutral beam of deuterium with the power in the range of 0.45–0.6 MW at the beam energy of 28–29 keV was injected into deuterium target plasma at the early stage of the discharge, which allowed the density to overcome the Greenwald limit. Line averaged densities in excess of 1.5 × 1020 m−3 were achieved, during the external gas puff. An ion temperature increase, measured by NPA was accompanied by a definite increase in the electron energy content, registered by Thomson scattering. Injection of a pure, highly ionized hydrogen plasma jet with a density up to 1022 m−3, total number of accelerated particles (1–5) × 1019 and a flow velocity of ~110 km s−1 was used as another instrument for density control. It increased plasma particle inventory in the Globus-M by ~50% (from 0.65 × 1019 to 1 × 1019) in a single shot without target OH plasma parameter degradation. The injection resulted in a fast density increase with the time much shorter than with gas puff fuelling, which was confirmed by Thomson scattering measurements.

In spherical tokamaks the conventional ICR plasma heating has a number of specific features and therefore requires additional investigations. For this aim the modelling of wave propagation and absorption was performed by the 1-D code developed at the Ioffe institute. All possible mechanisms of RF absorption (cyclotron absorption at fundamental and second harmonics, TTMP, Landau) were taken into account. The calculations demonstrated the possibility of effective RF power absorption both by ions and electrons in a broad range of plasma parameters (including relative hydrogen concentration).The ICRH experiments were performed on the low aspect ratio tokamak Globus-M (R = 0.36 m, a = 0.24 m, B0 = 0.3–0.4 T, Ip = 0.15–0.25 MA, vertical elongation 1.2–2, at RF power input level up to 200 kW at frequencies of 7.5–9.2 MHz. A 12-channel neutral particle analyser measured simultaneously hydrogen and deuterium fluxes and relative concentration of ion components. In the experiment the ion temperature increases twice, but the ion heating efficiency depends on the location of the second hydrogen cyclotron harmonic and on the concentration of the light ion component. It is shown that the position of the second hydrogen harmonic in front of the antenna decreases the efficiency of the ion heating. The increase in H-concentration in deuterium target plasma from 10% up to 70% does not influence ion heating efficiency essentially but seems to increase it moderately. The first results on 1.5-D transport ASTRA Modelling are described. They are in reasonable agreement with experimental data. The electron heating was not detected in the experiment due to comparatively low absorbed power with respect to OH one.

Progress in the theoretical and experimental development of the plasma jet source and injection of hydrogen plasma and neutral gas jets into the Globus-M spherical tokamak is discussed. An experimental test bed is described for investigation of intense plasma jets that are generated by a double-stage plasma gun consisting of an intense source for neutral gas production and a conventional pulsed coaxial accelerator. A procedure for optimizing the accelerator parameters so as to achieve the maximum possible flow velocity with a limited discharge current and a reasonable length of the coaxial electrodes is presented. The calculations are compared with experiment. Plasma jet parameters, among them pressure distribution across the jet, flow velocity, plasma density, etc, were measured. Plasma jets with densities of up to 1022 m−3, total numbers of accelerated particles (1–5) × 1019, and flow velocities of 50–100 km s−1 were successfully injected into the plasma column of the Globus-M tokamak. Interferometric and Thomson scattering measurements confirmed deep jet penetration and a fast density rise (

The Globus-M spherical tokamak has demonstrated practically all of the project objectives during the 15-year period of operation. The main factor limiting further progress in plasma performance is a relatively low toroidal magnetic field. The maximum toroidal magnetic field achieved on Globus-M was 0.4 T with the exception of a limited number of shots with 0.55 T, which led to damage of the toroidal field coil in 2002. The increase of the magnetic field up to 1.0 T together with the plasma current up to 0.5 MA will result in the significant enhancement of the operating parameters in the upgraded Globus-M2 machine. The experimental program will be focused on plasma heating and non-inductive current drive and will contribute to the creation of a physical and technological base for the compact fusion neutron source development. In the article a brief overview of the physical background for the machine upgrade is outlined. The current status of the project implementation is described. First experimental results on moderate magnetic field increase from 0.4 T up to 0.5 T in the existing Globus-M machine are discussed. The improvement of plasma confinement as well as enhancement of efficiency of the beam driven current is observed.

The article reports the results of experimental campaigns on plasma ohmic heating performed during 1999-2000 on the spherical tokamak Globus-M. Later experimental results with the tokamak fed by thyristor rectifiers are presented in detail. The toroidal magnetic field and plasma pulse duration in these experiments were significantly increased. The method of stray magnetic field compensation is described. The technology of vacuum vessel conditioning, including boronization of the vessel performed at the end of the experiments, is briefly discussed. Specific features of neutral gas column breakdown in spherical tokamaks by applied inductive voltage are outlined. Also discussed is the influence of ECR preionization on the breakdown conditions. Experimental data on plasma column formation and current ramp-up in different regimes of operation with the magnetic flux of the central solenoid limited to ~100 mWb are presented. A significant reduction of the background density after boronization (below 2 × 1018m-3) allows the density to be completely controlled with external gas puffing and makes the influence of the wall negligible. The magnetic flux consumption efficiency is discussed. The results of magnetic equilibrium simulations are presented and compared with experiment. Ramp-up of the plasma current of 0.25 MA for a time interval of about 0.03 s with about 0.02 s flat-top at a toroidal field strength of 0.35 T allows the conclusion that the target design parameters of Globus-M could be achieved in a double swing regime.

First experiments on non-inductive current drive (CD) in the spherical tokamak using lower hybrid (LH) waves at 2.45 GHz are described. The basic characteristics of the new experimental approach utilizing poloidal LH wave slowing-down scheme obtained by modeling are presented at the beginning of the paper. The experimental results for a poloidal slowing-down scheme are described. The CD efficiency comparable or even higher than theoretically predicted is demonstrated in the experiment using a 10 waveguide grill with 120° phase shift between neighboring waveguides.

The behaviour of the fast particle population during 18 keV hydrogen and 26 keV deuterium neutral beam injection in deuterium plasmas is investigated. Experiments reveal large fast ion losses. The experimental results are confirmed using different types of modelling: simulation using the NUBEAM module, solution of the Boltzmann kinetic equation and solution of the 3D fast ion tracking algorithm. The dynamics of the energetic particle redistribution and losses during sawtooth oscillation and toroidal Alfven eigenmodes are studied. A method to decrease fast ion losses under the current conditions (0.4 T, 0.2 MA) is shown. The influence of the plasma parameters on the energetic ion confinement rate is investigated. Modelling for the Globus-M2 conditions (1 T, 0.5 MA) is performed.

The results of geodesic acoustic mode (GAM) studies in the spherical torus Globus-M via Doppler reflectometry are presented. The intermittent character of the GAM evolution is similar to the limit-cycle oscillation behavior of zonal flows. The evident correlation between the GAM rotational velocity and both Dα emission and plasma density oscillations is exhibited and discussed. The obtained experimental results are compared with those from tokamaks with large aspect ratios.

The experimental and theoretical results obtained in the last two years on the interaction of neutral particle beams and high-frequency waves with a plasma using the spherical tokamak Globus-M are discussed. The experiments on the injection of low-energy proton beam of ~300 eV directed particle energy are performed with a plasma gun that produces a hydrogen plasma jet of density up to 3 × 1022 m−3 and a high velocity up to 250 km s−1. A moderate density rise (up to 30%) is achieved in the central plasma region without plasma disruption. Experiments on high-energy (up to 30 keV) neutral beam injection into the D-plasma are analysed. Modelling results on confinement of fast particles inside the plasma column that follows the neutral beam injection are discussed. The influence of the magnetic field on the fast particle losses is argued. A neutral beam injection regime with primary ion heating is obtained and discussed. The new regime with fast current ramp-up and early neutral beam injection shows electron temperature rise and formation of broad Te profiles until the q = 1 flux surface enters the plasma column. An energetic particle mode in the range of frequencies 5–30 kHz and toroidal Alfven eigenmodes in the range 50–300 kHz are recorded in that regime simultaneously with the Te rise. The energetic particle mode and toroidal Alfven eigenmodes behaviour are discussed. The toroidal Alfven eigenmode spectrum appears in Globus-M as a narrow band corresponding to n = 1. The first experimental results on plasma start-up and noninductive current drive generation are presented. The experiments are carried out with antennae providing mostly poloidal slowing down of waves with a frequency of 920 MHz, which is higher than a lower hybrid one existing under the experimental conditions. The high current drive efficiency is shown to be high (of about 0.25 A W−1), and its mechanism is proposed. Some near future plans of the experiments are also discussed.

Experiments and simulations to achieve high values of plasma parameters at the Globus-M spherical tokamak (ST) at moderate absolute auxiliary heating power (up to 0.8 MW) and high specific heating power (up to 2–3 MW m−3) are described. Important distinguishing features are the low edge safety factor range, which is unusual for STs, 2.7 < q < 5 and small plasma–outer wall space (3–5 cm). High ion heating efficiency with neutral beam injection (NBI) was demonstrated. Results of numerical simulation of fast ion trajectories are described and fast ion generation during the NBI and ion cyclotron resonance heating is discussed. Investigations on their confinement and slowing down are also presented. Reasons for achievement of high IC heating efficiency are outlined. Reliable H-mode regime achievement is described. Transport ASTRA modelling demonstrated that during NB heated H-mode ion heat diffusivity remains neoclassical and the particle diffusion coefficient inside transport barrier decreases significantly. Analysis was performed of divertor tile and special probe surfaces after irradiation by plasma during a large number of shots (3000–10 000 shots). Mixed layer composition is measured and deuterium retention in different tokamak first wall areas is estimated. Plasma jet injection experiments with upgraded plasma jet are described. Jet penetration to the plasma centre with immediate increase of density and temperature drop is proved and analogy with pellet injection is outlined.

The composition, morphology and structure of the mixed layers deposited onto the surface (deposits) after a preliminary carboboronization procedure (B/C : H layers deposition) and subsequent plasma–wall interaction in the different areas of the Globus-M spherical tokamak have been analysed. Globus-M is almost completely covered by recrystallized graphite protection tiles (currently about 90% of its inner vessel area facing to plasma). The deposit properties were examined by different diagnostic techniques (electron probe microanalysis, scanning electron microscope, x-ray diffraction, nuclear resonance reactions, thermal desorption spectroscopy, infrared Raman scattering and secondary ion mass spectroscopy) after sample exposure to plasma during 3000–10000 tokamak pulses. It was found that deposits existed even in high flux regions (separatrix strike points). The initial composition of the protective layers formed during boronization is dramatically modified during long-term plasma–wall interaction and resulted in significant intermixing of its components with materials of the vessel interior. The properties of the layers deposited onto the surfaces intensively interacting with plasma, as well as the composition of the layers deposited in the ‘shadowed’ zones are discussed. Deuterium retention in Globus-M was estimated relying upon the data on deuterium concentrations in the deposits that were measured by different methods. It is revealed that deuterium was absorbed only in the deposits and its concentration vanished in the bulk of the tiles. Conditions of deuterium desorption are analysed and the merits of recrystallized graphite are discussed.

A method is described for determining current distribution and plasma pressure in a tokamak from external magnetic measurements of poloidal and toroidal magnetic fluxes outside the plasma column and also of the total plasma current and the toroidal field. Apart from the magnetic measurements, also equilibrium conditions are applied, and the range of possible current distributions is restricted. Results of the initial measurements on the Tuman-3 tokamak are presented.