A.B. Mineev

Density limits and control in the Globus-M spherical tokamak

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.

Plasma formation and first OH experiments in the Globus-M tokamak

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.

Investigation of beam– and wave–plasma interactions in spherical tokamak Globus-M

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.

On-line plasma shape reconstruction algorithm in tokamaks and its verification in the Globus-M

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.

Globus-M plasma physics research for fusion application and compact neutron source development

During the past decade, plasma physics research promoting the physics base of ITER and developing novel concepts such as a compact fusion neutron source has been conducted on the Globus-M spherical tokamak (ST) (R = 36 cm, a = 24 cm, I p ≤ 250 kA, B T ≤ 0.4 T). Tokamak reconstruction is imminent. The upgraded tokamak Globus-M2 will have the same vacuum chamber and an enhanced magnetic system to provide B T = 1 T and I p = 500 kA. In this paper we outline the most important research directions and the main results obtained on Globus-M and make some predictions about the possibilities and parameters of Globus-M2.

Studies of runaway electrons in the Globus-M tokamak

Results are presented from experimental studies of runaway electrons in the ohmic heating regime in the Globus-M tokamak. The periodical hard X-ray bursts observed with the help of two hard X-ray spectrometers with high time resolution are attributed to MHD oscillations in the plasma core and at the periphery.

Methods for reconstructing equilibrium plasma configurations in the globus-M spherical tokamak

A magnetic diagnostics allowing one to reliably reconstruct equilibrium plasma configurations in a tokamak over a wide range of operating parameters is developed. The accuracy of determining the geometrical parameters and thermal energy of the tokamak plasma is analyzed in detail. The experimental data obtained in the Globus-M tokamak are processed the with help of the EFIT code. The influence of the plasma configuration on the intensity of the main impurity lines is investigated.

Characteristics of major plasma discharge disruption in the Globus-M spherical tokamak

The characteristics of the major disruption of plasma discharges in the Globus-M spherical tokamak are analyzed. The process of current quench is accompanied by the loss of the vertical stability of the plasma column. The plasma boundary during the disruption is reconstructed using the algorithm of movable filaments. The plasma current decay is preceded by thermal quench, during which the profiles of the temperature and electron density were measured. The data on the time of disruption, the plasma current quench rate, and the toroidal current induced in the tokamak vessel are compared for hydrogen and deuterium plasmas. It is shown that the disruption characteristics depend weakly on the ion mass and the current induced in the vessel increases with the disruption time. The decay rate of the plasma toroidal magnetic flux during the disruption is determined using diamagnetic measurements. Such a decay is a source of the poloidal current induced in the vessel; it may also cause poloidal halo currents.

Analysis of Dynamics of Plasma Current Quench in the Globus-M Spherical Tokamak

Data on the dynamics of the plasma current quench in the Globus-M tokamak are presented. The main current quench characteristics at different toroidal magnetic fields are compared. The distribution of the toroidal current induced in the vessel wall is determined from magnetic measurements, and the electromagnetic loads acting on the vessel wall during the current quench are calculated. By extrapolating the experimental data, the additional pressure on the vessel wall during the current quench in the upgraded Globus-M2 tokamak is estimated. It is shown that the current quench results in the appearance of bending stresses in the vessel domes. Using numerical simulations, it is shown that the best agreement between the measured and calculated plasma current dynamics during the current quench corresponds to the linear (in time) influx of the carbon impurity.

The Globus-M2 spherical tokamak: the first results

The Globus-M2 spherical tokamak is the considerably upgraded Globus-M facility. Its technical parameters were increased as much as possible to achieve the promising range of physical parameters (sub-fusion temperatures and collisionality of much less than unity). These parameters will be achieved in a compact magnetic configuration similar to that of the Globus-M tokamak, the plasma current and toroidal magnetic field amounting to 0.5 MA and 1 T, respectively. The demand to increase the magnetic field and plasma current in the Globus-M2 resulted in the need for a complete redesign of the electromagnetic system because the plasma equilibrium requirements have changed and the mechanical and thermal loads have considerably increased as compared to the Globus-M. The vacuum vessel and the in-vessel components of the new Globus-M2 tokamak remain the same. Power supplies were upgraded to provide the required currents in the toroidal field coil and the central solenoid. The Globus-M2 tokamak was build up and preliminary tests were carried out. New auxiliary heating systems and diagnostics were developed and installed to be used in future experiments. Fist plasma was achieved at the Globus-M2 in April 2018.