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Results of the experimental study of the ability of plasma jet of coaxial accelerator to breakdown and ionization the working gas at the initial discharge phase in Globus-M2 spherical tokamak are presented. Studies of the development of the initial stage of discharge initiated with a plasma jet showed that the plasma current in the discharge began to grow earlier by 2 ms than using the standard inductive breakdown. The breakdown voltage decreased from 6.1 to 3.1 V (-49%) at 4.7 kV accelerator electrode voltage. The emission intensity of line Dα decreased during formation of the discharge with the help of plasma jet without additional gas puffing. Radiation level CIII in the discharge generated by the plasma jet was the same as the radiation in discharge produced by induction breakdown of the gas. At the stage of increase in the plasma current the most intense visible radiation of the plasma was found near duct through which the jet was injected. Discharge radiation decreased and spread out along the cross section of the plasma column during plasma current plateau.

During modernization of the Globus-M tokamak, toroidal magnetic field and plasma current were increased, and a number of diagnostics were upgraded, which made it possible to study phenomena associated with the excitation of Alfvén waves in a spherical tokamak in a wider range of plasma parameters. In the experiments with neutral beam injection, the dependences of fast particle losses initiated by the toroidal Alfvén eigenmodes (TAE) on their magnitude in the magnetic field range of 0.4 – 0.7 T and currents of 180 – 330 kA were acquired. Resulting dependences confirm previously obtained results and indicate a decrease in losses with increasing magnetic field and plasma current. At the same time, a number of new phenomena, that have never been observed on Globus-M were detected. In experiments with neutral beam injection at the current ramp up stage, Alfvén cascades (AC) in the frequency range of 100 – 300 kHz were observed for the first time. By means of the Doppler backscattering diagnostics (DBS), it was shown that ACs are localized closer to magnetic axis unlike TAE. Also, during low hybrid wave current drive (LHCD) experiments, global Alfvén eigenmodes (GAE) were detected at a frequency close to 1 MHz, apparently driven in the resonance with runaway electron beam. Detected global eigenmodes are also able to arise in ohmic discharges, however, their magnitude is lower.

The conventional method for determining the effective ion charge Zeff is based on measurements of the bremsstrahlung intensity in the spectral regions free from line radiation. This paper describes the design of the Zeff diagnostics at the Globus-M tokamak as well as the results of the first measurements. With allowance for the emission intensity estimates, the geometry of the experiment was chosen; the filter monochromators for recording bremsstrahlung were designed and manufactured. Using the Thomson scattering data on T e (R) and n e (R) profiles, time dependences Zeff (t) were obtained in several shots of the Globus-M tokamak. The results of simulations Z eff performed using the ASTRA transport code are consistent with these measurements.

The results from calculations of the anisotropic fast particle distribution function in the Globus-M/M2 spherical tokamaks are presented. The calculations were performed using the NUEBAM code, the reasons for the anisotropy occurrence are discussed (injection geometry, low toroidal magnetic field and plasma current, presence of neutral particles). It is shown that the anisotropy decreases with increasing toroidal magnetic field and plasma current: the density of the slowed down fast ions with energies of E < 4 keV almost doubles and the averaged perpendicular velocity of fast ions increases.

Ion temperature is one of the most important parameters of high-temperature plasma. Information on the ion temperature spatial distribution is necessary for understanding and modelling of particle and energy transport processes, evaluating the effectiveness of plasma heating system operation. Active spectroscopy also known as CXRS (Charge eXchange Recombination Spectroscopy) is a powerful diagnostic tool for measurements of local values of ion temperature, and is widely used in experiments with magnetic confinement of high-temperature plasma. Active spectroscopy diagnostics on the Globus-M tokamak utilizes a tangentially injected heating beam of neutrals (hydrogen or deuterium atoms). The CXRS-system setup is described. The results of ion temperature measurements for the last Globus-M experimental campaign before the machine shutdown and further upgrading to higher values of magnetic field and plasma current are presented. Ion temperature significant rise in the experiments with moderate increasing of toroidal magnetic field and plasma current is detected and discussed.

A fast scanning probe which can penetrate inside the separatrix during discharge was developed for Globus-M2. The probe has been designed to move various sensor pins into the scrape-off layer. The driving force is provided by a crank-slider mechanism which is ensured maximum linear speed about 4 m/s. The paper presents probe head and drive system description.

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.

Analysis of stability of the peeling-ballooning modes in the edge plasma of the Globus-M tokamak is presented. Studies were performed using the ideal 3D MHD code written in the BOUT++ framework. The edge localized modes which are frequently observed in the Globus-M plasma could be described using MHD theory. It was found that the parameters of the Globus-M edge plasma are in the range of stability for the peeling-ballooning modes. The instability modes with a toroidal number of n = 12 and typical distances between filaments of approximately ~12 cm were found to be the most unstable. This statement agrees with the Doppler backscattering measurements. It was also considered whether the plasma in the Globus-M2 tokamak will be stable against the PB instability.

The latest results are presented from NBCD experiments at the Globus-M spherical tokamak. The Globus-M2 NBI upgrade program is discussed, in particular, the analysis of the injection geometry and choice of the optimal experiment layout. The results from numerical simulations of plasma heating and current drive by two neutral beams in the Globus-M2 shots are also analysed.

Electron temperature and density spatial distribution dynamics on the Globus-M (R =0.36 m, a=0.24 m, B =0.4 T, I= 0.2 MA) spherical tokamak was investigated during the NBI heating. The key tool of this research was the upgraded Thomson scattering diagnostics with variable intervals between probing pulses (20 pulses in total). The scattering points were placed along the major plasma radius from the inner to the outer plasma border (10 points in total). Simulation of the electron transport in the L and H-mode, based on experimentally measured ne(R) Te(R) profiles was carried out using the code ASTRA for the OH and NBI dischargers.