The paper is devoted to the IPM-2 ion source ion-optical system preparation for the plasma heating experiments with a neutral beam in the Globus-M2 spherical tokamak. Within the framework of this work, a complete renovation of the ion-optical system of the ion source was carried out, including its assembly, adjustment, and testing. To assess the quality of the work performed, measurements were made of the main electrical parameters of the high-energy beam, its energy spectrum and signals from secondary emission probes. Based on the data obtained, the beam power distribution profile and its characteristic dimensions are reconstructed, and the relative concentrations of the energy components of the beam are calculated.

Data on plasma disruption processes in the modernized Globus-M2 spherical tokamak are presented. Electron temperature and density profiles before the disruption, immediately after thermal quench and in the stage of plasma current quench are measured using the diagnostics of Thomson scattering of laser radiation. The dependence of the plasma current decay time during disruption on the pre-disruption current value is determined. The distribution of the toroidal current, which is induced during disruption, in the shell of the vessel is determined on the basis of magnetic measurements. Electromagnetic loads on the vessel are calculated.

A set of magnetic coils used to correct the error fields at the Globus-M2 spherical tokamak, which appear due to the imperfections of the production and assembly of the tokamak magnetic system, is described. The magnetic sensors that are used to monitor the locked helical MHD modes are also described. The results of experiments on detecting the locked modes in the discharges with plasma heating by neutral beam injection are presented. A correlation is found between the appearance of the locked modes accompanied by the loss of fast ions and the confinement of the main plasma.

It is shown that the radial profile of the electron temperature in the Globus-M2 spherical tokamak is related to the radial profile of the electron density by a power law T_e(ρ ) = const 1ptn_e(ρ )^1.65 . An analytical model of the density attractor, assuming the dependence of n_e on the specific poloidal volume v, previously tested on the TCV and JET tokamaks with a large aspect ratio, holds on the spherical tokamak, and the approximation of 162 experimental profiles has resulted in n_e(ρ )v(ρ )^1.06 = const . The proposed model makes it possible to calculate the spatial distribution T_e(ρ ) and n_e(ρ ) according to a known magnetic configuration.

With an increase of magnetic field up to 0.8 T and plasma currentto 400 kA, fast ion losses rate in the discharges with toroidal Alfveneigenmodes decreased in tokamak Globus-M2 comparing with Globus-M tokamakdischarges. Taking into account the data on the discharges with increasedmagnetic field and plasma current, the regression fit of neutral particleanalyzer flux drop in energy channel close to neutral beam energy onrelative eigenmode magnitude, the value of magnetic field and plasma currentwas analyzed. The power of flux drop dependence on TAE magnitude was foundto be ~0.5 and inverse proportional on the value of product of magneticfield and plasma current, which is highly likely is determined only byplasma current due to weak dependence on magnetic field. The resultobtained indicates that fast ion losses in Globus-M2, stimulated by toroidalAlfven eigenmodes are mostly determined by the shift of passing orbits tothe plasma edge. With the increase of plasma current and magnetic field,neutron flux drops arising in the moments of toroidal mode bursts have alsodecreased. Keywords: TAE, NPA, spherical tokamak, fast ion losses\

In experiments with neutral beam injection on the Globus-M2 spherical tokamak, sequences of long-lasting harmonics of toroidal Alfvén modes were discovered, equidistant from each other in frequency and shifted from zero by a constant value. Using microwave Doppler backscattering diagnostics, the central localization of toroidal modes was determined. In this work, the possibility of “splitting” of toroidal harmonics due to the Doppler shift caused by the toroidal rotation of the plasma is being discussed. It is found that the unshifted frequency of the toroidal Alfvén mode obtained from the spectrum of the magnetic probe signal is in good agreement with the frequency of the mode calculated at the mode location radius, and the toroidal rotation frequency, also determined from the spectrum of the magnetic probe signal, correlates well with the rotation frequency measured using charge exchange spectroscopy diagnostics, but differs by a constant amount. Possible reasons for the discrepancies are being discussed.

This paper highlights the most important results achieved at the spherical tokamak Globus-M2 with a high magnetic field. This paper also covers the most important topics of fusion research: thermal energy confinement in regimes with neutral beam injection, toroidal Alfvén eigenmode and correspondent fast ions confinement issues, L-H transition, turbulence suppression and edge-localized modes' behavior, experimental and theoretical study of regimes with nitrogen seeding that allow to significantly reduce thermal loads on the divertor plates, and experiments and simulations of lower hybrid current drive. The research results provide the basis for the next step toward a fusion neutron source—the development of the Globus-3 spherical tokamak.

The results of measuring the electron temperature and density spatial distributions in plasma of the Globus-M2 tokamak using the Thomson scattering diagnostics are presented. The diagnostics provides measurements throughout the entire tokamak discharge, starting from time of gas breakdown. The Thomson scattering data were analyzed in order to determine the positions of the last closed flux surface, the plasma magnetic axis, and the radius of inversion during the saw-tooth oscillations. The results of measurements performed during the internal reconnection of magnetic field lines are presents, as well as the dynamics of spatial distributions of electron temperature, density and pressure during the plasma transition to the H-mode. The results of measuring the electron temperature distribution in the scrape-off layer using the Thomson scattering diagnostics are also presented for distances up to 4 cm outside the last closed flux surface.

The first results of investigation of the turbulence structure using Doppler backscattering (DBS) on the Globus-M2 tokamak are presented. A one-channel DBS system with a variable probing frequency within the 18-26 GHz range was installed to investigate the edge plasma at normalized minor radii 0.9-1.1. Radial correlation Doppler reflectometry was used to study the changes in turbulence eddies after the LH transition. Correlation analysis was applied to the phase derivative of complex in-phase and quadrature (IQ) signals of the DBS diagnostic as it contains information about the poloidal plasma rotation velocity. In L-mode, the radial correlation length L r is estimated to be 3 cm and after transition to H-mode reduces to approximately 2 cm. Gyrokinetic modelling in a linear local approximation using code GENE indicates that the instability with positive growth rate at the normalized minor radius 0.75 in L-mode and H-mode on Globus-M2 was the ion temperature gradient (ITG) mode.

The first results of investigation of edge localized modes (ELMs) in the Globus-M2 tokamak using the Doppler backscattering method are presented in this paper. Specifically, ELMs that are initiated by sawtooth crashes in the H-mode are discussed. The goal of this paper is study plasma turbulence behaviour during ELMs and to showcase what ELM characteristics can be obtained using Doppler backscattering (DBS). An increase of the poloidal rotation velocity during an ELM burst and a decrease in the inter-ELM periods was observed. The effect of ELMs on the plasma turbulence was investigated and estimated to span around 6 cm inside the separatrix. This is to do with the fact that the sawtooth crashes which are responsible for initiating the ELMs take place in the core plasma. Additional experiments with standard reflectometry indicate that ELMs develop 3 cm inside the separatrix where the pedestal region is believed to be in Globus-M2. The direction of the expansion of the ELMs from the inner plasma region to the edge was determined and the velocity was estimated to be around 8 km s −1 . During a single ELM burst a series of filament structures were found in the peripheral DBS channels. In an attempt to understand the processes involved modelling of the reaction of the DBS signals to filaments was done using the BOUT ++ and IPF-FD3D full-wave codes, and the cases for both linear and nonlinear scattering were considered. The results show that the presence of nonlinear scattering during ELMs can lead to an overestimation of the measured velocity values in the region of filament existence near the separatrix.

The thermal energy stored in plasma Wp, normalized internal plasma inductance li and currentbeta bi are calculated via the free-boundary equilibrium PET code. The equilibrium reconstruction algorithmis iterative method of minimizing two parameters, the distance between the reconstructed plasmaboundary and that simulated by the PET code, as well as the difference between the plasma diamagnetic fluxfrom PET and the experimental one. The discharges from the Globus-M2 tokamak with a toroidal magneticfield up to 0.9 T and a plasma current of 0.3–0.4 MA in a mode with auxiliary heating by two atomic injectorsare analyzed. The possibility of using approximate formulas for estimating and is considered. The measureddiamagnetic flux is used to determine bdia and then to calculate Wdia . The normalized internal plasmainductance is additionally determined from the measured vertical magnetic field under the assumption that bp=bdia.

The pyGSS code constructed for computation of free-boundary plasma equilibrium in spherical Globus-M2 tokamak is described. Currents in the coils of the electromagnetic system, their coordinates, plasma current, positions of the limiter and current-conducting wall, etc., are used as the input parameters. Free parameters determining spatial distribution of equilibrium pressure and current density are selected in the course of code execution in such a way that the results of reconstruction would agree with the experimental measurements of the poloidal magnetic flux by means of toroidally closed loops. The results of computation of equilibrium are compared with those obtained by means of other codes and experimentally measured thermal plasma energy, position of the separatrix outer leg, the diamagnetic-loop signal, etc.