The paper is devoted to the Thomson scattering (TS) diagnostics recently developed for the Globus-M2 spherical tokamak and prototyping the ITER divertor TS diagnostics. The distinctive features of the system are the use of spectrometers, acquisition system and lasers that meet the base requirements for ITER TS diagnostics. The paper describes the diagnostic system that allows precise measurements of TS signals, as well as the results of the first measurements of electron temperature and density in both central region of the plasma column and scrape-off layer. The system provides measurements of electron temperature $T_{e}$ in the range of 5 eV to 5 keV and density $n_{e}$ in the range of $5{\cdot}10^{17}{\div}3.25{\cdot}10^{20} m^{-3}$. The use of two ITER-grade probing lasers of different wavelengths (Nd:YAG 1064.5 nm and Nd:YLF 1047.3 nm) allows reliable measurement of $T_{e}$ in multi-colour mode, i.e., assuming that spectral calibration is unknown.

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\

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 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.

Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges. The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density 〈 n e 〉 < 2 × 10 19 m − 3 in a wide range of toroidal magnetic fields and plasma currents. This type of instability was identified as compressional Alfvén waves, driven by electrons, accelerated during a sawtooth crush. It was found that the mode frequency is sweeping in time, according to the Berk–Breizman hole–clump nonlinear chirping model. The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column, when the plasma tends to decay at extremely low density 〈 n e 〉 < 2 × 10 18 m − 3 and, in fact, is an instability of the runaway electron beam. The exited modes cover the whole observed frequency range and are divided into several (two or three) frequency regions: approximately 0–30 MHz, 60–120 MHz and sometimes 30–60 MHz. Reconnection of the branches was also observed. Single chirps are more rapid than for 1 MHz Alfvén instability and follow an exponential law. This paper, to our knowledge, is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.

An important part of high-temperature plasma study is the determination of the electron temperature dynamics in the tokamak plasma. At spherical tokamaks, one can use Thomson scattering diagnostics as well as soft X-ray emission diagnostics (SXR). The capabilities of electron temperature measurement by the first diagnostics are limited by the repetition rate of laser pulses and their number in one tokamak discharge. Data of the second diagnostics are continuous in time and are determined by the time resolution of the detectors; however, obtaining the electron temperature using these data encounters a number of difficulties considered in this study. A method of combined processing of results of these diagnostics using machine learning algorithms was developed for overcoming these difficulties and applying the adVoprosy Atomnoi Nauki i Tekhniki, Seriya: Termoyadernyi Sintezages of both diagnostics. Training data include soft X-ray diagnostic data, hard X-ray diagnostic data, and CIII line emissivity diagnostic data. Thomson local scattering measurements were used as labels for supervised machine learning. The developed technique provides significant extension of the possibilities of determining the electron temperature at the Globus-M2 tokamak.

Absorbed power of the neutral-injection beam in spherical tokamaks Globus-M/M2 is estimatednumerically. Deceleration of fast particles is simulated by means of the NUBEAM code. The signal of analyzerof charge-exchange atoms is simulated by means of the FIDASIM code using the distribution functionof fast ions calculated by means of the NUBEAM code. Comparison of calculated and experimental signalsallowed determining the degree of influence of instabilities on confinement of fast particles along withabsorbed beam power.

Исследован нагрев плазмы одним и двумя инжекторами быстрых нейтральных атомов в сферическом токамаке Глобус-М2 при тороидальном магнитном поле 0.8–0.9 Тл и токе плазмы 0.35–0.4 МА. Измерение пространственных распределений температуры и концентрации электронов, выполненные диагностикой томсоновского рассеяния лазерного излучения, показало двукратный нагрев электронов плазмы при инжекции нейтральных частиц с энергией до 45 кэВ при мощности пучка 0.75 МВт по сравнению с омическим режимом. Дополнительное включение второго пучка с энергией частиц до 30 кэВ и мощностью до 0.5 МВт позволило получить режим с горячими ионами в диапазоне значений средней плотности плазмы 1.6–10 × 10 19 м –3 . По данным активной спектроскопии и корпускулярной диагностики температура ионов достигла величины 4 кэВ при плотности плазмы 8 × 10 19 м –3 в горячей зоне, превысив температуру электронов более чем в 2.5 раза.

NBI-assisted plasma heating with one or two injectors of fast neutral atoms was studied at the Globus-M2 spherical tokamak at the toroidal magnetic fields of 0.8–0.9 T and plasma currents of 0.35–0.4 MA. Measurements of the spatial temperature and electron density distributions, performed using the Thomson scattering diagnostics, showed a twofold increase in heating of plasma electrons during the injection of neutral particles with energies of up to 45 keV at the beam power of 0.75 MW, as compared to the ohmic heating regime. Switching on the second additional beam with the particle energy of up to 30 keV and power of up to 0.5 MW resulted in obtaining the hot ion mode in the range of mean plasma densities of (1.6–10) × 10 19 m −3 . According to the data of active spectroscopy and neutral particle analyzer diagnostics, in the hot zone, the ion temperature reached 4 keV at the plasma density of 8 × 10 19 m −3 , which is more than 2.5 times higher than the electron temperature.

The pyGSS code constructed for computation of free-boundary plasma equilibrium in sphericalGlobus-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 inthe course of code execution in such a way that the results of reconstruction would agree with the experimentalmeasurements of the poloidal magnetic flux by means of toroidally closed loops. The results of computationof equilibrium are compared with those obtained by means of other codes and experimentally measuredthermal plasma energy, position of the separatrix outer leg, the diamagnetic-loop signal, etc.

The paper provides an overview of the results obtained on the spherical tokamak Globus-M2 in 2019-2020. The experiments were performed with the toroidal magnetic field up to 0.8 T and plasma current up to 0.4 MA (80% of the design values). The temperature of electrons 1 keV and ions 800 eV at the plasma density of 10(20) m(-3) were recorded at neutral beam injection (850 kW, 28.5 keV). Heat conductivity analysis was made by means of the codes ASTRA 7.0, NCLASS, SPIDER, NUBEAM, 3D fast ion tracking algorithm on the basis of the experimental data. A scaling for spherical tokamaks, which demonstrates strong tau (E) dependence on magnetic field and moderate dependence on plasma current, has been confirmed for the magnetic field up to 0.8 T. For Globus-M/M2 it is tau EGLB similar to Ip0.43 +/- 0.22BT1.19 +/- 0.1 . The dependence of the normalized energy confinement time (B (T) tau (E)) on collisionality (nu*) in a wide range 0.02 < nu* < 0.2 was determined as BT tau E similar to nu*-0.74 <i . A non-inductively driven current was recorded during the launch of the electromagnetic waves of the lower hybrid frequency range (2.45 GHz) with the help of a toroidally oriented grill. The fraction of noninductively driven current has exceeded 70% in the discharge with a total current of 0.2 MA. The achieved values of efficiency eta = (0.15-0.4) x 10(19) A m(-2) W-1 are comparable with the results obtained on conventional tokamaks. This paper presents the results of experiments on the study of Alfven modes. The resulting scaling for the loss of fast ions caused by toroidal Alfven eigenmodes demonstrates their decrease with increasing magnetic field and plasma current. Observation of Alfven cascades made it possible to apply the method of MHD spectroscopy to determine the evolution of q (min) in a discharge. Also presented are the results of SOL investigation. Attention is also paid to the development of diagnostics.

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) × 10 19 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.

Hot ion mode was recently achieved at the Globus-M2 spherical tokamak when the toroidal magnetic field was as high as 0.9 T and plasma current 0.35 MA. The injection of two neutral beams with a total power of 0.7 MW and an energy of 30 keV into a plasma with major radius 0.36 m and minor radius 0.22 m (aspect ratio 1.6) made it possible to heat the plasma ions up to 4 keV, while electron density in the plasma center was as high as 8 × 1019 m−3. The achievement of high temperatures became possible due to the good thermal insulation of the plasma.

In Globus-M2 ohmic discharges with low density, by means of Mirnov coils array, magnetic field oscillations with frequencies in 1 MHz range were detected. Frequency range of these oscillations significantly exceed the range of TAE and RSAE frequencies, which were previously observed on Globus-M and Globus-M2 tokamaks, and their amplitude, contrary, turned out to be up to an order of magnitude lower. It was found that high frequency oscillations are interrelated with suprathermal electron fraction. At the same time the observed instability seems to have Alfvenic nature, since its frequency correlates well with Alfven frequency scaling. It was also found that magnetic perturbation always forms standing wave with predominantly low toroidal wavenumbers, including n = 0 structure, which makes gap (e.g. TAE) mode excitation impossible. Frequency chirping during single bursts with $$\delta \omega ~\sim ~\sqrt t $$ is consistent with hole-clump model predictions.

The article presents the results of studying the transfer of heat and particles in the Globus-M2 spherical tokamak in discharges with neutral injection at the current ramp up. An atomic beam was injected into the tokamak plasma at a fixed toroidal magnetic field of 0.7 T. The plasma current on the plateau was varied in the range 0.2–0.3 MA. Based on the electron temperature and concentration spatial distributions measured by the Thomson scattering method, the transport of heat and particles in plasma was simulated using the ASTRA code. The energy confinement time of the plasma was determined, as well as estimates of the coefficients of thermal diffusivity and diffusion was made.

The first results of studies of quasi-coherent fluctuations (QCFs) in the spherical Globus-M tokamak are presented. QCFs were observed as oscillations of the Doppler backscattering (DBS) amplitude. These oscillations were not detected in the spectra of the complex signal of the quadrature detector, the shift of which determines the plasma rotation velocity. The multi-frequency DBS scheme allowed us to determine the region of existence of the QCFs (ρ = 0.6–0.7). Bicoherence analysis of the backscattered power demonstrates a nonlinear interaction between broadband turbulence and QCFs. The detected QC fluctuations were interpreted as relatively long-wave oscillations that influence the backscattering on shorter-wave oscillations.