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.

A high-speed two-color pyrometer has been developed that allows measuring the intensity of thermal radiation in temperature range of 100-3500 o C with 2 μs time resolution. The analysis of the main factors affecting the measurement of surface temperature in the conditions of its interaction with plasma is carried out. It is shown that at plasma temperature of more than 10 eV and moderate concentration of particles, the wall plasma can be considered transparent to thermal radiation. Experimental studies on non-contact measurement of temperature of surface interacting with plasma on Globus-M2 tokamak and plasma gun test bench have been carried out. Sawtooth temperature fluctuations were detected on the wall of tokamak chamber near the exit of outer separatrix branch. The period of these flashes was 3-4 ms, and duration was 2 ms. The maximum wall temperature measured by a high-speed pyrometer exceeded the temperature measured by infrared camera. Measurements of temperature of the front surface of W-Li sample were carried out at plasma gun test bench under conditions of deuterium plasma jet cyclic action under loads simulating regimes of plasma current disruption in tokamak. The sample temperature reached more than 3000^oC during <1 ms. The thermal conductivity of the sample degraded with an increase in number of plasma exposure cycles, which was expressed in progressive decrease in target cooling rate after the pulse. Using a high-speed pyrometer, it is proposed to control the formation of heat-insulating foils on surface interacting with plasma Keywords: two-color pyrometer, thermal radiation, surface temperature, materials irradiation.

Technical solution were presented for a foil spectrometer installed on the Globus-M2 and TUMAN-3M tokamaks for measuring the electron plasma temperature. Measurements have been carried out of the time dependence of the plasma temperature in the central region of tokamaks. Using of integrated photodetectors and unique beryllium foils with a thickness of 14-80 μm made it possible to increase the sensitivity of the spectrometer. An important quality of the foils used were the increased values of strength, plasticity, homogeneity, and the absence of surface and internal defects. The combined use of the spectrometer with Thomson scattering diagnostics made it possible to carry out regular temperature measurements in the Globus-M2 tokamak with a high spatial and temporal resolution. The influence of impurities is estimated on the measurement of the electron temperature of the plasma. Keywords: foil spectrometry, tokamak, plasma, continuum, bremsstrahlung, electron temperature, soft-X-Ray, Si-photodiode.

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.

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.

Among the peripheral instabilities observed at the Globus-M2 tokamak, two types of edge localized modes (ELMs) are brought into focus: ELMs synchronized and desynchronized with the sawtooth oscillations. The desynchronized ELMs appear in regimes that are characterized by high values of pressure in the pedestal, p ped ≥ 3 kPa, and they are observed in discharges with the toroidal magnetic field B T > 0.6 T and plasma current I P > 0.3 MA. The desynchronized ELMs belong to the type-III/V with the dominating effect of the peeling mode. The synchronized ELMs were observed in a wider range of discharge parameters, including at B T < 0.6 T and I P < 0.3 MA. Calculations of the stability of the peeling-ballooning (PB) mode showed that at pedestal width ψ norm = 0.09 and p ped > 3.5 kPa, destabilization of PB modes is possible without additional influence. Experimental data shows that the microtearing mode plays a dominant role in the pedestal. The microtearing mode does not allow the pedestal at Globus-M2 tokamak to reach the state of the unstable kinetic ballooning mode (KBM), which explains the low predictive power of the EPED model at this tokamak.

A high-speed two-color pyrometer has been developed that allows measuring the intensity of thermal radiation in temperature range of 100-3500 0C with 2 µs time resolution. The analysis of the main factors affecting the measurement of surface temperature in the conditions of its interaction with plasma is carried out. It is shown that at plasma temperature of more than 10 eV and moderate concentration of particles, the wall plasma can be considered transparent to thermal radiation. Experimental studies on non-contact measurement of temperature of surface interacting with plasma on Globus-M2 tokamak and plasma gun test bench have been carried out. Sawtooth temperature fluctuations were detected on the wall of tokamak chamber near the exit of outer separatrix branch. The period of these flashes was 3-4 ms, and duration was 2 ms. The maximum wall temperature measured by a high-speed pyrometer exceeded the temperature measured by infrared camera. Measurements of temperature of the front surface of W-Li sample were carried out at plasma gun test bench under conditions of deuterium plasma jet cyclic action under loads simulating regimes of plasma current disruption in tokamak. The sample temperature reached more than 3000 0C during < 1 ms. The thermal conductivity of the sample degraded with an increase in number of plasma exposure cycles, which was expressed in progressive decrease in target cooling rate after the pulse. Using a high-speed pyrometer, it is proposed to control the formation of heat-insulating foils on surface interacting with plasma.

— The choice of a radiation high-speed non-contact method for measuring the surface temperature of an object under the influence of a hydrogen plasma jet is substantiated. A theoretical assessment was made of the contribution of the intensity of recombination and bremsstrahlung plasma radiation to the pyrometer readings. It is shown that under certain conditions hydrogen plasma can be considered transparent to the thermal radiation of the material in the mid-IR wavelength (MWIR) range of 3–5 μm. An infrared spectral ratio pyrometer based on an uncooled two-spectral MIR photodiode with a sandwich structure, designed to control the temperature of an object under study in an experimental setup of a hydrogen plasma gun, is considered. The experimental results of monitoring the surface temperature of some composite material samples in the process of pulsed plasma exposure are presented.

Cреди наблюдаемых на Глобус-М2 периферийных неустойчивостей выделяют краевые неустойчивости двух типов: синхронизированные и десинхронизированные с пилообразными колебаниями. Десинхронизированные срывы появляются в режимах, характеризующихся высокими значениями давления в пьедестале p ped > 3 кПа, и наблюдаются в разрядах с тороидальным магнитным полем B T > 0.6 Т и током по плазме I P > 0.3 МА. Десинхронизированные срывы краевой неустойчивости относятся к типу 3/5 с доминирующим влиянием пилинг-моды. Синхронизированные срывы наблюдались в более широком диапазоне параметров разряда, в том числе при B T < 0.6 Т и I P < 0.3 МА. Расчеты устойчивости пилинг-баллонной (ПБ) моды показали, что при ширине пьедестала ψ norm = = 0.09 и p ped > 3.5 кПа возможна дестабилизация ПБ-моды без дополнительного воздействия. Экспериментальные данные указывают на доминирующую роль микротиринговой неустойчивости в пьедестале. Микротиринговая мода не позволяет пьедесталу Глобуса-М2 достичь состояния неустойчивой кинетической баллонной моды, что объясняет низкую предиктивную силу модели EPED в токамаке Глобус-М2.

Absorbed power of the neutral-injection beam in spherical tokamaks Globus-M/M2 is estimated numerically. Deceleration of fast particles is simulated by means of the NUBEAM code. The signal of analyzer of charge-exchange atoms is simulated by means of the FIDASIM code using the distribution function of fast ions calculated by means of the NUBEAM code. Comparison of calculated and experimental signals allowed determining the degree of influence of instabilities on confinement of fast particles along with absorbed beam power.

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.

Experimental results are presented from studies of the plasma motion velocity in coaxial and rail accelerators. Dependences were obtained of velocity, kinetic energy, mass, and spectral composition of the jet on accelerator current. At high currents, it was found that the velocity decreases while kinetic energy of the jet increases and radiation intensity of impurities also increases compared to the radiation intensity of ionized component of working gas. Experimental results allowed us to claim that accelerated mass increased due to the erosion of accelerator wall and electrodes. This probably led to jet velocity limitation.

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.