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Abstract 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, pped ≥ 3 kPa, and they are observed in discharges with the toroidal magnetic field BT > 0.6 T and plasma current IP > 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 BT < 0.6 T and IP < 0.3 MA. Calculations of the stability of the peeling-ballooning (PB) mode showed that at pedestal width ψnorm = 0.09 and pped > 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.

Abstract 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) × 1019 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 × 1019 m−3, which is more than 2.5 times higher than the electron temperature.

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.

In this Letter, results of measurements of radiation losses Prad and effective ion charge Zeff in the conditions of magnetic field up to 0.7 T and plasma current up to 300 kA for a wide range of electron density first obtained on the Globus-M2 tokamak are presented. The analysis of the results shows the decrease in the radiation losses share of the input power on the Globus-M2 tokamak as compared with the same parameter for the Globus-M facility. A large dip at the center of the Prad power profiles is revealed. A decrease in the Zeff values is observed with an increase in electron density.

The Alfven oscillations were studied using the multifrequency Doppler backscattering (DBS) method on the Globus-M spherical tokamak during the neutral deuterium injection. The principles of Alfven mode registration by the DBS method are discussed. Microwave schemes of multifrequency probing are described. It is shown that the observed plasma rotation velocity oscillations are due to the oscillations of the drift velocity in crossed radial electric field of the Alfven wave and the magnetic field. The absolute values of the magnetic field perturbations of the toroidal Alfven eigenmode (TAE) in the internal regions of the tokamak discharge are determined. The obtained data are compared with the results of the magnetic field oscillations measurements obtained by a magnetic probe located outside the plasma region. The use of multifrequency probing in the DBS method made it possible to determine the radial distributions of the amplitude of magnetic field oscillations. These data were obtained for oscillations at the TAE fundamental frequency and at harmonic frequencies, which were found to correspond to different toroidal mode numbers. In general, the data obtained can be compared with the results of numerical modeling of the Alfven spectrum in the Globus-M tokamak. The prospects for the further use of the DBS method for studying Alfven oscillations in toroidal installations are discussed.

Globus-M2—a new 1-Tesla spherical tokamak—was recently launched. The main features and research directions of this machine in scope of fusion–fission reactor development are described. Main results of the first experimental campaign with toroidal magnetic field up to 0.73 T and plasma current up to 0.33 MA are discussed. Significant improvement of the discharge parameters as compared to Globus-M was achieved. Plasma total stored energy higher than 7 kJ was obtained. Energy confinement time increase was consistent with predictions by spherical tokamak scalings. Toroidal Alfven eigenmode-induced losses decrease with increase of plasma current and toroidal magnetic field. For the first time, LHCD with the toroidal wave slowing-down was successfully used at a spherical tokamak.

We developed high energy Q-switched 1064 nm and 946 nm Nd:YAG lasers for ITER Divertor Thomson Scattering (DTS). The 1064 nm Nd:YAG laser is designed to be used as the main DTS probing laser and features output energy of 2 J at 3 ns pulse duration. MOPA configuration with phase conjugation mirror provides divergence of 1.3 times the diffraction limit and high stability of the output beam parameters with repetition rate varied from 1 to 50 Hz. The 946 nm Nd:YAG laser designed for calibration by an auxiliary Thomson spectrum at the neighboring wavelength. The 946 nm laser consists of MO based on cavity dumping, high gain regenerative preamplifier and zero diffraction length ring cavity regenerative power amplifier with image-rotation providing 1 J × 50 Hz at 3 ns pulse duration with divergence of 1.5 the diffraction limit. As far as we know this is the highest value of the output energy obtained at 946 nm with nanosecond pulse duration until now. Both lasers use unified pump modules providing high maintainability of the system. The paper describes laser design approaches, state of the art and outlook of further developments.

Physical and engineering aspects of RF plasma cleaning developed for actively cooled first mirror serving as grounded DC-coupled electrode are under consideration. Feasibility of the water-cooling lines implemented as the coaxial notch filter for ITER in-vessel diagnostic mirrors was demonstrated for Capacitively Coupled Radio Frequency (CCRF) discharge in neon at 1–10 Pa. The impinging ions energy as a function of absorbed RF power was measured for both the DC-coupled and -decoupled schemes. Sputtering yield variation over rectangular surface of stainless steel mirror was studied experimentally. The measured sputtering yield profiles were found to be in qualitative agreement with the RF plasma numerical simulations. It was demonstrated that Al/Al2O3 used as Be/BeO proxy can be removed from the water cooled mirror surface by 100 eV neon ions with the use of the notch filter. The 14 h plasma exposure didn’t affect the mirror reflectivity.

Laser-induced fluorescence (LIF) on ITER will be used for local measurement of helium density nHe and ion temperature Ti in divertor. The LIF diagnostic is combined with the divertor Thomson scattering (DTS) via common laser injection and signal collection optics. Collisional-radiative model developed for helium ions is used to estimate laser parameters required for robust measurement of laser-induced signals against plasma background radiation. The estimations demonstrate feasibility of measuring nHeII with laser parameters provided by available optical parametric oscillator pumped by Nd:YAG. New approach for measuring Ti is proposed and the expected errors are estimated using numerical simulations.

Results of experimental studies of toroidal Alfven eigenmodes (TAEs) in the Globus-M spherical tokamak (R = 36 cm, a = 24 cm) are reported. The experiments were carried out in a wide range of plasma parameters at a magnetic field of up to 0.5 T and plasma current of up to 250 kA. Auxiliary plasma heating was performed by tangential injection of a deuterium beam with a power of Pb= 0.75 MW and particle energy of Eb = 28 keV into deuterium plasma. The experiments have shown that the TAE-induced loss of fast particles decreases with increasing plasma current and magnetic field. Using multifrequency Doppler backscattering diagnostics, it is established that the TAEs are localized at the plasma periphery. Results of simulations of the Alfven continuum and TAE structure by means of the modified KINX and CAXE codes agree satisfactory with the experimental data on the TAE frequencies and localization.

The results of experiments and simulations of the transport of heat and particles in the Globus-M spherical tokamak are presented. Investigations were carried out in the ohmic mode in hydrogen and deuterium plasma. It is shown that in the phase of current growth under the same initial conditions, as long as the condition is met at which the minimum of the safety factor exceeds unity, two scenarios of discharge development are possible: with an electron temperature dive or with an electron density peak. After the safety factor reaches the value of 1, the differences are no longer observed. Modeling of transport processes using the ASTRA code showed that the dive of the electron temperature and density is a consequence of the decrease in the transport of heat and particles in the plasma core (i.e., the presence of internal transport barriers).

The local measurements of electron density and temperature by Thomson scattering (TS) diagnostics are essential for research of energy confinement, plasma heating, and control at nuclear fusion devices. This paper presents a digital filter polychromator designed for TS spectrum registration. A distinguishing feature of such a digital polychromator is the use of an analog-to-digital converter (ADC) having analog memory with discretization frequency of 5 GHz and 12-bit sampling. The low heat dissipation and compact design of the ADC allow us to integrate the recording and data handling systems in the spectral unit to obtain a standalone device that is totally isolated from other systems and equipment in the voltaic sense. The scattered laser radiation received from the plasma comes to the entrance of the polychromator through a fiber-optic cable, and the processed scattering signals and calculated electron density and temperature are available on a digital interface. The polychromator was tested within the Thomson scattering system at the Globus-M tokamak using two probing lasers: Nd:YAG 1064 nm and Nd:Glass 1055 nm operating with pulse energy up to 2J and FWHM of 4 and 40 ns, respectively. The employed ADCs allowed us to perform superfast signal digitizing in the oscilloscope mode and to demonstrate the possibility of stray signal time separation despite the modest size (~1 m) of the vacuum chamber. The results of measurements performed during the plasma experiment at the Globus-M tokamak are presented, and their comparative analysis with the data obtained at the operating TS system is made.

ITER (International Thermonuclear Experimental Reactor) is a nuclear fusion research and engineering project. It is supposed to be the first fusion device to test the integrated technologies, materials and operational regimes necessary for the commercial production of fusion-based electricity. Front (explored in this paper) and Back diagnostic racks are used as supporting structures of the so-called ITER Divertor Thomson Scattering (DTS) Diagnostic System that is created to measure plasma characteristics. They will be attached to the lower port walls (ITER ports are used to locate diagnostic equipment and its supporting structures) through the so-called Divertor Rails. The developed design of the Front (Diagnostic) Rack locking (e.g. Rack attachment to the rails mechanism) was analyzed in this research to be consistent with thermal stresses, electromagnetic and seismic loads taking into account some specific assembly and maintenance loads including preloading under installation. The analysis of the rack locking design's resistance to the mentioned loads should prove correspondence of the suggested common rack design with the structure integrity criterions. Construction strength under the most severe loading scenario was estimated in this research and appropriate conclusions were made. It was found out that the stress criterion was not fully satisfied, so a new rack locking design concept was proposed. Its main details are also described at the end of this article.

Experimental study of thermal energy confinement in magnetic confinement devices is one of the fundamental problems in plasma physics. The data processing technique covering kinetic and magnetic measurements performed for the Globus-M tokamak is described. A zero-dimensional code has been developed on the basis of this approach making it possible to calculate important discharge parameters during the experiment (between discharges): the electron and ion stored thermal energy content, plasma effective charge, and confinement time. Good agreement of the zero-dimensional calculations and ASTRA modeling indicates that this approach can be applied for routine data processing in Globus-M in view of the specifics of the device.

A high-energy counter-NBI has been applied for the first time on the Globus-M spherical tokamak. An ELM-free H-mode has been obtained. However, no significant increase in the ion temperature and plasma energy content compared to ELMy H-mode has been observed. This is due to a high level of the fastion losses. Neither increase in the plasma current nor an increase in plasma-wall distance resulted in an increase in NB heating efficiency as it occurred during the co-NBI experiments.

Specific features of the energy distributions of fast ions during neutral beam injection at the Globus-М tokamak are considered. Different loss mechanisms that can lead to the formation of a specific shape of the energy spectrum of superthermal ions are analyzed. The effect of sawtooth oscillations on the loss of fast ions is discussed.

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.

ITER is a nuclear fusion research and engineering project. It is supposed to be the first fusion device designed for testing the integrated technologies, materials, and physical aspects necessary for development of the commercially available fusion-power plant. One of the important components of the project is optical diagnostic systems with collecting mirrors. These mirrors have to provide stability of optical systems under severe loads of different types that could possibly arise in the tokamak. The collecting mirrors of several ITER diagnostics have a large scale and should be installed into diagnostic ports. Thermal stress analysis of the mirror updated design is aimed to obtain deformation and rotation values of the mirrors’ reflecting surfaces in order to conduct ray tracing analysis and to edit mirrors alignment to provide correct functioning of the optical systems. The maximum temperature values of the Divertor Thomson Scattering collecting mirrors were estimated for the normal operation mode. The FE model of the second mirror takes into account all the force boundary conditions, basic kinematic boundary conditions and constraints. Boundary conditions taken for the simulation were applied on surfaces contacting with the diagnostic rack. Thermally stressed state was calculated and corresponding displacement and rotation distributions were obtained.

Radiation losses from the plasma of the Globus-M tokamak are studied by means of SPD silicon photodiodes developed at the Ioffe Institute, Russian Academy of Sciences. The results from measurements of radiation losses in regimes with ohmic and neutral beam injection heating of plasmas with different isotope compositions are presented. The dependence of the radiation loss power on the plasma current and plasma–wall distance is investigated. The radiation power in different spectral ranges is analyzed by means of an SPD spectrometric module. Results of measurements of radiation losses before and after tokamak vessel boronization are presented. The time evolution of the sensitivity of the SPD photodiode during its two-year exploitation in Globus-M is analyzed.

A diagnostic technique that is based on measuring the ratios of neutral-helium line strengths has been developed for peripheral distributions of electron temperature and density of tokamak plasma. The main components of the technique are a four-channel filter-lens imaging polychromator (FLIP-4) and a Phantom Miro M110 high-speed camera for recording the images. The polychromator has been assembled, adjusted, and tested on an optical test bench. The optical system was installed on the spherical Globus-M tokamak. Some preliminary experiments were carried out. Images of the plasma-gun jet were obtained at neutralhelium lines.

This note is devoted to a filter polychromator combining built-in optical, data acquisition and processing subsystems designed for electron temperature and density measurements in fusion plasma by Thomson scattering (TS). Special attention is paid to assessing the accuracy of TS signals measured in the polychromator spectral channels. The potential for estimating TS error caused by background emission is demonstrated using high-pass signal. The recent polychromator tests have shown APD gain stabilization better than ± 0.5% over the ambient temperature range from 17.5 °C to 35 °C

The paper describes the digital filter polychromator developed for Thomson scattering (TS) diagnostics. The device having ultra-low noise, high-speed detection system and up to 7 spectral channels is equipped with digitizer and single board computer for system control and data processing. The data acquisition system is capable to detect very short laser pulses of few ns width with low magnitude. This feature opens up new possibilities for the TS diagnostics design in modern fusion devices, which includes: operation with high background illumination, measurement of very low electron temperature and density at the plasma edge and scrape-off layer (SOL), separation of the TS signal and the stray light coming with a delay of several ns. The analysis of the TS signal waveform also gives a possibility to distinguish the TS and Mie scattering from the dust particles that is important for measurements in SOL. Owing to compactness and ultra-low energy consumption both optical and data processing units interfaced with optical Gigabit Ethernet were combined in a stand-alone device. Such all-in-one design with optical fiber input and optical digital output provides enhanced resistance to electromagnetic disturbances and simplify operation. A set of such polychromators can be mounted in a standard 19” rack and can be integrated with outer CODAC (Control, Data Access and Communication) system. Testing of the polychromators at the Globus-M2 tokamak has shown excellent performance of the developed serial devices.

Experimental data are presented that indicate the effect of the magnetohydrodynamic tearing mode in the Globus-M spherical tokamak on the occurrence of filamentary structures (filaments). The filaments are recorded by the Doppler backscattering method. The groups of filaments localized in the toroidal direction are discovered, the appearance of which is synchronized with the propagation of the tearing mode. Possible causes of the effect of low-frequency magnetohydrodynamic oscillations on the occurrence of filament groups are considered.

A new generation spherical tokamak Globus-M2 was launched in 2018. It keeps size of Globus-M, however new electromagnetic system allows achieving higher toroidal magnetic field and plasma current. First experimental campaign with the toroidal magnetic field 0.7 T and plasma current up to 300 kA was carried out in 2019. Increase in the toroidal magnetic field and plasma current resulted in overall improvement of the discharge parameters. Significant plasma temperature growth and neutron rate increase was obtained. Record stored energy, which is approximately one and a half times higher than in the Globus-M experiments, was reached. Toroidal lower hybrid wave launch provided noticeable current drive. Preparation for the next experimental campaigns with the higher toroidal magnetic field and plasma current are on the way.

We present the results of a comparative investigation of geodesic acoustic modes (GAMs) by the method of bicoherence analysis in tokamaks (Tuman-3M and Globus-M) with different aspect ratios. The obtained data provide indirect evidence of the mutual influence of GAMs and background turbulence of tokamak plasma. The cross-bicoherence spectra were calculated using the signals of Doppler reflectometers and temporal dependences of the velocity of plasma rotation in the electric field of GAMs. The level of bicoherence in both tokamaks increases before transition to the H-mode and then decays down to the noise level in the H-mode.

Neutral beam injection into the Globus-M spherical tokamak at the early stage of discharge leads to the development of instabilities in a frequency range of 50–200 kHz, which have been identified as toroidal Alfven eigenmodes (TAEs) [1]. The influence of these modes on the confinement of fast particles has been studied with the aid of a neutral particle analyzer (NPA) and a neutron detector. The isotope effect was studied using hydrogen and deuterium both in the injected beam and in the target plasma. A correlation analysis of signals from magnetic probes showed that the observed modes in most cases contain a single harmonic with toroidal number n = 1. Upon the injection of deuterium into deuterium plasma, the development of TAEs led to a decrease in the neutron flux by 25%, whereas the fluxes of high-energy recharge atoms decreased by 75%. After the injection of hydrogen, a decrease in the flux measured by NPA did not exceed 25%.

A complex of measures that utilize gas-dynamics effects for passive protection of optical elements, which are positioned deep in the diagnostic channels in the region of the ITER divertor, is considered. The proposed control methods are considered using an example of protection of optical elements for diagnostics of Thomson scattering at the ITER-tokamak divertor. To suppress the diffusive propagation of impurities, it is proposed to use the sorption properties of the walls of the diagnostic channels via an increase in their area. The presented experimental results are compared to the results of numerical calculations that confirm the efficiency of the proposed method. To suppress the convective propagation of impurities, it is proposed to use additional walls of the channel with a geometry that determines separation and deflection of flows and diaphragms for hydraulic deceleration of gas flows.

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.

Plasma parameters in the Globus-M spherical tokamak have been measured upon injection of a deuteron beam with energy about 30 keV and a power of up to 700 kW. Dependences of the ion temperature and neutron yield on the plasma current and density and the gap width between the wall and plasma are considered. Possible characteristics of fast-ion confinement in the modified Globus-M2 tokamak are discussed.

Multilayered prototypes of Al and Ag total reflection mirrors with a protective coating of amorphous ZrO2 oxide were analyzed using transmission electron (cross-sectional method), optical, and scanning (with electron probe microanalysis) microscopes. The prototypes’ reflectivity was measured by the recording of reflection spectra in the wavelength range 200–1100 nm. The structure of the mirrors was studied by X-ray diffraction reflection. High temperature annealing (8 h) did not change the stoichiometric composition of the protective coating. The reflectivity of the Al mirrors in the wavelength range 700–1100 nm and the Ag mirrors in the wavelength range of 500–1100 nm remained unchanged in spite of recrystallization of the polycrystalline metal films and the incipient process of amorphous ZrO2 crystallization. The mass of the crystalline oxide after annealing was several percent of the total amorphous oxide mass. At the same time annealing led to the occurrence of local voids on the aluminum/upper oxide interface. Such defects were not observed on the Ag mirrors. As a whole, the Ag mirrors turned out to be more stable to heating as compared to those made of Al.

The metal mirrors for deflecting laser radiation in the divertor Thomson scattering system of ITER are found to be inapplicable due to high density of laser radiation on a mirror. This is caused by the short distance from the laser focus to the mirror surface. We report on investigations of the possibility to use dielectric mirrors and protective sapphire plates in a rotary unit. We study the laser damage thresholds of optical elements, which are supposed to be used in the laser input channel. These optical elements are the dielectric mirrors deposited on different substrates (quartz, sapphire, and single-crystal molybdenum) and the protecting sapphire plate. A number of sapphire samples were irradiated by a neutron flux up to 1019 n/cm2 (E > 100 keV) and annealed. Laser damage thresholds of the elements were measured at room temperature and at the ITER operating temperature of 150°C.A YAG:Nd laser operating with 10-Hz repetition rate was used in the experiments. The laser pulse parameters ...

Calculations are made of the plasma spectral background, which is important for the Thomson scattering diagnostics in the ITER divertor. Theoretical grounds have been elaborated for computing the hydrogen spectral line shapes in the infrared spectral region for a divertor plasma in ITER. The shape of the P-7 Paschen line (transition n = 7 → n = 3) located near the laser scattering signal has been calculated for various lines of sight in the ITER divertor. Contributions from different mechanisms of broadening the P-7 line have been examined. The spectral intensities of bremsstrahlung and photorecombination continuum have been calculated. All calculations use data on the spatial distribution of temperatures and densities of all species of plasma particles computed with the SOLPS4.3 code for basic operation regimes of the ITER divertor.