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.

Doppler backscattering (DBS) was successfully previously used on the Globus-M tokamak. The diagnostic was utilised in the form of either a single-frequency or a four-frequency dual homodyne system. It was used primarily for the study of zonal flows, filaments and Alfve acute accent n eigenmodes. These phenomena are worth being studied both on the periphery and in the core region of the plasma in a tokamak. For this specific reason two multifrequency DBS systems were installed on the upgraded Globus-M2 tokamak. The first four-frequency system with dual homodyne detection had already been used on the Globus-M tokamak and has lower probing frequencies which provide measurements from the periphery plasma. The second and new six frequency DBS system was installed with a non-linear transmission line that was adapted to generate probing signals at frequencies 50, 55, 60, 65, 70 and 75 GHz. In general, the range of probing frequencies corresponds to the region of critical plasma densities from 5 x 10(18) to 7 x 10(19) m(-3) at normal incidence. The pyramidal horn antennas are located inside the vacuum vessel with a special cardan-like rotator outside the camera so as to tilt antennas in the toroidal and poloidal directions. A previously developed code was applied to simulate 3D raytracing for all frequency channels. Calculations were carried out for different angles of incidence and for different electron density distributions in order to investigate the possibilities of the implementation of radial and poloidal correlation Doppler reflectometry. Examples of the DBS system application for study of plasma properties in the Globus-M2 tokamak are presented.

Doppler backscattering (DBS) is a microwave diagnostics method typically used to study the plasma rotation velocity. Apart from conventional techniques, more advanced forms of DBS implementation were suggested on Globus-M. More specifically the study of a variety of oscillating processes was performed using DBS. In this review we present a detailed description of all of the methods and techniques employed in Globus-M alongside results obtained using DBS in all the years up until the shutdown of the tokamak. These include research similar to that done on other devices into the properties of such phenomena like geodesic acoustic modes or limit cycle oscillations, along with innovative works regarding the detection and investigation of Alfven eigenmodes and filaments that were the first of their kind and that provided important and novel results. Apart from that, the specific aspects of DBS application on a spherical tokamak are discussed. An in-depth look into the gradual change and improvement of the DBS diagnostics on Globus-M is also presented in this paper.

A new model for interaction between the internal reconnections caused by sawtooth and the edge-localized modes (ELM) was presented. The experimental evidence of the coupling between sawtooth crash and ELM events were observed in the Globus-M and Globus-M2 tokamaks. The numerical analysis of magnetic equilibrium showed that internal reconnections can induce the excess current density near the separatrix during the several hundreds of μs. The excess current destabilizes the peeling-ballooning (PB) instability. The PB stability analysis showed that the penetration depth of the induced current should be in the range of ψ norm = 0.8–0.95 to trigger the instability.

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.

New information on the development of Alfvén modes in the Globus-M2 spherical tokamak is presented. The data were obtained using a V-band Doppler reflectometer with probing radiation frequencies from 50 to 75 GHz. A microwave harmonic synthesizer was used as a multifrequency probing source. As a result of using a new reflectometer, the localization of the toroidal Alfvén eigenmode has been determined at a magnetic field of 0.5 T. The spectral components of the Alfvén mode with Doppler frequency shifts due to toroidal plasma rotation have been recorded. The so-called Alfvén cascades have been investigated in the central regions of the discharge.

This article considers a four-frequency microwave Doppler backscattering (DBS) system in the compact spherical tokamak Globus-M. The hardware was adequate for the purposes of studying the peripheral plasma in the tokamak. The multichannel DBS system is based on duplication of a dual homodyne detection circuit for four incident Ka-band frequencies. The ray tracing results for a spherical torus are described, and specific requirements for the antenna tilt adjustment are defined. Some new experimental results are given for using DBS diagnostics on the Globus-M tokamak in order to illustrate its efficiency.

The current work reports on the significant rise of the fusion triple product in experiments carried out on the compact spherical tokamak (ST) Globus-M2 with a twofold increase in the toroidal magnetic field. A tenfold rise in the n . T . τ E product was recorded during an increase in the magnetic field from 0.4 to 0.8 T and the plasma current from 0.25 to 0.4 MA at an unchanged auxiliary heating power value. Limited reasons may affect this positive trend, among which are energy confinement improvement and an increase in the efficiency of neutral beam heating. 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. It was experimentally confirmed that strong dependence of the energy confinement time on the magnetic field value is conserved at a higher magnetic field approaching 0.8 T. Enhancement of energy confinement is connected to a collisionality (ν *) decrease. While for conventional tokamaks the confinement dependence on collisionality becomes weaker with decreasing ν * dependence, in the ST, in contrast, we observe its strengthening.

Numerous theoretical and experimental studies have proved the important role of radial electric field inhomogeneity, or shear, in LH-transition initiation, and established heating power threshold for transition, although some experiments provide the observation of LH-transition dependency on particle source. It is necessary to apply a concerted approach to describe LH-transition initiation possibility and dynamics, considering E-r shear, particle source and turbulence properties as the main factors responsible for LH-transition initiation.

It is recognized that the filaments have a significant effect on the anomalous energy and particle transport in the tokamak periphery. They are actively investigated using various diagnostics in this regard. Recently studies of filaments using the Doppler backscattering method have been performed in the Globus-M and the ASDEX-Upgrade tokamaks. Backscattering from filaments manifests itself as a burst of quasi-coherent fluctuations of the signals of detectors. Such signals are easy to describe in the Born approximation using the diagnostic weighting function. However, the filaments in tokamaks differ noticeably in their size and intensity. With an increase in the amplitude of the filaments, it is necessary to consider the transition from linear scattering to nonlinear one and further up to the transition from backscattering to reflection from a moving filament. This problem can be solved only using a full wave code. Our simulation was carried out using the finite-difference time-domain code IPF-FD3D in slab geometry. We did not resort to using well-known non-linear MHD codes to determine filament parameters. In the simulation artificial filament-like perturbations were used, the parameters of which varied over a wide range. Modeling Doppler backscattering signal was focused on the identification of the influence of the amplitude of the filament and its size on the shape and the size of the Doppler backscattering output signal. The results obtained largely explain the similarity of the IQ detector data registered in different tokamaks.

On the spherical tokamak Globus-M2 in discharges with toroidal magnetic field B= 0.7 T by means of magnetic probes and a number of other diagnostics during neutral beam injection at the current ramp up stage, magnetic field oscillations in the range of 100-300 kHz having their frequency increasing in time, had been observed. Eigenmodes with wavenumbers $n = 1-3$ (toroidal number) and $m = 2-4$ (poloidal number) were being registered. These oscillations were identified as Alfven cascades. By means of multi-channel fluctuation reflectometer, observed eigenmodes were found to be localized near magnetic shear reversal radius. Application of MHD-spectroscopy technique allowed us to determine safety factor $q_{min}$ temporal evolution and experimental values are in well agreement with modelling results, provided by ASTRA transport code.

Here we report the results of the turbulence study in the high-confinement mode (H-mode) with and without edge localized modes (ELMs). The study was performed by the Doppler backscattering (DBS) method on the Globus-M tokamak. Two types of ELMs were observed in the Globus-M tokamak during NBI heating and accordingly, two different transitions to transient ELM-free modes were discovered. It was demonstrated that the transition from the H-mode with ELMs accompanied by small sawtooth oscillations to the ELM-free H-mode is characterized by the drop of the turbulence amplitude near the periphery while the transition in the case of ELMs is accompanied by large sawtooth oscillations, which occur without suppression of peripheral turbulence.

I.M. Balachenkov1,2, Yu.V. Petrov1, V.K. Gusev1, N.N. Bakharev1, V.V. Bulanin2, V.V. Dyachenko1, N.A. Khromov1, E.O. Kiselev1,2, A.N. Konovalov1, S.V. Krikunov1, G.S. Kurskiev1, V.B. Minaev1, M.I. Patrov1, A.V. Petrov2, A.M. Ponomarenko2, N.V. Sakharov1, P.B. Shchegolev1, A.Yu. Telnova1, V.A. Tokarev1, V.I. Varfolomeev1, A.Yu. Yashin2, N.S. Zhiltsov1,2 1 Ioffe Institute, 194021, St. Petersburg, Russia 2 Peter the Great St.Petersburg Polytechnic University, 195251, St. Petersburg, Russia e-mail: balachenkov@mail.ioffe.ru

The results of a study of toroidal Alfvén modes using the multifrequency method of Doppler backscattering at the Globus-M tokamak have been presented. The method for recording Alfvén modes in multichannel probing has been introduced. The possible causes of the observed oscillations of the poloidal velocity of plasma rotation at the frequencies of Alfvén waves have been discussed in detail. The data on the spatial distribution of Alfvén modes have been presented. The recommendations for further development of Doppler backscattering for a more detailed study of toroidal Alfvén modes at tokamaks have been defined.

A turbulence study was carried out on the Globus-M tokamak. The main method of turbulence studying was Doppler backscattering. Studies have been performed at different radii (in areas with different plasma parameters). It was found that during the transition from H-mode with edge localized modes (ELMs) to the improved confinement mode without ELMs, both turbulent fluctuations of the plasma density and velocity fluctuations are suppressed, which apparently leads to a decrease in anomalous transport.

Simulations of the microtearing instability developing in plasma of the Globus-M spherical tokamak were performed using the GENE gyrokinetic code in the flux-tube linear approximation mode. Under the effect of the instability, the magnetic islands form on the scale of the ion Larmor radius, and the magnetic field fluctuations occur that generate electron heat fluxes. The ion heat fluxes as well as the fluxes associated with the electrostatic fluctuations are negligible. The maximum growth rate of the microtearing instability is reached at a collision frequency within the experimental range of the collision frequency variation, within which the BT × τE ∝ ν*−0.4±0.1 scaling calculations were performed [1]. In similar calculations performed at the MAST tokamak, the growth rate decreases with decreasing collisionality in the entire range of the collision frequency variation, within which the BT × τE ∝ ν*−0.82 scaling calculations were performed [2]. This can explain why the dependences of energy confinement time on the collision frequency obtained for the MAST&NSTX and the Globus-M tokamaks are different.

The results of the study of toroidal Alfven modes (TAE) using the multi-frequency Doppler backscattering (DBS) in the Globus-M tokamak are presented. The article is focused on the presentation of the Alfven mode registration method for multichannel probing. The possible causes of the observed oscillations of the poloidal plasma rotation velocity at the Alfvén oscillation frequencies are discussed in detail. The data on the spatial distribution of Alfvén modes revealed by multi-frequency DBS are given. The recommendations for the further development of the DBS with the aim of a more detailed study of TAE in tokamaks were determined.