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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.

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.

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 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) × 1019 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.

Active NPA measurements of the fast ion distribution using the neutral beam as an additional charge exchange target are discussed. Expressions for the calculation of the NPA signal based on the fast ion distribution and for the reconstruction of the fast ion distribution from the NPA signal are provided. Demonstration of the described approach is carried out on the Globus-M2 spherical tokamak, where a scanning system for the NPAs was recently installed. Main features of the active NPA application on Globus-M2 are considered. The energy and spatial distributions of fast deuterium ions at dedicated pitch angles are obtained and compared with the calculated ones. Key traits of the obtained distributions are considered and explained.

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.

Active ion temperature measurement in the core plasma of a tokamak using active neutral particle analyzers (NPA) diagnostics is considered. Two approaches for local ion temperature reconstruction are reviewed: semi-analytical and iterative. A semi-analytical approach allows the calculation of ion temperature using simple expression, while a more complex iterative approach provides more flexibility. Problems associated with active NPA measurements and their possible solutions are discussed. An example of the core ion temperature profile reconstruction in the compact spherical tokamak Globus-M2 using both approaches is shown. Problems associated with active ion temperature measurements in a compact reactor and possible solutions 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.

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.