To control the plasma shape during a tokamak discharge, it is necessary to calculate the plasma shape in real-time. The rate requirements for the shape calculations are especially high for tokamaks ...with a small radius, such as Globus-M2 (St. Petersburg, Russia). A real-time magnetic plasma control system for the Globus-M2 tokamak with flux and current distribution identification (FCDI) algorithm for the plasma equilibrium reconstruction in feedback is presented. The control system contains discrete one-dimensional and matrix proportional-integral-derivative controllers synthesized by the matrix inequality method using the plasma LPV model calculated on experimental data, and carries out the coordinated control of the plasma position and shape as well as the compensation for the scattered field of the central solenoid. The FCDI algorithm is improved for the operation in the real-time mode, and makes it possible to reconstruct the plasma shape in 20 µs. The digital control system with a feedback algorithm was simulated on a real-time test bench, consisting of two Speedgoat Performance Real-Time Target Machines (RTTM), and demonstrated the average Task Execution Time (TET) value in 67 µs.
At the compact spherical Globus-M2 tokamak, a series of experiments was conducted to study the effect of the injection of nitrogen on the discharge parameters. The experiments were carried out in ...discharges in deuterium in the divertor configuration, and the auxiliary heating was performed by deuterium neutral beam injection. During the nitrogen seeding, a substantial decrease in electron temperature near the divertor was recorded as well as a sharp decrease of the heat flux onto the divertor plate, while the density and temperature of the main plasma changed insignificantly. Simulations by the SOLPS-ITER showed a satisfactory agreement with the experiment.
The diagnostic of the peripheral plasma parameters in terms of the relation of lines of neutral helium is included in the diagnostic complex of the tokamak Globus-M2. The first measurements of the ...peripheral plasma parameters are performed near the lower X point. The measured spatial distributions of the electron temperature and density are in satisfactory agreement with the simulation with the SOLPS-ITER code.
It is shown that the radial profile of the electron temperature in the Globus-M2 spherical tokamak is related to the radial profile of the electron density by a power law
. An analytical model of the ...density attractor, assuming the dependence of
on the specific poloidal volume
v
, previously tested on the TCV and JET tokamaks with a large aspect ratio, holds on the spherical tokamak, and the approximation of 162 experimental profiles has resulted in
. The proposed model makes it possible to calculate the spatial distribution
and
according to a known magnetic configuration.
Data on plasma disruption processes in the modernized Globus-M2 spherical tokamak are presented. Electron temperature and density profiles before the disruption, immediately after thermal quench and ...in the stage of plasma current quench are measured using the diagnostics of Thomson scattering of laser radiation. The dependence of the plasma current decay time during disruption on the pre-disruption current value is determined. The distribution of the toroidal current, which is induced during disruption, in the shell of the vessel is determined on the basis of magnetic measurements. Electromagnetic loads on the vessel are calculated.
The diagnostic complex of the Globus-M2 spherical tokamak (
R
= 36 cm,
a
= 24 cm), the only operating tokamak in Russia with a divertor plasma configuration, which operates in the range of ...subthermonuclear temperatures (
T
e
to 1.6 keV,
T
i
to 4.5 keV) and densities (
n
e
to 2 × 10
20
m
–3
), is described. The Globus‑M2 tokamak is the unique scientific facility, which is a part of the Federal Center for Collective Use of the Ioffe Institute, Russian Academy of Sciences “Materials Science and Diagnostics in Advanced Technologies.” This allows third parties to perform their research using it. The work contains a list of all diagnostics currently available on the tokamak. The description of the diagnostics is structured in such a way that the reader gets an idea of their capabilities for measuring plasma parameters with an emphasis on the limits and accuracy of the measured values, and also spatial and time resolution. At the same time, many technical details are omitted in order to save space; references are given to papers with a more detailed description of individual diagnostics.
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.
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 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.
