Vol 49, No 12 (2023)

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 (Te to 1.6 keV, Ti to 4.5 keV) and densities (ne to 2 × 1020 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.

The thermal energy stored in plasma Wp, normalized internal plasma inductance li and current
beta bi are calculated via the free-boundary equilibrium PET code. The equilibrium reconstruction algorithm
is iterative method of minimizing two parameters, the distance between the reconstructed plasma
boundary and that simulated by the PET code, as well as the difference between the plasma diamagnetic flux
from PET and the experimental one. The discharges from the Globus-M2 tokamak with a toroidal magnetic
field up to 0.9 T and a plasma current of 0.3–0.4 MA in a mode with auxiliary heating by two atomic injectors
are analyzed. The possibility of using approximate formulas for estimating and is considered. The measured
diamagnetic flux is used to determine bdia  and then to calculate Wdia . The normalized internal plasma
inductance is additionally determined from the measured vertical magnetic field under the assumption that bp=bdia.

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.

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.

The concept of next-generation spherical tokamak is being considered: the Globus-3 project,
which, in its characteristics, is compatible with the infrastructure existing at the Ioffe Institute, but differs
from the currently operating Globus-M2 tokamak in the stronger toroidal magnetic field (1.5–3.0 T) and
increased duration of plasma discharge. The parametric analysis data are presented that determined the preliminary
selection of the facility parameters. Three options for the electromagnetic system were considered:
with the warm copper coils, with the pre-cooled copper coils and with the coils made of high-temperature
superconductors. For the first option, the concept for designing the electromagnetic system and vacuum vessel
of the facility has been developed. The basic shot scenario with duration of up to 3 s at the field of 1.5 T
and plasma current of 0.8 MA is presented.