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.
The high-confinement mode in tokamaks (H-mode) is characterized by high pressure gradients at plasma edge, which results in the appearance of edge localized modes (ELMs). They are studied at the ...Globus-M2 spherical tokamak too, where edge localized modes are observed mainly in regimes with neutral beam injection. One of the ways for studying ELMs is the use of the Doppler backscattering (DBS) diagnostics installed at Globus-M2. It makes possible to estimate the amplitude of plasma density fluctuations and measure the radial electric field
E
r
. In this work, the effect of edge localized modes on the
E
r
field is studied in the radial range 0.4 < ρ < 1.1. It is shown that during ELMs the electric field increases in the entire measurement range. This indicates that ELMs affect the inner plasma regions as well. This is not consistent with the general ideas concerning the peripheral localization of ELMs, but is confirmed experimentally not only at Globus-M2. In addition, the results for the regime with ELMs are compared with those for the regime with limit cycle oscillations (LCOs) and it is shown that during LCOs such effect is not observed.
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 results of geodesic acoustic mode (GAM) studies in the spherical torus Globus-M via Doppler reflectometry are presented. The intermittent character of the GAM evolution is similar to the ...limit-cycle oscillation behavior of zonal flows. The evident correlation between the GAM rotational velocity and both Dα emission and plasma density oscillations is exhibited and discussed. The obtained experimental results are compared with those from tokamaks with large aspect ratios.
The temperature control system used for monitoring the thermal regime of operation of the electromagnetic system of the Globus-M2 tokamak is described. The system is a hardware-software complex built ...on the Arduino platform based on microcontrollers of the AVR family. The developed graphical interface continuously reflects the temperature at the controlled points and signals when it goes beyond the specified range. Automatic saving of measurement results by controlled events is provided.
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.
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,
p
ped
≥ 3 kPa, and they are observed in discharges with the toroidal magnetic field
B
T
> 0.6 T and plasma current
I
P
> 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
B
T
< 0.6 T and
I
P
< 0.3 MA. Calculations of the stability of the peeling-ballooning (PB) mode showed that at pedestal width ψ
norm
= 0.09 and
p
ped
> 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.
The thermal energy stored in plasma
, normalized internal plasma inductance
and current beta
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
and then to calculate
. The normalized internal plasma inductance
is additionally determined from the measured vertical magnetic field under the assumption that
.
The injection complex that heats plasma of Globus-M2 spherical tokamak consists of two injectors which supply high-energy beams of hydrogen (deuterium) atoms into the device’s plasma. Injectors are ...autonomous and allow for preparing and setting up the device and measuring the parameters of the atomic beam independently of the other injector and of the tokamak. The authors provide detailed analysis of each injector’s configuration, substantiate their selection of the experiment layout for introducing beams into the tokamak plasma, set forth characteristics of both injectors’ atomic beams injected into the tokamak plasma, and discuss the results of experiments on additional heating of Globus-M2 tokamak plasma obtained by injecting two atomic beams.
The results of experiments are described, which were performed at the Globus-M2 tokamak and aimed at studying the fast ion loss at the outer tokamak wall due to fast ions interaction with the ...toroidal Alfvén eigenmodes. The local heating of carbon tiles was experimentally measured, and the corresponding heat flux was calculated. It was shown how simulations of the lost particle orbits can explain the characteristic features of the spatial map of wall heating. The flux of lost fast particles onto the wall was studied as a function of the instability amplitude. It has been demonstrated that the simulations predict similar dependence of the fast ion flux on the instability amplitude and also correlate its nature to the peculiarities of fast ions spatial distribution.
