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.
With an increase of magnetic field up to 0.8 T and plasma current to 400 kA, fast ion losses rate in the discharges with toroidal Alfven eigenmodes decreased in tokamak Globus-M2 comparing with ...Globus-M tokamak discharges. Taking into account the data on the discharges with increased magnetic field and plasma current, the regression fit of neutral particle analyzer flux drop in energy channel close to neutral beam energy on relative eigenmode magnitude, the value of magnetic field and plasma current was analyzed. The power of flux drop dependence on TAE magnitude was found to be ~0.5 and inverse proportional on the value of product of magnetic field and plasma current, which is highly likely is determined only by plasma current due to weak dependence on magnetic field. The result obtained indicates that fast ion losses in Globus-M2, stimulated by toroidal Alfven eigenmodes are mostly determined by the shift of passing orbits to the plasma edge. With the increase of plasma current and magnetic field, neutron flux drops arising in the moments of toroidal mode bursts have also decreased.
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.
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.
An important part of high-temperature plasma study is the determination of the electron temperature dynamics in the tokamak plasma. At spherical tokamaks, one can use Thomson scattering diagnostics ...as well as soft X-ray emission diagnostics (SXR). The capabilities of electron temperature measurement by the first diagnostics are limited by the repetition rate of laser pulses and their number in one tokamak discharge. Data of the second diagnostics are continuous in time and are determined by the time resolution of the detectors; however, obtaining the electron temperature using these data encounters a number of difficulties considered in this study. A method of combined processing of results of these diagnostics using machine learning algorithms was developed for overcoming these difficulties and applying the adVoprosy Atomnoi Nauki i Tekhniki, Seriya: Termoyadernyi Sintezages of both diagnostics. Training data include soft X-ray diagnostic data, hard X-ray diagnostic data, and CIII line emissivity diagnostic data. Thomson local scattering measurements were used as labels for supervised machine learning. The developed technique provides significant extension of the possibilities of determining the electron temperature at the Globus-M2 tokamak.
The Alfvén oscillations were studied using the multifrequency Doppler backscattering (DBS) method on the Globus-M spherical tokamak during the neutral deuterium injection. The principles of Alfvén ...mode registration by the DBS method are discussed. Microwave schemes of multifrequency probing are described. It is shown that the observed plasma rotation velocity oscillations are due to the oscillations of the drift velocity in crossed radial electric field of the Alfvén wave and the magnetic field. The absolute values of the magnetic field perturbations of the toroidal Alfvén eigenmode (TAE) in the internal regions of the tokamak discharge are determined. The obtained data are compared with the results of the magnetic field oscillations measurements obtained by a magnetic probe located outside the plasma region. The use of multifrequency probing in the DBS method made it possible to determine the radial distributions of the amplitude of magnetic field oscillations. These data were obtained for oscillations at the TAE fundamental frequency and at harmonic frequencies, which were found to correspond to different toroidal mode numbers. In general, the data obtained can be compared with the results of numerical modeling of the Alfvén spectrum in the Globus-M tokamak. The prospects for the further use of the DBS method for studying Alfvén oscillations in toroidal installations are discussed.
First Globus-M2 Results Bakharev, N. N.; Balachenkov, I. M.; Chernyshev, F. V. ...
Plasma physics reports,
07/2020, Volume:
46, Issue:
7
Journal Article
Peer reviewed
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 Alfvén 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 paper presents the first ion temperature profile measurements results in the new spherical tokamak Globus-M2, which were obtained in discharges with neutral beam injection using the Charge ...eXchange Recombination Spectroscopy (CXRS) and the Neutral Particle Analyzer (NPA). Ion heat transport modeling using ASTRA code was carried out based on the obtained experimental ion temperature. The results of the modelling indicate the predominantly neoclassical ion heat transport.
We present new information on the development of Alfvén modes on the Globus-M2 spherical tokamak. Data have been obtained using a V-band Doppler reflectometer with probing radiation frequencies from ...50 to 75 GHz. Аs a multifrequency source of probing radiation, a microwave harmonic synthesizer has been used. As a result of using the new reflectometer, the toroidal Alfvén mode localization in a magnetic field of 0.5 T has been obtained. The spectral components of the Alfvén mode with Doppler frequency shifts caused by toroidal plasma rotation have been registered. In the central regions of the discharge, so-called “Alfvén cascades” have been investigated.
Results of experimental studies of toroidal Alfvén eigenmodes (TAEs) in the Globus-M spherical tokamak (
R
= 36 cm,
a
= 24 cm) are reported. The experiments were carried out in a wide range of plasma ...parameters at a magnetic field of up to 0.5 T and plasma current of up to 250 kA. Auxiliary plasma heating was performed by tangential injection of a deuterium beam with a power of
P
b
= 0.75 MW and particle energy of
E
b
= 28 keV into deuterium plasma. The experiments have shown that the TAE-induced loss of fast particles decreases with increasing plasma current and magnetic field. Using multifrequency Doppler backscattering diagnostics, it is established that the TAEs are localized at the plasma periphery. Results of simulations of the Alfvén continuum and TAE structure by means of the modified KINX and CAXE codes agree satisfactory with the experimental data on the TAE frequencies and localization.
