An array of ten built-in divertor probes was installed on the lower dome of the Globus-M spherical tokamak. It was used to measure the profiles of the floating potential, electron temperature, ion ...saturation current, and heat flux density to the divertor. It is demonstrated that there is a sharp change in the floating potentials of the probes located near the outer separatrix leg at the development of the peripheral instability.
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•Hierarchical robust switching control method with state vector matching is proposed.•Non-iterative Improved Moving Filaments method for plasma equilibrium reconstruction.•Plasma ...shape is controlled via Magnetic field at X-point and poloidal fluxes on the plasma separatrix.•Combination of advanced reconstruction code with isoflux control to achieve high performance.•New feedback control system simulation principle with LPV plasma model and reconstruction code.
The paper describes a new hierarchical tokamak plasma shape control method implemented in the system with switching robust controllers and matching state vectors. A system robustness was achieved via the H∞ loop-shaping design. A novel modeling approach was used for the feedback system simulation: tokamak plasma equilibrium was calculated by the reconstruction code receiving the sum of experimental scenario signals and the output signals of a linear parameter varying (LPV) model, and sending the output to the multivariable plasma shape controller to close the loop. The LPV plant model is based on plasma equilibria reconstructed from the experimental data of the spherical tokamak Globus-M and is used to simulate plasma responses to small deviations from tokamak discharge scenario. The control system components were designed and combined to achieve high-performance operation during short pulses of plasma with total duration of about 200 ms and the divertor phase time length of 20–25 ms. The non-iterative Improved Moving Filaments method was developed for the plasma equilibrium reconstruction with the simultaneous determination of coordinates and currents of filaments. The plasma shape is controlled via the magnetic field at X-point and poloidal fluxes on a plasma separatrix, without calculation of the gaps between the plasma separatrix and the first wall. The hierarchical feedback control method was used in simulations of the entire plasma discharge with the transition from the limiter to the divertor plasma magnetic configuration. The control system was discretized and simulated on a high-speed computer to be prepared for application in Globus-M experiments by a real time test bed.
Ion temperature is one of the most important parameters of high-temperature plasma. Information on the ion temperature spatial distribution is necessary for understanding and modelling of particle ...and energy transport processes, evaluating the effectiveness of plasma heating system operation. Active spectroscopy also known as CXRS (Charge eXchange Recombination Spectroscopy) is a powerful diagnostic tool for measurements of local values of ion temperature, and is widely used in experiments with magnetic confinement of high-temperature plasma. Active spectroscopy diagnostics on the Globus-M tokamak utilizes a tangentially injected heating beam of neutrals (hydrogen or deuterium atoms). The CXRS-system setup is described. The results of ion temperature measurements for the last Globus-M experimental campaign before the machine shutdown and further upgrading to higher values of magnetic field and plasma current are presented. Ion temperature significant rise in the experiments with moderate increasing of toroidal magnetic field and plasma current is detected and discussed.
Following paper represents a method to resolve spatial mode structure of plasma MHD instabilities employing cross-phase correlation analysis applied to signals of magnetic probes, mounted on Globus-M ...and Tuman-3M tokamaks. The data on observations of toroidal Alfven eigenmodes (TAEs), which were formerly identified in Globus-M tokamak discharges, has been analyzed. Possible interpretation of the phase plot, acquired from analysis of NBI-stimulated ion-cyclotron emission (ICE) on Tuman-3M is given as well. The method described below turns out to be useful instrument for resolution of spatial structure of plasma instabilities, especially in D-shaped tokamaks with low aspect ratio.
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.
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.
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.
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.
On the spherical tokamak Globus-M2 in discharges with toroidal magnetic field of 0.7 T by means of magnetic probes and a number of other diagnostics during neutral beam injection at the current ramp ...up stage, magnetic field oscillations in the range of 100–300 kHz having their frequency increasing in time, had been observed. Eigenmodes with wavenumbers
n
= 1–3 and
m
= 2–4 were being registered. These oscillations were identified as Alfvén cascades. By means of multi-channel fluctuation reflectometer, observed eigenmodes were found to be localized near the magnetic shear reversal radius. Application of MHD-spectroscopy technique allowed us to determine safety factor temporal evolution and experimental values are in well agreement with modelling results, provided by ASTRA transport code.
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.
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