G. S. Kurskiev

Investigation of beam– and wave–plasma interactions in spherical tokamak Globus-M

The experimental and theoretical results obtained in the last two years on the interaction of neutral particle beams and high-frequency waves with a plasma using the spherical tokamak Globus-M are discussed. The experiments on the injection of low-energy proton beam of ~300u2009eV directed particle energy are performed with a plasma gun that produces a hydrogen plasma jet of density up to 3 × 1022u2009m−3 and a high velocity up to 250u2009kmu2009s−1. A moderate density rise (up to 30%) is achieved in the central plasma region without plasma disruption. Experiments on high-energy (up to 30u2009keV) neutral beam injection into the D-plasma are analysed. Modelling results on confinement of fast particles inside the plasma column that follows the neutral beam injection are discussed. The influence of the magnetic field on the fast particle losses is argued. A neutral beam injection regime with primary ion heating is obtained and discussed. The new regime with fast current ramp-up and early neutral beam injection shows electron temperature rise and formation of broad Te profiles until the q = 1 flux surface enters the plasma column. An energetic particle mode in the range of frequencies 5–30u2009kHz and toroidal Alfven eigenmodes in the range 50–300u2009kHz are recorded in that regime simultaneously with the Te rise. The energetic particle mode and toroidal Alfven eigenmodes behaviour are discussed. The toroidal Alfven eigenmode spectrum appears in Globus-M as a narrow band corresponding to n = 1. The first experimental results on plasma start-up and noninductive current drive generation are presented. The experiments are carried out with antennae providing mostly poloidal slowing down of waves with a frequency of 920u2009MHz, which is higher than a lower hybrid one existing under the experimental conditions. The high current drive efficiency is shown to be high (of about 0.25u2009Au2009W−1), and its mechanism is proposed. Some near future plans of the experiments are also discussed.

Study of fast-ion losses in experiments on neutral beam injection on the Globus-M spherical tokamak

The paper presents a review of the main results on the heating of plasma ions and behavior of fast ions in experiments on neutral beam injection (NBI) carried out in 2003–2010 on the Globus-M spherical tokamak. It is noticed that, along with significant success achieved in NBI plasma heating, there is experimental evidence indicating significant losses of the power injected into the plasma. Most probably, the power is lost due to so-called first-orbit losses, i.e., losses of fast ions that are produced in plasma after ionization of beam atoms and occur in unconfined trajectories. Until recently, the absence of appropriate diagnostic equipment did not allow one to verify this hypothesis. The use of the ACORD-M charge-exchange analyzer directed tangentially to the plasma column made it possible to measure the spectra of fast ions slowed down in plasma and confirm the assumption on the presence of substantial orbit losses of fast particles (∼25–50% of the beam power). In addition to the review of the experimental results, the paper presents analysis of orbit losses on the basis of 3D simulations of fast-ion trajectories in plasma. The results of experiments on studying the influence of the magnitude of the tokamak magnetic field on the confinement of fast ions are also presented. Along with computer simulations, these experiments made it possible to formulate recommendations on the reduction of orbit losses in the Globus-M tokamak.

Experimental study of toroidal Alfvén modes in the Globus-M spherical tokamak

In the experiments carried out on the Globus-M tokamak in regimes with injection of 26-keV neutral beams with a power of 0.75–0.85 MW, two branches of instabilities excited by fast ions were observed in the early stage of a discharge: a low-frequency energetic particle mode (EPM) in the frequency range of 5–30 kHz and a high-frequency mode in the range of 50–200 kHz, identified as a toroidal Alfvén eigenmode (TAE). The TAE developed in the initial phase of the discharge at q(0) > 1 and terminated when sawtooth oscillations were excited at q(0) < 1. The spectrum and spatial localization of the mode agree with predictions of the linear theory. The modes observed in the Globus-M tokamak possess both properties common to other tokamaks and their own specific features.

The influence of toroidal Alfvén modes on the confinement of fast particles in the Globus-M spherical tokamak

Neutral beam injection into the Globus-M spherical tokamak at the early stage of discharge leads to the development of instabilities in a frequency range of 50–200 kHz, which have been identified as toroidal Alfvén eigenmodes (TAEs) [1]. The influence of these modes on the confinement of fast particles has been studied with the aid of a neutral particle analyzer (NPA) and a neutron detector. The isotope effect was studied using hydrogen and deuterium both in the injected beam and in the target plasma. A correlation analysis of signals from magnetic probes showed that the observed modes in most cases contain a single harmonic with toroidal number n = 1. Upon the injection of deuterium into deuterium plasma, the development of TAEs led to a decrease in the neutron flux by 25%, whereas the fluxes of high-energy recharge atoms decreased by 75%. After the injection of hydrogen, a decrease in the flux measured by NPA did not exceed 25%.

Hydrogen lines in the infrared region and spectral background for the thomson scattering diagnostics of the iter divertor plasma

Calculations are made of the plasma spectral background, which is important for the Thomson scattering diagnostics in the ITER divertor. Theoretical grounds have been elaborated for computing the hydrogen spectral line shapes in the infrared spectral region for a divertor plasma in ITER. The shape of the P-7 Paschen line (transition n = 7 → n = 3) located near the laser scattering signal has been calculated for various lines of sight in the ITER divertor. Contributions from different mechanisms of broadening the P-7 line have been examined. The spectral intensities of bremsstrahlung and photorecombination continuum have been calculated. All calculations use data on the spatial distribution of temperatures and densities of all species of plasma particles computed with the SOLPS4.3 code for basic operation regimes of the ITER divertor.

Observation of filaments on the globus-M tokamak by Doppler reflectometry

Experimental results showing evidence for the development of filaments in plasma of the Globus-M spherical tokamak have been obtained by the method of Doppler reflectometry. The correlation between the reflectometric data and results of probe measurements at the periphery of discharge in the tokamak is found. Experimental data were used to determine the rotation velocities of filaments, the moments of their appearance at the transition to improved confinement, and regions of filament formation.

Thomson scattering diagnostics in the Globus-M tokamak

Specific features of the Thomson scattering diagnostics, its main characteristics and capabilities, and the results of its experimental testing in the Globus-M tokamak are described. A powerful multipulse neodymium-glass laser is designed for investigating both fast and slow processes in the tokamak plasma. The laser is capable of generating up to 20 pulses uniformly distributed in time during one tokamak discharge. In order to investigate fast transient processes, the laser repetition rate can be increased within a specified time interval. The possibility of varying the time interval between laser pulses from 0.5 ms to 1.0 s makes this diagnostics highly informative. The optical scheme developed in the course of these studies allowed one to simplify the power supply system and create a comparatively inexpensive laser system. The use of avalanche photodiodes and filtering polychromators with a high optical transparency provides high sensitivity of the diagnostics. A special software was designed that allows automatic processing of several hundred signals during one shot and provides data on the electron density and temperature immediately in the course of measurements. The diagnostics allows one to trace the time behavior of the spatial profiles of the electron temperature and density in both ohmic discharges and discharges with auxiliary heating, as well as in experiments with particle injection with a plasma gun.

Characteristics of major plasma discharge disruption in the Globus-M spherical tokamak

The characteristics of the major disruption of plasma discharges in the Globus-M spherical tokamak are analyzed. The process of current quench is accompanied by the loss of the vertical stability of the plasma column. The plasma boundary during the disruption is reconstructed using the algorithm of movable filaments. The plasma current decay is preceded by thermal quench, during which the profiles of the temperature and electron density were measured. The data on the time of disruption, the plasma current quench rate, and the toroidal current induced in the tokamak vessel are compared for hydrogen and deuterium plasmas. It is shown that the disruption characteristics depend weakly on the ion mass and the current induced in the vessel increases with the disruption time. The decay rate of the plasma toroidal magnetic flux during the disruption is determined using diamagnetic measurements. Such a decay is a source of the poloidal current induced in the vessel; it may also cause poloidal halo currents.

Investigation of the plasma radiation power in the Globus-M tokamak by means of SPD silicon photodiodes

Radiation losses from the plasma of the Globus-M tokamak are studied by means of SPD silicon photodiodes developed at the Ioffe Institute, Russian Academy of Sciences. The results from measurements of radiation losses in regimes with ohmic and neutral beam injection heating of plasmas with different isotope compositions are presented. The dependence of the radiation loss power on the plasma current and plasma–wall distance is investigated. The radiation power in different spectral ranges is analyzed by means of an SPD spectrometric module. Results of measurements of radiation losses before and after tokamak vessel boronization are presented. The time evolution of the sensitivity of the SPD photodiode during its two-year exploitation in Globus-M is analyzed.

Formation of internal transport barriers in Globus-M tokamak in regime with early neutral heating beam switch-on

A series of experiments have been performed on the Globus-M spherical tokamak under conditions with additional plasma heating by a beam of neutral atoms at the stage of plasma current buildup. This regime allowed a region with negative magnetic shear to be formed in the inner plasma region adjacent to the magnetic axis. Under these conditions, the stability margin coefficient at the axis was greater than unity. This scenario of plasma heating leads to the transition to a regime of improved confinement with the formation of a broad temperature profile and peaked electron density profile due to the formation on internal barriers. Results of experiments and numerical simulations of transport processes using the ASTRA code are presented.