S. V. Krikunov

H-mode studies on TUMAN-3 and TUMAN-3M

The focus of the TUMAN-3 and TUMAN-3M tokamaks programme is on issues of improved confinement. The transition from an ordinary ohmic regime into improved confinement mode has been found in circular limiter configuration in a vessel with all-metallic walls and limiters. The signatures of the H-mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the radial electric field was found in experiments with internal probe biasing. Other techniques were demonstrated to trigger H-mode: short increase of the working gas puffing rate, minor radius magnetic compression and pellet injection. The scaling of the energy confinement time in ohmic H-mode was obtained, which differs dramatically from the scaling for the ordinary ohmic regime. A strong dependence of on plasma current was found. The scaling for the ohmic H-mode is consistent with the scaling proposed for devices with powerful auxiliary heating (JET/DIII-D H-mode scaling). The result shows that H-mode physics is universal in tokamaks with different geometries and heating methods. In 1994 a new vacuum vessel was installed in the TUMAN-3 tokamak. The modified device, TUMAN-3M, is able to produce higher and , up to 2 T and 0.2 MA, respectively. During the first operational period a plasma current of 0.15 MA was achieved at T, which corresponded to . The impact of the quality of wall coating on confinement was asserted. The longest energy confinement time (30 ms) was observed under the conditions of best boronization.

Evolution of geodesic acoustic mode in ohmic H-mode in TUMAN-3M tokamak

The behavior of a geodesic acoustic mode (GAM) in the TUMAN-3M tokamak has been experimentally studied using the heavy-ion beam probing technique. Oscillations of the electric potential under the action of a GAM localized at the plasma periphery have been detected. The GAM was observed in the regime of low confinement (L-mode) with low plasma density (∼0.8 × 1019 m−3) and disappeared upon the transition to a high confinement regime (H-mode). The possible role of GAM as a precursor of the improved confinement (LH-transition) is discussed.

Confinement of energetic ions in a tokamak plasma at magnetic field in the range of 0.7–1.0 T

We have experimentally studied the influence of toroidal magnetic field (BT) and plasma current (Ip) on the capture and confinement of energetic ions (EIs) formed upon ionization of a neutral beam injected in a tokamak. Based on the results of measurements of the flux of 2.45-MeV fusion neutrons, it is concluded that the amount of EIs significantly grows with increasing BT from 0.7 to 1.0 T and Ip from 140 to 180 kA. In addition to the classical Coulomb slowing down, a supplementary channel of EI losses is found that accounts for a 15% decrease in their confinement time.

Studies of runaway electrons in the Globus-M tokamak

Results are presented from experimental studies of runaway electrons in the ohmic heating regime in the Globus-M tokamak. The periodical hard X-ray bursts observed with the help of two hard X-ray spectrometers with high time resolution are attributed to MHD oscillations in the plasma core and at the periphery.

The influence of plasma horizontal position on the neutron rate and flux of neutral atoms in injection heating experiment on the TUMAN-3M tokamak

Horizontal displacement of plasma along the major radius has been found to significantly influence the fluxes of 2.45 MeV DD neutrons and high-energy charge-exchange atoms from neutral beam injection (NBI) heated plasma of the TUMAN-3M tokamak. An inward shift by ΔR = 1 cm causes 1.2-fold increase in the neutron flux and 1.9-fold increase in the charge-exchange atom flux. The observed increase in the neutron flux is attributed to joint action of several factors-in particular, improved high-energy ion capture and confinement and, probably, decreased impurity inflow from the walls, which leads to an increase in the density of target ions. A considerable increase in the flux of charge-exchange neutrals in inward-shifted plasma is due to the increased number of captured high-energy ions and, to some extent, the increased density of the neutral target. As a result of the increase in the content of high-energy ions, the central ion temperature Ti(0) increased from 250 to 350 eV. The dependence of the neutron rate on major radius R0 should be taken into account when designing compact tokamak-based neutron sources.

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.

Electron Density Modulation in Magnetic Islands in the TUMAN-3M Tokamak

MHD oscillations with m/n = 4/1 and 3/1 that arise at the periphery of the TUMAN-3M tokamak in the initial stage of a discharge are investigated. It is found that these oscillations lead to a significant modulation of the electron density ne, which is attributable to the accumulation of plasma within a magnetic island. Numerical simulations of the modulation structure made it possible to determine the radius of the resonant surface and the radial width of the island and to evaluate the characteristic density gradient in the island. The gradient was found to be ten times larger than that of the unperturbed profile of ne(r) near the resonant surface. This points to reduced plasma transport within the magnetic island.

Features of the microwave radiation under the fan instability development and accelerated electron interaction with magnetic ripples in the FT-2 tokamak ohmically heated plasma

Results of observation of anomalously intense microwave radiation (MR) in the electron plasma frequency range of 10–20 GHz that is generated continuously during ohmic heating of moderately dense plasma in the FT-2 tokamak are presented. Its appearance is associated with development of the “fan” plasma instability, and the presence of a large number of the local ripples of the toroidal magnetic field. It was found that MR is accompanied by the short gigantic and less intense flashes in the range of magnetic broadening of the first harmonic of the electron gyrofrequency (57–75 GHz). As is known, the latter occurs upon maser amplification of the synchrotron radiation of accelerated electrons interacting with the harmonics of local magnetic ripples in an autoresonance cyclotron mode. In our conditions, due to the nonlinear transformation of plasma waves into electromagnetic ones under the buildup of the fan instability, collective radiation (CR) is generated. Therefore, an abnormally high MR-intensity is probably due to CR maser amplification, whereas gigantic flashes occur during the self-excitation of maser amplifier under suitable conditions.

Features of accelerated electron beam formation in LHCD experiments on FT-2 tokamak

In experiments with lower hybrid current drive (LHCD) on the FT-2 tokamak, lower hybrid (LH) waves have been successfully used for the first time to ensure effective additional heating of plasma electrons from 450 to 600 eV (IPl = 32 kA, ΔtRF = 14 ms, PRF = 100 kW, F = 920 MHz). Several factors influencing the efficiency of plasma heating have been discovered. In particular, significant growth of radiation losses in the LHCD regime has been found, which is probably related to an increase in the intensity of synchrotron radiation from accelerated electrons. The increase in this intensity in the 53–156 GHz frequency range was accompanied by short spikes of microwave radiation, which were observed only in a narrower frequency range (53–78 GHz) and apparently resulted from interaction of a runaway electron beam with significant local mirrors of toroidal magnetic field. A model of the additional heating of plasma electrons due to absorption of the microwave radiation generated by a beam of accelerated electrons is proposed.

The globus-m diagnostics design

Abstract Diagnostics which is currently installed or under active development for a newly commissioned spherical tokamak (ST) Globus-M is described. Among the commonly implemented plasma diagnostics the priority is given to those of the enhanced locality and repetition rate response. Technical summaries of each diagnostics are given with special emphasis on innovative approaches.