V. S. Romanov

RF plasma production and heating below ion-cyclotron frequencies in Uragan torsatrons

In the IPP-Kharkiv there are two torsatrons (stellarators) in operation, and in both of them Alfven resonance heating under high-k∥ conditions is used. This method of heating is advantageous for small-size devices, since in contrast to the minority and second-harmonic heating it can be realized at lower plasma densities. A series of experiments has been performed at the Uragan-3M torsatron with an aim to investigate the features of the discharge with a three-half-turn antenna. Electron temperatures in the range are achieved at plasma densities . The plasma energy content has increased by a factor of 2 with respect to the plasma produced with the frame antenna. A new four-strap shielded antenna has been manufactured and installed in the Uragan-2M. A high-frequency discharge for wall conditioning is introduced in the Uragan-2M torsatron. The discharge is sustained by a specially designed small frame antenna, and efficient hydrogen dissociation is achieved. A self-consistent model has been developed for simulation of plasma production in ICRF. The model includes a set of particle and energy-balance equations for the electrons, and the boundary problem for the Maxwell equations. The first calculation results on RF plasma production in the Uragan-2M stellarator with the frame-type antenna are presented.

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.

Spectral and statistical analysis of fluctuations in the SOL and diverted plasmas of the Uragan-3M torsatron

In the l = 3/m = 9 Uragan-3M (U3-M) torsatron (R0 = 1 m, ā ≈ 0.12 m, ι(ā)/2π ∼ 0.3) with an open helical divertor and a plasma produced and heated by RF fields (ω ≲ ωci), studies of low frequency (5–100 kHz) density and potential fluctuations in the SOL plasma and in the diverted plasma flows (DPFs), have been carried out. It is shown, that in the SOL to more (less) distantly located points relative to the last closed magnetic surface, higher (lower) frequency fluctuations are inherent. Such a spectral splitting in two sub-ranges occurs in the DPFs too. A peculiarity of the spatial distribution of DPF fluctuation spectra is that lower frequency fluctuations dominate on the ion toroidal B × ∇B drift side. During L-H-like transition in U-3M simultaneously with strong Er shear formation, a suppression of lower frequency fluctuations and a decrease of local radial turbulent particle flux take place. Results are presented of investigation of plasma density fluctuations in SOL with the use of probability distribution function (PDF) analysis. Evaluations of skewness and kurtosis of fluctuations have been made. The analysis of Is fluctuations in DPF have been carried out in a similar way.

Studies of fast ion outflow to the helical divertor of the U-3M torsatron

In the l=3 Uragan–3M torsatron with RF-produced and heated plasmas (ω ≲ ωci), a two-temperature ion distribution with a suprathermal tail is formed. Faster ions (FIs) can be the characteristic of reactor-size stellarators in the long mean free path regime. The presence of the helical divertor offers new opportunities for studying FI loss by measuring ion fluxes and energies in the divertor plasma. Ion energy distributions were measured in divertor flows in two symmetric poloidal cross-sections in several field periods. It is shown that FI flows out to the divertor mainly on the ion B×∇B drift side in accordance with the assumptions of a determinative FI contribution to the plasma divertor flow vertical asymmetry inherent to torsatrons/heliotrons. Strong toroidal non-uniformities in flows and energies of ions outflowing into the divertor are observed. The island structure of the U-3M magnetic configuration and locality of RF power injection are considered as possible reasons for these non-uniformities.

Fast ion loss-driven H-mode transition in RF discharge plasmas of the Uragan-3M torsatron

In the l = 3 Uragan-3M (U-3M) torsatron with a helical divertor the plasma is produced and heated by RF field in the ω ≤ ωci range of frequencies. A two-temperature ion perpendicular energy distribution with a suprathermal tail sets in with heating. If the heating power is high enough, a spontaneous transition to an H-like confinement mode is observed. Recently, it has been supposed that the transition is connected with hotter and suprathermal ions (common name “fast ions“, FI) loss. The objective of this work is an experimental elucidation of the real link between the H-transition and FI loss. To do this, a transient regime of the RF discharge with two H-mode states is chosen, and the evolution is followed of electron density, FI content in the confinement volume, FI outflow to the divertor and edge potential. On the basis of juxtaposing of these processes, a conclusion is made that the H-mode transition in U-3M is really driven by FI loss. Possible mechanisms resulting in the transition are discussed, among them the ion orbit loss and the radial drift of helically-trapped ion orbits seem most probable.

Cleaning of inner vacuum surfaces in the Uragan-3M facility by radio-frequency discharges

A method for cleaning vacuum surfaces by a low-temperature (Te ∼ 10 eV) relatively dense (ne ≈ 1012 cm−3) plasma of an RF discharge was developed and successfully applied at the Uragan-3M torsatron. The convenience of the method is that it can be implemented with the same antenna system and RF generators that are used to produce and heat the plasma in the operating mode and does not require retuning the frequencies of the antennas and RF generators. The RF discharge has a high efficiency from the standpoint of cleaning vacuum surfaces. After performing a series of cleanings by the low-temperature RF discharge plasma (about 20000 pulses), (i) the intensity of the CIII impurity line was substantially reduced, (ii) a quasi-steady operating mode with a duration of up to 50 ms, a plasma density of ne ≈ 1012 cm−3, and an electron temperature of up to Te ∼ 1 keV was achieved, and (iii) mass spectrometric analysis of the residual gas in the chamber indicated a significant reduction in the impurity content.

Characteristics of the three-half-turn-antenna-driven RF discharge in the Uragan-3M torsatron

In the ℓ = 3 Uragan-3M torsatron hydrogen plasma is produced by RF fields in the Alfvén range of frequencies (ω ≤ ωci). The initial (target) plasma with the line-averaged density of units 1012 cm−3 is produced by a frame antenna with a broad spectrum of generated parallel wavenumbers. After this, to heat the plasma and bring its density to ~1013 cm–3, another, shorter wavelength three-half-turn antenna with large transverse currents is used. The behavior of the density, electron temperature, and loss of the plasma supported by the three-half-turn antenna is studied depending on the RF power fed to the antenna and initial values of the density and electron temperature supplied by the frame antenna.

Characteristic properties of the frame-antenna-produced RF discharge evolution in the Uragan-3M torsatron

In the ℓ = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ≲ ωci). To this end, a frame antenna with a broad spectrum of generated parallel wavenumbers is used. The RF discharge evolution is studied experimentally at different values of the RF power fed to the antenna (the anode voltage of the oscillator and the antenna current) and the initial pressure of the fueling gas. It is shown that, depending on the antenna current and hydrogen pressure, the discharge can operate in two regimes differing in the plasma density, temperature, and particle loss. The change in the discharge regime with increasing anode voltage is steplike in character. The particular values of the anode voltage and pressure at which the change occurs are affected by RF preionization or breakdown stabilization by a microwave discharge. The obtained results will be used in future experiments to choose the optimal regimes of the frame-antenna-produced RF discharge as a target for the production and heating of a denser plasma by another, shorter wavelength three-half-turn antenna.