Yu.K. Mironov

Plasma flow asymmetries in the natural helical divertor of an l = 3 torsatron and their relation to particle losses

In the l = 3/m = 9 Uragan-3M (U-3M) torsatron (R0 = 1m , ¯ a ≈ 0.12 m, Bφ = 0.72 T, ι(¯ a)/2π ≈ 0.4), an open helical divertor is realized. A hydrogen plasma with ¯ ne ≈ 2 × 10 18 m −3 , Te ≈ 0.3 keV, Ti ≈ 0.1 keV is produced and heated by RF fields (ω ≈ ωci). The flows of diverted plasma are detected by 78 plane Langmuir probes aligned poloidally in the spacings between the helical coils in two geometrically symmetric poloidal cross-sections of the torus. In measurements of the distributions of ambipolar (e.g. the ion saturation current Is) and non-ambipolar (e.g. the current to a grounded probe Ip) plasma flows, a strong vertical asymmetry of these distributions is observed, its main characteristics being a many-fold difference in the values of Is in the outgoing flows in the upper and lower parts of the torus and the opposite signs of Ip in these flows, with the positive current corresponding to the larger ambipolar flow of the diverted plasma. Reversal of the direction of the toroidal magnetic field results in the reversal of the asymmetry, with the larger flux (and Ip > 0) always flowing in the ion B × ∇B drift direction. On this basis, it is concluded that the asymmetry is related to direct (non-diffusive) losses of charged particles from the confinement volume. This conclusion is validated by numerical modelling of thermal and fast particle orbits in U-3M, where qualitative agreement has been revealed between the calculated distribution of the angular co-ordinates of lost particles and the measured poloidal distributions of the flows of diverted plasma.

Plasma heating effects on divertor flow vertical asymmetries in the Uragan-3M torsatron

In the l = 3/m = 9 Uragan-3M (U-3M) torsatron (R0 = 1 m, abar; ≈ 12 m, B = 0.7 T, ι(abar)/2π ≈ 0.4), an open helical divertor has been realized. Recently, under RF plasma production and heating conditions, a strong up–down asymmetry of diverted plasma flow has been observed as a result of measurements of distributions of this flow in two symmetric poloidal cross-sections of the U-3M torus. In many aspects, this asymmetry is similar to that observed in the l = 2 Heliotron E (H-E) heliotron/torsatron under neutral beam injection and electron cyclotron heating conditions. The main feature of the asymmetry is a predominant outflow of the diverted plasma in the ion toroidal drift direction. On this basis, the asymmetry can be related to non-uniformity of the distribution of direct charged particle losses in the minor azimuth. In the work reported, the magnitude of diverted plasma flow in U-3M and the degree of its vertical asymmetry are studied as functions of the heating parameter , P being the RF power absorbed in the plasma, and are juxtaposed with corresponding P-related changes in the density, , and suprathermal ion content in the plasma. As the heating power increases, both the temperature of the main ion group and the relative content of suprathermal ions increase. At the same time, a decrease in plasma density is observed, evidencing a rise of particle loss. The rise of particle loss with heating could result from both a shift of diffusion regime towards a lower collisionality and a rise of direct (non-diffusive) loss of high-energy particles. Outside the confinement volume, the total flow of diverted plasma increases together with an increase of vertical flow asymmetry towards the ion toroidal drift side. Such a mutual accordance between the processes in the confinement volume and in the divertor region validates the hypothesis on a dominating role of fast particle loss in the formation of vertical asymmetry of divertor flow in U-3M. In conclusion, the results obtained on U-3M are compared with those from similar research on H-E.

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.

A study of three-half-turn and frame antennae for ion cyclotron range of frequency plasma heating in the URAGAN-3M torsatron

Abstract Numerical and experimental results of Alfven wave heating of plasmas in the frequency range below the ion cyclotron frequency ( ω ω ci ) are presented. Two different types of antenna were used for plasma production and heating: a frame type antenna (FTA) conventionally used in the URAGAN-3M device and a three-half-turn antenna (THTA) proposed recently to avoid the deleterious effects of conversion of fast wave to slow wave in the plasma periphery and to perform plasma core heating more effectively. Numerical modeling of electromagnetic field excitation in the URAGAN-3M plasma by the FTA and THTA was performed using a one-dimensional code. The results of calculations showed better performance of the compact THTA compared with the FTA for the case of a high density plasma (approximately 10 13 cm −3 ). When using the THTA, the experiments performed showed the possibility of dense plasma production (more than 2 × 10 13 cm −3 ) and heating, which had not been obtained earlier in the URAGAN-3M. Shifting the power deposition profile deeper inside the plasma body with the THTA resulted in modification of the plasma density profile and an improvement in plasma confinement.

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.

Erosion and outgassing behavior of TiN-coated plasma facing components of the Uragan-3M torsatron

An erosion behavior of TiN-coated stainless steel (SS) surfaces was investigated during biased-limiter experiments within the Uragan-3M torsatron and during simulation experiments, which were performed with plasma-accelerator and glow-discharge (GD) plasmas. For a TiN-coated SS head-plate of a limiter, the arc ignition probability was found to be lower than 10 -4 per plasma pulse. Possible physical mechanisms of this effect had been discussed. Within special vacuum stands, using thermal-desorption and mass-spectrometry methods, there were performed measurements of an outgassing rate and hydrogen permeability of TiN-coatings. The specific outgassing-rate (q) of the TiN-coated SS samples, investigated after 3 h baking at 373 K, increased from 5 × 10 8 to 5 × 10 6 Torr 1/s cm 2 when the temperature was increased from 323 to 523 K. The negligible outgassing from TiN-coated SS samples, during their heating up to 473 K, was observed after a cleaning procedure with a molecular hydrogen inflow under a pressure of about 10 -4 Torr, contrary to the considerable increase of (q) rate for the irradiated samples. Measured values of the TiN-film hydrogen permeability were several times lower, and activation energy of the hydrogen permeation was considerably lower than that for the SS films (15 kJ/mole instead of 19.9 kJ/mole). The use of TiN-coated SS and diffusion membranes for the reduction of erosion, recycling, and hydrogen isotope inventory control, as well as for improvement of vacuum conditions, has been considered.

On the use of biased movable B4C-limiter for pumping O2 and H2O during discharge cleaning

Abstract The movable limiter, the head plate of which is formed with a boron carbide plate hot-pressed in vacuum, is used for studying plasma impurity removal and the possibility of boronization during discharge cleaning in Uragan-3M torsatron. Measurements of current-voltage characteristics, boron and oxygen spectroscopies, residual-gas mass-spectrometry were carried out for different biased and unbiased limiter positions. It is shown that at a negative limiter bias during pulsed discharges cleaning the boron spectral signal from the plasma increases due to arcing. At the same time the oxygen spectral line intensity decreases and it probably means desorbed oxygen and water effective trapping by boronized surfaces and the limiter B 4 C-plate, since the main plasma parameters do not change at this moment. The estimations of the limiter application for impurity pumping and solid target boronization shows that the negatively biased movable B 4 C-limiter can be used to improve pulsed discharge cleaning in Uragan-3M torsatron.