Uragan-3M Torsatron Research Articles

Calculations of 1/ν Transport in an l = 3 Stellarator Magnetic Field in the Presence of Magnetic Islands Caused by Magnetic System Errors

The 1/ν neoclassical transport (effective ripple) is calculated for the l = 3 Uragan-3M torsatron in the presence of magnetic islands caused by a small eccentricity in the vertical field coils. The technique used for the calculations is based on integration along magnetic field lines in a given stellarator magnetic field. For comparison, calculations are also performed for the case of negligible eccentricity.

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, Bϕ = 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.

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 = 1 m, ā≈0.12 m, Bϕ = 0.72 T, ι(ā)/2π≈0.4), an open helical divertor is realized. A hydrogen plasma with n̄e≈2×1018 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×\U0001d6c1B 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.

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 l / 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.

Damages of hot-pressed boron carbide during solid target boronization in Uragan-3M torsatron

The investigation of hot-pressed boron carbide erosion caused by the arc-regime of a solid target boronization (STB) procedure was carried out. The sample had a high density (2.44 g/cm 3, B 78.2 a.m.%, C 21.5 a.m.%) and was used as a head plate of the movable limiter during the pulsed discharge cleaning campaign in the Uragan-3M (U-3M) torsatron. The total number of pulses during STB was about 2 × 10 4 ( n e=2 × 10 12 cm −3, T e=10–15 eV, B=0.035 T, the plasma pulse duration t=50 ms, the negative pulsed biasing of B 4C-plate with an amplitude of up to 200 V had a time duration of 10–50 ms). As was shown by an optical examination of the surface morphology of the exposed sample, the character and intensity of damages of the limiter surface strongly varied depending on the distance from the plasma column axis. The most damaged sites were covered with many craters of 0.1–1.2 mm in diameter and spallings with a size up to 5 mm. The possible physical mechanisms of these damages and erosion behavior of hot-pressed in vacuum boron carbide caused by the arc-regime of STB are discussed.

On the use of biased movable B 4C-limiter for pumping O 2 and H 2O during discharge cleaning

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 4C-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 4C-limiter can be used to improve pulsed discharge cleaning in Uragan-3M torsatron.

Density behaviour and particle losses in RF discharge plasmas of the URAGAN-3M torsatron

URAGAN-3M (U-3M) is an l=3, m=9 torsatron with R0=1 m, ā ≈ 12 cm and ϵ(ā) ≈ 0.4. The entire magnetic system is enclosed in a large vacuum chamber and there is no material limiter, so that an open helical divertor is realized in this device. A plasma is RF produced in 100% hydrogen gas and heated in the multimode Alfven resonance regime (ω ≲ ωci) at a continuous working gas leak-in. With an RF power of ~200 kW at B = 0.45 T, a quasi-steady (up to 50 ms) state of the plasma parameters sets in with n̄e ≃ 2 × 1018 m-3, Ti(0) ≃ 130 eV and Te(0) equivalent to 300 eV. By using microwaves and Langmuir probes, the time and space behaviours of the electron density/ion saturation current are studied in the confinement volume, near the boundary of the confinement region, and in the divertor flux region. On the basis of ion saturation current profile measurements near the boundary of the confinement region the values of local particle flux density across the boundary are estimated: Γ⊥ ≈ 7 × 1020 m-2.s-1 far from the antenna and Γ⊥ approximately=1021 m-2.s-1, Γ⊥ ≈ 2 × 1021 m-2.s-1 near the antenna. Such flux densities and the corresponding diffusion coefficients are anomalously high, exceeding the neoclassical values by 2 to 3 orders of magnitude. On the other hand, these values are comparable with the flux densities and diffusivities obtained in edge plasma with similar parameters on other stellarators under NBI and ECR heating conditions. Therefore, it is claimed that the method of plasma heating that has been chosen in U-3M does not result in any substantial deterioration of particle confinement (with the possible exception of the region close to the antenna) as against other methods. Presumably, the anomalous particle transport across the boundary in U-3M is caused by destruction of magnetic surfaces and/or electrostatic drift wave turbulence. It is found thatincreasing the RF power results in an increase of the diverted plasma flow and of the hydrogen consumptionnecessary to maintain the average plasma density at a given level. This is considered as a manifestation of acommon regularity, according to which particle losses grow with heating power, this effect having been observedrecently on a stellarator type device under NBI and ECR heating conditions. An effect specific to U-3M, theincrease of the average electron density, being observed within 1 to 2 ms after RF pulse termination, is attributedmainly to particle transport reduction due to plasma cooling, while the intensity of neutral particle ionizationstill remains sufficiently high in the confinement volume at the initial phase of cooling under continuous hydrogenleak-in conditions.

Divertor studies in the l=3 URAGAN-3M torsatron

In the l=3, m=9 URAGAN-3M torsatron an open helical divertor is realized. Plasmas are RF produced and heated. The time and space characteristics of the plasma in the space between two coils of the helical winding and on the outward side of the helical winding are studied by microwave and probe techniques. The experimental measurements are preceded by numerical divertor field line tracing. As a result of the calculations and measurements, it is shown that the diverted plasma is formed due to (i) plasma transportation from the confinement volume; and (ii) neutral gas ionization by the near-antenna field. The flow of diverted plasma transported from the confinement volume increases with RF power. This is considered as direct evidence of an increase in particle loss with an increase in heating power, the effect having been deduced recently for a stellarator-type device under NBI and ECR heating conditions. An intense RF discharge along open field lines crossing the operating antenna results in a greatly increased diverted plasma density, in the appearance of a static positive plasma potential up to 150 V near the antenna, and possibly in significant additional RF power loss outside the confinement region.

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

Numerical and experimental results of Alfvén 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.

Dynamics of r.f. production of Stellarator plasmas in the ion cyclotron range of frequency

Abstract The present study investigated numerically the process of r.f. production of plasma in the URAGAN-3M torsatron in the frequency range below the ion cyclotron frequency (ω