L.I. Grigor'eva

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.

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.

Processes arising in the edge and diverted plasmas during ITB formation in the U-3M torsatron

Spontaneous changes in confined plasma parameters have been observed recently in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated plasma, these being interpreted as transition to an improved confinement mode due to ITB formation near the ι = 1/4 rational magnetic surface. In the work presented joint studies are carried out of changes in some edge and diverted plasma characteristics that accompany ITB formation. It is shown that ITB formation induces a hard Er bifurcation at the boundary presumably driven by the ion orbit loss. As a result, Er becomes more negative, and an Er shear layer occurs, where the low-frequency microturbulence and the turbulence-induced anomalous transport are suppressed, i.e. an ETB is formed. At the pre-bifurcation phase of transition a reduction of fast ion loss takes place. The bifurcation results in an improvement of electron confinement, while the ion loss increases.

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.

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.

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.

Edge plasma potential and associated ion fluxes to the surface during ICRF plasma production and heating in the Uragan-3 Torsatron

Abstract This paper gives a study of the dynamics of the edge plasma potential formation together with accompanying effects (higher harmonic generation, a d.c. potential acquired by the antenna, charged particle fluxes to the surfaces) during ICRF plasma production and heating in the URAGAN-3 (U-3) torsatron. An identification of low-frequency (ω ⪡ ωBi) electrostatic fluctuations arising during the RF pulse has also been made and the conclusion is drawn that the fluctuations are driven by a parametric instability of the pumping waves.

VHF discharges for wall conditioning at the Uragan-2M torsatron

The very high frequency (VHF) discharge for wall conditioning with hydrogen atoms is studied. It is driven by the RF power at frequencies ∼140MHz, higher than usually used in ICRF. For wall conditioning a special small size antenna is designed. The antenna is aimed to excite the slow wave that is damped via electron collisions with neutral gas. The wave excitation is modelled using a 1D numerical code. In the experiment, the discharge parameters are studied as functions of confining magnetic field and gas pressure. The Langmuir probe measurements give the radial profiles of plasma density and electron temperature. The discharge is volumetric: plasma occupies whole confinement volume and even steps out at the edge. The characteristic value of plasma density is 10 10 cm −3 , electron temperature varies in the range 3‐10eV. The temperature values of probe measurements are compatible with the results of optical diagnostics. Such parameters of discharge are favourable for wall conditioning in hydrogen. The discharge parameters did not reveal any sensitive dependence on neutral gas pressure and the toroidal magnetic field. The mass spectrometry of the residual gas is used for monitoring the wall conditioning effect of the VHF discharge.