In the l = 3/m = 9 Uragan-3M (U-3M) torsatron (R 0 = 1 m, ā ≈ 0.12 m, B φ = 0.72 T, ι( ā)/2π ≈ 0.4), an open helical divertor is realized. A hydrogen plasma with ne ≈ 2 × 10 18 m −3 , T e ≈ 0.3 keV, T i ≈ 0.1 keV is produced and heated by RF fields (ω ≈ ω ci ). 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 I s ) and non-ambipolar (e.g. the current to a grounded probe I p ) plasma flows, a strong vertical asymmetry of these distributions is observed, its main characteristics being a many-fold difference in the values of I s in the outgoing flows in the upper and lower parts of the torus and the opposite signs of I p 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 I p > 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.
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.
In the IPP-Kharkiv there are two torsatrons (stellarators) in operation, and in both of them Alfvén 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.
In the Uragan‐3M (U‐3M) and Uragan‐2M (U‐2M) torsatrons possibilities and prospects of Alfvén method utilization for wall conditioning, plasma production and heating are studied. In U‐3M the effect of fast ion loss on H‐like mode formation is investigated. In U‐2M the wall conditioning associated with the chemical reactivity of the atomic hydrogen to create volatile substances is used. A compact four‐strap antenna is proposed for Alfvén resonance heating in U‐2M (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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