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 = 2/m = 19 Heliotron E heliotron/torsatron device, with a currentless plasma heated by NBI and ECH, the poloidal distributions were measured of diverted plasma flows near the vacuum chamber wall in eight sections of one helical field pitch. A vertical asymmetry of these distributions was observed, contrary to what had been expected from the helical symmetry of the `ideal' (unperturbed) vacuum divertor magnetic configuration. Under NBI conditions, the density normalized divertor plasma flow Jd/n̄e was a rising function of the density normalized absorbed power Pabs/n̄e. This is considered to be a consequence of the power degradation of confinement. With second harmonic ECH, where the main part of the launched power was absorbed in the central region, a similar power dependence as for the NBI was observed. With fundamental ECH, where a considerable fraction of the microwave power was absorbed at the periphery, Jd/n̄e for some areas of the divertor trace distinctly exceeded those obtained under NBI-only conditions with the same Pabs/n̄e. On the other hand, two different time responses (fast and slow) of Jd were observed for the perpendicular NBI and fundamental ECH cases in some particular positions. The slow response is considered to be caused by a diffusion-like outflow of the bulk plasma to the divertor. The fast outflow of particles to the divertor might be caused by a loss of locally trapped high energy electrons and ions.

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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.