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.

Articles you may be interested inNonlinear behavior of resistive pressure driven modes in stellarator/heliotron plasmas with vacuum magnetic islands Phys.A typical feature of all current stellarator-type configurations with discrete toroidal or modular field coils is a multiple-toroidicity character of the magnetic field. It manifests itself in the presence of distant harmonics with high-order Nу2 toroidal mode numbers in a Fourier-decomposition of the magnetic field. The main purpose of the present paper is to derive convenient magnetic-field models, which reflecting adequately all specific features of the magnetic field in multiple-toroidicity stellarators, could allow one to reduce the adiabatic invariants of particle motion to elliptic integrals. Such models are derived in the paper for a number of current stellarator-type devices on the basis of the analysis of the structure of ripple wells along field lines. The use of these magnetic-field models can simplify investigations of the neoclassical transport in stellarators.

Possible reasons are considered for the vertically asymmetric helical field ripple to occur in torsatrons and heliotrons [K. Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)]. Vacuum magnetic configurations of present day l=2 torsatrons/heliotrons are analyzed using a harmonic expansion of the magnetic-field strength along a field line. It is shown that a typical feature of such configurations is a vertical asymmetry of their helical field ripple. This kind of vertical asymmetry of the ripple in torsatrons and heliotrons is caused by the presence of the helical satellite harmonics in a Fourier spectrum of the magnetic field. In configurations that are far from the quasihelically symmetric, due to these harmonics effects, the depth of the secondary magnetic wells along field lines can differ not only inside and outside the torus, but also in the upper and lower parts of the torus.

Direct losses of highly energetic helically trapped particles, especially of superbananas, are analytically investigated in torsatrons and heliotrons [Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)] with a vertically asymmetric helical field ripple. It is found that the vertically asymmetric helical ripple causes an additional net radial drift of suprathermal superbananas, leading to convective losses of these particles from the confinement volume mainly in the vertical direction. For a vertically asymmetric rippled torsatron, a location of the particle loss region in the coordinate space and a dominated direction of the particle escape from the confinement volume is predicted on the basis of the probabilities of the particle trapping/detrapping in/from the helical ripple wells. Analytical predictions are confirmed by numerical computations of direct losses of highly energetic helically trapped particles for a number of torsatrons/heliotrons of today.

Direct losses of fast ions are investigated numerically in current multiple-toroidicity stellarator configurations, in which magnetic field harmonic spectra can be described adequately by a sum of hundreds harmonics with different toroidal and poloidal mode numbers. For these configurations it is typical that amplitudes of distant harmonics with high-order N⩾2 toroidal mode numbers can exceed, near the plasma boundary, one-half of the amplitude of the main magnetic-field harmonic. This leads to the appearance of additional ripple wells along field lines which can trap charged particles (toroidally field ripple-trapped particles). An investigation of the role of these particles in plasma confinement in stellarators is the main goal of the paper. It is shown that a complication of the magnetic-field harmonic spectrum of a stellarator causes direct losses of transition and toroidally field ripple-trapped particles several times higher than losses of corresponding particles in rippled tokamaks.

Charged particle orbits are investigated in a torsatron with the complicated magnetic-field harmonic spectrum when distant, high-order harmonics are present. The structure of the local magnetic wells in such a torsatron is considered. Conditions for the appearance of the additional local magnetic wells along the field line (toroidal-ripple wells) due to the distant harmonics effect are obtained. It is shown that in a torsatron, the particles which are trapped in such wells have the largest radial deviations among all types of trapped particles. The analytical conditions under which the toroidal-ripple wells are dangerous for the particle confinement (criteria of the strong toroidal-field ripple) are derived.

Collisionless charged particle motion in torsatrons is studied with the use of a longitudinal adiabatic invariant and numerical integration of the guiding centre drift equations. Emphasis is placed on investigating the influence of satellite harmonics of the helical magnetic field and the radial electric field on particle confinement. From this standpoint, the particle orbits, the limiting cycles of equations of particle motion, and the shape and size of regions in phase space in which the particle orbits lie are considered as a function of the device parameters. A comparison is made of magnetic configurations of various types differing in the number of helical field periods, in the inhomogeneity ratio ϵt/ϵl and in their sets of satellite harmonics. The characteristics of the configurations compared are close to the parameters of actual torsatrons: Heliotron E, ATF and Uragan-2M