Plasma flow asymmetries in the natural helical divertor of anl= 3 torsatron and their relation to particle losses

Abstract

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.

Studies of fast ion outflow to the helical divertor of the U-3M torsatron

“…This work continues the works reported by Chechkin et al [5,6]. Unlike these works where total DPF were recorded (estimated by the ion saturation current to Langmuir probes), here flows and energies of ions selected from DPF were measured directly.…”

“…As discovered recently [6], a strong vertical asymmetry (up-down asymmetry) in the DPF spatial distribution is inherent to U-3M similar to other torsatrons/heliotrons [7,8]). The asymmetry manifests itself in that higher ambipolar DPF (detected by the ion saturation current on a Langmuir probe inserted into the flow) outflows in the direction of the ion toroidal B × ∇B drift and ions prevail in the corresponding non-ambipolar flow (positive current on the grounded probe).…”

“…The calculations were carried out for the plasma parameters and RF heating conditions similar to those realized in U-3M [6]. The orbits of monoenergetic particles with energies in the ranges of 0.1-0.5 keV (ions) and 0.3-1.2 keV (electrons) with the starting angular co-ordinates and pitch values taken from the ranges of (0, 2π ) and (−1, 1), respectively, were followed from the moment of their start on the r/ā = 0.7 magnetic surface till intersection with the last closed magnetic surface (r/ā = 1).…”

“…Both directions of the magnetic field were considered. The calculation procedure and obtained results are reported in [6].…”

Behavior of RF Discharge Plasmas in the Uragan‐3M and Uragan‐2M Torsatrons

“…3). The FI outflow to the divertor predominantly on the ion B×∇B drift side due to ion trapping into toroidal and helical non-uniformities of the magnetic field [4,5]. The effect is confirmed by an appreciable difference in the time behavior of parallel and perpendicular fluxes of energetic CX neutrals after RF pulse termination (Fig.…”