Azimuthal distribution of divertor plasma fluxes in an ℓ = 3 stellarator and torsatron

Abstract

A multicontact probe placed near the wall has been used to study the distribution of the divertor fluxes over the minor azimuth in stellarator and torsatron regimes of the Saturn machine. It has been found that the plasma flux towards the outer part of the torus is much higher than the flux measured along the direction to the inner part. The plasma fluxes are localized in the vicinity of the apexes of the magnetic surfaces and their azimuthal width near the wall does not exceed 1/18th of the vacuum chamber circumference.

Influence of Experimental Conditions on Distribution of Divertor Plasma Flow in the Heliotron-E Fusion Device

“…The inhomogeneity of divertor flow was observed in all the Heliotron-E experiments as well as in other helical devices (Saturn [22], Uragan-2 [25], and Uragan-3M [15]) where such measurements were carried out. 2.…”

“…The measurements similar to described above were carried out many years ago at the l ¼ 3 stellarator=torsatron Saturn [22] and recently at the l ¼ 3 Uragan-3M torsatron [15]. The different characteristics of these devices as well as methods of plasma production are the reason for a large difference in plasma parameters.…”

“…On the other hand, with the above-indicated plasma parameters, any effects caused by trapped particles should be negligible. That is why the change of ratios of main divertor flows observed near each OMS apexes was explained in [22] by a drift of the divertor plasma when moving along the open magnetic field lines to the wall.…”

Effect of alternating electric field on diffusion of weakly collisional plasma in ℓ = 1 Vint-20 torsatron

“…The above evaluations of the trapped-particle oscillation frequency range are very approximate, because of the complex distributions (in a stellarator configuration) of magnetic-field inhomogeneities, both along minor radius and minor azimuth. For example, in experiments in the Saturn stellarator [5], the greatest effect was observed when the average noise generator frequency was twice the calculated value. For this reason, in the present experiment, we used a noise generator that could be tuned up inside the 2-28 MHz frequency range with a spectrum half-width of ~ 1.5 MHz (the spectrum shape is shown in the insert in Fig.…”

“…To reduce superbanana plasma losses it is possible to convert trapped charged particles into circulating particles using an alternating longitudinal electric field, with an alternation frequency identical with or a multiple of the oscillation frequency of the particles trapped between local magnetic mirrors [3,4]. This possibility has been tested experimentally in the Saturn stellarator, where it was shown [5] that plasma losses in the superbanana diffusion regime can be reduced by 30-40% when an alternating electric field with a broad spectrum is excited in the plasma. In general, the degree of reduction observed in superbanana losses should depend largely on their contribution to overall plasma losses and should be determined by the relative value of the helical eh and toroidal e t inhomogeneities [6].…”