Plasma heating effects on divertor flow vertical asymmetries in the Uragan-3M torsatron

Abstract

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.

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

“…Recently, the role of helically trapped ions, their predominant loss in the ion toroidal B Â HB drift direction, in formation of the up-down asymmetry of divertor flows has been shown experimentally and confirmed by numerical simulation for the l ¼ 3 Uragan-3M torsatron [15]. In the Heliotron E device main particle losses, similar to those in U-3M [15], should be associated with fast toroidal banana-ions, which were trapped in the helical ripple wells at the plasma periphery, and the non-compensated ion B Â HB drift should be a determinative mechanism of particle losses and the vertical asymmetry of poloidal distributions of these losses.…”

“…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.…”

“…In the Heliotron E device main particle losses, similar to those in U-3M [15], should be associated with fast toroidal banana-ions, which were trapped in the helical ripple wells at the plasma periphery, and the non-compensated ion B Â HB drift should be a determinative mechanism of particle losses and the vertical asymmetry of poloidal distributions of these losses. However, in the H-E case, the degree of vertical asymmetry of ion loss distribution underwent a significant increase due to the natural vertical asymmetry of the helical magnetic field ripple, according to results of calculations in [16,17].…”

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

“…2; hereinafter, the hotter ions (T i2 ) and suprathermal ions (STI) are named as "fast ions" (FI)). The amount of FI increases with RF power [3] (Fig. 3).…”

“…The RF power P fed in the antenna attains 200 kW. With this, at the densityn e 10 12 cm −3 the electron temperature inferred from ECE measurements amounts T e (0) ≈ 600 eV in the central region, and a two-temperature ion energy distribution with a suprathermal tail is formed [2,3] ( Fig. 2; hereinafter, the hotter ions (T i2 ) and suprathermal ions (STI) are named as "fast ions" (FI)).…”