Main divertor flows in Heliotron E: their distribution and dependence on NBI and ECH

Abstract

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.

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

“…It follows from results presented in [1,4] that the maximum separation between the divertor legs took place for the Y ¼ 0 array. Contrary to this, for the Y ¼ 180 array the plasma flows in both divertor legs hit the N ¼ 3, 4, 5 CPs, occupying the central part of this array, but the measuring circuit of the N ¼ 3 CP was broken.…”

“…The spatial distribution of ISC response to an additional heating pulse was characterized by a strong poloidal inhomogeneity [3,4]. Such an inhomogeneity of relative and absolute ISC increments for different CPs in the Y ¼ 315 array can be seen in Figure 2.…”

“…2(b)). The reason for such a difference has been discussed in detail in [4] and can be related to the difference in the ECH power deposition profiles [7]: in the second harmonic ECH case, this profile was much more centrally peaked as compared with the fundamental ECH.…”

“…[1,3,4] for the initial ECH, NBI and NBI þ ECH regimes. By summing ISCs over all the collectors of each array, the poloidal distribution of the total plasma flow, G(Y,t) ¼ SI s (Y,t)=n e (t), can be obtained.…”

“…The divertor plasma flow (DPF) distribution in the Heliotron-E (H-E) device has been already investigated elsewhere [1][2][3][4][5]. In these publications a good qualitative correlation has been found between the DPF distribution and the configuration of open magnetic field lines; dependences of separate divertor flow components on the absorbed power have been studied in detail; a possible explanation has been done of the DPF time behavior when an electron-cyclotron heating pulse was superimposed on the neutral beam heated plasma; a strong vertical asymmetry in DPF distribution has been revealed and its possible explanations have been suggested.…”