The dependence of core rotation on magnetic configuration and the relation to the H-mode power threshold in Alcator C-Mod plasmas with no momentum input

Abstract

The observed toroidal rotation in Alcator C-Mod Ohmic L-mode plasmas has been found to depend strongly on magnetic configuration. For standard discharges with a lower single null and the ion B¢ÖB drift downward, and with B Ì = 5.4 T, I È = 0.8 MA and n = 1.4 ¢ 10 ¾¼ /m ¿ , the core toroidal rotation is measured to be in the range of 10-20 km/s (counter-current). Similar plasmas with upper single null have significantly stronger counter-current rotation, in the range of 30-50 km/s. The rotation depends very sensitively on the distance between the primary and secondary separatrices in near double null plasmas, with changes of 25 km/s occurring over a variation of a few millimeters in this distance. Application of ICRF power has been found to increase the rotation in the co-current direction. The transition to H-mode is seen to occur in these standard plasmas when the core rotation reaches a characteristic value, near 0 km/s, hence higher input power is needed to induce the transition in the upper single null configuration.

Magnetic topology effects on Alcator C-Mod flows

“…Alcator C-Mod has toroidal flow measurements both inside [5,6] and outside [7, 8] the separatrix for cases (i) and (ii). The core flow measurements in C-Mod are from the Doppler shifts of Argon (Ar 16+ and Ar 17+ ) X-ray transitions.…”

“…[6] the topology in Alcator C-Mod was changed from lower single null (LSN) to upper single null (USN) during the same Ohmic low confinement mode (L mode) discharges to find that the core toroidal flow became more strongly countercurrent (see Fig. 3 of Ref.…”

“…Sensitivity to magnetic topology is observed in both the core [5,6] and in the scrape-off layer [7,8] flows of Alcator C-Mod.…”

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

“…The dependence of the core toroidal rotation velocity on plasma current is shown in Fig.10, from a shot by shot scan at fixed magnetic field (5.2 T) and electron density (0.8-1.2×10 20 /m 3 ). There is a linear increase in the velocity with plasma current [29,30], trending in the co-current direction. The velocity reverses direction at around 0.9 MA for this range of density, consistent with the density ramp results at fixed current [14].…”

“…As the magnetic field was decreased, the core toroidal rotation velocity incremented in the co-current direction and eventually reversed. In general, with increasing magnetic field, the rotation velocity becomes more counter-current [29,34]. Shown in Fig.12 is the core rotation velocity as a function of magnetic field at fixed plasma current (1.0 MA) and electron density (1.17×10 20 /m 3 ) from a shot by shot scan.…”