Effects of LHRF on toroidal rotation in Alcator C-Mod plasmas

Abstract

Application of lower hybrid range of frequencies (LHRF) waves can induce both co-and counter-current directed changes in toroidal rotation in Alcator C-Mod plasmas, depending on the target plasma current, electron density, confinement regime and magnetic shear. For ohmic L-mode discharges with good core LH wave absorption, and significant current drive at a fixed LH power near 0.8 MW, the interior (r/a < 0.5) rotation increments (on a time scale of order the current relaxation time) in the counter-current direction if n e (10 20 m −3) > q 95 /11.5, and in the co-current direction if n e (10 20 m −3) < q 95 /11.5. All discharges with co-current rotation changes have q 0 > 1, indicating a good correlation with driven current fraction, unifying the results observed on various tokamaks. For high density (n e 1.2 × 10 20 m −3) L-mode target discharges, where core LH wave absorption is low, the rotation change is in the co-current direction, but evolves on a shorter momentum transport time scale, and is seen across the entire spatial profile. For H-mode target plasmas, both co-and counter-current direction increments have been observed with LHRF. The H-mode co-rotation is correlated with the pedestal temperature gradient, which itself is enhanced by the LH waves absorbed in the plasma periphery. The H-mode counter-rotation increment, a flattening of the peaked velocity profile in the core, is consistent with a reduction in the momentum pinch correlated with a steepening of the core density profile. Most of these rotation changes must be due to indirect transport effects of LH waves on various parameters, which modify the momentum flux.

Alcator C-Mod: research in support of ITER and steps beyond

“…However, in a minority of cases the H 98 -factor is raised significantly above 1 with the LHRF. In addition to the changes to T e,ped , T i,ped , n e,ped , the improvement is accompanied by the following effects in the edge: co-current pedestal rotation velocities increase [11]; power to the (outer) divertor target increases promptly with an increment that is ~1/2 of the injected LH power, qualitatively consistent with the inaccessibility of the LH waves; the central frequency of the edge-localized Quasi-Coherent Mode (QCM) down-shifts, becomes more coherent, and becomes more constant in time; and the broadband mid-frequency (100-300 kHz) density fluctuations in the pedestal region decrease. Some of these effects are shown in Figure 2.…”

20 years of research on the Alcator C-Mod tokamak

“…For target plasmas with high plasma current, the intrinsic rotation experiences a change in the counter-current direction, 227,228 while for low plasma current targets, the rotation increment is in the co-current direction. 229,230 This reversal of the change in rotation with LHCD has been traced to the current density, through the q profile. At low collisionality, ICRF can also cause core rotation to decrease markedly and even reverse direction.…”

Effects of Magnetic Shear on Toroidal Rotation in Tokamak Plasmas with Lower Hybrid Current Drive

“…Ion cyclotron range of frequencies mode conversion flow drive has been demonstrated [2], and rotation effects from electron cyclotron heating have been seen [3]. Toroidal rotation changes due to lower hybrid (LH) waves have also been observed [4][5][6][7][8][9][10], with velocities in both the co-and countercurrent directions. Even in plasmas with good core absorption of LH waves, the mechanism for rotation drive is not clear.…”

“…Even in plasmas with good core absorption of LH waves, the mechanism for rotation drive is not clear. Candidates include direct momentum input from the waves (calculated to be low [7]), electron orbit loss [4], trapped electron pinch effects [11], resonant electron radial drift [12,13], and less direct causes such as the turbulent equipartition pinch [8] or through modification of the q profile [9,10]. A challenge of these accounts is to explain the rotation changes observed in both directions.…”