Modification of H-mode pedestal structure with lower hybrid waves on Alcator C-Mod

Abstract

The application of lower hybrid range of frequencies (LHRF) waves in H-mode plasmas on Alcator C-Mod can result in a significant reduction in core particle inventory, with no significant degradation of energy confinement. This phenomenon has been observed in steady enhanced Dα (EDA) H-mode targets, which are sustained by ion cyclotron RF auxiliary heating, in which pedestal density nped is usually tied firmly to plasma current IP and shows a strong resilience to changes in the edge neutral source. Upon application of up to 1 MW LHRF power, nped is reduced by up to 30%, while the temperature profile increases simultaneously such that the pressure pedestal remains constant or is slightly increased. Steady EDA H-mode operation with no edge-localized modes can be maintained while edge collisionality is reduced by factors of reduction of 2–4. Elevation of scrape-off layer (SOL) density and electric currents accompany the application of LHRF (at levels as low as 400 kW) with a fast time response (∼10−2 s), while full density pedestal relaxation and core density reduction occur on longer time scales (∼10−1 s). A similarly prompt counter-IP change in the edge toroidal velocity is also observed in response to LHRF, followed on longer time scales by a counter-IP change in the central rotation. The range of time scales of the plasma response may indicate that the radial locations of LHRF interactions (i.e. SOL versus core), and power deposition mechanisms, are evolving in time. Understanding the responsible physical mechanisms and applying them to a broad range of discharges could provide a tool for improved H-mode density control.