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

The application of lower hybrid waves (LH) in H-mode plasmas on Alcator C-Mod can result in significant reduction of 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 1MW LH 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 (ELMs) can be maintained while edge collisionality is reduced by factors of reductions of 2—4. Elevation of scrapeoff layer (SOL) density and electric currents accompany the application of LH (at levels as low as 400kW) with a fast time response (~10 s), while full density pedestal relaxation and core density reduction occur on longer time scales (~10 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 LH interactions (i.e., SOL vs. core), and power deposition mechanisms, are evolving in time. Understanding the responsible physical mechanisms and applying them to a broad range of H-mode discharges could provide a tool for improving density control, affecting edge MHD stability and applying modulation for transport studies.