Observations of Lower Hybrid Wave Absorption in the Scrape Off Layer of a Diverted Tokamak

Abstract

The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod is designed to drive off-axis current in the core plasma, e.g. r/a<=0.8, however several interesting observations indicate that the LH waves are depositing significant power in the Scrape Off Layer (SOL) in high density L-Mode discharges. These phenomena are observed in accessible plasmas below the so-called “density limit” attributed to Parametric Decay Instabilities as omega-->2omegalh. Parallel current densities of ~500 kA/m [superscript 2] are measured on divertor Langmuir probes located just outside the separatrix during high power LH operation above n-bare~1×10[superscript 20] m[superscript −3]. These SOL currents rise at the same density at which the line integrated hard X-ray Bremsstrahlung emission drops sharply. Ray tracing/Fokker-Planck simulations over-estimate the line integrated X-ray emissivity by a factor of ~500 at n-bare = 1.4 10[superscript 20] m[superscript −3] as compared to a factor of 3–4 at n-bare = 0.6×10[superscript 20] m. Ray tracing simulations including a realistic SOL show similar trends in X-ray emission compared with the experimental observations. ©2009 American Institute of Physics