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

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 ω→2ωlh. Parallel current densities of ∼500 kA/m2 are measured on divertor Langmuir probes located just outside the separatrix during high power LH operation above ne∼1×1020 m-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 ne = 1.4 1020 m-3 as compared to a factor of 3–4 at ne = 0.6×1020 m. Ray tracing simulations including a realistic SOL show similar trends in X-ray emission compared with the experimental observations. [ABSTRACT FROM AUTHOR]