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 wit...