Lower hybrid wave edge power loss quantification on the Alcator C-Mod tokamak

For the first time, the power deposition of Lower Hybrid RF waves into the edge plasma of a diverted tokamak has been systematically quantified. Edge deposition represents a parasitic loss of power that can greatly impact the use and efficiency of Lower Hybrid Current Drive (LHCD) at reactor-relevant densities. Through the use of a unique set of fast time resolution edge diagnostics, including innovative fast-thermocouples, an extensive set of Langmuir probes, and a Ly? ionization camera, the toroidal, poloidal and radial structure of the power deposition has been simultaneously determined. Power modulation was used to directly isolate the RF effects due to the prompt (t 1.0 1020 [m?3]). Results will be shown addressing the distribution of power within the SOL, including the toroidal symmetry and radial distribution. These characteristics are important for deducing the cause of the reduced LHCD efficiency at high density and motivates the tailoring of wave propagation to minimize SOL interaction, for example, through the use of high-field-side launch

Lower hybrid current drive at high density in Alcator C-Mod

Experimental observations of lower hybrid current drive (LHCD) at high density on the Alcator C-Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line-averaged densities of 10[superscript 20] m[superscript −3] (ω/ω[subscript LH] ~ 3) in single null discharges with large (≥8 mm) inner gaps, well below the density limit previously observed on limited tokamaks (ω/ω[subscript LH] ~ 2). Modelling and experimental evidence suggest that the absence of LHCD driven fast electrons at high density may be due to parasitic collisional absorption in the scrape-off layer (SOL). Experiments show that the population of fast electrons produced by LHCD at high density ([bar over n][subscript e] > 10[superscript 20] m[superscript -3]) can be increased by operating with an inner gap of less than ~5 mm with the strongest non-thermal emission in inner wall limited plasmas. A change in plasma topology from single to double null produces a modest increase in non-thermal emission at high density. Increasing the electron temperature in the periphery of the plasma (0.8 > r/a > 1.0) also results in a modest increase in non-thermal electron emission above the density limit. Ray tracing/Fokker–Planck simulations of these discharges predict the observed sensitivity to plasma position when the effects of collisional absorption in the SOL are included in the model.United States. Dept. of Energy (Award DE-FC02-99ER54512)United States. Dept. of Energy (Award DE-AC02-76CH03073