Efficient lower hybrid current drive (LHCD) is demonstrated at densities up to ${\overline{n}}_e\approx 1.5\times {10}^{20}{\mathrm{m}}^{-3}$ in diverted plasmas on the Alcator C-Mod tokamak by operating at increased plasma current—and therefore reduced Greenwald density fraction. This density exceeds the nominal “LH density limit” at ${\overline{n}}_e\approx 1.0\times {10}^{20}{\mathrm{m}}^{-3}$ reported previously, above which an anomalous loss of current drive efficiency was observed. The recovery of current drive efficiency to a level consistent with engineering scalings is correlated with a reduction in density shoulders and turbulence levels in the far scrape-off layer. Concurrently, rf wave interaction with the edge and/or scrape-off-layer plasma is reduced, as indicated by a minimal broadening of the wave frequency spectrum measured at the plasma edge. These results have important implications for sustaining steady-state tokamak operation and indicate a pathway forward for implementing efficient LHCD in a reactor.

• Received 9 April 2018
• Revised 22 June 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.055001