10.7910/DVN/PZOMOIS. G. Baek, G. M. Wallace, P. T. Bonoli, D. Brunner, I. C. Faust, A. E. Hubbard, J. W. Hughes, B. LaBombard, R. R. Parker, M. Porkolab, S. Shiraiwa, S. WukitchHarvard Dataverse2019Physicslower hybrid heating & current driveparametric decay instabilityscattering of waves by turbulencescrape off layertokamak plasma2019-01-1816235144809121340456266403105631056application/pdfapplication/x-hdf5application/x-hdf5application/x-hdf5application/x-hdf5application/x-hdf51.0Efficient lower hybrid current drive (LHCD) is demonstrated at densities up to ne≈1.5×10^20 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 ne≈1.0×10^20 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.<a href="http://library.psfc.mit.edu/catalog/reports/2010/18ja/18ja022/abstract.php">PSFC REPORT PSFC/JA-18-22</a><br /><br />This research was conducted on Alcator C-Mod, a DOE Office of Science User Facility, and is supported by DOE Contract No. DE-FC02-99ER54512