First demonstration of efficient lower hybrid current drive at reactor relevant densities in a tokamak

For the first time, lower hybrid current drive (LHCD) is demonstrated in a reactor relevant diverted configuration at densities exceeding the previously reported “LH density limit” at the line-averaged density of n_e ≈ 1.0x10²⁰ m^-3. By achieving efficient LHCD beyond the LH density limit (above which anomalous loss of current drive efficiency is usually observed), these new experimental results show that current drive at a reactor relevant density can be attained by management of the scrape-off-layer (SOL) plasma at high B_t and I_p. In Alcator C-Mod, detailed studies have identified parasitic wave interactions with the diverted SOL plasma as a dominant contributor to the LH density limit. To avoid this, the new experiments focused on minimizing the SOL width and its associated turbulence level, based on the previously identified scaling of the SOL width with the Greenwald fraction (n_e/n_G, where n_G ≡ I_p/(πa²)) [B. LaBombard, et al., Phys. Plasmas 15 (2008) 056106]. Operation at high Bt (7.8 T) allows increased plasma current up to 1.4 MA at n_e ≈ 1.4x10²⁰ m^-3. At this low Greenwald fraction (n_e/n_G < 0.2), parasitic interactions are largely eliminated, evidenced by minimal broadening in the measured wave frequency spectrum. LHCD efficiency in these high density plasmas is found to match that achieved at low density: η₀=n_eI_pR₀/P_LH ≈ 2.5 (10¹⁹ MA MW^-1 m^-2). Further evidence of efficient current drive comes from increased non-thermal Bremsstrahlung emission rates at high I_p, whereas the emission remains low at low I_p. These new experimental results provide further motivation to place the LH antenna at the high-field-side (HFS) of the tokamak in a double null configuration. In this case, the HFS SOL exhibits a narrow SOL width and low levels of SOL turbulence, which is expected to provide an optimum SOL condition in attaining efficient current drive in a reactor plasma.