Measurement of the current profile in Alcator C-Mod with lower hybrid current drive using an upgraded motional stark effect diagnostic

Upgrades to the motional Stark effect (MSE) diagnostic on Alcator C-mod have enabled accurate measurement of the current profile using pitch angle constrained magnetic equilibrium reconstructions. The MSE diagnostic utilizes an intrashot calibration technique along with kinetic profiles to constrain the reconstruction throughout the discharge. The system was used to study the current profile in plasmas with substantial Lower Hybrid Current Drive (LHCD), including fully non-inductive discharges sustained for several current relaxation times. The current profile evolution is reconstructed using the MSE data with 100ms time resolution as LHCD is applied to the plasma and as the plasma current profile relaxes back to inductively driven after LHCD ceases. The LH driven current is observed to be off-axis and significantly broadens the plasma current profile, resulting in q_o above 1 and sawtooth suppression. In fully non-inductive discharges the reconstructed q profile is flat or non-monotonic. In plasmas with significant LHCD the plasma evolves towards a non-inductive equilibrium but in certain discharges large MHD instabilities coincide with a significant reduction in current drive. The reconstructed driven current profile was measured over a variety of parameters including plasma current, launched n_∥ and LH power. The plasma density was varied over a range where previous work shows the current drive efficiency decreases precipitously from the classic efficiency scaling proportional to 1/ne. As the density is increased the LHCD modification of the current profile is observed to decrease, consistent with decreases in non-thermal ECE emission and hard X-ray production indicating negligible current drive.