Measurement of the lower hybrid driven current profile density dependence on Alcator C-Mod

The plasma current profile has been reconstructed in plasmas with significant lower hybrid current drive (LHCD) using constraints from an improved motional Stark effect (MSE) diagnostic on Alcator C-Mod. The reconstructions demonstrate the application of LHCD significantly broadens the current profile, increasing q0 above 1 resulting in the suppression of sawteeth. Time resolved measurements of the current dynamics agree with the expected timescales. The current profile has a significant on-axis component in moderate density plasmas where LHCD drives 100{\%} of the current non-inductively for several current penetration times. The MSE-constrained reconstructions show a loss in current drive efficiency as the plasma density is increased consistent with previous 0-D observations. Additionally, the driven current profile moves radially outward as the density is increased. The current profile dependence on launched n$_{\vert \vert}$ is also discussed. These observations will constrain proposed theoretical models and simulations of LHCD and its observed degradation. This work is supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466.