Scaling of Global LHCD Efficiency in Alcator C-Mod*

A database of global current-drive efficiency by Lower Hybrid waves has been assembled covering nine years of C-Mod operation. Plasma conditions were averaged over 50-ms time slices during equilibrated current-profile time periods, excluding transient events such as Prad spikes. The database comprises 1800 time slices spanning: $PLH<1.1$ MW, n$_{\mathrm{\vert \vert }}=$1.5-2.3, Ip $=$ 0.3-1.0 MA, nebar $=$ 0.35-1.5e20. Nine percent of the data points are approximately non-inductive ($\Delta $V/V$>$0.9), while 17 percent experience low m,n MHD that degrades the LHCD efficiency. During LHCD, a simple Spitzer model is used to estimate the residual inductively-driven current which scales the pre-LH current by the ratio of the loop voltage to the pre-LH loop voltage, correcting also for the change in conductivity. The current-drive efficiency is defined as $\eta \quad =$ nebar R I$_{\mathrm{LHCD}}$/P$_{\mathrm{LH}}$ [10$^{\mathrm{20}}$ MA/m$^{\mathrm{2}}$ MW], where I$_{\mathrm{LHCD}}$ is the current driven by LH waves and P$_{\mathrm{LH}}$ is the forward-directed LH power. In approximately non-inductive, MHD-free plasmas, the global current drive efficiency shows a striking positive correlation with plasma current, $\eta \quad =$ 0.065 $+$ 0.40 x Ip [MA], reaching a value of $\eta =$0.47 at Ip$=$1.02 MA. A positive but weaker correlation between $\eta $ and Teo does not explain the $\eta $ dependence on Ip. Preliminary GENRAY/CQL3D simulations at Ip$=$1.0 MA predict 900 kA of driven current versus 1000 kA observed. Comparisons of $\eta $ to numerical simulations over a wide parameter range will be discussed.