Study of lower hybrid current drive efficiency over a wide range of FTU plasma parameters

The key quantities affecting the efficiency of Lower Hybrid (LH) radiofrequency waves in driving non-inductively the toroidal current in a tokamak have been recognized by means of a linear regression analysis over all the data available for the Frascati Tokamak Upgrade. The parameter space is bounded within the following ranges: line averaged plasma density 0.29 x 10(20) <= (n) over bar (e) <= 1.29 X 10(20) m(-3), central electron temperatures 1.1 <= T-e0 <= 7.4 keV, corresponding to volume averaged temperatures 0.27 <= (T-e) <= 1.2 keV, plasma current 0.3 I-p <= 0.7 MA, magnetic field 4 <= B-T0 <= 7.2 T, with a safety factor between 4.7 <= q(a) <= 10.7, LH power 0.4 <= P-LH <= 2.1 MW and LH parallel refraction index 1.32 <= N-parallel to 0 <= 2.42. The experimental current drive (CD) efficiency, reduced to the effective ion charge state Z(eff) = 1, varies for this data set within 0.12 <= eta(*)(CD) <= 0.34 A W-1 X 10(20) m(-2). A linear regression analysis gives a reliable scaling law for eta(*)(CD) with a correlation coefficient close to 0.9 that points out the importance of the various quantities. The CD efficiency is a significantly increasing function of (T-e) and B-T, and a decreasing one of q(a) and P-LH, while N-parallel to and (n) over bar (e) have limited influence. The physical reasons for the observed trend related to the variation of each parameter are recognized and discussed. The main causes are identified in the modification suffered by the N-parallel to spectrum along the ray trajectory before the power can be absorbed by the electrons and in the interaction with the edge plasma density fluctuations. The analysis also allows putting into evidence the synergy between the LH and electron cyclotron waves, when the latter are absorbed directly on the LH generated suprathermal electron tails and produce the highest values of eta(CD)*.