The lower hybrid current drive experiment was designed in order to test this method at particle density values higher than the other experiments and comparable with those of ITER.
The choice of such a frequency range has required a major technological effort, in particular with the new development of gyrotrons by Thales (former Thompson) Tube Electroniques.
The lower hybrid experiment started on FTU at the end of 1992 with a limited amount of power. During 1993 a discharge was produced with the plasma current entirely driven by lower hybrid waves at a moderate density (n~0.4x1020m-3). Since then, the system has been upgraded (up to 2.8 MW by the end of 1996). Maximum LH power absorbed by plasma has been 2.5 MW in 2001. So far, the interaction between lower hybrid waves and electrons has been observed up to density in the range n~1020m-3, without appreciable deterioration in the current drive efficiency. It should be stressed that this is the same density value envisaged for ITER.
In order to compare the FTU results with the results of other tokamaks, we recall the usual definition of the current drive efficiency as follows
The efficiency is expected to be an increasing function of the plasma temperature, according to general theoretical considerations. The obtained current drive efficiency in various experiment is shown in the figure below.
Larger device such as JET and JT-60U have observed larger values of the current drive efficiency, possibly due to the larger temperature values achievable in these devices. The shaded region corresponds to the expected domain which can be explored by FTU when the larger available heating power will allow the achievement of substantially larger temperatures.
Tests of technical solutions for the LH launching structures foreseen for ITER have also been programmed for FTU. In particular a Passive Active Multijunction grill (PAM) has been designed and built to study its operation and efficiency on FTU plasmas.