At the Frascati Tokamak Upgrade (FTU), scientists are investigating the effectiveness of Lower Hybrid Current Drives (LHCD) for driving plasma current. Recent experiments show that efficiency appears to decrease with increasing plasma density faster than theoretically predicted. Some concern has therefore been raised as to whether LHCD is a method capable of efficiently driving current in ITER.

A theoretical model that originated in Frascati has identified this reduced LH performance at high density as being due to unsuitable plasma edge conditions. It predicted that higher operating edge temperatures would allow LH waves to recover the expected current drive efficiency.

On this basis, new experiments have recently been performed on FTU, designed to develop methods useful for driving current on ITER. Two plasmas were created at FTU – one with a density profile close to the ITER steady state scenario and the other with a lower density at the plasma periphery. The first scenario exhibits high edge temperatures, while the other does not. Although detailed analysis is still being conducted, the experiment clearly shows that LH waves could indeed drive the plasma current in the first scenario. Hard x-ray radiation emitted by LH generated fast electrons is clearly measured. Despite the lower density in the second scenario, negligible or no effects are observed when operating under standard conditions (lower edge temperature). These results indicate that LH should be an efficient method of driving current in ITER steady state plasma, where high electron temperatures are expected.

Thanks to Angelo A. Tucillo, ENEA Frascati, for his contribution to this article