DIRECT LOSS OF ENERGETIC ELECTRONS DURING LOWER HYBRID CURRENT DRIVE IN JT-60U

Direct loss of energetic electrons during lower hybrid current drive in JT-60U is identified through measurements of X-ray signals from the divertor plates. It is found that the lost power decreases with increasing electron density and with increasing launched wave N(I) and is characterized by the slowing down time of the electrons accelerated by lower hybrid waves. At n(e)BAR approximately 0.5 x 10(19) M-3 and N(I) approximately 1.4, about 5-10% of the injected power is lost through the energetic electrons; the maximum current drive efficiency is obtained at a plasma current of 1.2 MA in JT-60U. It is also found that the confinement of high energy electrons is improved by increasing the plasma current. These results suggest that the power lost directly through energetic electrons will be negligible in ITER in steady state operation.