Wave accessibility effects on lower hybrid current drive in JT-60U

The effects of the wave accessibility condition on lower hybrid current drive (LHCD) are investigated in JT-60U. The density dependence of the detectable maximum photon energy of hard X rays in LHCD plasmas is consistent with that of the maximum electron energy which is limited by the accessibility condition. The current drive efficiency and the driven current profile are affected by the accessibility condition. Inaccessible wave power enhances impurity content, particle recycling and main plasma radiation loss, and in extreme cases triggers a MARFE. The wave intensity on the plasma periphery, which is detected by a Langmuir probe located on the divertor plate, is also consistent with an inaccessible power fraction going into the plasma interior. A ray trace analysis taking account of toroidal effect indicates that the inaccessible wave power is localized on the plasma periphery and finally propagates into the divertor plasma region