Investigation of the wave spectrum gap in the JT-60 LHCD plasma

To understand the physics of lower hybrid current drive (LHCD), an investigation into the wave spectrum gap was carried out in JT-60, using two lower hybrid wave launchers. From one launcher a wave with high phase velocity was injected; this wave can interact with high energy electrons to drive the current. Simultaneously, from the other launcher a wave with slower phase velocity was injected; this wave can interact with the bulk electrons. A high energy neutral beam was also injected in order to investigate the interaction of lower hybrid waves with ions. The results show that a fraction of the high phase velocity wave which is upshifted to fill the spectrum gap in the case where only one wave is injected is replaced by a slower wave when two waves are injected. As a result, the tail electron parameters (tail electron density and/or average tail electron energy) were improved while the interaction of the waves with ions decreased, because the frequency of the slower waves was chosen so that the wave did not interact with ions. (author). Letter-to-the-editor. 10 refs, 3 figs