Observation of ion heating using the difference frequency of two ICRF waves in GAMMA 10/PDX

GAMMA 10/PDX is a linear plasma confinement device that mainly uses slow waves in ion cyclotron range of frequencies (ICRF) for heating, and it can achieve a high ion temperature that is in the range of several keV. Although slow waves are effective for ion heating, it is difficult to excite them with antennas at high densities, such as those over 10¹⁹ m⁻³. In this study, we considered a way to excite a slow waves using the frequency difference of input waves. Two fast waves having different frequencies, which are both adequate for high-density plasma, were applied with antennas to excite a slow wave as a difference-frequency wave between the fast waves. As a result, the excitation of difference-frequency waves inside the plasma was confirmed from magnetic probe and reflectometry measurements, and an increase in diamagnetism was also observed. These results firstly demonstrate the possibility of slow-wave heating using a DF wave in a high-density linear plasma, where direct slow-wave heating is not feasible. (author)