Fast wave current drive experiment on JFT-2M

Current drive experiment by 200 MHz fast waves was carried out in JFT‐2M tokamak in order to investigate a capability of the fast wave current drive in hot and dense plasma. Two phased loop antennas were employed to excite fast waves, which had a loading resistance of 5 ohms. A plasma current of 30 kA was generated by an rf power of 40 kW at a density of 2×1012 cm−3. It is probable that the slow waves contribute to current drive at the low density below the LHW current drive density limit. The efficiency as high as that of slow LHW at a low density region decreases rapidly with the density and no rf current was generated above 3×1012 cm−3. This density limit is two times higher than that expected from LHW current drive scaling at the same frequency. It seems that the occurence of the density limit relates with the accessibility of the launched waves to the high density region and a lack of rf power. When the same loop antennas were employed to excite fast waves at a frequency of 750 MHz, we have not obser...