High harmonic fast wave heating and beam driven ion cyclotron emission behavior in the LHD

The electron heating with higher harmonic fast wave (HHFW) has been considered to be one of the useful approaches to heat electrons in the high density and high β regime, and initial direct electron heating with weak single-pass damping is achieved by the use of HHFW in the large helical device (LHD). ECH is worked as more effective than bulk electron heating and a substantial electron heating is observed in the central region of the plasma due to the synergy effect of ECH and HHFW. The heating efficiency of HHFW estimated by energy balance equations at break-in-slope using plasma stored energy which is measured by diamagnetic current. In these discharges, the heating efficiency of HHFW increases up to 0.55 as the electron density increases due to multi-pass damping. On the other hand, ion cyclotron emission (ICE), which produces fast wave of ion cyclotron range of frequencies (ICRF) and damping mechanism is similar to HHFW with weak single-pass damping, is observed in high ion temperature discharges. By connecting the ICRF heating antenna to the detector instead of the ICRF power supply, ICE is clearly detected as magnetic fluctuations at the resonance frequencies during high ion temperature discharge. The ICE intensity signal rapidly increases associated with the increase of central ion temperature which is measured with charge exchange spectroscopy. The ICE intensity has strong correlation to an ion temperature and it is interesting that the harmonic ICE signals strongly grow up rather than fundamental ICE signal. Since the radiation pattern in the direction of magnetic field is different in each ICE harmonic numbers for the ICE calculation in tokamak, this experimental result may indicate that the excited toroidal location of ICE is changed as ion temperature is increased in the LHD. (author)