Variation of Heating Efficiency of Magnetically Sheared CHS Plasmas by Polarization Control of 106GHz EC-Wave

To clarify the effect of polarization on electron cyclotron heating (ECH) in magnetized plasmas, experiment controlling the polarization of injected EC-waves is carried out in Compact Helical System (CHS). In the experiment, plasmas are generated and sustained only with 106.4 GHz ECH power. Magnetic field at the magnetic axis is 1.9 T so that the wave frequency is second harmonic. The optimum direction of linear polarization for the shortest time-delay of density start-up from the start of power injection and the optimum direction for the highest electron temperature and plasma stored energy during plasma duration show clear difference. The difference is attributed to the CHS magnetic configuration with strong shear and the plasma volume expansion from magnetic axis to the last closed flux surface

First Observation of ECH by Electron Bernstein Waves Excited via X-B Mode Conversion Process in LHD

In a magnetic field configuration of the Large Helical Device (LHD), when the extraordinary mode (X-mode) waves are obliquely injected from a bottom antenna, it can directly access the upper hybrid resonance (UHR) layer from the high field side and excite electron Bernstein waves (EBWs) without the need for any additional reflecting mirror antenna. A localized power absorption is observed in the low field side of the electron cyclotron resonance (ECR) layer. This result suggests electron cyclotron heating (ECH) by mode converted EBWs excited via X-B mode conversion process

Alignment Method of ECH Transmission Lines Based on the Moment and Phase Retrieval Method Using IR Images

New alignment methods of millimeter-wave transmission lines for Electron Cyclotron resonance Heating are proposed and evaluated on a high power level. These methods are based on the measured data of infrared images on the target, which is irradiated by the high power millimeter-waves at several positions. The first and second moments and retrieved phase obtained from these data are used to determine the propagation direction of the millimeter-wave beam along the waveguide axis. It is demonstrated that these methods have sufficient resolution to discriminate 0.1 deg., which is required to restrict the transmission loss below 1% over the 100 GHz range

Developments for collective Thomson scattering equipment with a sub-THz gyrotron in LHD

Plan of collective Thomson scattering (CTS) experiment for the Large Helical Device (LHD) in NIFS with a 303 GHz gyrotron is under way. Use of a sub-THz gyrotron expands the CTS-applicable region of plasma parameters. In LHD, sub-THz CTS can be applied to the high density operation region, plasmas with impurity hole, etc. Moreover, sub-THz CTS is expected to be free from ECE noise. Its “collective” use with 77 GHz and 154 GHz CTS will compose a powerful diagnostic system. A high power sub-THz gyrotron with a frequency of 303 GHz has been developed. Its maximum power is 320 kW. It oscillates in pulse mode and the maximum pulse width is around 100 μs, which is sufficient for use in CTS experiments. A whispering gallery mode TE22,2 was adopted for this gyrotron to avoid mode competition. Careful frequency measurement has proved purely single mode oscillation of the TE22,2 mode including turn-on and turn-off phases of the oscillation pulse. This is consistent with mode competition calculations taking account of a finite voltage rise time. A low loss transmission line is necessary for CTS. We have two possibilities. One is a new line with 1.25 inch corrugated waveguides that are optimized for the 300 GHz band. Transmission test with the 303 GHz gyrotron has been carried out and a sufficiently low loss coefficient has been confirmed. The other is to use an existing line with 3.5 inch corrugated waveguides for lower frequencies such as 77 GHz and 154 GHz. Transmission test has been carried out with the 303 GHz gyrotron and a sufficiently low loss coefficient has been confirmed also for 3.5 inch corrugated waveguides. An existing line with 3.5 inch corrugated waveguides will be used in the initial phase of 303 GHz CTS experiment

Neutral Particle Measurement in High Z Plasma in Large Helical Device

In Large Helical Device (LHD), the discharges of high Z are often used in order to obtain high ion temperature. The ion density is relatively smaller than the electron density in high Z plasma. Therefore the high ion temperature can be obtained since the input energy per ion atom is large. In the charge exchange neutral diagnostic, the ion temperature can be obtained by observing the spectra of neutral particles, which are generated by the charge exchange between the background neutral and the plasma ion, and assuming theMaxwellian distribution of the spectra. In calculation, we also consider the charge exchange between the partially ionized high Z ion and proton. The contribution of neutral particle from the charge exchange between argon and proton is small near the center but cannot be neglected near the peripheral region comparing with that of hydrogen charge exchang