Abstract
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 TE22,2was adopted for this gyrotron to avoid mode competition. Careful frequency measurement has proved purely single mode oscillation of the TE22,2mode 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.
Highlights
collective Thomson scattering (CTS) diagnostics using gyrotrons for electron heating around 100 GHz have been developed [1, 2]
We subsequently developed a fundamental harmonic prototype gyrotron with a whispering gallery mode (WGM) and used it to verify the design concept for stable single-mode oscillation [5]
Sub THz CTS experiment is under preparation with a pulse gyrotron in the 300 GHz band with collaboration between FIR UF and NIFS
Summary
CTS diagnostics using gyrotrons for electron heating around 100 GHz have been developed [1, 2]. The measured frequency was 303.3 GHz when Vk = 62 kV and the magnetic field at the cavity B = 11.59 T This frequency is equal to the designed TE22,2 mode frequency to within an accuracy of the order of 10 MHz. Frequency control is very important for application to CTS because a narrow band notch filter is used. At the first stage of the 300 GHz CTS experiment, an existing 3.5 inch CW line is used for power transmission Since this waveguide is not optimized for 300 GHz but for lower frequency gyrotrons for electron heating. It is composed of two elliptic mirrors to tailor the beam radiated from the gyrotron to a Gaussian beam with the waist at the waveguide entrance position and two plane mirrors to adjust the beam direction
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