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

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 s, 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.

Oscillation Characteristics of a High Power 300 GHz Band Pulsed Gyrotron for Use in Collective Thomson Scattering Diagnostics

“…Since the thickness of the vacuum window is optimized for the TE 22,2 mode, the amplitude of the TE 21,2 mode is very weak owing to the large reflection off the vacuum window. Strong oscillation of the TE 21,2 mode can be obtained with a double disk configuration [28]. Details of the double disk effect will be published elsewhere.…”

High-Frequency MW-class Coaxial Gyrotron Cavities Operating at the Second Cyclotron Harmonic

“…With the emergence of high-field tokamaks and the so-called high-field path to fusion energy [2], the need for MW-class gyrotrons operating at higher frequencies >250 GHz is becoming timely [3]. Such high-frequency high-power gyrotrons are also attractive for fusion plasma diagnostics based on collective Thomson scattering (CTS), since the submillimeter band exhibits low noise from electron cyclotron emission [4] and the high output power improves the system's signal-to-noise ratio. However, operating a MW-class gyrotron at such high frequencies requires increased magnetostatic field values (>10 T), which are currently not possible to produce cost-effectively from superconducting magnet systems with large bore-hole.…”