High-power pulsed gyrotron for 300 GHz-band collective Thomson scattering diagnostics in the Large Helical Device

Abstract

A high-power pulse gyrotron was developed to generate a probe wave for 300 GHz-band collective Thomson scattering (CTS) diagnostics in the Large Helical Device. In this frequency range, avoiding mode competition is critical to realizing high-power and stable oscillation with a narrow frequency bandwidth. A moderately over-moded cavity was investigated to ensure sufficient isolation of a desired mode from neighbouring modes, and to achieve high power output simultaneously. A cavity with the TE 14,2 operation mode, a triode electron gun with an intense laminar electron beam, and an internal mode convertor were designed to construct a prototype tube. It was experimentally observed that oscillation of the TE 14,2 mode was strong enough for mode competition, and provided high power with sufficient stability. The oscillation characteristics associated with the electron beam properties were compared with the numerical characteristics to find an optimum operating condition. As a result, single-mode operation with maximum output power of 246 kW was demonstrated at 294 GHz with 65 kV/14 A electron beam, yielding efficiency of ∼27%. The radiation pattern was confirmed to be highly Gaussian. The duration of the 130 kW pulse, which is presently limited by the power supply, was extended up to 30 µs. The experimental results validate our design concept and indicate the potential for realizing a gyrotron with higher power and longer pulse toward practical use in 300 GHz CTS diagnostics.

Recent progress in fast-ion diagnostics for magnetically confined plasmas

“…Examples are silicon detectors, for example, at JET, where neutrons with an energy exceeding a threshold of 7 MeV induce (n, ) and (n,p) reactions leading to signals which are detected in pulse mode (Jarvis 1994). Alternatively, scintillating fibers have been used (Nishitani et al 1996;Wurden et al 1995). Such instruments are based on detecting the light produced by a proton using photomultipliers.…”

Gyrotrons for High-Power Terahertz Science and Technology at FIR UF

“…It allows studying the distributions of thermal and fast ions with a high spatial resolution. Recently, several novel gyrotrons with output parameters that are appropriate for Collective Thomson Scattering (CTS) diagnostic in the Large Helical Device (LHD) at NIFS have been developed and used in a series of experiments [52,53]. One of them operates at 0.398 THz (second-harmonic) and delivers a power of 83 kW.…”

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

“…However, a competing fundamental harmonic mode prevented further increase in power [4]. 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]. The same design concept to use WGM mode was adopted in the design of a practical gyrotron and fabricated for use in CTS diagnostics in the LHD.…”

“…The oscillation mode is the TE22,2 mode, which is a moderately high order WGM similar to the TE14,2 mode used in the prototype gyrotron. WGM mode is suitable to satisfy requirements for mode competition avoidance and low cavity surface ohmic loss simultaneously [5]. WGM modes were used in the initial stage of the development of gyrotrons for fusion plasma heating and stable oscillations without mode competition were shown [7,8].…”