High power 303 GHz gyrotron for CTS in LHD

Abstract

A high-power pulsed gyrotron is under development for 300 GHz-band collective Thomson scattering (CTS) diagnostics in the Large Helical Device (LHD). High-density plasmas in the LHD require a probe wave with power exceeding 100 kW in the sub-terahertz region to obtain sufficient signal intensity and large scattering angles. At the same time, the frequency bandwidth should be less than several tens of megahertz to protect the CTS receiver using a notch filter against stray radiations. Moreover, duty cycles of ∼ 10% are desired for the time domain analysis of the CTS spectrum.At present, a 77 GHz gyrotron for electron cyclotron heating is used as a CTS wave source in the LHD. However, the use of such a low-frequency wave suffers from refraction, cutoff and absorption at the electron cyclotron resonance layer. Additionally, the signal detection is severely affected by background noise from electron cyclotron emission. To resolve those problems, highpower gyrotrons in the 300 GHz range have been developed.In this frequency range, avoiding mode competition is critical to realizing high-power and stable oscillation. A moderately over-moded cavity was investigated to isolate a desired mode from neighbouring modes. After successful tests with a prototype tube, the practical one was constructed with a cavity for TE 22,2 operation mode, a triode electron gun forming intense laminar electron beams, and an internal mode convertor. We have experimentally confirmed single mode oscillation of the TE 22,2 mode at the frequency of 303.3 GHz. The spectrum peak is sufficiently narrow. The output power of 290 kW has been obtained at the moment.

Development of 300 GHz Band Gyrotron for Collective Thomson Scattering Diagnostics in the Large Helical Device

“…Figure 1 shows the high power oscillation test results in addition to previously reported data [10]. Specifically, a new maximum power of more than 320 kW was achieved [11].…”

“…An electron gun was designed and optimized for the TE 22,2 mode using the design principle reported in [9]. High-quality electron beams with appropriate α values and small velocity spreads can be generated for I B up to 20 A [10].…”

“…A Gaussian like radiation pattern similar to that expected from the mode converter design calculations was obtained. The radiation pattern was somewhat elliptic and a small fraction of higher order modes was observed [10]. Quantitative mode content analysis is the next task.…”

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

“…[19]. The design predicted electron beams of high quality with the expected  values and sufficiently small velocity spreads up to IB = 20 A [20]. Figure 2 is a picture of the fabricated gyrotron.…”

“…Oscillation tests have been carried out and a maximum power higher than 320 kW for Vk = 65 kV and IB = 15 A was obtained [20,21]. The oscillation efficiency was higher than 30%.…”