Frequency measurements of the gyrotrons used for collective Thomson scattering diagnostics at TEXTOR and ASDEX Upgrade

Abstract

High resolution frequency measurements of the 110 GHz gyrotron at TEXTOR and the 105 GHz mode of the two-frequency gyrotron ͑Odissey-1͒ at ASDEX Upgrade ͑AUG͒ have been made in support of fast ion collective Thomson scattering diagnostics. Measurements were done by harmonic heterodyne methods using both fast Fourier transform spectroscopy with digital oscilloscopes and fast scanning spectrum analyzers. Accurate frequencies were obtained with a frequency counter. At TEXTOR, at 180 kW forward power the starting frequency was 109.970± 0.005 GHz and chirped down as much as 27 MHz depending on the duty factor. At AUG, at 500 kW forward power the frequency started at 104.786 GHz and chirped down 104 MHz, with 90% of the chirp occurring in the first 100 ms. Plasma perturbation of the TEXTOR gyrotron was observed when both ion cyclotron resonance heating antennas and neutral beam injection were operating, producing modulation at 29 and 58 MHz in the gyrotron output. Each gyrotron was observed to have an instrumental measurement limited linewidth of 120 kHz full width at half maximum.

Dynamics of fast ions during sawtooth oscillations in the TEXTOR tokamak measured by collective Thomson scattering

“…The NBI acceleration voltage is kept at 50 kV for all discharges and the injection power is typically about 1.3 MW. The CTS diagnostic uses a gyrotron with ν i = 109.97 GHz and P i ≈ 150 kW for probing radiation [36]. The probe and receiver beam patterns are arranged such that the scattering volume has a typical radial extension ∼10 cm.…”

Recent development of collective Thomson scattering for magnetically confined fusion plasmas

“…Once a stable gyrotron power level is reached the gyrotron frequency generally drifts additional tens of MHz [19]. The gyrotron power level is generally about 16 orders of magnitude above the power level of the detected CTS signal.…”

Spectral Characteristics of a 140-GHz Long-Pulsed Gyrotron

“…To keep the 10-kHz resolution of the locked LO, the sweep time of the SA (Agilent E4404B) was set to over 1 ms, and the resolution bandwidth is fixed at 10 kHz. For the long-pulse gyrotrons, a frequency downshift with longer time constant (> 1 s) is a common feature [16]- [18], but the physical mechanism of the slow-frequency downshift is still not completely understood, as mentioned in [19] and [20]. Possible explanations for the frequency downshift in this system are the following: 1) thermal expansion of the resonant cavity due to ohmic heating of the cavity walls; 2) neutralization of dc space charge fields of electron beam by impact ionization; and 3) resonance frequency modification of the cavity by the background plasma.…”