Issue |
EPJ Web of Conferences
Volume 32, 2012
EC-17 – 17th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating
|
|
---|---|---|
Article Number | 04002 | |
Number of page(s) | 10 | |
Section | Technology session | |
DOI | https://doi.org/10.1051/epjconf/20123204002 | |
Published online | 06 September 2012 |
https://doi.org/10.1051/epjconf/20123204002
Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons
1 Karlsruhe Institute of Technology (KIT): Institut für Hochfrequenztechnik und Elektronik (IHE), Engesserstraße 5, 76131 Karlsruhe, Germany
2 Karlsruhe Institute of Technology (KIT): Institute for Pulsed Power and Microwave Technology (IHM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a e-mail: andreas.schlaich@kit.edu
At IHM/KIT, high power gyrotrons with conventional cavity (e.g. 1 MW CW at 140 GHz for the stellarator Wendelstein 7-X) and coaxial cavity (2 MW shortpulse at 170 GHz for ITER) for fusion applications are being developed and verified experimentally. Especially with respect to the problem of parasitic RF oscillations in the beam tunnel of some W7-X tubes, investigations of the gyrotron RF output spectrum have proved to be a valuable source of diagnostic information. Signs of transient effects in millisecond pulses, like frequency switching or intermittent low-frequency modulation, have indicated that truly time-dependent measurements with high frequency resolution and dynamic range could give deeper insight into these phenomena. In this paper, an improved measurement system is presented, which employs a fast oscilloscope as receiver. Shorttime Fourier transform (STFT) is applied to the time-domain signal, yielding time-variant spectra with frequency resolutions only limited by acquisition length and STFT segmentation choice. Typical reasonable resolutions are in the range of 100 kHz to 10 MHz with a currently memory-limited maximum acquisition length of 4 ms. A key feature of the system consists in the unambiguity of frequency measurement: The system receives through two parallel channels, each using a harmonic mixer (h = 9 – 12) to convert the signal from RF millimeter wave frequencies (full D-Band, 110 – 170 GHz) to IF (0 – 3 GHz). For each IF output signal of each individual mixer, injection side and receiving harmonic are initially not known. Using accordingly determined LO frequencies, this information is retrieved from the redundancy of the channels, yielding unambiguously reconstructed RF spectra with a total span of twice the usable receiver IF bandwidth, up to ≈ 6 GHz in our case. Using the system, which is still being improved continuously, various transient effects like cavity mode switching, parasitic oscillation frequency variation, and lowfrequency modulation have been documented.
© Owned by the authors, published by EDP Sciences 2012
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.