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Frequency Measurements of the 110 GHz Gyrotron at TEXTOR

High resolution frequency measurements during TEXTOR plasma operation have been carried out of the 110 GHz gyrotron as used for fast ion collective Thomson scattering diagnostics. A pickoff sample of the gyrotron beam was frequency downshifted by both homodyne and heterodyne methods and fast Fourier transformed (FFT) by a 300 MHz bandwidth digitizing oscilloscope. The gyrotron frequency was found to be relatively clean and stable from plasma shot to plasma shot except for ±18 MHz components at about -40 dB below peak in some of the data. During a single plasma shot a downward frequency drift of 14 MHz was observed over a 936 ms period of about 90, 2 ms gyrotron pulses. An instrumentally limited instantaneous linewidth of <100 kHz at half maximum was also observed. The TEXTOR ICRH system did not fire reliably during these measurements to asses its effect on the gyrotron spectrum. Introduction The application of gyrotron radiation to fast ion collective Thomson scattering (CTS) diagnostics [1] requires precise knowledge and control of the gyrotron frequency and spectrum. The gyrotron frequency must be accurately tuned to the notch filter frequency of the receiver system and the gyrotron linewidth must be less than 100 MHz over a large dynamic range. Also there can be no spurious frequency components for a bandwidth of about ± 5 GHz around the center frequency to levels many orders of magnitude below the peak center frequency. Many factors determine the gyrotron frequency and spectrum. The gyrotron frequency is set by the dimensions of the resonator cavity and through the control of the operating parameters including the magnetic field, beam voltage, and current [2]. Suppression of spurious modes is also accomplished through control of the gyrotron operating parameters. Typically, the gyrotron operating parameters for optimum CTS frequency and clean spectrum do not coincide with the operating parameters for maximum gyrotron output power. In addition to the gyrotron operating parameters, external interference is possible in the environment of a major tokamak experimental facility from other high power electrical systems. Of particular concern is the ICRH system at TEXTOR interfering directly with the gyrotron. This report summarizes high resolution spectrum measurements over a 300 MHz bandwidth of the 110 GHz gyrotron obtained during TEXTOR plasma shots.