Speaker
Description
At the Wendelstein 7-X (W7-X) stellarator, a Phase-Locked-Loop (PLL) system has been implemented to stabilize the frequency of megawatt-class gyrotrons. The PLL system actively controls the accelerating voltage of the diode-type Magnetron Injection Gun (MIG) to counteract any unwanted frequency changes of the gyrotron output millimeter-wave beam. The accelerating voltage, which is applied between the cathode and anode of the MIG, is controlled by the body power supply.
After first proof-of-principle experiments, which are described in [1], the PLL system is used for the Collective Thomson Scattering (CTS) diagnostic at W7-X [2]. One of the Electron Cyclotron Resonance (ECRH) gyrotrons is operated at 174 GHz with repeated pulses, which have a duration of 10 ms and are repeated every 200 ms. At the CTS receiver entrance, a notch filter cuts out the high-power gyrotron signal to prevent any damage. Using the PLL system, the signal-to-noise ratio of the CTS diagnostic can be significantly increased, since the gyrotron frequency can be matched exactly to the notch filter and the width of the notch filter can be reduced. The results of the gyrotron frequency stabilization for the CTS diagnostic during the last operation phase (OP) 2.1 at W7-X are presented.
Furthermore, the next steps for the gyrotron PLL system are elaborated. The major challenge is to reduce the remaining noise in the frequency spectrum, which are sidebands at 135 kHz and multiples of 3.3 kHz. During the experiments, it was concluded that these sidebands originate from the noise of the switching cathode power supply. To further improve the gyrotron frequency spectrum, a fast amplifier is implemented in series to the existing body power supply, which allows faster control of the body voltage in order to stabilize the electron accelerating votlage.
References
[1] L. Krier, et al., Fusion Engineering and Design, vol. 192, p. 113828 (2023)
[2] S. Ponomarenko, et al., Review of Scientific Instruments, vol. 95, p. 013501 (2024)
