Past and future upgrades of the gyrotron high voltage cathode power supplies at the Forschungszentrum Karlsruhe

The high voltage/high power DC-cathode power supply for the Gyrotron Test Facility at the Forschungszentrum in Karlsruhe consists of a 12-pulse thyristor star-point-controller, a 130 kV capacitor-bank followed by a tetrode regulator. Originally designed for 80 kV/30 A CW (continous-wave) operation, its operating regime has been extended in line with gyrotron development to pulses of 65 kV/45 A/3 min and more recently to 65 kV/80 A/10 s without any changes to the main load bearing components (thyristors, transformers or power tetrode). This allows testing of gyrotrons in the 0.5 MW (CW), 1 MW (3 min) and 2 MW (10 s) output power range. per describes the system, its operation and some critical aspects of the last upgrade, such as the issue of harmonics in the 20 kV distribution mains-grid, the dynamic response of the thyristor-controller and the avoidance of microwave parasitic oscillations in the high power tetrode (a CQK-200-4, developed by ABB, Switzerland and now produced by Thales Electron Devices, France). Finally, an outlook is given on the extension-plans of the FZK Gyrotron Test Facility, namely to procure a Pulse-Step-Modulator (PSM) cathode power supply for 100 kV/100 A capable of CW operation for the development and testing of gyrotrons for fusion reactors (i.e. DEMO), with up to 4 MW CW output power. Some of the advantages and disadvantages of a PSM versus an analogue regulator system are being discussed.