X-Band LLRF Developments for High Power CLIC Test Stands and Waveguide Interferometry for Phase Stabilisation

Edwards, Amelia and Dexter, Amos and Catalan-Lasheras, Nuria (2022) X-Band LLRF Developments for High Power CLIC Test Stands and Waveguide Interferometry for Phase Stabilisation. PhD thesis, Lancaster University.

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Abstract

This thesis describes the upgrade of the first high power X-band RF test for high gradient accelerating structures at CERN, as required for the e+ e- collider research program; Compact Linear Collider, CLIC. Significant improvements to the control system and operation of the first test stand, Xbox-1, are implemented. The design and commissioning of the new Low Level Radio Frequency, LLRF, system is described in detail. The upgrade also encompasses software, interlock systems, timing, safety and control. The new LLRF requires an up-convertor to convert an input signal at 187.4 MHz to 11.806 GHz. The most common method is a phase locked loop, PLL, an alternative method was envisioned which uses single side-band up-convertor. This necessitated the design and manufacture of a custom cavity filter. The up-convertor and PLL are compared and both are implemented in the new LLRF. The new LLRF system is implemented at Xbox1 and used to RF condition a 50 MW CPI klystron, the final output power was 45 MW for a 50 ns RF pulse length. The phase and amplitude of the LLRF, TWT and klystron are characterised with both the PLL and up-convertor. The klystron phase stability was studied using a sensitivity analysis. The waveguide network between the klystron and the accelerating structures is approximately 30 m. This network is subject to environmental phase changes which affect the phase stability of the RF arriving at the structures. A single path inteferometer was designed which will allow a phase measurement pulse at a secondary frequency to be injected into the waveguide network interleaved with klystron pulses. The interferometer is commissioned in the lab and low power measurements validate its operation. The system is then integrated into the high power network at Xbox1 and used to measure phase shifts in the waveguide network which are correlated with temperature.