High Power X-band RF Test Stand Development and High Power Testing of the CLIC Crab Cavity Benjamin J. Woolley This thesis is submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy August 2015 Abstract This thesis describes the development and operation of multiple high power X-band RF test facilities for high gradient acceleration and deflecting structures at CERN, as re- quired for the e+ e- collider research programme CLIC (Compact Linear Collider). Signif- icant improvements to the control system and operation of the first test stand, Xbox-1 are implemented. The development of the second X-band test stand at CERN, Xbox-2 is followed from inception to completion. The LLRF (Low Level Radio Frequency) system, interlock system and control algorithms are designed and validated. The third test stand at CERN, Xbox-3 is introduced and designs for the LLRF and control systems are pre- sented. The first of the modulator/klystron units from Toshiba and Scandinova is tested. CLIC will require crab cavities to align the bunches in order to provide effective head- on collisions. An X-band travelling wave cavity using a quasi-TM11 mode for deflection has been designed, manufactured and tested at the Xbox-2 high power test stand. The cavity reached an input power level in excess of 50 MW, at pulse widths of 150 ns with a measured breakdown rate (BDR) of better than 10 -5 breakdowns per pulse (BDs/pulse). At the nominal pulse width of 200 ns, the cavity reached an input power level of 43 MW with a BDR of 10-6 BDs/pulse. These parameters are well above the nominal design pa- rameters of an input power of 13.35 MW with a 200 ns pulse length. This work also de- scribes surface field quantities which are important in assessing the expected BDR when designing high gradient structures. i Acknowledgements I would like to thank Amos Dexter for his supervision and for giving me the oppo r- tunity to partake in this PhD. I would also like to thank those others at Lancaster Uni- versity who have provided guidance including Graeme Burt, Praveen Ambattu and Shokrollah Karimian. In particular to Graeme and Praveen, who designed such an inter- esting cavity forming the basis of much of the work presented in this thesis. My special thanks go to Igor Syratchev for his supervision during my time at CERN. His guidance and ideas always left me with many new avenues of research to pursue and much to ponder about! Thanks also to Walter Wuensch and Alexej Grudiev, whose stimulating conversations led to the development of many of the ideas presented here. Thanks to Jan Kovermann for his teaching on the operation of Xbox-1 and his ‘handing of the baton’ to me to be- come the new Xbox control system designer and operator at CERN. Also thanks to everyone who helped to make operating and designing the test stands possible including Gerard McMonagle, Alberto Degiovanni, Nuria Catalan Lasheras, Stef- fen Doebert, Andrey Olyunin, Luis Navarro, Jorge Giner Navarro and Esa Paju. Further thanks go to Joseph Tagg from National Instruments for his significant con- tribution in the programming of the PXI hardware using his vast LabVIEW expertise. I would also like to thank the LLRF experts at CERN: Stephane Rey, Luca Timeo, Heiko Damerau and Alexandra Andersson for their discussions and help with LLRF system de- signs. Thanks also to Rolf Wegner for his expertise in tuning high gradient structures and in particular his management in tuning the crab cavity. Thanks also to Wilfrid Farabolini and Robin Rajamaki for their work on the breakdown cell location algorithms and results. Thanks to Germana Riddone and Anastasia Solodko for overseeing structure production and manufacturing and providing the test stands with devices to test. Thanks to Marek Jacewicz and Roger Ruber for adding such an interesting experiment to the test stands; the dark current spectrometer. Thanks to Valery Dolgashev, Sami Tantawi, Matt Franzi and the ASTA team for being so hospitable during my short stay at SLAC. I would also like to show my gratitude to STFC for granting me the scholarship and LTA funding that has supported me for the past 4 years. The research leading to these results has received funding from the European Com- mission under the FP7 Research Infrastructures project EuCARD-2, grant agreement no.312453. Finally, I would like to thank my family for their support throughout, especially to my girlfriend Eve Harrison whose patience and care has kept me content throughout my time researching this PhD. ii Contents Abstract .............................................................................................................. i Acknowledgements ............................................................................................... i List of Figures....................................................................................................vii List of Tables .................................................................................................... xix 1 Introduction ................................................................................................1 1.1 Lepton colliders ........................................................................................1 1.2 CLIC.......................................................................................................2 1.3 Accelerator Technology Choice ..................................................................3 1.4 RF Breakdown .........................................................................................5 1.5 Fabrication of High Gradient Structures.....................................................5 1.6 RF Conditioning of High Gradient Structures .............................................6 1.7 Standalone X-band Test Stands .................................................................7 1.7.1 Test Stand layout ..............................................................................8 1.8 Crossing angle and Crab Cavities...............................................................8 1.9 Other Applications of high gradient X-band technology ...............................9 1.10 High phase stability ............................................................................ 10 1.11 Summary ........................................................................................... 10 Chapter 2 .......................................................................................................... 12 2 Xbox-1: CERN’s first 12 GHz standalone test stand...................................... 12 2.1 Reasons for standalone test stands ........................................................... 12 2.2 Test Stand Design: High Level RF ........................................................... 13 2.2.1 Klystron ......................................................................................... 14 2.2.2 Modulator ....................................................................................... 15 2.2.3 Pulse Compressor Operation with a 180º Phase Flip.......................... 15 2.2.4 Pulse Compressor Operation with a Phase Ramp ............................... 19 2.2.5 Pulse Compressor Cavity Design ....................................................... 22 2.2.6 Waveguide Network Components ...................................................... 23 2.3 Test Stand Design: LLRF and Diagnostics................................................ 25 2.3.1 Pulse forming network (PFN) ........................................................... 25 iii 2.3.2 RF signal acquisition ....................................................................... 26 2.4 Test Stand Performance ......................................................................... 27 2.4.1 Calibration of the RF acquisition system ........................................... 27 2.4.2 Pulse forming network (PFN) Upgrade ............................................. 30 2.4.3 Pulse Compressor Operation ............................................................ 32 2.4.4 Structure Conditioning Algorithm .................................................... 36 2.5 Conclusion ............................................................................................ 38 Chapter 3.......................................................................................................... 40 3 Xbox-2 ..................................................................................................... 40 3.1 Increase of testing capacity ..................................................................... 40 3.2 Xbox-2 general layout ............................................................................ 40 3.2.1 Klystron/modulator ........................................................................ 41 3.2.2 Pulse Compressor and Waveguide components .................................. 42 3.2.3 Bunker and Device under Test ......................................................... 43 3.3 LLRF System Development .................................................................... 43 3.3.1 LLRF System Requirements............................................................. 43 3.3.2 LLRF System PXI Hardware ........................................................... 44 3.3.3 LLRF Up-conversion Hardware ........................................................ 45 3.3.4 LLRF Down-conversion Hardware .................................................... 47 3.3.5 LLRF system Tests ......................................................................... 48 3.3.6 RF Interlock Detection