Design of a high-power 48GHz gyroklystron amplifier for accelerator applications

Student thesis: Doctoral Thesis

Abstract

As the technology of radiofrequency linear accelerators (RF linacs) continues to improve, higher frequency acceleration systems become of interest as the achievable acceleration gradient has a dependence on frequency. Using a high driving frequency requires the consideration of many technological challenges. One such challenge is mitigating the effect of nonlinearities introduced during the electron acceleration and bunching process. To counteract the nonlinearity, an additional cavity at a harmonic of the main driving frequency can be included. This technique is known as harmonic linearisation. In existing C-band systems, harmonic linearisation can be achieved with an X-band structure, but if the main frequency is X-band, the lineariser must be Ka-band or higher. Linear klystrons are a well-developed technology and can reliably deliver tens of MW at X-band, but they are subject to a steep drop-off in achievable output power toward the Ka-band. The different interaction mechanism in a gyroklystron, based on phase-modulation of a helical beam, allows it to deliver multi-MW output power at significantly higher frequencies. The gyroklystron is therefore a strong candidate for delivering power to high-frequency linearising cavities. The international collaboration, CompactLight, is developing a design for a sophisticated X-ray Free Electron Laser (XFEL) with wide ranging research applications [1, 2]. The project required the consideration of both a 36GHz and 48GHz lineariser options. In each case, the development of new amplifiers was required to deliver sufficient power for the application. This thesis presents the design and analysis of a gyroklystron appropriate to drive a 48GHz linearising cavity. While the research presented in this thesis was performed with direct consideration of the CompactLight XFEL, its relevance is not exclusive to this project. With the performance of the microwave amplifier presented in this thesis, a lineariser at 48GHz could be a viable option for other C-band or X-band accelerator applications. Gyroklystron research was historically focused on radar applications. Since 48GHz lies in a frequency band unfavourable for atmospheric transmission, the development of components in this band has been lacking. The design presented in this thesis is the first published work on a MW-level amplifier at 48GHz and marks a step toward this frequency becoming a desirable choice for linearisation systems in future linacs. A gyroklystron design, including the electron source, vacuum windows, and input coupler has been designed through detailed simulation work. A triode-type magnetron injection gun compatible with a 2.02T axial guide magnetic field was designed and simulated. Applying -140kV to the cathode and -107.5kV to the modulating anode resulted in a gyrating electron beam with a current of 37A, guiding centre radius of 1.77mm, and velocity ratio spread of 8.9%. This resulted in a predicted gyroklystron output power of 2.0MW with a gain of 35dB at an efficiency of 38.6%.
Date of Award19 Oct 2021
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorAdrian Cross (Supervisor) & Liang Zhang (Supervisor)

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