S-Band Klystron Intra-Pulse Phase Feedback Upgrade at SPARCLAB Facility

One of the main technological challenges in plasma wakefield acceleration (PWFA) research and development is achieving stable and reproducible acceleration. In particular, for PWFA schemes based on particle-driven plasma wave excitation, beam stability and timing jitter are increasingly critical. In these configurations, magnetic or radio-frequency (RF) compression schemes are often used, and the beam time-of-arrival jitter at the end of the linear accelerator can be strongly correlated with the phase noise of RF accelerating structures operated off-crest. For this reason, since 2008, an RF phase fast-feedback system acting within each RF pulse has been successfully implemented at Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare (LNF-INFN) at the Sources for Plasma Accelerators and Radiation Compton with Laser And Beam (SPARCLAB) facility, operating on both S-band (2. 856 GHz) and C-band (5. 712 GHz) klystrons. This paper presents the upgrade and optimization of the fast-feedback system for an S-band klystron powered by a pulse-forming network modulator. This technology introduces significantly higher intrinsic phase noise than, for instance, solid state-based modulators. It is therefore essential to minimize such phase fluctuations to keep the machine stability under control. Both the feedback hardware (electronic boards and RF circuitry) and the software (controller and user interface) have been upgraded. Tests performed at SPARCLAB achieved a reduction in klystron-induced jitter of a factor of 30, reaching values below 15 fs rms on both power plants. Moreover, adding a remote control of the feedback loop enabled a straightforward optimization of the operating point, allowing the phase stability performance to be pushed close to its practical limits. A detailed analysis of RF phase noise measurements with the fast-feedback loop in operation is also presented.