New paper on an upgraded klystron phase feedback system for plasma accelerators
A new study led by EuPRAXIA-DN Fellow Xianghe Fang at INFN-LNF and Sapienza University of Rome presents an upgraded fast feedback system that reduces the phase jitter of the S-band klystrons at the SPARC_LAB facility by a factor of 30. The work, carried out with colleagues at the Laboratori Nazionali di Frascati (LNF-INFN), is published in Applied Sciences, in the Special Issue “New Challenges in Plasma Accelerators”, under the title “S-Band Klystron Intra-Pulse Phase Feedback Upgrade at SPARC_LAB Facility.”
Stable and reproducible acceleration is one of the main challenges in plasma wakefield acceleration. When magnetic or radio-frequency (RF) compression schemes are used, the beam arrival time jitter at the end of the linac is strongly correlated with the phase noise of RF accelerating structures operated off-crest. In klystrons powered by pulse-forming network modulators, high-voltage fluctuations are converted directly into phase fluctuations from pulse to pulse, so conventional slow feedback loops cannot correct them. The paper addresses this with an intra-pulse feedback that measures the klystron phase within each RF pulse, compares it with a stable reference and applies a real-time correction through a fast phase shifter.
The study builds on a first version of the system deployed at SPARC_LAB in 2008. The upgrade includes a new RF circuit design, improved error amplifier electronics, a new acquisition system and new control software integrated into the facility control system. A key addition is the remote control of the loop gain, which allows the operating point to be optimised during machine operation.
Tests on both S-band power plants showed klystron-induced phase jitter below 15 fs rms, compared with a native stability above 400 fs rms, and an improvement of a factor of 2.5 to 6 with respect to the previous version of the feedback. The system also operated stably over several hours and kept the jitter within the required range over a gain interval of about 1 V, showing that it is not highly sensitive to parameter tuning. With this upgrade, both plants meet the phase stability specifications of the EuPRAXIA@SPARC_LAB project.
These results are a key milestone for plasma wakefield acceleration applications that require ultra-low phase jitter. The team now plans to apply the same technique to RF stations driven by solid-state modulators, following a first prototype test on a C-band plant that measured a jitter below 10 fs rms.
Further information:
Fang, X., Bellaveglia, M., Gallo, A., Magnanimi, R., Michelotti, A., Quaglia, S., Scampati, M., Scarselletta, G., Serenellini, B., et al., Piersanti, L. “S-Band Klystron Intra-Pulse Phase Feedback Upgrade at SPARC_LAB Facility”, Applied Sciences, 16(12), 5733 (2026). https://doi.org/10.3390/app16125733
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