Impedance reduction of the High-frequency Cavities in the CERN PS by Multi-harmonic Feedback

TL;DR

This paper presents a multi-harmonic feedback method to reduce impedance in CERN PS RF cavities, improving beam quality for high-luminosity LHC operation.

Contribution

A novel multi-harmonic feedback system was developed and implemented to effectively lower impedance beyond traditional wide-band feedback capabilities.

Findings

Reduced cavity impedance and beam-induced voltage.

Improved longitudinal beam quality for LHC-type beams.

Dynamic phase adaptation enhances feedback effectiveness.

Abstract

In the framework of the "LHC Injectors Upgrade" (LIU) project, the CERN Proton Synchrotron (PS) is being prepared as a pre-injector for the Large Hadron Collider at high luminosity (HL-LHC). RF systems at 20 MHz, 40 MHz and 80 MHz are required for longitudinal beam manipulations to provide 25 ns bunch spacing and short bunches at transfer to the Super Proton Synchrotron (SPS). Beam induced voltage in these cavities causes transient beam-loading and uncontrolled longitudinal emittance blow-up. To reduce the impedance beyond the reach of the wide-band feedback around the power amplifier, a new multi-harmonic feedback has been developed and commissioned. Based on narrow-band filters treating each revolution frequency harmonic separately, the feedback phase at these harmonics can be dynamically adapted during acceleration, taking into account the phase of the cavity transfer function. The beneficial effect on longitudinal beam quality of LHC-type beams is shown.