Longitudinal emittance measurement for an external-bunching-based heavy-ion RFQ

Longitudinal emittance at the RFQ output is a very important parameter to determine the particle loss in the subsequent linac. To understand the quality of beams delivered by an external-bunching-based heavy-ion RFQ and to provide key information for High Intensity heavy ion Accelerator Facility (HIAF) superconducting linac, longitudinal emittance was measured at the protype of HIAF front-end at Institute of Modern Physics (IMP). An 81.25 MHz quarter wave resonator (QWR) re-buncher was developed and installed behind the RFQ. A voltage scan of the re-buncher and the corresponding bunch length measurement determine the longitudinal emittance. The measurements were implemented for different operation conditions of the pre-buncher which is installed upstream of the RFQ and is a Multi-Harmonic-Buncher (MHB) operating with two frequencies. Measured longitudinal emittance trends for different pre-bunching variants follow those expected from simulation but are systematically lower than estimated. Both simulation and measurement predict a relatively lower emittance while the MHB operating with the RFQ fundamental frequency. The reason has been discussed in this study.