Beam position monitors as precise phase pickups for beam energy measurement at the Compact Pulsed Hadron Source

The 13-MeV proton linac of the Compact Pulsed Hadron Source (CPHS) at Tsinghua University, China, is composed of a 50-keV electron cyclotron resonance proton source, a 3-MeV four-vane-type radio-frequency quadrupole (RFQ) accelerator, and a drift tube linac (DTL). Precise measurement of the beam energies at the exit of the RFQ and the DTL is critical for DTL commissioning. Two button-type beam position monitors (BPMs) installed downstream of the RFQ are used to perform the measurement using a time-of-flight method. The effects of several factors on phase measurement accuracy are analyzed. The phase measurement accuracy of the BPMs at CPHS is better than $\pm {1.03}^{\circ }$ at 325 MHz after corrections, corresponding to an energy measurement error of ± 0.07%. The beam energy measured at the exit of the RFQ is $2.994\pm 0.0022$ MeV, which is consistent with the design value.