Design, realization, and testing of a brazing-free C-band radio frequency photogun

Radio frequency (rf) photoguns are widely used in modern particle accelerator facilities, particularly in free electron lasers (FELs), as they provide very low-emittance, high-brightness electron bunches. Presently, the rf technology mostly used for photoguns is the S-band ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mo>∼</a:mo> <a:mn>3</a:mn> </a:math> GHz), with typical cathode peak fields of 80–120 MV/m and maximum repetition rates of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mo>∼</c:mo> <c:mn>100</c:mn> </c:math> Hz. According to beam dynamics studies, the higher the peak electric fields on the cathode, the better the quality of the beam emerging from the gun. In this respect, S-band rf guns represent the state-of-the-art, both in terms of reachable cathode peak fields and repetition rates. An innovative C-band rf gun, operating at 5.712 GHz and aiming at reaching cathode peak fields larger than 160 MV/m, with repetition rates exceeding 400 Hz, has been designed, realized, and high-power tested. Its device consists of a 2.5-cell standing wave (SW) cavity with a four-port mode launcher whose overcoupled design allows it to operate with short rf pulses (300 ns). Its realization has been based on the new brazing-free technology developed and successfully tested at the Frascati Laboratories of the National Institute of Nuclear Physics (INFN-LNF, Italy). In the paper, we illustrate the design and realization performed by INFN-LNF, and its high-power rf test, which was conducted at the C-band test facility at the Paul Scherrer Institute (Switzerland).