Overcoming Multipacting Barriers in SRF Photoinjectors

Abstract

Superconducting RF (SRF) photoinjectors are considered to be a potential breakthrough in the area of high brightness electron sources. However, there is always the very important question of the compatibility of SRF cavities and high quantum efficiency (QE) photocathodes. A deposition of active elements from high QE photocathodes on the surface of a cavity makes it more vulnerable to multipacting (MP) and could affect the operation of an SRF gun. On the other side, MP can significantly reduce the lifetime of a photocathode. It is well known in the SRF community that a strong coupling, high forward power and sufficient cleanliness of cavity walls are the key components to overcome a low-level MP zone. In this paper we present a theoretical model of passing a MP barrier which could help estimate the desirable conditions for successful operation of an SRF gun. We demonstrate our results for the 113 MHz SRF photo-injector for Coherent electron Cooling (CeC) alongside with the experimental observations and 3D simulations of the MP discharge in the cavity. The results of the theoretical model and simulations show good agreement with the experimental results, and demonstrate that, if approached carefully, MP zones can be easily passed without any harm to the photocathode. CEC POP SRF PHOTOINJECTOR The superconducting 113 MHz photoinjector is the essential part of the electron accelerator for the Coherent electron Cooling [1] Proof-of-Principal (CeC PoP) experiment at Brookhaven National Laboratory [2]. During the last couple of years, it has successfully delivered high-charge electron bunches with up to 10 nC per bunch, and demonstrated an excellent performance with the cathodes operating for months without significant loss of quantum efficiency [3]. Figure 1: Simplified geometry of the CeC PoP SRF photo injector. The gun is based on Quarter Wave Resonator (QWR) and operates at 1.25 MV of accelerating voltage. Figure 1 demonstrates the cross-section of the gun cavity and its major components: fundamental power coupler (FPC), cathode stalk and the cathode itself. The hollow FPC is located in the front of the cavity and allows for the propagation of the generated beam outside the cavity. The FPC couples power into the the gun from a 4 kW RF transmitter and additionally provides for a fine tuning of the resonant frequency. The CsK2Sb photo-cathode deposited on a molybdenum puck operates at room temperature, while the gun cavity operates at liquid helium temperature of 4 K. The cathodes are inserted into the gun using an ultra-high vacuum (UHV) transport system into a hollow stainless steel cathode stalk, which also serves as a half-wave RF choke with a pick-up antenna located outside of the gun cryostat. Figure 2: Experimentally observed oscillations of the cavity voltage around the lower MP bound of 20 kV (top plot). The bottom plot shows the pulse of the forward (black) and reflected (red) power, which also demonstrates the oscillations due to the electron avalanche build up. Even though the gun demonstrated an excellent performance in terms of the delivered beam parameters, the journey towards this achievement was not always flawless. In the beginning of 2017, we observed a significant multipacting (MP) activity in the gun, which was extensively studied experimentally and through numerical simulations [4]. The CW conditioning results for the FPC showed a significant vacuum activity at the gun voltage of about 120 kV, and at the voltages above 300 kV. We also observed a number of low level MP barriers at about 2 kV, 20-27 kV, 30 kV and 40 kV, which corresponded to the build up of the MP electrons in the front rounding of the cavity. An example of a MP event is shown in Fig. 2. In order to understand the system requirements which would eliminate the MP problem when turning on the gun, we decided to take a different approach, and analyze the com10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-TUPTS079 MC3: Novel Particle Sources and Acceleration Techniques T02 Electron Sources TUPTS079 2105 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I