Optimization of Cathodic Arc Deposition and Pulsed Plasma Melting Techniques for Growing Smooth Superconducting Pb Photoemissive Films for SRF Injectors

Superconducting photoinjectors have a potential to be the optimal solution for moderate and high current cw operating free electron lasers. For this application, a superconducting lead (Pb) cathode has been proposed to simplify the cathode integration into a 1.3 GHz, TESLA-type, 1.6-cell long purely superconducting gun cavity. In the proposed design, a lead film several micrometres thick is deposited onto a niobium plug attached to the cavity back wall. Traditional lead deposition techniques usually produce very non-uniform emission surfaces and often result in a poor adhesion of the layer. A pulsed plasma melting procedure reducing the non-uniformity of the lead photocathodes is presented. In order to determine the parameters optimal for this procedure, heat transfer from plasma to the film was first modelled to evaluate melting front penetration range and liquid state duration. The obtained results were verified by surface inspection of witness samples. The optimal procedure was used to prepare a photocathode plug, which was then tested in an electron gun. The quantum efficiency and the value of cavity quality factor have been found to satisfy the requirements for an injector of the European-XFEL facility

Deposition and Optimization of Thin Lead Layers for Superconducting Accelerator Photocathodes

A combination of a ultra high vacuum arc deposition system and a recrystallization method was used to optimize the smoothness and thickness of thin-layer lead cathodes for superconducting niobium electron injectors. A non-filtered arc system was chosen to deposit Pb films on niobium. The films then underwent melting and recrystallization by treating them with pulsed argon ion beams in a rod plasma injector

Coating in Ultra-High Vacuum Cathodic-Arc and Processing of Pb Films on Nb Substrate as Steps in Preparation of Nb-Pb Photocathodes for Radio-frequency, Superconducting e− ^− Guns

A photocathode composed of a Pb layer deposited on Nb substrate is an attractive solution proposed for fully superconducting, radio frequency electron gun to be used in the linear accelerator of the European X-ray free electron laser (Eu-XFEL) operating at Deutsches Elektronen Synchrotron (DESY) and in other, similar devices. Much effort has been put in development of deposition and post-processing of Pb films as superconducting photoemitters. These works led to a satisfactory solution based on lead deposition in a cathodic-arc followed by ex-situ smoothing of the obtained film through its re-melting with a pulsed plasma ion beam. Pb layers obtained by different procedures have been tested for their morphology, microstructure, dark current emission, quantum efficiency and impact on SRF e$^−$ gun performance

SRF Gun Development at DESY

A future upgrade of the European XFEL (E-XFEL) foresees an additional CW operation mode, which will increase the flexibility in the photon beam time structure [1–3]. One of the challenges of this operational mode is the need fora CW operating photo injector. We believe that using an SRF gun is the preferred approach as the beam parameters of normal conducting pulsed guns can be potentially met by SRF guns operating CW. For more than a decade DESY, incollaboration with TJNAF, NCBJ, BNL, HZB and HZDR, has performed R&D to develop an all superconducting RFgun with a lead cathode. In the frame of E-XFEL CW upgrade feasibility studies, the SRF-gun R&D program gained more attention and support. Within the next few years we would like to demonstrate the performance of the all superconducting injector required for the E-XFEL upgrade. The selected approach offers advantages w.r.t. the cleanliness of the superconducting surface, but requires a complete disassembly of a cryostat and stripping the gun cavity in a cleanroom to exchange the cathode. Thus it is practical only when the life time of the cathode is at least several months. In this paper we present the actual status of the R&D program, next steps and the longer term plans