An SRF Photocathode Gun for MeV Ultrafast Electron Microscopy

In order to achieve sharp, high resolution real-time imaging, electrons in a MeV UEM (ultrafast electron microscope) beamline need to minimize instabilities energy spread during generation and acceleration. The stability of the normal conducting photocathode gun which is currently used to produce short electron bunches has been one of main elements limiting the further improvement of beam instability. The Superconducting RF (SRF) photocathode gun is a promising candidate to produce highly stable electrons for UEM/UED applications. It operates in an ultrahigh Q, CW mode, and dissipates a few watts of RF power, which make it possible to achieve a 10s ppm level of beam stability by using modern RF control techniques. The main objective of Phase I was to develop a 1.3-GHz CW SRF photocathode gun producing MeV electrons for the UEM application. In the execution of the Phase I Work Plan, we have finished a complete RF design, performed beam tracking and analysis, carried out thermal analysis, designed the conduction cooling system, and designed a system for mechanical reinforcement. The design parameters used in the simulation (laser and gradient on Nb3Sn cavity, etc.) are based on known products and experiments, in order to evaluate the performance of the final delivered SRF photogun with as low risk as possible. During the construction of the Phase I commercialization plan, we have built and successfully tested an LLRF system for Fermilab 650-MHz conduction-cooled SRF cavity. This system is currently under upgrade to 1300 MHz. This LLRF system can be easily adapted to the SRF photogun in this Project. The overall objective is that by the end of Phase II, Euclid will deliver an operational 1.5-cell, 1.6-MeV conduction cooling Nb3Sn SRF CW Photogun. In the first year of Phase II, we will fabricate and weld the Nb half-cell to the existing 1.3 GHz single-cell cavity. Then the 1.5-cell Nb gun will be tested at the Euclid facility with conduction cooling at ~4K to investigate cooling efficiency and thermal contact resistance. In the 2nd year of Phase II, the cavity will be coated with Nb3Sn at Fermilabs or Cornell University and photoemission test will be performed at BNL, where the fs laser is available. The development of MeV-range ultrafast electron diffraction/microscopy (UED and UEM) has been identified as an enabling instrumentation in numbers of published DoE reports, which may lead to breakthroughs in fundamental science and applied technologies. The electron source we are proposing will significantly improve performance of the current MeV UEM.