Progress toward an All Niobium Superconducting Rf Photocathode Electron Gun*

Advanced Energy Systems Inc (AES) and Brookhaven National Laboratory (BNL) are currently working on a program to develop a Superconducting Photocathode Electron Gun using an all Nb cavity. The work is being performed as a Phase II SBIR and under a CRADA between AES and BNL. Results of initial efforts have been presented previously [1,2]. This paper will review the progress being made in this program. Results of revision and optimization of the Nb cavity will be shown. A brief overview of the thermal analysis being performed in support of this work will be included, a more detailed discussion will appear under a separate title [3]. The beam dynamics and anticipated beam performance will be discussed. An overview of the mechanical design of the cavity will be presented along with a discussion of, and figures showing, the current status of the fabrication process. Our approach to providing the cryostat in which the cavity will be tested will also be included along with a discussion of, and figures showing, the current status of the fabrication process. Finally, we will review our future plans which include the assembly and integration of the cryostat at AES and testing at BNL. 1 QUANTUM EFFICIENCY MEASUREMENTS To further validate and understand the QE of the niobium cathode we have designed a replacement cathode plate for the BNL ATF electron gun. This plate will exactly duplicate the geometry of the current cathode but will incorporate a brazed niobium slug as the cathode. The method for fabrication calls for nickel plating the tapered niobium plug and brazing it in a tapered hole in the backplate using Au-Cu alloy. The backplate will then be finish machined to remove sufficient copper and niobium to expose virgin material. This surface will then be prepared via standard BNL cathode preparation techniques. Figure 1 shows the design of the cathode testing backplate. We have designed a new fixture for testing of niobium cathode samples for quantum efficiency. One goal of the new series of tests will be to see if there is any measurable effect (positive or negative) associated with the process used to assemble the cathode test unit that will be tested at the ATF facility. The brazed assembly described previously will be used to fabricate several test cathodes for this purpose. 2 BEAM DYNAMICS CALCULATIONS The first step taken in the beam dynamics studies of the revised cavity was a brief parameter space search. For these studies, the field gradient at the cathode surface was 35 MV/m, the bunch charge was fixed at 0.01 nC, and the seed laser parameters set at a radius of 4.5 mm RMS with a pulse length of 6 ps FWHM. An emittance minimum between 45 and 50 became evident, while the energy spread continues decreasing. There is a certain trade-off to be made between the minimum transverse emittance and the minimum longitudinal emittance. It was found that a reduced pulse length at this low charge also Figure 1 – ATF backplate with brazed Nb cathode Figure 2 – Emittance scaling with beam radius 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5