BEAM COMMISSIONING PROGRAM OF THE 704 MHz SRF GUN* Wencan Xu #,1 , Z. Altinbas 1 , S. Belomestnykh 1,2 , I. Ben-Zvi 1,2 ,S. Deonarine 1 , L. DeSanto 1 , D. Gassner 1 , R. C. Gupta 1 ,H. Hahn 1 , L. Hammons 1 , Chung Ho 1 , J. Jamilkowski 1 , P. Kankiya 1 , D. Kayran 1 , R. Kellerman 1 , N. Laloudakis 1 , R. Lambiase 1 , C. Liaw 1 , V. Litvinenko 1,2 , G. Mahler 1 , L. Masi 1 , G. McIntyre 1 , T. Miller 1 , D. Phillips 1 , V. Ptitsyn 1 , T. Seda 1 , B. Sheehy 1 , K. Smith 1 , T. Rao 1 , A. Steszyn 1 , T. Tallerico 1 , R. Than 1 , J. Tuozzolo 1 , E. Wang 1 , D. Weiss 1 , M. Wilinski 1 , A. Zaltsman 1 1) Collider-Accelerator Department, Brookhaven National Lab, Upton, NY 11973, USA 2) Physics & Astronomy Department, Stony Brook University, Stony Brook, NY 11794, USA Abstract A 704 MHz superconducting RF photoemission electron gun for the R&D ERL project is under commissioning at BNL. Without a cathode insert, the SRF gun achieved its design goal: an accelerating voltage of 2 MV in CW mode. During commissioning with a copper cathode insert, it reached 1.9 MV with 18% duty factor, which is limited by multipacting in a choke-joint cathode stalk. A new cathode stalk has been designed to eliminate multipacting in the choke-joint. At the same time, a first beam test was carried out in May this year, and dark current from the photocathode was measured in the faraday cup. The SRF cavity was tested after the beam commissioning and shows no-degradation of the performance. This paper presents recent commissioning setup, results and plans for the future beam tests. INTRODUCTION The R&D ERL [1] at BNL is an accelerator with high average current, up to 500 mA. It serves as test bed for future RHIC projects: eRHIC [2], Coherent-Electron- Cooling [3], and Low Energy RHIC Electron Cooler [4]. The 704 MHz half-cell SRF gun is designed to provide 0.5 A, 2 MeV electron beam. Commissioning of the SRF gun is carried out in stages: without a cathode stalk (finished in early 2013), with a copper cathode stalk (finished in fall of 2013), and beam commissioning (started in mid-2014). The SRF gun without a cathode stalk reached the design voltage of 2 MV in CW mode. During commissioning with a copper cathode stalk, strong multipacting in the quarter-wavelength choke-joint was observed and it was understood with simulation. A multipacting-free choke-joint has been designed and an order was placed its fabrication. In the meantime, first beam commissioning with the existing cathode stalk coated with alkali antimonide photoemission layer took place in May of 2014. Dark current was observed, measured and conditioned. This paper describes the SRF gun commissioning results and plans. A multipacting-free choke-joint design is addressed as well. PERFORMANCE OF THE SRF GUN The SRF gun cryomodule is shown in Figure 1. It is built around the 704 MHz half-cell SRF cavity, including a quarter-wavelength choke-joint cathode insert, a pair of opposing fundamental power coupler (FPC) to deliver 1 MW of RF power, a high temperature superconducting solenoid (HTSS) to compensate space charge and a room- temperature ferrite HOM damper. The gun was successfully commissioned and reached the design goal (2 MV in CW mode) without a cathode stalk insert [5]. However, multipacting occured during commissioning with a copper cathode stalk. The main reason for multipacting was caused by distortion of grooves due to BCP and high SEY in the stainless steel area. After spending some time on mutipacting conditioning, the gun was able to operate at 1.9 MV with 18% duty factor. One important parameter for the cavity operation is its field stability. The amplitude stability of 2.3E-4 (rms) and the phase stability of 0.035 degree (rms) was demonstrated. Figure 1: SRF gun cryomodule. MULTIPACTING-FREE CATHODE STALK DESIGN Figure 2 shows the cathode stalk to be inserted into the SRF cavity. There are two folded half-wavelength chokes or four quarter-wavelength chokes, so the stalk has four gaps. The copper cathode substrate and an inner Nb cylinder compose the first gap. The second gap (end of the first half-wavelength choke) is formed by two Nb cylinders. The third gap is composed of the outer Nb cylinder and an inner stainless steel cylinder. Two stainless steel cylinders constitute the forth gap. The Nb cylinders are part of the MOPP012 Proceedings of LINAC2014, Geneva, Switzerland ISBN 978-3-95450-142-7 70 Copyright © 2014 CC-BY-3.0 and by the respective authors 01 Electron Accelerators and Applications 1B Energy Recovery Linacs