Author: Zhou, F.
Paper Title Page
WEPA08 Design and Operation Experience of a Multi-Collimator/YAG Screen Device on LCLS II Low Energy Beamline 631
  • X. Liu, C. Adolphsen, M. Santana-Leitner, L. Xiao, F. Zhou
    SLAC, Menlo Park, California, USA
  During the commissioning of the normal conducting VHF RF gun of LCLS II, it was observed that field emission (dark current) of roughly 2 µA level was present under normal operation of the gun. While the dark current of this level is deemed manageable with existing beamline configurations, it is desired in precaution to add a collimator on the low energy beamline to block the dark current, being concerned that the dark current situation might worsen with time. Since no spare longitudinal space is available, the new device takes place of the existing YAG screen. The new device is made of a 15 mm thick copper plate, with four round apertures of 6, 8, 10, and 12 mm radius respectively. At the end of the collimator plate, features are made for clamping two YAG screens and mounting their corresponding mirrors for beam/halo profile imaging. The collimator plate is electrically insulated from the chamber so that it can also be used for measuring the dark current. A motor-driven UHV compatible linear translator shifts the device between positions. Besides design details, related thermal, beam dynamics, and radiation analyses as well as operation experience will be presented.

* Work supported by US DOE under contract AC02-76SF00515.
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-WEPA08  
About • Received ※ 02 August 2022 — Revised ※ 09 August 2022 — Accepted ※ 12 August 2022 — Issue date ※ 13 September 2022
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WEPA10 Determination of LCLS-II Gun-2 Prototype Dimensions 637
  • L. Xiao, C. Adolphsen, E.N. Jongewaard, X. Liu, F. Zhou
    SLAC, Menlo Park, California, USA
  The LCLS-II spare gun (Gun-2) design is largely based on the existing LCLS-II gun (Gun-1), in which there is significant captured dark current (DC) that originates on the high field copper surface near the cathode plug gap opening. To help suppress DC, the Gun-2 cathode and anode noses and the cathode plug opening are elliptically shaped to minimize the peak surface field for a given cathode gradient. Stainless steel (SS) cathode and anode inserts are used in Gun-2 to further reduce dark current. The RF simulations were performed using a model that includes all the 3D features. The thermal and structural analyses were done to investigate the effects of the air pressure and RF heating. The multi-physics simulation results provided the information needed to compute the overall frequency change from the basic 2D model to the nominal frequency during operation. The Gun-2 cathode-to-anode gap distance will be made 1 mm longer than the nominal gap with the expectation that less than 1 mm will be machined off to meet the target frequency. In this paper, the Gun-2 frequency correction calculations are presented, and the cathode-to-anode gap determination is discussed.  
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-WEPA10  
About • Received ※ 30 July 2022 — Revised ※ 03 August 2022 — Accepted ※ 08 August 2022 — Issue date ※ 10 August 2022
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