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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