DUAL FEED RF GUN DESIGN FOR LCLS Liling XIAO, Zenghai LI Advanced Computations Department Stanford Linear Accelerator Center Nov , SLAC-LCLS Injector RF System Internal Review

L. Xiao and Z. Li LCLS Injector RF System Review 2 Outline 1.Introduction 2.Design via Simulation 3.Pulsed Heating 4.Quadruple mode 5.Dual Feed Gun Parameters 6. Summary

L. Xiao and Z. Li LCLS Injector RF System Review 3 1. Introduction Requirements and Parameters GHz operating frequency  mode field imbalance within 10% Coupling factor around 2  T < 50 o C due to pulsed heating for Ecathode=120MeV, t-pulse = 3  s with 120Hz pulse repetition rate

L. Xiao and Z. Li LCLS Injector RF System Review 4 D.T.Palmer, et al., “Microwave Measurements of the BNL/SLAC/UCLA 1.6 cell Photocathode RF Gun “ SLAC-PUB Previous Study R.Boyce, et al., “Design Considerations for the LCLS RF Gun”, LCLS TN 04-4, April 2004 J.Hodgson

L. Xiao and Z. Li LCLS Injector RF System Review 5 New Design Features Dual RF feeds - to eliminate the dipole modes Larger rounding of coupling iris - to reduce the pulsed heating Racetrack cell shape - to minimize the quadruple modes

L. Xiao and Z. Li LCLS Injector RF System Review 6 AA B B Results: f=2.8625GHz, β=2.28 (E0:E1=1:0.908),  T=150 o C for the single feed gun dimensions. (comparing the data on Slide 15) Laser Port RF Coupling Aperture 2. Design via Simulation Original Design

L. Xiao and Z. Li LCLS Injector RF System Review 7 2b 0 2b 1 lc hc r2 New Optimized Design- Adjust labeled dimensions by modeling with ACD’s parallel eigensolver Omega3P and parallel S-parameter solver S3P

L. Xiao and Z. Li LCLS Injector RF System Review 8 S 12 GHz  mode Model is generated and meshed using Cubit. Typical transmission curves calculated with S3P (solid curve) and fitted using Lorentzian profiles (dotted curve) Modeling with S3P

L. Xiao and Z. Li LCLS Injector RF System Review 9 b 0, b 1, lc, hc (inch) ,1.6435, 0.868, (starting point) , , 0.868, , , 0.82, , , 0.868, 0.09 F0 (GHz) Q beta E 0 /E 1 1: :0.9851:0.96 Iteration to optimized dimensions (Pulsed heating and quadruple correction not considered)

L. Xiao and Z. Li LCLS Injector RF System Review Pulsed Heating Previous Results from ANSYS on Temperature Distribution in the LCLS RF Gun (LCLS TN 04-4):

L. Xiao and Z. Li LCLS Injector RF System Review 11 Breakdowns due to the pulsed heating had been observed in the Fermilab RF Gun G4 J.P.Carneiro, et al., TESLA Note

L. Xiao and Z. Li LCLS Injector RF System Review 12 The temperature rise at the end of RF pulse is Hsmax(t) Maximum surface magnetic field located on the coupling aperture edge. D.T.Pritzkau, “RF Pulsed Heating”, SLAC-Report-577

L. Xiao and Z. Li LCLS Injector RF System Review 13 Temperature Rise vs. Rounding Radius of RF Aperture Assuming: Ecathode =120MV/m, f = 2.856GHz, β = 2,Q0 =13300, tp = 3  s Start point Final point

L. Xiao and Z. Li LCLS Injector RF System Review 14 Starting point: hc=0.056”, r2=0.022” -  Tmax=150 o C Final point: hc=0.2165”, r2=0.185”-  Tmax=42 o C

L. Xiao and Z. Li LCLS Injector RF System Review 15 hc= inch Coupling Factor vs. Length of Aperture hc=0.056 inch Field imbalance for all the points are within 3%. Final point Start point hc= inch 4. Quadruple Mode in Coupler Cell

L. Xiao and Z. Li LCLS Injector RF System Review 16  βr/mm Quadruple Moment in Cylindrical Cavity 4th order finite element basis functions in Omega3P have to be used to compute the quadruple fields accurately in the models. And the fields from Omega3P are being used in PARMELA emittance calculations.

L. Xiao and Z. Li LCLS Injector RF System Review 17  βr/mm b1 d Quadruple Moment in Racetrack Cavity

L. Xiao and Z. Li LCLS Injector RF System Review 18 Effect of Laser Ports 1: max  T=36 o C  βr/mm 2: Quadruple moment due to the laser ports is about 5% of that in the cylindrical cavity.

L. Xiao and Z. Li LCLS Injector RF System Review 19  βr/mm The racetrack corrects only the quadruple in the coupler cell. The skew quadruple effect in the half cell due to laser ports needs to be evaluated; racetrack shape can be added to the half cell if necessary.  βr/mm

L. Xiao and Z. Li LCLS Injector RF System Review Dual Feed “Corrected” RF Gun b1 d

L. Xiao and Z. Li LCLS Injector RF System Review 21 ParameterValue Race track arc radius b inch (original:1.6435”) Race track arc separation d0.134 inch (original: 0) Race track cell length l11.28 inch Half cell radius b inch (original:1.6335”, not consider the laser ports) Half cell length l inch RF coupling hole size (slot length) lc0.95 inch (original:0.868”) RF coupling hole radius of curvature rc inch RF coupling hole size (slot width) 2rc0.375 inch RF coupling hole thickness hc inch (original: 0.056”) RF coupling hole rounding radius r1 on up side0.022 inch RF coupling hole rounding radius r2 on down side0.185 inch (original: 0.022”) Cell iris radius a0.492 inch Disk thickness t0.868 inch Disk rounding radius r0.375 inch Laser port hole size (slot length) ll0.433 inch Laser port hole radius of curvature rl0.125 inch Laser port hole size (slot width) 2rl0.250 inch Laser port hole rounding radius r inch Laser port offset the cathode plate0.531 inch Laser port angle18 degree Waveguide2.840 inch*1.340inch RF-GUN Final Dimensions

L. Xiao and Z. Li LCLS Injector RF System Review 22 “Corrected” RF GUN Parameters: RF Parameters F0 (GHz) Q β2.07 Mode Sep.  f (MHz) 3.4  T max ( o C) 44 E0:E11:0.962

L. Xiao and Z. Li LCLS Injector RF System Review 23 On-axis Accelerating Field Profile in RF Gun Cavity E0:E1=1:0.962

L. Xiao and Z. Li LCLS Injector RF System Review 24 DimensionFreq. error per ” Field error per ” Full cell radius b1113KHz5.1% Half cell radius b070KHz5.2% Racetrack arc separation d45KHz2% Cell iris radius a40KHz0.3% Disk rounding radius r24KHz0.2% Frequency and Field Sensitivity to Major Gun Dimensions

L. Xiao and Z. Li LCLS Injector RF System Review 25  The original RF gun design has been optimized to: - eliminate the dipole modes - reduce pulsed heating - minimize the quadruple moment  A “corrected” design has been generated using SLAC’s parallel codes Omega3P & S3P with 4 th order elements  The steady-state thermal properties needs to be studied again for the new dual feed gun design. 6. Summary