LCLS RF Gun Thermal Analysis John Schmerge, SLAC November 3, 2004

1. LCLS RF Gun Thermal Analysis John Schmerge, SLACNovember 3, 2004 • LCLS Gun Description • Prototype gun • LCLS gun modifications • Gun design parameters • 120 Hz Thermal Analysis • Average and pulsed heating calculations • Reducing average power with rf pulse shaping • Stead state thermal and stress analysis with ANSYS • Summary John Schmerge, SLAC

2. LCLS Prototype Gun at GTF Single sided rf feed Beam Exit Cathode Plate Laser Entrance Port RF probes and tuners are not shown in figures John Schmerge, SLAC

3. LCLS RF Gun Modifications • Dual RF feed • 120 Hz cooling • Cathode Load Lock • High QE • Uniform Emission • Fast installation (< 2 hrs) • Motorized symmetric tuners with readbacks in both cells • Calibrated symmetrized field probes in both cells LCLS Gun John Schmerge, SLAC

4. Gun Design Parameters John Schmerge, SLAC

5. Ideal Field on Axis John Schmerge, SLAC

6. Required Thermal Stability dfp/dt = -52 kHz/˚C For sF< 1˚ requires sT < 0.15˚C for b = 2 Small amplitude effect with temperature variation Feedback on low level rf can further reduce the phase variation. Measured amplitude and phase variation for GTF gun over 20 minutes. Amplitude and phase oscillation due to 0.1˚C peak-peak temperature oscillation. John Schmerge, SLAC

7. Pulsed Heating • Stored Energy at 140 MV/m is 9.1 J. • Energy per macro pulse dissipated in the structure is 33 J with b = 1.3, t = 580 ns and 3 ms long rf pulse. • At 120 Hz this corresponds to 4 kW. • GTF gun with water cooling dissipates only 330 W at 10 Hz and 140 MV/m. • BNL, SHI, University of Tokyo gun with water cooling dissipates < 1 kW at 50 Hz and 100 MV/m. • Need a factor of 4 increase in average power dissipation. • Possible problems with thermal distortions changing resonant frequency and field distribution. • Possible problem with pulsed stress leading to Cu material failure. John Schmerge, SLAC

8. Thermal Distortion and Stress Analysis • Calculations with ANSYS – Finite Element Analysis Code • Assume 4 kW CW load distributed along the gun surface as determined by SUPERFISH and ANSYS • Calculate temperature distribution, thermal distortions and stresses in gun body • Limit distortions to < 100 kHz frequency shift to prevent the need for re-tuning the gun as the field is varied • Limit von Mises Stress to 2 107 Pa • Determine appropriate water cooling channel number and location to achieve goals • Compare with GTF performance John Schmerge, SLAC

9. Temperature Distribution for GTF and LCLS Water 20 ˚C GTF style cooling channels LCLS design using 4 cooling channels fp 0 power- fp full power = 800 kHz John Schmerge, SLAC

10. Temperature and Stress Distribution Water 30 ˚C no power and 14 ˚C full power fp 0 power - fp full power = 80 kHz Thermocouple On gun body Stress (Pa) Temperature (˚C) John Schmerge, SLAC

11. RF Coupler Temperature and Stress 3D model with GTF coupling iris thickness (56 mils) Stress (Pa) Temperature (˚C) John Schmerge, SLAC

12. RF Coupler Temperature and Stress 3D model with 2X GTF coupling iris thickness (112 mils) Stress (Pa) Temperature (˚C) John Schmerge, SLAC

13. Results • 4 cooling channels including 1 on the cathode plate. • Moving cooling channels to gun body OD increases stress by a factor of 2. • ANSYS predicts 230 W load changes the GTF gun frequency by 125 kHz and observe ≈ 50-100 kHz. • ANSYS predicts 80 kHz LCLS gun frequency shift when changing from no load with 30 ˚C water to 4 kW load with 14 ˚C water. • 3D ANSYS analysis shows rf coupling iris has the highest stress and temperature. • RF coupling iris must be thicker to reduce stress. John Schmerge, SLAC

14. Reducing Pulsed Heating with RF Pulse Shaping 1.8 kW 4.0 kW John Schmerge, SLAC

15. Optimum Pulse Shape 1.6 kW John Schmerge, SLAC

16. Pulse Shaping • Advantages • Factor 2 less power dissipated in structure • Less dark current • Reach higher peak fields due to shorter rf pulse length • Disadvantages • Possibly more mode beating effects between 0 and p modes • Klystron bandwidth will mitigate reduction in power • Extra effort to shape pulse John Schmerge, SLAC

17. Coupling Coefficient • Advantages of high b • Less average and pulsed power • Less dark current • Less sensitive to water temperature fluctuations • Less sensitive to resonant frequency shifts due to distortions • Shorter time constants reduce mode beating • Advantages of low b • Smaller field perturbation due to coupling holes • Less klystron power for a given cathode field • Less reflected power • Narrower resonance reducing amplitude of mode beating term John Schmerge, SLAC

18. Summary • LCLS gun modifications • Dual rf feed • 120 Hz cooling (4 kW average power) • Cathode load lock • Motorized symmetric tuners in both cells • Calibrated symmetrized field probes in both cells • 4 kW average power load • Stead state thermal and stress analysis acceptable for constant gun body temperature (variable water temperature) • Can reduce the average power load by ≈ a factor of 2 with an initial high power klystron pulse reduced to the steady state value after the field has reached the desired level John Schmerge, SLAC