FPC Coupler RF Dipole Kick RF Dipole Kick Variation due to microphonics cavity RF detuning Zenghai Li 08/03/2018 ZLi LCLS2 Coupler RF Kick 8/03/17

LCLS-2 cavity LCLS-2 9cell cavity QEXT=4.1E7 Couplers break field symmetry induce RF dipole kick to the beam Microphonics may cause detuning of cavity RF frequency more RF power needed to keep same gradient resulting in variation of RF dipole kicks New FPC inner conductor for LCLS-2 ZLi LCLS2 Coupler RF Kick 8/03/17

Asymmetry fields in coupler region On-axis TRANSVERSE E fields Non-zero indicating transverse deflection Deflection RF phase dependent Linear on gradient Vary with input power (to keep same gradient in detuned case) ZLi LCLS2 Coupler RF Kick 8/03/17

Cavity Resonant Curve Bandwidth: 32 Hz Detuned cavity requires more power for a given gradient Higher coupler fields Higher multipole kick But by how much ? 16-20% ??? (dF, Res_ampl) (dF, P/P0) ZLi LCLS2 Coupler RF Kick 8/03/17

FPC Coupler RF Kick Calculation Detuning only affects fields on the FPC coupler end Calculation only included the HOM and FPC couplers on the FPC end Cavity detuning: 10 Hz max Initial beam energy (before enter cavity) 0.75 MeV (first cavity in injector) (gradient=8MV/m) 50 MeV (beta ~1, dipole kick does not depends on beam energy any more at higher energy) ZLi LCLS2 Coupler RF Kick 8/03/17

E0=0.75MeV Gradient=8MV/m Crest phase: dF: 10 Hz max V=c particle: 00 E0=0.75MeV particle: - 560 dF: 10 Hz max Dipole kick deviation: ±2% ZLi LCLS2 Coupler RF Kick 8/03/17

Beta~1 particle Calculated for E0=50MeV Gradient=8MV/m (scale to 16MV/m by *2) Crest phase: 00 dF: 10 Hz max Dipole kick deviation: ±2% dipole kick deviation due to detuning much smaller than estimated using the change of input power levels to keep the same gradient ZLi LCLS2 Coupler RF Kick 8/03/17

Ex field in coupler region: on/off resonance Ex along “z” Ex along “x” in FPC ZLi LCLS2 Coupler RF Kick 8/03/17

|E| field along FPC: on/off resonance Significant differences in amplitude and phase in coaxial region ZLi LCLS2 Coupler RF Kick 8/03/17

3.9 GHz cavity Cavity with FPC and both HOM couplers Simulation model, included only the downstream FPC and HOM couplers ZLi LCLS2 Coupler RF Kick 8/03/17

3.9 GHz Calculated for E0=50MeV Gradient=10MV/m Crest phase: 00 dF: 30 Hz max Dipole kick deviation: ±0.2% ZLi LCLS2 Coupler RF Kick 8/03/17