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

2. 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 LCLS Coupler RF Kick
8/03/17

3. 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 LCLS Coupler RF Kick
8/03/17

4. 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 LCLS Coupler RF Kick
8/03/17

5. 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 LCLS Coupler RF Kick
8/03/17

6. 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 LCLS Coupler RF Kick
8/03/17

7. 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 LCLS Coupler RF Kick
8/03/17

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

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

10. 3.9 GHz cavity Cavity with FPC and both HOM couplers
Simulation model, included only the downstream FPC and HOM couplers
ZLi LCLS Coupler RF Kick
8/03/17

11. 3.9 GHz Calculated for E0=50MeV Gradient=10MV/m Crest phase: 00
dF: 30 Hz max
Dipole kick deviation: ±0.2%
ZLi LCLS Coupler RF Kick
8/03/17