DEFLECTING CAVITY OPTIONS FOR RF BEAM SPREADER IN LCLS II

1. DEFLECTING CAVITY OPTIONS FOR RF BEAM SPREADER IN LCLS II December 4th, 2013

2. RF Spreader System Requirements CDR CHAPTER 7: ELECTRON COMPRESSION AND TRANSPORT • Three rf cavity design options • Superconducting rf-dipole cavity • Normal conducting rf-dipole cavity • Normal conducting 4-rod cavity 3-Way Beam Spreader Vertical Beam Separation

3. Superconducting RF-Dipole Cavity • Beam aperture of 40 mm • Considering cavity processing • Low wakefield impedance budget • Any dimensional constraints ? • RF-Dipole Design • RF Fields and Surface Fields Cavity diameter = 34 cm Bar length = 41 cm Angle = 50 deg θ Bar height = 4.4 cm Cavity Length = 70 cm Magnetic field Electric field

4. Superconducting RF-Dipole Cavity • Required deflection can be achieved by one cavity • Compensation for beam loading • Only fundamental deflecting mode is considered • At a beam offset of 5 mm with a transverse voltage variation of δVT = 0.002VT • Average beam current of 0.02 mA • Multipacting is expected to be processed easily • No lower order modes • Widely separated HOMs • Reduced field non-uniformity with increased bar height 183 MHz

5. Field Non-Uniformity • Shaped loading elements • To reduce filed non-uniformity across the beam aperture • Suppress higher order multipole components • Voltage deviation at 20 mm • Horizontal: 5.0%  0.2% • Vertical: 5.5%  2.4% (A) (B)

6. Current RF-Dipole Cavities 400 MHz Crabbing Cavity for LHC High Luminosity Upgrade 499 MHz Deflecting Cavity for Jefferson Lab 12 GeV Upgrade • Deflecting voltage – 3.8 MV • Total crabbing voltage – 13.4 MV per beam per side • Per cavity – 3.4 MV 750 MHz Crabbing Cavity for MEIC* • Crabbing voltage • Electron beam – 1.5 MV • Proton beam – 8.0 MV *A. Castilla et.al., in Proceedings of the 3rd IPAC, New Orleans, Louisiana (2012), p. 2447.

7. Properties of RF-Dipole Cavity Designs 499 MHz Deflecting Cavity for Jefferson Lab 12 GeV Upgrade 44 cm 24 cm 400 MHz Crabbing Cavity for LHC High Luminosity Upgrade 53 cm 34 cm 750 MHz Crabbing Cavity for MEIC at Jefferson Lab* 35 cm 19 cm *A. Castilla et.al., in Proceedings of the 3rd IPAC, New Orleans, Louisiana (2012), p. 2447.

8. 499 MHz RF-Dipole Cavity • Multipacting was easily processed during the 4.2 K rf test • Design requirement of 3.78 MV can be achieved with 1 cavities • Achieved fields at 2.0 K • ET = 14 MV/m • VT = 4.2MV • EP = 40MV/m • BP = 61.3mT Quench 3.78

9. 400 MHz RF-Dipole Cavity • Multipacting levels were easily processed • Achieved fields at 4.2 K • ET = 11.6 MV/m • VT = 4.35 MV • EP = 47 MV/m • BP = 82 mT • Limited by rf power at 4.2 K • Achieved fields at 2.0 K • ET = 18.6 MV/m • VT = 7.0 MV • EP = 75 MV/m • BP = 131 mT Multipacting levels observed below 2.5 MV Multipacting levels observed below 2.5 MV Quench Limited by rf power Design goal – 10 MV 3.4 5.0

10. Normal Conducting RF-Dipole Cavity • Beam aperture of 25 mm • Due to the dependence on transverse shunt impedance (RT) • Considering cavity processing • RF-Dipole Design * • RF Fields and Surface Fields Cavity length = 37 cm Bar width = 6 cm Cavity height = 26 cm Bar height = 1.5 cm Cavity width = 15 cm Bar length = 31 cm Magnetic field Electric field * T. Luo, D. Summers, D. Li, “Design of a Normal Conducting RF-dipole Deflecting Cavity”, in Proceedings of the 2013 International Particle Accelerator Conference, Shanghai, China, WEPFI091

11. Normal Conducting RF-Dipole Cavity • Total deflection can be achieved by 6 cavities • Surface heating at the loading elements are reduced by curving and requires cooling • RF properties can be further improved with reduced beam aperture

12. Normal Conducting 4-Rod Cavity • Beam aperture of 25 mm • Due to strong relation with shunt impedance (RT) • 4-Rod Design * • RF Fields and Surface Fields Cavity length = 45 cm Cavity diameter = 45 cm Rod length = 21 cm Rod diameter = 3.1 cm Magnetic field Electric field Rod gap = 2 cm * C.W. Leemann, C. G. Yao, “A Highly Effective Deflecting Structure” in Proceedings of the 1990 Linear Accelerator Conference, Albuquerque, New Mexico, p. 232

13. Normal Conducting 4-Rod Cavity • Total deflection can be achieved by 4 cavities • Localized surface magnetic field has higher cooling requirements per cavity • Surface heating at the end of the rods requires cooling • RF properties can be substantially improved with reduced beam aperture, compared to NC RFD cavity

14. 499 MHz Normal Conducting 4-Rod Cavity • 499 MHz 2-cell 4-rod cavity* • Cu coated stainless steel can • Uses parallel cooling mechanism • RF power coupled using magnetic coupling at the end of the cavity • Maximum reached rf power = 5.2 kW • Limited by the cooling of rf power coupler * C. Hovater, G. Arnold, J. Fugitt, L. Harwood, R. Kazimi, G. Lahti, J. Mammosser, R. Nelson, C. Piller, L. Turlington, “The CEBAF RF Separator System”, in Proceedings of the 1996 Linear Accelerator Conference, Geneva, Switzerland, p. 77.

15. Summary • Total deflection of 4.0 MV can be achieved by one cavity using the SC RF-Dipole Cavity • Considering the distance between rf spreader system and end of linac needs to look into liquid He supply by • A transfer line • A separate refrigerator • NC RFD requires 6 cavities and has low rf power requirements • NC 4-Rod cavity requires 4 cavities • Similar rf cavity is currently being used successfully at Jefferson Lab rf separator system