RF R&D (Issues) for Muon Ionization Cooling Channels Derun Li Center for Beam Physics

1. RF R&D (Issues) for Muon Ionization Cooling Channels Derun Li Center for Beam Physics Lawrence Berkeley National Laboratory Nufact-08, Valencia, Spain July 1st 2008 D. Li – RF R&D for Muon Ionization Cooling Channels

2. Outline • Introduction • RF cavity for muon Ionization Cooling • Experimental study program • 805 MHz cavity design and fabrication • Achievable accelerating gradient in strong magnetic fields • Button tests with different materials and coatings • Thin beryllium windows for RF cavity • Curved thin beryllium windows • 201 MHz Cavity Program • Cavity design concept • Fabrication techniques • Preliminary high power test results of the cavity • Summary D. Li – RF R&D for Muon Ionization Cooling Channels

3. 50 0 100 m NF Ingredients Neutrino Factory comprises • Proton Driver • Primary beam on production target • Target, Capture, and Decay • Create π; decay into µ • Bunching and Phase Rotation • Conditioning: reduceE of bunch • Cooling (Ionization Cooling) • Reduce transverse emittance • MICE • Acceleration • 130 MeV ~ 20–50 GeV • Storage Ring • Long straight High gradient and low frequency RF cavities are needed, and they must be normal conducting and work in strong magnetic fields D. Li – RF R&D for Muon Ionization Cooling Channels

4. SC magnets SC magnets Muon Ionization Cooling Liquid Hydrogen Absorbers Muon beam Muon beam Low Frequency NC RF Cavities 4-D Cooling: • High gradient RF cavities to compensate for lost longitudinal energy • Strong magnetic field to confine muon beams • Energy loss in LH2 absorbers Goal: • Development of NC 201-MHz cavity operating at ~ 16 MV/m (~ 30 MV/m at 805-MHz)in a few-Tesla solenoidal B field D. Li – RF R&D for Muon Ionization Cooling Channels

5. RF Cavities in a Muon Ionization Cooling Channel LH2 Absorber Eight201-MHz RF cavities LH2 Absorber RFCC modules MICE Cooling Channel Courtesy of S. Q. Yang, Oxford Univ. D. Li – RF R&D for Muon Ionization Cooling Channels

6. SC coupling Coil Curved Be window Cavity Couplers Vacuum Pump 201-MHz cavity RFCC Module for MICE(Engineering Design) D. Li – RF R&D for Muon Ionization Cooling Channels

7. RF Cavity for Muon Ionization Cooling Channel • Requirements of RF cavity for muon ionization cooling channel • High cavity shunt impedance, high gradient and high field • Gradient at 201 MHz: ~ 16.5 MV/m (Kilpatrick criterion: 15 MV/m) • Gradient at 805 MHz: ~ 30 MV/m (Kilpatrick criterion: 26 MV/m) • Pillbox-like RF cavity with closed iris (iris terminated by curved thin Be windows) • Higher shunt impedance • Independent phase control, higher transit factor • Lower peak surface field • Highest Possible Gradient of Normal Conducting RF Cavity D. Li – RF R&D for Muon Ionization Cooling Channels

8. Cooling R&D Programs Normal conducting RF cavity studies • Goals: • Design and build high gradient RF cavities to explore engineering challenges and develop technical solutions • Operate the cavity in strong magnetic field to learn RF conditioning and operation in strong magnetic fields, and explore the gradient limits; RF breakdown physics (Palmer’s talk)  solutions & improve or update the cooling channel designs • Programs: • Experimental studies at 805 MHz using a pillbox cavity with curved thin beryllium (Be) windows • Tests are being conducting at MTA (MuCool Test Area) • 201 MHz cavity design, fabrication and tests • Be windows R&D (for both 201- & 805-MHz cavities) • Thermal and mechanical stabilities at high accelerating gradients • Scattering and limits • SC solenoids (coupling magnets) D. Li – RF R&D for Muon Ionization Cooling Channels

9. Review of Experimental RF Program at 805-MHz 805-MHz pillbox cavity design, fabrication and tests D. Li – RF R&D for Muon Ionization Cooling Channels

10. Experimental Programs • Development of the 805-MHz Pillbox cavity • Design and fabrication of the cavity • Highest possible shunt impedance and high acceleration gradient at the order of ~ 30+ MV/m • Allowing for testing of Be windows with different thickness, coatings, and other windows as well • Copper windows, flat Be windows, and curved Be windows • Study RF cavity operation and conditioning under the influence of strong external magnetic fields (a few Tesla) at both the solenoid and gradient modes • Be windows R&D • Mechanical stabilities under RF heating • Prototype and FEA modeling • Evolutions of Be windows D. Li – RF R&D for Muon Ionization Cooling Channels

11. Up to 12 MW peak power SC Solenoid 805 MHz pillbox cavity inside the SC solenoid Test Setup at MTA Conditioning and operation of the cavity in strong magnetic field (up to 5-T) and searching for materials and coatings that will withstand high peak electric fields in magnetic fields • Tests of curved thin beryllium windows • Tests of buttons with local field enhancement; different materials and coatings • Materials tested • Cu w. TiN coatings • W, Mo • More buttons are available Demountable button D. Li – RF R&D for Muon Ionization Cooling Channels

12. Experimental Results We have conducted experimental studies at 805 MHz at MTA, FNAL • Open 5-cell cavity reached 25 MV/m gradient (54 MV/m surface field) • Large dark current with surface and window damage • Pillbox cavity test exceeded its design gradient of 30 MV/m with no magnetic field and reached up to 40 MV/m • Thin Be windows with TiN-coated surface have been tested versus magnetic fields up to 4 Tesla • No surface damage was found on the Be windows • Little multipacting was observed; achievable accelerating gradient limit is a function of the external magnetic field Achievable gradients degrade with the increase of magnetic field D. Li – RF R&D for Muon Ionization Cooling Channels

13. Molybdenum buttons –LBNL TiN_Cu #2 Button Test Results Ti-N Cu buttons D. Li – RF R&D for Muon Ionization Cooling Channels

14. 201-MHz RF Cavity R&D 201-MHz cavity design, fabrication and tests D. Li – RF R&D for Muon Ionization Cooling Channels

15. The Cavity Body Profile Spherical section at the equator to ease addition of ports (±~ 6.5o) Elliptical-like (two circles) nose to reduce peak surface field Stiffener ring 2o tilt angle 42-cm 6-mm Cu sheet allows for uses of spinning technique and mechanical tuners similar to SCRF ones 121.7-cm De-mountable Pre-curved Be windows to terminate RF fields at the iris Low peak surface E-field at iris Derun Li - Lawrence Berkeley National Lab - July 1, 2008

16. Cavity Design Parameters • The cavity design parameters • Frequency: 201.25 MHz • β = 0.87 • Shunt impedance (VT2/P): ~ 22 MΩ/m • Quality factor (Q0): ~ 53,500 • Be window diameter and thickness: 42-cm and 0.38-mm • Nominal parameters for MICE and cooling channels in a neutrino factory • 8 MV/m (~16 MV/m) peak accelerating field • Peak input RF power: 1 MW (~4.6 MW) per cavity • Average power dissipation per cavity: 1 kW (~8.4 kW) • Average power dissipation per Be window: 12 watts (~100 watts) Derun Li - Lawrence Berkeley National Lab - July 1, 2008

17. 201 MHz Cavity Concept Spinning of half shells using thin copper sheets and e-beam welding to join the shells; extruding of four ports; each cavity has two pre-curved Beryllium windows, but also accommodates different windows Derun Li - Lawrence Berkeley National Lab - July 1, 2008

18. Fabrication of the MuCool Cavity • Two 6-mm thick copper shells are formed from annealed, flat sheet using a spinning technique • Two half shells are e-beam welded together at equator to form the cavity • Separate copper nose piece rings are e-beam welded to cavity iris (aperture) • RF and vacuum ports are formed by pulling a die through a hole cut across the equator weld (extruding) • Externally brazed tubes provide cooling • Cavity inside surfaces are finished by mechanically buffing and electro-polishing • Two thin, pre-curved beryllium windows are mounted on cavity aperture • Cavity is mounted between two thick aluminum vacuum support plates Derun Li - Lawrence Berkeley National Lab - July 1, 2008

19. 42-cm Curved Be Windows for 201-MHz Cavity • We have two successful windows • 21-cm and 0.38-mm thick • “Good” braze (between annular frames and foil) • Thin TiN coatings • Windows installed pointing to the same direction in the cavity • Already high power tested in 201-MHz cavity D. Li – RF R&D for Muon Ionization Cooling Channels

20. RF Test Setup at MTA The 805-MHz and 201-MHz cavities at MTA, FNAL for RF breakdown studies with external magnetic fields. 201 MHz cavity 805 MHz pillbox cavity Derun Li - Lawrence Berkeley National Lab - July 1, 2008

21. High Power RF Tests with Cu & Be Windows • The cavity was first tested with flat copper windows and reached ~ 16 MV/m quickly and quietly • The cavity then was tested with thin and curved Be windows and again reached to ~19 MV/m quickly • Cavity frequency had to be retuned • Cavity frequency was stable during the operation, however, we did observe frequency shift due to RF heating on the windows • Frequency shift of ~ 125 kHz (from 0 to ~ 19 MV/m, 150-micro-second pulse, 10-Hz repetition rate) in ~ 10 minutes, well within the tuning range (230 kHz/mm per side,  2-mm range) • With a few hundred Gauss stray field from Lab-G magnet, the cavity gradient can be maintained at 19 MV/m • To test with stronger external magnetic fields • Move the cavity closer to Lab-G magnet • SC coupling coil for MuCool Derun Li - Lawrence Berkeley National Lab - July 1, 2008

22. Lab-G magnet The 201-MHz cavity Tests with Stronger Magnetic Fields • New vacuum pumping system • Separation of the nearest curved Be window from the face of Lab-G magnet is 10-cm (before was 110-cm) • Maximum magnetic field near the Be window 1.5 Tesla (at 5 Tesla in magnet) • Test Results: • Multipactoring was observed over the entire magnetic field range up to 1.1-T at nearest Be window • A strong correlation exists between cavity vacuum and radiation levels • We have achieved ~ 14 MV/m at 0.75-T to the nearest curved thin Be window • The test results are very encouraging, data analysis is being conducted Derun Li - Lawrence Berkeley National Lab - July 1, 2008

23. Numerical Study with B Field • Preliminary studies, in collaboration with Dr. Z. Li and his colleagues at SLAC using Omega-3P and Track-3P codes • Cavity with flat windows: 5 MV/m on axis; 2-T uniform external magnetic field; scan of a few points from one cavity side Trajectories with external B = 2-T field Trajectories without external B field E field contour D. Li – RF R&D for Muon Ionization Cooling Channels

24. Summary • R&D Programs • Normal Conducting RF cavity R&D • Be window R&D • Experimental studies at 805 MHz using the pillbox cavity with buttons at MTA • The 201 MHz test cavity fabrication completed; tests will continue • Reached 16 MV/m quickly with Cu windows and without magnetic fields • Reached ~ 19 MV/m quickly again with curved and thin Be windows in low stray magnetic field and ~ 14 MV/m in stronger stray magnetic fields • Tests with SC coupling magnetic fields and beam • Understanding RF breakdown in strong magnetic field and gradient limits, plans are being developed (Palmer’s talk) • RF breakdown model and predictions • Magnetic shielding D. Li – RF R&D for Muon Ionization Cooling Channels