201 MHz NC RF Cavity R&D

1. 201 MHz NC RF Cavity R&D Derun Li Center for Beam Physics Lawrence Berkeley National Laboratory WG3 at NuFact 2004 July 28, 2004

2. Collaborators R. MacGill, J. Staples, S. Virostek, M. Zisman Lawrence Berkeley National Laboratory R. Rimmer, L. Philips, G. Wu Jefferson National Laboratory D. Summers University of Mississippi W. Lau, S. Yang Oxford University, UK 201 MHz NC RF Cavity R&D Derun Li

3. Outline • Introduction • 201 MHz cavity • Cavity design • Fabrication status • Progress on curved Be windows • FEA modeling and prototype for 805 MHz cavity • 21-cm curved Be windows • Experimental study at 805 MHz • Pillbox cavity with demountable windows • Cavity iris termination: foils and grids • Surface damage study with a button in the cavity • Summary 201 MHz NC RF Cavity R&D Derun Li

4. Introduction • Muon cooling channels call for normal conducting RF cavities with highest possible accelerating gradients • Muon beam: secondary, unstable and has short decay time • (~ 2 s at rest) • Created with LARGE 6-D phase space • High gradient with large beam aperture (iris) • Strong external magnetic field needed to confine muon beams • Normal conducting • Muon beam decays, manipulation must be done quickly, including cooling • High accelerating gradient • Goal: design and engineering for • RF cavity with high shunt impedance, large beam aperture and withstand high peak RF field • Higher gradient for the same input RF power (less RF power) • No surface breakdown 201 MHz NC RF Cavity R&D Derun Li

5. Introduction (cont’d) • Shunt impedance of an RF cavity • High peak RF field: Kilpatrick number • Required gradient at 201 MHz: ~ 16 MV/m • Kilpatrick: 15 MV/m • Required gradient at 805 MHz: ~ 30 MV/m • Kilpatrick: 26 MV/m 201 MHz NC RF Cavity R&D Derun Li

6. Be window Choice of NC RF Cavity • Conventional approach: NC “Ω” cavity with open iris • “High” shunt impedance for open iris cavities • high peak surface field (high Epk/Eacc ~ 2) • Shunt impedance reduces with the increase of iris → Very difficult (if not possible) to achieve the gradient required for a muon cooling channel • Taking advantage of muon beam’s penetration property, we choose a RF cavity with irises terminated by windows or grids • Pillbox like cavity • Lower peak surface field (low Epk/Eacc ~ 1) • Independent phase control, higher transit factor • High shunt impedance, but with large windows 201 MHz NC RF Cavity R&D Derun Li

7. 201 MHz Cavity R&D • Prototype of 201 MHz cavity with curved Be windows • Cavity design • Body profile • Be windows, grids • Ports • Coupler • RF (ceramic) windows • Tuners • Cavity fabrication going reasonably well • Cu sheets + spinning techniques • E-beam welding • Ports extruding • Cleaning, … • Progress and status of cavity fabrication • Placed purchase order of 21-cm radius of curved Be windows 201 MHz NC RF Cavity R&D Derun Li

8. 201 MHz Cavity Concept Spinning of half shells and e-beam welding At NuFact 2003 ! Water cooling channels Cavity design accommodates different windows 201 MHz NC RF Cavity R&D Derun Li

9. 201 MHz Cavity Design Spinning of half shells using thin Cu sheets and e-beam welding to join the shells. Four ports across the e-beam joint at equator. Cavity design uses pre-curved Be windows, but also accommodates different windows or grids. 201 MHz NC RF Cavity R&D Derun Li

10. Cavity Body Profile Spherical section at the equator to facilitate fabrication of ports (± ~ 6o) Elliptical-like nose shape to further reduce peak surface field Stiffener ring Spinning 2o tilt angle E-Beam welding 6-mm Cu sheet permits spinning technique and mechanical tuners similar to SCRF ones Port extruding Curved Be windows De-mountable pre-curved Be windows pointing in the same direction to terminate RF fields at the iris Bolted Be window 201 MHz NC RF Cavity R&D Derun Li

11. The cavity parameters The cavity design parameters (~1.2 m diameter, 0.43 m long) • Frequency: 201.25 MHz • β = 0.87 • Shunt impedance (Vacc2/Pw): ~ 22 M/m • Quality factor (Q0): ~ 53,000 • Curved Be window radius and thickness: 21-cm and 0.38-mm (better performance with significant savings, compared to pre-tensioned flat Be windows) Nominal parameters for a cooling channel in neutrino factory • 16 ~ 17 MV/m accelerating field • Peak input RF power ~ 4.6 MW per cavity (85% of Q0, 3τ filling) • Average power dissipation per cavity ~ 8.4 kW • Average power dissipation per Be window ~ 100 watts 201 MHz NC RF Cavity R&D Derun Li

12. Spun half shells + RF & CMM measurements 3 CMM scans per half shell conducted at 0o, 45o, 90o, respectively. Measured frequency: 196.97 MHz (simulated frequency: 197.32 MHz) CMM scans, RF frequency and Q measurements of half shells; Cu tape for better RF contacts. 201 MHz NC RF Cavity R&D Derun Li

13. E-Beam welding at JLab Stiffener ring Preparation for e-beam welding of the stiffener ring (left); after the e-beam Welding (above) 201 MHz NC RF Cavity R&D Derun Li

14. Recent progress for the welding fmeasured = 200.88 MHz 201 MHz NC RF Cavity R&D Derun Li

15. Recent Progress Port extruding We have successfully developed techniques to extrude ports across e-beam welded joints. 201 MHz NC RF Cavity R&D Derun Li

16. What’s Next? Cavity has been cleaned and ready for nose welding and ports annealing in next two weeks 201 MHz NC RF Cavity R&D Derun Li

17. Status • Cavity cleaning at J-Lab • 2~3 Months delay for using NASA e-beam welder • Extruding ports • Cosmetic welding • Continue engineering designs • Loop coupler: conceptual → engineering design • Supporting structure (vacuum): developed • RF windows (SNS type 4” coaxial window) • Tuner: mechanical • Cavity test at MTA this year 201 MHz NC RF Cavity R&D Derun Li

18. Window for muon RF cavity • Performance for an ideal window • Transparent to muon beams • Low-Z material • Perfect electric boundary to RF field • Good electrical conductivity • Mechanical strength and stability • No detuning of cavity frequency under RF heating • Beryllium is a good material for windows • High electrical & thermal conductivity with strong mechanical strength and low-Z • Engineering solutions being explored so far • Thin, flat Be foils (pre-tensioned) • Curved Be foils • Grids (Ph.D thesis of M. Alshaor’a at IIT) 201 MHz NC RF Cavity R&D Derun Li

19. Curved Be window R&D • Designed, fabricated and tested pre-tensioned flat Be windows • They work, but expensive; balance between thickness & RF gradient • Progress on FEA modeling and engineering design of all approaches • Fabricated pre-curved windows of S.S. and Be for 805 MHz cavity • Cu frames + curved Be foils: better performance with BIG savings Fabricated pre-curved Be window: 16-cm in diameter and 0.254 mm thick ANSYS simulations: mechanical vibration modes 201 MHz NC RF Cavity R&D Derun Li

20. More on Be Windows • Model validation • Preliminary measurements on mechanical vibration frequency of curved Be windows for 805 MHz cavity agree with FEA modeling • Ti-N coatings at LBNL recently • Curved Be windows • Pre-tensioned Be windows • Placed purchase order of 3 curved Be windows for 201 MHz cavity (21-cm radius, 0.38-mm thick) • Baseline design for MICE cavities 201 MHz NC RF Cavity R&D Derun Li

21. Summary • Good progress on NC RF R&D programs • Experimental study at 805 MHz • Tests of Ti-N coated curved and pre-tensioned Be windows at MTA • Tests on grids • 201 MHz cavity prototype • Be window: FEA modeling and prototype • Progress on 201 MHz test cavity fabrication; ready for testing this year (2004) • Test plans are being developed • The 201 MHz cavity has been used as baseline design for MICE 201 MHz NC RF Cavity R&D Derun Li