1. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 805-MHz Cavity Refurbishment and suggestions on future tests Derun Li and Robert Rimmer* Lawrence Berkeley National Laboratory *Jefferson Laboratory RF Workshop at Fermilab October 15, 2008

2. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 The 805-MHz Cavity Pillbox cavity Be (or Cu) windows Waveguide + window Coupler Thermo-couples or view ports Three more view ports on the equator Superconducting magnet Thin Cu window and end- plate film RF Power Up to three RF probes The cavity has been tested for over six years.

3. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 The 805-MHz Cavity with RF Button Cavity modified to house buttons

4. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Surface Damage: windows, RF button Windows Button holder Cu button

5. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Inspection of the Damaged Cavity Surface The cavity is at JLab now. Damage found at the Kidney- shape RF coupler and cavity surface that faces the coupler

6. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Cavity Repair Setup

7. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Cleaning of the Damaged Cavity Surface The cavity body surface has been cleaned The outer edge of the kidney-shape coupler has been rounded by 2-mm Inside edge is being polished by hands

8. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Cavity Repair Status The cavity surface has been cleaned very wellThe cavity surface has been cleaned very well –Should be as good as it was, Will condition well without magnetic fieldsWill condition well without magnetic fields With magnetic fields?With magnetic fields? –Do we want to do EP? Help prevent from field emissionHelp prevent from field emission Can be done at JLab, time to set up + chemicalsCan be done at JLab, time to set up + chemicals Possible problem?Possible problem? –Trapped chemicals due to the cavity history –Cleaning –Damaged button holder Exam with microscope to measure pit sizes per R. Palmer’s requestExam with microscope to measure pit sizes per R. Palmer’s request Should we make a new button holder?Should we make a new button holder?

9. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Tests with Buttons More tests using the repaired cavity with buttonsMore tests using the repaired cavity with buttons –Previous test results or gradient limits might be limited by damaged surface, not peak fields on buttons –How about a double button configuration? Two buttons for each material or coatingTwo buttons for each material or coating SUPERFISH simulations show f = 805.89-MHzSUPERFISH simulations show f = 805.89-MHz Inspect surface damage for each button experimentInspect surface damage for each button experiment button

10. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Do We Need A New Cavity? IF the repaired cavity does not work well as we expected, we should consider to build a new cavity at 805-MHz cavity. If the answer is yes,IF the repaired cavity does not work well as we expected, we should consider to build a new cavity at 805-MHz cavity. If the answer is yes, –What’s kind of cavity? –Would be nice to have a cavity that can be rotated inside Lab-G magnet to study ExB effects: Coupler designCoupler design –In fact, we may learn from any angle configurations, with help from powerful simulation tools

11. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 The 201-MHz Cavity Electric field distribution on axis (two curved windows) The MUCOOL prototype cavity (3-D model) f = 199.927 MHz

12. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 New Ideas for 201-MHz RF Programs?

13. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Test B-Field Effects Needs the SC coupling coilNeeds the SC coupling coil –Field map is not the same as in the cooling channel – How about a coaxial quarter wave cavity? Test 0, 90, and other anglesTest 0, 90, and other angles Mechanical stability Water cooling A folded quarter-wave cavity: more stored energy Coaxial quarter- wave cavity

14. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Coaxial Coupler in B Coaxial loop couplers for MICE 201-MHz cavities are close to SC coupling coil magnet –Potential RF breakdown in external magnetic fields –May need Ti-N coating? –What tests can be done? –Any suggestions?

15. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Multipacting Simulation for Muon Collider Cavity Lixin Ge and Zenghai Li Advanced Computations Department Stanford Linear Accelerator Center August 21, 2008

16. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Numerical Study with B Field Preliminary studies, in collaboration 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 E field contour Trajectories without external B field Trajectories with external B = 2-T field

17. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 High impact energy (heating?) Impact energy too low for MP Impact energy of resonant particles vs. field level w/o external B field w/ 2T external axial B field 2 types of resonant trajectories: Between 2 walls – particles with high impact energies and thus no MP Around iris – MP activities observed below 1 MV/m SEY > 1 for copper 2T Multipacting w/ & w/o External Axial Magnetic Field SEY > 1 for copper Resonant trajectory

18. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 SEY > 1 for copper w/ 2T B field at 10 degree w/ 2T transverse B field Multipacting w/ External Transverse Magnetic Field Impact energy of resonant particles vs. field level SEY > 1 for copper 2T 2 types of resonant trajectories: Between upper and lower irises Between upper and lower cavity walls Slight MP activities observed above 6 MV/m 2 types of resonant trajectories: One-point impacts at upper wall Two-point impacts at beampipe MP activities observed above 1.6 MV/m

19. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Simulation Condition Geometry: Pure pill box –Radius: 0.1425m –Length: 0.1m Field: –0 mode –Frequency: 805MHz Multipacting Simulation Condition: –Average Field Level Scan: 0-60MV/m, interval: 0.5MV/m –External Magnetic Field: 2T –Scan Angle: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 deg –Time step: 0.001 period to resolve external magnetic field –Total Particles: 164x1000x11x61x2=220,088,000 –Each field level running 30 periods

20. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Impact energy of resonant particles vs. field level Multipacting without External Magnetic Field Resonant trajectory at 5MV/m High impact energy (heating?) E magnitude field B magnitude field

21. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 With External Transverse Magnetic Field No resonant trajectory when angle < 7 deg 2T Single Impact Energy vs. field level for 4 deg

22. D. Li and R. Rimmer, RF Workshop, Fermilab, 10-15-2008 Multipacting with External Transverse Magnetic Field Impact energy of resonant particles vs. field level 2T Resonant Trajectory at 50 MV/m with 2T external magnetic field(10 deg angle)