1. 201 MHz NC RF Cavity R&D for Muon Cooling Channels
Derun Li
Center for Beam Physics
International Neutrino Factory Workshop
WG-3, Frascati, Italy
June 21-26, 2005

2. Outline Introduction Ionization cooling
Requirements of RF cavities for muon cooling
RF cavity R&D activities
805 MHz cavity
201 MHz cavity
Status of the prototype cavity
Be window R&D
RF Couplers
Summary

3. Introduction Ionization cooling principle LH Absorbers
RF Cavities
Strong magnetic field to confine muon beams
Lose energy in LH absorbers
High gradient RF cavities to compensate for lost longitudinal energy

4. RF Cavity for Muon Cooling
Requirements:
Intense muon beams
Have short lifetime and large phase space
Interact weakly with matter
Confined in focusing channels
High gradients
17 MV/m at 201 MHz and 34 MV/m at 805 MHz
Normal conducting
Rounded closed “pillbox” cavity
Large and thin Be foils (low-Z) to terminate RF fields
High cavity shunt impedance
Low peak surface field
High accelerating efficiency with independent phase control
Less RF power

5. NCRF Cavity R&D Activities
Experimental studies using an 805 MHz pillbox cavity with demountable windows at MTA (Lab G), FNAL
Lab G Magnet (MTA)
805 MHz cavity inside
Achievable accelerating gradient is a function of external B
Be windows withstand high field without surface damage
How to interpolate the results to 201 MHz cavities?
Does this apply to 201 MHz cavities?

6. cooling lines on cavity
201 MHz Prototype Cavity
Goal: Design, build a 201-MHz cavity with large Be windows, and condition and operate it at 16+ MV/m in a few Tesla magnetic field
Cavity and its sub-components
Cavity body + water cooling
lines
Four ports and flanges
RF loop couplers
Cavity support structure
Cavity tuners
Ceramic RF windows (~ 4”)
Curved Be windows
Possible LN temperature
operation
Cavity design
concept
Layout of water
cooling lines on cavity

7. The 201 MHz Cavity Parameters
The cavity design parameters
Frequency: MHz
β = 0.87
Shunt impedance (VT2/P): ~ 22 MΩ/m
Quality factor (Q0): ~ 53,000
Curved Be window with radius and thickness: 21-cm and 0.38-mm
Nominal parameters for cooling channels in a neutrino factory
For up to ~16 MV/m peak accelerating gradient
Peak input RF power ~ 4.6 MW per cavity (assuming 85% of Q0 and 3 times filling time)
Average power dissipation per cavity ~ 8.4 kW
Average power dissipation per Be window ~ 100 watts

8. The Cavity Fabrication
Finished equator welding
We have successfully developed extruding
technique for port pulling over e-beam joints
E-Beam Welding of Stiffener
Ring at J-Lab

9. The Cavity + Sub-components
Ceramic RF window
Loop coupler
Finished cavity port

10. EP at J-Lab
After 1st EP run last week (June 2005)
U-Shape Electrode

11. Current Status & Near Term Plans
Continue EP at J-Lab
High pressure water rinsing
Vacuum Assembly and base pressure measurement
Packing and Shipping
Couplers, RF probes, gauges and vacuum pump assembly at the MTA, FNAL
Baking at the MTA, FNAL
RF conditioning (Ti-N coated copper windows to start with) without magnetic field

12. Curved Be Windows for 201 MHz Cavity
Succeeded in two curved Be windows for the 805-MHz cavity
Placed purchase order of three Be windows for 201-MHz cavity:
0.38 mm thick, 420 mm diameter
at Brush-Wellman (~100 watts per window with ∆T~ 55 degrees at nominal Study-II parameters)
Window is formed by applying a die at elevated temperature
Copper frame is brazed to Be window
Be windows will be Ti-N coated
Present a perfect conducting BC for RF.
Min. scattering and mechanically strong
420 mm
420 mm diameter curved Be window for the 201 MHz cavity formed
at Brush-Wellman.
The formed Be foil is sandwiched (brazed) in two annular Cu frames
The curved Be window status:
warping (1st one)
cracking (2nd) on edge, but usable
Two good windows already
To be Ti-N coated

13. RF Coupler Design and Status
Loop couplers at critical coupling
Prototype coupling loop design uses standard off-the-shelf copper co-axial components
Coupling loop has integrated cooling lines
Two SNS style RF windows mfg. by Toshiba
received (no cost to us !)
Two couplers with RF windows are complete
High power tested up to 600 kW in TW mode and 2.4 MW (peak) in SW mode
Ready to ship to MTA, FNAL
Bellows connection required on MICE cooling channel (Study-II) for thermal and dimensional reasons
Ceramic RF window
Loop coupler

14. Coupler Conditioning Load
RF Power
Two couplers
Load
The two Couplers
Conditioning started in the week of May 16, 2005 at SNS
Good vacuum ~ low 10-8 T
Achieved 600 kW in TW mode (matched load)
Achieved 10 kW average power (~ 9 kW average for nominal NF parameters)
Achieved 2.4 MW peak power in SW mode (at variable short positions)
Ceramic windows work fine in nearly two weeks of the RF conditioning

15. Mucool Test Area (MTA) Facility to test all components of
805-MHz
201-MHz
Facility to test all components of
cooling channel, not ionization cooling
At high beam power
Designed to accommodate full Linac Beam
1.6 X 1013 Hz
2.4 X 1014 p/s
RF power from Linac: 201 and 805 MHz test stands
Waveguides pipe power to MTA
Clean room ordered
On site assembly of windows
Currently plan to operate either RF or LH2/H2 tests, but not both simultaneously.
805 MHz experimental study
will resume soon
201 MHz cavity expect to
arrive next month

16. Eight 201 MHz Cavities for MICE
Two SC coupling Coils
Power couplers
Eight 201 MHz cavities
The 201 MHz prototype cavity is the baseline design for MICE

17. Summary MTA is ready to accept high power RF test programs
805 MHz experimental programs will resume soon at MTA, FNAL
201 MHz prototype cavity progress well and expect to be shipped to MTA, FNAL for high power tests next month (July 2005)
Significant progress on fabrication of curved Be windows
RF couplers have been high power tested and ready for assembly at the MTA, FNAL
MUCOOL 201 MHz cavity is the baseline design for MICE