1. Booster Synchrotron Cavities: An Overview in the Context of PIP Mohamed Hassan, Timergali Khabiboulline, Vyacheslav Yakovlev 07/03/2013

2. Fermilabs Booster Parameters The Fermilab Booster is a synchrotron that accelerates protons from 400 MeV to 8 GeV The Booster circumference is 474.2 meters, the magnetic cycle is a biased 15 Hz and the RF operates at harmonic 84 of the revolution frequency

3. Proton Improvement Plan Objectives: Increase the Proton Source throughput while maintain good availability and acceptable residual activation through 2025. S. Hederson, Dec 2010 Goals: – PIP should enable Linac/Booster to deliver: 1.80E17 protons per hour (12 Hz) by May 1, 2013 deliver 2.25E17 protons per hour (15 Hz) by January 1, 2016 – while maintaining Linac/Booster availabilty > 85% and residual activation at acceptable levels and ensuring a useful operation life of the proton source through 2025. S. Hederson, Dec 2010 S. Henderson, Accelerator Advisory Committee, Nov. 7-9, 2011

4. Specifications for Design of New Accelerating Cavities for the Fermilab Booster CurrentModified Frequency Range37.80-52.82 MHzSame V acc 55 KV60 KV (possibly more) R/Q~50 Duty CycleEffectively 25%50% Repetition RateEffectively 7 Hz15 Hz Cavity TuningHorizontal BiasSame Beam Pipe Diameter 2.25>3 Higher Order Mode Impedance <1000 Ohm CoolingLCW at 95 F, Water flow up to 21 gpm Same

5. Brainstorming Frequency Tuning Variable Volume Variable Permeability Variable Permittivity Tuning Mechanism Piezoelectric Magnetostrictive FerromagneticFerroelectric Tuning Range <0.05%~40%~10%

6. Slow versus Fast Frequency Tuning Slow Using motor driven mechanism Response time ~60 s Fast Using piezoelectric/ magneto- strictive element Response time ~10 ms Faster Using a ferromagnetic material Response time ~ ms Fastest Using Ferroelectric material Response time ~ ns

7. Ferromagnetic Tuning Classical way of tuning microwave components using bias current that will change the permittivity of the material

8. Parallel Biased Cavities Bias Field is Parallel to the RF Field Ferrites with High Saturation Magnetization (Ni-Zn) Larger values of Mu (Larger Losses, Lower Q) Relatively limited by the heating in the ferrites Gradient is limited also by voltage breakdown in air H h

9. Perpendicular Biased Cavities Bias Field is Perpendicular to the RF Field Ferrites with Relatively Low Saturation Magnetization (Mn- Zn) Smaller values of Mu (Smaller Losses, Larger Q) Cooling is difficult Some environmental hazards because of Beryllium Oxide Only prototypes (up to our knowledge) H h

10. Comparison Between Booster Cavities FNAL BoosterTRIUMFSSCL LEBEHF-Booster Energy Range [GeV]0.4-8.00.45-3.00.6-111.2-9.0 BiasParallelPerpendicular Frequency [MHz]37.7-53.346.1-60.847.5-59.850.5-56.0 Peak Gap Voltage [kV] 2*2762.5127.52*36 Cavity Length [m]~2.4~1.23~1.25~3.25 Accelerating Time [ms] 35105020 Repetition Rate7501025 Ferrite MaterialNi-ZnYttrium Garnet Ferrite MaterialToshiba, Stackpole TT-G810 Cavity Q250-12002200-36002800-3420 Cavity R/Q503536 StatusOperatingPrototype

11. Tunable Booster Cavities Parallel BiasedPerpendicular Biased Bias Field is Parallel to the RF FieldBias Field is Perpendicular to the RF Field Ferrites with High Saturation Magnetization (Ni-Zn) Ferrites with Relatively Low Saturation Magnetization (Mn-Zn) Larger values of Mu (Larger Losses, Lower Q) Smaller values of Mu (Smaller Losses, Larger Q) H h H h

12. Voltage Breakdown In Air ~ 3 MV/m (30 KV/cm) In Vacuum (according to Kilpatrick) is ~ 10 MV/m (theoretical) 18 MV/m (measured) Theoretical Kilpatrick Theoretical Peter et. Al. Measured W. Peter, R. J. Fael, A. Kadish, and L. E. Thode, Criteria for Vacuum Breakdown in RF Cavities, IEEE Transactions on Nuclear Science, Vol. Ns-30, No. 4, Aug 1983

13. Max Field in Air Electric Field for 55KV 1.7 MV/m Assumed 0.25 Blend Radius upon John Reids recommendation

14. Max Electric Field Electric Field for 55kV 1.7 MV/m Electric Field for 60 kV 1.85 MV/m 3.3 MV/m 3.6 MV/m

15. Why Perpendicular Biased Cavity Could Achieve Higher Voltage Gradient? Vacuum fills most of the cavity volume (breakdown ~ 100 kV/cm) Vacuum windows are right away on the tuner connection Tuner is filled with dielectric Air fills most of the cavity volume (breakdown ~30 kV/cm) Vacuum windows are nearby the gap Tuner is filled with air

16. Possible Changes to the Current Design How about rounding the stem corners with large radius >0.25? How about enlarging the stem connection between the tuner and the cavity? How about moving the vacuum window position? How about filling the tuner with dielectric medium (though it might be a problem for cooling)? How about designing a perpendicular biased tuner to be used with the current cavity? How about using TRIUMF cavity (we have one somewhere here in FNAL)?

17. Conclusion Possible design changes have been identified Major changes in the current tuner have been suggested Perhaps TRIUMF cavity could be used as a test prototype for perpendicular-biased option

18. Ferroelectric Booster Cavity? High Q across the band ~1000 Fast Response ~ ns But 10% tunability Need high voltage to be applied 50 kV/cm Bias Circuit will be completely different Conceptual, No Prototype Very early developement Would require quite involved development Newsham, D., N. Barov, and J. S. Kim. "RAPIDLY TUNABLE RF CAVITY FOR FFAG ACCELERATORS."

19. Uranium Compounds under Low Temperature& High Pressure It might be a day that we see ferromagnetic superconducting cavities Aoki, Dai, and Jacques Flouquet. "Ferromagnetism and superconductivity in uranium compounds." arXiv preprint arXiv:1108.4807 (2011). Ferromagnetic Superconducting?