Booster Synchrotron Cavities: An Overview in the Context of PIP

Mohamed Hassan, Timergali Khabiboulline , Vyacheslav Yakovlev

07/03/2013

Fermilab’s 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

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

Specifications for Design of New Accelerating Cavities for the Fermilab Booster

Current Modified

Frequency Range 37.80-52.82 MHz Same

Vacc 55 KV 60 KV (possibly more)

R/Q ~50 ~50

Duty Cycle Effectively 25% 50%

Repetition Rate Effectively 7 Hz 15 Hz

Cavity Tuning Horizontal Bias Same

Beam Pipe Diameter

2.25” >3”

Higher Order Mode Impedance

<1000 Ohm <1000 Ohm

Cooling LCW at 95 F, Water flow up to 21 gpm

Same

Brainstorming

Frequency Tuning

Variable Volume

Variable Permeability

Variable Permittivity

Tuning Mechanism

PiezoelectricMagnetostrictive

Ferromagnetic

Ferroelectric

Tuning Range

<0.05%

~40%

~10%

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

Ferromagnetic Tuning

Classical way of tuning microwave components using bias current that will change the permittivity of the material

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

𝐻 φ̂+h φ̂=(𝐻+h) φ̂

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)

Hh

rotating (on cone) magnetic vector – Gyromagnetic Resonance H=f/2.8

Comparison Between Booster CavitiesFNAL Booster TRIUMF SSCL LEB EHF-Booster

Energy Range [GeV] 0.4-8.0 0.45-3.0 0.6-11 1.2-9.0

Bias Parallel Perpendicular Perpendicular

Frequency [MHz] 37.7-53.3 46.1-60.8 47.5-59.8 50.5-56.0

Peak Gap Voltage [kV]

2*27 62.5 127.5 2*36

Cavity Length [m] ~2.4 ~1.23 ~1.25 ~3.25

Accelerating Time [ms]

35 10 50 20

Repetition Rate 7 50 10 25

Ferrite Material Ni-Zn Yttrium Garnet Yttrium Garnet

Ferrite Material Toshiba, Stackpole

TT-G810 TT-G810

Cavity Q 250-1200 2200-3600 2800-3420

Cavity R/Q 50 35 36

Status Operating Prototype Prototype

Tunable Booster Cavities

Parallel Biased Perpendicular Biased

Bias Field is Parallel to the RF Field Bias Field is Perpendicular to the RF Field

rotating (on cone) magnetic vector – Gyromagnetic Resonance H=f/2.8

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

Hh

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 KilpatrickTheoretical 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

Max Field in AirElectric Field for 55KV

1.7 MV/m

Assumed 0.25” Blend Radius upon John Reid’s recommendation

Max Electric FieldElectric Field for 55kV

1.7 MV/m

Electric Field for 60 kV

1.85 MV/m

3.3 MV/m3.6 MV/m

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

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)?

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

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."

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?