1. FFAG Accelerator Proton Driver for Neutrino Factory
Alessandro G. Ruggiero
Brookhaven National Laboratory

2. A.G. Ruggiero -- NuFact 05 - Frascati
FFAG Accelerators
There is recently a renewed interest in Fixed-Field Alternating-Gradient (FFAG) Accelerators for a varied of applications in Nuclear and High-Energy Physics, Energy Technology and Medical Therapy. FFAG Accelerators have the capability to accelerate charged particles over a large momentum range (±30-50%) and the feature of constant bending and focusing fields. Thus magnets do not need to be ramped and particles can be accelerated very fast at the rate given by the limitation of the accelerating field from RF cavities placed in proper location between magnets. The performance of FFAG accelerators is thus to be placed between Linear Accelerators, with which they share the fast acceleration rate, and Synchrotrons as they allow the beam to re-circulate over few revolutions. They are similar to Cyclotrons but also take advantage of alternating focusing and bending for a more radial compact geometry, and free themselves from a rigid RF frequency – Path Length relation.
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

3. Initiatives at Brookhaven
Study of feasibility of FFAG Accelerators to accelerate intense beams of protons in the GeV energy range, for instance:
AGS Upgrade with a new FFAG injector spanning over the energy range of 400 MeV to 1.5 GeV corresponding to a momentum excursion of ±40%. The ring would be housed in the AGS tunnel and has henceforth a circumference of 807 m. The repetition rate is 2.5 or 5.0 Hz. (reference 1)
A site-independent 1.0-GeV Proton Driver capable to deliver as much as 10-MW of average beam power at the high repetition rate of 1 kHz. The injection energy is assumed at 200 MeV and the corresponding momentum excursion is then 45%. The circumference is 201 m. (reference 2)
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

4. A.G. Ruggiero -- NuFact 05 - Frascati
Reference Material
[1] A.G. Ruggiero, “1.5-GeV FFAG Proton Accelerator for the AGS Upgrade”, Invited Talk to EPAC-04, July 6-11, 2004, Lucerne, Switzerland.
[2] A.G. Ruggiero, “A 1-GeV 10-MWatt Proton Driver”, Invited Talk to ICFA-HB2004 Workshop, October 18-22, 2004, Bensheim, Germany.
[3] A.G. Ruggiero, “Design Criteria of a Proton FFAG Accelerator”, Proceedings of the FFAG’04 Workshop, October 13-16, 2004, KEK, Tsukuba, Japan.
[4] A.G. Ruggiero, “Adjusted Field Profile for the Chromaticity Cancellation in a FFAG Accelerator”, Proceedings of ICFA-HB2004 Workshop, October 18-22, 2004, Bensheim, Germany.
[5] A.G. Ruggiero, “Revised Adjusted Field Profile Estimate Criterion for FFAG Accelerators”, C-A/AP/208, BNL, March 25, 2005.
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

5. FFAG Lattices for Proton Beams
Two lattice configurations and magnet arrangements have been proposed:
Scaling Lattice that has the advantage of constant orbit parameters across the large momentum aperture but at cost of high bending fields, large magnet aperture and a limitation on available drift space. This lattice has been experimentally demonstrated at KEK with a pair of FFAG proton rings that have been commissioned.
Non-Scaling Lattice where orbit parameters vary considerably across the momentum aperture but with the benefit of lower bending fields, smaller magnet aperture and allowance for more drift space. The engineering and construction of a FFAG Accelerator based on this principle are greatly simplified and also expected to be more economical. Yet there is the concern of the beam stability when crossing a large number of resonances, some linear and others not, some driven by errors, misalignment and magnet imperfections, and others that appear to be structural. A Non-Scaling FFAG Accelerator has never been practically demonstrated.
Novel ideas have also recently been proposed, for instance to flatten the tune variation with an Adjusted Field Profile. In the case of proton beams, the concern of multiple resonance crossing is to be coupled to the longitudinal beam dynamics requiring a fast frequency-varying RF cavity system, and to the presence of space-charge forces at injection that, despite the fast rate by which the region of relevance is traversed, still may be significant.
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

6. FFAG Accelerators for Proton Drivers
3 FFAG Rings
GeV
GeV
GeV
Single-turn Transfer
400-MeV Injector
Multi-turn Injection
Single-turn Extraction
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

7. Three - FFAG Accelerator Rings (1)
Injector
Ring
Low-Energy
High-Energy
Kinetic Energy: Inj.
Ext.
GeV
0.40
1.50
4.45
11.6
 Inj.
0.7131
0.9230
0.9847
0.9972
p/p ±%
40.45
40.43
40.41
Circumference m
No. of Periods
136
Period Length
5.934
6.022
6.109
Harmonic Number
138
140 RF
 = m
MHz
36.02
46.03
49.75
50.38
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

8. Three - FFAG Accelerator Rings (2)
Injector
Ring
Low-Energy
High-Energy
Energy Gain / Turn
MeV
0.60
0.90
2.00
No. of Revolutions
1834
3278
3576
RF Peak Voltage
MVolt
1.20
1.80
4.00
Acceleration Period ms
6.137
9.398
10.001
Injection Period
1.144 --
Repetition Rate
kHz
0.137
0.106
0.100
Gap Voltage
kVolt 20 30 40
Gaps per Cavity 2
No. of Cavities 50
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

9. Three - FFAG Accelerator Rings (3)
Injector
Ring
Low-Energy
High-Energy
Protons / Cycle
1.0 x 1014
Average Current mA
1.60
Average Power MW
2.40
7.12
18.56
Nor. Emittance (full)
 mm-mrad
100
Act. Inj. Emittance
98.32
41.69
17.68
Bunching Factor
4.0
Tune-Shift
0.343
0.188
0.085
Half Vert. Beam Size cm
2.12
1.38
0.90
Half Hor. Beam Size
3.41
2.22
1.46
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

10. A.G. Ruggiero -- NuFact 05 - Frascati
RF Frequency & Power
Injector Ring
Low-Energy
Ring
High-Energy
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

11. Injector Ring and Injection
Injection Energy
MeV
400
H– Source Current mA 35
RFQ Transmission % 80
Chopping Ratio 50
Inject. Beam Current 14
Inj. Protons / turn
3.3 x 1011
Injected Turns
303
Pulse Length ms
1.144
Duty Cycle
0.1144
20 x 20 mm Foil
Injected Beam
400 MeV
1.5 GeV
Circulating Beam
12 cm x 25 cm Vacuum Chamber B1 B2
C1 Foil C2
From DTL
Injection Orbit
Bump Orbit
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

12. A.G. Ruggiero -- NuFact 05 - Frascati
Lattice Parameters D F S g
Non-Scaling Lattice
Injector
Ring
Low-Energy
High-Energy
Drifts: S g m
0.30
0.3044
1.3045
0.3088
F-sector: Length
Field min
Field max
Gradient kG
Kg/m
0.70
D-sector: Length
1.40
x max, in F
in D cm
17.22
13.88
17.46
14.07
17.69
14.26
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

13. A.G. Ruggiero -- NuFact 05 - Frascati
Lattice Functions
Phase Adv. / Cell H V o o
Betatron Tune, H
39.755
37.755
Nat. Chromaticity, H
Transition Energy, T i
All Rings
Linear Field Profile
Injection Orbit
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

14. Tune Variation & Radial Compactness same result for all Rings
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

15. Linear Field Distribution
Injector Ring
Low-Energy
Ring
High-Energy
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

16. Adjusted Field Profile (revised)
High-Energy
Ring
Same for all
Rings
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati

17. A.G. Ruggiero -- NuFact 05 - Frascati
Conclusions
* FFAG Accelerators for Proton Driver
up to 20 MW and 12 GeV are feasible
* Cost Estimate of 1.5 GeV FFAG is about 50 M$
* Issues:
Space Charge at Injection
Injector Linac (400-MeV DTL or ????)
Multi-turn Injection of H–
Magnet Feasibility
Adjusted Field Profile
Fast RF sweep (ferrite, RF power)
Numerical Tracking
* Repetition Rate --> 100 Hz
For Target one should raise it to > 1 kHz
Or CW Mode of Operation (Harmonic Number Jump)
June 23, 2005
A.G. Ruggiero -- NuFact 05 - Frascati