Non -Scaling FFAG Cyclotrons - Erice 2009

NonNon--Scaling FFAG Scaling FFAG

CyclotronsCyclotrons

Alessandro G RuggieroAlessandro G Ruggiero

Brookhaven National LaboratoryBrookhaven National Laboratory

Hadron Beam Therapy of CancerHadron Beam Therapy of Cancer

Erice, Sicily, Italy Erice, Sicily, Italy ------ April 24April 24--May 1, 2009May 1, 2009

The title of my presentation was chosen for me as

Non-Scaling FFAG Cyclotrons

FFAG stands for Fixed-Field Alternating-Gradient accelerator.

Cyclotrons were conceived about 70-80 years ago. They are Fixed-Field

accelerators with the good property that the guiding and focusing magnetic

field does not need to vary during the acceleration cycle, simplifying thus the

concept and the construction.

At the start they were Weak Focusing devices. But later when the principle of

Alternating-Gradient that allows Strong Focusing was discovered, they truly

became FFAG accelerators. There is thus the (general) consensus that, to

avoid semantic, Cyclotron and FFAG refer to the same identical device. In

reality there are some major differences like the Momentum Range…

The Alternating Focusing is provided either by Radial-Shaped sectors or by the

alternating edges of Spiral-Shaped elements.

Alessandro G. Ruggiero 2Erice – Sicily April 29, 2009

Fixed-Field Accelerators

Cyclotrons and Microtrons are FF Accelerators.

Originally they were conceived as Weak Focusing with

Constant Field Profile. That is they were not AG

RF

RF


Constant RF

Fixed-Field Alternating-Gradient

• Spiral Sector FFAG

• Radial Sector FFAG

• FFAG Betatrons

• FFAG Synchrotrons

– Acceleration in Phase (Protons)

– Gutter Acceleration (Muons)

Reverse Bend

Edge Focusing

D F

FD


There are two types of FFAG Cyclotrons according to their magnet lattice that

provides bending and focusing at the same time:

Scaling Lattice, and

Non-Scaling Lattice

Both were already known half-a-century ago when the first prototypes were built at

MURA. At that time Scaling Lattice was adopted, and even recently few Scaling

FFAG were built in Japan (KEK, Kyoto Univ.).

Scaling FFAG have the good feature that, by arranging the bending/focusing magnet

sequence with a proper profile, it is possible to cancel (to first order) the variation of

betatron oscillation frequency (tune) with beam momentum (Chromaticity). In this

mode it is possible to accelerate at constant betatron tunes, avoiding thus (in principle)

the crossing of major resonances; though the dependence of the betatron tunes with

betatron oscillation amplitude could still be appreciable.

But the required field profile for a Scaling Lattice is achieved with a considerable large

magnet size (aperture), and a high bending field. All existing/operating FFAG in the

world have a Scaling Lattice.


Scaling FFAG Field Profile: B = ± B0 (r0 / r)k

Non-Scaling FFAG Field Profile: B = ± B0 ( 1 + g r) Linear

FFAG Ring is a continuous unbroken sequence of Periods

FODODoubletsTripletsPimplets (G. Rees)….

Non-Scaling F D F

Scaling D F D

Triplet provides strongest Focusing with minimum Dispersion.It allows most compact MagnetsAlessandro G. Ruggiero 6Erice – Sicily April 29, 2009

Design of a Proton NS-FFAG Accelerator

� Sector Magnets. Parallel Entrance and Exit planes

� Trajectory is made of arcs of circle only on the Reference Orbit

� Sharp Magnet Edges

A. G. Ruggiero “Design Criteria of a Proton FFAG Accelerator”, Proc. of the Intern.

Workshop on FFAG Accelerators, Oct. 13-16, 2004, KEK, Tsukuba, Japan, pages 47-

63

� No Entrance and Exit Angles for Reference Orbit

� It minimizes Magnet Width

� It Allows more Drift between Triplets

F FD

Extraction Trajectory

Injection Trajectory


Photo of ERIT - taken by A.G. Ruggiero on Nov. 7, 2007 at KURRI


Boron Neutron Capture Therapy

Because of the lattice simplicity, the smaller magnet size, compactness, and the expectation

of a lower construction cost, Non-Scaling FFAG accelerators have recently received closer

scrutiny, and have been proposed for a variety of applications (Neutrino Factories, Muon

Colliders, Proton and Heavy Ion Drivers, …). Several feasibility studies were done, and

specific proposals were written; nevertheless the study remained mostly academic. There

was concerns about the beam stability and survival in the presence of multiple crossings of

low-order resonances. Thus there are not yet operating Non-Scaling FFAG Cyclotrons,

with the exception of a demonstration device scaled-down in energy and size using

electrons:

EMMA (Electron Model for Muon Acceleration, or

Electron Model for Multiple Application)

EMMA has two main goals:

Experiment with Multiple-Resonance Crossing

Demonstration of Fast Acceleration

The hope is that if the acceleration rate is large enough, when the beam crosses a major resonance, the

resulting growth and loss are kept to a minimum.


Electrons: 10 – 20 MeV C = 16.65 m 42 periods



The 10-MeV Proton Storage Ring for AI

Neutron Beam

Foil

MCS & ELS35-kV 10-mA Ion Source

2 MeV RFQ

2-10 MeV DTLInj. Kicker

Extr. Kicker

Circumference 10 m Kinetic Energy 10 MeV Circulating Protons 1010

(7.2 mA)

Beam Dump

200-300 kVolt 54 MHz RF Cavity

Magnets

p-Beam circulating in 12 bunches

N Spot Size 20-40 mm N Divergence 20-40 mrad N Prod. Rate 2.7 1012 /s

… Space Charge Limit at 10 12 circulating Protons …

Overall Dimensions 5m x 10m


Dejan Trbojevic


Dejan Trbojevic and G. Rees


Non-Scaling FFAG accelerators have of course also been proposed for

medical applications, namely Cancer Hadron Therapy, the topic of this

Workshop. There are two initiatives that I am aware of:

Work resulting from the collaboration Keil-Sessler-Trbojevic (KST), and

PAMELA in United Kingdom (see next Talk)

I like to report here about the KST study. This is at the stage of a feasibility

study with no obvious (to me) outcome (though Sessler may say something

about this). PAMELA project is already at the proposal stage (?) and ready

for funding (?) depending on the result of the EMMA experiment.

The goal of the KST approach is to provide two beams:

Protons at 250 MeV, and

Ions of Carbon at 400 MeV/u


KST


KST


KST


KST


KST


Alessandro G. Ruggiero Erice – Sicily April 29, 2009 21

I

Iron yoke and coil flux density (top), 4T

magnet inside LiHe vessel

2.0 m

straight

5.5 –

6.9 m

12C 6+

Flattening the Tune: NS Non Linear FFAGAdjusted Field Profile (AGR, G. Rees)

Carol Johnstone

KST


Acceleration of charged particles in a circular accelerator requires the following condition between

the revolution frequency f and the radio-frequency (rf) frf to be satisfied

frf = h f f = ββββ c / C (1)

where h, the harmonic number, is a positive integer. In the case of low-energy protons or heavy-ions the revolution frequency f varies during the acceleration cycle. If the harmonic number h is

kept at a constant value then the radio-frequency frf has to vary accordingly. That can be

accomplished with the use, for example, of ferrite-tuned rf cavities. This method lengthens

considerably the acceleration period because of the limitation on the peak rf voltage that can be

achieved with ferrite.

Faster acceleration can be obtained with higher radio-frequency and, eventually, the use of

superconducting rf cavities that can be operated only at constant frequency. In this case, the

harmonic number h should be allowed to vary also to compensate for the variation of the revolution

frequency f so to maintain frf constant during the acceleration cycle. Nevertheless, the harmonic

number h, being a positive integer, cannot vary continuously, but rather jump from an integer value

to another integer value. This method is called Harmonic Number Jump

(A. G. Ruggiero, Phys. Rev. ST-A&B 9, 100101 (2006) ).

Consider a single rf cavity located in one spot of the accelerator ring. The acceleration cycle is a

two-step process: (1) an energy kick at the cavity location, (2) the ring circumference that takes the

particle back to the cavity. The HNJ method requires that the energy gain ∆En during the n-th traversal of the cavity is adjusted to cause a change in the travel period Tn between consecutive

crossings of the cavity so that the particle is pushed forward or back exactly by ∆h rf harmonics and appears in an exactly identical rf bucket ahead or trailing by ∆h rf wavelengths. In linear

approximation (the exact calculation is done on the computer), this is accomplished by requiring:

∆En = βn2 γn3 E0 ∆h / hn (1 – αp γn2) = (Qe Vn / A) sin φnAlessandro G. Ruggiero Erice – Sicily April 29, 2009 23

Alessandro G. Ruggiero Erice – Sicily April 29, 2009 24

∆h = 1 constant

A. G. Ruggiero, Nucl. Phys. B, Proc. Suppl. 155, 315 (2006)

1.5 – GeV 200 – m NS-FFAG Proton Driver

TM010 + TM011

KST


KST


KST


r=4.81 m

34 cm

dispersion

βy

βx

C = 30.24 m

δp/p=+50%

3.6 m

r=4.28 m

Cavities

Extraction/injection Kickers

C=26.8m

Dejan Trbojevic


Proton

Carbon

Possible Advantages: 1.Amount of steal in the case of proton machine is smaller than the CYCLOTRON or scaling FFAG - Magnets are relatively small size. (see next…)2.Adjustment of the energy is done by changing the extraction time – reducing the total number of turns.3.Fast repetition rate ~1000 turns 4.Relatively small size – 30–50 meter circumference. (see next…)5.Easier to operate as the magnetic field is fixed (with respect to synchrotrons).6.Better preservation of the emittance than in the CYCLOTRON7.For carbon the superconducting machine is necessary to reduce the size but the orbit offsets are small - the magnets are small

Disadvantages:1.Large total power required for the cavities distributed around the circumference, this probably makes larger operating cost - this might not be true as shown in the poster at the workshop. 2.For protons, orbit offsets are not any more few millimeters (this was a compromise between the size, number of cells-periods) but +12 and – 6 cm for the lowest energy3.The momentum range is still limited to ± 50% with the kinetic energy range from 32-250 MeV, so it would require additional RFQ and Linac (or one more FFAG).



1.3 m Diameter

1.5-GeV AGS – NS-FFAG

Side View

Diagnostic & Steering Boxes

D-Sector Magnet

F-Sector Magnets

Flanges & Bellows

Vacuum Pump

10 cm

Top View

D-Sector Magnet

F-Sector Magnets

Flanges & Bellows

Vacuum Pump

30 cm


RF Cavity


D-Sector Magnet

F-Sector Magnets

Vacuum Pump

100 k$

500 k$

0 m 2.0 m 4.0 m 6.0 m


1.5-GeV AGS – NS-FFAG

B1

B2

C1 Foil C2

From DTL

Injection Orbit

Bump Orbit

Circulating Beam

Injected Beam

20 x 20 mm Foil

400 MeV 1.5 GeV

30 cm x 10 cm Vacuum Chamber

Kicker Septum

F D F F D F F D F


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Non -Scaling FFAG Cyclotrons - Erice 2009