Muon Acceleration and FFAG Shinji Machida KEK Nu

Muon Acceleration and FFAG Shinji Machida KEK Nu. Fact 05 Summer Institute June 12 -20, 2005

Content 1. 2. 3. 4. Acceleration of muons Evolution of FFAG as a muon accelerator Design example of muon acceleration • Reference (among others): – BNL-72369 -2004, FNAL-TM-2259, LBNL-55478 – Nu. Fact. J Design study report

1 Acceleration of muons

Requirement (1) • Acceleration: as quick as possible – Life time of muon is ~2. 2 us. – Example • At momentum of 0. 3 Ge. V/c • Lorentz factor g~3, Velocity b~0. 94. • Flight path length ~2000 m – That is even true on the lower momentum side.

Requirement (2) • Acceptance: as large as possible – Muons are produced as secondary particles of protons – Cooling before acceleration if necessary • Longitudinal emittance – dp/p ~ +-100% – dt or dx can be controlled by the width of primary proton: ~1 ns or 300 mm – dp/p * dx * bg = 1000 mm at 0. 3 Ge. V/c • Transverse emittance – 10 ~ 100 mm

Machine candidate (1) • Everyone knows modern high energy accelerator is synchrotron. Why not for muons? • VRCS (very rapid cycling synchrotron) – Rapid (or fast) cycling means time required for acceleration from injection to extraction is short. – The most rapid cycling machine at the present is ISIS at RAL, which has 50 Hz repetition rate. It still takes 10 ms to complete a whole cycle. Rep. rate ISIS J-PARC KEK-PS Fermilab AGS booster CPS booster 50 Hz 25 Hz 1 Hz 20 Hz 15 Hz ~7. 5 Hz

VRCS (continued) • In order to accelerate muons, rep. rate must be much faster. • 4600 Hz design exists. (D. J. Summers, et. al. ) Power supply and Eddy current are issues. d. I/dt is too much.

Machine candidate (2) • If we cannot use AC (ramping) magnet, the alternative is to use only RF cavities. This is a linear accelerator. • Linac (linear accelerator) – To accelerate muons to 20 Ge. V, the length becomes 4000 m with 5 MV/m accelerating cavity.

Linac (continued) • Linear collider assumes 35~45 MV/m, why not for muons? – Muon emittance is much larger than electron emittance in linar collider. – To make acceptance larger, RF frequency must be relatively lower (200 MH instead of 1. 5 GHz) and field gradient is lower as well. • Rule of thumb is that field gradient is proportional to square root of frequency. – Cost is another issue.

Machine candidate (3) • Synchrotron radiation is not a problem unlike electron. We can use bending arcs and reuse linac several time. • RLA (recirculating linear accelerator) – Use 400 m linac with energy gain of 2 Ge. V 10 times, we can accelerate muons to 20 Ge. V. – Need 10 arcs to bend 10 different momentum separately because we give up ramping magnet. This machine looks like JLAB machine.

RLA (continued) • This was a baseline for muon acceleration until a few years ago. • Switchyard becomes complex with more number of arcs and large muon emittance.

Machine candidate (4) • Suppose if we can make orbit in bending arc less sensitive to momentum, the same arc can be used for different momentum. • FFAG (fixed field alternating gradient) – Large field index in radial direction makes orbit shift as a function of momentum small. In accelerator terminology, dispersion function is small. – How small it should be? Beam size is something we can compare with. – Such an optics can be realized with high periodicity lattice. There is no clear separation of straight for acceleration and bending arc.

FFAG (continued) • Easy to understand with alternative bending. • Alternative bending with finite field gradient gives alternative focusing. RF RF

FFAG compared with others • Cost effective. Use RF cavity several times. • Large acceptance. • Machine is simple. – Fixed field magnet – No switchyard • Accelerating gradient is relatively low or must be low.

Acceleration of muons Summary • Muons have to be accelerated as quick as possible against muon life time. • Muon accelerator has to have large acceptance because a muon beam is produced as a secondary particle and emittance is huge. • Several schemes are considered: VRCS, Linac, RLA, and FFAG. At the moment, FFAG seems most feasible and cost effective. • Requirement for muon collider is different. Although machine is similar, muon collider has to assume small emittance to increase luminosity.

2 Evolution of FFAG

Invention • AG principle was invented in 1950 s. – By Courant, Synder, Christofilos – Combination of convex (focusing) and concave (defocusing) elements makes net focusing. horizontal vertical • FFAG principle was invented a few years later – By Ohkawa, Symon, Kolomenski

FFAG vs. ordinary AG • Fixed field (DC field) makes a machine simpler. – Cost of power supply for magnet is less. • No synchronization between magnet and RF frequency. – Repetition rate is only determined by RF frequency change. – Repetition rate of o. AG is determined by ramping speed of magnet. • Large momentum acceptance. – +-100% vs. +-1% • Magnet size tends to be large. – Even it is small, orbit moves in horizontal direction.

Field profile • Sharp rise of field makes orbit shift small. Bz(r) k >>1 r

Transverse focusing • Alternating gradient can be realized by two ways. • F(q) has alternating sign. radial sector Bz(r) r + r • Add edge focusing. spiral sector

Radial and spiral sector Radial sector consists of normal and reverse bends. machine center Spiral sector use edge as vertical focusing. machine center

MURA days (Midwest University Research Associate) • In US, electron model was constructed at MURA. – Radial sector (400 ke. V) – Spiral sector (180 ke. V) – Two beam accelerator (collider) • In Russia and Japan – Magnet design and fabrication.

“Two beam accelerator” Particles with the same charge can rotate in both directions. – Sign of neighboring magnets is opposite. – Outer radius has more bending strength. Colliding point

Extinction • People at that time aimed at high energy frontier. • Because orbit moves, magnet tends to be bigger. – Magnet of AG focusing machine has to be small compared with ZGS. – Magnet pole face has a bit complicated shape. • To accelerate protons, broadband RF cavity with high gradient has to be developed.

Revival • The right machine in the right place. • Large magnet can be made with 3 D modeling code. • RF cavity with new material. Three factors above are combined together in 2000.

The right machine in the right place • From 1980 s’, high intensity machine is demanded, not only high energy. • Ordinary AG machine needs large aperture magnet to accommodate large emittance beam.

Large magnet can be made with 3 D modeling code With an accuracy of 1%, 3 D design of magnet with complex shape becomes possible.

Gradient magnet with gap shape • A magnet with field index k=7. 6

RF cavity with new material (MA) Magnetic Alloy has • Large permeability ~2000 at 5 MHz • High curie temperature ~570 deg. • Thin tape ~18 mm • Q is small ~0. 6 Q can be increased with cutting core if necessary.

m. Qf (shunt impedance) • A m. QF remains constant at high RF magnetic RF (Brf) more than 2 k. G • Ferrite has larger value at low field, but drops rapidly. – RF field gradient is saturated.

Proton FFAG at KEK • With all those new technology, proton FFAG (proof of principle) was constructed and a beam is accelerated in June 2000.

Evolution of FFAG summary • FFAG is an old idea back to 1950 s. • FFAG concept was not fully appreciated because people want accelerator for energy frontier. • Technology was not ready yet. • RF cavity with new material and 3 D calculation tool make it possible to realize proton FFAG. • Proof of principle machine demonstrates that FFAG machine works as it designed.

3 FFAG as a muon accelerator

Scaling FFAG Originally, FFAG design satisfied scaling law, – Geometrical similarity r 0 : average curvature r : local curvature q : generalized azimuth – Constancy of k at corresponding orbit points k : index of the magnetic field The field satisfies the scaling law. Tune is constant independent of momentum: scaling FFAG

Resonance in accelerator • Why we need to keep constant tune during acceleration? • Because there are many resonances near operating tune. Once a particle hits one of them, it will be lost. In reality, however, operating tune moves due to imperfection of magnet (red zigzag line). ny nx

Non scaling FFAG • Muons circulate only a few turns in FFAG. • Is resonance really harmful to a beam? • Forget scaling law ! Let us operate ordinary AG synchrotron without ramping magnet. • Orbit shifts as momentum is increased. • Focusing force decreases as momentum increases.

Orbit for different momentum • Orbit shifts more at larger dispersion section.

Tune variation in a cycle • Tune decreases as a beam is accelerated.

Resonance crossing simulation • Animation • If the acceleration is fast, resonance is not a problem.

Acceleration (1) • Acceleration is so quick that RF frequency cannot be synchronized with revolution frequency of muons. • Revolution frequency changes because orbit shifts and path length changes although speed of mouns is already a speed of light. • If you look at orbits carefully, path length at the central frequency is shortest.

Acceleration (2) • In a first half of a cycle, path length becomes shorter and revolution frequency becomes higher. • In a second half of a cycle, path length becomes longer and revolution frequency becomes lower.

Acceleration (3) • Suppose we choose RF frequency that is synchronized with revolution frequency at the center. • In the first half of a cycle, a particle lags behind the RF. • At the center, a particle is synchronized with RF. • In the second half, a particle lags again. voltage low center high time

Acceleration (4) • This is called “Gutter acceleration”. dp/p (normalized) • In the longitudinal phase space, a particle follows the path with constant color. • If there is enough RF voltage, a particle can be accelerated to the top energy. Phase (1/2 pi)

FFAG as a muon accelerator summary • FFAG used to satisfy scaling law, that assures geometrical similarity of orbit and tune independent of momentum. • If resonance crossing is not harmful, scaling law is not necessary. • Just ordinary synchrotron without ramping magnet makes a new concept of FFAG, namely non-scaling FFAG. • Acceleration is o fast that RF frequency cannot be synchronized with revolution frequency. • “Gutter acceleration” is one possible way.

4 Design example of muon accelerator

Japanese scheme • Scaling FFAG • Acceleration with a bucket of low frequency RF, 5~20 MHz

Acceleration • No time to modulate RF frequency. • 1 MV/m (ave. ) RF voltage gives large longitudinal acceptance. • From 10 to 20 Ge. V/c within 12 turns.

Accelerator chain • Before acceleration – Target and drift – No cooling section • Four scaling FFAGs, – – 0. 3 - 1. 0 Ge. V 1. 0 - 3. 0 Ge. V 3. 0 - 10. 0 Ge. V 10. - 20. Gev • If physics demands, another FFAG – 20. - 50. Ge. V

Longitudinal emittance vs acceptance (after target and drift) Acceptance of US scheme is 0. 167 e. V. sec (150 mm). Difference comes from frequency of RF (5 vs. 201 MHz).

Transverse emittance ~100 mm (100, 000 pi mm-mrad)

Hardware R&D (1) Low frequency RF (ferrite loaded) Shunt impedance Ferrite core

Hardware R&D (2) Low frequency RF (air core)

Hardware R&D (3) Superconducting magnet

US scheme (Europe’s similar) • Combination of RLA (LA) and Non scaling FFAG • High frequency RF, 201 MHz

Accelerator chain • Before acceleration – Target, drift, buncher, rf rotator, and cooling • Linac – 0. 220 Ge. V - 1. 5 Ge. V • RLA – 1. 5 - 5. Ge. V • Two non-scaling FFAGs – 5. - 10. Ge. V – 10. - 20. Ge. V • If physics demands, another non-scaling FFAG – 20. - 50. Ge. V

Before acceleration • Three more stages compared to Japanese scheme.

A way to make small emittance fit into 201 MHz RF There is some stage to make longitudinal emittance smaller so that 201 MHz RF can be used.

Emittance evolution before FFAG injection • Cooling is also necessary to fit into the acceptance. longitudinal Emittance [mm] transverse Path length [m]

Acceleration system requirements From Reference 1. Initial momentum 0. 3 Ge. V/c Final momentum 20 Ge. V/c Normalized transverse acceptance 30 mm Normalized longitudinal acceptance 150 mm Bunching frequency 201. 25 MHz Maximum muons per bunch 1. 1 x 1011 Muons per bunch train per sign 3. 0 x 1012 Bunches in train 89 Average repetition rate 15 Hz Minimum time between pulses 20 ms

Scaling vs. non-scaling • Scaling machine principle is proven. • Large acceptance so that cooling is not needed. • Magnet tends to be larger. Cost more. • Non-scaling machine can be more compact. Cost less. • Need cooling to fit a beam into the acceptance. • Principle have to be proven. – Resonance crossing – Gutter acceleration – Demonstration by electron model is scheduled in UK.

Design example of muon acceleration summary • Japanese scheme assumes low frequency (~5 MHz) RF and no cooling is necessary. It uses scaling FFAG. • US and Europe scheme assumes high frequency (~200 MHz) RF. It uses non-scaling FFAG. • Hardware R&D is going on. • Proof of principle model for non-scaling FFAG is scheduled in UK.

Appendix FFAG as a proton driver

Requirement of proton driver (1) • Beam power = energy x current = energy x (particles per bunch) x (repetition rate) • Energy – MW using a few Ge. V or more energetic protons. • Particles per bunch and Repetition rate – From accelerator point of view, low ppb is preferable. – Probably rep. rate does not matter as long as the beam power above is obtained.

Requirement of proton driver (2) • Beam quality – Short bunch is preferable for smaller longitudinal emittance. – Momentum spread of protons is not important because that of muons can not be small. – Beam size (transverse emittance) is not important either.

Machine candidate (1) • Slow cycling synchrotron (0. 1 ~ 1 Hz) • J-PARC is one of examples – Maximum energy is 50 Ge. V. – Particles per bunch is high, 3 e 14 to obtain 0. 75 MW – Should be more to upgrade to a few MW facility – Space charge and beam instability are problems.

Machine candidate (2) • Rapid cycling synchrotron (10 ~ 50 Hz) • ISIS upgrade is one of examples – Maximum energy is 50 Ge. V. – Particles per bunch can be reduced, – Design of 30 Ge. V with 50 Hz is feasible.

Machine candidate (3) • Rapid cycling linac (10 ~ 50 Hz) • SPL is one of example – Maximum energy is limited to a few Ge. V. – More particle per bunch is needed compared with RCS – Space charge and beam instability problem are less because acceleration is quicker.

Machine candidate (4) • FFAG (100 ~ 1000 Hz) – Maximum energy can be as high as synchrotron. – Particles per bunch can be much less. – Space charge and beam instability problem are less because acceleration is quicker. SCS RCL FFAG energy ~50 Ge. V ~3 Ge. V ~20 Ge. V rep. rate 0. 1~1 10~50 50 100~1000 ppb high low much low Space charge etc. serious moderate less No problem

Exercise (1) • Life time of a muon is 2. 2 ms. However, it becomes longer when it is accelerated and Lorentz boosted. Calculate analytically or numerically what percentage of muons does survive when it is accelerated from 0. 3 Ge. V/c to 20 Ge. V/c assuming two cases of average energy gain. One is 1 Me. V/m and the other is 5 Me. V/m. • This exercise can be extended to more complex system. For example, assume there are two FFAGs, one from 0. 3 Ge. V to 3 Ge. V, and the other from 3 to 20 Ge. V. Also assume the number of RF cavity is 5 times more in the bigger ring and RF cost is proportional to square of average energy gain. To make the cost of muons minimum, how we can choose the average energy gain in the first and the second ring?

Exercise (2) • Consider periodic beam transport line consisting of focusing and defocusing quadrupole with the same absolute strength k (sign is opposite). There is drift space in between and separation is L. – Using thin lens approximation, show phase advance as a function of k and L. – Assume that non-scaling FFAG consists of the simple FODO cell. If phase advance per cell is limited between 30 degrees and 150 degrees, what is the maximum momentum ratio from injection to extraction? – Show Courant-Synder parameters a, b, g and phase advance m at the entrance of focusing and defocusing quadrupole at injection, extraction and at the center momentum.

Exercise (3) • Scaling FFAG has magnetic field shape as – Momentum compaction factor ac is defined as – Show momentum compaction factor of scaling FFAG. – RF bucket (half) height is where E is total energy, h is harmonic number, h is slippage factor defined as how much RF voltage is required to accelerate 10 to 20 Ge. V when h=20 and k=280.

Exercise • Any questions, you can send to shinji. machida@kek. jp

Subjects to be studied • Electron model of non scaling FFAG – New scheme of acceleration – Resonance crossing • High intensity operation • Optimization of scaling magnet • Make the magnet superconducting