Muon Acceleration and FFAG

1. Muon Acceleration and FFAG Shinji Machida KEK NuFact05 Summer Institute June 12-20, 2005

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

3. 1 Acceleration of muons

4. Requirement (1) • Acceleration: as quick as possible • Life time of muon is ~2.2 us. • Example • At momentum of 0.3 GeV/c • Lorentz factor g~3, Velocity b~0.94. • Flight path length ~2000 m • That is even true on the lower momentum side.

5. 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 GeV/c • Transverse emittance • 10 ~ 100 mm

6. 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.

7. 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. dI/dt is too much.

8. 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 GeV, the length becomes 4000 m with 5 MV/m accelerating cavity.

9. 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.

10. 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 GeV 10 times, we can accelerate muons to 20 GeV. • Need 10 arcs to bend 10 different momentum separately because we give up ramping magnet. This machine looks like JLAB machine.

11. 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.

12. 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.

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

14. 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.

15. Acceleration of muonsSummary • 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.

16. 2 Evolution of FFAG

17. Invention • AG principle was invented in 1950s. • 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

18. 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 oAG 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.

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

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

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

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

23. “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

24. 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.

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

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

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

28. Gradient magnet with gap shape • A magnet with field index k=7.6

29. 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.

30. mQf (shunt impedance) • A mQF remains constant at high RF magnetic RF (Brf) more than 2 kG • Ferrite has larger value at low field, but drops rapidly. • RF field gradient is saturated.

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

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

33. 3 FFAG as a muon accelerator

34. Scaling FFAG Originally, FFAG design satisfied scaling law, • Geometrical similarity r0 : 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

35. 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

36. 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.

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

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

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

40. 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.

41. 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.

42. 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. low center high voltage time

43. Acceleration (4) • 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. • This is called “Gutter acceleration”. dp/p (normalized) Phase (1/2 pi)

44. FFAG as a muon acceleratorsummary • 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.

45. 4 Design example of muon accelerator

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

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

48. Accelerator chain • Before acceleration • Target and drift • No cooling section • Four scaling FFAGs, • 0.3 - 1.0 GeV • 1.0 - 3.0 GeV • 3.0 - 10.0 GeV • 10. - 20. Gev • If physics demands, another FFAG • 20. - 50. GeV

49. Longitudinal emittance vs acceptance(after target and drift) Acceptance of US scheme is 0.167 eV.sec (150 mm). Difference comes from frequency of RF (5 vs. 201 MHz).

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