1 Introduction and overview of FFAG accelerators S. Machida CCLRC-ASTeC 7 February, ffag/machida_ ppt & pdf

2 Contents 1.Cyclotron, synchrotron, and FFAG (11) 2.Revival (6) 3.Recent activities (12) 4.Non-scaling FFAG for muon acceleration (13) 5.Non-scaling FFAG for other applications (1) 6.Summary (1)

3 1.Cyclotron, synchrotron, and FFAG (11) 2.Revival (6) 3.Recent activities (12) 4.Non-scaling FFAG for muon acceleration (13) 5.Non-scaling FFAG for other applications (1) 6.Summary (1)

4 Accelerators of medium energy (< GeV) In uniform and fixed field, revolution frequency is constant. Cyclotron produces continuous beams, but fixed energy. No focusing in longitudinal direction. Weak focusing in transverse direction. Bunch current is limited by longitudinal space charge. 590 MeV is maximum. Magnetic fields increases synchronized with beam momentum. Beams go through a fixed orbit. Accelerated beams are available only as a pulse. Focusing in longitudinal direction. Strong focusing in transverse direction. Bunch current is limited by transverse space charge. Energy frontier machine. cyclotronsynchrotron Cyclotron, synchrotron, and FFAG

5 FFAG (Fixed Field Alternating Gradient) Fixed field like cyclotron –No feedback between magnet and RF. Operation is easier. –Cost of power supply is low. –Repetition can be higher and make high average current. (strong) Focusing in both longitudinal and transverse direction like synchrotron –More particles can be accelerated. –Beam size is smaller and vacuum chamber is smaller. Variable energy like synchrotron –Medium energy machine is usually multi-purpose. Cyclotron, synchrotron, and FFAG

6 Comparison CyclotronSynchrotronFFAG Fieldfixedvariedfixed Repetitioncontinuousslow pulse <50Hz fast pulse ~1kHz Focusing in longitudinal noyes Focusing in transverse weakstrong Average current mediumlowhigh? (not demonstrated) AcceptanceLarge? in H Small in V Small in H Small in V Large in H Large in V Energyfixedvariable strong point weak point Cyclotron, synchrotron, and FFAG

7 FFAG accelerator Invented in early 1950s. –Ohkawa in Japan, Symon in US, and Kolomenski in USSR. Research program at MURA (Midwestern University Research Associate) in US –Construction of electron FFAG of 180, 400 keV, and 40 MeV. –Proposal of 30 GeV proton FFAG Cyclotron, synchrotron, and FFAG

8 Good old days at MURA Bohr Chandrasekhar 400 keV radial sector180 keV spiral sector 40 MeV two beam accelerator All are electron FFAG. Cyclotron, synchrotron, and FFAG

9 How does FFAG work? (field profile) Bending radius cannot be constant for all momentum. However, sharp rise of field makes orbit shift small. Focusing force can be constant if the field gradient increases with radius. Proposed field profile in radial direction is k >1 Bz(r) r Cyclotron, synchrotron, and FFAG Gradient of high p Gradient of low p Orbit of low p Orbit of high p

10 How does FFAG work? (transverse focusing) Alternating gradient can be realized by two ways. F(  ) has alternating sign. radial sector Add edge focusing. spiral sector Bz(r) r r + Cyclotron, synchrotron, and FFAG

11 How does FFAG work? (radial and spiral sector) machine center Radial sector consists of normal and reverse bends. Spiral sector use edge to have vertical focusing. Cyclotron, synchrotron, and FFAG

12 How does FFAG work? (cardinal conditions) Geometrical similarity   : average curvature  : local curvature  : 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 Cyclotron, synchrotron, and FFAG

13 A way to change output energy Change k value by trim coils. Low momentum particle will reach the outer (extraction) orbit with low k. Bz(r) r k (high) k (low) injection radius extraction radius extraction momentum with high k. (Bz(r) h *r ex ) extraction momentum with low k. (Bz(r) l *r ex ) Cyclotron, synchrotron, and FFAG

14 Three reasons to stop development Development is stopped in late 1960s because, 1.Magnet design was complicated. It was hard to get desired 3-D fields profile in practice. 2.No material for RF cavity. It requires high shunt impedance, high response time, and wide aperture. 3.Synchrotron was more compact and better choice for accelerator of high energy frontier. Cyclotron, synchrotron, and FFAG

15 1.Cyclotron, synchrotron, and FFAG (11) 2.Revival (6) 3.Recent activities (12) 4.Non-scaling FFAG for muon acceleration (13) 5.Non-scaling FFAG for other applications (1) 6.Summary (1)

16 Revival in late 1990s Technology becomes ready and enough reason to re-start development, 1.3-D calculation code such as TOSCA becomes available. Static fields can be modeled precisely. 2.RF cavity with Magnetic Alloy (FINEMET as an example) has most suitable properties for FFAG. 3.Growing demands for fast cycling, large acceptance, and high intensity in medium energy accelerator regime. revival

17 Magnet can be made with 3-D modeling code With an accuracy of 1%, 3-D design of magnet with complex shape becomes possible. Spiral shaped magnet for Kyoto-U FFAG (yoke with blue). revival

18 RF cavity with new material (MA)  QF 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. Magnetic Alloy also has High curie temperature ~570 deg. Thin tape (large core can be made) ~18  m Q is small (broadband) ~0.6 revival

19 Magnet for large acceptance From 1980s’, high intensity machine is demanded, not only high energy. Ordinary AG machine needs large aperture magnet to accommodate large emittance beam. Quad of J-PARC 3 GeV synchrotronMagnet of 150 MeV FFAG revival

20 First proton FFAG at KEK With all those new technology, proton FFAG (proof of principle) was constructed and a beam is accelerated in June revival

21 What we achieved from PoP FFAG Design procedure. –FFAG accelerator works as we expected. –3-D modeling of magnet is accurate enough. 1 ms acceleration (1 kHz operation) is possible. –MA cavity gives enough voltage. Enough acceptance in both longitudinal and transverse. Beam dynamics study –Multi-bucket acceleration –Acceleration with fixed frequency RF bucket –Resonance crossing, preliminary result revival

22 1.Cyclotron, synchrotron, and FFAG (11) 2.Revival (6) 3.Recent activities (12) 4.Non-scaling FFAG for muon acceleration (13) 5.Non-scaling FFAG for other applications (1) 6.Summary (1)

23 Three new programs started in Japan Hadron therapy prototype –150 MeV (initially aimed at 200 MeV) –Status: Completed. Muon phase rotation –PRISM –Status: Under construction. ADSR (accelerator driven sub-critical reactor) –Three cascade FFAGs to 150 MeV as a neutron source –Status: 1st spiral FFAG just starts commissioning. Recent activities

24 Hadron therapy prototype 150 MeV, 100 Hz, ~10 nA Why FFAG for hadron therapy ? –Easy operation. –High average intensity (more dose, more patients per year). –Spot scanning with high repetition pulses is possible. –Variable energy and acceleration of many ion species. Recent activities

25 Broad beam method (Conventional) vs. Spot scanning method Inevitable irradiation outside of the treatment field. Each patient needs his own shaped bolus. A small beam spot makes it possible to irradiate a well defined area. Non-uniform irradiation in the area is possible. Final collimator Bolus Ridge filter vs. Recent activities

26 Acceleration and extraction Beam signal during acceleration. Extraction efficiency is ~60% at the moment. (1.5 nA.) Recent activities

27 PRISM Momentum acceptance of +- 30%. Central momentum is 68 MeV/c. Why FFAG for phase rotation ? –Large acceptance in longitudinal and transverse. –Multiple use of RF cavity. –Prototype of muon accelerator. Injection and extraction kicker are necessary. r cm Recent activities

28 Accelerator Driven Sub-critical Reactor 150 MeV (1 GeV in future), 1  A (100  A in future). Why FFAG for ADSR ? –Stable operation (a fewer trip) compared with linac. –Almost DC beams. No difference between DC and 1 kHz for target. –1 GeV machine is no problem compared with cyclotron. –High average current. Recent activities

29 Spiral injector FFAG Commissioning just started. Variable energy with different k value is demonstrated. Recent activities

30 More projects are coming Hadron therapy machine in Ibaraki prefecture Neutron source for BNCT Industrial applications Neutrino factory Recent activities

31 Hadron therapy machine in Ibaraki prefecture Recent activities

32 Neutron source of BNCT (Boron Neutron Capture Therapy) Reactor was the only neutron source. With FFAG, similar neutron intensity is expected. Kyoto-U reactor Proposed neutron source (Mori) Recent activities

33 Neutrino factory In 2001, Japanese proposed neutrino factory based on FFAG muon accelerator. Recent activities

34 Problems to be solved (there are still many) Interference between main magnet and peripheral devices such as injection, extraction, and RF elements. Beam diagnostics. High intensity operation. H - injection. More efficient RF cavity. … Projects and R&Ds are going on in parallel. Recent activities

35 1.Cyclotron, synchrotron, and FFAG (11) 2.Revival (6) 3.Recent activities (12) 4.Non-scaling FFAG for muon acceleration (13) 5.Non-scaling FFAG for other applications (1) 6.Summary (1)

36 All of those FFAGs are conventional scaling FFAG If we can break scaling law, FFAG will be much simpler and magnet will be smaller. Why do we keep scaling (constant tune) during acceleration? Because of resonance in accelerator. Bz(r) r r No gentle slope at low momentum. - Orbit excursion is shorter. Constant gradient. - Linear magnet. Non-scaling FFAG for muon acceleration

37 Resonances in accelerators There are many resonances near operating tune. Once a particle hits one of them, (we think) it will be lost. In reality, however, operating tune moves due to imperfection of magnet (red zigzag line). Particles can survive after crossing resonances if resonance is weak and crossing is fast. x y Tune diagram of 150 MeV FFAG Non-scaling FFAG for muon acceleration

38 Non-scaling FFAG Muons circulate only a few (~15) turns in FFAG. Is resonance really harmful to a beam? Maybe not. Forget scaling law ! Let us operate ordinary AG synchrotron without ramping magnet. Orbit moves as momentum increases. –Large  p makes the orbit shift small. Focusing force decreases as momentum increases. Non-scaling FFAG for muon acceleration

39 Orbit for different momentum Orbit shifts more at larger dispersion section. No similar shape unlike scaling FFAG. non-scaling Non-scaling FFAG for muon acceleration low p high p

40 Tune variation in a cycle Tune decreases as a beam is accelerated. d (tune)/dT(turn)~1 for muon rings. Non-scaling FFAG for muon acceleration low phigh p

41 Acceleration (1) Acceleration is so quick that RF frequency cannot be synchronized with revolution frequency of muons. 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. 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 time voltage time of flight momentum Non-scaling FFAG for muon acceleration

42 Acceleration (2) 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) injection extraction Non-scaling FFAG for muon acceleration

43 Beam dynamics issues Acceleration out of RF bucket. “Gutter” acceleration. –Mismatch in longitudinal and transverse. –With finite initial transverse amplitude. Crossing of many resonances during acceleration. –Structure resonance has some effects. –With alignment errors, integer resonances have to be considered. Huge acceptance (30,000  mm-mrad) for muons. –Dynamic aperture without acceleration at injection energy. Non-scaling FFAG for muon acceleration

44 “Gutter” acceleration Finite transverse amplitude without transverse amplitudewith finite transverse amplitude Longitudinal phase space (phi, momentum) Horizontal is 5,000 pi mm mrad Vertical is zero 5 to 10 GeV ring Non-scaling FFAG for muon acceleration

45 Vertical is 5,000  mm-mrad, normalized, zero horizontal emittance. Shows the coupling due to nx-2ny=0 (structure) resonance. If we start finite horizontal and zero vertical emittance, no exchange of emittance. Resonance crossing without errors 5 GeV 10 GeV vertical emittance horizontal emittance Non-scaling FFAG for muon acceleration

46 Resonance crossing without errors, amplitude dependence 5,000 pi mm-mrad500 pi mm-mrad 0.5 pi mm-mrad Non-scaling FFAG for muon acceleration

47 Beam has to face many integer tunes. Resonance crossing with alignment errors tune per cell tune per ring Non-scaling FFAG for muon acceleration

48 Resonance crossing with alignment errors, envelope Horizontal is 10,000  mm-mrad, normalized, zero vertical emittance. Errors of 0, 0.05, 0.10, 0.20 mm (rms). 0. mm0.05 mm 0.20 mm 0.10 mm Horizontal phase space (x, xp) Non-scaling FFAG for muon acceleration

49 1.Cyclotron, synchrotron, and FFAG (11) 2.Revival (6) 3.Recent activities (12) 4.Non-scaling FFAG for muon acceleration (13) 5.Non-scaling FFAG for other applications (1) 6.Summary (1)

50 Issues Emittance is much smaller than muons. –30,000 pi mm-mrad vs. 300 pi mm-mrad –Half of the problems go away Acceleration is much slower. –15 turns vs. 15,000 turns –RF frequency modulation is possible. Resonance crossing is much more serious problem. –Alignment tolerance –Errors of fields strength Non-scaling FFAG for other applications

51 Summary FFAG has a potential as medium energy accelerator. Several projects are currently running in Japan. It still needs R&Ds (even for scaling FFAG). Non-scaling FFAG was proposed for muon acceleration. Simulation study for muon acceleration is going on. Need study to apply non-scaling FFAG to other applications.