Status of MEIC Beam Synchronization

1. Status of MEIC Beam Synchronization V.S. Morozov on behalf of MEIC study group (summary of the results of a series of special MEIC R&D meetings) MEIC Collaboration Meeting, JLab October 5-7, 2015

2. Introduction Overview of synchronization requirements and schemes Harmonic jump Beam path-length adjustment Scanning synchronization RF cavity tuning range Engineering aspects of moving magnets Conclusions Outline

3. Issue: energy-dependence of ion velocity desynchronizes ions with electrons Schemes involving magnet movement Moving magnets in the ion collider ring Moving whole arcs or a small number of magnets in chicane(s) With or without harmonic jump Moving magnets in the electron collider ring & adjusting RF in both rings Moving (almost) whole arcs or a small number of magnets in chicane(s) With or without harmonic jump Some combination of the above two schemes Schemes with small or no magnet movement Bypass beam lines Scanning synchronization Problem & Synchronization Schemes

4. Time between collisions: Synchronized Beams Electrons Ions

5. Suppose ion energy set significantly lower: Desynchronization Electrons Ions

6. Ion path length change such that Synchronization Option I Electrons Ions

7. Electron path length change along with ion and electron frequency adjustment such that Synchronization Option II Electrons Ions

8. Harmonic jump such that Harmonic Jump at “Magic” Energies Electrons Ions

9. Harmonic jump Pros: simplifies synchronization, highly beneficial to detection and polarimetry Cons: potential for a dynamic instability, luminosity loss Moving ion magnets Pros: does not require changes in RF and CEBAF injection Con: moving superconducting magnets not trivial Moving electron magnets Pro: warm magnets simpler to move Cons: requires adjustment of RF and possibly CEBAF injection Moving (almost) whole arcs (arguments reversed for chicane) Pros: relatively small change in magnet spacings Con: have to deal with a large number of magnets Bypass beam lines Pro: less magnet movement Cons: additional magnets and beam line complexity Scanning synchronization Pro: no magnet movement Cons: technical challenges and potential detection issues Pros & Cons

10. 100 GeV/c protons: L = 2153.78 m, f = 476.3 MHz, h = 3422 (bunch spacing = 62.94 cm) Observations A path-length chicane probably not practical without harmonic jump for the whole momentum range When moving whole arcs without harmonic jump Maximum transverse shift R = L /  = 24.9 cm where  = 523.4 With ~256 gaps between arc dipoles and quadrupoles, max gap change = 8.9 mm When moving whole arcs with harmonic jump R = (bunch spacing) /  = 69 mm Max gap change = 2.5 mm Synchronization Parameters

11. Synchronization – highly desirable Smaller magnet movement: (bunch spacing)/2 Smaller RF adjustment Detection and polarimetry – highly desirable Cancellation of systematic effects associated with bunch charge and polarization variation – great reduction of systematic errors, sometimes more important than statistics Simplified electron polarimetry – only need average polarization, much easier than bunch-by-bunch measurement Dynamics – question Possibility of an instability – needs to be studied Luminosity reduction by about twice the beam gap size (instead of one) Implications of Harmonic Jump

12. Framing Dynamic Problem One solution for synchronization is different number of bunches in MEIC collider rings Two different numbers of bunches in collider rings: N1, N2 Beam-beam collisions precess All bunch combinations cross if N1, N2 are incommensurate Can create linear and nonlinear instabilities (K. Hirata & E. Keil (1990), Y. Hao et al. (2014)) Analysis similar to coupled bunch instabilities driven by impedance But flat frequency dependence T. Satogata

13. Linear Simulation Results Raising number of bunches in linear simulation quickly produced instabilities – as low as N=(10,11)! A verification but of course many details left out T. Satogata

14. Things Missing… Linear model doesn’t look good BUT MEIC is strong focusing (transverse and longitudinal, e and p) Landau damping may damp instabilities faster than even the pessimistic growth rates Typical damping times are o(1/Q) (chromatic dominates nonlinear) Hadron rebunching should be performed without e- beam Nonlinear beam-beam tune spread may help Many dynamical effects were not included in H/L/P paper Nonlinear beam transport 6D effects (e.g. chromatic tune spread, tune modulation) Higher order moment instabilities Assumed only round beams T. Satogata

15. More realistic simulation needed by challenging In case of MEIC, 1 turn = ~3000 beam-beam interactions + non-linear dynamics Non of the existing codes seem adequate Developing a new code GHOST in collaboration with ODU Accuracy: high-order transfer map, symplecticity, bunch slicing Speed: Bassetti-Erskine solution for each pair of slices, single-term map, GPU Simulating Non-Pair-Wise Collisions GHOST& BeamBeam3D, 10 cm bunch40k particles B. Terzic et al.

16. Ion Arc Chicane x = 52 cm x = -69 cm Total bending angle and z length are fixed • Using current 22.8 m FODO cell design (8 m 3 T dipoles with 3.4 m separation) • One chicane per arc (also helps with synchronization of the 2nd IP), one extra FODO cell per arc • Takes up cells, which could otherwise be used for chromaticity compensation

17. Electron Arc Chicane • Using current 15.2 m FODO cell design (5.4 m dipoles with 2.2 m separation) • One chicane per arc (also helps with synchronization of the 2nd IP), one extra FODO cell per arc • Takes up cells, which could otherwise be used for chromaticity compensation x = 44 cm x = -50 cm Total bending angle and z length are fixed

18. Total bending angle and start and end points fixed Keep field in edge magnets maximum and adjust it in middle magnets with energy Extreme case with a factor of 5 energy swing shown (intermediate cases possible) Probably most efficient way to adjust the path length Provides almost complete path length adjustment over the whole range without HJ Sagitta in edge dipoles is an issue: grows ~linearly with bending angle Chicane Option II L = 1.97 m x = 8.26 m d = 11 cm A. Hutton

19. Bypass Electron Beam Lines Two (or more) beam lines at one location: “path length jump”. Electron beam passes through one of them. Several “jumps” distributed around the ring. “Jumps” can be combined with each other and small adjustable chicanes to produce the necessary path length adjustment merger Switch Y. Zhang

20. Path Length Jump Illustration Present CEBAF path length chicane: 1 cm chicane “Path length jump” Spin rotator Spin rotator variable +/-1 cm CCB fixed +4 cm Tune trombone & Straight FODOs R=155m fixed +8 cm e- 81.7 Future 2nd IP Arc, 261.7 fixed +8 cm RF RF Spin rotator Spin rotator IP fixed +8 cm variable +/-1 cm Electron collider ring w/ major machine components Y. Zhang 20

21. Bypass Electron Line Option II • Can be used for synchronization without harmonic jump Bending magnet 4, 3, 2, 1, 1, 2, 3, 4 J. Guo

22. Scanning Synchronization • Position of the orbit crossing point adjusted for every incident bunch pair making the IP move periodically in time • No magnet movement • Requires kicker technology development and detector design compatible with moving IP Ya.S. Derbenev et al.

23. Impact on Ring Design • Space for and locations of chicanes and/or bypass lines • Linear and non-linear optics • Synchrotron radiation and its effect on electron emittance and spin • Magnet strengths • Cost of additional elements

24. PEP-II Cavity Tuning Range • PEP-II cavities have one fixed and one movable tuner • Each tuner has ~ 500 kHz range • Adequate for retuning to 476.3 MHz plus beam loading • May be OK to tune over whole energy range without HJ Moving plunger tuner Fixed tuner Moving tuner R. Rimmer and J. Guo

25. SRF Cavity Tuning Range Typical JLab SRF cavity tuner 200 kHz at 1497 MHz, limited by switches. Could be 400 kHz? (T. Powers) Where is the warm/cold elastic limit? Analysis needed. Scales to ~ 250 kHz at 952.6 MHz. 140 kHz required for harmonic change of 1, 280kHz for even harmonics Probably OK for tuning over even harmonics Not sufficient for full energy range without HJ Need to change the design to allow larger tuning range Decrease stiffness of cavity (in a controlled way) Longer stroke tuners Use plungers or external reactive tuners? Have some ideas… R. Rimmer and J. Guo

26. Moving radially inward and outward is feasible Use slides and measurement scheme to make repeatable Rotation of magnets will be required as well Considerations for interfaces between magnets and to outside world Beamline Vacuum – bellows between adjacent beam pipe vacuum chambers Insulating Vacuum – bellows between adjacent Arc Half-Cells (?) Power – “service loop” for conductors Cryogens – may require flex hoses between adjacent Arc Half-Cells and assessment of U-Tubes Repositioning Superferric Magnets T.J. Michalski

27. Moving radially inward and outward is feasible Use slides and measurement scheme to make repeatable Rotation of magnets will be required as well – if doing a short string Considerations for interfaces between magnets and to outside world – no major issues Vacuum – assess capacity to expand/contract RF shielded bellows between rafts Potential to use different bellows for different energies Repositioning PEP-II Magnets With a single bellows in a Arc Half-Cell, Raft and Dipole must be moved as a pair. Length of Bellows is .125m or 4.92” Location of Bellows Corrector PEP-II HER Bellows T.J. Michalski

28. It is preferable to Have magnets that are being repositioned at the bottom of the stack in the arcs Move electron magnets Move a shorter section by a larger amount, i.e. use a chicane Approximate effort: 10-15 shifts for a 3-person crew Preliminary Engineering Conclusions T.J. Michalski

29. There are a number of synchronization solutions for both equal and different numbers of bunches in the electron and ion rings. Synchronization adjustments are expected to happen on a half a year to a year time scale and can be made in a few days during a shutdown. Running with different bunch numbers simplifies synchronization and provides a significant physics benefit Based on the physics and engineering considerations, our preliminary baseline choice is Harmonic jump (to be demonstrated in simulations) Adjusting electron path length using a movable chicane RF tuning range meets the synchronization requirements Conclusions