Synchronization issues in meic
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Synchronization Issues in MEIC. Andrew Hutton, Slava Derbenev and Yuhong Zhang MEIC Ion Complex Design Mini-Workshop Jan. 27 & 28, 2011. The Problem. Electrons travel at the speed of light Protons and ions are slower There are three areas that need to be addressed

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Synchronization Issues in MEIC

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Synchronization Issues in MEIC

Andrew Hutton, SlavaDerbenev

and Yuhong Zhang

MEIC Ion Complex Design Mini-Workshop

Jan. 27 & 28, 2011

The Problem

  • Electrons travel at the speed of light

  • Protons and ions are slower

  • There are three areas that need to be addressed

    • In collider ring  matching electron & ion beams at multiple IPs

    • During acceleration

    • Cooling  matching ion beam and cooling electron beam

  • Assumptions

    • MEIC collider ring circumference is around 1 km

    • Large booster (LEIC) is the same circumference as MEIC

    • Electron ring is the same circumference as MEIC

    • Superconducting RF systems have limited frequency swing

Harmonic Numbers

  • Assuming circumference of the MEIC collider ring is about 1 km

  • For an RF frequency of 1497 MHz

    • The best harmonic number is 4860 = 2x2x3x3x3x3x3x5

    • Corresponds to a circumference of 971.98 meter

  • For an RF Frequency of 748.5 MHz

    • The harmonic number is 2430

  • For an RF Frequency of 499 MHz

    • The harmonic number is 1620

Orbit Differences in MEIC

  • MEIC design parameters

    Proton energy 20 to 60 GeVBunch repetition rate 748.5 MHz

    Deuteron energy 10 to 30 GeV/uCollider ring circumference ~1000 m

    Lead energy 7.9 to 23.8 GeV/u Harmonic number2500

  • Orbit difference from 1000 m ring @ 60 GeV proton design point

    proton 60 GeVdesign point

    20 GeV -97.9 cm  2.44 bunch spacing  2 unit of HN

    deuteron: 30 GeV/u  -36.7 cm  0.92 bunch spacing  1 unit of HN

    10 GeV/u  -429 cm  10.7 bunch spacing  11 unit of HN

    Lead: 23.8 GeV/u  -65.7 cm  1.64 bunch spacing  2 unit of HN

    7.9 GeV/u  -692cm  17.3 bunch spacing  17 unit of HN

  • MEIC Circulator Cooler

    Energy range 4.3 to 32.7 MeVBunch repetition rate 748.5 MHz

    γ 8.4 to 63.9Circulator ring circumference ~ 50 m

    β 0.9929 to 0.9999 Harmonic number125

    Orbit difference

    cooling [email protected] GeV/u -4.9 cm  0.1 wavelength  no change of HN

    cooling [email protected] GeV/u -35 cm  0.86 wavelength  1 unit of HN

Harmonic Number vs. Proton Energy

  • The proton energy that corresponds to a harmonic number of 1 less than the nominal is

    • 43.32 GeV for 1497 MHz

    • 31.77 GeV for 748.5 MHz

    • 25.77 GeV for 499 MHz

  • For 750 MHz, change of harmonic numbers is not a viable solution for the 20 – 60 GeV energy range

    • It is a viable solution at lower energies

Two Interaction Regions

  • The two Interaction Regions are 180°apart for both beams in the present configuration

    • Arcs are equal and straight sections are equal

      • Offsetting the beam in the Arcs would work

    • Putting two Interaction Regions in a single straight will not work without an additional variable chicane

      • Chicane is complicated in this region

        • Magnet offset ~1 meter for 2 mm path length change

  • MEIC can have up to two interaction regions

    • Must be equidistant in ring

  • There can be one more interaction region in LEIC

Change Ion Ring Path Length

  • It is possible to change the path length in the ion ring

    • For one Interaction Point, need +/- 20 cm

    • For two Interaction Points, need +/- 40 cm

  • If path length is created in the arcs

    • 20 cm corresponds to an offset of about ±25 mm

    • 40 cm corresponds to an offset of about ±50 mm

  • Increasing the bore of a 6 Tesla magnet by 30 mm is expensive!

    • 60 mm may be prohibitive

  • Need to mount all the magnets on movers

    • Unpleasant, but possibly affordable

Three Ring Collider Proposal

  • The MEIC ring should be used to cover the higher energies

    • RF frequency will be fixed

    • Electron ring and ion ring will use SRF cavities

    • Ion ring magnets will be on movers to accommodate velocity change

  • The LEIC ring will be used to cover lower energies

    • The LEIC ring will need variable RF frequency

      • Ion ring will require RF cavities that can span a wide frequency range

      • Could be a sub-harmonic of MEIC ring

        • Injected bunch trains would be interleaved using an RF separator

Alternate Solution: Change of Electron Path & RF Frequency

The scheme does not require change of the ion orbit which is considered far more difficult to realize for SC magnets. It rather varies

  • RF frequency(less than ±10-3)

  • Ion ring harmonic number

  • Electron orbit (less than half wavelength for one IP

    and one wavelength for two IPs)

  • Circulator cooler ring circumference (less than half bunch spacing)

Change of ring radius

MEIC with One IP

MEIC with 2 IPs (Half Ring Apart)

Harmonic number has to be changed by unit of 2

Change of Collision Frequency & Electron Ring

One IP

Two IPs

Electron Cooling

  • Electron cooling requires exact matching of the electron and ion velocities

  • The time between adjacent buckets is 1/frequency

    • Therefore RF frequencies must also be matched

  • In the MEIC ring, if the RF frequency is constant (749.5 MHz) so the same electron cooling system will work at all energies

    • Fixed frequency SRF cavities will work for energy recovery of the electron beam used for cooling

Circulator Ring Circumference

  • The length of the circulator ring will need to be changed to accommodate different electron velocities

    • The maximum change will be 1/hion

    • The circumference change in the circulator ring is heλ/hion

  • Numerical example

    • MEIC is ~900 metres long, hion = 4500

    • Circulator ring is ~20 meters long, he = 100

    • Circulator ring must change circumference by 4.5 mm for a one wavelength change in MEIC circumference

    • This is a radius change of ~0.7 mm

  • This is a small number so it can easily be accommodated within the circulator ring magnet bore

LEIC Electron Cooling

  • The RF frequency in the LEIC ion ring has to change

    • The circumference change in the circulator ring can be accommodated within the magnet bore

  • The RF frequency in the electron cooling system has to change

  • The RF frequency of the electron linac must change

    • SRF cavities will not work

  • Electron energy is low

    • Propose no energy recovery for the electron beam

    • Extend the number of turns that the electron beam is in the circulator ring

  • Electron cooling would then be available throughout the acceleration cycle

Circulator Ring

  • Assume racetrack layout as proposed in the ZDR

    • Electron cooling occurs on one straight section

    • Electron beam injected/extracted on opposite straight section

  • Straight sections must have zero dispersion

    • If injected beam is on axis, it will be on axis for cooling

    • Injection orbit is independent of beam energy

  • However, correct longitudinal position is not guaranteed by good injection orbit

    • Requires Arcs to be achromatic, but not isochronous

    • Arc energy setting must lead beam energy during ramp so path length shortens to maintain correct timing

Clearing Gaps

  • Colliders usually have one (or more) gaps in the bunch train

    • Ion clearing in electron beams

    • Electron cloud clearing in proton or positive ion beams

    • Required for aborting high power beams

  • MEIC will have gaps, probably ~10% of the circumference

    • Will reduce MEIC luminosity by ~10%

      • RF frequencies are the same so gaps are synchronous

  • LEIC will have gaps, also about 10% of the circumference

    • Will reduce LEIC luminosity by at least 20%

      • Gaps are asynchronous

      • Could increase beam-beam effects

        • Needs study

Impact of Clearing Gaps

  • The clearing gaps impact the RF systems

  • Stored energy in the cavities changes along the bunch train

    • Bunch energy changes along the bunch train

    • Transverse position in regions of non-zero dispersion changes along the bunch train

    • Polarization precession changes along the bunch train

  • Effect minimized with RF systems with high stored energy

    • SRF cavities

    • Copper cavities with storage cavities

      • It is difficult to vary the frequency of both types of cavity

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