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 &amp; 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

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 GeV Bunch 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 number 2500

• 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 63.9 Circulator ring circumference ~ 50 m

β 0.9929 to 0.9999 Harmonic number 125

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)

MEIC with 2 IPs (Half Ring Apart)

Harmonic number has to be changed by unit of 2

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