1. MEIC New Baseline: Performance and Accelerator R&D
Yaroslav Derbenev, Fanglei Lin, Vasiliy Morozov, Yuhong Zhang

2. New MEIC Baseline New major components Energy range Design points
Collider ring circumference: 2200 m
Electron collider ring: PEP-II magnets and vacuum chambers
Electron ring RF: PEP-II, (476 MHz base frequency)
Ion collider ring: super-ferric magnets
Warm ion linac
Single booster ring: 285 MeV to 8 GeV
Energy range
Electron: to 12 GeV
Proton: (8) 20 to 100 GeV
Lead ions: up to 40 GeV
Design points
Low energy: GeV x 4 GeV (space charge dominated)
Medium energy: GeV x 5 GeV (beam-beam dominated)
High energy: GeV x 10 GeV (SR dominated)
Reference: GeV x 5 GeV

3. Collider Performance: The Big Picture
Luminosity
Polarization
Mitigation
Circumference
(1.4 km  2.2 km)
Space charge
Cooling efficiency
down up to 33%
improving e-pol lifetime
Hour-glass
Traveling focusing
PEP-II magnets
High current
Large emttiance
No 12 GeV for quad
Yes
Very bad!
Different lattice
PEP-II RF stations
750 MHz  496 MHz
Large bunch charge
Down up to 33%
Super-ferric magnets
Still slow ramping
Single booster
May crossing γt
High energy injection to full size ring
More options for cooling
crossing γt should have no or small impact
imaginary γt
Warm ion linac
Below are TBD
Stack/cooler ring
Improve duty factor

4. Low Energy (Ecm~21.9 GeV) Parameters
Detector type
Full acceptance
Large Acceptance
Proton
Electron
Beam energy
GeV 30 4
Collision frequency
MHz
476
Particles per bunch
1010
0.41 3
Beam Current A
0.32
2.3
Polarization %
> 70
~ 80
RMS bunch length cm
1.2
emittance, normalized
µm rad
0.35 / 0.14
74 / 29.6
0.35 / 0.12
Emittance aspect
2.5
Horizontal and vertical β*
5 / 2
5.8 / 2.3
2 / 0.8
2.8 / 0.93
Spot size at IP, horiz. and vert. µm
23.4 / 9.36
14.8 / 5.9
Vertical beam-beam tune shift
0.015
0.018
Laslett tune shift
0.06
Very small
Distance from IP to 1st FF quad m 7
3.5
4.5
Hour-glass effect
0.76
0.49
Luminosity/IP, HG corrected, 1033
cm-2s-1
1.6
2.6
Slide 4 4

5. Design Considerations: Low Energy
Low proton energy, space charge dominated, proton bunch charge limited
Optimization: longer bunch  more charge but there is a hour-glass effect
double proton bunch length, 2 cm  4 cm, which can double the proton bunch charge,
hour glass effect is 0.76
net luminosity boosting: 2 * 0.76 = 1.52

6. Medium Energy (Ecm~44.7 GeV) Parameters
Detector type
Full acceptance
Large Acceptance
Proton
Electron
Beam energy
GeV
100 5
Collision frequency
MHz
476
Particles per bunch
1010
0.66
3.9
Beam Current A
0.5 3
Polarization %
> 70
~ 80
RMS bunch length cm 2
1.2
emittance, normalized
µm rad
0.35 / 0.07
144 / 28,8
144 / 28.8
Emittance aspect
Horizontal and vertical β*
17.5 / 3.5
7.5 / 1.5
9 / 1.8
4 / 0.8
Spot size at IP, horiz. and vert. µm
24 / 4.8
33.2 / 6.6
17.2 / 3.4
24.3 / 4.9
Vertical beam-beam tune shift
0.012
0.033
0.011
0.034
Laslett tune shift
0.027
Very small
Distance from IP to 1st FF quad m 7
3.5
4.5
Hour-glass effect
0.87
0.72
Luminosity/IP, HG corrected, 1033
cm-2s-1
5.0
8.0
Slide 6 6

7. Design Considerations: Medium Energy
Optimization: not match the spot size of two colliding beams
strong-weak beam-beam regime
electron beam-beam tune-shift is an order of magnitude smaller than the design parameter limit (0.018 vs. 0.15)
Electron emittance is very large, so the beam spot size is very large
Will loss luminosity (about 15% to 26%) if matching the large electron beam spot size
We don’t match the spot size of two colliding beams (ratio is 0.72)
Electron β*y is reduced to 1.5 cm

8. High Energy (Ecm~63.3 GeV) Parameters
Detector type
Full acceptance
Large Acceptance
Proton
Electron
Beam energy
GeV
100 10
Collision frequency
MHz
159
Particles per bunch
1010
2.0
2.8
Beam Current A
0.5
0.72
Polarization %
> 70
~ 80
RMS bunch length cm
3.5
1.2
emittance, normalized
µm rad
0.5 / 0.1
1152 / 230
Aspect ratio 5
Horizontal and vertical β*
22.5 / 4.5
7.5 / 1.5
12 / 2.4
4 / 0.8
Spot size at IP, horiz. and vert. µm
32.6 / 6.5
66.4 / 13.3
15.9 / 3.2
27.2 / 5.4
Vertical beam-beam tune shift
0.003
0.027
Laslett tune shift
0.033
Very small
Distance from IP to 1st FF quad m 7
4.5
Hour-glass effect
0.75
0.58
Luminosity/IP, HG corrected, 1033
cm-2s-1
0.97
1.4
Slide 8 8

9. Design Considerations: High Energy
Optimization: reducing bunch repetition rate (number of bunches in ring)
electron synchrotron radiation dominated regime
electron current is limited to 0.72 A at 10 GeV
L ~ f N1 N2 ~ I1 I2 / f
Proton bunch length needs to be adjusted to satisfy the boundaries of beam effects (space charge, etc.)
Optimization: not matching beam spot size at IP
weak beam-beam regime
electron beam-beam tune-shift is an order of magnitude smaller than the design parameter limit (0.006 vs. 0.15)
Proton beam-beam tune-shift is close to the limit
We don’t match the spot size of two colliding beams (ratio is 0.5)
Electron β*y is reduced to 1.5 cm

10. Reference Point Parameters
Detector type
Full acceptance
Large Acceptance
Proton
Electron
Beam energy
GeV 60 5
Collision frequency
MHz
476
Particles per bunch
1010
0.52
3.9
Beam Current A
0.4 3
Polarization %
> 70
~ 80
RMS bunch length cm 2
1.2
emittance, normalized
µm rad
0.35 / 0.07
144 / 28,8
144 / 28.8
Emittance aspect
Horizontal and vertical β*
15/ 5
7.5 / 1.5
6 / 1.2
4 / 0.8
Spot size at IP, horiz. and vert. µm
28.7 / 5.7
38.4 / 7.6
18.1 / 3.6
24.3 / 4.9
Vertical beam-beam tune shift
0.013
0.024
Laslett tune shift
0.06
Very small
Distance from IP to 1st FF quad m 7
3.5
4.5
Hour-glass effect
0.89
0.71
Luminosity/IP, HG corrected, 1033
cm-2s-1
3.1 (5.5)
6.1 (12.2)
Slide 10
Old design 10

11. Limiting Factors & Design Optimization
SR power of the high energy electron beam
 limiting average current, but permitting higher bunch charge
 Reducing bunch repetition rate can increase luminosity as long as ion space charge is OK
Space charge effect for low energy proton beam
 limiting bunch charge, but permitting higher average current (higher bunch frequency)
 Increase bunch length for storing more charge in a bunch (hour glass effect)
 Increase bunch repetition rate
Beam-beam effect
 limiting bunch charge, but permitting higher average current (bunch frequency)

12. Limiting Factors & Design Optimization
Unusually higher electron emittance
 Due to the large bending angle of PEP-II dipoles and also its lattice
 Squeezing through the IR magnets (final focusing) is a challenge
Ion beam formation/electron cooling
Space charge limit at low energy
Cooling time (increasing by 33%)
Electron beam instabilities
Too long damping time at low energy (3, 4 GeV)
Interaction region

13. Short Term R&D: Reduction of Electron Emittance
Problem
A factor of 2.5 larger than the previous baseline, requiring large aperture
Affecting luminosity (matching beam spot size at IP)
It does not meet the requirement of the present IR design
Possible mitigation approaches
Hardware approach
New short dipoles
Lattice approach
TME lattice
Mixed FODO-TME lattice
Insertion approach
Damping wiggler
Related problem: improving damping time for low energy

14. Parameters with Small Emittance
Detector type
Full acceptance
Large Acceptance
Proton
Electron
Beam energy
GeV
100 5
Collision frequency
MHz
476
Particles per bunch
1010
0.66
3.9
Beam Current A
0.5 3
Polarization %
> 70
~ 80
RMS bunch length cm 2
1.2
emittance, normalized
µm rad
0.45 / 0.09
55 / 11
Emittance aspect
Horizontal and vertical β*
10 / 2
7.5 / 1.5
5.5 / 1.1
4 / 0.8
Spot size at IP, horiz. and vert. µm
20.6 / 4.1
15.2 / 3.1
20.5 / 4.1
Vertical beam-beam tune shift
0.015
0.044
0.043
Laslett tune shift
Very small
0.027
Distance from IP to 1st FF quad m 7
3.5
4.5
Hour-glass effect
0.89
0.72
Luminosity/IP, HG corrected, 1033
cm-2s-1
8.6
12.8
Slide 14 14

15. Luminosity with Smaller Emittance
Energy
Electron emttiance
Luminosit at full acceptance detector
Luminosit at full at large acceptance detector
mm mrad
1033 cm-2s-1
30x4
74 / 28
1.6
2.4
100x5
144 / 55
5 / 8.6
8 / 12.8
100x10
1152 / 440
0.97 / 2.2
1.4 / 3.1
60x5
3.1 / 4.2
6.1 / 8.0

16. MEIC SC Ion Linac

17. Up to 200 MeV/u (proton to uranium)
CW operation
Up to 200 MeV/u (proton to uranium)

18. CERN Heavy Ion Accelerating Facility
MEIC warm ion linac
Could we develop a similar ion injector like this?
(Optional) an extra compact booster for heavy ions
Particularly, having a cost effective warm ion linac
MEIC booster ring
MEIC collider ring

19. IR Design for Achieving Higher Luminosity
Optimization of electron IR for accommodation of large emittance
Different permanent quad covering the energy range
Reducing electron β*y to 1.2 cm or even smaller
Taking advantage of smaller detector space
Traveling focusing (TF) scheme
Presently, 10% to 50% loss of luminosity due to hour glass effect
Proton bunch length is longer than beta-star
Electron beam must be TF scheme friendly (short bunch length and large beta-star)
Large Piwinski angle/crab waist

20. Beam-Beam New design elements in IR
Unmatched beam spot sizes at IP
Long bunch length and hour-glass effect
Traveling focusing
Coherent instabilities due to change of harmonic numbers

22. Consideration of MEIC Upgrade Path
Present approach
First stage baseline (MEIC) up to 100 GeV x 12 GeV (10 years)
Energy upgrade (EIC) up to 250 GeV x 20 GeV
Alternative approach
First stage baseline (MEIC) up to 100 GeV x 10 GeV (5 years)
Luminosity upgrade (HL-MEIC) same, luminosity x 3~ (5 years)
Energy upgrade (HE-MEIC=EIC) up to 250 GeV x 14(?) GeV
What is the first stage baseline?
Very conservative, all based on “ready-to-build” technologies
Acceptable starting performance
Significant relaxation of the machine design
Minimum technical uncertainty and R&D requirement (weak cooling)
Minimum cost (PEP-II RF station (476 MHz)

23. Consideration of MEIC Upgrade Path
What constitutes a luminosity upgrade? (A factor of 4 to 5)
Doubled the bunch repetition rate (476 MHz  952 MHz)
Increase of the beam current
Reduction of beam emittance (better cooling? Better electron lattice?)
Redesign of IR (beta-star reduction by a factor 2?), including DA
Unmatched beam size?
More aggressive hour-glass effect?
A little large beam-beam?
Space charge compensation? Electron lens?
Small cost (a fraction of energy upgrade cost)