1. More on MEIC Beam Synchronization
Yuhong Zhang
MEIC Accelerator R&D Meeting
August 6, 2015

2. Facts about Path Length Adjustment for Achieving Beam Synchronization
Nominal bunch spacing (for electrons and ions): MHz rep rate
Range of path length adjustment: cm (1 IP with harmonic number change)
18 to 100 GeV up to 254 cm (without harmonic number change)
12 to 100 GeV up to 555 cm (without harmonic number change)
Consider the case with harmonic number change, path-length adjustment +/-31.5 cm
Due to symmetry, each half ring is responsible for +/ cm (still big!)
(CEBAF path length correction +/-1 cm, Rublin)
Needs three chicane (Morozov) of 66 to 110 m length to cover +/ cm
(requiring to move up to 24 dipoles and 27 quads, and other elements)
Present design, one arc is 755 m in e-ring, 3 chicane is up to 22% of the two arc
We are effectively moving up to 1/5 of the arc!
# of FODO cells
# of magnet moved
Path length change
Radial movement range
Maximum change in magnet spacing 3
4 dipoles, 5 quads
10 cm
+52 / -69 cm
20 mm 4
6 dipoles, 7 quads
+36 / -39 cm
14 mm 5
8 dipoles, 9 quads
+28 / -29 cm
11 mm
66 m 88 m
110 m

3. Fixed Bypass Beam Line for Reducing the Range of Magnet Moving
We introduce a set of fixed bypass beam lines for discrete increments of path length for assisting the (large) path length adjustment
The movable chicanes will be required to cover much smaller ranges of path length adjustment. We hope these chicanes are not much more than the CEBAF chicanes (15 m long for +/- 1 cm)
A switch yard will direct electrons to different orbit which is most closed to the required path length.
There is an extra cost for these additional bypass beam lines, but it may make the problem less harder. And it is better done at the e-ring for cost effectiveness.
These bypass beam lines can be staged
Switch yard
merger

4. An Economy Way of Using Bypass Beam Lines
Add multiple (>2) bypass beam lines at one location prevents simultaneously use of them.
We propose only two beam lines at one location. It can be called “path length jump”. The electrons will go either one of two beam lines.
We distribute several “jumps” in the ring (left and right symmetrically), then the increments of the path length can be added up to produce a much large amount of increase.
If the jumps are chosen properly, any path length adjustment should be covered
Switch
merger
Units
Path length adjustment
Sum of Chicanes cm
(+/-1)x2 ==> 4
4.5 5 6
Jump 1 4
Jump 2 8 9 10
Jump 3
13.5 12
Jump 4
Total 32
31.5
Building blocks
Path length (cm)
Chicanes
0 to 4.5
Chicanes + Jump 1
4.5 to 9
Chicanes + Jump 2
9 to 13.5
Chicanes + Jump 3
18 to 22.5
Chicanes + Jump 2&3
22.5 to 27
Chicanes + ump 1&2&3
27 to 31.5

5. An Illustration in Electron Collider Ring
Present CEBAF path length chicane: +/-1 cm
chicane
“Path length jump” e-
R=155m RF
Spin rotator
CCB
Arc, 261.7
81.7 IP
Tune trombone & Straight FODOs
Future 2nd IP
variable +/-1 cm
Fixed +4 cm
Fixed +8 cm
Fixed +8 cm
Fixed +8 cm
Variable +/-1 cm
Electron collider ring w/ major machine components

6. Harmonic Number Change Without Gear Change (A Backup Plan)
Still allow change of harmonic number in the ion ring
Change the harmonic number in the electron ring to match the harmonic number in the ion ring (both rings always have the same number of bunches)
Change electron ring path length to accommodate additional bunches
Advantage: no gear change, restoring one-to-one collision pattern
Advantage: gaps will be synchronized, no extra luminosity loss
Have multiple by-pass beam lines (jumps) to absorb large path length change
Have 2 or 4 chicanes to adjust +/- half bunch spacing (for one IP) for covering energies between harmonic energies.
Why change harmonic numbers in the ion ring?
Making the RF frequency change not too large
Why change harmonic numbers in the electron ring?
Have the same bunch numbers in two collider rings

7. Change of Electron Path Length for No-Gear Change Scheme
From 18 to 100 GeV, there are 4 harmonic numbers
The electron ring needs to cover 4x63=252 cm
 3x63 cm /-31.5 cm
Each arc: 1.5x63 cm / cm
Add two large “path-length jumps” in each arc
0.5x63 cm = 31.5 cm
1 x63 cm = 63 cm
Ion Energy
Bunches
GeV/u
47.25
3416
29.28
3417
22.91
3418
19.38
3419
Units
Path length adjustment
Sum of Chicanes cm
31.5 4
4.5 6
Jump 1
Jump 2 8 9 10
Jump 3
13.5
Jump 4
Large jump 1
Large jump 2 63
Total
242

8. What is the Impact on the Ring Design?
Issues
Space and location for large by-pass beam lines
Cost of additional beam lines (magnet power supplies may be shared within a jump)
Optics (nonlinear beam dynamics)
Magnet field strength
Synchrotron radiation and its impact on the emittance and spin
From Vasily Morozov’s “Looks very good. Of course, engineering issues especially near the spreader and recombiner points need to be checked. My only potential concern is about how this may affect the chromaticity compensation scheme. That is if we place sextupoles in the arcs we will need to make sure that the bypasses do not affect the phase advance between the sextupoles, etc. We should also take a quick look at the optics of the bypass sections to make sure there is no significant impact on the emittance, spin, etc.”

9. Harmonic Numbers with 1 IP (J. Guo)
Energy(GeV/u)
max dfe (kHz)
139.42
max dfi (kHz)
278.83
62.93
ions γ β Ne Np
fe (MHz)
dfe (kHz)
fp (MHz)
dfp (kHz)
Le (m)
dLe (cm)
100
3416
69.719 90
64.943 80
58.289 70
48.628
97.255 60
33.827
67.654 50
9.475
18.949
-4.277
0.000 40
15.728
31.465
3417 32
36.381
72.763 28
9.384
31.447
3418
31.428
3419
31.410
3420
31.392

10. Fitting the Physics Operation Scenario
A possible operation scenario (P. Nadel-Turonski)
100 GeV proton
66.7 GeV proton and Helium-3
50 GeV all A/Z = 0.5 nuclei including d, p
40 GeV all stripped Pb ions, protons
20-30 GeV (?) low-energy setting
A better choice of 1st harmonic number is 52.7 GeV
 reducing path-length adjustment to +/-23.6 cm (25% reduction) for 40 to 100 GeV
Low energy (20 to 30 GeV) be curved by three harmonic energies
Ion energy (GeV/u)
Harmonic number
30.5 +1
23.5 +2
19.71 +3
17.30 +4
15.57 +5
14.25 +6
13.22 +7
12.36 +8
Ion energy (GeV)
Path length adjustment per arc (cm)
Chicanes (+/-1 cm) per arc
5 cm Jumps per arc
100
11.8 1 2
66.7
6.1 50
1.8 - 40
-11.8
Even number for two IPs!
Are these discrete energies are good enough?
This works for two IPs!

11. What is a New (Ugly) Baseline for MEIC Beam Synchronization?
Do not change the ion ring circumference, but do allow change of its harmonic number (ion bunch number)
Allow adjusting the electron ring circumference up to +/-0.5 bunch spacing for one IP (and up to +/-1 bunch spacing for two IPs)
Allow adjusting the RF frequency in both collider rings
Set the 1st harmonic number to 47.3 GeV, thus, no harmonic number change from to 100 GeV
Electron ring circumference will be adjusted by multiple chicanes. Optionally using the path-length jump
If “gear-change” must be avoid,
increase harmonic number in the electron collider ring to make same bunch numbers in both rings (“harmonic number jump without gear change”);
Increase electron ring circumference to accommodate extra electron bunches; add large “path-length-jumps”