1. Polarized Positrons in JLEIC
Fanglei Lin, Yuhong Zhang, Jiquan Guo, Vasiliy S. Morozov,
JLEIC R&D Meeting, April 21, 2016
F. Lin

2. Positron Beams
The question always comes up “what about positron beams at an EIC?”
We can dream about polarized positron beams of similar luminosity and polarization as the electron beams foreseen for EIC, and some science would drive/require that, but…
We should give our accelerator colleagues some ROUGH idea about a reasonable lower threshold for
Luminosity
Polarization
requirements for valuable science at an EIC to give them a target.
Caveat: we do NOT want our accelerator colleagues to make this a direct priority, but more have them to keep such options in the back of their minds. So, a discussion on what thresholds are in the ballpark will be useful for them.
Here my bold proposal for valuable science with polarized positron beams:
Luminosity > 1033
Polarization > 40%
from Rolf Ent

3. JLEIC Primary Positron Parameters
For a full acceptance detector
Achieved with a single pass ERL cooler
CM energy
GeV
33.5 40
52.9 p e+
Beam energy 70 4
100 7
Collision frequency
MHz
476/3=159
Particles per bunch
1010
1.8
0.59
2.0
Beam current A
0.46
0.15
0.5
Polarization %
>70%
~40%
Bunch length, RMS cm 2
1.2
Norm. emitt., vert./horz. μm
0.5/0.25
36/18
190/95
Horizontal & vertical β*
4/2
5.8/2.9
2/4
4.1/2.0
7.1/3.55
2.4/1.2
Vert. beam-beam
0.002
0.03
Laslett tune-shift
0.056
small
0.028
Det. space, up/down m
3.6/7
3/3.2
Hour-glass reduction
0.89
0.87
0.82
Lumi./IP, w/HG, 1033
cm-2s-1
0.9
0.7
For a high(er) luminosity detector
Horizontal & vertical β* cm
1.6/0.8
2.3/1.2
1.6/0.82
4.8/2.4
Vert. beam-beam
0.002
0.15
0.03
Det. space, up/down m
±4.5 3
Hour-glass reduction
0.67
0.63
0.71
Lumi./IP, w/HG, 1033
cm-2s-1
1.7
2.3
0.9

4. Luminosity of JLEIC w/ Positrons
Assume that the positron beam current is 0.15 A

5. Formation of Positron Beam
collider
CEBAF
Beam current mA
150
0.05
Bunch freq.
MHz
158.8
22.68
Bunch spacing ns
6.3
44.1 m
1.89
13.2
Bunch number
1138
Bunch charge pC
945
2.2
Bunch intensity
109
5.9
0.014
Energy
GeV 3 5
6.95
9.3 10
Electron current A
1.4
0.95
0.71
SR power MW
0.16
2.65
Damping time ms
376 81 26 14
Polarization lifetime
hour 66 8
2.2
0.9
0.3
Beam lifetime
Positron current mA
150
SR power w/o wigglers
0.017
0.13
0.5
1.6
2.1
Initial injection time w/o wigglers
min
43.6
9.4
3.5
1.5
1.2
Damping time w/ wigglers 20
Damping time reduction
19.6
4.2
2.6
SR power w/ wigglers
0.32
0.54
0.75
4.0
4.3
Initial injection time w/wigglers
2.3
0.6
Initial injection times are still reasonable short comparing to the beam and polarization lifetimes without wigglers.
Dipole wigglers in the straight should not affect the polarization direction at IPs, but may affect the polarization lifetime.

6. Injected Polarized Positron Structure
τd=20ms
44 ns
22.7 MHz bunch train, 3.23µs,
73 bunches (Ipulse = 50 A)
injector pulse train up polarization from CEBAF ……
injector pulse train down polarization from CEBAF
2.2 pC bunch
73 bunches (Ipulse = 50 A)
τd=20ms
25 Hz, 40 ms, Iave=8.1 nA
Positron
Polarization %
> 40
Pulse current Ipulse A 50
Bunch charge pC
2.2
Bunch rep. rate in pulses
MHz
22.7
Bunch spacing in pulses ns 44
Pulse rep. rate Hz 25
Average current Iave nA
8.1

7. Continuous Injection Polarization w/ continuous injection
Equilibrium polarization
A relatively low average injected beam current of tens-of-nA level can maintain a high equilibrium polarization in the whole energy range.

8. Positron Production Scheme I
It seems easier to stack “cold” electrons than “hot” positrons
1000 turn phase space painting sounds aggressive but it is “only” a factor of ~10 increase of 2D emittance (1  ~10 m) in case of 6D phase space painting or a factor of ~32 increase of 2D emittance (1  ~32 m) in case of 4D phase space painting
Option with optimistic assumption about electron stacking
26.5 m accumulator ring
with 1000 turn phase-space painting
bunch focusing
and compression
if needed
e+ collection
system
x 10-2
1 mm W
target
x 10-3
to CEBAF
5-7 MeV Polarized e+
2 x kHz
= 10 nA
10 MeV polarized e-
MHz
= 1 mA
10 MeV pol e-
2 x 44.1 nC bunches
@ 11.3 MHz
= 1 A
10 MeV polarized e-
2 x kHz
= 1 mA
0-9.5 MeV polarized e+
2 x kHz
= 1 A

9. Positron Production Scheme II
Option with less optimistic assumption about electron stacking
26.5 m accumulator ring
with 100 turn phase-space painting
bunch focusing
and compression
if needed
e+ collection
system
x 10-2
1 mm W
target
x 10-2
to CEBAF
~60 MeV polarized e+
2 x kHz
= 10 nA
10 MeV polarized e-
MHz
with 10% duty factor
= 0.1 mA
acceleration
to 100 MeV
10 MeV pol e-
2 x 4.41 nC bunches
@ 11.3 MHz
= 0.1 A
10 MeV polarized e-
2 x kHz
= 0.1 mA
MeV polarized e+
2 x kHz
= 1 A

10. Experimental Test Minimum experimental setup e+ collection system
1 mm W
target
x 10-3
to diagnostics
5-7 MeV Polarized e+
MHz
= 1 nA
10 MeV polarized e-
MHz
= 100 A
0-9.5 MeV polarized e+
MHz
= 0.1 A