1. Joe Grames, Jiquan Guo, Fanglei Lin, Vasiliy Morozov, Yuhong Zhang
Compact High-Intensity CW Polarized Positron Source for New Frontier Research
Joe Grames, Jiquan Guo, Fanglei Lin, Vasiliy Morozov, Yuhong Zhang
February 14, 2017
F. Lin

2. Research Impact and Benefits
Physics interest in the CEBAF (E. Voutier)
allows to uniquely distinguish the quantum interference when more than one Quantum Electro-Dynamics based mechanism contributes to a reaction process
isolate the interference contributions betwen the known Bethe-Heitler and the unknown virtual Compton amplitudes
Provide the measurement of the effective electron-quark coupling qualifying charge-conjugation violation
search a U-boson or heavy photon, candidate of Standard Model Dark Matter interaction mediator
Physics interest in the EIC (Y. Furletova, W. Melnitchouk, R. Ent)
Study the flavor separation of the pion and kaon structure
Study flavor decomposition of the nucleon’s quark and antiquark distribution
Study the gluon structure of nucleons and the diffraction mechanism in QCD
Search for right-handed W-boson exchange
Study other physics beyond the standard model
Physics interest in the applied physics (F.A. Selim)
facilitate the development of spintronic devices
study ferromagnetism at surfaces and interfaces and provide an effective method for measuring spin densities

3. Polarized Positrons at JLEIC
A group (J. Grames, J. Guo, F. Lin, V. Morozov, E. Voutier, Y. Zhang) is exploring a polarized positron injector suitable for Jefferson Lab Electron Ion Collider (JLEIC).
L ≥1033cm-2s-1 Pe+≥40%
CEBAF
Bunch structure from the injector:
37 s
17 MHz bunch train, 1.75 us
30 bunches (I = 3.4 uA)
0.2 pC
1.1 ms
19 ms
~ 50 Hz, Iave= 9 nA …
×30
A pulsed beam with low average current (~10 nA) is required for injection into JLEIC with a reasonably short injection time and reasonably high equilibrium polarization (>40%).

4. Positron Production Scheme
10 MeV polarized e-
MHz
2 nC bunches
@ MHz
Accumulator Ring (18m)
500-turn
phase-space painting
Bunch
Management
2 17 MHz
Positron Conversion/Collection Efficiency ~ 10-4
5-7 MeV Polarized e+
MHz
to CEBAF
Harmonic kicker
Extraction
Polarized Electron
10 MeV Injector
Polarized
Electron Source
Accumulator Ring
Electrons at
Converter
Positron Source
R&D
Challenge
JLEIC
MHz
3 mA
w/ DF = 5%
MHz
1.5 A
17MHz
34 mA
w/ DF = 0.26%
17MHz
3.4 μA
Electron accumulator
Harmonic extraction
Target: 340 kW peak
CEBAF
250MHz
1 – 10 mA (cw)
Not necessary
250MHz
250MHz
100nA – 1uA (cw)
High-QE photocathode
High voltage gun
Target: kW ave
This scheme is based on the Polarized Electrons for Polarized Positrons (PEPPo) technique.

5. JLEIC Positron Parameters
Primary Conservative Version
CM energy
GeV
21.9
44.7
63.3 p e+
Beam energy 40 3
100 5 10
Collision frequency
MHz
476
Particles per bunch
1010
0.98
1.3
0.93
Beam current A
0.75 1
0.71
Polarization %
80%
>40%
Bunch length, RMS cm 2
Norm. emitt., vert./horz. μm
0.3/0.3
24/24
0.5/0.1
54/11
0.9/0.2
432/86
Horizontal & vertical β*
8/8
13.5/13.5
6/1.2
5.1/1
10.5/2.1
4/0.8
Vert. beam-beam
0.005
0.092
0.069
0.002
0.009
Laslett tune-shift
0.061
3e-4
0.028
7e-4
0.015
2e-5
Det. space, up/down m
3.6/7
3/3.2
Hour-glass reduction
0.87
Lumi./IP, w/HG, 1033
cm-2s-1
0.9
7.6
1.5

6. JLEIC Positron Parameters
Primary Aggressive Version
CM energy
GeV
21.9
44.7
63.3 p e+
Beam energy 40 3
100 5 10
Collision frequency
MHz
476/4=119
Particles per bunch
1010
1.6
3.7
2.1
Beam current A
0.3
0.7
0.4
0.71
Polarization %
80%
>40%
Bunch length, RMS cm 1 2
Norm. emitt., vert./horz. μm
0.3/0.3
24/24
0.5/0.1
54/11
0.9/0.2
432/86
Horizontal & vertical β*
8/8
13.5/13.5
6/1.2
5.1/1
10.5/2.1
4/0.8
Vert. beam-beam
0.015
0.15
0.008
0.014
Laslett tune-shift
0.096
9e-4
0.058
7e-4
0.025
2e-5
Det. space, up/down m
3.6/7
3/3.2
Hour-glass reduction
0.87
0.75
Lumi./IP, w/HG, 1033
cm-2s-1
11.4
2.4

7. Key Challenges
High bunch charge and high repetition rate polarized electron source
QE decays quickly at high mA beam current
Operation of spin polarized GaAs beams at high current and/or high bunch charge
Polarized electron accumulator ring
Beam dynamics during the multi-turn injection
Spin dynamics
Harmonic stripline RF kicker for the extraction of the electron ring
Polarized positron source and collection (from 10-6 to 10-4)
Improve collection efficiency of positrons
Optimize IP^2 figure of merit
Increase electron beam energy at the target (at the expense of radiation/activation)

8. Anticipated Outcomes
Polarized electron source (P > 80%) operating at high voltage (> 300 kV) with various repetition rates, high bunch charge (> 1 pC) with good operating lifetime (> 200 C) at high average current (> 1 mA)
Electron accumulation ring with high gain (> 100), preservation of polarization in accumulator ring using Siberian snake (P > 80%), and suitable injection/extraction (< 100 ns)
PEPPo pair-creation target and a liquid metal target with collection optimized for final yield, polarization, emittance, momentum spread, bunch length and bunch train structure

9. Timetable of Activities

10. Back Up

11. Positron Production Scheme
10 MeV polarized e-
MHz
2 nC bunches
@ MHz
Accumulator Ring (18m)
500-turn
phase-space painting
Bunch
Management
2 17 MHz
Positron Conversion/Collection Efficiency ~ 10-4
5-7 MeV Polarized e+
MHz
to CEBAF
Harmonic kicker
Extraction
Polarized Electron
10 MeV Injector
Polarized
Electron Source
Accumulator Ring
Electrons at
Converter
Positron Source
R&D
Challenge
MHz
3 mA peak
Up to 0.09mA avg
~40ns×500×30 train
748.5 MHz
1.5 A peak
30/45 bunches
20ns gap
17MHz
34mA peak
up to 0.09mA avg
1.75μs, 525m train
17 MHz
3.4μA peak
up to 9nA avg
Electron accumulator
Harmonic extraction
Target: 340 kW peak
20ns gap allows more reasonable kicker rise time
17MHz allows reducing collider ring bunch reprate by factor of 4, may help increase luminosity for low beam current/high energy collision cases, when beam-beam is not a bottleneck.

12. Multi-Turn Injection x x
Concept: an orbit bump created near a septum and then slowly reduced as beam being injected (phase-space painting)
A number of painting schemes have been developed
Process can also be simultaneously occurring in vertical and longitudinal dimensions
CERN’s LEIR has a design for 75-turn injection of Pb54+, we plan to push this number to a few hundred to a thousand using low electron emittance
Main injection system components
Magnetic or electrostatic septum
Four bumper magnets with ~1 s rise time, reasonably fast fall-off time and ~10 mrad maximum deflection x x
> Septum thickness + bunch width
Injected beam
Accumulator ring

13. Harmonic Kicker Extraction
Y. Huang. Phys. Rev. Accel and Beams, 19, (2016)
Harmonic kicker that kicks every 3rd bunch
Empty ring buckets already kicked out to linac
Ring buckets still occupied by bunches
Empty linac buckets
Linac buckets occupied by extracted bunches

14. High Power Target Liquid Metal Target – lead-bismuth eutectic (LBE)
High Z = 82, 83
Low melting point: 124°C
High boiling point: 1670°C
Multiple LBE targets tested on various accelerators
Natural Circulation
Mechanical Pumping
Electromagnetic Pumping
Approaching 10 kW power level, CW
Stainless Steel Windows (0.25mm)
LBE (2mm)
Input (e-)
Output (e-,e+,γ)

15. High Polarization Photocathode R&D
6.4% QE & 84% polarization at 776 nm from strained GaAs/GaAsP superlattice photocathode with GaAsP/AlAsP Distributed Bragg Reflector (DBR)
The highest QE & FOM of any reported high polarization photocathode
Possible to improve both QE & Pol
SBIR partnership
Photo-cathode
DBR
Substrate
Paper for SPIN and APL submission: W. Liu, S. Zhang, M. Stutzman, M. Poelker, Y. Chen, W. Lu, and A. Moy

16. Higher Voltage (300 kV) Inverted Photo Gun
8 MV/m
11 MV/m
CsK2Sb 1mA
First insulator commissioned to 325 kV
New insulator, getting installed now, supports higher voltage operation