Accelerator Physics Opportunities at JLab
and the
Compact Positron Source at SLAC
Spencer Gessner, Sophie Crisp SLAC
LEEPP Workshop, JLab
March 26, 2026
Plasma Acceleration
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JLAB SRF Cavities 10-20 MV/m
Plasma Accelerator 1-100 GV/m
Future Colliders
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ILC Electron-Positron Collider
250 GeV - 1 TeV
Plasma Collider
10 TeV
Positron Acceleration in Plasma
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Electron acceleration
Positron acceleration
Plasma electron trajectory
Plasma electron trajectory
Plasmas are composed of mobile electrons and immobile ions.
The plasma response to beams of opposite charge is asymmetric.
Experimental results on e+ acceleration at FACET
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S. Corde et al., Nature 524, 442 (2015)
A. Doche et al., Nat. Sci. Rep. 7, 14180 (2017)
S. Gessner et. al. Nat. Comm. 7, 11785
6 GeV
energy gain!
Review of Positron Acceleration in Plasma
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quality gap
Positron acceleration in plasma experiments lags behind electron acceleration in plasma, in part due to a lack of experimental opportunities.
AWAKE at CERN
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400 GeV protons
10 m plasma
σz > λp
Electron Injection
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Seed of an Idea
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A. Gorn et. al. Phys. Plasmas 25, 063108 (2018)
Off-axis electron injection trajectory
On-axis electron injection trajectory (not used)
AWAKE Electron Injector
“This would be a lot easier with positrons.”
Seed of an Idea
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A. Gorn et. al. Phys. Plasmas 25, 063108 (2018)
Off-axis electron injection trajectory
On-axis electron injection trajectory (not used)
AWAKE Electron Injector
GBAR Experiment
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GBAR Positron Accumulator
10 MeV Electron Linac
GBAR positron source is few-meter scale. Fits in AWAKE tunnel.
Positron Beams from Traps
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R. Hessami at CERN (2019)
Now Stanford Ph.D. student
Penning-Malmberg Trap
GBAR positron bunch is too long → investigate bunch compression.
Positron Beams from Traps
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Longitudinal phase space during bunch compression
Final bunch length < 1 ps with 1E8 positrons.
AWAKE operates at 1 pulse every 30 seconds.
Implies 3.3E6 trapped positrons/second.
Consistent with GBAR parameters ✓
FCC-ee
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91 km circumference e+e- collider
Colliding beams are unpolarized
FCC-ee Z-pole operation
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J. Keintzel, CERN
FCC-ee Z-pole operation
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J. Keintzel, CERN
Part-per-million energy calibration!
FCC-ee Z-pole operation
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How to measure ppm energy changes?
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Measure spin precession of polarized electrons and positrons.
Polarized Positrons at FCC?
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Option 1: Inject unpolarized positrons into FCC ring and wait from them to polarize via Sokolov-Ternov Effect.
Problem: Polarization is 250 hours without wigglers and 12 hours with wigglers. Too long!
Option 2: Pre-polarize positrons in 2.86 GeV damping ring. This is CERN’s baseline design.
Problem: Damping ring adds significant cost to experiment. Not needed for electrons!
Option 3: JLab bremsstrahlung-based polarized positron source.
Opportunity: FCC needs relatively few polarized positron bunches. Save money and provide new physics capabilities!
US HFCC program can provide seed-funding for polarized positron design effort at JLab.
Back to SLAC
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SLAC is a world-leading laboratory for ultrafast science
LCLS-II
FACET-II
LCLS
Back to SLAC
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SLAC is a world-leading laboratory for ultrafast science
LCLS-II
FACET-II
LCLS
Positron target and damping ring
LDRD: Multi-Disciplinary Science with Compact Positron Source
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An upgrade for FACET-II e⁺ is uniquely positioned to enable study of positron acceleration in high-gradient plasmas.
Accelerator R&D
Ultrafast Materials Science
Novel Treatment
Modalities
Advanced Accelerator Physics
Review Article
Laboratory Astrophysics
Review Article
Proposal aligns with SLAC’s strength as an ultrafast science laboratory.
Challenges
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Challenge #2
Compress and accelerate positrons from trap while preserving the beam quality.
Challenge #1
Produce, capture, and cool a record number of slow positrons
Production Rate Challenge
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The positron trap provides high-quality beams, but at relatively low rate. The current state-of-the-art is around 1 × 108 e+/s.
The FACET-II positron source based on the SLC target system with a new damping ring can provide 3 × 1010 e+/s.
We can purse high-impact accelerator R&D with only 1 × 109 e+/s, a ten-fold improvement over the current state-of-the-art.
GBAR Source
Requires kW-scale drive linac.
Compression and Emittance Growth
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How do we compress low-energy beams while preserving beam quality?
Test Area: XTA Linac at NLCTA
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XTA Beam (60 MeV)
Free space
LDRD proposal requests 1 kW beam at 100 MeV to generate 109 e+/s
XTA can provide 60 MeV beam with ~1 Watt of beam power.
Fewer positrons per second, but option to start immediately and test out components.
Details in Sophie’s talk.
Questions: Ultrafast Science with Positron Beams
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Y. Fukaya, J. Phys. D: Appl. Phys. 52, 013002
Can we compress positron bunches to sub-picosecond duration and synchronize with external laser?
Opportunities: Science with single, well-timed positrons
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Craig Levin, Stanford Medicine
Radiology
PET Expert
Ryan Coffee, SLAC
LCLS
Detector Expert
We are partnering with experts from Stanford Medicine and SLAC to provide single positrons for Time-of-Flight Positron Emission Tomography (ToF-PET) detector testing.
Other opportunities include:
Do some science soon.
Enable more science later.
Research Roadmap
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We are here!
Conclusions
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SLAC is pursuing a compact positron source for science with ultrashort positron beams.
Ultrashort positron beams will enable new opportunities in:
SLAC’s work on targets, moderators, and beam physics is aided by collaboration with KEK and synergistic with JLab’s aims.
The JLab positron source can provide unique opportunities for positron plasma acceleration R&D and FCC-ee polarized positron source.
We are excited to continue our partnership with JLab on electron-driven positron sources and the exciting science case for low-energy positrons!