Plans for the Compact Positron Source at SLAC
Spencer Gessner, Rafi Hessami, Aaron Lindenberg, Chris Adolphsen, Joel England, Mark Hogan, SLAC
LCWS2024, Tokyo
July 10, 2024
Motivation: Access to Positron Beams for Accelerator R&D
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Positron PWFA experiments have only taken place at SLAC by using existing SLC infrastructure.
FACET-II*
FFTB
FACET
M. J. Hogan et. al. Phys. Rev. Lett. 90 205002 (2003).
B. Blue et. al. Phys. Rev. Lett. 90 214801 (2003).
P. Muggli et. al. Phys. Rev. Lett. 101 055001 (2008).
S. Corde et. al. Nature. 524 442445 (2015).
S. Gessner et. al. Nat. Comm. 7 11785 (2016).
A. Doche et. al. Nat. Sci. Rep. 7 14180 (2017).
C. A. Lindstrøm et. al. Phys. Rev. Lett. 120 124802 (2018).
S. Gessner et. al. arXiv:2304.01700 (2023).
*E333 experiment planned for filament regime positron PWFA.
Motivation: Access to Positron Beams for Accelerator R&D
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Positron PWFA experiments have only taken place at SLAC by using existing SLC infrastructure.
FACET-II*
FFTB
FACET
M. J. Hogan et. al. Phys. Rev. Lett. 90 205002 (2003).
B. Blue et. al. Phys. Rev. Lett. 90 214801 (2003).
P. Muggli et. al. Phys. Rev. Lett. 101 055001 (2008).
S. Corde et. al. Nature. 524 442445 (2015).
S. Gessner et. al. Nat. Comm. 7 11785 (2016).
A. Doche et. al. Nat. Sci. Rep. 7 14180 (2017).
C. A. Lindstrøm et. al. Phys. Rev. Lett. 120 124802 (2018).
S. Gessner et. al. arXiv:2304.01700 (2023).
*E333 experiment planned for filament regime positron PWFA.
High Energy e- Linac
High Power Target
e+ Return Line
e+ Damping Ring
e+ Compression
e+ Experiments
Approximately $50M to return e+ capabilities to FACET-II
Motivation: Multi-Disciplinary Science
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SLAC is uniquely positioned to deliver positron beams available nowhere else in the world for high-impact research.
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
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)
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)
Positron Traps for Antimatter Experiments
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GBAR Positron Source at CERN
Schematic for trapping and cooling positrons
A multi-cell trap for increasing positron beam rate
Penning-Malmberg traps are well established technology for accumulating and manipulating low-energy positron beams.
Low-Energy Positron Beams for Materials Science
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Low-energy positron beams are probes for materials science research and are a particularly useful tool for studying surfaces.
KEK Slow-Positron Facility
Penning-Malmberg Traps for Positron Beams
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R. Hessami at CERN (2019)
Now Stanford Ph.D. student
Penning-Malmberg Trap
Are Penning-Malmberg traps viable sources of positron beams?
100 nm emittance!
Challenges
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Challenge #1
Produce high quality positron bunches at high rate
Challenge #2
Compress and accelerate positrons from trap while preserving the beam quality.
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
A 100 MeV electron beam with 1 kW power will produce 109 slow positrons per second.
x 10
Positron Capture
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The traditional method for capturing and cooling positrons is with a buffer gas trap.
The positrons in the trap cool faster than they annihilate with the gas.
But it takes a long time to accumulate enough positrons. . .
Positrons are lost while we wait to accumulate.
Schematic for trapping and cooling positrons
Efficient Positron Capture
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The AIST group in Japan has developed a novel method for trapping positrons that is optimized for linac sources (New Jour. Phys, 24, 123039 2022).
The technique takes advantage of the pulsed nature of the linac source (as opposed to CW radionuclide sources).
The potentials inside the trap are synchronized with the incoming positron beam.
The AIST technique is a 5X improvement over traditional BGT with pulsed beams.
Challenges
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Challenge #1
Produce high quality positron bunches at high rate
Challenge #2
Compress and accelerate positrons from trap while preserving the beam quality.
Producing 109 e+/s is challenging but possible with state-of-the-art techniques and 100 MeV e- driver.
Compression and Acceleration
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The beam inside the trap is cold and at rest.
The beam must be extracted from the trap, accelerated, and compressed in time.
Our PRAB paper explored electrostatic bunching, but we have not performed a detailed study of realistic devices.
Emittance Growth
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The GPT simulation shows emittance growth occurs at the start of the RF cavity.
Solution (yet to be implemented): stronger focusing at entrance of s-band cavity.
Intrinsic Angular Momentum
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The positrons are cooled in the PM trap before being injected.
The cooling process means that the positrons are born in a solenoidal magnetic field. They have intrinsic angular momentum.
The intrinsic angular momentum is much greater than the thermal emittance of the beam.
Solution: Round-to-Flat Beam Transformer
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Beams with intrinsic angular momentum can be partitioned such that the vertical emittance is small (same as thermal emittance) and the horizontal emittance is large (size of angular momentum term).
Round-to-flat beamlines use skew quadrupoles to transform the beam.
Beams with intrinsic angular momentum are proposed for “damping-ring-free” linear colliders.
Challenges
19
Challenge #1
Produce high quality positron bunches at high rate
Challenge #2
Compress and accelerate positrons from trap while preserving the beam quality.
Producing 109 e+/s is challenging but possible with state-of-the-art techniques and 100 MeV e- driver.
Solutions exist at conceptual level, but detailed models and beamline simulations are still needed.
Plan at SLAC
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We have requested $500k LDRD funding to pursue a technical design of the compact positron source:
We will pursue a demonstrator at SLAC (NLCTA) or through collaboration with universities.
Research Roadmap
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We anticipate that by the end of the LDRD, we will have already applied for funding such that construction and implementation of the compact positron source can begin in FY26 or FY27.
A stand-alone system could be installed at NLCTA or B44 or at a university in support of beam physics R&D, first materials science studies, and first tests of positron tracer beams for radiation therapy.
An integrated system could be deployed at FACET-II, consistent with LAF downtimes and constraints from LCLS-II. Science with positron beams could begin in FY28.
Backup
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Approach: Linac Source
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FACET-II
Replace switchable dipole with kicker to select 125 MeV electron bunches for positron target. This is compatible with FACET operation: 30 Hz at injector with 10 Hz for experiment and 20 Hz for positron generation.
Stand-alone System
Meter-scale, cryo-cooled, distributively coupled RF structure produces 100 MeV beams with 2.5 kW beam power. A compact solution using C3 technology!
Approach: Targetry and Radiation Protection
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The target must withstand 1 kW of incident beam power.
We will leverage our existing collaboration with KEK and JLab through the “Advanced Electron and Positron Sources” US-Japan grant.
A thin (1 mm), rotating tungsten target with water cooling is likely sufficient for our needs.
We will work with Radiation Protection to develop a compact shielding solution for 1 kW of losses at 125 MeV beam energy.
KEK rotating target
We just learned that our follow-on grant “Advanced Positron Sources” will be funded.
Approach: Beam Extraction and Modeling
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Our PRAB paper modeled the acceleration and compression of the beam with the GPT code.
We observe a blow-up in emittance at the entrance of the s-band accelerator structure. We aim to address this issue with iterative beam dynamics studies.
After initial acceleration, the beam undergoes a round-to-flat transformation with a skew quadrupole lattice. The round-to-flat lattice should be as compact as possible.
The LDRD will fund a postdoc to carry out a detailed study of open beam physics questions.
Ultimate Parameters
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Our study will establish the expected performance parameters for a system to be built and demonstrated at SLAC. Performance parameters include:
Our study will also provide a roadmap for future R&D and upgrades that will extend the capabilities of the trap to provide even higher quality beams. Future R&D directions may include:
Surko Group Cryogenic Trap
Surko Group Multi-Cell Trap
Positrons at FACET-II
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Positron Damping Ring Magnet
Sodium-22 Source
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Questions: Ultrafast Science with Positron Beams
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Y. Fukuya, J. Phys. D: Appl. Phys. 52, 013002
Can we compress positron bunches from PM traps to sub-picosecond direction and synchronize it with external laser probe?
Questions: Ultrafast Science with Positron Beams
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Y. Fukuya, J. Phys. D: Appl. Phys. 52, 013002
Can we compress positron bunches from PM traps to sub-picosecond direction and synchronize it with external laser probe?
First science opportunities include:
How do surface dynamics differ from bulk? Not straightforward to disentangle in x-ray or electron scattering experiments.
Ferroelectrics, Krapivin et al, PRL (2022)
Heterostructures, Wu et al, Sci, Adv. (2022)
Phase Change Materials, Qi et al, PRL (2022)
Catalysis, Diesen et al, PRL (2021)