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THOMAS JEFFERSON NATIONAL ACCELERATOR FACILITY

PROPOSAL FOR FY2021 LABORATORY DIRECTED RESEARCH AND DEVELOPMENT FUNDS

TITLE: HIGH MAGNETIC FIELD POLARIZATION OF 3HE FOR NUCLEAR STRUCTURE MEASUREMENTSTOPIC: ADDRESSING R&D ISSUES RELEVANT FOR NEW RESEARCH

LEAD SCIENTIST: JAMES MAXWELLPhone: 757-269-5036Email: [email protected]: 5/31/2020Department/Division: PhysicsOther Personnel:Mentor (if needed)

Proposal Term: From: 10/2020Through: 9/2023

Division Budget Analyst Susan BrownPhone: 757-269-7668Email: [email protected]

This document and the material and data contained herein were developed under the sponsorship of the United States Government. Neither the United States nor the Department of Energy, nor the Thomas Jefferson National Accelerator Facility, nor their employees, makes any warranty, express or implied, or assumes any liability or responsibility for accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use will not infringe privately owned rights. Mention of any product, its manufacturer, or suppliers shall not, nor it is intended to imply approval, disapproval, or fitness for any particular use. A royalty-free, non-exclusive right to use and disseminate same for any purpose whatsoever, is expressly reserved to the United States and the Thomas Jefferson National Accelerator Facility.

1 of 12 JLab LDRD Proposal


AbstractTechniques for the spin polarization of nuclei are indispensable tools in

the study of the quark and gluon structure of matter, and they have been an integral part of Jefferson Lab measurements since its inception. This proposal will investigate a new method for the creation of high-density, polarized 3He gas capable of operating within magnetic fields above 1T by combining two existing technologies: double-cell, cryogenic polarized 3He targets and new high-field optical pumping methods developed at ENS in Paris. This effort, if successful, will result in a new type of polarized target which will expand JLab’s scientific reach, removing a critical constraint on the use of polarized 3He in large acceptance detectors, while also benefiting broad applications of 3He polarization techniques, such as medical imaging, neutron spin filters and polarized fusion research.



Summary of ProposalDescription of ProjectThe spin polarization of 3He gas has broad applications in diverse fields

such as medical imaging, magnetometry, neutron spin filtering, and polarized fusion [1]. In high energy and nuclear physics scattering experiments, polarized 3He is used as a crucial surrogate for polarized neutrons, and polarized 3He targets have been integral components of Jefferson Lab experiments for decades. Traditional 3He gas polarization techniques have had limitations on their use in high magnetic field environments---a significant issue within or near magnetic spectrometers, MRI magnets, or tokamaks, for example. A novel 3He polarization system, conceived by this author and R. Milner of MIT, would take advantage of recent improvements in high-field metastability exchange optical pumping (MEOP) [2] and proven double-cell cryotarget methods [3] to create high-density polarized 3He gas in magnetic fields above 1T [4], suitable to be used as a scattering target.

In the course of this proposal, a prototype polarization system will be built to allow investigation of this technique, with emphasis on its use in Jefferson Lab experimental target conditions. Gas will be polarized within a 5 T solenoid in a glass pumping cell at room temperature and 100 mbar before being convectively transferred to an aluminum target cell held at 5 K through heat exchange with a liquid helium supply. Such a system is expected to provide 60% polarized gas at high field and in a sufficient density to compete with traditional, low-field polarized gas target methods.

Expected ResultsAlthough this novel system is based on proven techniques---high field

MEOP and double-cell cryotargets---this proposal will investigate their combination, looking into a number of outstanding questions that must be addressed before they can be used for scattering applications. This proposal will measure the performance of the prototype system, including maximum polarization, pumping rate, and relaxation time in various magnetic fields, temperatures and pressures. Understanding the rate of gas transport between these cells, and methods for controlling it, is a crucial step to ensure sufficient polarization in the target cell. Methods for the measurement of the polarization, and the degree of accuracy with which they can be performed, will also be investigated. The final result of this project will be a well-understood, working design for a new class of polarized target which will directly expand Jefferson Lab's ability to explore the quark and gluon interactions in the nucleon, will enable future polarized 3He scattering experiments in high magnetic fields common in large



acceptance spectrometers, and will benefit broad applications of 3He polarization technology.



Proposal NarrativePurpose/GoalsPolarized Helium-3 offers an attractive target medium for accessing the

neutron's spin properties. About 89% of the time, 3He is in a spatially-symmetric S-state, where its two protons spins are anti-aligned in a spin singlet and the neutron spin is aligned with the nuclear spin. This makes polarized 3He an invaluable surrogate for a polarized free neutron target. Two techniques for aligning spins using laser light are available to create polarized 3He gas for targets: metastability exchange optical pumping (MEOP) and spin-exchange optical pumping (SEOP) [1]. In MEOP, an RF discharge excites a population of gas atoms into a metastable state which can be optically pumped to produce polarization, and this polarization is transferred to the larger ground state population by metastability-exchange collisions. In SEOP, rather than producing metastable atoms to pump, alkali metals which can be directly optically pumped are evaporated into the gas, and polarization is transferred via Fermi-contact hyperfine interactions between the alkali electron and the 3He nucleus. The largest operational difference between MEOP and SEOP is the gas pressure, and ultimately target density, at which these techniques can be performed. While MEOP is historically limited to near 1 mbar, SEOP works as high as 13 bar. This has made SEOP the technique of choice for high luminosity scattering experiments: SEOP targets were used for 13 experiments in Jefferson Lab's Hall A in the 6 GeV era, and 7 more are already approved to run at 12 GeV.

The prevalence of large magnetic fields in high energy and nuclear physics detector packages highlights a key limitation of current polarized 3He targets. High magnetic fields are used in spectrometers like prisms, separating particles of different momenta. This not only provides crucial information on the scattering interaction, but also constrains background particles from overwhelming detectors. Large acceptance spectrometers such as CLAS12 at JLab, sPHENIX at RHIC, and CMS at the LHC utilize strong solenoid fields around the interaction region as integral detector elements. While polarized 3He targets have been used extensively at JLab in Halls A and C, this has been with small acceptance spectrometers where the magnetic field at the target is low. Because SEOP is only performed at low magnetic fields, no polarized 3He target is available for Hall B's large acceptance spectrometer, CLAS12.

In November of 2019, Richard Milner and I proposed a concept for a new type of polarized 3He target to operate in high magnetic fields [4]. This concept would overcome the limitations of traditional MEOP methods by combining the density increase of double-cell cryotargets---such as the MEOP target used at Bates in the early 1990's [3]---with high magnetic field MEOP methods recently developed at ENS [2]. In our proposed scheme,



atoms will be polarized directly inside the 5T field of CLAS12's central solenoid to create 60% longitudinally polarized gas at 100mbar. By transferring the room temperature, polarized gas to a cold target cell, the gas density will be increased to meet the luminosity requirements for scattering experiments. In bringing polarized 3He into a high-luminosity, large-acceptance spectrometer for the first time, this target will open a wide class of new observables to study the quark and gluon structure of the neutron at JLab's precision frontier, and these measurements will be only the first use of a broadly applicable technique.

My aim in this proposal is to build a double-cell, high-magnetic-field 3He polarization system prototype and to test its performance with an emphasis on Jefferson Lab applications such as CLAS12. Taking advantage of my experience utilizing high-field MEOP for a polarized ion source for the EIC, this work will first explore the limits of room-temperature polarization achievable at high fields and pressures. Jefferson Lab's world-leading expertise in polarized and cryogenic targets will be harnessed to build a double-cell test system, allowing the transfer of room temperature polarized 3He gas into a cryogenic cell cooled via liquid 4He condensed with a pulse-tube cryocooler system. While the central thrust of this work is to directly expand the polarized scattering program at Jefferson Lab, this research offers broad potential benefits not only to high energy and nuclear physics experimental programs, but to myriad other applications of polarized 3He, from neutron spin filters to medical imaging to polarized fusion research.

Approach/MethodsThe proposed high-field, high-density 3He polarization system synthesizes

existing techniques to create a novel, high-luminosity, polarized 3He target that operates in a high magnetic field environment. The figure below shows a diagram of the proposed design, with two gas cell volumes in convective contact—one cooled by a liquid helium heat exchanger, the other heated and optically pumped. Using 100 mbar gas in a 20 cm long aluminum target cell at 5 K will result in a target thickness of 3x1021 3He/cm2, which is sufficient to reach CLAS12's maximum per nucleon luminosity limit at a beam current of 2.5uA, for example. Studies at ENS in Paris have indicated that 60% polarization should be possible at 100 mbar and 5 T without any further improvement in the method [1].



The main sources of polarization relaxation come from wall interactions, transverse magnetic field gradients, and ionization in the beam. To avoid depolarization on the cell walls, the room temperature pumping cell will be made of borosilicate glass, and the aluminum pumping cell will be coated with a cryogenic layer of H2, which has been shown to yield days long relaxation times between 2 and 6 K [5]. The CLAS12 solenoid field map has been used to assess the rate of relaxation from transverse field gradients in the target region, and showed that the field uniformity is more than sufficient [4]. CEBAF's electron beam can induce spin relaxation in the 3He target gas through the production of molecular 3He2

+. This effect was studied extensively for the Bates 88-02 target [3], and was found to create a 2000 second relaxation time in 2.6 mbar gas under a beam current of 5uA. While the molecular production increases with density, increasing the magnetic field has been shown to reduce the depolarization due to diatomic molecules [6]. The rate of communication between the cells, delivering polarization from the pumping cell to the target cell, must be studied to ensure that convection is sufficient to maintain high polarization. Measurements of the polarization in the high-field pumping cell can be performed using a probe laser, a method with which the author has significant experience [7], and the polarization in the target cell can be inferred based on the rate of communication between the cells [8].

The below figure shows a cross sectional diagram of the high-field, double-cell prototype polarization system to be built, which will follow the design of Hall D's cryotarget (to simplify design and construction) with the addition of a MEOP pumping cell. The table shows the costs of the major components (above $10,000). This proposal takes advantage of cryogen-free cooling systems for the magnet and cryostat, reducing the operating costs, mitigating cryogen supply complications and avoiding the use of non-renewable resources. The magnetic field will be provided by a cryogen-free, 5T, 3 inch warm-bore, superconducting solenoid, sourced from Cryomagnetics Inc of Oak Ridge, TN. The vacuum chamber will be inserted into the magnet's warm bore, and evacuated with a Pfieffer turbopump set. The Cryomech PT-420 pulse tube's first, 40K stage will be used to cool a radiative heat shield which will enclose the lower stage, the liquid helium


MAJOR EQUIPMENT COSTSuperconducting Magnet

$160k

Pulse Tube Cryocooler $64kCustom Cryostat $45kLumibird 10W Laser $28kToptica DFB Laser $25kWavelength Meter $27kPfeiffer Turbopump Set

$20k

CTI Cryopump $12k


transfer line and the target cell. The target cell will be cooled to below 5K using liquid helium condensed on the second stage of the pulse tube. The MEOP pumping cell will be outside the heat shield, and accessible to the pump and probe laser brought into the cryostat in fiber feed-throughs.

Procurement will be staged throughout the first two years to allow testing of components as they arrive, with two major systems to test individually before they are combined: the polarization system and the cryogenic system. Year one of the effort will include design, procurement and setting up the laser lab, with a milestone of first 3He gas polarization in a sealed cell at high field. The goal of polarized gas within the first year is aggressive, but is much aided by my recent experience building a similar system for the polarized 3He ion source for EIC at Brookhaven. Year two will be the construction of the cryostat and polarization tests at high field and varied pressures, with a milestone of cryostat completion by the end of the year. Year three will be tests with the full apparatus, with the goal of a final assessment of its performance.

Goals for FY2021 Quarter 1

o Finalize design of high-field MEOP test apparatuso Send out for bids for superconducting magneto Order pumping laser and probe laser systemso Begin laser lab preparations

Quarter 2o Select bid for superconducting magnet systemo Order optics for pump and probe systemso Complete laser safety setup for lab

Quarter 3o Receive and test pumping laser, probe lasero Procure and begin work on DAQ hardware/software for probeo Procure remaining equipment for polarizer

Quarter 4o Receive and test superconducting magnet systemo Test prototype probe DAQo First polarization of gas in sealed test cell at 1 mbar, 2 T

Required ResourcesThis effort requires a laboratory space that is suitable for work with class

4 lasers, as well as a small amount of cryogens. Because the superconducting magnet is cryogen free, the ODH hazard will be much reduced. Inside the cryostat will be 3He gas for polarization and 4He gas for heat exchange, both of which are small volumes (at STP, approximately 6 L



of 3He and 20 L of 4He). The pulse-tube and compressor system itself contains about 80 L at STP of helium. Considerable laser safety equipment is budgeted in this proposal, assuming a suitable lab is available.

Anticipated Outcomes/ResultsThe primary results of this project will be a prototype high-field, high-

density 3He polarization system and its assessment for experimental use. This effort will aim to satisfy the requirements of new polarized 3He experiments proposed to PAC 48 to use this technique in Hall B, and publish the results to benefit wide applications of polarized gas technologies. The measurements taken with the prototype system will include:

Maximum steady-state polarization and pumping rate versus discharge intensity, magnetic field strength and pressure

Depolarization rate with and without discharge, with and without cryogenic hydrogen coating

Gas transfer rate between cells Probe polarization measurement uncertainty

Budget ExplanationThe bulk of this proposal’s budget is dedicated to the procurement of

necessary equipment for the construction of the prototype polarization system. The MEOP polarization components include pumping and probe lasers, a wavelength meter, an RF source and amplifier, vacuum pumps, and a cryopump to maintain gas purity, as well as material costs for the production of glass cells, optics and laser safety equipment. The cryogen-free, warm-bore superconducting magnet is the largest expenditure in the project, and is crucial to the success of the tests. This magnet will allow our tests to be performed without need for large-volume cryogen deliveries which present open-ended continued operational costs, supply and timing difficulties, and a large laboratory space (to avoid ODH hazards) which will be difficult to enclose for laser safety issues.

The cryogenic portion of the apparatus will require a Cryomech pulse tube system for cooling, additional vacuum pumps, and the cryostat itself. The cryostat will include a custom outer vacuum chamber, and support and gas flow components, all designed in house and sent out for machining from outside contractors. The price of the cryostat is based on the costs of the similar system built for the Hall D cryotarget. Other materials required will include thermometry, pressure gauges and flow meters, data acquisition hardware and custom gas cells. The budget also includes support for the PI at 25% effort per year, as well as a total of 6 months effort of a technician to aid in construction and operation, and 3 months effort of a designer to collaborate on the PI’s apparatus design.



References

[1]

T. Gentile, P. Nacher, B. Saam and T. Walker, "Optically polarized 3He," Rev. Mod. Phys., vol. 89, p. 045004, 2017.

[2]

A. Nikiel-Osuchowska and others, "Metastability exchange optical pumping of 3He gas up to hundreds of millibars at 4.7 Tesla," Eur. Phys. J. D, vol. 67, p. 200, 2013.

[3]

R. Milner, R. McKeown and C. Woodward, "A polarized 3He target for nuclear physics," Nuclear Instruments and Methods in Physics Research Section A, vol. 274, p. 56, 1989.

[4]

J. Maxwell and R. Milner, "Conceptual Design of a Polarized 3He Target for the CLAS12 Spectrometer," arXiv:1911.06650, 2019.

[5]

V. Lefevre-Seguin and J. Brossel, "Attempts to increase the nuclear relaxation time of 3He gas at low temperatures," Journal of low temperature physics, vol. 72, p. 165–188, 1988.

[6]

K. Bonin, T. Walker and W. Happer, "Relaxation of gaseous spin-polarized 3He targets due to ionizing radiation," Physical Review A, vol. A, 1988.

[7]

J. Maxwell and others, "Enhanced polarization of low pressure 3He through metastability exchange optical pumping at high field," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 959, p. 161892, 2020.

[8]

J. Cathleen and others, "A Measurement of the Spin-Dependent Asymmetry in Quasielastic Scattering of Polarized Electrons from Polarized 3He," Ph.D. Thesis CalTech, 1992.

AttachmentsPlease find attached a letter of support from Richard Milner of MIT.


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