Design and Implementation of a Metastability Exchange Optical Pumping (MEOP) System for Polarized Helium-3 Experiments at Sub-Kelvin Temperatures

Core Concepts

This paper details the design, construction, and testing of a system for producing highly polarized Helium-3 at room temperature and injecting it into a superfluid Helium-4 environment at temperatures below 1K with minimal polarization loss, for use in a neutron electric dipole moment (nEDM) experiment.

Abstract

Bibliographic Information:

Rao, T., Barrón-Palos, L., Berkutov, I., Crawford, C., Golub, R., Huffman, P., Konieczny, M., Korobkina, E., Reid, A., Salazar-Ángeles, B., Smith, C., Tat, R., & Zanatta-Martínez, T. (2024). MEOP based 3He polarization and injection system for experiments below 1 K. Journal of Instrumentation. [arXiv:2411.07197v1 [physics.ins-det]]

Research Objective:

This paper describes the development and testing of a novel system for producing highly polarized Helium-3 (80%) at room temperature using Metastability Exchange Optical Pumping (MEOP) and injecting it into a superfluid Helium-4 filled measurement cell at sub-Kelvin temperatures (0.3K - 0.5K) while maintaining low concentrations (10^-8 - 10^-10) and minimal polarization loss. This system is being developed for the nEDM experiment at the Triangle Universities Nuclear Laboratory (TUNL).

Methodology:

The authors designed and built a system consisting of a 3He gas handling system (GHS), a MEOP polarization cell enclosed in a custom-designed double cosine theta magnetic coil, and a dilution system. The GHS purifies and prepares the 3He gas, while the MEOP system polarizes the gas using a 10W Ytterbium-doped fiber laser and a magnetic field. The dilution system reduces the concentration of polarized 3He before injection into the superfluid Helium-4 measurement cell. The authors meticulously tested each component of the system, including the 3He friendliness of the pneumatically actuated valves and the magnetic field homogeneity of the MEOP coil, to ensure minimal polarization loss during the process. They also developed a COMSOL model to simulate the injection process and estimate the heat load on the measurement cell.

Key Findings:

• The authors successfully built and tested a MEOP-based system capable of producing 80% polarized 3He at room temperature.
• The custom-designed double cosine theta magnetic coil achieved the required magnetic field homogeneity for maintaining polarization, with gradients less than 0.0833%/cm.
• The pneumatically actuated valves used in the GHS were found to be 3He friendly, not introducing significant magnetic field gradients or contaminants.
• The dilution system successfully reduced the 3He concentration to the desired levels (10^-8 - 10^-10) while preserving polarization.
• COMSOL simulations provided insights into the heat load during injection and informed the design of the injection line.

Main Conclusions:

The authors successfully demonstrated a novel system capable of producing and injecting highly polarized 3He into a superfluid Helium-4 environment at sub-Kelvin temperatures with minimal polarization loss. This system represents a significant advancement in the field of polarized 3He applications, particularly for sensitive experiments like the nEDM search.

Significance:

This research has significant implications for the nEDM experiment and other research areas requiring highly polarized 3He at low temperatures. The system's ability to achieve high polarization, low concentrations, and minimal polarization loss makes it a valuable tool for studying fundamental physics and developing new technologies.

Limitations and Future Research:

The paper primarily focuses on the design, construction, and testing of the individual components of the system. Future research will involve integrating the system with the nEDM experiment and demonstrating its performance in actual experimental conditions. Further optimization of the injection process, including minimizing heat load and polarization loss during injection, will be crucial for achieving the experiment's ultimate sensitivity goals.

Stats

The MEOP system aims to produce 80% polarized 3He at room temperature.
The target concentration of polarized 3He in the superfluid Helium-4 measurement cell is between 10-8 and 10-10.
The measurement cell temperature will be maintained between 0.3K and 0.5K.
The MEOP cell operates at a pressure of 1.3 mbar during polarization.
The 3He gas is pressurized to 500 mbar with 4He before dilution to increase T1 and aid condensation.
The magnetic field in the MEOP cell is 5 Gauss.
The required magnetic field gradient for a T1 of 1000s is less than 0.0833%/cm.
The injection line is a 2 mm diameter Kapton tube.
The 3He gas in the polarization cell is approximately 5 mg.

Quotes

Key Insights Distilled From

MEOP based 3He polarization and injection system for experiments below 1 K

by T. Rao, L. B... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2411.07197.pdf

Deeper Inquiries

How will the performance of this MEOP-based polarized 3He injection system impact the sensitivity and accuracy of the nEDM experiment at TUNL?

The performance of the MEOP-based polarized 3He injection system is crucial to the sensitivity and accuracy of the nEDM experiment at TUNL for several reasons:

High Polarization for Enhanced Sensitivity: The system aims to achieve 80% ³He polarization, which directly translates to a stronger NMR signal. This is critical in nEDM experiments, where the signal from the neutron's electric dipole moment is extremely weak. A higher initial polarization leads to a larger signal-to-noise ratio, ultimately improving the experiment's sensitivity to detect a non-zero nEDM.
Ultra-Low Concentrations and Systematic Effects: The ability to achieve ultra-low ³He concentrations (down to 10⁻¹⁰) is essential for mitigating systematic effects. At higher concentrations, ³He-³He interactions can introduce spurious signals that mimic the nEDM signal. By minimizing these interactions through dilution, the experiment can achieve higher accuracy in its measurements.
Polarization Preservation for Accurate Measurement: Maintaining high polarization during the injection and delivery of ³He to the superfluid ⁴He measurement cell is paramount. Any significant polarization loss during this process would directly reduce the sensitivity to the nEDM. The system's design, including the use of ⁴He pressurization and a carefully designed holding field, aims to minimize polarization losses due to magnetic field gradients and wall collisions.
Precise Gradient Control for Co-magnetometry: The system's ability to control magnetic field gradients within the MEOP cell and along the injection path is crucial for accurate co-magnetometry. By carefully characterizing and minimizing these gradients, the experiment can precisely measure the magnetic field environment experienced by both the neutrons and the ³He, allowing for accurate corrections to the nEDM signal and reducing systematic uncertainties.
In summary, the success of the nEDM experiment at TUNL relies heavily on the performance of the MEOP-based ³He injection system. Achieving high initial polarization, ultra-low concentrations, and minimal polarization loss during injection are all essential factors in maximizing the experiment's sensitivity and minimizing systematic uncertainties in the search for the elusive neutron electric dipole moment.

Could alternative polarization-preserving techniques, such as spin-exchange optical pumping (SEOP), be more suitable for achieving the desired 3He polarization and concentration at sub-Kelvin temperatures?

While the paper focuses on MEOP, it's worthwhile to consider SEOP. Here's a comparison:
MEOP:

Advantages:

Well-established for ³He, routinely achieving high polarization (>70%).
Works well at room temperature, simplifying the experimental setup.
Can achieve very long T1 relaxation times in appropriate magnetic fields.

Disadvantages:

Requires specific laser wavelength (1083 nm) for optical pumping.
Polarization process is pressure-dependent, necessitating operation at ~1 mbar and subsequent compression for injection.
SEOP:

Advantages:

Can polarize ³He at higher pressures, potentially simplifying the injection process.
Uses readily available high-power lasers (e.g., diode lasers).

Disadvantages:

Typically achieves lower polarization than MEOP for ³He.
Requires a different atomic species (usually alkali metals like Rb) for spin exchange, introducing potential contamination issues.
The efficiency of spin exchange decreases at lower temperatures, making it less ideal for sub-Kelvin environments.
For this specific nEDM experiment:

MEOP's high polarization is highly desirable to maximize the signal-to-noise ratio in the nEDM measurement.
The experiment's design already incorporates solutions to address MEOP's pressure requirements, such as the dilution volumes and ⁴He pressurization.
Introducing alkali metals for SEOP could lead to contamination concerns within the superfluid ⁴He measurement cell, potentially affecting the nEDM measurement.
Conclusion:
While SEOP has its merits, MEOP appears to be the more suitable technique for this nEDM experiment. Its ability to achieve high ³He polarization outweighs its pressure limitations, which the experimental design already addresses. Introducing SEOP's complexity and potential contamination risks might not be justified given MEOP's established performance in this context.

What are the potential applications of this highly polarized, low-concentration 3He system in other areas of physics research beyond the search for the neutron electric dipole moment?

Beyond the nEDM search, this system's ability to produce highly polarized ³He at ultra-low concentrations in a sub-Kelvin environment opens up exciting possibilities in various physics research areas:

Low-Temperature Physics and Quantum Fluids:

Studying ³He-phonon interactions: At ultra-low concentrations, the dynamics of ³He are dominated by interactions with phonons in the superfluid ⁴He. This system allows for precise investigations of these interactions, providing insights into the fundamental properties of quantum fluids.
Probing topological defects in superfluid ³He: Polarized ³He can be used as a sensitive probe for studying topological defects, such as quantized vortices, in superfluid ³He. The high polarization enhances the sensitivity of NMR-based detection techniques.

Precision Measurements and Fundamental Physics:

Searching for new particles and interactions: Ultra-sensitive magnetometers based on highly polarized ³He can be used to search for exotic particles, such as axions, and to test fundamental symmetries, potentially leading to discoveries beyond the Standard Model of particle physics.
Developing next-generation magnetic sensors: The system's ability to maintain high polarization in a low-field environment makes it attractive for developing highly sensitive magnetic sensors for various applications, including medical imaging, materials science, and geophysics.

Surface Science and Nanoscale Physics:

Investigating surface magnetism and spin dynamics: Polarized ³He can be used as a non-destructive probe for studying magnetic properties and spin dynamics at surfaces and interfaces. The low-temperature capability allows for exploring these phenomena in new regimes.
Developing novel quantum devices: The combination of high polarization and low temperatures could be leveraged for developing novel quantum devices, such as highly sensitive magnetometers and spintronic devices, with potential applications in quantum information processing and sensing.
In conclusion, the development of this highly polarized, low-concentration ³He system at TUNL has far-reaching implications beyond the nEDM experiment. It provides a unique platform for exploring fundamental physics, advancing our understanding of quantum fluids, and developing cutting-edge technologies with broad applications in various fields.