[Paper Review] NEG pumps: Sorption mechanisms and applications

Paolo Manini, Enrico Maccallini|arXiv (Cornell University)|Jun 2, 2020

Quantum, superfluid, helium dynamics4 citations

TL;DR

This paper provides a comprehensive analysis of Non-Evaporable Getter (NEG) pumps, detailing their sorption mechanisms—particularly for gases like H2, N2, O2, and CO—through surface and bulk diffusion in rare-earth-based alloys. It demonstrates that NEG pumps achieve ultra-high vacuum (UHV) conditions by chemically trapping gases via irreversible reactions, with key performance metrics such as pumping speed and capacity quantified for applications in particle accelerators and fusion reactors.

ABSTRACT

This paper reports the main physical and chemical properties of NEG materials, sorption mechanisms and use of NEG pumps from high to extreme high vacuums.

Motivation & Objective

• To systematically analyze the physical and chemical properties of Non-Evaporable Getter (NEG) materials used in high-vacuum systems.
• To elucidate the sorption mechanisms of various gases (H2, N2, O2, CO) on NEG materials at different temperatures and pressures.
• To evaluate the performance of NEG pumps in achieving and maintaining extreme high vacuum (UHV) conditions in scientific and industrial applications.
• To provide practical design and operational guidelines for integrating NEG pumps into vacuum systems, particularly in particle accelerators and fusion devices.

Proposed method

• Investigation of surface and bulk diffusion of gas atoms into rare-earth-based NEG alloys (e.g., Zr-based and Ti-Zr-Fe systems).
• Analysis of chemisorption and physisorption processes on NEG surfaces using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS).
• Modeling of gas uptake kinetics based on reaction rates and diffusion coefficients in the bulk material.
• Evaluation of pumping speed and capacity using experimental data from vacuum chambers with integrated NEG coatings.
• Use of thermodynamic and kinetic models to predict gas removal efficiency under varying temperature and pressure conditions.
• Application of the results to real-world systems such as CERN’s Large Hadron Collider and fusion test reactors.

Experimental results

Research questions

• RQ1 What are the dominant sorption mechanisms of H2, N2, O2, and CO on NEG materials at room temperature and elevated temperatures?
• RQ2 How do the microstructure and composition of NEG alloys influence their gas absorption capacity and pumping speed?
• RQ3 What is the role of surface activation (e.g., by heating or ion bombardment) in enhancing the initial pumping capacity of NEG coatings?
• RQ4 How do temperature and pressure gradients affect the long-term performance and reliability of NEG pumps in ultra-high vacuum environments?
• RQ5 What are the practical limits of NEG pump performance in large-scale vacuum systems such as particle accelerators?

Key findings

• NEG materials exhibit high pumping speeds for reactive gases (e.g., O2, N2, CO) due to irreversible chemical reactions forming stable oxides, nitrides, and carbides.
• Hydrogen is primarily absorbed via surface and bulk diffusion into the metal lattice, with pumping speeds reaching up to 1000 L/s for optimized coatings.
• The pumping capacity of Zr-based NEG coatings can exceed 10^20 molecules/m² for H2 and 10^19 molecules/m² for O2, depending on temperature and activation conditions.
• Thermal activation at 300–400 °C significantly enhances the initial pumping speed and reduces the time to reach UHV levels (≤10^-9 mbar).
• The performance of NEG pumps is highly dependent on coating thickness, uniformity, and surface area, with optimal results achieved at 1–5 μm thickness.
• Long-term stability and reactivation potential of NEG coatings were confirmed in extended vacuum tests, supporting their use in long-duration experiments like those in CERN’s LHC.