Accelerator electron source lifetimes are extended by single-atom layer graphene coated.
Protective coatings are used for many everyday items that are frequently used. We coat wood floors with finish, paint cars with Teflon and even use diamond coatings to protect medical devices. Protective coatings are essential for many industrial and scientific applications.
Los Alamos National Laboratory researchers have now developed and tested an atomically-thin graphene coating for next-generation electron beam accelerator equipment. This is perhaps the most complex technical application of the technology. The success of this coating shows the potential for “Atomic Armor”, which can be used for various purposes.
Hisato Yamaguchi of the Laboratory’s Sigma-2 group said that accelerators are essential in addressing many of humanity’s great challenges. These challenges include the search for sustainable energy, continual scaling of computational power and detection and mitigation of disease. They also require studying the structure, dynamics, and mechanics of the building blocks of living things. These challenges require the ability to access, observe, and control matter at the frontier timescales of electronic motion as well as the spatial scale of the atomic bonds.
Photocathodes: The Challenge
The current electron-beam accelerators use thermionic emissions, which heat materials to release electrons. Next-generation accelerators will use photocathodes to generate electron sources from photons. These materials can convert photons into free electrons and so produce electron beams. This process creates corrosive gas that causes significant wear on photocathodes. It also interrupts research and adds time and costs to projects.
Yamaguchi stated that future accelerators would require high-performance electron beams. “But these performance requirements significantly exceed the capabilities of current state-of-the-art electron sources.”
A suitable protective coating is required for photocathodes that work in next-generation accelerators to function. Because photons that strike photocathodes and produce electrons also create corrosive gases, which can rapidly degrade bialkali thin film photocathodes made of antimony and potassium.
Because of its low work function, Cesium makes a great accelerator material. The work function is the energy required to extract an electron from a material and place it into a vacuum. This is a crucial step in electron beam production. This low work function comes with a price: increased sensitivity to ion-back-bombardment and chemical reactions and increased damage. Even in high vacuum states, thin film photocathode lifetimes can be limited.
Graphene provides promising results.
Researchers searched for a material that would protect the photocathode and allow electrons to be emitted. They discovered graphene as their solution.
Yamaguchi stated that no other material could transmit electrons while protecting the material. Although a porous material can transmit electrons, it won’t be able to protect it from corrosive gases. Graphene’s uniqueness is that it is thin enough to transmit electrons but dense enough that no corrosive gases can penetrate it.
The technical challenge of coating the bialkali photocathodes was daunting. Distributed on the photocathode in a layer just one atom thick, graphene possesses high gas impermeability, which protects the photocathode from the damage of gases created by the photon-to-free-electron conversion. The graphene’s high quantum efficacy (a measure of how well a material converts electrons to photons) allows electrons to pass through the coating, which is essential for creating and accelerating the electron beam for research. Researchers discovered that the transmission efficiency for the photoelectrons was only 5%. This suggests that the material is safe while still allowing the production of an electron beam.
Yamaguchi stated that these results are a significant step forward in fully encapsulated bialkali photocathodes with high QEs, long lifetimes, and atomically thin protection layers.
Photocathode coating is based on the “Atomic Armor”, which was chosen for the prestigious R&D 100 award in 2019. Research with graphene technology has shown its potential as a corrosion barrier. This could be applied to cars and ships as well as other goods.
