Nanoscale Diamonds Generate More Efficient Electron Beams

Beam-driven wakefield acceleration methods are potential candidates for upcoming large-scale machines, such as linear colliders and X-ray free-electron lasers, as they are capable of improving efficiency and decrying operation costs.

Electron beamlets as observed on YAG screens at varying distances from a cathode source. Image Credit: Argonne National Laboratory.

A major factor that fuels this improved efficiency involves exploiting the temporal distribution of electron beams. In the last few decades, scientists have examined several different mechanisms that effectively create temporally shaped electron beams of different quality with different restrictions.

In a recent study performed by the Argonne and Los Alamos national laboratories of the U.S. Department of Energy (DOE), researchers have applied a phenomenon, known as field emission, to investigate the application of arrays of microscopic diamond tips to create what they expect to be a transversely shaped beam of electrons.

This beam will be subsequently delivered to an emittance exchange beamline to transform the transverse distribution into a temporal distribution.

Field emission operates by reducing the quantum barriers that electrons can, according to the laws of probability, intermittently tunnel through.

It’s as if by applying these fields we can change a brick wall into drywall—it’s much easier to go through it.

Jiahang Shao, Study Author and Accelerator Physicist, Argonne National Laboratory

In other techniques used to produce electrons, either thermionic cathodes, which utilize hot filaments similar to those employed in incandescent light bulbs, are used to knock off electrons from photoelectric cathodes, or a solid, which make use of ultra-short laser pulses to spring the electrons loose.

According to Shao, the benefit of field emission cathodes is that they neither need a heat source nor a costly laser setup.

We’re using electric fields regardless when it comes time to accelerate the electrons. It’s not much more inconvenient to use them to generate them in the first place.

Jiahang Shao, Study Author and Accelerator Physicist, Argonne National Laboratory

To effectively apply the field emission approach, the team needed to use an ultra-strong concentrated electric field directly on the cathode surface. To achieve this, they produced a film of diamond containing diamond pyramids that measure around 10 µm on a side with nanocale tips placed on top and organized into a 1-mm equilateral triangle.

The experimental analysis was conducted on the Argonne Cathode Test-stand (ACT) beamline installed at the Argonne Wakefield Accelerator facility.

Generating a transversely shaped beam by field emission is the first step of the project, and we are exploring different emitter geometries as well as (radio-frequency) rf gun operation parameters.

Jiahang Shao, Study Author and Accelerator Physicist, Argonne National Laboratory

According to Manoel Conde, an accelerator scientist from the Argonne National Laboratory and the co-author of the study, the team is striving to balance two isolated but competing phenomena by applying these diamond field-emitter arrays.

The researchers needed to create an extreme current of electrons exiting the material, but at the same time, they also wanted to reduce the expelling force between the electrons to sustain the triangle shape at the time of emission and transportation.

A new article based on the study titled,“Demonstration of transport of a patterned electron beam produced by diamond pyramid cathode in an rf gun,” has featured in the January 2020 issue of the Applied Physics Letters journal. It described the effective demonstration of creation and transportation of a transversely shaped beam of electrons from a diamond field-emitter arrays cathode in an rf gun.

Yet another article titled,“Shaped beams from diamond field-emitter array cathodes,” was published in the July2 020 issue of the IEEE Transactions on Plasma Science and described the constant geometry optimization of the diamond field-emitter arrays.

Apart from Shao and Conde, other authors from the Argonne National Laboratory included Darrel Doran, Gwanghui Ha, Wanming Liu, John Power, and Eric Wisniewski.

Others who contributed to the study included Heather Andrews, Kimberley Nichols, Dongsung Kim, and Evgenya Simakov from the Los Alamos National Laboratory, and also Sergey Antipov from Euclid Techlabs, and Gongxiaohui Chen from the Illinois Institute of Technology.

The study was partly conducted at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the DOE Office of Science. Researchers in this center utilized Fluorine Inductively Coupled Plasma for silicon etching, Nova NanoSEM 450 for scanning electron microscopy, and Karl Suss Mask Aligner for photolithography.

The study was financially supported by a Research and Development program directed by the Los Alamos National Laboratory. Los Alamos National Laboratory—an affirmative action equal opportunity employer—is handled by Triad National Security, LLC for the U.S. Department of Energy’s NNSA.


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