In the production of high-quality electron beams, as found in the most advanced scientific equipment such as free electron lasers, ultrafast electron diffraction and imaging, and wake field accelerators, scientists regard photocathode technology as a way of converting light into electrons. method. These tools provide researchers with a way to penetrate materials, atomic structure, and behavior more deeply under actual conditions.

The photocathode works through a process called the photoelectric effect, in which photons usually emitted by a laser strike the material, exciting electrons away from its surface. Photocathodes are superior to other forms of cathodes because they allow scientists to better control the quality of the electron beam. However, the photocathode still has room for improvement.

Scientists trying to create a new photocathode need to develop a material that can meet three different parameters. First, it must have a high "quantum efficiency", that is, the proportion of electrons produced by each incident photon. Second, it must have a low intrinsic emissivity to measure how much the beam may diverge after it is generated. Finally, the photocathode must not be able to withstand the conditions under ideal vacuum conditions.

In a new study by the Argonne National Laboratory, researchers have demonstrated a new material that has an excellent balance between these parameters.

The all-nitrogen-doped ultra-nanocrystalline diamond photocathode assembly was tested on the beamline of the Argonne cathode test bench. The nitrogen-doped UNCD sample is the dark middle part of the image. (Photo: Shao Jiahang/Argong National Laboratory)

This material itself is called ultra-nanocrystalline diamond or UNCD, which is a patented material by Argonne. Researchers at the Center for Nanoscale Materials (CNM) in Argonne, U.S. Department of Energy’s Office of Science User Facilities, have synthesized UNCD through chemical vapor deposition. Shao Jiahang, a physicist at Argonne University, said that UNCD materials have existed for many years, but this research is the first time they have been used in the photocathode environment of an RF photocathode gun. "UNCD was developed at Argonne for other applications, but due to its unique properties, we found that it is also suitable for the needs of advanced photocathodes."

According to Shao, most previous photocathodes can be metallic or semiconductor. He said that each has advantages and disadvantages. Metal photocathodes have a longer lifespan because they can survive in harsh vacuum environments, but semiconductor photocathodes have higher quantum efficiency.

Since UNCD-based photocathodes can be chemically converted to semi-metallic methods, they can obtain benefits that may not be seen in pure metal or semiconductor photocathodes. He is currently a postdoctoral fellow at Argonne and the first author of this study (application Physics) letter, "Demonstration of a nitrogen-doped ultra-nanocrystalline diamond photocathode in a radio frequency gun environment").

Chen said: "Usually, pure diamond acts as an insulator." "But for UNCD, it can be adjusted by different doping techniques to make it behave like a semi-metal. In the latest technology, nitrogen-doped UNCD shows a better Some of the best metal photocathodes have higher quantum efficiency values, excellent vacuum tolerance, and are superior to all semiconductors and even some metal photocathodes, as well as metal and semiconductor photocathodes with moderate intrinsic emissivity."

The research was carried out on the Argonne cathode test bench. Future work includes testing on an increased cathode surface field using an improved cathode assembly design, measuring cathode response time, and characterizing surface terminated cathodes.

Link to this article： Nanodiamonds are very suitable for the needs of advanced photocathodes

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