Cornell Researchers Reveal Future of Photocathodes With HERACLES Beamline   

The Newman Lab/Cornell University

A visual representation of the HERACLES beamline, a machine that emulates harsh environments of the largest particle colliders.

Researchers at the Newman Lab are currently experimenting on different photocathode materials and their degradation to improve their durability in harsh environments. In doing so, they will better understand phenomena, such as ion back bombardments, that only run at high currents.

Photocathodes, surfaces that emit electrons when hit by light, are used for many scientific instruments today — such as X-ray machines, free electron lasers, semiconductor manufacturing and electron microscopy. Shining specific types of lasers on these photocathodes will emit electrons based on the properties of the laser and photocathode. Photocathodes, however, become damaged when exposed to these laser beams for long periods of time.

The High ElectRon Average Current for Lifetime ExperimentS (HERACLES) beamline is a test accelerator that can create an environment similar to that of the photo injectors used in some of the world’s largest particle colliders. HERACLES is a testing facility, used primarily in the development of foundational knowledge of photocathodes behavior in particle accelerators.

“Generally, this environment is incredibly harsh on the photocathode, leading to performance degradation,” said Sam Levenson grad, who works in the Newman Lab. “By replicating those conditions in a controlled fashion, we can perform research aimed at improving the photocathodes robustness.”

Photocathodes can be split into two families: metal photocathodes and semiconductor photocathodes. Metal photocathodes are a family of photocathodes made up of metals, such as coppers and magnesium. Semiconductor photocathodes are typically made up of gallium arsenide, gallium nitride and cesium antimonide.

The Newman Lab used quantum efficiency — a metric used to evaluate the ratio of the number of emitted electrons to the number of photons — to measure the sensitivity of the photocathode to light.

Their study found that metal cathodes last for long periods of time but do not exhibit high quantum efficiency, meaning they are not very effective at converting photons to electrons. Semiconductor cathodes, however, have very high quantum numbers but do not last very long. As the photocathode dies, quantum efficiency decreases, so the cathode is no longer sensitive to light or able to effectively convert the photons into electrons, leaving the photocathode ineffective.

HERACLES emulates these harsh environments of particle accelerators by running at high currents with powerful lasers. This, however, can have negative effects on the photocathodes.

“When the emitted [HERACLES] beam collides with residual gas molecules, it will make them positively charged. Since the ions have the opposite charge, they are accelerated towards the cathode,” Levenson said.

This interaction, called ion back bombardment, causes damage to the photocathode.

The Newman Lab is currently testing different locations of a growth chamber in relation to HERACLES, as well as different photocathode coatings, to promote advanced photocathode growth. High-efficiency photocathodes must be kept in vacuum to mitigate the effects of chemical poisoning from gas molecules, which can quickly degrade a photocathode.

The photocathodes are grown in a vacuum chamber located on a different floor of the lab in order to mitigate the effects of chemical poisoning, and they must be transported with a vacuum suitcase that connects to the back of HERACLES. This process takes time, leading to the degradation of the photocathodes. The construction of an attached growth chamber will allow for the photocathodes to be tested immediately after growth.

The researchers are also testing different semiconductor photocathode coatings to determine their sensitivity. For example, gallium arsenide requires a layer of cesium on the surface, which is an extremely sensitive chemical element that oxidizes quickly and easily. This leaves it extremely vulnerable to the ion back bombardment that degrades these photocathodes.

This area of research provides insight into the potency of photocathode sources on X-rays, electron microscopy and more devices that rely on photocathodes. Having photocathodes that can withstand more time in harsh environments will create more effective instruments not only in particle physics facilities, but also in hospitals and laboratories and other institutions that rely on this technology.