• Number 393  |
• July 22, 2013

John Smedley, master of engineering electron-emitting photocathodes

For twenty years, John Smedley, a physicist in Brookhaven Lab’s Instrumentation Division, has been researching and designing novel accelerator photocathodes—the miniature electron emitters at the heart of accelerator experiments that probe the fundamental laws of nature and the properties of materials for applications in energy technologies and other areas.

When photocathodes are struck by photons—quantum packets of light—they produce electrons, a conversion from light to electric current that has enabled a long history of technological advances. Building these devices requires the delicate manipulation of materials, some composed of layers as thin as a single atom.

“Our group has been involved in almost all of the major accelerator projects at Brookhaven, including the Relativistic Heavy Ion Collider (RHIC), the National Synchrotron Light Source (NSLS), and the upcoming NSLS-II. Accelerator science has had a major impact on Brookhaven’s history, and I think it is also a major element of its future. What we’re doing now is using existing technologies to design better photocathodes, which will lead to the next generation of accelerator and light source technologies.”

In accelerators, a photocathode exploits the photoelectric effect to produce an electron beam that is used to create a collision with other particles, produce x-ray pulses, or scatter off targets to determine their structure, among other applications. This cathode must be built to withstand the accelerator environment’s harsh electric fields, high currents, and chemical contaminants without melting or malfunctioning, and must also be able to generate a consistent high-quality electron beam over a long lifetime. 

Smedley’s photocathode designs have met these challenges, and benefit from his position in the Instrumentation Division because of his ability to work closely with other groups at Brookhaven.  Using the tools of modern materials science available at NSLS and the Center for Functional Nanomaterials (CFN), Smedley can look at a cathode’s development using x-rays in real time, rather than simply judging their effectiveness solely by observing the end result, and use what he learns to engineer and improve photocathode performance. This technique has been used in semiconductor design and growth for years, but Smedley's team has pioneered its application in photocathode design. 

“John is an extremely talented physicist, making vital contributions to the laboratory’s core capabilities. His enthusiasm for science enlivens junior and senior colleagues alike,” said Instrumentation Division Head Graham Smith.

Since so many experiments—including the Source Development Lab (SDL) linear accelerator in the Lab’s Photon Sciences directorate and the Laser-Electron Accelerator Facility (LEAF) in Chemistry—rely on the same fundamental electron source technology, Smedley’s work has robust multidisciplinary applications. 

“When I came to the NSLS in 2004, I knew nothing about x-ray science,” Smedley said. “My training was in accelerator physics. Over the past eight years, I’ve become a materials scientist, a testament to the dedication and skill of my colleagues. I’ve used 20 beamlines at the NSLS, and several facilities at the CFN. Brookhaven is the only DOE national lab that has an instrumentation division, so this sort of work isn’t possible anywhere but here, where we have such a broad array of capabilities.”

Most recently, Smedley has been involved in the development of a cutting-edge diamond amplified photocathode that may revolutionize fourth-generation light source technology. By using synthetic diamonds, which provide a level of uniformity and purity that make them ideal components for electron amplifiers, Smedley's team was able to create amplifiers capable of increasing an electron beam’s current more than 300 times. This kind of amplification would also be useful for linear accelerators, or linacs, including the energy recovery linac that is being developed for RHIC.

In addition, should a proposal to add an electron accelerator ring to RHIC move forward—making it the world’s only electron-ion collider, to be known as eRHIC—Smedley's group in Instrumentation will be challenged to design the most sophisticated photocathode yet: a device capable of producing a 50-milliamp polarized electron source. To put that in perspective, current polarized electron sources max out at about one milliamp.

Smedley first came to Brookhaven as part of the Science Undergraduate Laboratory Internship program (SULI) when he was a student at Johns Hopkins University. He earned his BA in physics and mathematics at Johns Hopkins in 1994, and a Ph.D. in accelerator physics from Stony Brook University in 2001. He officially joined the Lab as a Research Associate in 2001. He was recently recognized for his contributions to the advancement of photocathode technology with the Charles Hirsch Award from the Institute of Electrical and Electronics Engineers (IEEE). In his spare time, he is an avid hiker and scuba diver, and enjoys playing ultimate Frisbee. — Angela Leroux-Lindsey

Submitted by DOE's Brookhaven National Laboratory