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Toward Photocathode Engineering

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Siddharth Karkare, Eric Sawyer, and Teresa Esposito (inset) work at the Photocathode Development and Diagnostics lab.

Future accelerators, such as the Energy Recovery Linac (ERL), will be powered by bright electron beams obtained from photoinjectors. The ultimate brightness of these beams is limited by the quality of the electrons emitted from a photocathode when a laser beam strikes the photoemissive surface. In today’s advanced photoinjectors, like the Cornell ERL injector, the photocathode must meet stringent demands on high quantum efficiency (defined as a number of emitted electrons per incident photon), low transverse energy spread (corresponding to highly parallel trajectories out of the photocathode), and a prompt response time (faster than a trillionth of a second), as well as a photocathode that is robust to adverse conditions when generating very intense beams.

GaAs and related materials activated to negative electron affinity (the condition when an electron excited by a photon prefers to leave the material instead of staying bound to it) are among the most promising photocathodes. Although widely used in several photoinjectors worldwide, many photoemission characteristics from such photocathodes have remained poorly understood until now. The ERL photocathode team at Cornell University has developed a detailed Monte-Carlo computer code that successfully predicts photoemission from such GaAs surfaces for the first time and does so without the use of adjustable parameters. The research group is now working on extending these simulations to allow calculating not only III-V semiconductors (GaAs and similar materials) but also the layered structures that can be grown using modern methods of semiconductor manufacturing like molecular beam epitaxy. Such photocathode “engineering” is believed to open the door towards more than a hundred-fold increase in the achievable beam brightness, enabling new science ranging from coherent x-ray production to ultrafast electron diffraction of large proteins.

These results have been recently published in the Journal of Applied Physics, vol 113 (2013) 104904. The work was performed by a 3rd-year Cornell physics PhD student Siddharth Karkare with key contributions from undergrads Eric Sawyer (a Cornell physics senior) and Teresa Esposito (a senior from Florida Institute of Technology, a Research for Undergraduate Experience student at CLASSE in the summer of 2012) and other team members and collaborators. The photocathode research at Cornell is led by Professor Ivan Bazarov.