532nd Brookhaven Lecture

The discovery of the law governing the photoelectric effect warranted Albert Einstein the award of the Nobel Prize in 1921. One hundred years later, electrons produced by photoelectric effect are playing a pivotal role in accelerator physics enabling the realization of advanced electron accelerators experiments.

The unique properties of electron beams generated by photoelectric effect have been leveraged in the last decades to produce the high intensity and high brightness electron beams enabling the realization of the first Free Electron Laser operating in X-ray region. Since then, the landscape of accelerator-based applications has been constantly expanding. With the development of superconducting accelerator technology, and electron sources which allow operation at very high repetition rates, up to few GHz, the central role of photocathodes in accelerator physics is becoming more and more evident. Advanced photocathode materials and structures are required for producing the electrons beams with the property of brightness, charge, average current, and electron spin polarization necessary for present and future experiments.

Brookhaven National Laboratory hosts several photoelectron-based advanced accelerators facilities and has a vested interest in leading the development of photocathode materials. The upcoming Electron Ion Collider will be largely relying on advanced photoelectron sources. BNL is playing a primary role in the R&D of photocathodes required to operate EIC's and other advanced electron sources around the world.

The lecture will introduce the audience to basic properties of photocathodes illustrating their relevance for different applications in accelerator physics, it will review the speaker contributions to the field that have enabled among other achievements the world record in the average current delivered by a photocathode in an electron gun, the first measurement of a sub-thermal electron intrinsic emittance from a cryogenically cooled photocathode and the demonstration of a record high efficiency from a superlattice photocathode for highly spin polarized electron beams. Some of these results have already enabled the realization of bunched beam electron cooling experiments at RHIC, others promise to allow extending the science reach of other accelerator facilities expanding their current capabilities.