High Brightness Photocathodes in Photoinjectors

High Brightness Photocathodes in Photoinjectors High Brightness Photocathodes in Photoinjectors The performance of existing and future ultrafast science instruments like X-ray Free Electron Lasers (XFELs) and Ultrafast Electron Diffraction (UED) and microscopy (UEM) experiments is limited by the brightness of electron beams produced from photocathodes in photoinjectors. Generation of brighter electron beams from photoinjectors requires the development of photocathodes capable of emitting electrons with the smallest possible mean transverse energy (MTE), or equivalently, smallest possible intrinsic emittance. With the goal of improving the performance of the above applications, over the last decade, a significant effort has gone into understanding the process of photoemission and reducing the MTE of electrons emitted from cathodes. As a result, several ways of achieving low MTEs of 20-30 meV have been developed and MTE as low as 5 meV has been demonstrated. Despite these large improvements in MTE, photoinjectors still use cathodes with a large (~500 meV) MTE. This proposal aims to identify and resolve the issues involved in using these low-MTE cathodes in photoinjectors and develop cathodes for low-MTE (sub-25 meV) operation in new photoinjectors, resulting in nearly 25 times brighter electron beams. The ultimate goal of the proposal is to demonstrate low-MTE (and thus high-brightness) operation in the existing and new photoinjectors being developed to power future XFEL and UED/UEM facilities. The proposed increase in brightness will result in a dramatically increased X-ray pulse energy in existing XFELs maximizing their potential to study atomic and electronic structural dynamics in quantum materials, electronic and nuclear coupling in biological and chemical processes and energy materials in situ. It is also critical for the development of compact XFELs to make them widely accessible. For UED/UEM applications, the proposed increase in beam brightness will imply increase in the spatio-temporal resolution greatly increasing the scientific reach of these ultrafast electron scattering techniques. Thus, this work will transform the capabilities of various existing and upcoming DOE facilities opening new frontiers in ultrafast studies of quantum materials, energy technologies and critical biological processes.