Predictive High-Fidelity Modeling Capability for High-Brightness Photoinjectors

"High brightness and high average power Free Electron Lasers known as x-ray FELs, developed by DOE, are intently desired by the scientific community for their unique capabilities and for the opportunities they enable in the medical and biological sciences, such as imaging biological molecules or chemical reactions. Beam optics codes (particularly Particle-in-Cell codes) couple non-uniformity and surface structure to current and emittance in a complex manner and are the de facto tool of choice in device design, but such codes neglect time-dependent contributions to the emission current due to the difficulty of accounting for multiple scattering events on the emission distribution. Such a limitation
undermines device design, degrades predictions of performance via simulation, and impairs the ability to account for
mechanisms (performance degraders) that can lead to real damage to the larger device (like particle ejection from the
beam core, or “halo”).
General statement of how this problem is being addressed:
An improved simulation capability is proposed that bridges the gap between current capability and that needed to capture the effects needed in simulation to have a predictive capability. New physics models proposed captures the time- dependent (delayed) emission of the photoemission process, as well as the effect of sub-micron geometrical features that have a great effect on intrinsic emittance, which up to now has been predicted within a factor of two from the simulation codes. The new emission models will be joined with a comprehensive of emission physics models already in the MICHELLE code, which includes thermal-field, thermionic, and semiconductor secondary emission models, including the effect of surface roughness, crystal face orientation (leading to variations in work function, etc.), laser irradiance variations, and laser jitter. The effort will package all these models together for application in the MICHELLE code and for use in other beam optics modeling codes.
What is to be done in Phase I?
Phase I will develop the first generation of the photoemission time-dependent (delayed) emission models. The software architecture will be developed to prototype the combining of all emission models into a central framework and be a stand-alone, separable software entity. That framework will add in the new time-dependent model. The framework will then be implemented into the MICHELLE beam-optics code, and the result tested against other codes. Functioning software will be made available to beta testers and researchers in the DOE community to model high-brightness sources.
Commercial Applications and Other Benefits:
The application of the DOE future synchrotron light sources and free electron lasers has a very significant benefit to mankind. In the sciences, imaging biological molecules or chemical reactions or even viruses requires a photon flux intensity sufficiently high that the molecules or structures can be “seen” before the sample is destroyed by the energy flux. The structures of man-made nanoscale objects, other molecules, and even that of atoms may be resolved by free electron lasers capable of imaging sub-nanometer regions with sub-picosecond time resolutions. Conventional synchrotron x-ray sources fall short of the requirements because of time resolution limits. Specific examples include
resolving ultrafast biochemistry, time-resolved spectroscopy of correlated electron materials, magnetization dynamics,
and imaging chemical reactions. High brightness, high rep-rate photocathode sources are likewise one of the two technologies requiring improvement (the other being mirrors) to enable high performance FELs desired by directed energy applications, such as the Navy's Innovative Naval Prototype Free Electron Laser. This effort specifically targets the development of the next generation of such machine by developing a predictive modeling and simulation capability for the electron beam sources. If the project is carried over to Phase II or Phase III, this new capability will be widely available by the software framework to be developed that will fit into many beam optics codes to model such devices.

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