Emission, Emittance, Trajectories, and the Modeling of Electron Emission John J. Petillo1, Donald A. Shiffler2, , Zhigang Pan3, John W. Luginsland4, Kevin L. Jensen 1

Leidos, Billerica, MA Air Force Research Laboratory, Kirtland AFB, NM 3 U. Maryland, College Park, MD 4 Air Force Office Sci. Res., Arlington, VA

2

For short wavelength and high average power Free-Electron Lasers (FEL), good gain arises from the overlap between the electron bunches and the light wave field in the optical resonator being well matched, which requires the beam emittance to be small. The importance of modeling the cathode emission physics and electron dynamics to understand the interplay between emittance and space charge is therefore important in simulation codes. The highest brightness beams are produced by photoinjectors, for which the total emittance is a combination of the thermal emittance, the rf-defocusing induced emittance, and the space charge force induce emittance. The electron source used by the rf gun sets the limit on the minimum possible emittance and in particular, the intrinsic emittance of the photocathodes are emerging as the primary limitation to realizing the short wavelength FEL's and Energy Recovering Linac (ERL) sources of x-rays and makes describing the causes of cathode emittance important particularly with regards to their modeling and simulation. Surface roughness on a photocathode can contribute to additional unwanted effects apart from emittance increases: given that low emission barriers of the photocathode often employ cesium to lower the work function, the field enhancement of protrusions couple with the reduced barrier and the large surface fields to cause dark current which, because it is accelerated at wrong parts of the rf cycle, contributes to halo formation, is redirected back to the photocathode and damages surface coatings, and is accelerated into the side walls where it creates secondaries. Field emission is not only associated with the cathode, but can also arise elsewhere in high gradient accelerator structures and rf cavities where field emission sites are generated by various mechanisms. Although the impact of field enhancement and surface variation can be explicitly considered in beam optics codes, such studies are often done for single tips or a very small number in close proximity because of limitations imposed by the cell size of the simulations. The minimum cell size on the cathode is crudely governed by a product of factors related to the ability to resolve the electron beam and small structures within it: typically, a scale factor of $10^4$ exists between the largest (overall device) and smallest (near the cathode) grid elements so that devices measuring 10's of cm use grids on the cathode of 10's of microns. Modeling emission and emittance at the cathode in beam optics codes entails several needs. First, a model of how emittance scales with feature geometry generally absent except for stylized geometries. Second, emission structures can have dimensions and apex shapes which require a generalization of the Point Charge Model (PCM) for conical emitters. Third, space charge close to the emitters, be they conical or fiber-like, affects trajectories in a complex manner associated with space charge. Fourth, high QE semiconductors entail two electron populations that

contribute to emission: those electrons that are directly photoemitted, and those that arrive to the surface after scattering, perhaps repeatedly. Lastly, coupling transport within bulk material to emission into vacuum is not easy and often pays no attention to quantum effects that surely matter, especially in a trajectory interpretation. In this presentation, we shall present physics-based modeling proposals that may enable addressing each of these effects in Beam Optics and emission codes used to simulate photoinjectors.