Emittance, surface structure, and electron emission for thermal, photo, and field emission processes

Summary form only given. The emittance of high brightness electron sources such as field emitters and photocathodes is increased by geometric features on the emitter surface that give rise to local field enhancement. Knowledge of the emittance is important for accelerators and electron beam devices, and particularly, the intrinsic emittance (or that portion due to the cathode), is of increasing importance because it cannot be eliminated by subsequent beam manipulations. Applications for which the reduction of intrinsic emittance is paramount include, but are not limited to, high frequency vacuum electronics, High Power Microwave (HPM) devices, Free-Electron Lasers (FEL), and electron beam lithography. Particularly for field and photoemission, emittance is strongly coupled to space charge effects. The present work analyzes a simple geometrical model to identify how roughness and field enhancement increases emittance, and to vet methods designed for the future incorporation of space charge modifications. In particular, an analytic model of surface structure to be described allows for the identification of various mechanisms that increase emittance based on shape, quantifies the impact of field of thermal, field, and photoemission processes, and enables estimations of the impact of space charge. The methodology is by design intended to be amenable to inclusion in beam optics codes. The analytic model is based on a hemispherical protrusion, but the techniques are applicable to other approaches, particularly long wires that characterize field emission from carbon fibers. An indication of how that is accomplished is discussed. The methodology predicts emittance as a function of emission angle, launch velocity, and an impulse correction factor. The method can be extended to treat non hemispherical sources and the inclusion of space charge forces, and an indication of how that may be possible will be discussed.