Photoemission Theory
Kevin Jensen, Donald Feldman, Eric Montgomery, Patrick O'Shea, Joan Yater (NRL), and Jon Shaw (NRL)

Green Laser Impinging Upon Cesiated Tungsten Cathode

Theoretical Predictions of Photoemission (solid lines) Closely Agree with Experimental Data (circular markers)

Photocathodes capable of high brightness and low emittance electron beams are needed for RF photo-injectors in free electron lasers and accelerator applications: rugged and self-rejuvenating photocathodes with high quantum efficiency (QE) using the longest wavelength drive laser would have substantial impact. For a megawatt-class FEL, the ideal electron bunch is a flat, or top-hat distribution of approximately 10 ps duration containing approximately 1 nC of charge (100 A peak current): the rise and reponse times of the photcathode therefore are also critical. Our theoretical and experimental efforts are directed towards the development of a custom-engineered photocathode based on a controlled porosity dispenser cathode architecture with the requisite properties. The theoretical models developed for the photocathode effort are in support of experimental photocathode programs at the University of Maryland and the Naval Research Laboratory to develop high QE, fast response photocathodes. The joint theoretical/experimental program is focused on the analysis of photoemission characteristics of several dispenser-based photocathode configurations, cesiated surfaces and their behavior, and the development of models of QE and emittance and their validation by comparison to experiment. The present work has two purposes. First, it shall describe the status of models and their use to analyze experimental data related to environmental degradation and replenishment mechanisms, emission non-uniformity over the surface of the cathode, surface and geomemtrical effects, properties affecting quantum efficiency (such as bulk transport and scattering), and the properties and performance of cesiated surfaces. Second, it shall describe the development of modules by which these models can be transitioned into beam simulation codes such as MICHELLE and VORPAL.

This research is supported by the Office of Naval Research and the Department of Defense Joint Technology Office.

Cathode Fabrication UHV Chamber with Robotically Controlled Laser Modules

Comparison of Experimental Data (markers) to Theoretical Prediction (solid lines)

Photocathodes are a critical component in photo-injectors for free electron lasers and other accelerator applications that require a high current, low emittance electron beam. An ideal photocathode would have high efficiency in the visible range, long operational lifetime in a typical RF vacuum environment, and prompt electron emission. Efficiency is improved by adding a photosensitive, cesium-based compound to the cathode surface. Because this layer is chemically active, however, it is vulnerable to evaporation and contamination, causing the efficiency to degrade with time. Reapplication of this surface layer is tedious in an RF gun and motivates the need for an in situ rejuvenation technique that would extend the effective cathode lifetime. A novel approach is the controlled-porosity, multi-alkali dispenser photocathode: cesium is stored in a reservoir beneath the cathode and diffuses to the surface during a rejuvenation process to replace that which is lost. Cathode porosity and temperature are adjusted such that approximately one mono-layer of cesium is maintained at the surface. In preparation for the dispenser approach, the behavior of cesiated metal photocathodes was modeled and experimentally verified using an in-house test and fabrication facility. Quantum efficiency as a function of temperature, pressure, cesium coverage, laser intensity, and laser wavelength was measured and found to agree strongly with recent theory. As predictions for photoemission are refined and verified, theory will guide the selection of cathode materials, instead of simple trial-and-error. Recent measurements suggest that cesiated silver cathodes show promise in terms of efficiency and lifetime and also support the continued theoretical development. We demonstrate experimental agreement with photoemission theory for various cesiated metal photocathodes and present the current design for a multi-alkali dispenser photocathode.

This research is supported by the Office of Naval Research and the Department of Defense Joint Technology Office.

Diagram of NRL's Proposed Scanning Photoemission Microscope

Photoemission of GaN with metal Lines as Seen Using the Photoemission Microscope

Photocathodes capable of high brightness and low emittance electron beams are needed for RF photo-injectors in free electron lasers and accelerator applications. The cathodes should be easy to maintain in the gun environment and provide high quantum efficiency (QE). To provide high QE with modest photon energy (e.g., 2.3 eV / λ = 532 nm), a large surface dipole must be created normally by coating the surface with a layer of Cs. Any such dipole layer is inherently reactive, and so becomes contaminated and must be reapplied periodically. Variations in the substrate and coating across the surface can contribute to non-uniform QE distributions. Patchy QE is undesirable since it can increase beam emittance. To make the Cs easy to apply and replenish, we hope to build a Cs dispensing system by coating the backside of the cathode with Cs and providing a set of holes connecting the front and back surfaces of the cathode, allowing Cs to diffuse from a backside reservoir and across the surface. Ideally the dispensing system would work continuously, by holding the cathode at a constant temperature high enough to allow diffusion but low enough to prevent excessive evaporation. To design such a dispenser, knowledge of the diffusion and evaporation rates of Cs on target surfaces at different temperatures is required. We have built a scanning photoemission microscope to aid in making these measurements. As the dispensing holes must be small, good resolution is required. The microscope consists of a single mode 532 nm laser and focusing microscope mounted on a UHV chamber via computer-controlled scanning stage. Specimens can be introduced into the UHV chamber via load lock, heated, and Cs applied. Synchronizing the emission current with the stage position creates a map of the photoemission QE. Such QE maps are useful as a general cathode characterization tool. Assuming the QE is related to the Cs coverage, the surface diffusion and evaporation rates can be deduced by comparison of the QE maps measured at different times and at different specimen temperatures following application of Cs to a limited area.

This research is supported by the Office of Naval Research and the Department of Defense Joint Technology Office.

Commercial "M-Type" Thermionic Dispenser Cathode

Experimental and Theoretical Photoemission Results of Various Commercially Available Thermionic Dispenser Cathodes

A great deal of research has been devoted to developing thermionic catho des based on the dispenser principle to produce cathodes with low work function and long operating lifetimes. In recent years, there has been interest in determining whether commercial thermionic cathodes would have suitable properties as photocathodes (Travier, Leblond). As a preliminary step to the development of new cesium dispenser based photocathodes, we studied the photoemissive properties of the most promising, recently developed thermionic cathodes, i.e., Scandate and M-type cathodes. While not efficient enough for very high average current devices, they show some promise to be superior to present metal photocathodes. One is now under test in the gun test facility at Argonne National Laboratory.

This research is supported by the Office of Naval Research and the Department of Defense Joint Technology Office.

Gallium nitride, when activated with cesium, is a very efficient negative electron affinity (NEA) photoemitter at UV wavelengths. Because it responds only to UV light, it has not been studied as thoroughly as gallium arsenide cathodes. It has great potential, however, as an accelerator cathode for a number of reasons. NEA GaN has a very high quantum efficiency -- up to .5 in excellent conditions, which is nearly the maximum achievable quantum efficiency from a conventional cathode. Secondly, the cesium bonds strongly to the substrate, which makes this cathode resistant to contamination. Finally, unlike other NEA cathodes, this material is prompt enough to be used in an RF gun.