Nathan A. Moody

A photoemission model for low work function coated metal surfaces and its experimental validation

Photocathodes are a critical component many linear accelerator based light sources. The development of a custom-engineered photocathode based on low work function coatings requires an experimentally validated photoemission model that accounts the complexity of the emission process. We have developed a time-dependent model accounting for the effects of laser heating and thermal propagation on photoemission. It accounts for surface conditions (coating, field enhancement, and reflectivity), laser parameters (duration, intensity, and wavelength), and material characteristics (reflectivity, laser penetration depth, and scattering rates) to predict current distribution and quantum efficiency (QE) as a function of wavelength. The model is validated by (i) experimental measurements of the QE of cesiated surfaces, (ii) the QE and performance of commercial dispenser cathodes (B, M, and scandate), and (iii) comparison to QE values reported in the literature for bare metals and B-type dispenser cathodes, all for vari...

Photoemission from metals and cesiated surfaces

A model of photoemission from coated surfaces is significantly modified by first providing a better account of the electron scattering relaxation time that is used throughout the theory, and second by implementing a distribution function based approach (“Moments”) to the emission probability. The latter allows for the evaluation of the emittance and brightness of the electron beam at the photocathode surface. Differences with the Fowler-Dubridge model are discussed. The impact of the scattering model and the Moments approach on the estimation of quantum efficiency from metal surfaces, either bare or partially covered with cesium, are compared to experiment. The estimation of emittance and brightness is made for typical conditions, and the derivation of their asymptotic limits is given. The adaptation of the models for beam simulation codes is briefly discussed.

Theory of photoemission from cesium antimonide using an alpha-semiconductor model

A model of photoemission from cesium antimonide (Cs3Sb) that does not rely on adjustable parameters is proposed and compared to the experimental data of Spicer [Phys. Rev. 112, 114 (1958)] and Taft and Philipp [Phys. Rev. 115, 1583 (1959)]. It relies on the following components for the evaluation of all relevant parameters: (i) a multidimensional evaluation of the escape probability from a step-function surface barrier, (ii) scattering rates determined using a recently developed alpha-semiconductor model, and (iii) evaluation of the complex refractive index using a harmonic oscillator model for the evaluation of reflectivity and extinction coefficient.

Prototype dispenser photocathode: Demonstration and comparison to theory

A method to significantly extend the operational lifetime of alkali-based photocathodes by diffusing cesium to the surface at moderate temperature is presented and shown to restore the quantum efficiency (QE) of cesiated tungsten. Experimental measurements of QE as a function of surface cesium coverage compare exceptionally well with a recent theoretical photoemission model, notably without the use of adjustable parameters. A prototype cesium dispenser cell is demonstrated and validates the concept upon which long-life dispenser photocathodes can be based.

Theoretical model of the intrinsic emittance of a photocathode

A theoretical expression for the intrinsic emittance of a photocathode is developed based on a method of evaluating moments of the emission distribution function. The method is first used to reevaluate the well-known rms emittance of a thermionic source, and then, using analogous approximations but with an updated theoretical model of photoemission, an equation for the intrinsic emittance and brightness of a photocathode of comparable simplicity to the thermionic case is obtained in the limit of weak field and low temperature.

Factors affecting performance of dispenser photocathodes

Usable lifetime has long been a limitation of high efficiency photocathodes in high average current accelerator applications such as free electron lasers, where poor vacuum conditions and high incident laser power contribute to early degradation in electron beam emission. Recent progress has been made in adapting well known thermionic dispenser techniques to photocathodes, resulting in a dispenser photocathode whose photosensitive surface coating of cesium can be periodically replenished to extend effective lifetime. This article details the design and fabrication process of a prototype cesium dispenser photocathode and describes in detail the dominant factors affecting its performance: activation procedure, surface cleanliness, temperature, and substrate microstructure.

Multiple scattering effects on quantum efficiency and response time for cesiated metal photocathodes

An oft used approximation to predict quantum efficiency (QE) from bare metals or those with a low work function coating such as cesium is to assume that photo-excited electrons have not scattered prior to their emission. Monte Carlo simulations are used to assess that approximation, and show that, while good for bare metals, for cesiated metals a photoexcited electron may undergo several scattering events and yet be emitted. Neglecting scattered electrons therefore underestimates QE. Emitted electrons that have undergone scattering before emission elongate the response time by giving rise to a long time tail, low energy contribution to the faster non-scattered emission, for which a model is developed. The theory is applied to study variations in QE as a function of wavelength measured from cesiated metal surfaces. The extension of the findings to semiconductor photocathodes is briefly discussed.

Application of a general electron emission equation to surface nonuniformity and current density variation

Using a recently developed model of emission that includes field, thermal, and photoemission effects simultaneously for arbitrary magnitudes of field, temperature, and laser intensity, we perform a study of the consequences of emission site variation on the subsequent electron beam. The electron emission model incorporated into the particle-in-cell (PIC) code MICHELLE, which is a conformal mesh finite-element (FE) two-dimensional (2-D) and 3-D electrostatic PIC code for modeling steady-state electron guns (and collectors), is described in detail.The addition of the generalized emission model therefore allows for assessing the impact of local thermal, field, and work function variation on the resultant electron beam.

Active bialkali photocathodes on free-standing graphene substrates

The hexagonal structure of graphene gives rise to the property of gas impermeability, motivating its investigation for a new application: protection of semiconductor photocathodes in electron accelerators. These materials are extremely susceptible to degradation in efficiency through multiple mechanisms related to contamination from the local imperfect vacuum environment of the host photoinjector. Few-layer graphene has been predicted to permit a modified photoemission response of protected photocathode surfaces, and recent experiments of single-layer graphene on copper have begun to confirm these predictions for single crystal metallic photocathodes. Unlike metallic photoemitters, the integration of an ultra-thin graphene barrier film with conventional semiconductor photocathode growth processes is not straightforward. A first step toward addressing this challenge is the growth and characterization of technologically relevant, high quantum efficiency bialkali photocathodes on ultra-thin free-standing graphene substrates. Photocathode growth on free-standing graphene provides the opportunity to integrate these two materials and study their interaction. Specifically, spectral response features and photoemission stability of cathodes grown on graphene substrates are compared to those deposited on established substrates. In addition, we observed an increase of work function for the graphene encapsulated bialkali photocathode surfaces, which is predicted by our calculations. The results provide a unique demonstration of bialkali photocathodes on free-standing substrates, and indicate promise towards our goal of fabricating high-performance graphene encapsulated photocathodes with enhanced lifetime for accelerator applications.Graphene in accelerator technology: A new material for enhanced photocathode performance and lifetimeGraphene has shown potential to unlock new capabilities of electron sources and other aspects of accelerator technology. This report focuses on integrating graphene with high performance photocathodes with the goal of extending lifetime by thousands of hours.Scientists at Los Alamos National Laboratory, USA, and colleagues succeeded in growth of chemically susceptible photocathodes on free-standing graphene substrates while maintaining state-of-the-art performance. Successful growth on graphene is a critical step toward a material-centric approach to photocathode design: enhancing lifetime without compromising efficiency or other performance metrics. Graphene, an atomically thin sheet of carbon, is an emerging material that has inspired new cathode design capabilities, including heterostructuring, resonant tunneling, and impermeable gas barriers. Conventional photocathode materials have no performance regimes. The next step is complete graphene encapsulation of photocathode films and demonstration of lifetime enhancement in the operating environment of accelerator facilities.

Single layer graphene protective gas barrier for copper photocathodes

Photocathodes can benefit from a thin protection layer and attain long-term stability. Graphene is potentially a good candidate for such application. We report direct growth of single-layer graphene on single crystal Cu(110) photocathodes using chemical vapor deposition and the effective protection of copper photocathodes with graphene against degradation under atmospheric conditions. Due to the interaction and charge transfer between graphene and copper, the graphene-protected cathodes have 0.25 eV lower work function and 17% higher quantum efficiency at 250 nm compared with bare Cu cathodes. The graphene coating can protect copper photocathodes from degradation for more than 20 min in an exposure to 200 Torr of air. The validation of graphene-photocathode compatibility opens a new route to the lifetime-extension for photocathodes.