The minimum achievable particle beam emittance in an electron accelerator depends strongly on the intrinsic emittance of the photocathode electron source. This is measurable as the mean longitudinal and transverse energy spreads in the photoemitted electron beam (MLE and MTE respectively); consequently, MLE and MTE are notable figures of merit for photocathodes used as electron sources in particle accelerators. The overall energy spread is defined by the sum of the MTE and the MLE, and the minimization of MTE is crucial to reduce emittance and thus generate a high-brightness electron beam. Reducing the electron beam emittance in an accelerator that drives a Free-Electron Laser (FEL) delivers a significant reduction in the saturation length for an x-ray FEL, thus reducing the machine's construction footprint and operating costs while increasing the x-ray beam brightness. The ability to measure the transverse energy distribution curve of photoelectrons emitted from a photocathode is a key enabler in photocathode research and development that has prompted the Accelerator Science and Technology Centre (ASTeC) at the STFC Daresbury Laboratory to develop the Transverse Energy Spread Spectrometer to make these crucial measurements. We present details of the design for the upgraded TESS instrument with measured data for copper (100), (110), and (111) single-crystal photocathodes illuminated at UV wavelengths around 266 nm.

Transverse electron cooling of heavy molecular ions has been studied at the Test Storage Ring (TSR). Electron beams from a cold GaAs:(Cs,O) photocathode, with kinetic energies down to 31 eV, have been used for cooling of singly-charged ions of masses up to 41 u. We believe that these are the heaviest singly-charged ions for which successful electron cooling has been reported so far. Transverse ion-beam emittances << 1 mu m were reached after typically several seconds of cooling time. The measured transverse cooling rates agree with a simple binary-collision model, assuming a transverse electron temperature of approximately 1 meV/k(B). The results serve as benchmark for electron cooling at the new Cryogenic Storage Ring, which uses the same photocathode electron source and is targeting singly-charged ions of even higher mass.

Мы изготовили p-GaAs(Cs,O) фотокатод, активированный до состояния эффективного отрицательного электронного сродства (χ∗<0), а затем подвергли его деградации точно контролируемыми дозами кислорода. Этим мы смогли смоделировать одну из возможных траекторий деградации фотокатода во время работы ускорителя и отследить последующие изменения в разбросе электронов по энергиям

Abstract The photoelectron processes in a p-GaN(Cs) photocathode with the effective negative electron affinity were studied experimentally within the 90–295 K temperature range. It was found that the photocathode illumination at the photon energies, which are below the energy gap of the p-GaN layer, increases the band bending at a semiconductor surface due to the photoemission from surface states.

Abstract—We report the development of a photosensor device based on a position-sensitive detector with a gallium arsenide (GaAs) cathode and a 16-element anode. In the case of asymmetric heterostatic circuit (electrodes combined in fours) its working field was limited by 10 mm (for a 18-mm photocathode). Implementing a scheme of analog coding of the coordinates of the centroids of electron avalanches arriving to the anode in the photosensor device made it possible to increase the field size to 14 mm and achieve a spatial resolution of 50 μm. The resulting photosensor device is used as the principal component of a multimode field photopolarimeter in observations with a microsecond time resolution on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We report some of the results obtained in the process of this work.

Полупрозрачные p-GaAs(Cs,O) - фотокатоды (ПФК) c эффективным отрицательным электронным сродством (ОЭС) широко используются в современных фотоприёмниках различного назначения и поэтому, поиск путей повышения их технических характеристик является актуальной научной задачей. В настоящее время принято считать, что основные характеристики p-GaAs(Cs,O) - фотокатодов, такие как вероятность выхода фотоэлектронов в вакуум и угловое распределение эмитированных фотоэлектронов, ограничены шероховатостью эмитирующей поверхности p-GaAs - слоя [1]. В данной работе впервые экспериментально показана возможность формирования атомарно – гладкой эмитирующей поверхности p-GaAs - слоя полупрозрачного фотокатода на подложке из «толстого» стекла без введения дислокационной сетки в полупроводниковую структуру. В экспериментах использовались многослойные гетероэпитаксиальные структуры (ГЭС), выращенные методом МОС - гидридной эпитаксии. В качестве просветляющего покрытия ГЭС был использован SiO - слой. Сочленение ГЭС со стеклянной подложкой выполнено электродиффузионной сваркой. Выбранные материалы и режимы сварки исключали введение дислокаций в ГЭС. Для удаления GaAs-подложки и «стопорного» AlGaAs - слоя использовались селективные травители на основе NH4OH : H2O2 и HCl, соответственно. Среднеквадратичная шероховатость поверхности активного pGaAs - слоя после использования селективных травителей, измеренная атомно – силовым микроскопом (АСМ), была близка к ~ 0.2 нм. Финишное «выглаживание» эмитирующей поверхности p-GaAs - слоя включало два этапа. На первом этапе мы использовали химикомеханическое полирование (ХМП) p-GaAs-слоя в разбавленном щелочном растворе оригинального состава без использования абразивных материалов. После ХМП среднеквадратичная шероховатость поверхности p-GaAs - слоя не превышала ~ 0.1 нм. Дальнейшее «выглаживание» поверхности p-GaAs - слоя ПФК проводилось путём его прогрева в «равновесных» условиях. Для обеспечения этих условий поверхность р-GaAs - слоя ПФК «прикрывалась» р-GaAs - слоем идентичного состава и помещалась в «самодельную» печь, заполненную чистым водородом. Прогрев проводился в равновесных условиях, в которых потоки мышьяка, галлия и цинка из p-GaAs - слоя ПФК и «прикрывающего» p-GaAs - слоя уравновешивали друг друга. Рельеф эмитирующей поверхности р-GaAs-слоя ПФК после второго этапа «выглаживания», измеренный методом АСМ, показан на рисунке. Из рисунка следует, что на поверхности p-GaAs - слоя сформировались регулярные террасы атомной высоты. Появление атомарно – гладких террас на поверхности p-GaAs - слоя указывает на то, что снижение удельной свободной энергии поверхности обусловлено снижением её удельной энтальпии. Фотолюминесцентное изображение p-GaAs - слоя «показало» отсутствие следов дислокационной сетки в фотокатодной структуре. Мы полагаем, что дальнейшее совершенствование предложенной методики финишной обработки поверхности p-GaAs-слоя позволит создать ПФК с физически предельными характеристиками.

Spontaneous changes in photoemission properties of a р -GaN(Cs)–vacuum interface with effective negative electronic affinity induced by rearrangements of its atomic structure have been studied for the first time. The optimum Сs coating that ensures both the maximum photoelectron escape probability and its stability has been found. A thermodynamic model has been proposed to escape the relation of the photoemission properties of the р -GaN(Cs)–vacuum interface to its free energy and entropy.

Photoelectron scattering in a p-GaN(Cs,O) photocathode was studied by photoelectron emission spectroscopy at low temperatures. It has been experimentally established that the transfer of photoelectrons from the bulk of a p-GaN(Cs,O) photocathode to vacuum is accompanied by the emission of a cascade of optical phonons in the bulk and at the GaN-vacuum interface. Based on how the high-energy features of the photoelectron energy distribution shift with the photon energy, we find m(hh)/m(0) = 2.3 +/- 0.3.

We present measurements of the transverse and longitudinal energy spread of photoelectrons emitted from a GaAsP photocathode as a function of its degradation state. The cathode was initially activated to a state of negative electron affinity in our photocathode preparation facility, achieving a quantum efficiency of 3% at a wavelength of 532 nm. It was then transferred under XHV conditions to our transverse energy spread spectrometer, where energy spread measurements were made while the photocathode was progressively degraded through a controlled exposure to oxygen. Data have been collected under photocathode illumination at 532 nm, and the changing photoelectron energy distribution associated with the changes in the level of electron affinity due to quantum efficiency degradation through an exposure to 0.25 L of oxygen has been demonstrated. Our experiments have shown that GaAsP boasts a significantly higher resilience to degradation under exposure to oxygen than a GaAs photocathode, though it does exhibit a higher level of mean transverse energy. Coupled with the favourable published data on GaAsP photoemission response times, we conclude that GaAsP is a viable candidate material as a particle accelerator electron source.

It has been experimentally established that the transfer of photoelectrons from the bulk of a p-GaN (Cs,O) photocathode to vacuum is accompanied by the emission of a cascade of optical phonons. In the quantum efficiency spectrum of the p-GaN (Cs,O) photocathode, an exciton peak has been identified, indicating a significant contribution of the electron–hole interaction to the generation of free electrons in heavily doped p-GaN.

It has been established that the probability of Cs-induced chemisorption of molecular oxygen on a p-GaAs(Cs) surface is mainly determined by the probability of the dissociation of the molecule during its collision with the surface. With an increase in the amount of adsorbed oxygen on the p-GaAs(Cs, O) surface, the probability of its chemisorption decreases and, depending on the value of the cesium coating, can either be still determined by the probability of the dissociation of the molecule or be limited by the probability of the "capture" of oxygen atoms by the local chemisorption centers or depend on these two processes.

Monte Carlo simulation of smoothing and step-terraced morphology formation on the Kossel crystal surface is carried out. The simulation results are compared with the experimental data on GaAs surface smoothing in equilibrium conditions. Despite the simplicity of the Kossel crystal model, the simulation qualitatively describes the experiment. The full length of monatomic steps and the mean size of islands on terraces are explored for quantitative characterization of the surface relief evolution. The comparison of the simulation and experiment yields surface diffusion activation energy E-d = 1.3 +/- 0.05 eV, lateral bond energy E-b = 0.32 +/- 0.02 eV and adatom desorption energy E-des = 2.1 +/- 0.05 eV, which are in reasonable agreement with the values obtained earlier from GaAs growth experiments and ab initio calculations. (C) 2015 Elsevier B.V. All rights reserved.

The (Cs,O)-activation procedure for p-GaAs(Cs,O)-photocathodes was studied with the aim of demarcating the domains of validity for the two practical models of the (Cs,O)-activation layer: The dipole layer (DL) model and the heterojunction (HJ) model. To do this, the photocathode was activated far beyond the normal maximum of quantum efficiency, and several photocathode parameters were measured periodically during this process. In doing so, the data obtained enabled us to determine the domains of validity for the DL- and HJ-models, to define more precisely the characteristic parameters of the photocathode within both of these domains and thus to reveal the peculiarities of the influence of the (Cs,O)-layer on the photoelectron escape probability.

The technique for preparing step-terraced GaAs surfaces by annealing in the conditions close to equilibrium is further developed. The kinetics of GaAs(0 0 1) surface flattening is experimentally measured. The step-terraced surface morphology formation is characterized by the evolution of root mean square roughness, total length of monatomic steps, and by Fourier and correlation analyses. It is shown that the excess of monatomic step length over the value for the ideal vicinal surface is the most adequate characteristic of the surface smoothing because it decreases inversely proportional with annealing duration. (C) 2012 Elsevier B. V. All rights reserved.

Domains of validity for dipole layer and heterojunction models of the (Cs,O) – activation layer for GaAs – photocathode are determined. Two – step photoelectron escape model from NEA-photocathode is proved. Dominated elastic and inelastic scattering processes, which are accompanied the photoelectron escape, are revealed.

ASTeC has spent several years developing a GaAs Photocathode Preparation Facility (PPF) which routinely produces cathodes with quantum efficiencies (Q.E. )u p to 20 % at 635 nm [1]. The goal is to use these and other cathode materials in high–average–current high–brightness injectors for particle accelerators. Electron injector brightness is limited by source beam emittance, and brightness will be increased significantly by reducing the longitudinal and transverse energy spread in the emitted electrons, thereby creating a cold beam. We are constructing an experimental system which is compatible with the PPF for measurement of the energy distribution in electrons emitted from photocathodes at room and LN2-temperature. The photocathode will be illuminated by a small, variable–wavelength light spot. The electron beam image will be projected onto a detector comprised of a 3-grid energy filter, a microchannel plate and a phosphor screen. A low–noise CCD camera will capture screen images, and the spatial and energy distribution of the emitted electrons will be extracted through analysis of these images as a function of the grid potentials. The system will include a leak valve to progressively degrade the cathode, and thus allow its properties to be measured as a function of gas exposure.

Accelerator drivers for Energy Recovery Linac (ERL) and Free-Electron Laser (FEL) based light sources demand electron injectors which deliver high brightness bunches on the several hundreds of picocoulomb scale, at repetition rates between 1 MHz and 1 GHz (or higher), corresponding to an average current between 0.1 and 100 mA. Simultaneous satisfaction of these injector requirements is considerably beyond the current state-of-the-art. Daresbury Laboratory is concentrating efforts on the development of high average current III-V and XnY3−nSb photocathode-based DC and SRF photocathode guns for ERL applications. The ultimate goal of this research is their integration with the ALICE ERL.