Search

We report on the growth and characterization of a new class of photocathode structures for use as electron sources to produce high brightness electron beams for accelerator applications. The sources are realized using III-nitride materials and are designed to leverage the strong polarization field, which is characteristic of this class of materials when grown in their wurtzite crystal structure, to produce a negative electron affinity condition without the use of Cs, possibly allowing these materials to be operated in radio frequency guns. A Quantum Efficiency (QE) of about 1 × 10 − 3 and an emitted electrons' Mean Transverse Energy (MTE) of about 100 meV are measured at a wavelength of 265 nm. In a vacuum level of 3 × 10 − 10 Torr, the QE does not decrease after more than 24 h of continuous operation. The lowest MTE of about 50 meV is measured at 300 nm along with a QE of 1.5 × 10 − 5 . Surface characterizations reveal a possible contribution to the MTE from surface morphology, calling for more detailed studies. [ABSTRACT FROM AUTHOR]

We report the design and performance of a time-resolved electron diffraction apparatus capable of producing intense bunches with simultaneously single digit micrometer probe size, long coherence length, and 200 fs rms time resolution. We measure the 5d (peak) beam brightness at the sample location in micro-diffraction mode to be 7 × 10 13 A / m 2 rad 2 . To generate high brightness electron bunches, the system employs high efficiency, low emittance semiconductor photocathodes driven with a wavelength near the photoemission threshold at a repetition rate up to 250 kHz. We characterize spatial, temporal, and reciprocal space resolution of the apparatus. We perform proof-of-principle measurements of ultrafast heating in single crystal Au samples and compare experimental results with simulations that account for the effects of multiple scattering. [ABSTRACT FROM AUTHOR]

Recent research on photocathodes for photoinjectors has focused on the understanding of the photoemission process at low energy (i.e. at photon energy close to the material's work function) as well as on the study of ordered and innovative photocathode materials, with the aim of minimizing the emittance at the cathode. We here present a preliminary study on low energy photoemission from (100) oriented Ba1−xLaxSnO3 thin films, characterizing their quantum efficiency and the mean transverse energy of the photoelectrons. The aim of the study is to pave the way for future experiments on innovative photocathodes based on perovkite oxides. [ABSTRACT FROM AUTHOR]

The requirement of a short response time of a cathode with respect to the laser beam for the generation of the photoelectrons limits, in the case of SPARC photoinjectors, the choice of the cathode materials to metals that usually present response time on the order of a few tens of femtoseconds. While copper and magnesium have been extensively studied, new research activities were devoted to study the photoemission properties of yttrium and to explore the feasibility of a photocathode based on this metal. Nevertheless, to date little data has been reported in literature. We present, in this work, the results of our investigation of Y films as suitable materials for the production of bright electron beams. Bulk and thin films of Y, prepared by pulsed laser deposition on Si substrates, have been tested as photocathodes in an ultra-high vacuum photodiode chamber at 10-7 Pa. Systematic laser cleaning treatments improved the film quantum efficiency (QE) from 10-6 to 4.5×10-4 at 266 nm, comparable to those of the corresponding bulk. Preliminary QE measurements carried out in the visible range at 406 nm will be illustrated. [ABSTRACT FROM AUTHOR]

Abstract: Mg-based films have been prepared by pulsed laser deposition technique for photocathode applications. We have investigated the influence of pulse laser duration on morphology and photoemissive properties. Two laser sources have been used, generating pulses of 30ns at 308nm (XeCl excimer laser), 5ps and 500fs at 248nm (KrF excimer laser) to grow Mg films onto Si and Cu substrates in high vacuum (∼10−7 Pa) and at room temperature. Morphological investigations carried out by scanning electron microscopy (SEM) have revealed that, in our experimental conditions, the number and the mean size of the droplets on the films surfaces decreases as the pulse laser duration shortens. The contamination level of Mg film surfaces have been studied by energy dispersive X-ray spectroscopy (EDX). The photoelectron performances in terms of quantum efficiency (QE) and emission stability have been tested in a UHV DC photodiode cell (10−7 Pa). Measures of the QE of the samples surfaces have revealed a decrease on the initial value for Mg-based photocathodes prepared by fs laser (from 7.8×10−4 to 6.6×10−4) PLD with respect to ps (from 6.2×10−4 to 7.4×10−4) and ns lasers (from 5.0×10−4 to 1.6×10−3). A comparison among Mg-based photocathodes prepared by ns, ps and fs PLD for the production of high brightness electron beams has been presented and discussed. [Copyright &y& Elsevier]

Sodium potassium antimonide photocathodes with Quantum Efficiency (QE) in the range of few percent have been grown, and their photoemission properties are measured. We report the intrinsic emittance and response time of electron bunches extracted from this material. It is possible to recover the QE of an overheated cathode by simple potassium addition, and the cathode is rugged enough to deliver tens of mA of average current with no or minimal degradation. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we review the status of bright electron sources. High peak current (up to kA) low emittance (< 1 mm rad) electron beams have been one of the critical components in the development of XFELs. The outlook for the field is even more promising as progress in the understanding of photoemission physics, development of novel photocathode materials, gun and laser technology, enable further progress in advanced light sources as well as novel applications such as ultrafast electron diffraction and microscopy. We outline here the more recent research trends which pave the way for potentially orders of magnitude brightness improvements with respect to the current state-of-the-art. [ABSTRACT FROM AUTHOR]