The P3 organizing committee is pleased to announce that Los Alamos National Lab will be hosting the next Photocathode Physics for Photoinjectors (P3) Workshop, October 15th-17th 2018. This year's workshop will be the fifth workshop of the extremely successful series that began in 2010 at Brookhaven National Laboratory.
Photocathode research remains a vibrant and critical field for future...
Ultrafast electron scattering (UES), including diffraction, imaging, and energy-loss spectroscopy, are emerging new techniques for visualizing dynamics of matter at atomic levels. These rapidly advancing new developments have generated scientific outcomes with broad and profound impacts in quantum material, solar energy conversion, chemical reaction, high energy density physics, and biology,...
Photocathode physics plays a critical role in the formation of space and time resolution in time resolved stroboscopic electron scattering experiments like electron diffraction, microscopy, and spectroscopy. As effective photoemission temperatures approach single meV values, it is feasible to consider sources for <= 1 electron per pulse which approach the uncertainty principle emittance limit,...
The brightness of electron beams is limited by the smallest possible mean transverse energy (MTE) of electrons emitted from the photocathode. In this talk, I will show the various factors that limit MTE and show how by minimizing each of these factors we obtained a record low MTE of 6 meV from the (100) surface of Cu. I will also describe the instrumentation developed to measure such a small...
Operating photoemission electrons sources with photon energy near the work function has been shown to minimize the intrinsic emittance, but at the cost of several orders of magnitude lowered quantum efficiency. Thus, modern femtosecond photocathode electron sources would require very high intensity laser pulses to extract significant charge, which results in dynamic changes to the electronic...
n-type ultra-nano-crystalline diamond (n-UNCD, a synthetic polycrystalline, a mix of sp3 and sp2 phases, diamond
with semimetallic electron conductivity) has emerged as a negative electron a±nity (NEA) photocathode platform
that can be engineered toward a speci¯c targeted application. This presentation summarizes our experimental results
related to (i) quantum e±ciency (QE); (ii)...
The majority of materials used as photocathodes for accelerators have been discovered through empirical methods. This process is often trial and error, without significant input from computational materials science. We have developed an approach that is heavily informed by computational resources such as density functional theory (DFT), materials databases, and high-throughput techniques to...
One way of increasing the electron beam brightness in radiofrequency electron guns is by shrinking the photoemission area. Unfortunately, sharp tips are not good candidates to be used in high field RF environments due to field emission and stability issues.
We use Surface Plasmon Polariton wave interference to produce sub-micrometer emission areas. Radially symmetric nano-structured patterns...
Photocathodes are the source of electrons for nearly all modern accelerators. They are widely used due to their flexibility to meet a broad variety of beam requirements and are particularly important for machines relying on high-brightness beams. In this presentation, we will investigate the requirements and challenges for photocathodes used in linac-driven free electron lasers. The cathode...
The Pegasus photoinjector laboratory has a variety of activities related to photocathode technology. In the upcoming months we plan to test the performances of advanced photocathode materials such as alkali antimonide cathodes in the high gradient radiofrequency photoinjector using a recently developed ultrahigh vacuum load-lock chamber. I will also present an update on a different project...
Although low effective mass semiconductors offer the promise of photocathodes with subthermal mean transverse energy (MTE), the effect has never been observed in practice. One proposed explanation for this is that the cesium coating used on many photocathodes, while decreasing their work function, increases the MTE of the photoemitted electrons. In this talk, we discuss the ongoing effort at...
Operational lifetime of cathodes for high brightness electron sources are often limited by three factors – ion bombardment of the cathode, chemical contamination and thermal decomposition. The recent development of 0.5 nm thick crystalline two-dimensional nanoporous silica and aluminosilicate structures has the potential of reducing these detrimental effects and thus significantly increase the...
The performance of x-ray light sources, such as free electron lasers, ultrafast electron diffraction systems and ultrafast electron microscopy, is limited by the brightness of the electron beam. Given the improvements in photocathode design and synthesis, the source surface roughness has become a key limiting factor on the intrinsic emittance, specifically the mean transverse energy (MTE), of...
Electron sources driven by femtosecond laser have important applications in many aspects, and the research about the intrinsic emittance is becoming more and more crucial. The intrinsic emittance of polycrystalline copper cathode, which was illuminated by femtosecond pulses (FWHM of the pulse duration was about 100 fs) with photon energies above and below the work function, was measured with...
Many physics experiments at Jlab require high current polarized electron beams and we have been actively engaged in research and development that are intended to improve the existing photocathode performance as well as exploring new photocathodes with the goal to improve the operation capability and the quality of accelerator facilities. Since last P3 workshop, some interesting studies have...
This contribution describes Jefferson Lab polarized electron sources where the world record of high current polarized electron beam was delivered. Polarized GaAs Superlattice photoguns with long lifetime represents a significant challenge for proposed facilities that must operate in excess of tens of mA of polarized average current. Damage due to ion bombardment is the dominant mechanism that...
The quantum efficiency (QE) spectral response of five thin GaAs photocathodes with different active layer thickness and dopant density was experimentally evaluated, and to better appreciate the experimental results, a Monte Carlo model was developed to simulate electron transport and emission. The simulation accurately predicts expected behavior, namely QE is enhanced for thicker GaAs...
We have demonstrated that the SPTEM can provide both TEM images and diffraction patterns. The TEM images were obtained at a spatial resolution of 1 nm with a 30-kV acceleration voltage. The apparatus has an electron beam energy width below 114-meV in the TEM without any monochrometors. The energy width indicates that the temporal coherence is approximately 34 fs. The brightness is measured by...
Using spin-polarized electron beams in a low energy electron microscope provides opportunities to measure microscopic spin structures of domain walls and related spin textures, as SPLEEM permits imaging the vector orientation of the magnetization with high spatial- and angular resolution. This talk outlines key principles of relevant magnetic properties and opportunities in photocathode and...
Cesium telluride (CsTe) photocathodes has been the first hand choice for electron sources by worldwide accelerators, due to its high quantum yield, stable performance at complicated operation environment and long lifetime. In this work we compared the results of in situ x-ray characterization of the traditional sequential and co-evaporation growth of CsTe photocathodes. We were able to achieve...
The future upgrade at Relativistic Heavy Ion Collider (RHIC) calls for the implementation of electron cooling strategies, where ultra-cold electron beams will be generated by photoinjectors. To meet this requirement, the photocathode in the injector is required to deliver a high average current of 50 mA. Bialkali cathodes can deliver the required current, however owing to their short lifetime,...
We report on our recent advancements in development of a fully automated growth procedure for alkali antimonide photocathodes. Our goal is to create a capability that allows fabrication of uniform fully stoichiometric alkali antimonide films of any given thickness at sufficiently low substrate temperatures, where the compounds are stable. We show that co-evaporation procedure at low substrate...
We examine the potential of thin surface coatings to increase the QE of Nb while minimizing any additional RF losses. It is found that ultra-thin layers of Cs2Te on Nb, as low as 1.5 nm, still exhibit QE~6%. A phenomenological model of the Nb/Cs2Te bilayer is in good agreement with the measured thickness dependence. We also explore metal overlayers to exploit the superconducting proximity...
Interference enhanced, etalon photocathodes are promising due to the potential to enhance quantum efficiency without increasing emittance. This is likely so long as emittance is dominated by the excess energy (ħω-φeff) rather than being thermalization dependent, since the etalon effect does not directly change the band structure of the material. Enhancement occurs both by increasing absorption...
Photocathodes are important electron sources for X-ray free electron laser (XFEL) and X-ray energy recovery linacs (XERL), which generate brilliant, ultrafast, and coherent X-rays for exploration of matter with ultrahigh resolutions in both space and time. Quantum efficiency (QE) and operational lifetimes are two of the key figures of merit of photocathodes performance. Whereas alkali-based...
Alkali antimonide photocathodes have attractive properties, such as low-emittance and high quantum efficiency, which makes them excellent candidates for high-brightness electron sources. Less attractively, these materials are highly reactive and require ultra-high vacuum conditions to prevent irreversible oxidation, which precludes ex-situ characterization. Such limitations have stymied a...
The Photo Injector Test facility at DESY in Zeuthen (PITZ) was built to develop and optimize high brightness electron sources for short wavelength, SC linac driven free electron lasers like FLASH and the European XFEL. High quantum-efficiency Cs2Te photocathodes are driven by a UV laser to produce up to 5 nC charge per single electron bunch in the PITZ gun. Experimental characterization of the...
A New Photocathode R&D program at the Argonne Wakefield Accelerator facility: photocathode fabrication chamber and high-power testing with a 1.3 GHz NCRF injector test stand abstract -> A new program is under development at the Argonne Wakefield Accelerator (AWA) facility to fabricate and test photocathodes. The goal is to provide high quality beams for the in-house structure-based wakefield...
We will describe the LBNL experimental setup for high brightness beams production and measurements. The system includes an high repetition rate radiofrequency electron gun (the APEX gun), with 2 beamlines used for Ultrafast Electron Diffraction experiments, beam shaping and characterization.
We are also developing a 30 kV testbed for testing novel photo-emitters, before the high voltage-high...
After successful tests of Cs2Te photocathodes at the SwissFEL Test Injector Facility (SITF) in 2014, decision was made to use these types of semiconductor photocathodes for SwissFEL operation, rather than copper. Since SwissFEL first beam in winter 2016, only two photocathodes were used. Quantum efficiency (QE) degradation of 40 % has been observed after 6 months of operation or after 15 mC of...
In the past decade, X-ray free electron lasers (XFEL) has seen a rapid development in China. The center of the activities is at Shanghai, led by Shanghai Institute of Applied physics (SINAP) of the Chinese Academy of Sciences (CAS). The starting point was a test facility, where state-of-the-art experiments of FEL physics have been conducted. Since then, SINAP has led the effort of successfully...
In this talk, we will introduce our recent works in our lab on photocathode physics, including the normal conducting RF gun development, the surface roughness effects, the improved model of photoemission of semiconductors, the two photo-injector beam lines in our lab and their capabilities for the possible experiments.
Many recent efforts to improve the intrinsic emittance of photocathodes have centered on lowering the excess energy of the emitted electrons by using photon energies at or below threshold. However, this comes at the significant expense of quantum efficiency. Thus, for short pulse applications, nonlinear photoemission is a concern, due to the commensurate increase in excess energy in higher...
The HZB accelerator project bERLinPro (berlin Energy Recovery Linac Prototype) is continuously developing and is aiming for first operation in 2019. The goal of bERLinPro is to demonstrate a superconducting ERL with high current and low emittance. Within this highly complex R&D project a key component has been developed in-house: the photocathode, which is the electron source, and...
Bunched-beam electron cooling is a key feature of all proposed designs of the future electron-ion collider, and a requirement for achieving the specified collision luminosity. For the Jefferson Lab Electron Ion Collider (JLEIC), fast cooling of ion beams will be accomplished via so-called 'magnetized cooling' implemented using a recirculator ring that employs an energy recovery linac. In this...
Quality of photocathodes is one of the critical issues for the stability and reliability of the light source facility. In 2014, SRF gun-I with Cs2Te provided stable electron beams successfully for IR FEL at HZDR [1]. Cs2Te worked in SRF gun for more than one year without degradation. Currently, Mg photocathodes with QE up to 0.5% are applied in SRF Gun II, which is able to generate e- beam...
Progress in attosecond science in the past decade has enabled the study of ultrafast electronic
processes with unprecedented temporal resolution [1]. We extended an interferometric two-photon
technique named RABBITT [2] based on attosecond XUV pump pulses and a phase-matched IR probe
field from gas phase to solid surfaces to study the dynamics of the photoelectric effect [3].
Experiments on...
In calculating materials properties, a density-functional theory (DFT) has come to represent many functional theories, as well as many-body concepts that go beyond ground-state and density-dependent properties. The average practitioner should be aware of the complexion of DFT. We will outline the modern usage of DFT and its applicability to Alkali Antimonides; specifically, the density of...
Measurements of the spectral dependence of the mean transverse energy (MTE) from single crystal Mo(100) and W(100) photocathodes display a non-uniform increase with excess photoemission energy. The observed behavior is attributed to the bulk electronic band structure of the emitting states in the Γ-H direction of these body-centered cubic (bcc) metal crystals; specifically, the influence of...
Many applications, such as compact accelerators and electron microscopy, demand high
brightness electron beams with small source size and ultra-low-emittance. Diamond emitters
manufactured from the semiconductor process can be employed as such a compact beam source.
The micron-scale pyramid structure of the emitter allows enhancement of the external field
compared to that at the substrate,...
The state of the art in creating next-generation high-brightness electron beams requires that electrons emerge with mean transverse energy (MTE) of ~10 meV or lower. Identification of new, promising materials requires predictive understanding of the physics underlying photoemission. This talk will present our first-principles ab initio solid-state calculations of MTE for a variety of proposed...
Abstract
Obtaining high efficiency of polarized electron sources is a priority task, especially for the upcoming Electron-Ion Collider to be able to reach the luminosity needed to reach its physics goals [1].
In this presentation, we review several proposed methods and related simulations aimed at enhancing quantum efficiency (QE) of polarized GaAs photocathodes. Among recent significant...
Laser induced electron emission is important to the development of novel plasma and vacuum devices [1, 2],
compact electromagnetic radiation sources and accelerators, and time-resolved electron microscope. The rapid
development in nanotechnology and ultrafast laser optics has brought great opportunities to control electron
emission at ultrashort spatiotemporal scales and offers unprecedented...
The thermal limit of the intrinsic emittance of photocathodes represents an important property
to measure experimentally and to understand theoretically. Detailed measurements of intrinsic
emittance have become possible in momentatron experiments. Moreover, recent developments
in material design have allowed growing photoemissive layers with controlled surface roughness.
Although analytical...
The high quantum efficiency of modern cesiated Semiconductor photocathodes is a consequence of the
deeper penetration depth of the drive laser, a smaller (or absent) surface barrier, and - critically - the
tendency of the dominant scattering mechanism during electron transport to the surface to be less
effective at draining the initial energy of the photoexcited electron. As a consequence, a...
High brightness photo-generated electron bunches for next generation and x-ray FEL's and light sources rely on particle-in-cell codes used for their modeling and design. The codes in turn require predictive and accurate models of photocathode yield, emittance, emission promptness and beam characteristics based on accurate material and environmental parameters, but in a but in a manner that...
Dielectrics at optical frequencies withstand fields up to two orders of magnitude larger than metal cavities of conventional particle accelerators at microwave frequencies. Making use of the advances in nanofabrication and ultrafast lasers, extremely compact accelerating structures can be fabricated on chip including waveguides for laser power delivery. Acceleration [1-3], deflection [3] and...
In this contribution we report on experimental demonstration of generation and detection of attosecond electron pulses. It is reached via inelastic ponderomotive scattering of electrons at an optical traveling wave formed by two laser pulses at different frequencies. This scheme represents an analogy to the classical Kapitza-Dirac effect [1], in which the electron beam diffracts/reflects at an...
Carefully designed and fabricated semiconductor heterostructures are key to tunable and optimal optoelectronic devices, often enabling ways to overcome the performance bottlenecks defined by imperfections and defects native to the semiconductor crystals. Visualizing structure, corelated with function at the relevant length, time and energy scales remain a key challenge, but also are an...
We present the results of experimental observation of emission from single diamond field emitter tips when
triggered by an ultra-short laser pulse. Diamond field emitter array (DFEA) cathodes were originally proposed
for applications that require large current densities. DFEAs represent periodic arrays of diamond pyramids with
micron-size dimensions and tips with diameters of the order of tens...
Recent research on photocathode materials has focused on the achievement of very low intrinsic emittance. Future research goals require the setting up of a measurement apparatus capable of measuring very low mean transverse energy values, as low as 1 meV. Furthermore, cooling the photocathode to cryogenic temperature is also a way to reduce the intrinsic emittance and to control and study the...
