Showing papers by "Howard A. Padmore published in 2016"

Near atomically smooth alkali antimonide photocathode thin films

Abstract: Nano-roughness limits the emittance of electron beams that can be generated by high efficiency photocathodes, such as the thermally reacted alkali antimonide thin films. However there is an urgent need for photocathodes that can produce an order of magnitude or more lower emittance than present day systems in order to increase the transverse coherence width of the electron beam. In this paper we demonstrate a method for producing alkali antimonide cathodes with near atomic smoothness with high reproducibility.

Bi-alkali antimonide photocathode growth: An X-ray diffraction study

Abstract: Bi-alkali antimonide photocathodes are one of the best known sources of electrons for high current and/or high bunch charge applications like Energy Recovery Linacs or Free Electron Lasers. Despite their high quantum efficiency in visible light and low intrinsic emittance, the surface roughness of these photocathodes prohibits their use as low emittance cathodes in high accelerating gradient superconducting and normal conducting radio frequency photoguns and limits the minimum possible intrinsic emittance near the threshold. Also, the growth process for these materials is largely based on recipes obtained by trial and error and is very unreliable. In this paper, using X-ray diffraction, we investigate the different structural and chemical changes that take place during the growth process of the bi-alkali antimonide material K2CsSb. Our measurements give us a deeper understanding of the growth process of alkali-antimonide photocathodes allowing us to optimize it with the goal of minimizing the surface roughness to preserve the intrinsic emittance at high electric fields and increasing its reproducibility.

Refraction effects in soft x-ray multilayer blazed gratings.

Abstract: A 2500 lines/mm Multilayer Blazed Grating (MBG) optimized for the soft x-ray wavelength range was fabricated and tested. The grating coated with a W/B4C multilayer demonstrated a record diffraction efficiency in the 2nd blazed diffraction order in the energy range from 500 to 1200 eV. Detailed investigation of the diffraction properties of the grating demonstrated that the diffraction efficiency of high groove density MBGs is not limited by the normal shadowing effects that limits grazing incidence x-ray grating performance. Refraction effects inherent in asymmetrical Bragg diffraction were experimentally confirmed for MBGs. The refraction affects the blazing properties of the MBGs and results in a shift of the resonance wavelength of the gratings and broadening or narrowing of the grating bandwidth depending on diffraction geometry. The true blaze angle of the MBGs is defined by both the real structure of the multilayer stack and by asymmetrical refraction effects. Refraction effects can be used as a powerful tool in providing highly efficient suppression of high order harmonics.

X-ray diffraction gratings: Precise control of ultra-low blaze angle via anisotropic wet etching

Abstract: Diffraction gratings are used from micron to nanometer wavelengths as dispersing elements in optical instruments. At shorter wavelengths, crystals can be used as diffracting elements, but due to the 3D nature of the interaction with light are wavelength selective rather than wavelength dispersing. There is an urgent need to extend grating technology into the x-ray domain of wavelengths from 1 to 0.1 nm, but this requires the use of gratings that have a faceted surface in which the facet angles are very small, typically less than 1°. Small facet angles are also required in the extreme ultra-violet and soft x-ray energy ranges in free electron laser applications, in order to reduce power density below a critical damage threshold. In this work, we demonstrate a technique based on anisotropic etching of silicon designed to produce very small angle facets with a high degree of perfection.

Real-time data-intensive computing

Abstract: Today users visit synchrotrons as sources of understanding and discovery—not as sources of just light, and not as sources of data. To achieve this, the synchrotron facilities frequently provide not just light but often the entire end station and increasingly, advanced computational facilities that can reduce terabytes of data into a form that can reveal a new key insight. The Advanced Light Source (ALS) has partnered with high performance computing, fast networking, and applied mathematics groups to create a “super-facility”, giving users simultaneous access to the experimental, computational, and algorithmic resources to make this possible. This combination forms an efficient closed loop, where data—despite its high rate and volume—is transferred and processed immediately and automatically on appropriate computing resources, and results are extracted, visualized, and presented to users or to the experimental control system, both to provide immediate insight and to guide decisions about subsequent experim...

Multiplexed high resolution soft x-ray RIXS

Abstract: High-resolution Resonance Inelastic X-ray Scattering (RIXS) is a technique that allows us to probe the electronic excitations of complex materials with unprecedented precision. However, the RIXS process has a low cross section, compounded by the fact that the optical spectrometers used to analyze the scattered photons can only collect a small solid angle and overall have a small efficiency. Here we present a method to significantly increase the throughput of RIXS systems, by energy multiplexing, so that a complete RIXS map of scattered intensity versus photon energy in and photon energy out can be recorded simultaneously1. This parallel acquisition scheme should provide a gain in throughput of over 100.. A system based on this principle, QERLIN, is under construction at the Advanced Light Source (ALS).

R+ D Progress Towards a Diffraction Limited Upgrade of the ALS

Abstract: Author(s): Steier, C; Anders, A; Byrd, J; Chow, K; Duarte, R; Jung, J; Luo, T; Nishimura, H; Oliver, T; Osborn, J; Padmore, H; Pappas, C; Robin, D; Sannibale, F; De Santis, S; Schlueter, R; Sun, C; Swenson, C; Venturini, M; Waldron, W; Wallen, E; Wan, W; Yang, Y | Abstract: Improvements in brightness and coherent flux of about two orders of magnitude over operational storage ring based light sources are possible using multi bend achromat lattice designs [1]. These improvements can be implemented as upgrades of existing facilities, like the proposed upgrade of the Advanced Light Source, making use of the existing infrastructure, thereby reducing cost and time needed to reach full scientific productivity on a large number of beamlines. An RaD program was started at LBNL to further develop the technologies necessary for diffraction-limited storage rings. It involves many areas, and focuses on the specific needs of soft x-ray facilities [2]: NEG coating of small chambers, swap-out injection, bunch lengthening, magnets/radiation production, x-ray optics, and beam physics design optimization. Hardware prototypes have been built and concepts and equipment was tested in beam tests on the existing ALS.

High efficiency diffraction grating for EUV lithography beamline monochromator

Abstract: A blazed diffraction grating for the EUV lithography Beamline 12.0.1 of the Advanced Light Source has been fabricated using optical direct write lithography and anisotropic wet etching technology. A variable line spacing pattern was recorded on a photoresist layer and transferred to a hard mask layer of the grating substrate by a plasma etch. Then anisotropic wet etching was applied to shape triangular grating grooves with precise control of the ultralow blaze angle. Variation of the groove density along the grating length was measured with a Long Trace Profiler (LTP). Fourier analysis of the LTP data confirmed high groove placement accuracy of the grating. The grating coated with a Ru coating demonstrated diffraction efficiency of 69.6% in the negative first diffraction order which is close to theoretical efficiency at the wavelength of 13.5 nm. This work demonstrates an alternative approach to fabrication of highly efficient and precise x-ray diffraction gratings with ultra-low blaze angles.

APEX Phase-II Commissioning Results at the Lawrence Berkeley National Laboratory

Abstract: Science needs in the last decade have been pushing the accelerator community to the development of high repetition rates (MHz/GHz-class) linac-based schemes capable of generating high brightness electron beams. Examples include X-ray FELs; ERLs for light sources, electron cooling and IR to EUV FEL applications; inverse Compton scattering X-ray or gamma sources; and ultrafast electron diffraction and microscopy. The high repetition rate requirement has profound implications on the technology choice for most of the accelerator parts, and in particular for the electron gun. The successful performance of the GHz room-temperature RF photoinjectors running at rates <~100 Hz, cannot be scaled up to higher rates because of the excessive heat load that those regimes would generate on the gun cavity walls. In response to this gun need, we have developed at the Berkeley Lab the VHF-Gun, a lower-frequency roomtemperature RF photo-gun capable of CW operation and optimized for the performance required by MHz-class Xray FELs. The Advanced Photo-injector EXperiment (APEX) was funded and built for demonstrating the VHF gun performance, and the results of its last phase of commissioning are presented. INTRODUCTION APEX, the Advanced Photo-injector EXperiment [1] at the Lawrence Berkeley National Laboratory (LBNL) is an electron injector test facility dedicated to the characterization of the VHF-Gun [2-4], a new concept electron RF gun developed at LBNL for the generation of high-brightness high-repetition rate electron beams. Specifically, the VHF-Gun was the response to the need of a source capable to generate at MHz-class repetition rate the high quality beams required to operate high-duty cycle X-ray FELs [5]. Indeed, an injector based on the VHF-Gun is in the baseline of the LCLS-II project at SLAC [6]. In the VHF-Gun, the electron bunches are generated by laser-induced photo-emission on high quantum efficiency (QE) cathodes. The particles are then accelerated over a 4 cm gap up to the energy of 750 keV by the 20 MV/m electric field excited in a room-temperature continuous wave (CW) RF cavity resonating at 186 MHz (the 7 subharmonic of 1.3 GHz or the 8 of 1.5 GHz, the dominant superconducting linac technologies). The low frequency choice allowed addressing the two most challenging requirements imposed by the highrepetition rate application: the capability of the gun of running in CW, and the achievement of the extremely low vacuum pressures necessary to operate reactive and delicate high QE photocathodes with acceptable lifetimes. Indeed, at this frequency, the cavity is large enough to lower the power density on the cavity walls to a level that conventional cooling techniques can be used to run in CW mode, while maintaining the high accelerating fields required for the high brightness performance. Also, the long wavelength allows for large apertures on the cavity walls with negligible field distortion. Such apertures provide the vacuum conductance necessary to achieve the desired low pressures. The APEX project was organized in 3 stages (Phase 0, I and II), with the first two dedicated to the characterization and testing of the VHF-Gun, to cathode testing and to electron beam characterization at the gun energy. In Phase II, a buncher and a linac were added to the VHF-Gun to compress and accelerate the beam to more relativistic energies, reducing space charge forces and allowing characterizing the gun/injector brightness and compression performance. The experimental results of the first two phases of APEX are reported elsewhere and include demonstration of the VHF-Gun performance [7], characterization of the APEX dark current [8], and demonstration of the capability of Cs2Te cathodes to operate at the LCLS-II regime [9]. In this paper we report the results of the recent two-month beam measurement campaign to demonstrate the requirements of one of the LCLS-II modes of operation. APEX PHASE-II DESCRIPTION Figure 1 shows a CAD layout and a panoramic view of APEX Phase-II beamline. The vacuum loadlock that allows replacing cathodes without breaking vacuum, and the VHF-Gun are visible on the left part. Following the beam path exiting the gun, a focusing solenoid, a 1.3 GHz CW 2-cell buncher cavity [10], and a second focusing solenoid follow. A small pulsed linac composed by two 1m-long 1.3 GHz normal-conducting standing-wave accelerating sections (a modified version of the Argonne AWA structures [11]) follows. ___________________________________________ *Work supported by the Director of the Office of Science of the US Department of Energy under Contract no. DEAC02-05CH11231 †fsannibale@lbl.gov Proceedings of IPAC2016, Busan, Korea TUOCA02 02 Photon Sources and Electron Accelerators T02 Electron Sources ISBN 978-3-95450-147-2 1041 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s Figure 1: APEX Phase-II beamline. Top: CAD view. Bottom: panoramic photo of the beamline. Originally the layout included a third accelerating section but due to the delayed delivery by the manufacturer this section was not installed. As shown in Table 1, the VHF-Gun and the buncher run in CW mode, while the linac operates in pulsed mode with 10 Hz repetition rate. The rationale behind this configuration is that the electron beam 6-D brightness is a single bunch beam property that can be measured at any repetition rate. This allowed the use of a room temperature copper linac with a strong cost reduction and system simplification. Table 1: Phase-II Beamline Main Operational Parameters Parameter Value Units Nominal beam energy 16 MeV Nominal VHF-Gun energy 750 keV VHF-Gun and buncher mode of operation Continuous Wave (CW) Linac mode of operation Pulsed

Innovative diffraction gratings for high-resolution resonant inelastic soft x-ray scattering spectroscopy

Abstract: High-resolution Resonant Inelastic X-ray Scattering (RIXS) requires diffraction gratings with very exacting characteristics. The gratings should provide both very high dispersion and high efficiency which are conflicting requirements and extremely challenging to satisfy in the soft x-ray region for a traditional grazing incidence geometry. To achieve high dispersion one should increase the groove density of a grating; this however results in a diffraction angle beyond the critical angle range and results in drastic efficiency loss. The problem can be solved by use of multilayer coated blazed gratings (MBG). In this work we have investigated the diffraction characteristics of MBGs via numerical simulations and have developed a procedure for optimization of grating design for a multiplexed high resolution imaging spectrometer for RIXS spectroscopy to be built in sector 6 at the Advanced Light Source (ALS). We found that highest diffraction efficiency can be achieved for gratings optimized for 4th or 5th ord...

The VHF-Gun, the LBNL High-Brightness Electron Photo-Injector for MHz-Class Repetition-Rate Applications*

Abstract: Science needs are pushing the development of MHz-class repetition-rate linac-based facilities generating high-brightness electron beams. The successful lower repetition-rate RF gun schemes cannot be scaled up to MHz rates. At LBNL, we developed the VHF-Gun, a room-temperature RF gun designed for CW operation and high-brightness beam performance.