V. O. Kostroun

Monolithic crystal monochromators for synchrotron radiation with order sorting and polarizing properties

Two novel types of x‐ray monochromators which eliminate unwanted higher orders usually present in crystal monochromatized synchrotron radiation beams have been used successfully in spectroscopic studies. The design of the monochromators is based on the refractive index correction’s dependence on wavelength and relative orientation of crystal surface to reflecting Bragg planes. In addition to the order sorting property, both types of monochromators can be used to decrease or increase the reflected beam cross section by Bragg focusing, and in one type, to determine the polarization state of the diffracted beam.

Record high-average current from a high-brightness photoinjector

High-power, high-brightness electron beams are of interest for many applications, especially as drivers for free electron lasers and energy recovery linac light sources. For these particular applications, photoemission injectors are used in most cases, and the initial beam brightness from the injector sets a limit on the quality of the light generated at the end of the accelerator. At Cornell University, we have built such a high-power injector using a DC photoemission gun followed by a superconducting accelerating module. Recent results will be presented demonstrating record setting performance up to 65 mA average current with beam energies of 4–5 MeV.

Simple numerical evaluation of modified Bessel functions Kν(x) of fractional order and the integral ʃx∞Kν(η)dη

Abstract Theoretical expressions for the angular and spectral distributions of synchrotron radiation involve modified Bessel functions of fractional order and the integral ʃ x ∞ K ν (η) d η . A simple series expressions for these quantities which can be evaluated numerically with hand-held programmable calculators is presented.

Photocathode behavior during high current running in the Cornell energy recovery linac photoinjector

The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beam line gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (x-ray fluorescence spectroscopy, x-ray diffraction, atomic force, and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.

Order sorting, focusing and polarizing monolithic monochromators for synchrotron radiation

Abstract Two novel types of monochromators which eliminate higher orders usually present in crystal monochromatized synchrotron radiation beams have been used successfully in spectroscopic studies in the 1–4 A X-ray range. The design of the monochromators is based on the refractive index corrections dependence on wavelength and relative orientation of crystal surface to reflecting Bragg planes. The monochromators can be used to select a particular order, eliminate higher orders, to decrease or increase the reflected beam cross section by Bragg focusing, and in one type, to manipulate the polarization state of the diffracted beam. The general theory and summary of operating experience with these monochromators in synchrotron radiation studies at the 12 GeV electron synchrotron at the Wilson Synchrotron Laboratory at Cornell University and the 1.8 GeV storage ring DCI at Orsay are presented.

An electron beam ion source for laboratory experiments

The design and properties of an electron beam ion source (EBIS) capable of producing low‐energy, highly charged ions such as Ar16+ and Xe30+ are described. The source, to be used in laboratory experiments in atomic and surface physics, utilizes a conventional, 0.42‐T solenoid and an externally launched electron beam. Ultrahigh vacuum in the ionization region is created by a distributed sputter‐ion pump. The source design is relatively simple, and the source is small and easy to operate. The concept of a saturated electron beam is introduced in order to explain the observed argon and xenon ion charge state evolution. Very high ion charge states can be produced by confining ions for times up to 1 s in an electron beam in which the degree of space‐charge neutralization by ions is determined by potentials applied to the drift tubes.

Identification of “spurious” reflections in “channel-cut” monochromators

Abstract The vector method is used to calculate the position of reflections from possible hkl planes on a screen placed behind a “channel-cut” monochromator, and the reflections are identified with the aid of a stereographic projection of planes along some primary direction of interest.

A cryogenically cooled high voltage DC photoemission electron source

Linear electron accelerators and their applications such as ultrafast electron diffraction require compact high-brightness electron sources with high voltage and electric field at the photocathode to maximize the electron density and minimize space-charge induced emittance growth. Achieving high brightness from a compact source is a challenging task because it involves an often-conflicting interplay between various requirements imposed by photoemission, acceleration, and beam dynamics. Here we present a new design for a compact high voltage DC electron gun with a novel cryogenic photocathode system and report on its construction and commissioning process. This photoemission gun can operate at ∼200 kV at both room temperature and cryogenic temperature with a corresponding electric field of 10 MV/m, necessary for achieving high quality electron beams without requiring the complexity of guns, e.g., based on RF superconductivity. It hosts a compact photocathode plug compatible with that used in several other laboratories opening the possibility of generating and characterizing electron beam from photocathodes developed at other institutions.

An angle-resolved translational energy spectrometer for investigating low-energy, highly charged ion--atom (molecule) collisions

An experimental apparatus for investigating low‐collision energy gas‐phase reactions of highly charged ions with atoms and molecules by angle‐resolved translational energy spectroscopy is described. The basic function of the apparatus is to slow down highly charged ions extracted at 2–3 kV from the Cornell superconducting solenoid, cryogenic electron beam ion source (CEBIS) to energies below 100 qeV. The slowing down and monochromatization of the beam is done by an ion optical system that consists of two 180° hemispherical electrostatic analyzers with a tuneable deceleration lens located between them. The low‐energy beam available for experiments has an angular spread of 0.5° and an energy spread of 0.4 qeV. The ion optical system is mounted on a 508 mm by 576 mm electrically isolated platform inside a large vacuum chamber, and the modular nature of the apparatus allows for quick and easy rearrangement of the ion optical elements for different experiments. A brief overview of in‐plane scattering kinematic...

Atomic physics experiments with the Cornell electron beam ion source

Abstract Over the past three years, a superconducting solenoid, cryogenic electron beam ion source has been designed, constructed and tested at Cornell University. The source is designed for atomic physics experiments requiring low energy, very highly charged ion beams. The present status of the source is discussed and its use in some preliminary atomic physics experiments is described.