Mike V. Fisher

First results of SRC’s new high‐energy resolution variable line density grating monochromator beamline: HERMON

A new high‐resolution soft x‐ray beamline utilizing a variable line density grating has been constructed and tested at SRC. In addition to normal grating rotation, the grating housing mechanism allows a translation of the grating. This additional motion of the grating can be used in such a way that grating aberration effects such as defocus, coma, and spherical aberrations are minimized over the entire scan range. In order to achieve the theoretical resolving power of 105–5000 over the photon energy range of 280–1150 eV, extreme care had to be exercised in positioning and controlling the grating scan angle (<0.12 arcsec) and focus drive position (<10 μm). Using a spherical grating with a figure error of <0.2 arcsec and 10 μm slits, we were able to experimentally reproduce our theoretical predicted energy resolution over a wide energy range. We present photoabsorption data of the K‐shell edges and associated Rydberg states of Ne, O2, and CO. The high‐resolution monochromator unveils structures which were p...

Combining rotation and translation in a variable line space high-resolution soft-X-ray monochromator: Design requirements and performance evaluation of a novel grating mount

Abstract We describe a new fixed-slit single-element spherical-grating monochromator which has been designed, built and tested for the new high-resolution soft X-ray beamline at the University of Wisconsin Synchrotron Radiation Center. A novel grating mount utilizes a combined rotation and translation of a variable line space grating to minimize major aberration terms. The resolving power of the aberration-corrected monochromator is then primarily determined by the finite figure error and the mechanical performance of the grating mechanism. Assuming a 0.2 arc sec figure error, a resolving power ranging from E ΔE = 10 4 to 4.6 × 103 over the energy range of 500 to 1150 eV requires grating rotation and translation to be synchronized with a precision and accuracy of the order of 0.1 arc sec and 10 μm respectively. Extensive testing has verified precision and allowed appropriate calibrations to be developed to obtain the required accuracy. Calibrations minimize the impact of specific component behavior such as the compression of linear crossed roller bearings, the characteristic signature of a lead screw and the elastic response of the support structure.

Commissioning low emittance beam at Aladdin

The Aladdin storage ring is now routinely run in a low emittance configuration at 800 MeV. Vertical beam sizes and lifetime are comparable to the original lattice, while the horizontal beam size is reduced by a factor of three. Tools used to commission the new lattice include model based correction to obtain the design machine functions, and model independent correction to set the desired transverse coupling. Newly installed optical profile and position monitors, shunts to trim individual magnets, as well as implementation of a new control system scripting language, were important in achieving the desired results. Special attention was given to operation of the fourth harmonic bunch lengthening cavity used to improve the beam lifetime, and noise reduction in the RF system to improve photon beam quality on the infrared beamlines. In addition, compensation of undulators allows their strengths to be varied with minimum perturbation to the beam outside the regions of the undulators. Details of bringing the low emittance lattice to operational readiness are presented.

Precision motion and position control for the Plane Grating Monochromator at SRC

A nearly stigmatic Plane Grating Monochromator (PGM) under commissioning for the new undulator beamline at the Synchrotron Radiation Center will provide a resolving power > 10000 as it scans from 8 to 240 eV. Scanning requires the precise, simultaneous rotation of a plane mirror and a combined rotation-translation of a plane grating in close proximity to one another inside a UHV chamber. The required scanning motions are significant due to the large energy range covered by a single grating. The mirror and grating rotate nearly 23 and 33 degrees respectively and the grating translates approximately 200 mm. Sub-arcsec angular resolution allows several steps to be taken across the monochromators energy resolution. Both rotations utilize a combination stepping motor-piezoelectric actuator scan drive that is controlled with a feedback loop using a laser interferometer to measure the in-situ rotation of the optics. The grating mechanism is supported via bellows to an external stepping motor driven stage that travels directly on an inspection grade granite block. The stage position is controlled with a feedback loop using a precision linear encoder. A positional accuracy and vertical stability of a few micrometers is achieved on the grating translation which prevents the image from shifting at the exit slit and introducing energy calibration errors.

Undulator 4 meter NIM at SRC

A new 4 meter normal incidence monochromator (NIM) has recently been commissioned at the Synchrotron Radiation Center (SRC). The beamline utilizes two indexing spherical gratings to cover the energy range of 6–50 eV. A resolution of 0.35 meV at 15.765eV has been measured, which agrees well with the theoretical value. The source for the beamline is a 3.5 m long, 10.9 cm period electromagnetic undulator. The device can be scanned through the full energy range at any time with beamline based controls.

Mechanical design of a plane grating monochromator for the new undulator at Aladdin

A nearly stigmatic plane grating monochromator under construction for the new undulator beamline at Aladdin will provide a resolving power ≳10000 as it scans from 8 to 240 eV with a single grating. Scanning requires the precise, simultaneous movements of both a plane mirror and a plane grating in close proximity to one another inside a UHV chamber. The mirror, which absorbs up to 16.5 watts, is internally water cooled to minimize thermally induced slope errors. The radiatively cooled grating absorbs less than a watt. Careful examination of the focusing requirements revealed that the monochromator could be scanned either in the conventional mode of rotating‐translating the mirror and rotating the grating or in a modified mode of rotating the mirror and rotating‐translating the grating. The latter mode was chosen for simplicity of design. The mirror and grating rotate nearly 30 and 40 degrees, respectively, with subarcsec resolution. Both utilize a stepping‐motor lead‐screw piezoelectric actuator scan drive...

COMPUTER CONTROL OF THE SRC HIGH-RESOLUTION BEAMLINE

We discuss the complexities of controlling the new high‐resolution soft x‐ray beamline at the University of Wisconsin Synchrotron Radiation Center. A monochromator at the heart of the beamline utilizes a combined rotation and translation of a variable line spaced grating to minimize major aberration terms. A rotational accuracy of 0.12 arcsec and a translational accuracy of 10 μm are required for the combined motions to obtain the desired resolution. A rotational resolution of better than 0.01 arcsec was achieved with the use of a laser interferometer and piezoelectric actuator for submicrometer feedback control. The translation control uses a linear encoder with a resolution of 0.1 μm and a motorized feedback loop. A calculation overhead of less than 100 μs for each movement was obtained by using a spline fit to approximate the rotational and translation positions to the required accuracy.

Recent Developments at Aladdin

Following on the success of lower emittance operation at 800 MeV, SRC is pursuing a number of additional enhancements to the performance of the Aladdin storage ring. Work on Aladdin has included development of low emittance lattices at 1 GeV, which will maximize the capabilities of a recently installed spectromicroscopy beamline and a proposed high-resolution keV beamline. Installation of one-meter long insertion devices in the short straight sections within the quadrant arcs of the four sided storage ring is being pursued to increase the number of undulator beamlines from four to possibly eight. Studies have been made to determine what is the minimum insertion device gap that does not interfere with nominal ring operation (injection, ramping, and lifetime at full energy), and indicate that smaller-gapped devices for higher photon energy are reasonable. Lifetime increases or further emittance reductions appear possible with modest aperture increases at a small number of points on the ring. Finally, planning is under way for long term projects such as a new injector or a next generation VUV/soft-xray source for the Midwest. Details are presented.

Current status of the Synchrotron Radiation Center

The Synchrotron Radiation Center operates the Aladdin electron storage ring at energies of 800 meV or 1 GeV in support of a broad range of national and international research programs with a major focus on the study of valence electrons, spectromicroscopy, and nanolithography. Upgrades to the storage ring have improved the stability of the source, and experiments with low emittance lattice configurations show the feasibility of increased brightness for new or enhanced research. Three recently installed undulators, two pure permanent magnet devices and an electromagnetic device, and the associated instrumentation offer experimentalists high flux combined with high resolution. The status of the existing instrumentation, recent scientific results, and an overview of plans for new undulator-based instruments to cover the photon energy range from 7.8 to 400+ eV will be presented.

Comparison of blazed and laminar profile varied-line-spacing gratings

The performance of a beamline containing a blazed and a laminar grating is reported in this paper. Two varied line spacing (VLS) gratings are currently installed in the SRC High Energy Resolution Monochromator (HERMON). The blazed grating was ruled on a conventional ruling engine. The laminar profile grating was created holographically. The grating blanks were cut from a single spherical substrate having a figure error of less than 0.2 arcseconds RMS. The theoretical resolving power of 5000 to 10000 has been achieved in the energy range of 245 to 550 eV for the medium energy grating and 500 tol 150 eVfor the high energy grating indicating correct application of the polynomial groove density to the gratings. The groove profile for the laminar grating has been measured using atomic force microscopy and is in agreement with specifications. The beamline achieves near theoretical throughput throughout its scan range. A third grating which was designed to be blazed and ion etched is currently being evaluated.