D.E. Eisert

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...

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.

Operation of Aladdin at lowered emittance

To increase the available photon flux density for users, alternate lattice tunings of the Aladdin synchrotron light source have been developed with horizontal emittances significantly lower than the present value of 127 /spl pi/ nm-rad. Reduction of the horizontal emittance by a factor of three has been obtained experimentally. When the fourth harmonic Landau cavity is used to lengthen the bunch, the observed beam lifetime with the new lattice is not significantly changed from that of the existing lattice. The present goal is to achieve a factor of four horizontal and a factor of two vertical emittance reduction routinely. Progress in making this new configuration fully operational is discussed, including the use of quadrupole shunts with correction software for beta function and dispersion correction, understanding of a mode coupling instability in higher-harmonic RF systems, RF clearing of ions, and fabrication of new optical monitoring stations.

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.

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.

A new real-time operating system and Python scripting on Aladdin

We are in the process of upgrading the VME processors on the Aladdin electron-storage-ring control system. The last major redesign of the control system occurred in the mid 1980s. At that time we converted to VME microcomputers and VAX/VMS workstations communicating via Ethernet. This is the second upgrade since then of the VME processor. As upgrades of the Motorola 680/spl times/0 processor are no longer available we have decided to switch to the Intel Pentium III. This change allowed us to reconsider our use of the rather primitive /spl mu/C/OS kernel and implement a commercial realtime OS. We decided to use QNX primarily as it was a good match to our existing software and was zero cost. In addition to upgrading the CPUs we have also added a new scripting language to our main control application. We used SWIG (Simplified Wrapper and Interface Generator) to create wrapper code for the scripting software. SWIG can create wrapper code for many scripting languages so our initial choice of a scripting language was not critical. We decided to start by using Python due to the many available add-on libraries and the apparent ability to support larger projects. We will discuss our evaluation process and the challenges we encountered.

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.

First measurements and observations from the new permanent magnet undulator at the SRC

In December of 2000 the University of Wisconsin Synchrotron Radiation Center (SRC) installed a new, state-of-the-art undulator in long straight section 2 of the Aladdin storage ring. This undulator replaced the first Aladdin undulator, which was on loan from the Stanford Synchrotron Radiation Laboratory and had been in use since 1986 [K. Halbach et al., IEEE Trans. Nucl. Sci. NS-28, 3136 (1981); H. Winick et al., Nucl. Instrum. Methods Phys. Res. A 208, 127 (1983); M. A. Green et al., Nucl. Instrum. Methods Phys. Res. A 266, 91 (1988)]. The new undulator was designed, constructed, and tested by Danfysik (Danfysik A/S, Moellchaves 31A, DK-4040 Jyllinge, Denmark). The undulator uses NdFeB permanent magnets for the magnetic structure and is 3.52 m long with 50 periods, each of 68.3 mm. The new undulator is intended to provide high flux from 7.8 to 500 eV. To cover this wide energy range we use the first through ninth odd harmonics. The magnetic structure has been phase-corrected to better than 2° rms and sho...

Improvements to the Aladdin synchrotron light source

Aladdin is an IR to soft x-ray synchrotron light source operated by the University of Wisconsin at Madison. As part of the ongoing program of upgrades and improvements, several changes have recently been made to the ring. It had previously been determined that physical apertures (BPMs) at the QF quadrupoles were limiting beam lifetime when the ring was operated in its low emittance configuration. Increasing the size of these apertures has resulted in a significant increase in lifetime. Also as part of the aperture opening process, a number of ring components were redesigned and replaced, lowering the ring impedance. This has led to an increase in the threshold beam current for microwave instability. An insertion device for EUV lithography has been incorporated into one of the Aladdin short straight sections, and an elliptically polarizing undulator will be installed in another short straight section for a new VLS- PGM beamline. An innovative infrared beamline is under construction, which will extract 320 (H) times 25 (V) mrad2 from a bending magnet by as 12 beamlets, which are combined in an IR microscope. Another modification to Aladdin was the design and installation of discrete trim coils on the quadrupole pole-tips to facilitate using the quads as steering correctors. Details of these improvements are presented.