Walter S. Trzeciak

Rapidly modulated variable‐polarization crossed‐undulator source

The needs of research teams in many disciplines now mandate the construction of rapidly modulated variable-polarization crossed-undulators for polarization-sensitive experiments. Such a source is being proposed for the Aladdin storage ring at the Synchrotron Radiation Center (SRC) to provide arbitrary polarization, modulated at 10 Hz, with a first harmonic tunable from 8-40 eV. An outline for an entire systems design is presented. The crossed-undulator design uses two planar undulator sections, each producing linearly polarized radiation, rotated with respect to one another by 90 degrees about their common longitudinal axis, and variably phased along the same axis, as illustrated. The proposed system will result in a comprehensive facility for the production and utilization of variably polarized radiation.<<ETX>>

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.

Vacuum ultraviolet high‐resolution and high‐flux beamline for the Aladdin storage ring

A new vacuum ultraviolet beamline for Aladdin based on an electromagnetic undulator and a 4 m normal incidence monochromator is described. The combination of the undulator, new optics, and an improved version of the currently operational monochromator is expected to deliver more than 3×1011 photons/s at a resolving power of 3×104 in the photon energy range 6–30 eV. At a lower resolving power of 104, more than 1011 photons/s will be available between 30 and 40 eV. Expecting a very high demand for the new beamline, two branches are planned to allow for time sharing.

Implementation of an undulator beamline on Aladdin

Abstract The first undulator monochromator on the Aladdin storage ring has been installed and recently tested. The design and alignment of an undulator beamline presented several unique problems not normally encountered in the design of bending magnet beamlines. The primary problem is the lack of visible radiation from the undulator source when it is operated under normal conditions. In this case a temporary beamline was installed to measure the intensity of undulator light transmitted through a pinhole which could be horizontally and vertically scanned through the undulator beam. This was used to adjust the steering of the beam to be centered in the port and to do some nondispersive characterization of the undulator. This temporary line was designed to permit precision alignment of the components of the beamline according to the measured position of the full energy beam. The monochromator used on this beamline is a 6 m toroidal grating monochromator (TGM). It allows further dispersive testing of the undulator and provides dispersed undulator flux for user experiments. The beamline is identical in terms of expected reflectivity losses and grating efficiencies to two similar beamlines implemented on bending magnets. This allows direct comparison of the flux available to experimenters from undulator sources to flux from bending magnets. Initial measurements indicate that with the undulator gap set for 50 eV, at the peak of the first harmonic, the undulator beamline produces 40 times more flux than a similar beamline using a bending magnet source.

Measurements and modelling of undulator radiation with a filter-photodiode detector

Abstract Owing to coherence effects, undulator radiation is energy-angle correlated with a quasi-monochromatic central cone of very small divergence. In correct combination, the energy-dependent responses of a filter and photodiode can aid in spatially resolving this radiation. Experience and measurements with a filter-photodiode monitor, gained over several years of monitoring radiation from the SSRL/LBL undulator on the Aladdin storage ring, are presented. Modeling of the detection process is described together with comparison to the data.

Rapidly-modulated variable-polarization crossed-undulator source

The needs of research teams in many disciplines now mandate the construction of rapidly modulated variable-polarization crossed-undulators for polarization-sensitive experiments. Such a source is being proposed for the Aladdin storage ring at the Synchrotron Radiation Center (SRC) to provide arbitrary polarization, modulated at 10 Hz, with a first harmonic tunable from 8-40 eV. An outline for an entire systems design is presented. The crossed-undulator design uses two planar undulator sections, each producing linearly polarized radiation, rotated with respect to one another by 90 degrees about their common longitudinal axis, and variably phased along the same axis, as illustrated. The proposed system will result in a comprehensive facility for the production and utilization of variably polarized radiation. >

User operation of the first undulator on Aladdin

The first undulator on Aladdin recently began operating as a user facility. The device is equipped with a 6-meter toroidal grating monochromator beamline. The undulator and its beamline have several unique characteristics. Furthermore, the adjacent bending magnet beamline is almost a twin of the undulator beamline, with similar grating efficiencies and reflection losses. This makes it easy to directly compare undulator and bending magnet source beamlines in a realistic environment. The results indicate that the undulator beamline produces between 20 and 100 times more flux in the first harmonic than a similar beamline on a bending magnet. The excellent polarization of 99.5% for the first harmonic makes this beamline extremely useful for polarization studies. User programs based on the undulator radiation included angle-resolved photoemission in gas phase and tests of optical components. The beamline is now entirely dedicated to soft X-ray spectromicroscopy (project MAXIMUM).

Implementation and effects of undulator scanning at Aladdin

Abstract The stability of the Aladdin closed orbit while scanning the U2 undulator gap is reported. Over the range of undulator gap settings the entire optimum closed orbit has been corrected to 20 μm rms vertically and 40 μm rms horizontally. The correction scheme utilizes computer control of the two undulator end correctors for the horizontal orbit and two of the regular ring correctors for the vertical orbit. Data showing the output of several other beam lines while the undulator is scanned over its entire range is presented. Small fluctuations in the intensity observed in some user beamlines has been attributed to the discrete steps used by the orbit correctors. A change in vertical beam size with undulator gap is shown to be related to a sextupole component in the undulator field. Possible remedies for these problems are discussed.

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.