Synchrotron Radiation Center

Undulator Beamline Construction Steps Up Pace

A number of important milestones have been reached in the construction of the SRC "Variable Polarization Beamline," which features a 3.5 meter-long permanent magnet undulator. The project has several interesting aspects, starting with a plan to provide high intensity, high resolution light with polarization that can be varied from linear to purely circular. This is accomplished by converting the high intensity, linearly polarized light from the undulator into a circularly polarized beam after the monochromator, with an efficiency that exceeds what could have been obtained with a crossed-field undulator.

The undulator, which is being manufactured by Maxwell-Brobeck Division, is nearing completion for delivery by January of next year. The undulator magnetic fields have been specified to guarantee that the undulator gap can be scanned during normal operation of the Aladdin storage ring. The 3.5 meter long undulator (see photo) will make use of all the available space in long straight section 3.

 

The monochromator for this beamline will be a fully stigmatic (fixed exit focus) plane-grating monochromator (PGM), the first of its kind at SRC. The optical design, produced by the SRC-Optics group under the direction of Ruben Reininger, has significant benefits for spectroscopy. These include improved higher-order suppression at long wavelengths, a resolving power of higher than ten thousand, and the ability to scan the entire photon energy range from 8 to 250 eV with a single grating.

The optical elements of the plane grating monochromator have all been ordered. The substrates of the first two optical components, the cylindrical mirror and the plane mirror, will be composed of Glidcop and will be internally water cooled. These components are being manufactured by Photon Sciences, of Tucson, Arizona. The grating substrate will be made of a single crystal of silicon with a holographic ruling. This optic will be manufactured by Photon Sciences and Hughes Aircraft. Tinsley Laboratories of Richmond, California will manufacture the ellipsoidal mirrors.

The new undulator beamline represents a major investment of resources at SRC, in order to provide a state-of-the-art VUV and soft X-ray source with some "first-of-its-kind" features. In anticipation of a very heavy demand for this new facility, a second branch for the beamline has been designed, to allow for time sharing of the beam by two experiments. The new branch includes a plane mirror that can be inserted into the beam, a second exit slit and a refocusing toroidal mirror. The fixed-focus feature of the PGM is retained on the branch line, with a spot size of less than 1 mm (horizontal) by 0.1 mm (vertical). About 2 meters separates the two branch-line focal points, so that two very large experiments can be operating in tandem.

 

The Engineering Group is currently working on the beamline components and precision mechanisms. The scanning of the monochromator requires simultaneous rotation and translation of either the plane mirror or the grating, and rotation of the other element. Since the plane mirror is actively water cooled, the monochromator has been designed so that the mirror only rotates and the grating rotates and translates. Both rotations will be continuously monitored by integral laser interferometers.

Because of the complexity of water cooling, a high precision optical component under ultra-high vacuum, a full-scale prototype of the first mirror rotation mechanism, was constructed to test the design concept. Mike Fisher is currently evaluating this part of the monochromator design. "The aluminum prototype will be used to evaluate the precision of the motion, the water cooling method, and to find out what works and what doesn’t. Not every problem can be eliminated this way," Mike points out, "but it is much simpler to catch problems in aluminum than in stainless steel." This process of prototyping will also be used to finalize the design of the grating translation and rotation hardware.

Some beamline components have already been manufactured. Greg Rogers reports that the two mirror box chambers and stands have already been located in the SRC vault. Where it is possible, successful designs from earlier SRC beamline construction projects are being used to speed up the process of completion. Final assembly of components, and initial commissioning of the beamline, are scheduled for the spring of next year.