D. H. Bilderback

Energy recovery linacs as synchrotron radiation sources (invited)

Practically all synchrotron x-ray sources to data are based on the use of storage rings to produce the high current electron (or positron) beams needed for synchrotron radiation (SR). The ultimate limitations on the quality of the electron beam, which are directly reflected in many of the most important characteristics of the SR beams, arise from the physics of equilibrium processes fundamental to the operation of storage rings. It is possible to produce electron beams with superior characteristics for SR via photoinjected electron sources and high-energy linacs; however, the energy consumption of such machines is prohibitive. This limitation can be overcome by the use of an energy recovery linac (ERL), which involves configuring the electron-beam path to use the same superconducting linac as a decelerator of the electron beam after SR production, thereby recovering the beam energy for acceleration of new electrons. ERLs have the potential to produce SR beams with brilliance, coherence, time structure, and source size and shape which are superior to even the best third-generation storage ring sources, while maintaining flexible machine operation and competitive costs. Here, we describe a project to produce a hard x-ray ERL SR source at Cornell University, with emphasis on the characteristics, promise, and challenges of such an ERL machine.

Liquid gallium cooling of silicon crystals in high intensity photon beams (invited)

The high‐brilliance, insertion‐device‐based photon beams of the next generation of synchrotron sources (Argonne’s APS and Grenoble’s ESRF) will deliver large thermal loads (1–10 kW) to the first optical elements. Considering the problems that present synchrotron users are experiencing with beams from recently installed insertion devices, new and improved methods of cooling these first optical elements, particularly when they are diffraction crystals, are clearly needed. A series of finite element calculations were performed to test the efficiency of new cooling geometries and various cooling fluids. The best results were obtained with liquid Ga metal flowing in channels just below the surface of the crystal. Ga was selected because of its good thermal conductivity and thermal capacity, low melting point, high boiling point, low kinetic viscosity, and very low vapor pressure. Its very low vapor pressure, even at elevated temperatures, makes it especially attractive in UHV conditions. A series of experiment...

The potential of cryogenic silicon and germanium X-ray monochromators for use with large synchrotron heat loads

Abstract It appears feasible to construct cryogenic silicon and germanium monochromators which may withstand 100 to 1000 times more power loading than is possible at room temperature.

Tests of a prototype pixel array detector for microsecond time-resolved X-ray diffraction

X-ray test results from a prototype 92 × 100 pixel array detector (PAD) for use in rapid time-resolved X-ray diffraction studies are described. This integrating detector is capable of taking up to eight full-frame images at microsecond frame times. It consists of a silicon layer, which absorbs the X-rays, bump-bonded to a layer of CMOS electronics in which each pixel has its own processing, storage and readout electronics. Tests indicate signal performance characteristics are comparable with phosphor-based CCD X-ray detectors, with greatly improved time resolution, comparable linearity and enhanced point spread. This prototype is a test module en route to a larger detector suitable for dedicated operation. Areas of needed improvement are discussed.

Microbeam high-resolution x-ray diffraction in strained InGaAlAs-based multiple quantum well laser structures grown selectively on masked InP substrates

Structural and optical properties of the InGaAlAs-based multiple quantum well (MQW) 1.3μm laser structures produced on InP (001) substrates by metal organic vapor phase epitaxy (MOVPE) technique in the regime of selective area growth (SAG) have been studied. An x-ray beam of 10μm diameter generated by a microbeam high-resolution x-ray diffraction (μ-HRXRD) setup based on an imaging one-bounce capillary optic and a three-bounce channel cut Si(004) analyzer crystal has been utilized to measure the diffraction curves from MQW structures grown between oxide mask stripes. The high angular resolution achieved in our experiments allowed accurate measurements of θ–2θ scans over a broad range of angles that was necessary for utilization of fitting algorithms for quantitative analysis of the strain and thickness of individual layers in the MQW structures. The thickness and strain variations in the quantum well and the barrier layers of the MQW SAG structure have been analyzed as a function of the oxide mask width i...

Performance of a hard x-ray undulator at CHESS (invited)

A 3.3‐cm period Nd‐Fe‐B hybrid undulator has been designed and successfully operated in the Cornell Electron Storage Ring (CESR). This 2‐m‐long, 123‐pole insertion device is a prototype of one of the undulators planned for the Advanced Photon Source. In dedicated operation, the undulator produced the expected brightness at 5.437 GeV with the fundamental x‐ray energy ranging from 4.3 to 7.9 keV corresponding to a change in gap from 1.5 to 2.8 cm.

The Cornell ERL prototype project

Synchrotron light sources based on Energy Recovery Linacs (ERLs) show promise to deliver X-ray beams with both brilliance and X-ray pulse duration far superior to the values that can be achieved with storage ring technology. Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a prototype ERL. This 100MeV, 100mA CW superconducting electron accelerator will be used to study and resolve the many accelerator physics and technology issues of this type of machine. These studies are essential before ERLs can be confidently proposed for large-scale applications such as synchrotron light sources. Key issues include the generation of high average current, high brightness electron beams; acceleration and transport of these beams while preserving their brightness; adequate damping of higher order modes (HOMs) to assure beam stability; removal of large amounts of HOM power from the cryogenic environment; stable RF control of cavities operating at very high external Q; reduction of beam losses to very low levels; and the development of precision non-intercepting diagnostics to allow beam setup, control and characterization. Our prototype design allows us to address these and other issues over a broad range of parameter space. This design, along with recent progress on understanding these issues, will be presented.

The performance of a separated crystal X-ray monochromator during wiggler heating — Preliminary results☆

Abstract We report on the performance of a double crystal monochromator exposed to large heat loads from a 6 pole wiggler magnet running at 15.6 kG peak field and a 5.3 GeV electron beam. This source of synchrotron radiation can produce between 1.8 and 4.5 kW of X-ray power depending on beam conditions. Various single crystals of silicon have been tested in a nondispersive double crystal arrangement over a wide energy range along with several different cooling schemes. We report preliminary measurements of the performance of silicon crystals cooled by ambient helium gas or water cooled through conductive greases or through a 10 μm thick layer of liquid indium and gallium. A side cooled, thick crystal geometry shows good stability for photon energies from 20 to 45 keV. A thin, bottom cooled geometry shows promise at 7.4 keV. We have observed monochromator performance (rocking curve width, shift in edge, X-ray flux output vs electron beam current, harmonic rejection capabilities, etc.) under a variety of heating conditions. We summarize our suggestions for making effective use of monochromators under severe operating conditions.

Developments in tapered monocapillary and polycapillary glass X-ray concentrators

Abstract Tapered glass capillaries can be used to condense both wide-band and monochromatic hard (> 5 keV) X-ray beams. These long, hollow, needle-like optics have been used in a variety of experiments to demonstrate their utility in areas such as protein crystallography, hard X-ray imaging, and Laue diffraction. Monocapillary versions of these X-ray concentrators have yielded intensity gains of up to 1000 and X-ray beam sizes as small as 50 nm. Recently, in an effort to make larger condensed beam sizes (> 10 μ m), polycapillary tubing has been tapered and a 68 μm diameter X-ray beam with an intensity gain of 4.6 was observed using 6 keV synchrotron radiation. Results from developments of both types of tapered capillaries and from ray-tracing design efforts are presented.

Microchannel water cooling of silicon x‐ray monochromator crystals

The use in silicon x‐ray monochromator crystals of water cooling channels with dimensions optimized for efficient heat transfer from silicon to water has been investigated. Such channels are typically about 40 μm wide and 400 μm deep. Procedures have been found for reliably producing microchannel‐cooled crystals with very small amounts of residual strain. These crystals have been tested at a high‐power wiggler beam line at the Cornell High Energy Synchrotron Source, using an x‐ray beam having total power in excess of 250 W and normal‐incidence power density greater than 5 W/mm2. Under these conditions, the surface‐temperature rise of a typical microchannel‐cooled crystal was less than 5 °C, and degradation of the (111) rocking curve at 12 keV was very slight. The cooling efficiency is consistent with analytic calculations.