Currently, the ID09B time-resolved X-ray beamline at the European Synchrotron and Radiation Facility (ESRF) in Grenoble, France is the only facility in the world capable of acquiring time-resolved macromolecular structures with 150-ps time resolution and < 2 Angstrom spatial resolution. Over the past decade, the Anfinrud group has been instrumental in helping to develop this capability on that beamline. Our effort has involved the design of X-ray shutters and choppers, timing electronics based on a Field-Programmable-Gate-Array (FPGA), and laser systems. Till now, all of our time-resolved X-ray studies have been conducted at the ESRF. Unfortunately, the ESRF operates in a mode that is optimized for these studies only 14 days out of each year, and we have had access to only a small fraction of this limited amount of beam time. To expand significantly the amount of beam time available for our studies, we have partnered with the Advanced Photon Source (APS) in Argonne, IL to develop picosecond time-resolved X-ray capabilities on BioCARS, an NIH-funded beamline that is headed by Prof. Keith Moffat and is operated by the University of Chicago. Dr. Marvin Gershengorn, Director of Intramural Research at NIDDK, has provided in excess of $1M to procure the capital equipment needed to develop picosecond time-resolved X-ray capability on this beamline, which occupies Sector 14 at the APS. This project, which has been ongoing for two years, is soon nearing completion and first experiments are planned soon after the start of FY2008. ? ? Our vision was to achieve picosecond time-resolved X-ray capabilities comparable to that realized at the ESRF when the APS is operated in 24-bunch mode, a common operating mode that is in use 132 days per year. This goal requires that we be able to isolate a single bunch of X-rays from a train of pulses separated by only 153 ns. To that end, we upgraded a high-speed X-ray chopper with new, low phase-noise electronics and a new, specially designed rotor. In July 2007, we succeeded in isolating a single bunch of X-rays during their 24-bunch mode. To achieve X-ray fluence comparable to that generated at the ESRF when they operate in their exotic 4-bunch mode, we had to replace the existing U33 undulator (Undulator A, which has a 33 mm magnetic period) with two newly designed U23 and U27 undulators. When the gaps of these undulators are tuned to generate 12 keV X-ray photons, the X-ray fluence is predicted to exceed that generated at the ESRF during their 4-bunch mode. When the APS operates in their exotic hybrid mode, which is scheduled approximately 31 days per year, the X-ray fluence will be a factor of 4 higher than that available with 4-bunch mode. Our first opportunity to use the beamline in that mode will occur the end of November, 2007. Our BioCARS collaborators have upgraded their X-ray optics and developed a heat-load chopper, both of which are essential to these efforts. ? ? The infrastructure needed to pursue picosecond time-resolved X-ray studies goes far beyond delivering single X-ray pulses to the experimental hutch. We installed a picosecond laser system in a laser hutch located near the X-ray hutch, and are in the process of installing an array of laser performance diagnostics that will aid in the operation of the laser. We have also developed an FPGA-based timing system that will synchronize all time-critical components to the X-ray pulses. For example, the FPGA will drive the heat-load chopper, the high-speed chopper, the picosecond laser system, a millisecond shutter, and all other component that must be synchronized with the experiment. Importantly, we can set the time delay between X-ray and laser pulses from picoseconds to seconds with a precision of 10 ps. We have also designed and constructed the diffractometer used to acquire time-resolved X-ray diffraction images. This effort included the design of a millisecond shutter, a motorized support for the high-speed X-ray chopper, a support for motorized X-ray slits, detectors for non-invasively monitoring the laser and X-ray pulse energy and relative time delay, a motorized stage for the X-ray detector, supports for a collimator pipe and X-ray beam stop, beam conditioning optics that tailor the laser pulses in both space and time, and beam delivery optics that focus the laser pulses onto the sample. ? ? After the infrastructure we have developed is fully operational, BioCARS will emerge as the premier site for pursuing time-resolved X-ray studies of biomolecules.
| Cammarata, Marco; Eybert, Laurent; Ewald, Friederike et al. (2009) Chopper system for time resolved experiments with synchrotron radiation. Rev Sci Instrum 80:015101 |
