Up until FY 2008, the ID09B time-resolved X-ray beamline at the European Synchrotron and Radiation Facility (ESRF) in Grenoble, France was 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 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. All of our prior time-resolved X-ray studies were conducted at the ESRF. During that era, the ESRF operated in a mode that was optimized for these studies only 14 days out of each year, and we had access to only a small fraction of this limited amount of beam time. To expand the amount of beam time available for our studies, we partnered with the Advanced Photon Source (APS) in Argonne, IL and BioCARS to develop picosecond time-resolved X-ray capabilities on Sector 14 at the APS. BioCARS is an NIH-funded beamline headed by Prof. Keith Moffat, and is operated by the University of Chicago. In FY2005, Dr. Marvin Gershengorn, Director of Intramural Research at NIDDK, committed in excess of $1M to procure the capital equipment needed for this effort. ? ? 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 used 132 days per year. This goal requires that we 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. Our BioCARS collaborators upgraded their X-ray optics and developed a heat-load chopper, both of which were essential to these efforts. 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 replaced the existing U33 undulator (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 exceeds 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 is a factor of 4 higher than that available with the ESRF 4-bunch mode. These two undulators were used in tandem for the first time in Nov. 2007. ? ? 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 have installed an array of laser diagnostics that aid in the optimization of the laser performance. We have also developed an FPGA-based timing system that synchronizes all time-critical components to the X-ray pulses. For example, the FPGA drives the heat-load chopper, the high-speed chopper, the picosecond laser system, a millisecond shutter, and various other motion controls 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 also developed 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. We are currently developing a rapid translation scheme for time-resolved WAXS studies, which will allow us to integrate scattering patterns with a move-stop-acquire sequence that repeats at frequencies up to 83 Hz. Finally, we have developed the software used to control the beamline. This software package, written in Python, is generalized for both time-resolved Laue crystallography and time-resolved WAXS studies. ? ? Though we continue to refine beamline components as well as methodologies for acquiring time-resolved X-ray data, we are rapidly moving beyond the commissioning phase and expect to shift our focus back to scientific problems in FY2009.
| Cammarata, Marco; Eybert, Laurent; Ewald, Friederike et al. (2009) Chopper system for time resolved experiments with synchrotron radiation. Rev Sci Instrum 80:015101 |
