The experimental set-up located at station 14-ID-B is represented in Fig. 9 by a three-dimensional solid model. Starting from the right, a large-aperture 0.5 mm-thick beryllium window (Brush Wellman Electrofusion Products) separates the KB mirror vacuum segment from the experimental apparatus. To reduce mechanical stress when supporting an external pressure up to 1 atm, the beryllium window was brazed to a convex cylindrical frame with a 101.6 mm radius of curvature. With a clear aperture of 44 mm (H) × 90 mm (V), this window accommodates the direct white beam and all possible mirror angles up to 4 mrad in each direction.
The components in Fig. 9 are mounted on an optical table that is motorized with six degrees of freedom (three translational, x
, plus pitch, roll and yaw angles). A change in both KB-mirror angles is coupled to combined motions of the table to follow the X-ray beam trajectory. Additionally, since the distance from the X-ray beam to the top of the Jülich chopper rotor must be maintained to within a few micrometers in order to isolate single pulses cleanly (see §2.5.3
), the optical table must be stable to the same degree. The position of the monochromatic beam is measured by a fluorescence-based beam-position monitor (BPM) (Alkire et al.
). An attenuator box is located downstream of the BPM, followed by the millisecond shutter and Jülich chopper. Two Huber mechanical stages provide horizontal and vertical motion of the Jülich chopper. The vertical motion changes the chopper open time in tunnel-less mode, and the horizontal motion selects the chopper mode: bypass, tunnel-less or tunnel chopping modes. A rigid pedestal downstream of the Jülich chopper supports an in-vacuum four-blade JJ-slit (manufactured by JJ X-ray, Denmark) assembly and non-invasive diagnostic X-ray detectors. The layout is shown schematically in Fig. 10, in which the X-ray beam traverses the components from right to left. A 10 mm-diameter beryllium window separates the vacuum segment required by the Jülich chopper from atmospheric pressure on the detector side. Two X-ray-sensitive detectors are located downstream of the beryllium window. A Bicron BC422Q scintillator coupled to a microchannel plate photomultiplier is positioned just below the X-ray beam; it detects scattered radiation from the Be window and produces pulses with a 275 ps rise time. This detector is used to record non-invasively the pump–probe delay for each measurement during an experiment. The second detector, a PIN diode manufactured by Canberra (model ANFD300-20-300RM) with a 4 mm beam-pass-through hole at its center, is relatively slow but provides pulse intensity information on a pulse-by-pulse basis for I
Figure 10 Schematic view of the high-speed chopper, clean-up slits and non-invasive X-ray detectors, which record simultaneous shot-by-shot X-ray flux and pulse time-of-arrival signals during data collection. The scale shows the position in meters of each component (more ...)
For conventional synchrotron beamlines, where the X-ray flux is considered to be quasi-continuous, an integrating detector such as an ion chamber is often used to measure the beam intensity. The readout electronics for these detectors typically consists of a transimpedance amplifier followed by a voltage-to-frequency converter and a computer-controlled counter. Such an approach does not work well for this time-resolved beamline, where the average current produced by the chopped X-ray beam is very low but the peak current is sufficiently high to easily saturate a transimpedance amplifier. Instead, we use a digital oscilloscope (LeCroy WaveSurfer) to digitize and integrate the area under the signal generated by each X-ray pulse from the I
0 detector. The area is proportional to the total charge collected and therefore to the number of photons in the X-ray pulse. An EPICS (Experimental Physics and Industrial Control System) application running directly on the oscilloscope makes it resemble a standard EPICS detector, whose output can be recorded in conjunction with any other EPICS device, for example, one of the instrument positioners.
The goniometer is a standard Huber 515.2 kappa model in which a motorized XYZ-translation stage mounted on the ϕ-stage facilitates centering of the crystal. Two microscope CCD cameras with 1 µm and 6 µm resolution are mounted to the rigid pedestal and are used to aid in crystal centering. Diffracted X-rays are imaged on a CCD detector (at present a MAR165 CCD) mounted on a 0.8 m-long Velmex dovetail translation stage used to adjust the crystal-to-detector distance. The MAR165 CCD is a 165 mm-diameter phosphor-coupled CCD detector and is run in 2 × 2 pixel binning mode with an effective pixel size of 80 µm. The MAR165 CCD has a full well capacity of 45000 photons pixel−1 at 12 keV and a readout time of 2.5 s per frame. For typical crystals of small molecules or for a well ordered crystal of small biological macromolecules, it is possible to saturate a single pixel with diffracted beam produced by one 100 ps X-ray pulse in 24-bunch mode. This corresponds to an instantaneous flux >4.5 × 1014 photons s−1 pixel−1 at the most intense region of the Bragg spot. This flux is far beyond the capabilities of any photon-counting pixel-array detector presently available commercially. Thus, an integrating detector with a large well capacity is best suited for these time-resolved experiments.
3.1. Laser systems
Two laser systems are presently operational and available for BioCARS users: a modified Spectra-Physics (SP) Spitfire Pro picosecond laser coupled to a TOPAS optical parametric amplifier (OPA), and a nanosecond OPOTEK Nd:YAG/optical parametric oscillator (OPO). The SP picosecond laser enables picosecond time resolution for the first time at BioCARS. Both lasers are housed in an external temperature-stabilized laser hutch. Picosecond laser pulses are transported to the 14-ID-B X-ray station by mirrors while nanosecond laser pulses are transported via fiber optics.
The SP picosecond laser system is composed of several components. A SP Millennia CW laser pumps a SP Tsunami mode-locked Ti:sapphire laser which generates a femtosecond seed beam for the Spitfire Pro. This seed beam is spectrally narrowed to about 12.5 cm−1 FWHM bandwidth to produce transform-limited ~1.2 ps seed pulses. The Spitfire Pro enclosure contains both a Ti:sapphire regenerative preamplifier and a double-pass Ti:sapphire power amplifier, each of which is pumped by a SP Empower laser. The seed pulses are stretched, amplified, then compressed to produce ~1.2 ps pulses at 780 nm with an energy of 5 mJ and a repetition frequency up to 1 kHz. The amplified pulses pump the TOPAS OPA, which produces tunable signal and idler pulses. Nonlinear mixing schemes such as second-harmonic generation of the signal or idler pulses or sum-frequency mixing of the signal or idler pulses with the residual pump beam extends the tunability from greater than 2000 nm down to ~450 nm with >300 µJ pulse energy. The multi-wavelength TOPAS output is passed through a set of dichroic mirrors to isolate the desired wavelength, which is transported ~30 m to the 14-ID-B station by a periscope mirror system.
Laser beam-conditioning optics located above the diffractometer in station 14-ID-B stretch the pulses to 35 ps in an echelon and, depending on the experimental requirements, are focused to either a circular or elliptical focal spot. The polarization of the laser pulse (linear or circular) is controlled via a New Focus Berek compensator located before the final focusing optic. Two computer-controlled gradient neutral density filters adjust the laser power at the sample.
A broadly tunable Vibrant laser system (OPOTEK), which generates ~4 ns laser pulses (FWHM), is also available. The frequency-tripled output from a flash-lamp-pumped Nd:YAG laser pumps an OPO to produce pulses tunable from 400 to 650 nm at 10 Hz with ~35 mJ pulse−1. With appropriate modules this output can be frequency doubled to produce UV pulses from 240 to 380 nm with an average energy of 4 mJ pulse−1. The beam is presently transported to the laser hutch via a 30 m-long optical fiber of diameter 300 µm which limits the pulse energy at the sample to ~150 µJ. The laser beam is focused further to 200 µm diameter, providing power densities at the sample of up to ~4.8 mJ mm−2. Table 3 lists the pulse parameters for the lasers.
3.2. Experimental geometry and timing
To maximize the extent of photoactivation in pump–probe time-resolved X-ray measurements, it is crucial to match the penetration depth of the laser to that of the X-ray beam. To that end, we employ an orthogonal pump–probe geometry and tune the laser wavelength so that its optical penetration depth is comparable with the vertical spot size of the X-ray beam. A 25 µm tungsten pinhole mounted on the goniometer is used to align the laser and X-ray beams. The pinhole is first positioned at the goniometer center of rotation with the horizontal ϕ rotation axis perpendicular to both X-ray (+z) and laser beam (−y) directions. Next, the cross hairs for microscope cameras oriented at two different angles (+30° and +60°) relative to the X-ray beam are centered on the pinhole. To align the X-ray beam, the pinhole is rotated to transmit the X-ray beam and the KB mirror angles are adjusted to maximize transmission through the pinhole. The dimensions of the X-ray beam can be recorded by mounting a small phosphor screen on the goniometer center of rotation and measuring the X-ray spot profile observed on the microscope camera. The laser beam steering motors position the laser focal spot at the center of the microscope cross hairs. Centering the sample on the microscope cross hairs is sufficient to ensure optimal spatial overlap between the pump and probe pulses. The laser position suffers from long-term drift owing to the >30 m path separating the laser and X-ray hutches. To compensate for this drift, the sample is periodically retracted from the beam and an additional microscope camera mounted underneath the crystal records the laser beam position; position errors exceeding about 10% of the beam size are corrected.
Once spatial overlap is achieved, the relative time of arrival Δt of the laser pump and X-ray probe pulses is measured and used to calibrate the time delay. This measurement employs a fast Hamamatsu metal–semiconductor–metal InGaAs photodetector (G7096 series) positioned at the goniometer center of rotation and read out using an Agilent Infiniium 6 GHz 40 GSa s−1 oscilloscope (details to be published elsewhere). The precision of the time delay is limited by the 10 ps resolution of a digital delay generator. The RMS jitter of the measurement was found to be <10 ps, a value small compared with the ~100 ps duration X-ray pulse.
3.3. FPGA control and synchronization
As discussed in previous sections, synchrotron-based pump–probe time-resolved techniques require precise synchronization of multiple X-ray shutters and laser triggers. Additionally, it is also important to be able to quickly change the time delay between the X-ray and laser pulses. These goals have been achieved through the use of a Xilinx Virtex-II Pro field-programmable gate array (FPGA) with an imbedded PowerPC processor. The FPGA technology was implemented via a Suzaku-V FPGA project kit from Atmark Techno that integrates the Xilinx chip into a complete package running Linux with an Ethernet interface and external memory. In addition, an external GigaBaudics, 3 GHz Programmable Delay Line, model PADL3-10-11, is used to generate pump–probe delays over a 20.47 ns range with 10 ps resolution.
All timing is synchronized to the 271.55 kHz P0 signal and 43.99 MHz (storage-ring RF frequency divided by 8) signal provided to every beamline by the APS. The P0 signal provides one pulse every storage-ring orbital period, and is used to phase the FPGA output signals. The 44 MHz signal is multiplied by 8 to produce the original 351.93 MHz RF frequency, which is then divided down to generate all of the sub-harmonic frequencies required to synchronize the chopper controllers, laser mode locker and various triggers.
3.4. Control software and user interfaces
Beamline hardware such as stepper motors, counters, analog-to-digital converters, etc. are controlled using the EPICS package developed in part at the APS. For high-level data acquisition two software packages, written in-house, are routinely used at BioCARS. LaueCollect has been developed primarily for time-resolved pink-beam experiments while xControl is employed mostly for monochromatic data collection. Both programs are written in Python and utilize channel access to communicate with EPICS via the CaPython wrapper (KEK Accelerator Laboratory, Japan). Each program serves as an interface to the MAR165 CCD detector to collect and read out CCD frames and rotate the sample on the goniometer.