Dennis M. Mills

A unique polarized x-ray facility at the advanced photon source

To use the unique element-specific nature of polarized x-ray techniques to study a wide variety of problems related to magnetic materials, we have developed a dual-branch sector that simultaneously provides both hard and soft x-ray capabilities. This facility, which is located in sector 4, is equipped with two different insertion devices providing photons in both the intermediate (0.5–3 keV) and hard x-ray regions (3–100 keV). This facility is designed to allow the simultaneous branching of two undulator beams generated in the same straight section of the ring.

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 historical development of cryogenically cooled monochromators for third-generation synchrotron radiation sources

In the period of the late-1980s, before the construction of multi-GeV third-generation storage rings with their intense insertion-device sources, the perceived number one problem for X-ray instrumentation was proper cooling of the first optical element in the beamline. This article, first given as an acceptance speech for the Compton Award ceremony at the Advanced Photon Source, presents a somewhat historical and anecdotal overview of how cryogenically cooled monochromator optics have been developed to provide a monochromator cooling solution adequate for todays power levels. A series of workshops and international collaborations were the key components for the progress and final success of this development.

Time‐resolution experiments using x‐ray synchrotron radiation

Many important biological, chemical and physical phenomena take place on time scales of nanoseconds or picoseconds. Those working to unravel the time development of such fast processes have long recognized that pulsed electromagnetic radiation and particle beams often make more incisive probes than do continuous emissions. During the last decade, a powerful new device joined the arsenal of modulated radiation sources available to scientists attacking problems that require good temporal resolution—the high‐energy storage ring.

Feasibility study into the use of mechanical choppers to alter the natural time structure of the APS

In standard operating mode, the Advanced Photon Source (APS) is projected to be with 20 equally spaced bunches of positrons separated by 177 ns. This time structure may not be optimal for all experiments, particularly those experiments that require long periods (microseconds to milliseconds) of darkness preceding and/or succeeding the x‐ray burst. As a result, an ongoing study is being made into the possibility of altering the natural time structure of the x‐ray bursts from the APS. One possibility is through the use of fast kicker magnets (wobblers). A more passive approach is through the use of high‐speed mechanical choppers. Using choppers rotating between 18 000 and 20 000 rpm (typical rotational speeds for currently operational neutron choppers), a single burst of x rays can be passed while blocking the surrounding bursts. Interpulse periods are adjustable from approximately 3 to 300 μs depending on the slit configuration of the chopper wheels. Even longer periods of darkness can be achieved by using...

Dynamical diffraction of ultrashort X-ray free-electron laser pulses

Calculations are presented for the femtosecond time-evolution of intensities of beams diffracted by perfect Bragg crystals illuminated with radiation expected from X-ray free-electron lasers (XFELs) operating through the self-amplified spontaneous emission (SASE) process. After examining the case of transient diffraction of an electromagnetic delta-function impulse through flat, single- and double-crystal monochromators, the propagation of a 280 fs-duration SASE XFEL pulse of 8 keV photons through the same optics is discussed. The alteration of the sub-femtosecond spiky microbunched temporal structure of the XFEL pulse after it passes through the system is shown for both low-order (broad bandwidth) and high-order (narrow bandwidth) crystal reflections. Finally, the shot-to-shot statistical fluctuations of the integrated diffracted intensity is simulated. Implications of these results for XFEL applications are addressed.

A new high-speed beam chopper for time-resolved X-ray studies

A new high-speed X-ray beam chopper, which can be phase locked to the temporal structure of the Advanced Photon Source (APS) storage ring, has been developed and tested. The open window time of the chopper is 2450 ns, which corresponds to approximately 67% of the revolution time of the APS storage ring. By phase locking the rotation of the beam chopper to the storage-ring orbital frequency, any portion of the storage-ring fill pattern can be positioned within the beam-chopper transmission-time window.

Test of a high-heat-load double-crystal diamond monochromator at the Advanced Photon Source.

We have tested the first diamond double-crystal monochromator at the Advanced Photon Source (APS). The monochromator consisted of two synthetic type 1b (1 1 1) diamond plates in symmetric Bragg geometry. The single-crystal plates were 6 mm × 5 mm × 0.25 mm and 6 mm × 5 mm × 0.37 mm and showed a combination of mosaic spread/strain of the order of 2–4 arcsec over a central 1.4 mm-wide strip. The monochromator first crystal was indirectly cooled by edge contact with a water-cooled copper holder. We studied the performance of the monochromator under the high-power X-ray beam delivered by the APS undulator A. By changing the undulator gap, we varied the power incident on the first crystal and found no indication of thermal distortions or strains even at the highest incident power (200 W) and power density (108 W/mm2 in normal incidence). The calculated maximum power and power density absorbed by the first crystal were 14.5 W and 2.4 W/mm2, respectively. We also compared the maximum intensity delivered by this monochromator and by a silicon (1 1 1) cryogenically cooled monochromator. For energies in the range 6–10 keV, the flux through the diamond monochromator was about a factor of two less than through the silicon monochromator, in good agreement with calculations. We conclude that water-cooled diamond monochromators can handle the high-power beams from the undulator beamlines at the APS. As single-crystal diamond plates of larger size and better quality become available, the use of diamond monochromators will become a very attractive option.

Performance of a gallium-cooled 85° inclined silicon monochromator for a high power density X-ray beam

We have made double crstal rocking curve measurements on a gallium-cooled silicon monochromator in both the normal flat geometry and an 85° inclined geometry on the X-25 focused wiggler beamline at the National Synchrotron Light Source. At 192 mA ring current, the focused wiggler delivers about 37.7 W of power into a spot size of FWHM 0.4 × 0.8 mm2, resulting in an average power density of about 118 W/mm2. The inclined crystal geometry spreads the beam footprint on the surface of the crystal while maintaining a b = −1 symmetric Bragg reflection. At an 85° inclination angle, the beam footprint is 11.5 times larger than that for the flat geometry. In the case of the flat geometry at a ring current of 156 mA, we see, via an infrared camera, an increase in temperature of 56°C above the nominal silicon temperature. The rocking curve this case were significantly broadened (FWHM for 15 keV Si(333) = 35 arcsec) due to the thermally induced strain in the silicon. In the inclined crystal, the thermal peak on the crystal was only about 2.7°C above the nominal silicon temperature. In this case, the rocking curve width for the 15 keV Si(333) reflection was measured to be FWHM = 2.7 arc sec compared with the theoretical width of FWHM = 1.0 arcsec. The residual strain is totally due to the mounting of the crystals and not the heating from the X-ray beam.

Thermal Design of Synchrotron Radiation Exit Ports at CESR

CESR, running at the maximum design parameters (8 GeV, 100 ma), produces 177 watts/mrad of synchrotron radiation at the exit ports for CHESS (Cornell High Energy Synchrotron Source). Due to the low angle of incidence, this corresponds to a linear heat loading of 55 watts/cm at the normal vacuum chamber wall. At the exit line crotch, radiation striking at normal incidence results in an average linear load of 885 watts/cm. For a beam height of 0.12 mm this translates to a power density of 740 watts/mm2. We present a design for a crotch which can effectively dissipate this high power density and will be compatible with the ultra-high vacuum system of CESR. The structure is a composite of a beryllium heat diffuser and an axially cooled copper cylinder. At 8 GeV and 100 ma we anticipate no component temperatures higher than 330°C.