Michael R. Furlanetto

Spin exchange rates in proton–hydrogen collisions

The spin temperature of neutral hydrogen, which determines the optical depth and brightness of the 21-cm line, is determined by the competition between radiative and collisional processes. Here, we examine the role of proton‐hydrogen collisions in setting the spin temperature. We use recent fully quantum-mechanical calculations of the relevant cross-sections, which allow us to present accurate results over the entire physically relevant temperature range 1‐10 4 K. For kinetic temperatures T K 100 K, the proton‐hydrogen rate coefficient exceeds that for hydrogen‐hydrogen collisions by about a factor of 2. However, at low temperatures (T K 5 K) H‐H + collisions become several thousand times more efficient than H‐H and even more important than H‐e − collisions.

Free-surface optical scattering as an indicator of the shock-induced solid-liquid phase transition in tin

When highly polished metal surfaces melt upon release after shock loading, they exhibit features that suggest that significant surface changes accompany the phase transition. The reflection of light from such surfaces changes from specular (preshock) to diffuse upon melting. A familiar manifestation of this phenomenon is the loss of signal light observed with a velocity interferometer system for any reflector, which occurs at pressures high enough to melt the free surface. Unlike many other potential material phase-sensitive diagnostics (e.g., reflectometry and conductivity) that show relatively small changes, the specularity of reflection provides a more sensitive and definitive indication of the solid-liquid phase transition. Data are presented that support the hypothesis that specularity changes indicate melt in a way that can be measured easily and unambiguously.

Design and assembly of a telecentric zoom lens for the Cygnus x-ray source

Cygnus is a high-energy radiographic x-ray source. The rod-pinch x-ray diode produces a point source measuring 1 mm diameter. The target object is placed 1.5 m from the x-ray source, with a large LYSO scintillator at 2.4 m. Differentsized objects are imploded within a containment vessel. A large pellicle deflects the scintillator light out of the x-ray path into an 11-element zoom lens coupled to a CCD camera. The zoom lens and CCD must be as close as possible to the scintillator to maximize light collection. A telecentric lens design minimizes image blur from a volume source. To maximize the resolution of test objects of different sizes, the scintillator and zoom lens can be translated along the x-ray axis. Zoom lens magnifications are changed when different-sized scintillators and recording cameras are used (50 or 62 mm square format). The LYSO scintillator measures 200 × 200 mm and is 5 mm thick. The scintillator produces blue light peaking at 435 nm, so special lens materials are required. By swapping out one lens element and allowing all lenses to move, the zoom lens can also use a CsI(Tl) scintillator that produces green light centered at 550 nm. All lenses are coated with anti-reflective coating for both wavelength bands. Two sets of doublets, the stop, and the CCD camera move during zoom operations. One doublet has XY compensation. The first three lenses use fused silica for radiation damage control. The 60 lb of glass inside the 340 lb mechanical structure is oriented vertically.

A fisheye lens as a photonic Doppler velocimetry probe

A new fisheye lens design is used as a miniature probe to measure the velocity distribution of an imploding surface along many lines of sight. Laser light, directed and scattered back along each beam on the surface, is Doppler shifted by the moving surface and collected into the launching fiber. The received light is mixed with reference laser light in each optical fiber in a technique called photonic Doppler velocimetry, providing a continuous time record. An array of single-mode optical fibers sends laser light through the fisheye lens. The lens consists of an index-matching positive element, two positive doublet groups, and two negative singlet elements. The optical design minimizes beam diameters, physical size, and back reflections for excellent signal collection. The fiber array projected through the fisheye lens provides many measurement points of surface coverage over a hemisphere with very little crosstalk. The probe measures surface movement with only a small encroachment into the center of the cavity. The fiber array is coupled to the index-matching element using index-matching gel. The array is bonded and sealed into a blast tube for ease of assembly and focusing. This configuration also allows the fiber array to be flat polished at a common object plane. In areas where increased measurement point density is desired, the fibers can be close packed. To further increase surface density coverage, smaller-diameter cladding optical fibers may be used.

Design, construction, alignment, and calibration of a compact velocimetry experiment

A velocimetry experiment has been designed to measure shock properties for small cylindrical metal targets (8-mm-diameter by 2-mm thick). A target is accelerated by high explosives, caught, and retrieved for later inspection. The target is expected to move at a velocity of 0.1 to 3 km/sec. The complete experiment canister is approximately 105 mm in diameter and 380 mm long. Optical velocimetry diagnostics include the Velocity Interferometer System for Any Reflector (VISAR) and Photon Doppler Velocimetry (PDV). The packaging of the velocity diagnostics is not allowed to interfere with the catchment or an X-ray imaging diagnostic. A single optical relay, using commercial lenses, collects Doppler-shifted light for both VISAR and PDV. The use of fiber optics allows measurement of point velocities on the target surface during accelerations occurring over 15 mm of travel. The VISAR operates at 532 nm and has separate illumination fibers requiring alignment. The PDV diagnostic operates at 1550 nm, but is aligned and focused at 670 nm. The VISAR and PDV diagnostics are complementary measurements and they image spots in close proximity on the target surface. Because the optical relay uses commercial glass, the axial positions of the optical fibers for PDV and VISAR are offset to compensate for chromatic aberrations. The optomechanical design requires careful attention to fiber management, mechanical assembly and disassembly, positioning of the foam catchment, and X-ray diagnostic field-of-view. Calibration and alignment data are archived at each stage of the assembly sequence.

18th APS-SCCM and 24th AIRAPT

This second joint conference between the APS Topical Group on Shock Compression of Condensed Matter and the International Association for the Advancement of High Pressure Science and Technology (AIRAPT) demonstrates that static and dynamic compression of condensed matter continues to be a vibrant field of science and engineering. It is also by its nature an interdisciplinary field, incorporating chemistry, materials science, solid mechanics, plasma physics, and condensed matter physics, and utilizes theoretical, computational, and experimental tools. Recent years have brought about many advances in loading platforms, diagnostics, and computations that are leading to the emergence of many new avenues of research. These advances are also breathing new life into traditional topics such as equations of state, phase transformations, and chemistry at extreme conditions. The plenary lectures by Gennady Kanel, Karl Syassen, David Ceperley, Jon Eggert, Duck Young Kim, and Richard Kraus spanned the disciplines of static and dynamic high pressure physics and illustrated the breadth of the field. They also showed that interesting and important problems remain for researchers of the future to solve. The main guiding principal in the organization of this conference was to intertwine static and dynamical experimental alongside computational and theoretical studies of similar materials. To achieve this goal, we arranged the conference to include static, dynamic, and computational components in the same sessions, quite often taking presenters out of their comfort zone. The three special sessions on Deep Carbon Budget (organized by Giulia Galli and Rus Hemley), High Energy Density Materials (organized by Raymond Jeanloz and Jon Eggert), and Dynamic Response of Materials (organized by Yogendra Gupta and John Sarrao) furthered this guiding principal. We also endeavored to represent the breadth of static and dynamic high pressure science and technology, notably beyond that done at national laboratories. To this end, a significant fraction of the plenary, invited and contributed presentations showcased work done in academia, defense laboratories and industry, as well as internationally. Although travel distance and visa issues always present difficulties, the conference had strong representation from a record number of international participants, including sizable groups from Russia and China (thanks to Tony Zocher and Frank Cherne), as well as Japan, the United Kingdom, France, Canada, Germany, Israel, and Italy. It is our sincere hope that international interactions that occurred at the conference will lead to further collaborations in the future. Finally, we strived to increase student participation at the conference. Through the leadership of Scott Alexander and his committee, a new all-day student symposium was held the day before the main conference, with only student attendees and presenters, in order to acclimate the students to conference participation and help them network with their peers. In cooperation with the APS Topical Group and the AIRAPT and with additional support from DTRA and the AWE, the conference was able to provide financial assistance to a large number of students to attend the conference and present their research. This aid helped increase the number of student attendees significantly over previous conferences. Finally, the conference sponsored a networking lunch for students and representatives from a number of laboratories and other institutions, which was well attended. Seattle proved itself to be an excellent venue for the conference. The international flavor of the city provided ample dining options and numerous activity choices outside of the conference sessions. The major international airport made travel as easy as possible, as Seattle is a convenient central location for attendees from Europe and Asia. The conference was truly a team effort with critical contributions from many individuals. We deeply appreciate their contributions to the success of the conference and the publication of these proceedings. Gilbert (RIP) Collins David S Moore Choong-Shik Yoo

Design, assembly, and testing of a photon Doppler velocimetry probe

A novel fiber-optic probe measures the velocity distribution of an imploding surface along many lines of sight. Reflected light from each spot on the moving surface is Doppler shifted with a small portion of this light propagating backwards through the launching fiber. The reflected light is mixed with a reference laser in a technique called photon Doppler velocimetry, providing continuous time records. Within the probe, a matrix array of 56 single-mode fibers sends light through an optical relay consisting of three types of lenses. Seven sets of these relay lenses are grouped into a close-packed array allowing the interrogation of seven regions of interest. A six-faceted prism with a hole drilled into its center directs the light beams to the different regions. Several types of relay lens systems have been evaluated, including doublets and molded aspheric singlets. The optical design minimizes beam diameters and also provides excellent imaging capabilities. One of the fiber matrix arrays can be replaced by an imaging coherent bundle. This close-packed array of seven relay systems provides up to 476 beam trajectories. The pyramid prism has its six facets polished at two different angles that will vary the density of surface point coverage. Fibers in the matrix arrays are angle polished at 8°to minimize back reflections. This causes the minimum beam waist to vary along different trajectories. Precision metrology on the direction cosine trajectories is measured to satisfy environmental requirements for vibration and temperature.

A data-driven approach for determining time of initial movement in shock experiments using photonic Doppler velocimetry

Photonic Doppler Velocimetry is an interferometric technique for measuring the beat frequency of a moving surface, from which the calculated velocity profile of the surface can be used to describe the physical changes the material undergoes after high-impact shock. Such a technique may also be used to characterize the performance of small detonators and determine the time at which the surface began moving. In this work, we develop a semi-automated technique for extracting the time of initial movement from a normalized lineout of the power spectrogram near the offset frequency of each probe. We characterize the response bias of this method and compare with the time of initial movement obtained by hand calculation of the raw voltage data. Results are shown on data from shock experiments such as gas gun setups and explosives-driven flyer plates.