Point-projection Backlighting Research Articles

Performance of an elliptical crystal spectrometer for SGII X-ray opacity experiments

A new crystal spectrometer for application in X-ray opacity experiments is proposed. The conditions necessary to yield broad spectral coverage with a resolution${>}$500, strong rejection of hard X-ray backgrounds and negligible source broadening for extended sources are formulated. In addition, the design, response modeling and reporting of an elliptical crystal spectrometer in conjunction with a linear detector are presented. The measured results demonstrate the performance of the new crystal spectrometer with a broad energy coverage range, high spectral resolution, and high luminosity (good collection efficiency). This spectrometer can be used in combination with point-projection backlighting techniques as utilized in X-ray opacity experiments. Specifically, the X-ray source, transmission and self-emission spectra of the sample can be measured simultaneously in a single shot, which can reduce the experimental uncertainties from shot-to-shot fluctuations. The new crystal spectrometer has been used in the X-ray opacity experiment to precisely measure the aluminum$K$-absorption edge shift in the energy range around 1.560 keV in strongly compressed matter. It is demonstrated that the spectrometer can be used to realize measurements of new and unpredictable physical interactions of interest, as well as basic and applied high-energy-density science.

End-on x-ray backlighting experiments for axial diagnostics of wire-array Z-pinch plasma on PPG-1**Project supported by the National Natural Science Foundation of China (Grant Nos. 11177086 and 51177086), Guangdong Innovative Research Team Program of China (Grant No. 2011S013), the National Key Research and Develop Program of China (Grant No. 2016YFC0105102), the National Basic Research Program of China (Grant Nos. 2012AA02A604 and

An X-pinch axial backlighting system has been designed to quantitatively measure the density distribution of wire-array Z-pinch plasmas. End-on backlighting experiments were carried out on a 200 kA, 100 ns pulsed-power generator (PPG-1) at the Tsinghua University. Compared with side-on backlighting, end-on measurements provide an axial view of the evolution of Z-pinch plasmas. Early stages of 2-, 4-, and 8-wire Z-pinch plasmas were observed via point-projection backlighting radiography with a relatively high success rate. The density distribution of Z-pinch plasma on the r–θ plane was obtained directly from the images with the help of step wedges, and the inward radial velocity was calculated. The ablation rates obtained by X-pinch backlighting experiments are compared in detail with those calculated by the rocket model and the results show consistency.

Development of a Big Area BackLighter for high energy density experiments.

A very large area (7.5 mm(2)) laser-driven x-ray backlighter, termed the Big Area BackLighter (BABL) has been developed for the National Ignition Facility (NIF) to support high energy density experiments. The BABL provides an alternative to Pinhole-Apertured point-projection Backlighting (PABL) for a large field of view. This bypasses the challenges for PABL in the equatorial plane of the NIF target chamber where space is limited because of the unconverted laser light that threatens the diagnostic aperture, the backlighter foil, and the pinhole substrate. A transmission experiment using 132 kJ of NIF laser energy at a maximum intensity of 8.52 × 10(14) W/cm(2) illuminating the BABL demonstrated good conversion efficiency of >3.5% into K-shell emission producing ~4.6 kJ of high energy x rays, while yielding high contrast images with a highly uniform background that agree well with 2D simulated spectra and spatial profiles.

Streaked radiography of an irradiated foam sample on the National Ignition Facility

Streaked x-ray radiography images of annular patterns in an evolving tantalum oxide foam under the influence of a driven, subsonic radiation wave were obtained on the National Ignition Facility. This is the first successful radiography measurement of the evolution of well-defined foam features under a driven, subsonic wave in the diffusive regime. A continuous record of the evolution was recorded on an x-ray streak camera, using a slot-apertured point-projection backlighter with an 8 ns nickel source (7.9 keV). Radiography images were obtained for four different annular patterns, which were corrected using a source-dependent flat-field image. The evolution of the foam features was well-modeled using the 3D KULL radiation hydrodynamics code. This experimental and modeling platform can be modified for scaled high-energy-density laboratory astrophysics experiments.

The experimental research of pinhole point backlight based on Shenguang-Ⅲ proto-type facility

A new type of pinhole-assisted point backlighter developed and optimized based on experimental research performed on Shenguang-Ⅲ proto-type facility is presented. High quality images of tungsten micro wires and capsule are acquired with a 4.75 keV X-ray point source produced by 1600 J/1 ns/351 nm laser interaction with 3 μm Ti target. Detailed parameters of this backlighter are also obtained using a multiple diagnostic device. The results indicate that pinhole-assisted point-projection backlighter has advantages of high intensity and high spatial resolution, and can be widely used in high energy density physics research.

Experimental studies on the opacity of dense aluminum compressed by a laser-driven shock waves

The high-precise opacity of the dense plasma has important applications in the design and simulation of fusion research, and in plasma diagnostics. Base on the novel technique of point-projection backlighting, a broadband high-resolution elliptical crystal X-ray spectrometer, which is used to measure simultaneously the self-emission spectrum, the backlighting source spectrum, and the transmission spectrum in one shot, is designed on the Shengguang-II laser facility. The process of the colliding-shock-compressed sample by laser-driven shock waves is also investigated using a one-dimensional radiation hydrodynamics code MULTI. In the measurement, the dense plasma, produced in aluminum by colliding shocks driven by laser beams, reaches a peak density several times that of a solid, and the short backlighting from the 3d-4f transition bands of ytterbium is used as an absorption source for time- and space-resolving diagnostics. Several experimental results are obtained, they are the X-ray source spectrum, the transmission spectrum, and the self-emission spectrum of the dense Al sample in one shot obtained by using the point-projection method, as well as X-ray-absorption fine-structure spectra, and the changes in the K-shell photo-absorption edge of aluminum as it was compressed by a laser-driven shock waves. The transmissivity distribution and red- shift around 80 m (with respect to the cold value of 1.56 keV) of the dense aluminum are also obtained. The data obtained are further analysed. As a result, a new theoretical model is developed.

神光-Ⅱ激光装置上的针孔点背光成像技术实验研究

神光-Ⅱ激光装置上的针孔点背光成像技术实验研究

Long duration backlighter experiments at Omega

We have successfully demonstrated a 7.5 ns duration pinhole-apertured backlighter at the Omega laser facility. Pinhole-apertured point-projection backlighting for 8 ns will be useful for imaging evolving features in experiments at the National Ignition Facility. The backlighter consisted of a 20 microm diameter pinhole in a 75 microm thick Ta substrate separated from a Zn emitter (9 keV) by a 400 microm thick high-density carbon piece. The carbon prevented the shock from the laser-driven surface from reaching the substrate before 8 ns and helped minimize x-ray ablation of the pinhole substrate. Grid wires in x-ray framing camera images of a gold grid have a source-limited resolution significantly smaller than the pinhole diameter due to the high aspect ratio of the pinhole, but do not become much smaller at late times.

Numerical Simulations and Astrophysical Applications of Laboratory Jets at Omega

We have conducted experiments on the Omega laser at the University of Rochester that have produced jets of supersonic Ti impacting and being deflected by a ball of high density plastic. These mm-sized jets of dense plasma are highly complex, have large Reynolds numbers, and, given sufficient time and shear, should produce a fully turbulent flow. The experiments are diagnosed with a point-projection backlighter, resulting in a single image per shot. Simulations of the 3D hydrodynamics capture the large-scale features of the experimental data fairly well while missing some of the smaller scale turbulent-like phenomena. This is to be expected given the limited characterization of the targets as well as the finite resolution of the 3D simulations. If Euler scaling holds, these experiments should model larger astrophysical jets in objects such as HH 110 where an outflow can be seen colliding with a molecular cloud. However, Euler scaling demands that not only the isothermal internal Mach numbers of the two systems be similar but also that any dissipative mechanisms, such as radiative cooling or viscous dissipation, be of equal importance relative to each other. Similar equations of state are required as well. We discuss such issues in the context of these experiments and simulations.

High-energy point-projection radiography of a driven, shielded Hohlraum

Point projection backlighting is a high magnification, high resolution x-ray backlighting configuration that provides detailed images of large objects at a reduced energy budget compared to area backlighting. However, point projection backlighting has the drawback that the camera aperture is large and open to more sources of noise. This issue has made it difficult to image driven targets without shielding. We present 9keV x-ray images of the rear end of a driven Hohlraum that employs shielding. We also present data of a 10.3keV x-ray image of an undriven Hohlraum.

Use of X-pinches of diagnose behavior of low density CH foams on axis of wire array Z-pinches

X -pinch radiography was used to analyze the interaction between streams of coronal plasma and on-axis foam targets in wire array z-pinch experiments on the MAGPIE generator (1 MA,240 ns). The implosion of the x-pinch, used in place of a current return conductor to the load, provided a short (<2 s) small (∼5 μm) intense burst of soft x-rays, ideal for point projection backlighting. Timimg of the x-pinch was adjusted via the mass of its wires, allowing us to study the evolution of the foam during the experiment. Choice of the x-pinch materials, filters, and recording film determined the probing radiation, and hence the plasma/foam densities were resolved. Quantitative results will be discussed.

Improved pinhole-apertured point-projection backlighter geometry

Pinhole-apertured point-projection x-ray radiography is an important diagnostic technique for obtaining high resolution, high contrast, and large field-of-view images used to diagnose the hydrodynamic evolution of high energy density experiments. In this technique, a pinhole aperture is placed between a laser irradiated foil (x-ray source) and an imaging detector. Future high energy density experiments that utilize more opaque materials will require backlighters with improved contrast as compared to what is currently used. In this article, we present an improved backlighter geometry that utilizes a tilted pinhole for debris mitigation and a front-side illuminated backlighter foil for improved photon statistics.

National Ignition Facility scale hohlraum asymmetry studies by thin shell radiography

A necessary condition for igniting indirectly driven inertial confinement fusion (ICF) capsules on the National Ignition Facility (NIF) is controlling drive asymmetry to the 1% level [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. Even flux-asymmetry modes (e.g., Legendre modes P2, P4, P6, and P8) must be reduced by hohlraum design and laser beam pointing. Odd flux-asymmetry modes (e.g., Legendre modes P1, P3, P5, etc.) are theoretically removed by reflection symmetry across the hohlraum midplane [S. M. Pollaine and D. Eimerl, Nucl. Fusion 38, 1523 (1998)], but will be produced by power imbalance, laser beam pointing errors, and target fabrication errors. An experimental campaign is now being conducted on the University of Rochester’s Omega laser to measure higher order (P4 and higher) flux asymmetry modes inside hohlraums that approximate the conditions of a NIF hohlraum during the 90 eV early drive phase [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. These experiments use a new point-projection backlighting technique [O. L. Landen, D. R. Farley, S. G. Glendinning et al., Rev. Sci. Instrum. 72, 627 (2001)] to cast high quality 4.7 keV radiographs of thin 2 mm diameter Ge-doped CH shells designed to enhance sensitivity to drive asymmetries. Distortions in the position of the limb of the shells resulting primarily from drive asymmetries are measured to an accuracy of 2 μm. The linearity and sensitivity of thin imploding shells to flux asymmetry makes it possible to achieve this degree of accuracy, which is sufficient for NIF ignition symmetry tuning. The promising results to date permit the comparison of measured and predicted distortions and, by inference, drive asymmetries for the first eight asymmetry modes.

10 and 5 μm pinhole-assisted point-projection backlit imaging for the National Ignition Facility

In pinhole-assisted point-projection backlighting, pinholes are placed a small distance (of order 1 mm) away from the backlighter source to produce images with large field of view. Pinholes placed close to high-power backlighter sources can vaporize and, if sufficiently small, close due to x-ray driven ablation, thereby potentially limiting the usefulness of this method. A study of streaked one-dimensional backlit imaging of 25 μm W wires using the OMEGA laser at the University of Rochester is presented. The pinhole closure time scale for 10 μm pinholes placed 0.45 and 1 mm distant from a 0.6 TW Ti backlighter is 1.3 and 2.2 ns, respectively. Similar time scales for 5 μm pinholes is also presented. Successful wire imaging prior to pinhole closure is clearly demonstrated.

Study of K alpha absorption structures in a subcritical-density laser-produced plasma.

The study of K\ensuremath{\alpha} line absorption by point-projection backlighting is presented for a laser-produced aluminum plasma at electronic densities below 5\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The appearance of ionic species as a function of distance from the target is measured absolutely by the use of a knife edge as a spatial fiducial. These data are used to determine the gradient of the average charge as a function of space. A non-local-thermodynamic-equilibrium ionization model has been developed to determine temperature and densities consistent with this measure. Comparisons with hydrodynamic codes are presented, along with a discussion of the inferred temperature.

X-ray absorption spectroscopy of laser-produced plasmas: A study of the experiment and data analysis

The novel technique of point-projection backlighting is used to probe a laser-produced plasma. A spatially restricted source of x rays is created by irradiating a fiber which is used to backlight a plasma. Temporal resolution of the absorption spectrum is obtained by creating the backlight with a pulse that is delayed in time from the main, plasma-producing, laser. The backlight can also be of shorter duration than the main laser pulse providing for enhanced time resolution and therefore less temporal averaging. The results for low-Z aluminum and silicon targets are presented. The low-Z absorption spectra using both low-Z (e.g., aluminum) and high-Z (e.g., molybdenum) elements for backlighting are studied. A description of this technique and the analysis of the data obtained is presented.