Flash Radiography Research Articles

Compact hard x-ray flash radiography device based on wire-shorted low-impedance rod pinch diode.

A rod pinch diode (RPD) is a feasible load configuration to generate a high-brightness, small-size hard x-ray radiation source. In this paper, the radiography performance of a wire-shorted low-impedance RPD on a compactly designed table-top driver (WRPD-1) is demonstrated for the first time. The driver consists of four high-power discharge branches connected in parallel, with each branch consisting of two metal-film capacitors and one multigap field-distortion switch in series. The four branches are triggered synchronously to generate a fast-rising current pulse: the inductance of the load section at the short circuit is ∼10 nH, and the short-circuit current amplitude is ∼325kA at ±90kV charging voltage, with a 10%-90% rise time of 110ns. With a low-impedance RPD shorted by an 18-µm-diameter aluminum wire, a quasi-spherical x-ray focal spot with diameter <0.6mm (width of the half-maximum grayscale) and a pulse duration of ∼25ns (half-width of the radiation pulse) is obtained at ±70kV charging voltage, and the imaging resolution excels 10 lp/mm under 1.56× magnification. According to the transmission-absorption x-ray spectrum estimation, the average emitted photon energy is ∼30keV with a distinct peak in the 10-15keV range that corresponds to the L-lines of tungsten, and the total energy of photons >10keV reaches ∼1.16J. The present results show that the device can serve well for the flash radiography diagnosis and potentially as an efficient light source for dynamic x-ray diffraction.

Achieving high-strength porous tungsten with wide porosity range via selective dissolution W-Ti-Fe precursor

Porous tungsten with wide porosity variation is considered a candidate for high-energy X-ray flash radiography. This work combines tape casting and dealloying to selectively dissolve a W-Ti-Fe precursor, achieving high-strength porous tungsten with a wide porosity range. The PVB exists on the surface of particles in the form of an amorphous carbon film after pyrolysis, providing conditions for the W-Fe-C interface reaction. The porosity of porous tungsten is 50.7–80.4%. The compressive strengths of porous tungsten with porosity of 50.7% and 80.4% are 221.9 MPa and 10.6 MPa, respectively. The dissolution of the Ti located in the ligament can enhance the porosity of porous tungsten. The coating composed of the solid solution formed at the interface of W and Ti during heat treatment and Fe3W3C can enhance the ligament strength. This work provides a theoretical reference for preparing porous tungsten with a wide porosity range and high performance.

Fragmentation of a molten metal droplet in an ambient water flow

The influence of an ambient fluid flow on the fragmentation of hot molten tin droplets (initially at 800°C) and cold low melting point alloy droplets (initially at 70°C) in water is investigated with high-speed photography and flash radiography. The water is accelerated using a converging nozzle to a constant speed of up to 30 m/s using a double piston arrangement designed to eliminate the formation of a shock wave that is present in most earlier studies. At low flow velocities, the fragmentation of hot droplets is governed by thermal effects and vapour formation, growth, and collapse. At high flow velocities, vapour formation is suppressed and the droplet fragmentation is determined by hydrodynamic effects in which hydrodynamic instabilities (Rayleigh-Taylor and Kelvin-Helmholtz) and wavecrest stripping all play a role in the droplet breakup. At intermediate flow velocities, both thermal and hydrodynamic effects play a role. Quantitative image analysis of the radiographs is used to determine the spatial distribution of the droplet mass during the fragmentation process. Comparison with earlier work in which the ambient flow is preceded by a strong shock wave indicates that the transition from thermal to hydrodynamic breakup is strongly dependent on the pressure field.

Sub-Millimeter Hard X-Ray Source Based on Wire-Shorted Low-Impedance Rod Pinch Diode

Rod pinch diode (RPD) is an intense X-ray point source with a millimeter spot diameter, tens of nanosecond pulsewidth, and tens of keV to MeV photon energy, resulting in extraordinary flash radiography performance. Classical RPD is featured with a high-impedance vacuum gap between the annular cathode and the central anode rod, which limits the electron current and electrical power density, therefore is not conducive to the improvement of brightness. Plasma prefilling can effectively reduce the RPD impedance, thereby increasing the electron current and realizing an impedance match of RPD with low-impedance, high-power-density drivers. In this article, an experimental study on plasma-prefilled RPD is carried out based on a fast linear transformer driver (FLTD) with relatively low charging voltage; the RPD anode and cathode are initially connected by metal wires, which are transformed into plasma shortly after the onset of discharge current. A quasi-spherical X-ray spot is obtained with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 0.45 mm diameter, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 30 ns pulsewidth, 0.6 rad@1 m (LiF) dose, and 55–60 keV equivalent mono-energy photon energy. The radiography performance is further characterized by a tungsten rolled-edge and slit plate. The preliminary results proved the feasibility of low-impedance RPD as a flash radiography source within the relatively low photon energy range.

Pulse Risetime and Corkscrew Excitation of Beam Breakup in Linear Induction Accelerators

Beam breakup (BBU) in linear induction accelerators (LIA) used for flash radiography is problematic because the high-frequency beam motion can blur the source spot, thereby degrading resolution. BBU can be excited by the fast risetime of an offset current pulse, it can grow from noise due to other sources of RF, or it can be excited by the corkscrew motion of the beam head. In this article, numerical experiments are used to examine BBU excited by the fast risetime of the current pulse expected to be injected into an advanced LIA. Excitation by high-frequency corkscrew on the beam head is also investigated. The results of these simulations indicate that BBU can be expected to be excited by both fast risetimes and corkscrew.

Dual-pulse generation from a velvet cathode with a new inductive voltage adder for x-ray flash radiography applications

The evolution of two intense pulsed electron beams for an x-ray flash radiography application is characterized in a coupling of experimental and simulation studies. For this purpose, experiments were performed at Commissariat \`a l'Energie Atomique et aux Energies Alternatives with a new dual-pulse inductive voltage adder named ``Mi2.'' This experimental setup has been designed and built to produce two intense pulsed electron beams of 2 kA, 700 keV, and 80 ns full width at half maximum from a velvet cold cathode. The delay between the pulses ranges from tens of nanoseconds to a few microseconds. This study quantifies the effect of the plasma dynamics on the beam current of the second pulse, which represents an important parameter from the beam transport point of view. A multidimensional particle-in-cell simulation model is used to provide a numerical analysis in order to determine the influence of velvet plasma dynamics on the second pulse. Analysis of the experimental results indicates that the motion of the electron extraction zone within the expanding plasma remains lower than 1 mm for $3\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ delay, inducing a minor impact on the beam current extracted from the velvet cathode. Based on the developed model, this plasma expansion induces an increase of 5% for the current extracted from the second pulse with a delay of $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ for two pulses having the same voltage, which is consistent with linear induction accelerator injector parameters for multiple pulse use.

Quasi-isentropic compression of LiH above 400 GPa using magnetocumulative generator.

The knowledge of high-pressure behavior of LiH is significant for the validation of fundamental theoretical models and applications in thermonuclear materials and potential energy supplies. The compressibility of 7LiH under isentropic compression at high pressure was investigated experimentally and theoretically. The experimental technique for quasi-isentropic compression with low-density materials was developed using the magnetocumulative generator CJ-100 and x-ray flash radiography. The x-ray images and extracted interface of the sample target in dynamic flash radiography experiments were obtained. According to each interface size of the target both before and after compression, the compression ratio of 7LiH and reference material aluminum was obtained. The density of the reference and using its known isentropic curve provide the pressure in the reference. The pressure in 7LiH was deduced from the pressure in the reference and using the calculated gradient correction factor. The quasi-isentropic data point at 438 GPa was obtained experimentally. A semiempirical three-term complete equation of state was constructed and validated for 7LiH using the theory of Mie-Grüneisen-Debye with experimental data from the literature. The quasi-isentrope data point is reasonably consistent with the theoretical results. The quasi-isentropic experimental techniques and results broaden the existing research scope and are practical and helpful to further validate theoretical models in the future.

Beam Breakup Simulations for the Scorpius Flash-Radiography Accelerator

Beam breakup (BBU) in linear induction accelerators (LIAs) used for flash radiography is problematic because the high-frequency beam motion can blur the source spot, thereby degrading resolution. Amplification of BBU depends directly on details of cell design and is suppressed in operation by the solenoidal magnetic field focusing on the electron beam. Therefore, much effort has gone into design of the cells and magnetic focusing for the new Scorpius radiography LIA. In this article, we use computer simulations to demonstrate that BBU in Scorpius should be no more than in present radiography LIAs at the Los Alamos National Laboratory.

Study on particle generation model of anode in rod-pinch diode

&lt;sec&gt;Flash radiography technology is commonly used in detonation physics experiments and nondestructive testing, in which an X-ray diode is an integral part of flash radiography equipment. Its function is to convert the electric energy stored in the front power supply into X-rays through the bremsstrahlung effect. Rod-pinch diode is one of the most commonly used X-ray diodes in 1–4 MV. It has the characteristics of a small focal spot and high imaging resolution. The anode ions of the rod pinch diode come from the anode plasma, and the anode plasma electrons are generated at the same time as the anode plasma ions. Before the establishment of the bipolar current between the anode and cathode of the rod pinch diode, these electrons are mainly absorbed by the anode; however, after the formation of the bipolar current, due to the zero electric field on the anode surface, plasma electrons will accumulate near the anode surface and will not be absorbed. Given the theoretical derivation of bipolar current in the gap between anode and cathode of rod pinch diode in the early stage, it is necessary to study the electrons in anode plasma.&lt;/sec&gt;&lt;sec&gt;The simulation of the rod-pinch diode is an essential tool for improving the performance of the rod-pinch diode. To improve simulation accuracy, it is necessary to study the emission mechanism of cathode and anode particles and continuously optimize the simulation model. In this paper, PIC and Monte Carlo simulation are used. An anode plasma model is proposed in this work based on the anode ion emission model of the rod-pinch diode and the characteristics of space charge bipolar flow, that is, when the anode plasma environment is fully established, the electric field on the anode surface is zero, and ions and electrons will accumulate on the anode surface. The new model is analyzed in detail and compared with the anode ion emission model in rod-pinch diode current, electromagnetic field distribution between cathode and anode, electron energy spectrum, motion state of electron incident anode rod, dose, and spot size. The results show that the calculation results from the new model are closer to the experimental results, which shows that the role of electrons accumulated near the anode in the plasma cannot be ignored in the numerical calculation of the rod-pinch diode anode rod surface plasma.&lt;/sec&gt;

Design and performance of a pulsed power-driven x-ray source for flash radiography

The design and test results of a pulsed power-driven X-ray source which is named Hawkeye-I are described. The Hawkeye-I is developed to execute flash X-ray radiography. It is a six-stage induction voltage adder and terminated by a positive polarity rod-pinch diode. The six stages are identical and the prime energy of each stage is provided by a tesla transformer elevating the output voltage from a discharge brick which consists of two $4\text{ }\text{ }\ensuremath{\mu}\mathrm{F}$ capacitors and a three-electrode gas switch. Comparing to the induction voltage adders driven by Marx generators, the numbers of gas switches and capacitors in the energy storage sections of Hawkeye-I decrease significantly. The Hawkeye-I can produce a maximum output voltage of about 4.2 MV. The corresponding X-ray source spot size and dose at 1 m are 1.4 mm and 17.9 rad, respectively. Fifty consecutive shots have been conducted to evaluate the reliability of the machine and 98% of the shots are successful. Furthermore, as the energy propagating in the accelerator is mainly determined by seven laser-triggered gas switches, the Hawkeye-I is capable of controlling the timing of the X-ray pulse accurately.

Electron-Beam Corkscrew Motion in an Advanced Linear Induction Accelerator

Scorpius is a multipulse linear induction accelerator (LIA) under development for flash radiography. Because it has substantially more cells than present LIAs, higher magnetic focusing fields are needed to suppress beam breakup (BBU). Therefore, it is more susceptible to corkscrew motion of the beam, which also depends on beam energy spread and focusing magnet misalignments. For energy spread and alignment tolerances expected for Scorpius, a magnetic tune designed to suppress BBU is shown to produce corkscrew motion within the range that can be controlled through the use of steering dipoles on existing LIAs. A gap-voltage modulation scheme is shown to almost completely eliminate chromatic effects such as corkscrew.

Beam Envelope Stability in an Advanced Linear Induction Accelerator

A new linear induction accelerator (LIA) is under development for multipulse flash radiography. Because it has substantially more cells than present LIAs, higher magnetic focusing fields are needed to suppress beam breakup (BBU). It is, therefore, more susceptible to the parametric beam envelope instability (PEI), which has an instability threshold that has usually been typified by the vacuum phase advance per cell exceeding some large fraction of $\pi $ . Here we derive a threshold criterion for PEI that depends not only on the magnetic field, but also on the beam space charge and emittance. A tune designed to suppress BBU in Scorpius is shown to be stable to the PEI according to this criterion, and also by the lack of emittance growth in particle-in-cell (PIC) code simulations.

Investigation of Rayleigh-Taylor instability in copper plate under explosive loading

The Rayleigh–Taylor instability in metal is of great practical interest to a diverse range of fields, and it is significantly different from the traditional fluid interface instability. In this paper, a set of high explosives driven Rayleigh–Taylor instability experiments in copper plate are presented for intensively understanding the perturbation growth behavior of metal interface instability. The perturbation growth history on copper interface at a specific time is recorded by flash radiography. The experimental results show that the evolution of interface perturbation is sensitive to its initial perturbation characters under a given driving pressure, the larger the initial perturbation amplitude, the faster the perturbation grows, but the perturbation wavelength of the interface remains almost unchanged at the explosive loading. The perturbation on the front interface will influence the interface features of the back free interface over time. Namely, the position corresponding to the perturbation trough on the front interface gradually evolves into a peak and vice versa.

Uncertainty Quantification Enforced Flash Radiography Reconstruction by Two-Level Efficient MCMC.

Flash Radiography inspections stand to gain from inversion to infer density distribution of object based on X-ray transmission image. It is indispensable to be able to reliably provide uncertainties associated with the inversions. Although many inversion algorithms have been devised, they often perform poorly due to either their sensitivity to regularization parameter chosen in variational optimization or prohibitive computation and noisy results in stochastic simulation. In this paper, we present a gradual reconstruction algorithm, called TLE-Gibbs (two-level efficient Gibbs sampling), for flash radiography. At its core, TLE-Gibbs is a stochastic approach based on efficient Gibbs sampling and reconstruction refinement. A two-level scheme is proposed that enables high-resolution image to be constrained with uncertainty estimation from high-level reconstruction. Furthermore, a splitting variant that increases flexibility and precision is considered in the two-level scheme. An efficient Markov chain Monte Carlo (MCMC) endowed with first-order truncated conjugate gradient (CG) optimizer is developed to achieve minimal cost per sample and to approximate the posterior distribution. Finally, we adopt an effective refinement method to remove noises remained in the sample meanwhile maintaining sharp edges. For performance evaluation, TLE-Gibbs is applied on both synthetic data in which the influence of system blur is specially investigated and real data, and comparison with state-of-the-art reconstruction methods demonstrates the superiority of the proposed method.

High-resolution X-ray flash radiography of Ti characteristic lines with multilayer Kirkpatrick–Baez microscope at the Shenguang-II Update laser facility

Abstract High-resolution X-ray flash radiography of Ti characteristic lines with a multilayer Kirkpatrick–Baez microscope was developed on the Shenguang-II (SG-II) Update laser facility. The microscope uses an optimized multilayer design of Co/C and W/C stacks to obtain a high reflection efficiency of the Ti characteristic lines while meeting the precise alignment requirement at the Cu Kα line. The alignment method based on dual simulated balls was proposed herein, which simultaneously realizes an accurate indication of the center field of view and the backlighter position. The optical design, multilayer coatings, and alignment method of the microscope and the experimental result of Ti flash radiography of the Au-coned CH shell target on the SG-II Update are described.

Optimization based on Monte Carlo simulation of a pixelated scintillator array for megavolt X-ray flash radiography

Pixelated scintillators arrays are widely used in X-ray radiographic applications and provide the sensitivity and resolution of detectors, which are determined by the pixel size and thickness. The design of pixelated scintillator arrays is impeded by the absence of a suitable method to simulate and predict the performance. A method of design and optimization based on a Monte Carlo simulation of a pixelated lutetium yttrium oxyorthosilicate scintillator array for MeV X-ray flash radiography has been established. The variance of the gain and spatial resolution of scintillators with different structural parameters have been studied. The proposed method was subsequently validated using flash radiographic experiments. The results of both agree well. Furthermore, the results indicate that an array with a large pixel size and small thickness exhibits strong scintillator gain, sufficient spatial resolution, and excellent uniformity of response.

Suppression of Beam Breakup in Linear Induction Accelerators by Stagger Tuning

Beam breakup (BBU) in linear induction accelerators (LIA) is suppressed by the solenoidal magnetic-field focusing of the electron beam. Amplification of BBU instability can also be significantly reduced by using cells having different transverse impedances. This so-called “stagger tuning” occurs naturally on one of the LIAs used for flash radiography at the Los Alamos National Laboratory. In this article, we use computer simulations to demonstrate how the natural stagger tuning functions, and how it might be used on advanced LIAs under consideration. Other means for engineering stagger tuning into LIAs are also investigated. Results of these simulations show promise for mitigation of BBU by reduction of the effective impedance, thereby relieving the necessity for extraordinarily strong solenoidal fields for designs with a large number of cells.

Implementation of a point spread function method to analyze flash radiography images: Image enhancement, movie generation, and projection detangling

This work investigates how one can postprocess a series of flash-generated X-ray radiographs with known point spread functions and collection geometries to produce radiographs with enhanced image quality and/or to approximate a multiframe X-ray movie by spatially correlating temporally sequenced images. To produce images with enhanced quality, one collects multiple individual projections, simultaneously, and then uses the known point spread function and acquisition geometry to correlate and integrate the numerous projections as if they were acquired from approximately the same perspective. If using sources of similar spectral characteristics, the outcome correlates with that which would be produced from a multiflash integration, where one benefits from an increase in flux of X-ray photons. If using sources of different spectral characteristics, a composite radiograph can be produced where the unique source spectrum can be matched to known material absorption cross-sections to accentuate radiographic features within the target material. To produce multiframe movies, similar mathematics are applied to images collected throughout a timespan over which the object changes. The aforementioned processes' mathematics and examples are demonstrated for the case in which the individual projections are completely separable on the detector with no overlap. Finally, a potential method is discussed for separation of images in the case where multiple radiograph projections are overlapped and entangled on the detector.

The Story of the LTD Development

In this article, the story of linear transformer driver (LTD) development is presented, which began in the 1980s when the inductive energy storage (IES) systems with plasma opening switches (POSs) were the focus of pulsed power research all over the world. The accompanying problems of this technology resulted in the development of microsecond LTDs by Boris Kovalchuk, which was the first crucial step toward the development of this new technology. Next came the fast (~100 ns) LTDs, eliminating completely the need for any pulse compression and power amplification. Most recently, the LTD technology was further developed to generate fast rising output pulses with a flat or trapezoidal (rising or falling) top. Such LTD cavities, called Square Pulse LTDs, are better suited for applications such as flash radiography, Z-pinch, high-power microwaves, etc. A number of recent new suggestions important for the viability of LTD technology are also presented.

Development of a 4-MV, 80-kA-Induction Voltage Adder for Flash X-ray Radiography

An induction voltage adder (IVA) has been developed to drive rod-pinch diodes (RPDs), to produce X-rays for flash radiography. The IVA facility consists of four-stage induction cavities stacked in series, each of which is single-point driven by a 6- $\Omega $ pulseline. The prime pulsed-power source comprises two Marx generators and four de-ionized water pulselines. Each Marx generator fast charges two pulselines to about 2.2 MV in less than 370 ns. Four electrically triggered, SF6-insulated gas switches serve as energy-transfer switches from pulse-forming lines to output transmission lines, and thus without an additional intermediate energy storage. A stepped inner stalk is utilized to make the vacuum impedance of the secondary transmission line increase from $30~\Omega $ at the first cavity to $120~\Omega $ at the last cavity, to prevent the electron emission from cathode surfaces. The RPD is chosen to create bremsstrahlung X-rays, which are believed to be optimal at voltage levels of about 4 MV. In this article, the design details and experimental results concerning the main components, such as the Marx/PFLs, IVA, and RPD, are demonstrated. The electrical performances of the facility are characterized. Presently, the IVA provides voltages of up to 4.3 MV and currents of up to 85 kA. The X-ray dose is up to 15.5 Rad [lithium fluoride (LiF)] at 1 m with a spot size of 1.4 mm and a FWHM time of 55 ns.