Time-integrated Pinhole Cameras Research Articles

Neutron source reconstruction using a generalized expectation-maximization algorithm on one-dimensional neutron images from the Z facility.

Magnetized Liner Inertial Fusion experiments have been performed at the Z facility at Sandia National Laboratories. These experiments use deuterium fuel, which produces 2.45MeV neutrons on reaching thermonuclear conditions. To study the spatial structure of neutron production, the one-dimensional imager of neutrons diagnostic was fielded to record axial resolved neutron images. In this diagnostic, neutrons passing through a rolled edge aperture form an image on a CR-39-based solid state nuclear track detector. Here, we present a modified generalized expectation-maximization algorithm to reconstruct an axial neutron emission profile of the stagnated fusion plasma. We validate the approach by comparing the reconstructed neutron emission profile to an x-ray emission profile provided by a time-integrated pinhole camera.

Magnetic implosion of thin aluminum foil liners

Metal liner implosions driven by a pulsed current generator are used in research on inertial confinement fusion, generation of soft x-ray pulses and megagauss magnetic fields. The energy density in the plasma column formed during the liner stagnation is largely determined by the initial radius and the radial convergence of the liner. When the liner implosion time is close to the current rise time, the liner initial radius is proportional to the current rise time. The paper presents the results of experiments on the fast implosion of cylindrical liners on the MIG generator (2 MA, 80 ns) at the Institute of High Current Electronics, Tomsk, Russia. Liners 0.7–1.0 mm in diameter were made from a 1.8 to 2.5 μm thick aluminum foil. A low-density plasma was pre-injected into the liner area. As the Lorentz force sweeps away the pre-injected plasma, the current switches to the liner in 1–3 ns. Then the liner is imploded in 3–7 ns by a current close to the peak generator current. The stagnation radius measured via time-integrated pinhole camera images was found to be not more than 25 µm. Such a radius of the stagnated plasma assumes that the plasma mass density is several times higher than the solid aluminum density.

Radiative Characteristics of Reversed Polarity Gas-Puff Ar and Kr Plasmas

Experiments with regular and reversed polarities were carried out on the SHOTGUN-III Z-pinch device at Nihon University, Japan. SHOTGUN’s storage section consists of the 12- $\mu \text{F}$ fast capacitor bank, which was charged to 25 kV for these experiments and provided current up to 200 kA. The Ar and Kr gases were puffed between the electrodes through a hollow nozzle using a fast gas valve. Time-resolved X-ray detectors, time-integrated X-ray pinhole camera and two spectrometers, and an optical framing camera were applied for the study of radiative properties of Ar and Kr gas puffs. K-shell Ar spectra (3.9–4.2 A) and L-shell Kr spectra (6.3–7.6 A) show much intense He- and Li-like Ar and Ne- and Na-like Kr spectral features, respectively, and the evidence of the electron beams in reversed polarity experiments. The influence of reversed polarity on gas-puff plasma radiative characteristics and X-ray spectroscopic measurements is discussed. The advantages of using K-shell Ar and L-shell Kr for hot electron beam diagnostics are highlighted.

High energy X-ray pinhole imaging at the Z facility.

A new high photon energy (hν > 15 keV) time-integrated pinhole camera (TIPC) has been developed as a diagnostic instrument at the Z facility. This camera employs five pinholes in a linear array for recording five images at once onto an image plate detector. Each pinhole may be independently filtered to yield five different spectral responses. The pinhole array is fabricated from a 1-cm thick tungsten block and is available with either straight pinholes or conical pinholes. Each pinhole within the array block is 250 μm in diameter. The five pinholes are splayed with respect to each other such that they point to the same location in space, and hence present the same view of the radiation source at the Z facility. The fielding distance from the radiation source is 66 cm and the nominal image magnification is 0.374. Initial experimental results from TIPC are shown to illustrate the performance of the camera.

Gold planar wire array radiation sources at university scale generators and their applications

Various configurations of planar wires arrays (PWA) made from wires in a broad range of atomic numbers up to 74 (W) have been extensively tested on Zebra generator at the University of Nevada, Reno. In this work, we present the results of our experiments with PWA made of the wires with an even higher atomic number, specifically 79 ( Au ). Two different configurations (single and double PWAs) are considered. The diagnostic set included: x-ray detectors, laser shadowgraphy, time-gated and time-integrated x-ray pinhole cameras, time-integrated spatially resolved and time-gated spatially integrated x-ray spectrometers, and a bolometer. The total x-ray yield was measured along and perpendicular to the load plane and reached the highest value measured on Zebra so far for the single PWA (perpendicular to the plane, 32 kJ). Theoretical tools included non-LTE kinetic and wire ablation dynamic models. The full set of data using aforementioned diagnostics is analyzed and specific implosion and radiative characteristics of Au PWAs. The application of gold PWAs in compact hohlraum configurations on Zebra is highlighted. VisRad (Prism Computational Sciences), a 3-D view factor code, is used to simulate the multi-dimensional radiation environment with a special emphasis on radiation temperature calculations and uniformity at the re-emission target surface.

Observation of energetic protons trapped in laboratory magnetic-tower jets

Preliminary results of the self-emission of charged particles from magnetically driven plasma jets has been investigated. The jets were launched and driven by a toroidal magnetic field generated by introducing a ∼ 1.4 MA, 250 ns electrical current pulse from the MAGPIE generator into a radial wire array. This configuration has shown to reproduce some aspects of the astrophysical magnetic-tower jet launching model, in which a jet is collimated by a toroidal magnetic field inside a magnetic cavity. The emission of ions and protons from the plasma was recorded onto Columbia Resin 39 plates using time-integrated pinhole cameras. In addition a fly-eye camera, an array of 25–496 cylindrical apertures allowed estimating the location of the ion emitting source. The results show the ion emission comes from both the jet and its surrounding magnetic cavity, with the emission extending to a height of at least ∼ 9 cm from the initial position of the wires. The emission of ions is consistent with the dynamics of the jet obtained from time-resolved imaging diagnostics, i.e. optical laser probing and self-emission of the plasma in the extreme ultra-violet. These preliminary results suggest the ions are trapped inside the cavity due to the strong toroidal magnetic field which drives the jet. In addition these studies provide first estimates of the energy and fluence of protons for future laser-driven proton probing diagnostics aimed at measuring the magnetic field in these experiments.

Radiation Anisotropy at the Implosion of Multiwire Arrays

We present the results of studying the dynamics of soft X-ray radiation (SXR) emission in the axial and radial directions in the current implosion of multiwire tungsten arrays with discharge currents of up to 3.5 MA at the Angara-5-1 facility. The size of the region emitting soft X-rays was determined by the measurements of the radiation anisotropy. It was found that the size of the emitting region is ~ 4-5 mm in the SXR range for cylindrical multiwire arrays at peak X-ray power, which corresponds to approximately two to threefold radial compression of the plasma. The time-integrated pinhole camera images show the presence of a significant amount of emitting plasma around the pinch core-trailing mass. We measured the time-integrated radial resolved pinch spectra. It is shown that the presence of trailing mass decreases the spectral power of radiation and changes the characteristics of the spectrum. The size of the emitting region received from the radial resolved spectra is identical to those obtained from the radiation anisotropy and time-integrated pinhole camera images.

Laser-produced plasma helium-like titanium Kα x-ray source and its application to Rayleigh-Taylor instability study

Several experiments are performed on the ShenGuang-II laser facility to investigate an x-ray source and test radiography concepts. X-ray lines emitted from laser-produced plasmas are the most practical means of generating these high intensity sources. By using a time-integrated space-resolved keV spectroscope and pinhole camera, potential helium-like titanium Kα x-ray backlighting (radiography) line source is studied as a function of laser wavelength, ratio of pre-pulse intensity to main pulse intensity, and laser intensity (from 7.25 to ∼11.3 × 1015 W/cm2). One-dimensional radiography using a grid consisting of 5 μm Au wires on 16 μm period and the pinhole-assisted point projection is tested. The measurements show that the size of the helium-like titanium Kα source from a simple foil target is larger than 100 μm, and relative x-ray line emission conversion efficiency ξx from the incident laser light energy to heliumlike titanium K-shell spectrum increases significantly with pre-pulse intensity increasing, increases rapidly with laser wavelength decreasing, and increases moderately with main laser intensity increasing. It is also found that a gold gird foils can reach an imaging resolution better than 5-μm featured with high contrast. It is further demonstrated that the pinhole-assisted point projection at such a level will be a novel two-dimensional imaging diagnostic technique for inertial confinement fusion experiments.

High energy density plasmas generated by radial foil explosions

High energy density (HED) plasmas, produced by diverse techniques, such as lasers or pulsed power generators, can help scientists to better understand extreme states of matter as well as astrophysical phenomena. While fast Z-pinches are the most common approach to generating HED plasmas in the pulsed power community, radial foil configurations can yield plasma pressures on the order of 0.5 Mbar on a 1 MA, 100 ns current rise time generator, similar to wire array configurations producing Z-pinches. In this experimental setup, a thin metallic foil stretched onto a circular anode connects to a very small ‘pin’ cathode at the center. Radial currents flow in the foil then down the pin cathode, thereby generating an axi-symmetric toroidal magnetic field under the foil. Initial experimental results (Gourdain et al 2010 Phys. Plasmas 17 012706) showed that the foil current interacts with this field and the resulting J × B force lifts the foil upward. Very rapidly the foil plasma turns into a bubble-shaped cavity above the central pin, and that bubble expands at 300 km s−1, until instabilities destroy the axial symmetry, bursting the bubble open. This paper complements these initial results by using time-integrated x-ray pin-hole cameras and a focusing spectrometer with spatial resolution. In addition to laser interferometry, these new data help to provide a better estimate of the plasma electron density inside the bubble, above 1020 cm−3, and of the electron temperature, between 300 and 400 eV inside the central plasma column. We also discovered the presence of ‘bright’ spots in the plasma, with densities larger than 5 × 1021 electrons cm−3 and temperatures above 1 keV. Finally laser interferometry gives a precise mapping of the initial plasma jet and bubble areal densities.

Implosion dynamics and radiation characteristics of wire-array Z pinches on the Cornell Beam Research Accelerator

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator (COBRA) [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4to16mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar (64% iron, 36% nickel). Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-ray peak powers (300–500GW) and total energy yields (6–10kJ) were obtained using 4-mm-diameter arrays that stagnated before peak current. Additional findings include a decrease in soft x-ray radiation prior to stagnation as the initial wire spacing was changed from 1.6mmto785μm, and a timing correlation between the onset of energetic electrons, hard x-ray generation, and the arrival of trailing current on axis—a correlation that is likely due to the formation of micropinches. The details of these and other findings are presented and discussed.

Experimental study on imploding characteristics of wire-arrayZpinches on Qiangguang-1 facility

To investigate the imploding characteristics of cylindrical wire array, experiments with load current varying from 1.5 MA to 1.7 MA were carried out on the Qiangguang-1 facility. The complicated temporal-spatial distribution of x-ray radiation was measured by the one-dimensional (1D) x-ray imaging system. Other diagnostic equipments including the x-ray power meter (XRPM) and the time-integrated pinhole camera were used to record time-resolved x-ray power pulse and pinhole x-ray images. Analysis shows that the fast leading edge of the local x-ray radiation pulse is of primary importance in sharpening x-ray power pulse rather than the temporal synchrony and the spatial uniformity of implosion. Experimental results indicated that the better axial imploding synchrony, the faster the increase of x-ray power for an array consisting of 32 tungsten wires of 5 μm diameter than for the others, and the higher the x-ray radiation power with maximal convergence ratio (r0/r1) of 10.5. A `zipper-like' effect of x-ray radiation extending from the anode to the cathode was also observed.

Low microdivergence medium-mass ion beam produced from a N2O cryogenic diode

Medium-mass ion beams including nitrogen and oxygen were produced from a cryogenic diode with N2O ice as the ion source. The nominal diode voltage was 300–400 kV, and the peak ion current was 240 A. The beam divergence angle was measured with a five-aperture time-integrated pinhole camera. The five camera images were analyzed to estimate the spatial distribution of the beam source divergence angle along the anode radius, yielding a value of 5–6 mrad for the average microdivergence. This is low enough for this ion source to be studied further in the near future. If possible, we want to consider this as one of the probable candidate ion sources for ion beam drivers for future inertial confinement fusion (ICF) and inertial fusion energy (IFE) applications.

Determination of the effective size of a hot spot in hot dense plasmas

A new method for determining the dimension of a hot spot in hot dense plasmas is presented. A time-integrated pin-hole camera was used and the experimental densitograms from the pin-hole pictures of hot spots were computationally simulated using a specific source intensity distribution function. The method was used in plasma focus DPF-78. The best estimate for the hot spot size is the effective size, the FWHM of the intensity distribution function. The effective radius of hot spots in DPF-78 varies in the range 30 - .

PBFA II lithium beam characterization from inner-shell x-ray images

The Particle Beam Fusion Accelerator (PBFA II) is now driving targets with ICF-relevant lithium ion beams. During the most recent lithium beam target series, time-integrated x-ray pinhole cameras viewed the ion-induced inner-shell x-ray fluorescence from the central gold cone target and a titanium-coated strip. Ion-beam profiles at a nominal 10-mm radius and fixed azimuthal direction were obtained from images of the Ti Kα fluorescence of a Ti-coated Al diagnostic wire. The gold cone gave us beam profiles at a nominal 3-mm radius and at all azimuthal angles from the Au Lα fluorescence. From these profiles, we obtained the ion-beam vertical focus position, the full width at half maximum, and the degree of azimuthal uniformity for the lithium target shots. For these initial results, beam steering problems were evident. Azimuthal uniformity was measured from the ion-beam footprint on the outer Au case (predominantly Au Lα) of the hohlraum target and was found to be in the same range (up to 30%) as for previous proton beam target series.

Measurement of beam properties and reproducibility on recent PBFA II target shots

Recent proton experiments on Particle Beam Fusion Accelerator II used a 2 μm gold foil cone to characterize the ion beam. Using the ion beam images obtained by viewing beam-induced characteristic line radiation emitted by such foils with time-integrated x-ray pinhole cameras, the beam centroid axial location and azimuthal symmetry have been analyzed for a recent series of target shots. Azimuthal symmetry on the target midplane on individual shots varied from 6% to 29%. Averaged over the entire series of shots, inferred intensities on the target midplane varied by 24% to 37% from quadrant to quadrant. The beam profiles and beam reproducibility are vital to the interpretation of the results of these target experiments.

Collision and interpenetration of plasmas created by laser-illuminated disks

Supersonic, counterstreaming plasmas were produced by ablating plasma from the inside surfaces of two parallel disks made of aluminum and magnesium, respectively, with a 0.53 μm laser at an intensity of 1014 W/cm2 for 1.3 nsec. Diagnostics included holographic interferometry, a time-integrated x-ray pinhole camera and a gated x-ray crystal spectrograph with imaging slits. The plasmas interpenetrate for the first half of the laser pulse but stagnate once the electron density exceeds 5×1020 cm−3. Spectroscopic measurements suggest a coronal electron temperature of ∼800 eV and an ion temperature of ∼15 keV in the stagnated plasma. The observations are in good agreement with a two ion fluid model of interpenetrating plasmas in which the dominant slowing down process is ion–ion collisions.

Solenoid gas puff imploding liner x-ray source

There is a worldwide effort to produce a soft x-ray source for submicron microlithography to manufacture future generation large scale integrated microchips. The gas puff plasma discharge has been suggested as a viable alternative for low volume facilities which require relatively low throughputs with a single aligner station. The repetition rate for such systems is limited by the large gas flow into the vacuum system with the standard electromechanical puff valve. A unique puff arrangement is being developed which will limit the dead gas flow and, in principle, will allow for the development of high repetition rate systems. In this arrangement, the gas flows continuously from a low-pressure plenum through an annular aperture into the Z-pinch electrode gap. A discharge through a single turn solenoid coil mounted on the outside of the plenum inductively heats the gas, temporarily increasing the mass flow to produce a low mass (<1 μg), preionized liner which is imploded by a Z-pinch current discharge. (Multiturn coils have also been used. However, all the experiments reported in this paper used a single turn configuration.) Experiments were carried out with a variety of low-energy (<2.5 kJ) drivers including capacitor banks, peaking capacitors, and dc charged paper/castor oil Blumleins. Radiation measurements in the ultrasoft and soft x-ray regions were made using x-ray diodes, p-i-n diodes, and a time integrated pinhole camera. A slug model coupled to a capacitor bank or Blumlein circuit solver is used to estimate the liner dynamics as well as the load current and voltage.

Efficient x-ray production from ultrafast gas-puff Z pinches

The Proto-II accelerator has been used to implode krypton and xenon annular gas puffs. A significant fraction of the machine electrical energy was converted first to plasma kinetic energy and then to x rays when the plasma pinched on axis. Quantitative measurements using time-resolved bolometers have shown as much as 10% of the total radiation yield near 1 keV in Xe and 2 keV in Kr. We have compared this radiation yield to the predictions from one-dimensional magnetohydrodynamic code calculations. The implosions were also observed with both time-integrated pinhole cameras and spectrographs. No hard x-ray (E>10 keV) output was observed.