Search

In this paper, we report on a crystal based x-ray imaging system fielded at the OMEGA EP laser facility. This new system has a pointing accuracy of +/100 μm, a temporal resolution down to 100 ps (depending on backlighter characteristics), variable magnification, and a spatial resolution of 21.9 µm at the object plane at a magnification of 15×. The system is designed to use a crystal along the crystal plane that satisfies the Bragg condition for the x ray of interest. The thin crystal is then bent into a spherical geometry and attached to a glass backing substrate to hold it in the diagnostic, and the x rays are imaged onto a charge coupled device. We report on data acquired with the new Los Alamos National Laboratory supplied spherical quartz crystal to image the Mn He-α 6.15 keV line emission.

We examine the effects of varying the magnitude of an applied, uniform, axial magnetic field ( $B_{z}$ ) on the formation of laboratory plasma jets produced by a 1-MA 100-ns rise time pulsed power generator in a radial foil configuration. A Helmholtz coil applies the external magnetic field (0 to 2 T). With a small-enough applied field, the foil surface ablation is relatively azimuthally uniform, and $J\times B$ forces drive the ablated plasma inward and upward to form a well-defined azimuthally symmetric jet. Using applied field strengths larger than a critical $B_{z} = 1.1\pm 0.1$ T with aluminum foils, the plasma ablation is not azimuthally uniform, discrete bursts of plasma initiate from the foil surface, and the formation of a well-defined jet is disrupted. The critical $B_{z}$ for this plasma jet formation disruption correlates with the foil material’s electrical resistivity and equation of state (EOS). To better understand the material-dependent phenomenon, we compare the experimental results with 3-D extended magnetohydrodynamics simulations of ablation of a 2 mm $\times 2$ mm by 25- $\mu \text{m}$ slab that represents a section of the radial foil. The simulations initialize the slab in the solid state and include the material resistivity and EOS from the solid to plasma phases. As is observed in the experimental disruption, these simulations show enhanced nonuniform plasma ablation with an applied $B_{z}$ , which can inhibit the azimuthal uniformity necessary to produce a well-defined plasma jet. Furthermore, the simulations also show a material and resistivity dependence on the ablation process similar to the trend shown by the experiments. These results demonstrate the necessity of accurate, detailed modeling of material properties during the transition from solid to plasma phases.

Ar gas puff z-pinch experiments were conducted on the 0.9-MA, 200-ns COBRA generator at Cornell University. In the experiments, a triple-nozzle gas valve was used to produce the z-pinch load with concentric outer and inner gas puffs and a central gas jet. We used a planer laser-induced fluorescence to measure the initial gas profile of the load, two four-frame 4-ns gated extreme ultraviolet cameras to image the imploding plasma, and filtered photoconducting diamond detectors to measure the X-ray output. We have observed the shock heating and/or photoionization of the dense center jet by the imploding plasma. With an outer-to-inner plenum pressure ratio of 1:3, deacceleration of the imploding plasma occurred when the outer plasma shell imploded on the inner shell resulting in a stable implosion. By varying the plenum pressure for the center jet, the X-ray emission was optimized. An Ar K-shell X-ray (3–4 keV) yield of 77 ± 17 J was produced at a current level of 0.87 ± 0.2 MA and an implosion time of 200 ns.

Thomson scattering measurements were performed on plasma jets created from a 15- $\mu \text{m}$ -thick radial Al foil load on a 1-MA pulsed power machine. The laser used for these measurements has a maximum energy of 10 J at 526.5 nm. Using the full energy, however, significantly heats the $5\times 10^{18}$ cm−3 jet by inverse bremsstrahlung, creating a density bubble in the jet. To measure the evolving plasma parameters of this laser-heated jet, a streak camera was used to record the scattered spectrum, resulting in the sub-ns time-resolved Thomson scattering. Analysis of the streak camera image showed that the electron temperature of the jet was about 25 eV prior to the laser pulse. The laser then heated the plasma to 80–100 eV within about 2 ns. The electron temperature then stabilized for about 0.5 ns prior to falling at the end of the laser pulse. Jets made from a radial Ti foil showed more heating by the laser than the Al jets, going from 50 to over 150 eV, and heating was detected even when only 1 J of laser energy was used. Also, the ion-acoustic peaks in the scattered spectrum from the Ti jets were significantly narrower than those from Al jets, a result of several possible differences in the plasma created from these two materials.

In the present research, plasma jets form from Joule heating and ablation of a radial foil (approximately 15 μm thin disk) using a pulsed power generator (COBRA) with 1 MA peak current and 100 ns rise time. We study the effects on jet dynamics resulting from varying an applied uniform axial magnetic field (Bz) from 0 to 2 T. We empirically observe a disruption of the plasma jet collimation, for which plasma is ej ected from the foil as discrete bursts and does not form an azimuthally symmetric plasma jet. The critical Bz for the disruption depends upon the foil material (Al, Ti, Ni, Cu, Zn, Mo, W). The disruption initiates from the foil surface and is dependent upon material properties such as electrical conductivity and equation of state. The applied Bz acts with the higher conductivity materials to facilitate nonuniform plasma and current filamentation that breaks the azimuthal symmetry needed to form the plasma jet. The plasma filamentation likely originates from initial perturbations (in density and resistivity) in the higher-density solid-liquid phases. 3D numerical simulations in Cartesian coordinates (using the extended magnetohydrodynamics code, PERSEUS) of a section of the ablating foil reproduce some of the experimental trends including the external magnetic field dependence and material resistivity dependence of the generation of plasma and current nonuniformities.

Streaked Thomson scattering measurements have been performed on plasma jets created from a 15 $\mu\mathrm{m}$ thick radial Al, Ti, or Cu foil load on COBRA, a 1 MA pulsed power machine. The streaked system enables collecting scattered light from two separate laser pulses separated in time by between 3 and 14 ns from two different scattering angles. The time separation is created by splitting the initial 2.2 ns full width half max duration, 10 J, 526.5 nm laser beam into two separate pulses, each with 2.5 J, and delaying one beam relative to the other. To collect streaked spectra from two angles simultaneously a split fiber bundle was used to couple light from the plasma to the spectrometer, and the size of the fibers, 100 $\mu \mathrm{m}$ diameter, was used as the effective spectral slit width. The 2.5 J laser pulse is shown to heat the plasma jet by inverse bremsstrahlung radiation, as measured by the streaked Thomson scattering system. Analysis of the streak camera image showed that the electron temperature of the Al jet was increased from 20 eV up to 50 eV within about 2 ns for both laser pulses when they were separated by 12 ns in time. Initial results from the Cu jets showed more heating then the Al jets while Ti jets showed the most heating of these three materials. In addition to this increased heating, the scattering profiles from Ti and Cu jets had ion-acoustic features that were more difficult to interpret then those from Al jets. Results from the two scattering angles showed reasonable agreement of the measured electron temperature, and implied that the electron density was at least $2\times 10^{18}\text{cm}^{-3}$ . Results will also be presented with the laser pulses separated by only 4 ns, creating a more continuous laser pulse. In addition, the effects of the current polarity in the radial foil on the measured electron temperature will be shown.

Thomson scattering measurements have been performed on laboratory plasma jets in order to measure the flow velocities and electron temperatures inside and near the edge of the jet. These jets were created using $\mathrm {a}15 \mu \mathrm {m}$ thick radial Al foil load on COBRA, a 1 MA pulsed power machine. The laser used for these measurements had a maximum energy of 10 J at 526.5 nm. Using this 10 J laser, however, significantly heats the $5\mathrm {x} 10 ^{18}$ cm $^{-3}$ jet by inverse bremsstrahlung, creating a density bubble in the jet. This was demonstrated by reducing the probe laser energy to 1 J, which reduced the measured electron temperature in the jet from about 40 to 20 eV and nearly eliminated the bubble. These measurements were made using a time gated spectrometer, with at least a 5 ns gate window, in order to encompass the entire 3 ns laser pulse. Therefore, no time dependent electron temperature measurements could be made from the Thomson scattering spectra. We are now developing an experimental arrangement to send the collected scattered spectra into a streak camera, in order to determine the plasma temperature as a function of time throughout the 3 ns laser pulse. With the streak camera data, we hope to track the changing plasma parameters during the laser heating of the plasma jet. In the future we hope to extend this diagnostic arrangement to other configurations, enabling time resolved measurements of the plasma parameters of various plasmas.

In the present research, plasma jets form from Joule heating and ablation of a radial foil (approximately 15 μm thin disk) using a pulsed power generator (COBRA) with 1 MA peak current and 100 ns rise time. We study the effects on jet dynamics resulting from varying an applied uniform axial magnetic field (Bz) from 0 to 2 T. We empirically observe a disruption of the plasma jet collimation. The critical Bz for the disruption depends upon the foil material (Al, Ti, Ni, Cu, Zn, Mo, W). The disruption initiates from the foil surface and is dependent upon material mass as well as the solid electrical or thermal resistivities. We consider how the different foil materials affect the plasma magnetization, radiative losses, and formation time of surface plasma. The axial magnetic field larger than the critical Bz can break the azimuthal symmetry of the foil ablation.

On the 20 MA Z machine, the helical instability features that are observed in magnetized metal liner experiments were mitigated using a dielectric coating. 1In the experiments to be presented, we compare results using liners with and without a dielectric coating and without an applied axial magnetic field on the 1 MA, 100-200 ns COBRA pulsed power generator at Cornell University. 2Diagnostics used in these experiments include 4-frame extreme ultraviolet imaging, 3-frame laser imaging, and high resolution X-ray radiography. This combination of diagnostics enables investigation of plasma formation and evolution ranging from the higher density $( > 10 ^{19} /$cm $^{3})$region close to the initial liner radius to the lower density regions $\sim 0.3$mm from that radius.

We have conducted Ar gas puff z-pinch studies on the COBRA generator. A triple-nozzle is used to produce tailored gas density distributions with two concentric annular nozzles and a center gas jet. We report suppression of Rayleigh-Taylor instabilities by inner dense plasmas, and photo-ionization of the inner gas puffs by the hot outer imploding plasma. By varying the initial gas density profiles, we have optimized the Ar K-shell x-ray radiation, and produced stable 200-ns implosions.

We present a method to seamlessly self-assemble, disassemble, transport, and reconstruct ordered 2-D structures of fluid-borne microspheres on a surface using an array of magnetic zigzag wire traps. Competition between and control over: 1) the trapping forces of underlying magnetic patterns; 2) magnetic dipole repulsion; and 3) Brownian motion gives rise to reproducible cluster structures. Weak external magnetic fields $({ tune the spacing between particles and enable the assembled structures to be remotely manipulated and reassembled on the platform. This method could be used in biological and photonic applications by utilizing the microspheres, for example, as carriers of host biomolecules or as linkers to fluorescent emitters.

In the present research, plasma jets form from Joule heating and ablation of a radial foil (∼15 μm thin disk) using a pulsed power generator (COBRA) with 1 MA peak current and 100 ns rise time. We study the effects on jet dynamics resulting from varying an applied uniform axial magnetic field (Bz) from 0 to 1.8 T. We empirically observe a disruption of the plasma jet collimation. The critical Bz for the disruption depends upon the foil material (Al, Ti, Ni, Cu). We discuss physical reasons for the material dependence of the disruption.

The dynamics of Magnetized Liner Inertial Fusion (MagLIF)1, a new and promising approach to pulsed power fusion, are presently under detailed study at Sandia National Laboratories. Alongside this, a comprehensive analysis of the influence of the specific liner design geometry in the MagLIF system on liner initiation is underway in the academic community.

A single channel sub-nanosecond time-resolved Thomson scattering system used for pulsed power-driven high energy density plasma measurements has been upgraded to give electron temperatures at two different times and from two different angles simultaneously. This system was used to study plasma jets created from a 15 μm thick radial Al foil load on a 1 MA pulsed power machine. Two laser pulses were generated by splitting the initial 2.3 ns duration, 10 J, 526.5 nm laser beam into two pulses, each with 2.5 J, and delaying one relative to the other by between 3 and 14 ns. Time resolution within each pulse was obtained using a streak camera to record the scattered spectra from the two beams from two scattering angles. Analysis of the scattering profile showed that the electron temperature of the Al jet increased from 20 eV up to as much as 45 eV within about 2 ns by inverse bremsstrahlung for both laser pulses. The Thomson scattering results from jets formed with opposite current polarities showed different laser heating of the electrons, as well as possibly different ion temperatures. The two-angle scattering determined that the electron density of the plasma jet was at least 2 × 1018 cm-3.

As part of the work on the Magnetized Liner Inertial Fusion (MagLIF) concept1, Awe et al. found on the 20 MA Z-machine that applying an externally generated axial magnetic field to a liner produces a helical striation pattern in the liner2. We are investigating this phenomenon using 10 mm long cylindrical metal liners having 4 mm diameter and varying wall thickness on the 1 MA, 100–230 ns COBRA pulsed power generator at Cornell University. In addition, we will present experimental results on a configuration that includes a wire initially connecting the cathode and anode on the axis of a cylindrical liner that does not initially connect the cathode to the anode. Results of these experiments both with and without external magnetic fields will be shown. Extreme ultraviolet (XUV) imaging as well as optical streak and 12 frame camera imaging are used to observe and analyze liner surface emission. Axial and side-on interferometry are used to determine the distribution of plasma both near the liner surface and inside the liner, assessing, among other things, the impact of non-uniformities during the plasma ablation phase of the experiments.

Tests of the magnetized liner inertial fusion (MagLIF) concept will make use of the 27 MA Z-machine at Sandia National Laboratories, Albuquerque, to implode a cylindrical metal liner to compress and heat a preheated plasma contained within it [1]. While most pulsed power machines produce much lower currents than the Z-machine, there are questions that can still be answered on smaller scale setups. Recent work on the Cornell Beam Research Accelerator (COBRA) has made use of cylindrical metal liners made in-house to study the initiation of plasma on the liner surface and axial magnetic field compression. Although such liners are not sufficiently uniform for MagLIF-relevant studies of liner dynamics during implosion, they can make early-time studies of cylindrical liner implosions viable for smaller laboratories. This paper will present experimental results of imploding liners having imperfect uniformity to assess the impact that has on initiation and ablation. Extreme ultraviolet (XUV) imaging as well as optical streak imaging is used to assess when and where non-uniform-driven emission occurs. Axial interferometry is used to determine the distribution of plasma near the liner surface, including the impact of non-uniformities during the plasma initiation and ablation phases of the experiments.

Tests of the magnetized liner inertial fusion (MagLIF) concept will make use of the 27 MA Z machine at Sandia National Laboratories, Albuquerque, to implode a cylindrical metal liner to compress and heat preheated, magnetized plasma contained within it. While most pulsed power machines produce much lower currents than the Z-machine, there are issues that can still be addressed on smaller scale facilities. Recent work on the Cornell Beam Research Accelerator (COBRA) has made use of 10 mm long and 4 mm diameter metal liners having different wall thicknesses to study the initiation of plasma on the liner’s surface as well as axial magnetic field compression [P.-A. Gourdain et al., Nucl. Fusion 53, 083006 (2013)]. This report presents experimental results with non-imploding liners, investigating the impact the liner’s surface structure has on initiation and ablation. Extreme ultraviolet (XUV) imaging and optical 12 frame camera imaging were used to observe and assess emission non-uniformities as they developed. Axial and side-on interferometry was used to determine the distribution of plasma near the liner surface, including the impact of non-uniformities during the plasma initiation and ablation phases of the experiments.