[en] Adding a wire on the axis of wire arrays significantly affects the x-ray emission of the conical arrays, and much less that of the cylindrical ones. The radiation of the conical wire arrays increases with the thickness of the central wire, surpassing that of the equivalent cylindrical arrays. Significant energy is emitted early on, around the time of the conical shock formation, before the pinch stagnation.

[en] Localized, intensely radiating regions are often observed in Z pinches. High resolution images of such areas have been recorded at least as far back as the 1970s. However, there is as yet no widely accepted consensus on the nature of these “bright spots” or how they are formed. This phenomenon has also been referred to “hot spots” or “micropinches.” To shed further light on this issue, we have analyzed axially resolved K-shell spectra from 4 Z pinches driven by the refurbished Z generator (“ZR”) at Sandia National Laboratories, and the previous version of the Z machine (“Z”). The atomic numbers of the loads varied from 13 to 29. We find that higher spatial K-shell intensity in the Al pinch correlates with density. The K-shell intensity within a copper shot taken on ZR correlates strongly with increased electron temperature, but another, somewhat less well-diagnosed copper shot from Z shows correlation with density. The bright spots in a Ti pinch correlate with neither density nor temperature, but do correlate with the product of density and diameter (proportional to opacity). This opacity correlation is also observed in the other 3 pinches.

[en] Adding a center wire on the axis of a conical wire array produces conditions suitable for studying shear flow stabilization of the Z-pinch. The conical wire array produces and axial plasma flow while the center wire introduces a radial variation of the axial velocity. Experiments of this array configuration were preformed on the 1 MA Zebra Z-pinch generator and showed stabilization of the kink instability when a center wire was present. Comparison with equivalent cylindrical wire arrays indicates that the shear flow stabilization plays a role in the stabilization of the kink instability.

[en] Large diameter multi-shell gas puffs rapidly imploded by high current (∼20 MA, ∼100 ns) on the Z generator of Sandia National Laboratories are able to produce high-intensity Krypton K-shell emission at ∼13 keV. Efficiently radiating at these high photon energies is a significant challenge which requires the careful design and optimization of the gas distribution. To facilitate this, we hydrodynamically model the gas flow out of the nozzle and then model its implosion using a 3-dimensional resistive, radiative MHD code (GORGON). This approach enables us to iterate between modeling the implosion and gas flow from the nozzle to optimize radiative output from this combined system. Guided by our implosion calculations, we have designed gas profiles that help mitigate disruption from Magneto-Rayleigh–Taylor implosion instabilities, while preserving sufficient kinetic energy to thermalize to the high temperatures required for K-shell emission

[en] Wire array z-pinches have been used successfully for many years as a powerful x-ray source, as a dynamic hohlraum, and as an intense K-shell radiation source. Significant progress has been made in the effective modeling of these three-dimensional (3D) resistive plasmas. However, successful modeling also requires an accurate representation of the power delivered to these loads from the generator, which is an uncertainty potentially as large as the magnetohydrodynamic (MHD) implosion dynamics. We present 3D resistive MHD simulations of wire arrays that are coupled to transmission line equivalent models of the Z generator, driven by voltage sources derived directly from electrical measurements. Significant (multi-mega-ampere) current losses are shown to occur in both the convolute and the final feed. This limits the array performance and must be correctly accounted for to accurately represent the generator response to the load. Our simulations are validated against data for compact: 20 mm diameter, 10 mm long wire arrays that have produced the highest x-ray power densities on Z. This is one of the most comprehensive experimental data sets for single and nested wire arrays and includes voltage, current, x-ray power and energy, and multiple mass distribution measurements. These data tightly constrain our simulation results and allow us to describe in detail both the implosion and stagnation, and how energy is delivered to, and radiated from z-pinch loads. We show that the radiated power is consistent with the kinetic energy delivered to a distributed 3D mass profile over its implosion and stagnation. We also demonstrate how the local inductance of the transmission line connecting to the wire array is responsible for delivering more than 50% of the total radiated power. This makes the power output dependent on the design of specific elements of the generator, and their response to the imploding load, and not just on the peak current that can be delivered.

[en] In order to understand the formation and dynamics of plasma in wire array z-pinch experiments, measurements of the distribution of current throughout the array are required. We present details of two Faraday probing diagnostics aimed at exploring the magnetic fields and hence distribution of current in an array. An imaging Faraday system utilizes a short laser pulse to make estimates of the current distribution in the precursor column formed on axis before implosion. In a second system, a rod of high Verdet constant glass is placed close to the wires of an array and the polarization of a cw laser passing through the rod is monitored to examine the variance of current with time

[en] Advancements have been made in the diagnostic techniques to measure accurately the total radiated x-ray yield and power from z-pinch implosion experiments at the Z machine with high accuracy. The Z machine is capable of outputting 2 MJ and 330 TW of x-ray yield and power, and accurately measuring these quantities is imperative. We will describe work over the past several years which include the development of new diagnostics, improvements to existing diagnostics, and implementation of automated data analysis routines. A set of experiments on the Z machine were conducted in which the load and machine configuration were held constant. During this shot series, it was observed that the total z-pinch x-ray emission power determined from the two common techniques for inferring the x-ray power, a Kimfol filtered x-ray diode diagnostic and the total power and energy diagnostic, gave 449 TW and 323 TW, respectively. Our analysis shows the latter to be the more accurate interpretation. More broadly, the comparison demonstrates the necessity to consider spectral response and field of view when inferring x-ray powers from z-pinch sources

[en] We describe a pair of time-integrated transmission spectrometers that are designed to survey 7-28 keV (1.9 to 0.43 A) x-ray photons produced by experiments on the Sandia Z pulsed power facility. Each spectrometer uses a quartz 10-11 crystal in a Cauchois geometry with a slit to provide spatial resolution along one dimension. The spectrometers are located in the harsh environment of the Z vacuum chamber, which necessitates that their design be compact and rugged. Example data from calibration tests and Z experiments are shown that illustrate the utility of the instruments.

[en] Experiments with cylindrical copper wire arrays at the 1-MA Zebra facility show that high temperatures exist in the precursor plasmas formed when ablated wire array material accretes on the axis prior to the stagnation of a z pinch. In these experiments, the precursor radiated approximately 20% of the >1000 eV x-ray output, and time-resolved spectra show substantial emission from Cu L-shell lines. Modeling of the spectra shows an increase in temperature as the precursor forms, up to ∼450 eV, after which the temperature decreases to ∼220-320 eV until the main implosion

[en] Recent experiments on the Z accelerator have produced high-energy (17 keV) inner-shell K-alpha emission from molybdenum wire array z-pinches. Extensive absolute power and spectroscopic diagnostics along with collisional-radiative modeling enable detailed investigation into the roles of thermal, hot electron, and fluorescence processes in the production of high-energy x-rays. We show that changing the dimensions of the arrays can impact the proportion of thermal and non-thermal K-shell x-rays