Imploding wire arrays on the 20 MA Z generator have recently provided some of the most powerful and energetic laboratory sources of multi-keV photons, including ∼375 kJ of Al K-shell emission (hν ∼ 1–2 keV), ∼80 kJ of stainless steel K-shell emission (hν ∼ 5–9 keV) and a kJ-level of Mo K-shell emission (hν ∼ 17 keV). While the global implosion dynamics of these different wire arrays are very similar, the physical process that dominates the emission from these x-ray sources fall into three broad categories. Al wire arrays produce a column of plasma with densities up to ∼3 × 1021 ions/cm3, where opacity inhibits the escape of K-shell photons. Significant structure from instabilities can reduce the density and increase the surface area, therefore increase the K-shell emission. In contrast, stainless steel wire arrays operate in a regime where achieving a high pinch temperature (achieved by thermalizing a high implosion kinetic energy) is critical and, while opacity is present, it has less impact on the pinch emissivity. At higher photon energies, line emission associated with inner shell ionization due to energetic electrons becomes important.

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.

Multiple experimental campaigns have been executed to study the implosions of initially solid beryllium (Be) liners (tubes) on the Z pulsed-power accelerator. The implosions were driven by current pulses that rose from 0 to 20 MA in either 100 or 200 ns (200 ns for pulse shaping experiments). These studies were conducted in support of the recently proposed Magnetized Liner Inertial Fusion concept [Slutz et al., Phys. Plasmas 17, 056303 (2010)], as well as for exploring novel equation-of-state measurement techniques. The experiments used thick-walled liners that had an aspect ratio (initial outer radius divided by initial wall thickness) of either 3.2, 4, or 6. From these studies, we present three new primary results. First, we present radiographic images of imploding Be liners, where each liner contained a thin aluminum sleeve for enhancing the contrast and visibility of the liner's inner surface in the images. These images allow us to assess the stability of the liner's inner surface more accurately and more directly than was previously possible. Second, we present radiographic images taken early in the implosion (prior to any motion of the liner's inner surface) of a shockwave propagating radially inward through the liner wall. Radial mass density profiles from these shock compression experiments are contrasted with profiles from experiments where the Z accelerator's pulse shaping capabilities were used to achieve shockless (“quasi-isentropic”) liner compression. Third, we present “micro-Ḃ” measurements of azimuthal magnetic field penetration into the initially vacuum-filled interior of a shocked liner. Our measurements and simulations reveal that the penetration commences shortly after the shockwave breaks out from the liner's inner surface. The field then accelerates this low-density “precursor” plasma to the axis of symmetry.

Recent experiments at the 20 MA Z Accelerator have demonstrated, for the first time, implosion velocities up to 110–130 cm/μs in imploding stainless steel wire arrays. These velocities, the largest inferred in a magnetically driven implosion, lead to ion densities of 2 × 1020 cm−3 with electron temperatures of ∼5 keV. These plasma conditions have resulted in significant increases in the K-shell radiated output of 5–10 keV photons, radiating powers of >30 TW and yields >80 kJ, making it the brightest laboratory x-ray source in this spectral region. These values represent a doubling of the peak power and a 30% increase in the yield relative to previous studies. The experiments also included wire arrays with slower implosions, which were observed to have lower temperatures and reduced K-shell output. These colder pinches, however, radiated 260 TW in the soft x-ray region, making them one of the brightest soft x-ray sources available.

High current pulsed-power generators efficiently store and deliver magnetic energy to z-pinch targets. We review applications of magnetically driven implosions (MDIs) to inertial confinement fusion. Previous research on MDIs of wire-array z-pinches for radiation-driven indirect-drive target designs is summarized. Indirect-drive designs are compared with new targets that are imploded by direct application of magnetic pressure produced by the pulsed-power current pulse. We describe target design elements such as larger absorbed energy, magnetized and pre-heated fuel, and cryogenic fuel layers that may relax fusion requirements. These elements are embodied in the magnetized liner inertial fusion (MagLIF) concept [Slutz et al. "Pulsed-power-driven cylindrical liner implosions of laser pre-heated fuel magnetized with an axial field," Phys. Plasmas, 17, 056303 (2010), and Stephen A. Slutz and Roger A. Vesey, "High-Gain-Magnetized Inertial Fusion," Phys. Rev. Lett., 108, 025003 (2012)]. MagLIF is in the class of magneto-inertial fusion targets. In MagLIF, the large drive currents produce an azimuthal magnetic field that compresses cylindrical liners containing pre-heated and axially pre-magnetized fusion fuel. Scientific breakeven may be achievable on the Z facility with this concept. Simulations of MagLIF with deuterium-tritium fuel indicate that the fusion energy yield can exceed the energy invested in heating the fuel at a peak drive current of about 27 MA. Scientific breakeven does not require alpha particle self-heating and is therefore not equivalent to ignition. Capabilities to perform these experiments will be developed on Z starting in 2013. These simulations and predictions must be validated against a series of experiments over the next five years. Near-term experiments are planned at drive currents of 16 MA with D-2 fuel. MagLIF increases the efficiency of coupling energy (=target absorbed energy/driver stored energy) to targets by 10-150X relative to indirect-drive targets. MagLIF also increases the absolute energy absorbed by the target by 10-50X relative to indirect-drive targets. These increases could lead to higher fusion gains and yields. Single-shot high yields are of great utility to national security missions. Higher efficiency and higher gains may also translate into more compelling (lower cost and complexity) fusion reactor designs. We will discuss the broad goals of the emerging research on the MagLIF concept and identify some of the challenges. We will also summarize advances in pulsed-power technology and pulsed-power driver architectures that double the efficiency of the driver.

The implosions of initially solid beryllium liners (tubes) have been imaged with penetrating radiography through to stagnation. These novel radiographic data reveal a high degree of azimuthal correlation in the evolving magneto-Rayleigh-Taylor structure at times just prior to (and during) stagnation, providing stringent constraints on the simulation tools used by the broader high energy density physics and inertial confinement fusion communities. To emphasize this point, comparisons to 2D and 3D radiation magnetohydrodynamics simulations are also presented. Both agreement and substantial disagreement have been found, depending on how the liner's initial outer surface finish was modeled. The various models tested, and the physical implications of these models are discussed. These comparisons exemplify the importance of the experimental data obtained.

The Z pulsed power driver at Sandia National Laboratories is used for a variety of high energy physics experiments. The Z system stores 20 megajoules at the present nominal ±85 kV Marx charge voltage. The Z system consists of 36 basically identical modules, each with a DC-charged Marx generator, water insulated intermediate store capacitor, laser triggered gas switch, and water insulated pulse-forming section. High current drivers such as Z are able to create energy densities of megajoules per cubic centimeter in ~1 cm3 volumes, used for creating extremes of temperature and pressure on a nanosecond time scale. The Z driver has delivered currents up to 27 MA with 85 nanosecond rise time (10%-90%) into fixed inductance and imploding plasma loads. Z has generated over 300 TW peak X-ray power, with more than 2 MJ total energy radiated from imploding tungsten plasma shells. Z is also used for studying dynamic compression of solid materials at megabar levels, using magnetic pressure from current densities of tens of megamperes per cm. Z is a confluence of pulsed energy storage and switching technologies. DC charged Marx generators are triggered with ten-nanosecond precision, transferring energy to water-insulated coaxial capacitors. Six megavolt laser triggered gas switches perform the final nanosecond time synchronization, and self-closing water switches further compress the pulse. Issues with the system include improving the reliability and performance of the components, and improving our conceptual understanding of the entire machine. Many of the Z components are unique: Z requires laser triggered gas switches operating at six million volts, water switches operating at three million volts, and large-area solid-vacuum interfaces operating reliably at 140 kV/cm. Synchronization with fast load diagnostics requires nanosecond predictability of the load current timing, while the pre-fire probability of any switch must be extremely small. We will describe- the state of the machine, and specific measurements of subsystem performance.

A recent publication [D. B. Sinars et al., Phys. Rev. Lett. 105, 185001 (2010)] describes the first controlled experiments measuring the growth of the magneto-Rayleigh–Taylor instability in fast (∼100 ns) Z-pinch plasmas formed from initially solid aluminum tubes (liners). Sinusoidal perturbations on the surface of these liners with wavelengths of 25–400 μm were used to seed single-mode instabilities. The evolution of the outer liner surface was captured using multiframe 6.151 keV radiography. The initial paper shows that there is good agreement between the data and 2-D radiation magneto-hydrodynamic simulations down to 50 μm wavelengths. This paper extends the previous one by providing more detailed radiography images, detailed target characterization data, a more accurate comparison to analytic models for the amplitude growth, the first data from a beryllium liner, and comparisons between the data and 3D simulations.

The first controlled experiments measuring the growth of the magneto-Rayleigh-Taylor instability in fast (∼100  ns) Z-pinch plasmas are reported. Sinusoidal perturbations on the surface of an initially solid Al tube (liner) with wavelengths of 25-400  μm were used to seed the instability. Radiographs with 15  μm resolution captured the evolution of the outer liner surface. Comparisons with numerical radiation magnetohydrodynamic simulations show remarkably good agreement down to 50  μm wavelengths.

Experimental data is presented that illustrates important displacement current phenomena in the magnetically insulated transmission lines (MITLs) of the refurbished Z accelerator [D. V. Rose et al., Phys. Rev. ST Accel. Beams 13, 010402 (2010)]. Specifically, we show how displacement current in the MITLs causes significant differences between the accelerator current measured at the vacuum-insulator stack (at a radial position of about 1.6 m from the Z axis of symmetry) and the accelerator current measured at the load (at a radial position of about 6 cm from the Z axis of symmetry). The importance of accounting for these differences was first emphasized by Jennings et al. [C. A. Jennings et al., IEEE Trans. Plasma Sci. 38, 529 (2010)], who calculated them using a full transmission-line-equivalent model of the four-level MITL system. However, in the data presented by Jennings et al., many of the interesting displacement current phenomena were obscured by parasitic current losses that occurred between the vacuum-insulator stack and the load (e.g., electron flow across the anode-cathode gap). By contrast, the data presented herein contain very little parasitic current loss, and thus for these low-loss experiments we are able to demonstrate that the differences between the current measured at the stack and the current measured at the load are due primarily to the displacement current that results from the shunt capacitance of the MITLs (about 8.41 nF total). Demonstrating this is important because displacement current is an energy storage mechanism, where energy is stored in the MITL electric fields and can later be used by the system. Thus, even for higher-loss experiments, the differences between the current measured at the stack and the current measured at the load are often largely due to energy storage and subsequent release, as opposed to being due solely to some combination of measurement error and current loss in the MITLs and/or double post-hole convolute. Displacement current also explains why the current measured downstream of the MITLs (i.e., the load current) often exceeds the current measured upstream of the MITLs (i.e., the stack current) at various times in the power pulse (this particular phenomenon was initially thought to be due to timing and/or calibration errors). To facilitate a better understanding of these phenomena, we also introduce and analyze a simple LC circuit model of the MITLs. This model is easily implemented as a simple drive circuit in simulation codes, which has now been done for the LASNEX code [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)] at Sandia, as well as for simpler MATLAB (R)-based codes at Sandia. An example of this LC model used as a drive circuit will also be presented.

Blast waves (BWs) form when the wave speed of an initially diffusive, supersonic radiation wave becomes subsonic and creates a radiographically-visible, hydrodynamic shock wave. BWs are a novel diagnostic in radiation-flow, code validation experiments that use Sandia's Z-accelerator's dynamic hohlraum (DH) as a radiative source. The physics models being tested are sensitive to delivered energy and power changes of better than +/- 10%; therefore, precise in-situ radiative power and energy measurements are required for quantitative comparisons between simulation and experiment. The energy sensitive BW diagnostic complements bolometric and x-ray radiometric diagnostics in providing these measurements. Recent comparisons between BW qualification experiments and simulations have revealed a spatial dependence on the radiation source. We discuss the experimental design and sensitivities for the BW diagnostic and experimental results in comparison to simulations and other diagnostics.

Dynamic bohlraum (DH) z-pinches on the Z machine at Sandia National Laboratories have proven to be an excellent radiation source for driving high energy density experiments. The Z DH source produces as much as 100 W with powers up to 17 TW into a 4-mm diameter, axially-located radiation exit hole above the source. The Sandia Laboratories' ZR upgrade of Z may be able to similarly produce 200 U. The complex Z DH radiation source varies in spectral power and timing from shot-to-shot. We are characterizing the source physics through simulation and experiment in order to minimize this variability. We are studying how the complex spatially and temporally-dependent behavior of the DH source affects the performance of a number of axially-locatcd experiments. Using the Z DH as a source, we developed the new blast wave diagnostic, and applied it in a number of novel experiments. A large number of physics issues can be addressed by the Z DH and blast wave diagnostic, including experiments to study opacity, radiation transport and hydrodynamics. We will show how some experiments are quite sensitive to the source details while others are much less so. A synopsis and analysis of recently completed experiments as well as proposals for future work are presented.

The quality of high wire-number z-pinch implosions on Z using a dynamic hohlraum (DH) configuration [Sanford, et al., Phys. Plasmas 9, 3573 (2002)] is significantly affected by the method of holding the wires. The three arrangements discussed here have led to differences in radial and axial x-ray powers of factors of 1.6±0.2 and 1.5±0.2, respectively. An increase in power is accompanied by reductions in rise time and pulse width, and improvements in shot-to-shot reproducibility. Higher powers are produced by fixtures that enable the wires to be maintained taut, which also produce superior current contacts at the electrodes (and in particular at the cathode) prior to implosion. The increased axial power, and decreased variation in power and pulse shape, correlate with decreased wire-plasma material observed at the axial radiation exit holes of the DH.

Summary form only given. Experiments have been performed on a relativistic magnetron using innovative metal cathodes (Al) rather than the cotton fiber cathode previously used. The new cathode is fabricated by projecting a pattern on the cathode and ablating the metal by a KrF laser; defined here as projection ablation lithography (PAL). The PAL cathode utilizes microtexturing of a solid metal cathode substrate to provide electric field enhancement. The potential advantages of this cathode are that, unlike microtips, which can overheat, explode and create plasma, the PAL cathode emission regions are heat-sinked to the base cathode material. Furthermore, the PAL cathode consists of pure metal, so after an initial bakeout or plasma cleaning (or a few conditioning shots), there is low gas emission (unlike fabric cathodes). The relativistic, 6-vane, (Titan) magnetron is powered by the MELBA-C accelerator with parameters: 0.3 MV, 1-10 kA and 0.5 /spl mu/s pulselength. The microwave startup time for PAL cathode has decreased to an average of 118 ns compared to 233 ns for the cotton cathode. Microwave pulselength average for the PAL cathode has increased to 217 ns, from 144 ns experimentally observed for the cotton cathode. With these improvements in microwave startup-time and pulselength, the average microwave power has remained approximately the same as conventional cloth cathodes (100-500 MW total, peak microwave power extracted). A new, triple-azimuthal-emission region has been tested as means of pre-bunching the electrons into the 3 spokes desired for pi-mode operation in a 6-cavity magnetron. This Tri-PAL cathode has demonstrated the lowest average microwave starting-times (109 ns). Cathode plasma closure for some shots has been dramatically decreased, from 3 /spl mu/s/cm to less than 0.5 /spl mu/s/cm; certain shots have even shown zero plasma closure.

Relativistic magnetron experiments performed on a six-cavity device have generated over 300 MW total microwave power near 1 GHz. These experiments were driven by the long-pulse electron beam from an accelerator with parameters as follows: voltage of *300 kV, current of 1-10 kA, and typical pulselength of 0.5 ms. This paper reports investigations of high-power microwave generation, mode competition, and pulse shortening for the relativistic magnetron with a ceramic insulator compared to a plastic insulator. The ceramic insulator improves the vacuum by a factor of ten (to 10/sup *7/ torr range) and flattens the voltage of the accelerator. Relativistic magnetron performance with the ceramic insulator shows increased microwave power and pulselength over the plastic insulator. Effects of RF breakdown in the extraction waveguide on peak microwave power and pulselength are also investigated by utilizing SF/sub 6/ in one or both of the extraction waveguides.

Microfabricated folded waveguide traveling-wave tubes (TWTs) are potential compact sources of wide-band, high-power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with delayed feedback. Simulations of a 560-GHz oscillator and experimental evaluation of the concept at 50 GHz are presented. Additionally, results from various fabrication methods that are under investigation, such as X-ray lithography, electroforming, and molding (LIGA), UV LIGA, and deep reactive ion etching are presented. Observations and measurements are reported on the generation of stable single-frequency oscillation states. On varying the feedback level the oscillation changes from a stable single-frequency state at the threshold to multifrequency spectra in the overdriven state. Simulation and experimental results on amplifier characterization and dynamics of the regenerative TWT oscillator include spectral evolution and phase stability of the generated frequencies. The results of the experiment are in good agreement with the simulations.

Initial results are presented of an innovative cathode operating in a relativistic magnetron powered by an accelerator with parameters: −0.3 MV, 1–10 kA, and 0.5 μs pulse length. This cathode is fabricated by ablating a pattern on the cathode using a KrF laser. This projection ablation lithography (PAL) cathode has demonstrated fast current turn-on and microwave startup times have decreased from an average of 193 to 118 ns. The pulselength of 1 GHz microwave oscillation has increased from a 144 ns average to 217 ns. With these improvements in microwave startup and pulse length, the microwave power has approximately remained the same compared to the previously used cloth cathodes. A new triple-azimuthal emission region is tested as means of prebunching the electrons (“cathode priming”) into the three spokes desired for pi mode operation in a six-cavity magnetron. The Tri-PAL cathode priming results in the fastest startup and highest efficiency of relativistic magnetron microwave generation.