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We present a technique for experimentally measuring two-dimensional radiation temperatures in dynamic Hohlraums on Z. In principle the technique can be applied to any radiation source. Total radiated power from the source is measured by normalizing the area under an x-ray diode signal to energy yield measured by a bolometer. The radiated power as a function of time, which is just the normalized x-ray diode signal, can then be used to normalize gated microchannel plate x-ray pinhole camera images. The procedure is most accurate when the gated x-ray pinhole camera has the same filter as the x-ray diode and when the filter is transmissive near the peak of the Planckian radiation temperature being measured. We present results for two-dimensional radiation temperatures as a function of time for dynamic Hohlraum experiments on Z. In these experiments a z pinch consisting of nested tungsten wire arrays driven by the 20 MA, 100 ns Z accelerator implodes onto cylindrical foam located on axis. X-ray diodes and bolo...

As a result of advances in fast pulsed-power technology and cylindrical load fabrication, the Z pulsed-power accelerator at Sandia National Laboratories drives currents approaching 20 MA with a rise time of approximately 100 ns through cylindrically-symmetric loads (typically a cylindrical array consisting of a few hundred wires) to produce plasma densities in excess of , x-ray output energies approaching 2 MJ, radiation pulses as short as 4 ns and peak x-ray powers as high as . More than 15% of the stored electrical energy in the Z pulsed-power accelerator is converted into x-rays. The plasma pressures at peak compression are several TPa with electron temperatures that can exceed 3 keV at containment magnetic fields exceeding 1000 T. Depending on the atomic number and composition of the imploding plasma, these z-pinches can be tailored to produce intense sources of thermal x-rays, keV x-rays or neutrons. Although applications of these x-ray sources have included research in radiation material interaction, equations of state, opacity, astrophysics and x-ray lasers, the principal focus of the present research is to use them for indirect-drive inertial confinement fusion (ICF).

Al:Mg alloy wire arrays of mass loads 1.3–3.6 mg/cm have been imploded with peak currents of 19 MA on the 60 TW Z generator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The large mass loads have resulted in the highest K-shell x-ray line optical depths (∼103) produced to date in Z-pinches. Analysis of the time-resolved spectrum of a 2.1 mg/cm shot near the time of peak compression has yielded a temperature–density profile of the pinch that approximately reproduces all features of the x-ray data except the continuum above 5 keV, which is underpredicted. The Ly α/He α ratio for Al is shown to be enhanced relative to that of Mg by two mechanisms: photopumped ladder ionization and absorption of the Al He-like line in a cool outer halo. This analysis and comparisons to some Ti shots demonstrates that the K-shell yield of Al is significantly reduced by line and continuum self-absorption, but that of Ti is not.

Nested-wire arrays on the 20-MA Z -pinch accelerator have produced x-ray powers up to 280{plus_minus}40 TW (a 40{percent} increase over a single array) and an x-ray pulse width of 4thinspthinspns. The short x-ray pulse widths are associated with the formation of tight (1-mm-diameter), uniform pinches at stagnation. Two-dimensional radiation magnetohydrodynamic calculations suggest that the inner array mitigates the growth of implosion instabilities thereby leading to smaller diameter pinches that radiate at higher power than single-wire arrays. {copyright} {ital 1998} {ital The American Physical Society }

Doubling the number of tungsten wires from 120 to 240, keeping the mass fixed, increased the radiated X-ray power relative to the electrical power at the insulator stack of the Z accelerator by (35/spl plusmn/15)% for 8.75- and 20-mm radii Z-pinch wire arrays. One-dimensional radiation magneto hydrodynamic calculations suggest that the arrays were operating in a quasi "plasma-shed" regime, where the plasma generated by the individual wires partially merge prior to the inward implosion of the entire array.

Time-resolved x-ray pinhole photographs and time-integrated radially resolved x-ray crystal-spectrometer measurements of azimuthally symmetric aluminum-wire implosions suggest that the densest phase of the pinch is composed of a hot plasma core surrounded by a cooler plasma halo. The slope of the free-bound x-ray continuum, provides a time-resolved, model-independent diagnostic of the core electron temperature. A simultaneous measurement of the time-resolved K-shell line spectra provides the electron temperature of the spatially averaged plasma. Together, the two diagnostics support a one-dimensional radiation–hydrodynamic model prediction of a plasma whose thermalization on axis produces steep radial gradients in temperature, from temperatures in excess of 1 kV in the core to below 1 kV in the surrounding plasma halo.

A systematic experimental study of annular aluminum-wire Z-pinches on a 20-TW electrical generator shows that the measured spatial characteristics and emitted x-ray power agree more closely with rad-hydro simulations when large numbers of wires are used. The measured x-ray power increases first slowly and then rapidly with decreasing interwire gap spacing. Simulations suggested that this increase reflects the transition from implosion of individual wire plasmas to one of an azimuthally symmetric plasma shell. In the plasma-shell regime, x-ray powers of 40 TW are achieved.

We describe herein a system of self-magnetically insulated vacuum transmission lines (MITLs) that operated successfully at 20 MA, 3 MV, and 55 TW. The system delivered the electromagnetic-power pulse generated by the $Z$ accelerator to a physics-package load on over 1700 $Z$ shots. The system included four levels that were electrically in parallel. Each level consisted of a water flare, vacuum-insulator stack, vacuum flare, and 1.3-m-radius conical outer MITL. The outputs of the four outer MITLs were connected in parallel by a 7.6-cm-radius 12-post double-post-hole vacuum convolute. The convolute added the currents of the four outer MITLs, and delivered the combined current to a single 6-cm-long inner MITL. The inner MITL delivered the current to the load. The total initial inductance of the stack-MITL system was 11 nH. A 300-element transmission-line-circuit model of the system has been developed using the tl code. The model accounts for the following: (i) impedance and electrical length of each of the 300 circuit elements, (ii) electron emission from MITL-cathode surfaces wherever the electric field has previously exceeded a constant threshold value, (iii) Child-Langmuir electron loss in the MITLs before magnetic insulation is established, (iv) MITL-flow-electron loss after insulation, assuming either collisionless or collisional electron flow, (v) MITL-gap closure, (vi) energy loss to MITL conductors operated at high lineal current densities, (vii) time-dependent self-consistent inductance of an imploding $z$-pinch load, and (viii) load resistance, which is assumed to be constant. Simulations performed with the tl model demonstrate that the nominal geometric outer-MITL-system impedance that optimizes overall performance is a factor of $\ensuremath{\sim}3$ greater than the convolute-load impedance, which is consistent with an analytic model of an idealized MITL-load system. Power-flow measurements demonstrate that, until peak current, the $Z$ stack-MITL system performed as expected. tl calculations of the peak electromagnetic power at the stack, stack energy, stack voltage, outer-MITL current, and load current, as well as the pinch-implosion time, agree with measurements to within 5%. After peak current, tl calculations and measurements diverge, which appears to be due in part to the idealized pinch model assumed by tl. The results presented suggest that the design of the $Z$ accelerator's stack-MITL system, and the tl model, can serve as starting points for the design of stack-MITL systems of future superpower accelerators.

We have developed a spatially and temporally resolved crystal spectrometer for analyzing a variety of pinch experiments on Z. The spectrometer uses a convex curved crystal to disperse spectra onto a flat microchannel plate (MCP) framing camera detector. A single wide, 1 cm, strip on the MCP is gated to provide temporal resolution. The spectral range governed by the 4 cm length of the MCP strip varies with the central Bragg angle and crystal. For a KAP crystal a typical range is 1500–2000 eV. This range can be shifted by translating the crystal along the optical axis to access different Bragg angles. The spectrometer can therefore measure K shell spectra of a wide variety of elements such as Al, Ti, and Fe. The short 1 cm width of the strip is spatially resolved with an imaging cross slit. With a 500 μm cross slit and magnification 1 the spatial resolution at the pinch is 1 mm. The instrument may also be fielded with seven time frames using a seven strip-line microchannel plate as the detector by sacrificing the spatial resolution. We will present data obtained from an aluminum pinch on Z.

High-energy photons and electrons on the Sandia National Laboratories “Z” accelerator, a z-pinch device, will deposit energy into a capsule and fuel; this may create a potential preheat problem for inertial confinement infusion (ICF). In this article we discuss heating of the capsule and fuel by high-energy photons and electrons. The fuel is heated to

We have developed a system of differential-output monitors that diagnose current and voltage in the vacuum section of a 20-MA 3-MV pulsed-power accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuum-insulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The inner-MITL monitors are located 6 cm from the axis of the load. Each of the stack and outer-MITL current monitors comprises two separate $B$-dot sensors, each of which consists of four 3-mm-diameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100-ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the $B$-dot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Common-mode-noise rejection is achieved by combining these signals in a $50\mathrm{\text{\ensuremath{-}}}\ensuremath{\Omega}$ balun. The signal cables that connect the $B$-dot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single $50\mathrm{\text{\ensuremath{-}}}\ensuremath{\Omega}$ cable transmits the output signal of each balun to a double-wall screen room, where the signals are attenuated, digitized ($0.5\mathrm{\text{\ensuremath{-}}}\mathrm{ns}/\mathrm{sample}$), numerically compensated for cable losses, and numerically integrated. By contrast, each inner-MITL current monitor contains only a single $B$-dot sensor. These monitors are fielded in opposite-polarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for common-mode-noise rejection after digitization. All the current monitors are calibrated on a 76-cm-diameter axisymmetric radial transmission line that is driven by a 10-kA current pulse. The reference current is measured by a current-viewing resistor (CVR). The stack voltage monitors are also differential-output gauges, consisting of one 1.8-cm-diameter $D$-dot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MV on dedicated accelerator shots with a short-circuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outer-MITL $B$-dot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA to a low-impedance $z$-pinch load, the peak lineal current densities at the stack, outer-MITL, and inner-MITL monitor locations are 0.5, 1, and $58\text{ }\text{ }\mathrm{MA}/\mathrm{m}$, respectively. On such shots the peak currents measured at these three locations agree to within 1%.

We have developed a diagnostic system that measures the spectrally integrated (i.e. the total) energy and power radiated by a pulsed blackbody x-ray source. The total-energy-and-power (TEP) diagnostic system is optimized for blackbody temperatures between 50 and 350 eV. The system can view apertured sources that radiate energies and powers as high as 2 MJ and 200 TW, respectively, and has been successfully tested at 0.84 MJ and 73 TW on the $Z$ pulsed-power accelerator. The TEP system consists of two pinhole arrays, two silicon-diode detectors, and two thin-film nickel bolometers. Each of the two pinhole arrays is paired with a single silicon diode. Each array consists of a $38\ifmmode\times\else\texttimes\fi{}38$ square array of 10-$\ensuremath{\mu}\mathrm{m}$-diameter pinholes in a 50-$\ensuremath{\mu}\mathrm{m}$-thick tantalum plate. The arrays achromatically attenuate the x-ray flux by a factor of $\ensuremath{\sim}1800$. The use of such arrays for the attenuation of soft x rays was first proposed by Turner and co-workers [Rev. Sci. Instrum. 70, 656 (1999)]. The attenuated flux from each array illuminates its associated diode; the diode's output current is recorded by a data-acquisition system with 0.6-ns time resolution. The arrays and diodes are located 19 and 24 m from the source, respectively. Because the diodes are designed to have an approximately flat spectral sensitivity, the output current from each diode is proportional to the x-ray power. The nickel bolometers are fielded at a slightly different angle from the array-diode combinations, and view (without pinhole attenuation) the same x-ray source. The bolometers measure the total x-ray energy radiated by the source and---on every shot---provide an in situ calibration of the array-diode combinations. Two array-diode pairs and two bolometers are fielded to reduce random uncertainties. An analytic model (which accounts for pinhole-diffraction effects) of the sensitivity of an array-diode combination is presented.

A combination of a 400 ns, 300 mJ, 640 nm dye laser, and an optical streak camera have been used to demonstrate that time-resolved shadowgrams can be made of the implosion phase of tungsten wire arrays. Initial experiments have shown that mirror lifetime and spatial resolution are issues for this diagnostic technique. Nonetheless, these experiments have provided new information on wire array dynamics; specifically, they show that even with a 0.46 mm wire spacing, the high density regions formed by the wires, are separate until 30 ns into the main drive current. Peak currents of 6.6 MA were obtained 40 ns after the start of the current, while peak radiated powers of 85 TW were measured at 50 ns.

The authors have explored the high velocity implosion limit of krypton gas-puff z-pinch implosions in an effort to efficiently generate K-shell radiation from high-Z elements. Using Saturn, a fast electrical generator, they imploded krypton gas-puff z pinches attaining implosion velocities as high as 125 cm/{mu}s. They observed the magneto Raleigh-Taylor instability in the fastest implosions. Despite the high implosion velocities, the plasma was never observed to attain the He-like ionization state, most likely because of inadequate density giving too long an ionization time. The dominant ionization state at peak compression was Ne-like Kr. They measured radiation yields as high as 100 J from Kr K{sub {alpha}} transitions. They used two different nozzle configurations for these experiments: 2.5-cm mean diameter nozzles and 3.5-cm mean diameter nozzles. Both sets of nozzles had exits with a 1-cm width. These nozzles are designed to have a 5-degree inward tilt to reduce the effect of zippering. The gas valve timing was adjusted relative to the generator so that the implosion occurred during the time when the gas density was increased. This also acted to reduce the effect of zippering. VUV preionization of the Kr was used for all of these shots. The pinch length wasmore » 2 cm. Current in the z pinch was measured with both B-dot current monitors and piezoelectric pressure transducers.« less

We have developed wire-array $z$-pinch scaling relations for plasma-physics and inertial-confinement-fusion (ICF) experiments. The relations can be applied to the design of $z$-pinch accelerators for high-fusion-yield $(\ensuremath{\sim}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{GJ∕shot})$ and inertial-fusion-energy $(\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{GJ∕shot})$ research. We find that $({\ensuremath{\delta}}_{a}∕{\ensuremath{\delta}}_{\mathrm{RT}})\ensuremath{\propto}{(m∕\ensuremath{\ell})}^{1∕4}{(R\ensuremath{\Gamma})}^{\ensuremath{-}1∕2}$, where ${\ensuremath{\delta}}_{a}$ is the imploding-sheath thickness of a wire-ablation-dominated pinch, ${\ensuremath{\delta}}_{\mathrm{RT}}$ is the sheath thickness of a Rayleigh-Taylor-dominated pinch, $m$ is the total wire-array mass, $\ensuremath{\ell}$ is the axial length of the array, $R$ is the initial array radius, and $\ensuremath{\Gamma}$ is a dimensionless functional of the shape of the current pulse that drives the pinch implosion. When the product $R\ensuremath{\Gamma}$ is held constant the sheath thickness is, at sufficiently large values of $m∕\ensuremath{\ell}$, determined primarily by wire ablation. For an ablation-dominated pinch, we estimate that the peak radiated x-ray power ${P}_{r}\ensuremath{\propto}{(I∕{\ensuremath{\tau}}_{i})}^{3∕2}R\ensuremath{\ell}\ensuremath{\Phi}\ensuremath{\Gamma}$, where $I$ is the peak pinch current, ${\ensuremath{\tau}}_{i}$ is the pinch implosion time, and $\ensuremath{\Phi}$ is a dimensionless functional of the current-pulse shape. This scaling relation is consistent with experiment when $13\phantom{\rule{0.3em}{0ex}}\mathrm{MA}\ensuremath{\leqslant}I\ensuremath{\leqslant}20\phantom{\rule{0.3em}{0ex}}\mathrm{MA}$, $93\phantom{\rule{0.3em}{0ex}}\mathrm{ns}\ensuremath{\leqslant}{\ensuremath{\tau}}_{i}\ensuremath{\leqslant}169\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, $10\phantom{\rule{0.3em}{0ex}}\mathrm{mm}\ensuremath{\leqslant}R\ensuremath{\leqslant}20\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$, $10\phantom{\rule{0.3em}{0ex}}\mathrm{mm}\ensuremath{\leqslant}\ensuremath{\ell}\ensuremath{\leqslant}20\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$, and $2.0\phantom{\rule{0.3em}{0ex}}\mathrm{mg}∕\mathrm{cm}\ensuremath{\leqslant}m∕\ensuremath{\ell}\ensuremath{\leqslant}7.3\phantom{\rule{0.3em}{0ex}}\mathrm{mg}∕\mathrm{cm}$. Assuming an ablation-dominated pinch and that $R\ensuremath{\ell}\ensuremath{\Phi}\ensuremath{\Gamma}$ is held constant, we find that the x-ray-power efficiency ${\ensuremath{\eta}}_{x}\ensuremath{\equiv}{P}_{r}∕{P}_{a}$ of a coupled pinch-accelerator system is proportional to ${({\ensuremath{\tau}}_{i}{P}_{r}^{7∕9})}^{\ensuremath{-}1}$, where ${P}_{a}$ is the peak accelerator power. The pinch current and accelerator power required to achieve a given value of ${P}_{r}$ are proportional to ${\ensuremath{\tau}}_{i}$, and the requisite accelerator energy ${E}_{a}$ is proportional to ${\ensuremath{\tau}}_{i}^{2}$. These results suggest that the performance of an ablation-dominated pinch, and the efficiency of a coupled pinch-accelerator system, can be improved substantially by decreasing the implosion time ${\ensuremath{\tau}}_{i}$. For an accelerator coupled to a double-pinch-driven hohlraum that drives the implosion of an ICF fuel capsule, we find that the accelerator power and energy required to achieve high-yield fusion scale as ${\ensuremath{\tau}}_{i}^{0.36}$ and ${\ensuremath{\tau}}_{i}^{1.36}$, respectively. Thus the accelerator requirements decrease as the implosion time is decreased. However, the x-ray-power and thermonuclear-yield efficiencies of such a coupled system increase with ${\ensuremath{\tau}}_{i}$. We also find that increasing the anode-cathode gap of the pinch from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}4\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$ increases the requisite values of ${P}_{a}$ and ${E}_{a}$ by as much as a factor of 2.

We have measured the x-ray power and energy radiated by a tungsten-wire-array $z$ pinch as a function of the peak pinch current and the width of the anode-cathode gap at the base of the pinch. The measurements were performed at 13- and 19-MA currents and 1-, 2-, 3-, and 4-mm gaps. The wire material, number of wires, wire-array diameter, wire-array length, wire-array-electrode design, normalized-pinch-current time history, implosion time, and diagnostic package were held constant for the experiments. To keep the implosion time constant, the mass of the array was increased as ${I}^{2}$ (i.e., the diameter of each wire was increased as $I),$ where $I$ is the peak pinch current. At 19 MA, the mass of the 300-wire 20-mm-diam 10-mm-length array was 5.9 mg. For the configuration studied, we find that to eliminate the effects of gap closure on the radiated energy, the width of the gap must be increased approximately as $I.$ For shots unaffected by gap closure, we find that the peak radiated x-ray power ${P}_{r}\ensuremath{\propto}{I}^{1.24\ifmmode\pm\else\textpm\fi{}0.18},$ the total radiated x-ray energy ${E}_{r}\ensuremath{\propto}{I}^{1.73\ifmmode\pm\else\textpm\fi{}0.18},$ the x-ray-power rise time ${\ensuremath{\tau}}_{r}\ensuremath{\propto}{I}^{0.39\ifmmode\pm\else\textpm\fi{}0.34},$ and the x-ray-power pulse width ${\ensuremath{\tau}}_{w}\ensuremath{\propto}{I}^{0.45\ifmmode\pm\else\textpm\fi{}0.17}.$ Calculations performed with a time-dependent model of an optically thick pinch at stagnation demonstrate that the internal energy and radiative opacity of the pinch are not responsible for the observed subquadratic power scaling. Heuristic wire-ablation arguments suggest that quadratic power scaling will be achieved if the implosion time ${\ensuremath{\tau}}_{i}$ is scaled as ${I}^{\ensuremath{-}1/3}.$ The measured 1\ensuremath{\sigma} shot-to-shot fluctuations in ${P}_{r},$ ${E}_{r},$ ${\ensuremath{\tau}}_{r},$ ${\ensuremath{\tau}}_{w},$ and ${\ensuremath{\tau}}_{i}$ are approximately 12%, 9%, 26%, 9%, and 2%, respectively, assuming that the fluctuations are independent of $I.$ These variations are for one-half of the pinch. If the half observed radiates in a manner that is statistically independent of the other half, the variations are a factor of ${2}^{1/2}$ less for the entire pinch. We calculate the effect that shot-to-shot fluctuations of a single pinch would have on the shot-success probability of the double-pinch inertial-confinement-fusion driver proposed by Hammer et al. [Phys. Plasmas 6, 2129 (1999)]. We find that on a given shot, the probability that two independent pinches would radiate the same peak power to within a factor of $1\ifmmode\pm\else\textpm\fi{}\ensuremath{\alpha}$ (where $0l~\ensuremath{\alpha}\ensuremath{\ll}1)$ is equal to $\mathrm{erf}(\ensuremath{\alpha}/2\ensuremath{\sigma}),$ where \ensuremath{\sigma} is the 1\ensuremath{\sigma} fractional variation of the peak power radiated by a single pinch. Assuming \ensuremath{\alpha} must be $l~7%$ to achieve adequate odd-Legendre-mode radiation symmetry for thermonuclear-fusion experiments, \ensuremath{\sigma} must be $l3%$ for the shot-success probability to be $g~90%.$ The observed ${(12/2}^{1/2})%=8.5%$ fluctuation in ${P}_{r}$ would provide adequate symmetry on 44% of the shots. We propose that three-dimensional radiative-magnetohydrodynamic simulations be performed to quantify the sensitivity of the x-ray emission to various initial conditions, and to determine whether an imploding $z$ pinch is a spatiotemporal chaotic system.

We have measured the x-ray power and energy radiated by a tungsten-wire-array z pinch as a function of the peak pinch current and the width of the anode-cathode gap at the base of the pinch. The measurements were performed at 13- and 19-MA currents and 1-, 2-, 3-, and 4-mm gaps. The wire material, number of wires, wire-array diameter, wire-array length, wire-array-electrode design, normalized-pinch-current time history, implosion time, and diagnostic package were held constant for the experiments. To keep the implosion time constant, the mass of the array was increased as I2 (i.e., the diameter of each wire was increased as I), where I is the peak pinch current. At 19 MA, the mass of the 300-wire 20-mm-diam 10-mm-length array was 5.9 mg. For the configuration studied, we find that to eliminate the effects of gap closure on the radiated energy, the width of the gap must be increased approximately as I. For shots unaffected by gap closure, we find that the peak radiated x-ray power P(r) proportional to I1.24+/-0.18, the total radiated x-ray energy E(r) proportional to I1.73+/-0.18, the x-ray-power rise time tau(r) proportional to I0.39+/-0.34, and the x-ray-power pulse width tau(w) proportional to demonstrate that the internal energy and radiative opacity of the pinch are not responsible for the observed subquadratic power scaling. Heuristic wire-ablation arguments suggest that quadratic power scaling will be achieved if the implosion time tau(i) is scaled as I(-1/3). The measured 1sigma shot-to-shot fluctuations in P(r), E(r), tau(r), tau(w), and tau(i) are approximately 12%, 9%, 26%, 9%, and 2%, respectively, assuming that the fluctuations are independent of I. These variations are for one-half of the pinch. If the half observed radiates in a manner that is statistically independent of the other half, the variations are a factor of 2(1/2) less for the entire pinch. We calculate the effect that shot-to-shot fluctuations of a single pinch would have on the shot-success probability of the double-pinch inertial-confinement-fusion driver proposed by Hammer et al. [Phys. Plasmas 6, 2129 (1999)]. We find that on a given shot, the probability that two independent pinches would radiate the same peak power to within a factor of 1+/-alpha (where 0or =alpha1) is equal to erf(alpha/2sigma), where sigma is the 1sigma fractional variation of the peak power radiated by a single pinch. Assuming alpha must beor =7% to achieve adequate odd-Legendre-mode radiation symmetry for thermonuclear-fusion experiments, sigma must be3% for the shot-success probability to beor =90%. The observed (12/2(1/2))%=8.5% fluctuation in P(r) would provide adequate symmetry on 44% of the shots. We propose that three-dimensional radiative-magnetohydrodynamic simulations be performed to quantify the sensitivity of the x-ray emission to various initial conditions, and to determine whether an imploding z pinch is a spatiotemporal chaotic system.

Summary form only given. Implosion and ignition of an indirectly-driven ICF capsule operating near a Fermi-degenerate isentrope requires initial Planckian-radiation-drive temperatures of 70-to-90 eV to be present for a duration of 10-to-15 ns prior to the main drive pulse. Such capsules are being designed for high pulsed-power generators such as X1. This foot-pulse drive capability has been recently demonstrated in a NIF-sized (/spl phi/=6-mm 1=7-mm), gold hohlraum, using a one-sided static-wall hohlraum geometry on the Z generator. The general arrangement utilized nested tungsten-wire arrays of radii (mass) 20 mm (2 mg) and 10 mm (1 mg) that had an axial length of -10 mm. The arrays were driven by a peak current of -21 MA and were made to implode on a 2-/spl mu/m-thick Cu annulus (mass=4.5 mg), which had a radius of 4 mm and was filled with a low-density CH foam, all centered about the z-axis. The gold hohlraum was mounted on axis and above the Cu/foam target. A 2.9-mm-radius axial hole between the top of the target and hohlraum permitted the X-rays generated from the implosion to enter the hohlraum. The radiation within the hohlraum was monitored by viewing the hohlraum through a 3-mm diameter hole on the lateral side of the hohlraum with a suite of diagnostics. The radiation entering the hohlraum was estimated by an additional suite of on-axis diagnostics, in a limited number of separate shots, when the hohlraum was not present. Additionally, the radiation generated outside the Cu annulus was monitored, for all shots, through a 3-mm diameter aperture located on the outside of the current return can.

We have demonstrated operation of a 3.35-m-diameter insulator stack at 158 kV/cm with no total-stack flashovers on five consecutive Z-accelerator shots. The stack consisted of five +45/spl deg/-profile 5.715-cm-thick crosslinked-polystyrene (Rexolite-1422) insulator rings, and four anodized-aluminum grading rings shaped to reduce the field at cathode triple junctions. The width of the voltage pulse at 89% of peak was 32 ns. We compare this result to a new empirical flashover relation developed from previous small-insulator experiments conducted with flat unanodized electrodes. The relation predicts a 50% flashover probability for a Rexolite insulator during an applied voltage pulse when E/sub max/e/sup -0.27/d/(t/sub eff/C)/sup 1/10/=224, where E/sub max/ is the peak mean electric field (kV/cm), d is the insulator thickness (cm), t/sub eff/ is the effective pulse width (/spl mu/s), and C is the insulator circumference (cm). We find the Z stack can be operated at a stress at least 19% higher than predicted. This result, together with previous experiments conducted by Vogtlin, suggest anodized electrodes with geometries that reduce the field at both anode and cathode triple junctions would improve the flashover strength of multi-stage insulator stacks.

Summary form only given, as follows. The quest to reduce the complexity, cost and risk of higher current accelerators drives the need for implosion times that are longer than the canonical 100 ns of present waterline generators. Experiments have been performed at 100 to 200 ns implosion times on the 10-MA Saturn and the 20 MA Z accelerators. These experiments have shown that 7 to 10 ns FWHM pulses can be produced even at the longest implosion times. The experimental data will be presented along with calculations which indicate that stability, uniformity and plasma opacity are critical to determining the radiation efficiency of these implosions.

Summary form only given. The radiation from uniform-fill argon gas puffs on the Saturn accelerator with a 4.5-cm diameter nozzle are compared with that generated from a previously optimized 2.5-cm diameter annular nozzle. The pressure range of the uniform fill spanned 1300 to 2900 Torr and that of the annular nozzle was set to 1650 Torr-the pressure that previously maximized the K-shell radiation yield. B-dot monitors measured current in the MITLs and 4.5 cm upstream of the load. A bolometer and duplicate sets of filtered XRDs and PCDs, spanning the energy range of 200 eV to 6 keV, monitored the temporal characteristics of the radiation. A suite of time-integrated and time-resolved, filtered, fast-framing, X-ray pinhole cameras, and crystal spectrometers monitored the spatial and spectral structure of the radiation. The radial density profile of the initial gas profile was measured on a test stand at NRL using a two-color interferometer.

The US Department of Energy has supported a substantial research program in Inertial Confinement Fusion (ICF) since the early 1970s. Over the ensuing 25 years, pulsed power approaches to inertial fusion have remained of interest primarily because of the high energy, efficiency, and relatively low cost of the technology when compared to the mainline ICF approach involving large glass lasers. These compelling advantages, however, have been tempered with the difficulty in concentrating the energy in space and time to create the high energy and power density required to achieve temperatures useful in indirect drive ICF. Since the Beams '96 meeting, the situation has changed dramatically, and extremely high X-ray power (290 TW) and energy (1.8 MJ) have been produced in fast z-pinch implosions on the Z accelerator. These sources have been utilized to heat hohlraums to >150 eV and have opened the door to important ICF capsule experiments. Although light ion beams offer a long term potential for fusion energy, we are suspending our ion beam research this year to maximize progress with z pinches.

Summary form only given. The characteristics of 95% Al:5% Mg alloy wire-array z-pinches formed by the Z generator and their associated X-ray emission are discussed. The arrays consisted of 180 to 266 wires, with each wire being 20-mm long and mounted at radii of 15, 20, or 25 mm. The implosion time of the loads explored was maintained at /spl sim/106 ns (the time which maximized the energy coupling of the generator to the load) by keeping M*R*R constant. M is the array mass that decreased from 7.1 to 4.2 to 2.6 Mg as the radius r was increased.

Summary form only given, as follows. Davis et al. (1997) Douglas et al. (1997) and Hammer et al. (1997) have shown the advantages of nested configurations for high current Z-pinches. Experiments have begun on the 20-MA Z accelerator to study these configurations using two nested wire arrays. For the first set of experiments, the outer array was chosen to be a 40 mm diameter with 240 wires which has been found to be the optimum single array which produced 200/spl plusmn/30 TW of X-rays. A second array was mounted internal to this array at a diameter of 20 mm. An initial scan keeping the final implosion time constant but varying the masses of the inner and outer arrays produced 250/spl plusmn/40 TW and reduced the X-ray pulsewidth from 6.5 to 4.8 ns for the lightest internal array, 470 /spl mu/g/cm. In this scan, the early implosions of the outer array, as its mass was decreased as the inner mass was increased, caused power coupling issues. A subsequent series of experiments kept the outer array mass constant at 1850 /spl mu/g/cm and varied the inner mass. This methodology revealed a new optimum inner mass of 1030 /spl mu/g/cm with a peak X-ray power of 290/spl plusmn/40 TW in a 4 ns pulse. The effects of wire array dynamics have also been studied by varying the number of wires in the inner array from 60 to 240 wires. In this paper, these results will be reviewed and compared to different 2D (r-z) radiation MHD codes and to results from a wire code.

The authors have designed and tested a 10 nH, 1.5 m radius vacuum section for the Z accelerator. The vacuum section consists of four vacuum flares, four conical 1.3 m radius magnetically-insulated transmission lines (MITLs), a 7.6 cm radius 12-post double-post-hole convolute which connects the four outer MITLs in parallel, and a 5 cm long inner MITL which connects the output of the convolute to a z-pinch load. The authors have demonstrated they can deliver a 100 ns rise-time 20 MA current pulse to the baseline z-pinch load. They can reproduce the peak MITL current to within /spl plusmn/1.6%. Power-flow measurements indicate the vacuum section performs as expected until peak current. Afterward, measurements and simulation results diverge.

Summary form only given. Laboratory experiments were performed to study the effect of emission mitigation protocols used on 20-MA Z. These laboratory experiments were utilized to determine the viability of the methods employed on Z's central convolute region load hardware. In addition, this examination is designed to be an aid in determining an experimental test matrix on Z. The convolute hardware's material, comprised of stainless steel 304L, was determined to have varying emission properties when different methods were applied. The particular focus of the study centered on the sulfur levels inherent in the stainless steel alloy. The effects of electrochemical electropolishing, hydrogen firing, vacuum firing and gold coating on voltage standoff thresholds were studied. A second low sulfur metal, stainless steel 304L SCQ, was studied and compared to the results of the currently used stainless steel 304L alloy. The results of these studies will be presented.

The U. S. Department of Energy is supporting research at Sandia National Laboratories (SNL) to investigate the possibility of using pulsed power driven magnetic implosions (z-pinches) to drive inertial confinement fusion (ICF) targets.1 This research is being conducted on SNL’s Z facility that is capable of delivering peak currents of 20 MA in z-pinch loads producing implosion velocities as high as 7.5×107 cm/s, x-ray energies of 1.6 MJ, and x-ray powers exceeding 200 TW. Several different target concepts that employ DT cryogenic capsules (2–4 mm in diameter) indirectly driven by radiation fields generated by z-pinch hohlraum sources are being pursued.1

PBFA Z, a new 60-TW/5-MJ electrical accelerator located at Sandia National Laboratories, is now the world’s most powerful z-pinch driver. PBFA Z stores 11.4 MJ in its 36 Marx generators, couples 5 MJ into a 60-TW/105-ns FWHM pulse to the 120-mΩ water transmission lines, and delivers 3.0 MJ and 50 TW of electrical energy to the z-pinch load. Depending on load parameters, we attain peak load currents of 16–20 MA with a current rise time of ∼105 ns with wire-array z-pinch loads. We have extended the x-ray performance of tungsten wire-array z pinches from earlier Saturn experiments. Using a 2-cm-radius, 2-cm-long tungsten wire array with 240, 7.5-μm diameter wires (4.1-mg mass), we achieved an x-ray power of 210 TW and an x-ray energy of 1.9 MJ. Preliminary spectral measurements suggest a mostly optically-thick, Planckian-like radiator below 1000 eV. Data indicate ∼100 kJ of x rays radiated above 1000 eV. An intense z-pinch x-ray source with an overall coupling efficiency greater than 15% has been demonstrated.

Summary form only given. The generation of intense bursts of warm X-rays (10 to 100 keV) with power in the 1-TW regime are of interest for the study of in-depth nuclear radiation effects. Results from high-atomic-number single-wire experiments carried out at 0.8 MA on Gamble II in the 1970s showed 0.25% efficient production of nonthermal, bremsstrahlung-like lines and continuum in the 5- to 100-keV regime. This high efficiency in combination with suggested Z/sup 2/ and I/sup 2/ scaling of the nonthermal radiation motivated the present experiment to measure and model the radiation from the Z pinch formed from compact high-Z wire arrays at high current. In the experiment, tungsten wire arrays of length 20 mm on a mounting radius of 2 mm were imploded over the mass range 1 to 16 mg on the Saturn accelerator operating with a peak discharge current of 5 MA. As in the Gamble-II experiments, bright spots were observed to form at /spl sim/1-mm intervals along the z axis at the time of a first implosion and to be the source of the hard radiation measured. Maximum radiation occurred for masses less than or approximately equal to 4 mg. The experiment was simulated using the LASNEX and TIP numerical codes with a nonthermal model.

Modeling of load behavior in Z‐pinch plasma radiation sources driven by high current generators requires accurate measurement of fast‐rise‐time multimegampere electrical currents close to the load. Using a novel applications of high pressure technology, we have demonstrated that fast‐response piezoelectric stress transducers can measure such currents under conditions of extremely high current density, induced electric fields, and bremsstrahlung radiation where conventional current diagnostics fail. Large signal, nanosecond‐time‐resolution lithium niobate piezoelectric stress gauges are employed to directly measure the magnetic pressure B2/dμ0=μ0I2/ 8π2r2 generated at radius r by a curret I flowing in a radial transmission line near the load of a pulsed power current source.With a current diagnostic consisting of a pure tungsten electrode of a Y‐cut lithium niobate stress gauge, current densities up to I/2πr=78 MA/m can be measured before the electrode yield strength and piezoelectric operating stress limi...

Our experiments show energy losses between 2 and 10 times that of the resistive time predictions. The experiments used hydrogen, helium, air, nitrogen, SF6, polyethylene, and water for the switching dielectric. Previously underestimated switch losses have caused overpredicting the accelerator outputs. Accurate estimation of these losses is now necessary for new high-efficiency pulsed power devices where the switching losses constitute the major portion of the total energy loss. We found that the switch energy losses scale as (V peak I peak) 1.1846. When using this scaling, the energy losses in any of the tested dielectrics are almost the same. This relationship is valid for several orders of magnitude and suggested a theoretical basis for these results. Currents up to .65 MA, with voltages to 3 MV were applied to various gaps during these experiments. Our data and the developed theory indicate that the switch power loss continues for a much longer time than the resistive time, 'tr, with peak power loss generally occurring at peak current in a ringing discharge instead of the early current time.

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.

The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state ...

The first Particle Beam Fusion Accelerator (PBFA-I) has recently completed its operational series of tests. It provided 30 TW into 36 power flow lines terminated in electron beam loads at the accelerator's center. The outputs were 2 MV at 15 MA for a 40 ns pulse which provided 1 MJ of output energy. During the accelerator develcpament, several ways of modifying and reconnecting this modular, few MV output pulsed power were discovered. The data from PBFA-I tests show the flexibility and reliability of pulsed power. This technology base could provide a 100 TW accelerator known as PBFA-II which will be the light ion beam test bed for breakeven experiments for the Department of Energy's Inertial Confinement Fusion Program. Pulsed power technology continues to be flexible and adaptable to a wide variety of basic research problems. It is a cost effective technology for producing large pulsed energy outputs.