Imploding Load Research Articles

Large time-varying inductance load for studying power flow on the Z machine

Interest in studying power flow dynamics has grown in recent years, with new power flow diagnostics being developed at Sandia National Laboratories for the $Z$ Pulsed Power Facility. Presently, the only power flow loads that have been studied are cylindrical static or imploding loads that are driven by synchronous short pulse (100 ns rise time). Presented is a design that utilizes the dynamic materials properties program's stripline geometry in a high voltage pulsed shaped (asymmetric asynchronous) driving mode. This design has exhibited repeatable current loss with a large time-varying inductance that is well matched to the machine at pulse initialization but which triples to high inductance in 800 ns. Evidence is presented that plasma not captured in the magnetohydrodynamic approximation and ill represented by any of our existing predictive pulsed power codes is adversely affecting load current delivery. The authors believe this design could be of great interest to the experimental and modeling communities for studying power flow dynamics.

Realization of quasi-spherical implosion using pre-shaped prolate wire arrays with a compression foam target inside

Quasi-spherical (QS) implosion of wire arrays and its impact on the foam target have been studied on the 100 ns 1.5 MA Qiangguang-I facility, which suggests that a high quality impact between the QS implosion and foam target can be achieved by adjusting load's initial shape carefully to match the external magnetic pressure. Implosions of loads with H/d ∼ 1.2 were studied with a self-emission x-ray pinhole image system and a dark field schlieren system. The radially developed spike-like instabilities indicate the spherical convergence of plasma. The observed radiation on the foam target surface suggests satisfying implosion symmetry and wire-foam impact simultaneity. An average implosion speed of 10.5 × 106 cm/s was obtained with an optical streak image system. The derived peak kinetic energy density ∼2.1 kJ/cm is remarkably higher than cylindrical cases, which agree with the expectations.

Experimental Studies of an Ultrahigh-Speed Plasma Flow

In 1991, Turchi et al. reported evidence for a 2000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS). The 2008 article by Turchi et al. posits that if such high $Z$ plasma could instead be composed of deuterium or a deuterium–tritium mixture, then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several experiments executed in an effort to achieve an ultrahigh-speed flow in a deuterium plasma. The first phase of this research concentrated on extension of the earlier work to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. The experiments met with limited success, exhibiting evidence of a 550 km/s plasma flow which convected a small fraction of the total magnetic field. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low-energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. These experiments demonstrated that the hardware assembled for this investigation was unlikely to forestall breakdown in the injected gas as required by Turchi et al. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low-energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultrahigh-speed plasma flow concept.

Architecture, implementation, and testing of a multiple-shell gas injection system for high current implosions on the Z accelerator

Tests are ongoing to conduct ~20 MA z-pinch implosions on the Z accelerator at Sandia National Laboratory using Ar, Kr, and D2 gas puffs as the imploding loads. The relatively high cost of operations on a machine of this scale imposes stringent requirements on the functionality, reliability, and safety of gas puff hardware. Here we describe the development of a prototype gas puff system including the multiple-shell nozzles, electromagnetic drivers for each nozzle's valve, a UV pre-ionizer, and an inductive isolator to isolate the ~2.4 MV machine voltage pulse present at the gas load from the necessary electrical and fluid connections made to the puff system from outside the Z vacuum chamber. This paper shows how the assembly couples to the overall Z system and presents data taken to validate the functionality of the overall system.

Study of the implosion of foam-wire loads at the Angara-5-1 facility

Results are presented from experiments on studying the compactness of compression of imploding nested foam-wire loads at currents of up to 4 MA at the Angara-5-1 facility. The degree of pinch compression was estimated from the dynamics of the spatial distribution of the current (magnetic field) and the shape of the soft X-ray pulse. The load consisted of nested cascades, one of which being a wire array and the other being a hollow or solid low-density cylinder made of agar-agar foam with a wall thickness of 100–200 μm. In some experiments, one of the cascades was made of C20H17O6 solid-state organic acid foam. The radial distribution of the magnetic field inside the nested cascades of the imploding foam-wire load (both between the cascades and inside the inner cascade) was measured using tiny magnetic probes. The measured radial distributions of the magnetic field are compared with the magnetic field configuration calculated using a one-dimensional MHD code simulating the implosion of a nested foam-wire load. It is shown that the spatial structure of the current and magnetic field during the implosion of such a load is determined by the development of supersonic and subsonic magnetized plasma flows in its cascades. The specific features of pinch formation and methods for the compensation of the nonsimultaneous pinch compression between the anode and the cathode (the zipper effect) during the implosion of a nested foam-wire load are analyzed.

Particle-in-cell simulation of outer magnetically insulated transmission line of Z-pinch accelerator

In order to implement the particle-in-cell(PIC) simulation of four outer magnetically insulated transmission lines(MITL) of Z-pinch accelerator, we adopt the backward-going transmission line voltage wave at the double post-hole convolute(DHPC) travelling in the outer MITL direction, which is gained in circuit simulation to model the part from DHPC to Z-pinch load. For the purpose of enhancing the accuracy of equivalent circuit model of DHPC and Z-pinch load, the backward-going transmission line voltage wave that is computed by circuit simulation is rescaled by PIC simulation. The insulator voltage and current waves that are obtained in the PIC simulation of four outer MITLs of PBFA Z can match the electrical measurements better than those obtained by the circuit simulation. Moreover, the influence of Z-pinch load implosion on the process of pulsed power coupling in the outer MILT is analyzed and explained.

Implosion dynamics in double planar wire array Z pinches

The double planar wire array (DPWA) Z pinch is a highly efficient radiation source with unique implosion dynamics and precursor formations. The inductively divided current successively ablates the wires and injects the material to the interior of the array. Three uniquely imploding DPWA load types were identified and classified according to the critical load parameter: low, intermediate, or high aspect ratio. Radiation pulse shaping was obtained from secondary precursors: off-axis mass carrying high current densities during the implosion phase. Time-gated EUV spectroscopy of off-axis mass accumulations is used to assess a 60 eV electron temperature prior to the implosion phase. These structures are shown to form by a different mechanism than the secondary precursors. High yields, compact size, and shaped radiation pulses merit further consideration of the DPWA as a candidate for inertial confinement fusion research.

Simulations of the implosion and stagnation of compact wire arrays

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-ampère) 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.

Studies of penetration of the magnetic field into electrically imploded loads in the Angara-5-1 facility

Results are presented from measurements of the distributions of the azimuthal magnetic field in aluminum, copper, molybdenum, tungsten and other wire arrays electrically imploded at currents of up to 3 MA in the Angara-5-1 facility. It is shown that the time during which the magnetic field of the current pulse reaches the array axis depends on the material of the wires or wire coating. The current of the precursor formed on the array axis before the implosion of the main load mass is measured. It is shown that the penetration of the load material with the frozen-in magnetic field into a polymer (agar-agar) foam liner is drastically different from that in the case of a wire array. It is found that the rate of current transfer to the array axis is maximum for tungsten wire arrays. The rates of plasma production during implosion of loads made of different materials are compared.

Transportation of an electromagnetic pulse to the load in the Angara-5-1 facility

One of the main problems in Z-pinch experiments is to transport power and energy from the generator to the load. As the pulse produced in a double forming line propagates to the load along a water-vacuum insulator, its power and energy decrease due to current leakage in the plasma shortening the gap and during the establishment of magnetic self-insulation in regions with a zero magnetic field. Only a fraction of the delivered energy is spent on the load implosion, whereas the rest of the energy goes on creating the magnetic field around the load. In this work, an analysis is made of what is the fraction of the generator energy that reaches the liner, what fraction is radiated, and what are losses of energy and current in different stages of transporting the electromagnetic pulse to the load of the Angara-5-1 facility.

Power Flow Design Constraints for a Recyclable Transmission Line for Z-Pinch IFE

A recyclable transmission line (RTL) carries power from the pulsed-power driver to the fusion target in a z-pinch-driven inertial-confinement fusion energy (IFE) system. In order to minimize the driver voltage, the RTL inductance must be small, requiring a short, low-impedance, magnetically insulated transmission line (MITL). However, the large linear current density that flows in the electrodes at small radius near the load resistively heats the anode surface, leading to anode plasma formation and ion emission. If the impedance of the RTL is too small, large ion current losses can occur and large electron flow currents can be launched into the z-pinch load region. These problems are avoided by choosing the line impedance at the load end of the RTL to be well above the effective impedance of the imploding load. By gradually reducing the impedance along the line moving from the load to the driver, the RTL inductance can be controlled. But, if the impedance is varied too rapidly along the line, significant electron flow current losses can occur. The impact of these constraints on the RTL design of an IFE system is discussed and a compromise design with reasonable power coupling efficiency is established.

Power Flow in a Magnetically Insulated Recyclable Transmission Line for aZ-Pinch-Driven Inertial-Confinement-Fusion Energy System

Power flow in a recyclable transmission line (RTL) for a z-pinch-driven inertial-confinement fusion energy (IFE) system is studied. In a magnetically insulated transmission line, plasma forms by explosive emission on the cathode but emission from the anode does not occur. However, in an RTL, the large linear current density that flows in the electrodes at small radius near the load resistively heats the anode surface, leading to anode plasma formation and ion emission. If the impedance of the RTL is too small, large ion current losses can occur and large electron flow currents can be launched into the z-pinch load region. If only the boundary current drives the z-pinch load, then these large electron flow currents can introduce a polarity effect with more bound current in the anode than the cathode. While being mindful of the IFE system requirement to maintain a small RTL inductance, these problems are avoided by choosing the line impedance at the load end of the RTL to be well above the effective impedance of the imploding load. In this case, the ion current losses are tolerable and the electron flow current is negligibly small. For the present baseline design with a peak current of 60 MA driving a 100-ns implosion, these power flow constraints require a gap of order 2 mm or more at the load end of the RTL

Symmetric aluminum-wire arrays generate high-quality Z pinches at large array radii

A Saturn-accelerator study of annular, aluminum-wire array, Z-pinch implosions, in the calculated high-wire-number plasma-shell regime [Phys. Rev. Lett. 77, 5063 (1996)], shows that the radiated x-ray pulse width increases from about 4 nsec to about 7 nsec, when the radius of the array is increased from 8.75 to 20 mm at a fixed array mass of 0.6 mg. Eulerian radiation- magnetohydrodynamic code (E-RMHC) simulations in the r-z plane suggest that this pulse-width increase with radius is due to the faster growth of the shell thickness (that arises from a two-stage development in the magnetic Rayleigh–Taylor instability) relative to the increase in the shell implosion velocity. Over the array radii explored, the measured peak total x-ray power of ∼40 TW and energy of ∼325 kJ show little change outside of a ±15% shot-to-shot fluctuation and are consistent with the E-RMHC simulations. Similarly, the measured peak K-shell (lines plus continuum) power of ∼8 TW and energy of ∼70 kJ show little change with radius. The minimal change in K-shell yield is in agreement with simple K-shell radiation scaling models that assume a fixed radial compression for all initial array radii. These results suggest that the improved uniformity provided by the large number of wires in the initial array reduces the disruptive effects of the Rayleigh–Taylor instability observed in small-wire-number imploding loads.

US/Russian collaboration in high-energy-density physics using high-explosive pulsed power: ultrahigh current experiments, ultrahigh magnetic field applications, and progress toward controlled thermonuclear fusion

A collaboration has been established between the All-Russian Scientific Research Institute of Experimental Physics (VNIIEF) and the Los Alamos National Laboratory (LANL), the two institutes which designed the first nuclear weapons for their respective countries. In 1992, when emerging governmental policy in the United States and Russia began to encourage lab-to-lab interactions, the two institutes quickly recognized a common interest in the technology and applications of flux compression, the technique for converting the chemical energy released by high-explosives into intense electrical pulses and intensely concentrated energy. In a period of just over three years, the two institutes have performed more than fifteen joint experiments covering research areas ranging from basic pulsed power-technology to solid-state physics to controlled thermonuclear fusion. Using flux compression generators, electrical currents ranging from 20 to 100 MA were delivered to loads of interest in high-energy-density physics. A 20-MA pulse was delivered to an imploding liner load with a 10-90% rise time of 0.7 /spl mu/s. A new, high-energy concept for soft X-ray generation was tested at 65 MA. More than 20 MJ of implosion-kinetic energy was delivered to a condensed matter imploding liner by a 100-MA current pulse. Magnetic flux compressors were used to determine the upper critical field of a high-temperature superconductor and to create pressure high enough that the transition from single particle behavior to quasimolecular behavior was observed in solid argon. A major step was taken toward the achievement of controlled thermonuclear fusion by a relatively unexplored approach known in Russia as MAGO (MAGnitnoye Obzhatiye, or magnetic compression) and in the United States as MTF (Magnetized Target Fusion). Many of the characteristics of a target plasma that produced 10/sup 13/ fusion neutrons have been evaluated. Computational models of the target plasma suggest that the plasma is suitable for subsequent compression to fusion conditions by an imploding pusher.

Experimental Results from SHIVA Star Vacuum Inductive Store/Plasma Flow Switch Drven Implosions

Using a 1313-μF, 3-nH, 120-kV, 9.4-MJ SHIVA Star capacitor bank, we have performed vacuum inductive store/plasma flow switch (PFS) driven implosions of low mass (200-400 μg/cm2) cylindrical foil liners of 2-cm height and 5-cm radius. This technique employs a coaxial discharge through a plasma armature, which stores magnetic energy over 3-4 μs and rapidly switches it to an imploding load as the plasma armature exits the coaxial gun muzzle. The current transferred to the load by the PFS has a rise time of less than 0.2 μs. With 5-MJ stored energy, we have driven fast liner implosions with a current of over 9 MA, obtaining an isotropic equivalent 2.7-TW 0.5-MJ X-ray yield.

Nonaxisymmetric wave propagation in pulsed power machines

We analyze, in the absence of space charge, nonaxisymmetric wave propagation in pulsed power machines in the long wavelength limit (λ≫anode-cathode gap dimensions). The conditions for generalized TEM waves are derived for both straight (coaxial) and convergent (double-disk) transmission lines. We examine the asymmetries resulting from the lack of switch synchronization as it affects the wave propagation and power delivery to a small radius symmetric imploding load. We apply the long wavelength equations to the output water line and vacuum feed sections of a machine resembling BLACKJACK V and calculate the reduction in power delivered to the load as a function of the degree of synchronization. An important, but simple, consequence of the theory is that for a small symmetric load the angular average of the voltage at the switches is approximately the effective driver of the power reaching the load.

Optimum coupling of imploding loads to pulse generators

Dense, high temperature plasmas can be produced by accelerating axisymmetric arrays of wires, cylindrical foils, etc., in the radial direction through the action of magnetic forces. The energy of such magnetically imploded plasmas arises almost entirely from the thermalization of the kinetic energy of implosion. If such a process is to achieve high plasma temperatures it must be efficient, i.e., a sizable fraction of the available electromagnetic energy must be converted to kinetic energy. The present work addresses the problem of deriving a criterion for coupling such imploded loads to the common types of pulse generators. This criterion is obtained through the numerical solution of two coupled nonlinear ordinary differential equations, an equation of motion, and a circuit equation. Solutions are presented for two types of generators and two types of load and are compared with experimental results where available.