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Los Alamos “Ranchero” Magnetic Flux Compression Generators (FCGs) have been used to power imploding liner loads. The fundamental FCG design is based on a cylindrical detonation system that expands the armature simultaneously into a coaxial generator volume and has been shown to generate currents as high as 76 MA. Analysis of the 76 MA test results revealed a weakness in the design at the output glide plane. To prevent premature shorting at the output current slot of the generator, the armature/glide plane interface was originally designed to lag the leading edge of the armature. However, 2D-MHD calculations reveal that at very high currents a magnetically driven aneurism develops in this lagging section which reduces the performance of the generator. A new model Ranchero is being developed to correct this weakness and provide enhanced performance. In the new model, the output glide plane is eliminated and the armature is extended along the FCG axis, with its radius increasing along a curve until it reaches the current output slot. A cylindrical detonation system of the type required for earlier designs continues to be used, and the high explosive (HE) in the extended section is detonated by the last point of the cylindrical detonator. The stator of the FCG is contoured, allowing the contact point of the armature to zipper from the input to the output end in the last few μs of flux compression. In addition, the new model Ranchero is intended to use PBX 9501 (9501) for the HE and also remove the smoothing layer, which has been part of all Ranchero HE systems to date. Both of these factors lead to increased performance. 9501 is more energetic than the PBXN 110 used in Ranchero generators to date, and both calculations and experiments have shown that the smoothing layer is not needed when the detonator point spacing is 18 mm. Tests of original model Rancheros using PBXN 110 castable HE, with an imbedded smoothing layer, demonstrated an armature expansion velocity of 3.1 mm/μs. Further tests show that removal of the smoothing layer increases the speed to 3.3 mm/μs, and replacement of the cast PBXN 110 with 9501 without a smoother gives a velocity of 3.8 mm/μs. Designs, concerns, and experimental results facilitating the new model Ranchero are presented. In addition, performance estimates are given for the initial imploding liner tests to be conducted, and further computational details are presented in a companion paper given by C. L. Rousculp and others at this conference.

A computationally efficient method for transporting radiation in multidimensional plasmas has been developed and evaluated. The basis of this method is a uniform plasma approximation that allows one to utilize existing escape probability techniques that are successfully used in one-dimensional (1D) calculations to approximately solve the multidimensional radiation transport problem. This method is superior to diffusion methods because (1) the probability of escape technique insures that the plasma goes to the correct optically thin and thick limits, (2) the effects of line absorption due to photoexcitations are modeled, and (3) this method uses source functions that are based on a self-consistent nonlocal thermodynamic equilibrium calculation, not an ad hoc assumption that the source functions are Planckian. This method is highly efficient because equation of state information from 1D calculations is tabulated as a function of plasma internal energy, ion density, and the line probability of escape from a ...

The classical Richtmyer‐Meshkov ~RM! instability develops when a planar shock wave interacts with a corrugated interface between two different fluids. A larger family of so-called RM-like hydrodynamic interfacial instabilities is discussed. All of these feature a perturbation growth at an interface, which is driven mainly by vorticity, either initially deposited at the interface or supplied by external sources. The inertial confinement fusion relevant physical conditions that give rise to the RM-like instabilities range from the early-time phase of conventional ablative laser acceleration to collisions of plasma shells ~like components of nested-wire-arrays, double-gas-puff Z-pinch loads, supernovae ejecta and interstellar gas!. In the laser ablation case, numerous additional factors are involved: the mass flow through the front, thermal conduction in the corona, and an external perturbation drive ~laser imprint!, which leads to a full stabilization of perturbation growth. In contrast with the classical RM case, mass perturbations can exhibit decaying oscillations rather than a linear growth. It is shown how the early-time perturbation behavior could be controlled by tailoring the density profile of a laser target or a Z-pinch load, to diminish the total mass perturbation seed for the Rayleigh‐Taylor instability development. © 2000 American Institute of Physics. @S1070-664X~00!93305-6#

Mitigation of the Rayleigh–Taylor (RT) instability of Z-pinch loads imploded from large initial radii through tailoring initial load density profiles in radial and axial directions is studied numerically. These methods could be helpful for a variety of applications of high-power Z-pinches, from producing large amounts of K-shell radiation to imploding inertial confinement fusion pellets. Radial density tailoring is demonstrated to delay the onset of the RT instability development at the expense of reducing the energy available for conversion into radiation. Axial density tailoring can fully stabilize acceleration of a fraction of the initial load mass. For a better tradeoff between stability and radiative performance of the loads, the density profiles could be tailored in two dimensions, combining the advantages of both methods. Post-processing of the radiation-magnetohydrodynamic simulation results demonstrates that an appreciable K-shell argon radiation power could be generated with a stabilized argon load imploded by a 5 MA current from a ∼10 cm initial radius in about 0.5 μs.

A load structure with tailored density profile which delays the onset of the Rayleigh-Taylor instability development in imploding {ital Z} pinches by inverting acceleration of the magnetic field/plasma interface is proposed and studied numerically. This approach makes it possible to start gas-puff implosions from large radii (say, 8cm) and produce significant {ital K}-shell yield with current pulse duration of 250ns and longer. It could also be used to mitigate imprinting of initial perturbations into laser fusion targets. {copyright} {ital 1996 The American Physical Society.}

The stability and radiative performance of structured Z‐pinch plasma loads heated by high‐current (≳20 MA) pulsed power generators are investigated. A limited mapping of parameter space is made for the regions of stability for loads configured as thin shells, uniform fills, and multiple shells. Although large diameter thin shell loads are shown to be the most efficient radiators of K‐shell x rays, they are susceptible to disruption by the Rayleigh–Taylor instability. Large diameter uniform fill loads are shown to be more stable and very good radiators.

A two‐dimensional magnetohydrodynamic code that includes resistivity and viscosity has been used to study the growth of m=0 perturbations of a Z pinch in which the current increases with time. Stability for all wave numbers was found when the product SR was less than about 1000, where S is the Lundquist number and R is the Alfven–Reynolds number. Since SR depends only on the line density N, the stability criterion translates as N

The evolution of a dynamic X‐pinch plasma, formed by two or more intersecting crossed copper wires, is investigated theoretically and numerically in two dimensions to determine the behavior at the point of intersection. An X‐pinch plasma for conditions corresponding to (a) pure hydrodynamic and (b) nonlocal thermodynamic equilibrium radiation hydrodynamic simulations are considered when driven by an externally applied current.

We have created large diameter, long implosion time, PRS loads that produce a stable pinch and a narrow X-ray pulse. Operation scaled successfully with current from 2 MA on DM2 to 6 MA on Saturn. The K-shell yields are comparable to those produced by short-pulse operation at a similar current. This approach, therefore, allows substantial savings to be realized in the pulsed-power driver cost, complexity and risk.

Summary form only given, as follows. We have continued our development and testing of uniform-fill nozzles for long-implosion time PRS sources on the 4 MA Double-EAGLE generator at Maxwell Physics International. By adjustment of switch gaps in the waterline, the nominal 100 ns electrical pulse of Double-EAGLE can be extended to 200 or 250 ns with little decrease in peak current. We have used these modes to test 7 and 10 cm diameter uniform fill gas puffs in well diagnosed experiments. The 7 cm nozzle generated a K-shell yield in excess of 15 kJ in a 16 ns FWHM pulse at 188 ns implosion time and demonstrated I/sup 4/ scaling to the testing limit of 4 MA. The final implosion diameter was about 2 mm. The compression ratio was therefore better than that achieved in short pulse operation (2.5 cm initial shell diameter to 1 mm final pinch diameter). We will discuss these Z-pinch results, the 10 cm nozzle results, and features of the gas valve and nozzles employed to produce them.

The stability of a Z pinch carrying the Pease–Braginskii current is examined using a two‐dimensional compressible magnetohydrodynamic (MHD) code that includes resistivity and bremsstrahlung. It is found that the growth rate of m=0 perturbations depends upon the Lundquist number S and that the pinch is stable when S is less than about 6 for ka= (1)/(3) and less than about 1 for ka=1, where k is the wave number and a is the pinch radius.

The performance of plasma radiation source (PRS) loads imploded by a long current pulse machine is investigated theoretically. The emission tolerant (with oil) Decade Quad design of Pulse Sciences Incorporated is an example of such a long current rise time machine. It has a peak short circuit current of 9 MA that is reached in 300 nanoseconds. Large radius loads must be employed for this type of machine in order to insure that high implosion velocities are reached and that the available energy is utilized. However, large radius implosions are known to be susceptible to the Rayleigh Taylor instability. The extent to which this instability affects the ability of an argon load to radiate K-shell photons is not known and is the subject of this model investigation.

We discuss the design of structured Z-pinch loads capable of mitigating the detrimental effect of Rayleigh-Taylor (RT) instability on the performance of fast Z-pinch devices used as plasma radiation sources. The stabilizing effects of density tailoring in both the radial and the axial directions are considered. Our 2-D numerical simulations demonstrate that using a structured gaseous load with a radial density profile specifically tailored for a given current wave form, it is possible to delay the onset of the RT instability development while a shock wave propagates through the load. Once the acceleration of the magnetic field/plasma interface is inverted, perturbations are shown to oscillate rather than to grow exponentially. Our simulation results indicate the possibility of high-quality implosions producing significant Ar K-shell yield from initial radii in the range between 4 and 8 cm, with current pulse duration of 250 ns and longer. Axial density tailoring can actually mitigate the RT instability, or even suppress it completely, at the expense of decreased hydrodynamic efficiency of acceleration. Seeking the best trade-off between stability and performance, it would be natural to combine the two approaches.

Non‐LTE radiation hydrodynamic numerical simulations in 1‐D and 2‐D are performed for multi‐terawatt driven argon and krypton gas puff loads. The influence of enhanced transport coefficients on the plasmas' implosion dynamics and the effects it has on the radiation yield and spectral distribution are discussed. Also presented are simulations for the performance of a krypton gas puff driven by a class of future generators.

Recent observations are given for an argon double-shell gas puff imploded with up to 4 MA in 200 ns on the Double Eagle generator [G. B. Frazier et al., Digest of Technical Papers, Fourth IEEE Pulsed Power Conference (IEEE, Piscataway, NJ, 1983), p. 583]. Good K-shell x-ray output with good pinch quality was observed. A novel experimental procedure was used to selectively seed the inner or outer gas plenums with a chlorine tracer. The tracer data provide the first direct experimental evidence that the mass initially closest to the axis is the dominant contributor to the hot core of the radiating pinch.

An investigation has been conducted to determine the feasibility of generating conditions favorable to population inversion, gain, and x‐ray lasing in a two‐component plasma consisting of sodium and neon. An intense laser beam is assumed incident on both sides of a Na/Ne slab target separated by a carbon thermal buffer. Temperature and density profiles of the laser target interaction are calculated using a Lagrangian hydrodynamics model. Level populations and radiative emissions are evaluated using a collisional‐radiative equilibrium model including opacity effects. It appears possible to achieve measurable gain at 82, 58 and 230 A from the 3‐2, 4‐2, and 4‐3 transitions of heliumlike neon, respectively. Additionally, by pulse tailoring and target design, it should be possible to enhance the Na pumping power. off

The dependence of the growth rate of the n = 1(lambda = 2..pi..R/sub 0/) kink instability on plasma ..beta.. has been investigated experimentally in a rectangular-cross-section high-toroidal-..beta.. (..beta../sub t/ (axis) approximately-greater-than 10%) device and compared with theoretical predictions. The ..beta../sub t/ for the onset of instability as well as the estimated values of the growth rates observed experimentally are in good agreement with predictions from a numerical ideal magnetohydrodynamic model.

A two‐dimensional resistive MHD code is used to examine the stability of a z‐pinch to m=0 perturbations under conditions of rising current. The current waveform is chosen to rise according to the one‐third power of time in order to keep the unperturbed radius of the pinch constant. One‐dimensional calculations show that regardless of the initial shape of the current density profile, the plasma column recovers the self‐similar profile dictated by this current rise. In two‐dimensions, initial perturbations corresponding to m=0 instability modes oscillate without growing or grow weakly at low values of the current in apparent contradiction to ideal MHD theory. At high current, the pinch is unstable with a growth rate predicted by ideal MHD. This is put forward as a possible explanation of experimental results which shown anomalously stable Z‐pinches. Enhanced stability is also seen for low values of the Lundquist number when the current is set to the Pease‐Braginskii value where the ohmic heating and radiati...

The implosion dynamics of a hollow cylindrical neon gas‐puff plasma are investigated using a 1‐D non‐local thermodynamic equilibrium radiation hydrodynamic model. Two cases are considered: the implosion of the gas puff with and without a low‐density central core plasma. It is shown that the implosion efficiently converts the kinetic energy of runin into radiation and that the radiation plays a significant role in the implosion energetics. For each case, the history of the implosion dynamics and emission spectra are presented as well as the distribution of radiative energy in the L and K shells. Finally, a comparison is made with purely hydrodynamic calculations.