Nested-array Z pinches are concentric, two layer wire assemblies -an exterior (driver) layer and an interior (target) layer. On the 20-MA Z accelerator they produce record-high x-ray power, 40% over a single array, but the mechanism of power enhancement remains unclear. The observed kinematics of the nested arrays is best described by a model that allows the drive mass to penetrate the target mass. At assembly the load filament's mutual inductance requires current to switch rapidly from the drive to the target array. PACS numbers: 52.25.Nr, 52.50.Lp As plasma radiation sources (PRS), dynamic Z-pinch loads are routinely imploded to generate a large yield of soft x rays [1]. Further development of high power PRS loads recently achieved a milestone with the successful production of about 280 TW of x rays from a double nested wire-array implosion at the Sandia National Laboratories (SNL) Z facility. Remarkably, the plasma was uniform and stable. While conventional multiwire arrays have consistently produced about 2 MJ of soft x rays, most such dynamic Z-pinch plasmas lack uniformity and tend to be unstable, at least in the outer regions. However, in the double nested wire scenario, the plasma is so stable and uniform as to rekindle visions of producing an intense x-ray source for application in a number of activities, including x-ray lasers. The majority of successful x-ray laser concepts to date have focused not on Z pinches but rather on laser produced plasmas and, more recently, on capilliary discharges.The performance of any PRS is known to be very sensitive to implosion stability. Better stability often means a tighter, more uniform Z pinch at the stagnation phase and higher K-shell radiative output. Two-and threedimensional instabilities and asymmetries can broaden x-ray pulsewidths, lower x-ray powers, and lower yields compared to the predictions of one-dimensional calculations. However, these instabilities can also act, in some cases, to slow the load assembly, limit the dynamic range of load inductance, and allow more energy into the load via a smoother squeezing than is delivered as primary kinetic energy. Thus, while some enhancement of available energy may arise through two-and three-dimensional effects, the higher powers in experiment are seen with tighter pinches. This indicates a considerable potential for improving radiative performance by making the PRS implosions more uniform and stable [2].Early experiments, with structured gas-puff Z-pinch loads (either as an annular shell onto an inner concentric column [3] or onto a similar inner shell [4]), demonstrated that such loads increase the uniformity of the stagnated plasma by mitigating instability growth. Rus-sian gas-puff-on-wire-array experiments have shown a remarkable aluminum K-shell yield and radiative power increase due to fast switching of the pinch current from the outer gas shell to the inner wire array [5]. Recent efforts on this path have been fielded on the "Z" facility [6], using fully nested wire-array loads [7,8].As discussed by De...
In this paper, a theoretical model is described and demonstrated that serves as a useful tool for understanding K-shell radiating Z-pinch plasma behavior. Such understanding requires a self-consistent solution to the complete nonlocal thermodynamic equilibrium kinetics and radiation transport in order to realistically model opacity effects and the high-temperature state of the plasma. For this purpose, we have incorporated into the MACH2 two-dimensional magnetohydrodynamic ͑MHD͒ code ͓R. E. Peterkin et al., J. Comput. Phys. 140, 148 ͑1998͔͒ an equation of state, called the tabular collisional radiative equilibrium ͑TCRE͒ model ͓J. W. Thornhill et al., Phys. Plasmas 8, 3480 ͑2001͔͒, that provides reasonable approximations to the plasma's opacity state. MACH2 with TCRE is applied toward analyzing the multidimensional implosion behavior that occurred in Decade Quad ͑DQ͒ ͓D. Price et al., Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA, edited by C. Stallings and H. Kirbie ͑IEEE, New York, 1999͒, p. 489͔ argon gas puff experiments that employed a 12 cm diameter nozzle with and without a central gas jet on axis.Typical peak drive currents and implosion times in these experiments were ϳ6 MA and ϳ230 ns. By using Planar Laser Induced Fluorescence measured initial density profiles as input to the calculations, the effect these profiles have on the ability of the pinch to efficiently produce K-shell emission can be analyzed with this combined radiation-MHD model. The calculated results are in agreement with the experimental result that the DQ central-jet configuration is superior to the no-central-jet experiment in terms of producing more K-shell emission. These theoretical results support the contention that the improved operation of the central-jet nozzle is due to the better suppression of instabilities and the higher-density K-shell radiating conditions that the central-jet configuration promotes. When we applied the model toward projecting argon K-shell yield behavior for Sandia National Laboratories' ZR machine ͑ϳ25 MA peak drive currents, ϳ100 ns implosion times͒ ͓D. McDaniel et al., for experiments that utilize the 12 cm diameter central-jet nozzle configuration, it predicts over 1 MJ of K-shell emission is attainable.
Electrical connections from the interior of a pressure vessel to .atmospheric pressure typically place many design constraints on high pressure experimentation. A simplified version (1)(2)(3)
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