Cylindrical Wire Arrays Research Articles

Monochromatic x-ray imaging experiments on the Sandia National Laboratories Z facility (invited)

The Z facility is a 20 MA, 100 ns rise time, pulsed power driver for z-pinch plasma radiation sources. The Z facility can make >200 TW, 1–2 MJ, near-blackbody radiation sources through the compression of cylindrical wire arrays. These sources are being used as drivers to study inertial-confinement fusion capsule implosions, complex radiation–hydrodynamic jet experiments, and wire-array z-pinch physics tests. To backlight plasmas in this environment we have built diagnostics based on spherically bent crystals that provide high spatial resolution (9–10 μm), a narrow spectral bandpass (<0.5 eV), and a large field of view (4 mm×20 mm). These diagnostics use the 2 TW, multi-kJ Z-Beamlet laser to produce x-ray emission sources at 1.865 or 6.151 keV for backlighting.

Relation between the electric parameters of a Z-pinch discharge and plasma production in the load during the implosion of a cylindrical wire array

A study of the process of implosion of a cylindrical tungsten wire array by electrical and optical methods shows that it involves two phases. In the first phase, the plasma is produced from the dense wire cores under the action of the heat flux from the current-carrying plasma. This plasma then fills the internal space of the liner array. The measured inductance of the liner and its visible diameter vary only slightly in this phase. During the second phase, the total material of the liner is compressed toward the axis and the inductance of the discharge gap increases. The process of the implosion of wire arrays is studied by analyzing the electric parameters (current and voltage) of the load in the Angara-5-1 facility. The time behavior of the load inductance, the average current radius, and the start time of the liner compression are determined. The compression start time determined from the visible size of the liner is found to coincide with that determined from electric measurements. The compression ratio of the liner in terms of the average current radius turns out to be lower than that measured by optical and X-ray diagnostics. The reason is that, by the instant of maximum compression, only a portion of the current flows at the periphery of the initial wire array.

Experimental and numerical studies of plasma production in the initial stage of implosion of a cylindrical wire array

The features are studied of plasma production in the initial stage of implosion of hollow cylindrical wire arrays at electric-field growth rates of 1012 V/(cm s). The results are presented from the analysis of both UV emission from the wire plasma and the discharge parameters in the initial stage of the formation of a Z-pinch discharge. It is found that, a few nanoseconds after applying voltage to a tungsten wire array, a plasma shell arises on the wire surface and the array becomes a heterogeneous system consisting of metal wire cores and a plasma surrounding each wire (a plasma corona). As a result, the current switches from the wires to the plasma. A further heating and ionization of the wire material are due primarily to heat transfer from the plasma corona. A model describing the primary breakdown along the wires is created with allowance for the presence of low-Z impurities on the wire surface.

Prolonged plasma production at current-driven implosion of wire arrays on Angara-5-1 facility

Results of experimental investigation and modeling of prolonged plasma production during implosion of cylindrical wire arrays are presented. Results of the radiography of dense cores of imploding wire array and the measurements of internal azimuthal magnetic field in wire array give new experimental evidences of prolonged plasma production phenomenon. This phenomenon is an important property of current-driven implosion of the wire arrays at current rise rates /spl sim/(0.5-1)10/sup 14/ A/s. The prolonged plasma production can determine the current and the density profiles before final stage of a Z pinch compression, and also in the moment of Z pinch stagnation. From this point of view, the requirement that residual uncompressed plasma should not shunt the current at the discharge periphery becomes of the greatest importance. The conditions exist when the prolonged plasma production isn't an obstacle for the achievement of high-power X-ray emission from Z pinch. Presented experimental results on multiwire array implosion can be explained on the basis of prolonged plasma production without referring to multiwire array azimuthal structure.

Multiwire screw-pinch loads for generation of terawatt x-ray radiation

A multiwire screw pinch (a variant of a Z pinch) for high-current 100 ns pulsed power generators used for terawatt x-ray radiation is proposed and discussed. Wires, twisted along a curved load surface, are suggested to be an effective way to create the initial axial magnetic field, to begin the rotation of the plasma, and to mitigate the growth of Rayleigh–Taylor instabilities. A screw-pinch design is predicted by a zero-dimensional model to provide more efficient energy transfer from the pulse power generator to the load compared with a straight, cylindrical wire array.

ZAPP: An Inexpensive Versatile X-Ray Source

The Z-pinch atomic physics project (ZAPP) in E-Division at the Lawrence Livermore Laboratory has been established to study the physics of high temperature high density plasmas. Using a linear hollow gas puff or a cylindrical wire array we can obtain electron temperatures in the keV region and densities of 1020 -1022/cm3 for virtually any element or mixtures of elements. At the center of this project is a 100 kV, 55 kJ capacitor bank which discharges through low inductance switches and transmission lines into the load developing peak currents in the 2.5 × 106 amp range. One of our goals is to have all major diagnostics temporally and spatially resolved. Included in this list are pinhole photography, grazing incidence and bent crystal spectroscopy, laser interferometry and holography, and pulsed laser scattering. We have also implemented an array of PIN diodes, XRD detectors and Rogowski coils. With a clean, repeatable, well diagnosed plasma we can address a wealth of questions concerning both plasma and atomic physics. The main experimental emphasis centers on the investigation of plasma properties such as line broadening and shifting of spectral lines and in general testing the ability of existing codes to predict observed spectra of both lines and continuum radiation. Other plasma properties to be studied are the formation of hot spots and the production of stabilized plasmas. Atomic physics experiments include the study of radiative and relativistic effects in highly stripped heavy ions, radiation transport phenomena, collisional excitation rates and level populations to test inversion schemes.