Aluminum Wire Array Z-pinch Research Articles

The energy distribution of the electron beams in z-pinches with different load geometries

A wide variety of z-pinches are host to energetic electron beams whose energies and origins have yet to be sufficiently characterized. Knowing the electron energies and currents of these electron beams can give insight into the mechanism responsible for the acceleration of the electrons to such high energies. Measurements with a magnetic analyzer have been carried out to determine the energy distribution of electron beams on a variety of aluminum wire array z-pinch geometries at the Nevada Terawatt Facility at the University of Nevada, Reno. These measurements show that as the geometry of the wire array is changed so that the initial angle of inclination, or necking shape, is enhanced, there is an increase in electron beam activity and electron energies. Measurements such as these can be useful to benchmark spectroscopic techniques used to characterize these energetic beams and give insight into the mechanisms responsible for the generation of such energetic beams in z-pinch plasmas.

Spatially resolved single crystal x-ray spectropolarimetry of wire array z-pinch plasmas.

A recently developed single-crystal x-ray spectropolarimeter has been used to record paired sets of polarization-dependent and axially resolved x-ray spectra emitted by wire array z-pinches. In this measurement, two internal planes inside a suitable crystal diffract the x-rays into two perpendicular directions that are normal to each other, thereby separating incident x-rays into their linearly polarized components. This paper gives considerations for fielding the instrument on extended sources. Results from extended sources are difficult to interpret because generally the incident x-rays are not separated properly by the crystal. This difficulty is mitigated by using a series of collimating slits to select incident x-rays that propagate in a plane of symmetry between the polarization-splitting planes. The resulting instrument and some of the spatially resolved polarized x-ray spectra recorded for a 1-MA aluminum wire array z-pinch at the Nevada Terawatt Facility at the University of Nevada, Reno will be presented.

Opacity and gradients in aluminum wire array z-pinch implosions on the Z pulsed power facility

Aluminum wire array z pinches imploded on the Z generator are an extremely bright source of 1–2 keV radiation, with close to 400 kJ radiated at photon energies >1 keV and more than 50 kJ radiated in a single line (Al Ly-α). Opacity plays a critical role in the dynamics and K-shell radiation efficiency of these pinches. Where significant structure is present in the stagnated pinch this acts to reduce the effective opacity of the system as demonstrated by direct analysis of spectra. Analysis of time-integrated broadband spectra (0.8–25 keV) indicates electron temperatures ranging from a few 100 eV to a few keV are present, indicative of substantial temperature gradients.

Increasing the K-shell yield of line radiation in Z-pinch implosions using alloyed Al/Mg wire-arrays

The variation of the K-shell yield of pure aluminum wire-array Z-pinch implosions with load parameters is discussed. The mechanism and the efficiency of increasing the K-shell yield using alloyed Al/Mg wire-arrays are numerically investigated. It has been shown that the maximum K-shell yield from a pure aluminum wire-array Z-pinch implosion can be obtained at an optimal load mass for a given generator and at a fixed initial wire-array radius. This optimal load mass is determined by the load energy coupling with the generator, the capability of Z-pinch plasmas to emit the K-shell radiation, and the self absorption of K-shell lines. For different generators, the optimal load mass increases as the drive current increases, and the line absorption limits the further increase of K-shell radiation. The coupled energy per ion is likely decreasing with increased mass, so the plasma might not be able to ionize into the K-shell. Also, the ability of the plasma to radiatively cool can increase with mass, thus, making it difficult for the plasma to ionize into and remain in the K-shell during the stagnation phase of the implosion. Alloyed Al/Mg wire-arrays were thus suggested to be used to decrease the opacity of K-shell lines and to increase the overall K-shell yield. In this paper, we show that using alloyed Al/Mg wire-arrays will decrease the opacity and increase the K-shell yield remarkably if the plasma is optically thick. We will also show that the efficiency of increasing the K-shell yield with alloyed Al/Mg wire-arrays cannot increase indefinitely. The ratio of K-shell yield from an alloyed Al/Mg wire-array to that from a pure aluminum wire-array reaches a limit. For example, we show that when the mass share of magnesium is 10% then this limit is 1.2, and for a 50% mass share, the limit is 1.3.

Spatially-resolved spectroscopic diagnosing of aluminum wire array Z-pinch plasmas on QiangGuang-I facility

Spatially-resolved crystal spectrometers with a high spectral resolution are developed to diagnose K-shell x-ray radiation from Z-pinch plasmas. These diagnostic apparatuses are successfully applied to aluminum wire array Z-pinch experiments on QiangGuang-I facility, a driver with a pulsed current up to about 1.5 MA in 80 ns. Time-integrated experimental results show that the K-shell x-ray emission lines of aluminum Z-pinch plasmas are dominated by line emissions from helium-like ionisation state. Bright spots that might have higher electron temperature or density are produced randomly in location and size along the z-axis during implosions. According to the experimental data, the electron temperature and the ion density are estimated to be between 250 eV and 310 eV, and between 7.0 × 1019 cm−3 and 4.0 × 1019 cm−3 respectively, while the ion temperature is inferred to be about 10.2 keV, which is much higher than the electron temperature.

Preliminary studies on the mechanism of radiation production in aluminum wire array Z-pinch implosion

The mechanism of energy conversion and radiation emission of aluminum wire array Z-pinch implosion has been studied by using a one-dimension non-equilibrium magnetohydrodynamic code. The energy transfer processes have been discused carefully, which shows that the accelerator feeds its stored energy to plasma via Poynting energy flux, and plasma kinetic energy is converted to plasma internal energy through pdV work and shock heating, thus producing radiation via several atomic processes including line radiation, recombination radiation and bremsstrahlung radiation. With the information of atomic populations, the mechanism of producing radiation has been anzlyzed. It is shown that in the implosion stage the ionization and excitation processes dominate, leading to the dominant yield of line radiation.While at stagnation most ions are in naked states, so the recombination radiation is the main radiation source. Corresponding to the two peak fractional populations of H-like and He-like ions, the line radiation also exhibits two radiation peaks at the time near peak compression. In the expansion stage, it is the radiative recombination that dominates the atomic processes, thus decreasing the share of line radiation in total radiation.

Experimental study on K-shell radiation production of aluminum wire array Z-pinch at Qiangguang-I facility

Experimental study of aluminum wire array Z-pinch was conducted on Qiangguang-I facility （1.5 MA, 80 ns） to produce —1.7 keV characteristic X-ray radiation. With fixed wire diameter of 20 μm and annular diameter of 12 mm, K-shell yield up to 0.9 kJ/cm and 1.1 kJ/cm was obtained for wire number N=8 and N=12 arrays, respectively, which are much higher than that for N=16 and N=24 loads. X-ray power waveforms and time-resolved images showed that several implosions occurred due to the existence of trailing mass. During 60%—80% of the whole implosion time，the wire array almost stayed at its initial position rather than running inwards. Main implosion was completed in the next 25—30 ns, with parts of load mass left behind and forming so-called trailing mass. At later time of main implosion, load current was transferred to trailing mass left in outer radii, and caused secondary even tertiary implosion, which also contributed to total K-shell yield. Appearance of trailing mass might be caused by azimuthally correlated axial modulation, consisting of hot spots and gaps on wire plasma column, as well as the development of other inhomogeneities.

Enhanced energy coupling and x-ray emission in Z-pinch plasma implosions

Recent experiments conducted on the Saturn pulsed-power generator at Sandia National Laboratories [R. B. Spielman et al., in Proceedings of the Second International Conference on Dense Z Pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3] have produced large amounts of x-ray output, which cannot be accounted for in conventional magnetohydrodynamic (MHD) calculations. In these experiments, the Saturn current had a rise time of ∼180 ns in contrast to a rise time of ∼60 ns in Saturn’s earlier mode of operation. In both aluminum and tungsten wire-array Z-pinch implosions, 2–4 times more x-ray output was generated than could be supplied according to one-dimensional (1D) magnetohydrodynamic calculations by the combined action of the j×B acceleration forces and ohmic heating (as described by a classical Braginskii resistivity). In this paper, we reexamine the problem of coupling transmission line circuits to plasma fluid equations and derive expressions for the Z-pinch load circuit resistance and inductance that relate these quantities in a 1D analysis to the surface resistivity of the fluid, and to the magnetic field energy that is stored in the vacuum diode, respectively. Enhanced energy coupling in this analysis, therefore, comes from enhancements to the surface resistivity, and we show that plasma resistivities approximately three orders of magnitude larger than classical are needed in order to achieve energy inputs that are comparable to the Saturn experiment x-ray outputs. Large enhancements of the plasma resistivity increase the rate of magnetic field and current diffusion, significantly modify the qualitative features of the MHD, and raise important questions as to how the plasma fluid dynamics converts enhanced energy inputs into enhanced x-ray outputs. One-dimensional MHD calculations in which resistivity values are adjusted phenomenologically are used to illustrate how various dynamical assumptions influence the way enhanced energy inputs are channeled by the fluid dynamics. Variations in the parameters of the phenomenological model are made in order to determine how sensitively they influence the dynamics and the degree to which the calculated x-ray outputs can be made to replicate the kinds of large variations in the experimental x-ray power data that were observed in three nominally identical aluminum wire shots on Saturn.

Theoretical studies of aluminum wire array Z-pinch implosions with varying masses and radii

A one-dimensional radiation magneto-hydrodynamics code coupled to an atomic kinetics code was developed, whose simulation results were compared with the experimental observations. Based on the simulation results, the physical processes and physical scheme of Z-pinch implosions were studied. During the implosion phase, the plasmas are heated and ionized, and before the stagnation, the greatly ionized plasmas of high temperature and high density are created. Then the continuously rising density leads to the recombination of plasmas and the emission of an intense X-ray pulse. Our simulation shows that there are two necessary conditions for producing high yield of K-shell energy. Firstly, highly ionized plasmas must be prepared before the beginning of recombination. Secondly, the density of the plasmas which are in recombination phase should be high enough to make the plasmas recombine with a large rate so that more internal energy can be drawn before stagnation. As another result of our study, the atropic processes play a role in cooling free electrons during the recombination phase.

Phenomenological modeling of turbulence in Z-pinch implosions

A phenomenological investigation into the effects of magnetohydrodynamic (MHD) turbulence on the initial stagnation dynamics of aluminum wire array and argon gas puff Z-pinch implosions is performed. The increases that turbulence produces in the plasma viscosity, heat conductivity, and electrical resistivity are modeled by using multipliers for these quantities in one-dimensional (1-D) MHD calculations. The major effect of these increases is to soften the 1-D implosions by decreasing the densities that are achieved on axis at stagnation. As a consequence, a set of multipliers can be found that reasonably duplicates the average electron temperatures, ion densities, and mass of the K-shell emission region that were measured at stagnation for a variety of Physics International aluminum wire array and argon gas puff experiments. It is determined that the dependence of these measured quantities on the multipliers is weak once a level of enhancement is reached, where agreement between calculations and experiments is attained. The scaling of K-shell yield with load mass for a fixed implosion velocity is then reexamined, and the minimum load mass needed to efficiently produce K-shell emission by thermalization of kinetic energy is calculated for aluminum and argon using this phenomenological soft implosion modeling. The results show an upward shift in the minimum mass by a factor of 6 when compared to the original nonturbulent hard implosion calculations.