Implosion Of Tungsten Wire Arrays Research Articles

10.1007/s11452-008-3001-5

A study is made of the Z-pinch plasma expansion after the current is switched off. Measurements were carried out in experiments on the implosion of tungsten wire arrays in the Angara-5-1 facility. It is found experimentally that, at a distance of 2 m from the pinch, the ion velocity in the expanding Z-pinch plasma is about (2.5–4.0) × 107 cm/s, which substantially exceeds the thermal velocity of tungsten ions. A model describing the plasma expansion process is proposed that is based on the ambipolar acceleration mechanism. The results of numerical simulations are compared with the experimental data.

Experimental study of the ion flux parameters and anode plasma dynamics in the Angara-5-1 facility

Results are presented from experimental studies of the anode plasma dynamics and measurements of the ion flux ejected along the axis of a high-current Z-pinch. Pinch discharges were formed by the implosion of tungsten wire arrays in the Angara-5-1 facility. It is shown that the ion energy spectrum depends on the mass and configuration of wire arrays, as well as on the diameter of the anode aperture. The shape of the ion spectrum indicates that the plasma propagates in the form of a compact plasmoid. Shadow and X-ray images of the plasma show that the axial velocity of the plasma outflowing through the anode aperture is comparable with the velocity of radial plasma compression and, for tungsten ions, can reach a value corresponding to an energy of 100 keV. The experimental data indicate that the ion energy spectrum mainly forms due to the electrodynamical acceleration of the plasma and cumulative jets. A possible mechanism for the production of compact plasma formations in the course of electrodynamic plasma acceleration is discussed.

Collective ion acceleration during the decay of a high-current Z-pinch

A study is made of the Z-pinch plasma expansion after the current is switched off. Measurements were carried out in experiments on the implosion of tungsten wire arrays in the Angara-5-1 facility. It is found experimentally that, at a distance of 2 m from the pinch, the ion velocity in the expanding Z-pinch plasma is about (2.5–4.0) × 107 cm/s, which substantially exceeds the thermal velocity of tungsten ions. A model describing the plasma expansion process is proposed that is based on the ambipolar acceleration mechanism. The results of numerical simulations are compared with the experimental data.

Measurements of the Axial Magnetic Field during the Implosion of Wire Arrays in the Angara-5-1 Facility

Results are presented from measurements of the axial magnetic field during the implosion of tungsten wire arrays in the Angara-5-1 facility at currents of 2.5–4.5 MA. The azimuthal structure of the plasma produced from the wires is examined using the effect of the compression of the axial magnetic flux by this plasma. It is shown that the plasma starts to penetrate into the axial region of the wire array at the very beginning of implosion. A mechanism other than the formation of a closed current-carrying shell is proposed for describing the transfer of the external axial magnetic field to the central region of the array.

The role of strong coupling in z-pinch-driven approaches to high yield inertial confinement fusion

Peak x-ray powers as high as 280±40 TW have been generated from the implosion of tungsten wire arrays on the Z Accelerator at Sandia National Laboratories. The high x-ray powers radiated by these z-pinches provide an attractive new driver option for high yield inertial confinement fusion (ICF). The high x-ray powers appear to be a result of using a large number of wires in the array which decreases the perturbation seed to the magnetic Rayleigh-Taylor (MRT) instability and diminishes other 3-D effects. Simulations to confirm this hypothesis require a 3-D MHD code capability, and associated databases, to follow the evolution of the wires from cold solid through melt, vaporization, ionization, and finally to dense imploded plasma. Strong coupling plays a role in this process, the importance of which depends on the wire material and the current time history of the pulsed power driver. Strong coupling regimes are involved in the plasmas in the convolute and transmission line of the powerflow system. Strong coupling can also play a role in the physics of the z-pinch-driven high yield ICF targeL Finally, strong coupling can occur in certain z-pinch-driven application experiments.

Power enhancement by increasing the initial array radius and wire number of tungstenZpinches

Tungsten wire array implosions on the 7- to 8-MA Saturn generator have been optimized using wire number and array diameter variations to produce $75\ifmmode\pm\else\textpm\fi{}10\mathrm{TW}$ of x rays with total energy outputs of $450\ifmmode\pm\else\textpm\fi{}50\mathrm{kJ}.$ By increasing the number of wires in a 12.5-mm-diam array from 24 to 70 and simultaneously decreasing the individual wire diameter from 13 to 7.5 \ensuremath{\mu}m, the total radiated power increased from $20\ifmmode\pm\else\textpm\fi{}3$ to $40\ifmmode\pm\else\textpm\fi{}6\mathrm{TW}$ and the x-ray pulse width decreased from 18 to 8.5 ns. In addition, a diameter scan at an implosion time of $50\ifmmode\pm\else\textpm\fi{}5\mathrm{ns}$ showed that the pulse width has a strong dependence on collapse velocity and wire thickness. For the largest diameter load of 17.5 mm with 120 5-\ensuremath{\mu}m-diam wires, a 4-ns pulse width with a peak power of $75\ifmmode\pm\else\textpm\fi{}10\mathrm{TW}$ was achieved: four times power gain over the 20-TW electrical power generated by the pulsed power system. Time-resolved pinhole photography confirms that the power enhancement with increased wire number is associated with the plasma achieving a tighter compression and better axial uniformity. For the higher-velocity implosions, we infer from two-dimensional radiation-magnetohydrodynamic calculations that the plasma becomes hotter and hence radiates at a higher brightness temperature. Zero- and two-dimensional load models coupled with a detailed circuit model have shown expected radial kinetic energies in the range of 100--200 kJ. The total radiated energy of $>400\mathrm{kJ}$ in a 4--20-ns FWHM pulse exceeds the total kinetic energy by more than a factor of 2. Two-dimensional, three-temperature simulations reproduce the observed trends in powers and pulse widths by using a variable initial random density perturbation. These calculations also indicate that the radiated energy is accounted for by the total work done on the plasma by the magnetic field.