Power enhancement by increasing the initial array radius and wire number of tungsten<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi></mml:math>pinches

Abstract

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 $&gt;400\mathrm{kJ}$ 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.