We describe the injtial experiments to study the Z-pinch-drjven hohlraum ligh-yield jnertjal confinement fusion (ICF) concept of Hammer and Porter [J. H. Hammer et al., Phys. Plasmas, 6, 2129]. We show that the relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling ("hohlraurn energetic") is well understood from O-D semi-analytic, 2-D viewfactor, and 2-D radiation magneto-hydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pjnch driven secondary (2655 TW), indicating the concept could scale to fusion yields of 400 MJ. We have also developed a novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry. This source wjll perrnjt investigation of the pinch power balance and hohh-aum geometry requirements for ICF reIevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [R. B.Spielman. er al.. Phys. Plasmas. 5,2105Plasmas. 5, (1998].

We present observations for 20-MA wire-array z pinches of an extended wire ablation period of 57%+/-3% of the stagnation time of the array and non-thin-shell implosion trajectories. These experiments were performed with 20-mm-diam wire arrays used for the double- z -pinch inertial confinement fusion experiments [M. E. Cuneo, Phys. Rev. Lett. 88, 215004 (2002)] on the Z accelerator [R. B. Spielman, Phys. Plasmas 5, 2105 (1998)]. This array has the smallest wire-wire gaps typically used at 20 MA (209 microm ). The extended ablation period for this array indicates that two-dimensional (r-z) thin-shell implosion models that implicitly assume wire ablation and wire-to-wire merger into a shell on a rapid time scale compared to wire acceleration are fundamentally incorrect or incomplete for high-wire-number, massive (>2 mg/cm) , single, tungsten wire arrays. In contrast to earlier work where the wire array accelerated from its initial position at approximately 80% of the stagnation time, our results show that very late acceleration is not a universal aspect of wire array implosions. We also varied the ablation period between 46%+/-2% and 71%+/-3% of the stagnation time, for the first time, by scaling the array diameter between 40 mm (at a wire-wire gap of 524 mum ) and 12 mm (at a wire-wire gap of 209 microm ), at a constant stagnation time of 100+/-6 ns . The deviation of the wire-array trajectory from that of a thin shell scales inversely with the ablation rate per unit mass: f(m) proportional[dm(ablate)/dt]/m(array). The convergence ratio of the effective position of the current at peak x-ray power is approximately 3.6+/-0.6:1 , much less than the > or = 10:1 typically inferred from x-ray pinhole camera measurements of the brightest emitting regions on axis, at peak x-ray power. The trailing mass at the array edge early in the implosion appears to produce wings on the pinch mass profile at stagnation that reduces the rate of compression of the pinch. The observation of precursor pinch formation, trailing mass, and trailing current indicates that all the mass and current do not assemble simultaneously on axis. Precursor and trailing implosions appear to impact the efficiency of the conversion of current (driver energy) to x rays. An instability with the character of an m = 0 sausage grows rapidly on axis at stagnation, during the rise time of pinch power. Just after peak power, a mild m = 1 kink instability of the pinch occurs which is correlated with the higher compression ratio of the pinch after peak power and the decrease of the power pulse. Understanding these three-dimensional, discrete-wire implosion characteristics is critical in order to efficiently scale wire arrays to higher currents and powers for fusion applications.

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A radiation source has been developed on the 20-MA Z facility that produces a high-power x-ray pulse, generated in the axial direction primarily from the interior of a collapsing dynamic hohlraum (DH). The hohlraum is created from a solid cylindrical CH2 target centered within an imploding tungsten wire-array Z pinch. Analyses and interpretation of measurements made of the x-ray generation within and radiated from the hohlraum target have been done using radiation-magnetohydrodynamic-code simulations in the r-z plane that take account of the magnetic Rayleigh–Taylor (RT) instability. These analyses suggest that a significantly reduced RT seed (relative to that used to explain targetless Z-pinch data on Z) is required to explain the observations. Although some quantitative and qualitative agreement with the measurements is obtained with the reduced RT seed, differences remain. Initial attempts to include into the simulations a precursor plasma, arising from wire material driven ahead of the main implosion, did not ameliorate the differences. Modification of the simulated W/CH2 interface may be required to properly explain the measured axial radiation pulse. This pulse, which exits a 4.5-mm2 hole centered above the target, begins ∼5 ns prior to stagnation (as defined by peak radial radiation power). The 5-ns interval leading to stagnation represents the duration when the imploding tungsten plasma acts as a hohlraum wall, trapping radiation within the interior of the foam target. The hohlraum radiation exiting the hole at 6 degrees to the z-axis reaches a maximum intensity of 3.1±0.6 TW/str (associated with an average hohlraum temperature of 215±10 eV), 1.4±0.4 ns prior to stagnation. (The uncertainties represent rms shot-to-shot variations.) This radiation pulse, characterized here, is useful for performing radiation-transport experiments with drive temperatures in excess of 200 eV.

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