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.
The hypothesis that wire array Z-pinch radiation sources can be represented as an ablating mass source embedded within a Lorentz force field is examined and the effects that this has upon the trajectory and spatial structure of the ensuing implosion are studied. Two-dimensional (2D) resistive magnetohydrodynamic (MHD) simulations of the ablating core regions and of the array cross-section indicate that the core ablation rate is determined by force balance at the ablation surface. This implies a weak dependence of the ablation velocity (the ratio of the magnitude of the Lorentz force to the mass ablation rate) on the array parameters (current, radius, mass, etc.). In the case of a constant ablation rate, the radial profiles in the flow region between the wires and the axis are found to converge to a set of time independent equilibria. These profiles represent a unique solution to the ideal MHD equations for super-Alfvénic flow in cylindrical geometry. Comparisons of simulated implosion trajectories with experimental optical streak photography data are used as a code validation exercise and show important deviations from the scenario of invariant ablation velocity. The importance of the number of wires in the array in determining the ablation rate and thus the trajectory and structure of the implosion is highlighted. The effects upon the inferred implosion symmetry and the x-ray pulse shape and peak power are discussed.
The mass ablation phase of a wire-array Z pinch is investigated using steady-state ͑r , ͒ simulations. By identifying the dominant physical mechanisms governing the ablation process, a simple scaling relation is derived for the mass ablation rate ṁ with drive current I, in the case where radiation is the primary energy transport mechanism to the wire core. In order to investigate the dependence of ṁ on wire core size, a simplified analytical model is developed involving a wire core placed in a heat bath and ablating due to radiation. Results of the model, simulation, and experiment are compared.
The design of a 6.5-MV linear transformer driver (LTD) for flash-radiography experiments is presented. The design is based on a previously tested 1-MV LTD and is predicted to be capable of producing diode voltages of 6.5 MV for a 50-Ω radiographic-diode load. Several fault modes are identified, and circuit simulations are used to determine their effect on the output pulse and other components. For all the identified fault modes, the peak load voltage is reduced by less than 5%.Index Terms-Circuit simulation, electron accelerator, pulse power systems, radiography, X-ray applications.
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