Characteristics and scaling of tungsten-wire-array z-pinch implosion dynamics at 20 MA
- Pulsed Power Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico 87195-1193 (United States); and others
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 et al., Phys. Rev. Lett. 88, 215004 (2002)] on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. This array has the smallest wire-wire gaps typically used at 20 MA (209 {mu}m). 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 {approx}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 {mu}m) and 12 mm (at a wire-wire gap of 209 {mu}m), 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{sub m}{proportional_to}[dm{sub ablate}/dt]/m{sub array}. The convergence ratio of the effective position of the current at peak x-ray power is {approx}3.6{+-}0.6:1, much less than the {>=}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.
- OSTI ID:
- 20641428
- Journal Information:
- Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Vol. 71, Issue 4; Other Information: DOI: 10.1103/PhysRevE.71.046406; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1063-651X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABLATION
ACCELERATION
COMPRESSION
COMPRESSION RATIO
CONVERGENCE
IMPLOSIONS
INERTIAL CONFINEMENT
KINK INSTABILITY
PEAK LOAD
PLASMA
PLASMA DIAGNOSTICS
PULSE RISE TIME
STAGNATION
THREE-DIMENSIONAL CALCULATIONS
TRAJECTORIES
TUNGSTEN
TWO-DIMENSIONAL CALCULATIONS
WIRES
X RADIATION
X-RAY SOURCES