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Modeling of implosion dynamics and the magneto-Rayleigh-Taylor instability in single and double liner-on-target, gas-puff Z-pinches

  • Author(s): Narkis, Jeff
  • Advisor(s): Beg, Farhat N
  • et al.

The gas-puff Staged Z-pinch (SZP) is a magneto-inertial fusion concept in which one or more annular gas-puff liners implode onto a cylindrical target of fusion fuel. This dissertation addresses three topics essential to the viability of the concept: shock preheating of the target, stability of the implosion, and scalability to multi-megaampere drivers.

Shock preheating of the target is necessary because it reduces the required convergence ratio to reach fusion-relevant conditions by raising its adiabat. The strength of the shock is determined by inertial and magnetic forces acting on the target. Initial 1-D magnetohydrodynamic (MHD) simulations of SZP implosions of thin liners show that, while liner resistivity can affect the onset of shock compression, the final target adiabat is controlled by liner inertia.

As in the conventional Z-pinch, the SZP liner is prone to the magneto-Rayleigh-Taylor (MRT) instability, whose growth must be sufficiently mitigated to maintain liner integrity throughout the implosion. Here, multiple mitigation mechanisms are considered. The first is inherent to a gas-puff Z-pinch liner - modes of wavelengths below a certain minimum can resistively diffuse away. 2-D MHD simulations suggest that Kr and Xe liners have a higher minimum wavelength than Ne and Ar due to their higher resistivity, and later simulations with Ne liners suggest this effect could reduce dominant mode growth below the classical rate, $\sqrt{gk}$.

Two additional mechanisms are implemented: axial premagnetization, which reduces growth via field-line tension, and inclusion of a second liner, which alters the dynamics. 2-D simulations with Ne liners show that stabilization of single and double-liner configurations requires a $B_{z0}$ of 0.5 and 0.2 T, respectively. This reduction is important because there is a rapid drop-off in DD neutron yield for large $B_{z0}$ due to reduced fuel compression.

Finally, a semi-analytic model is developed to assess the scalability of the SZP. After presenting the model, test problems are compared with well-established codes and show reasonable agreement. Then, a simple design study is performed, in which DD neutron yield is optimized on a 850-kA driver and then scaled to 20 MA. Predicted yields agree well with published scaling models and 1-D MHD simulations.

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