Increasing plasma parameters using sheared flow stabilization of a Z-pinch

Abstract

The ZaP and ZaP-HD Flow Z-pinch experiments at the University of Washington have successfully demonstrated that sheared plasma flows can be used as a stabilization mechanism over a range of parameters that has not previously been accessible to long-lived Z-pinch configurations. The stabilization is effective even when the plasma column is compressed to small radii, producing predicted increases in magnetic field and electron temperature. The flow shear value, extent, and duration are shown to be consistent with theoretical models of the plasma viscosity, which places a design constraint on the maximum axial length of a sheared flow stabilized Z-pinch. Measurements of the magnetic field topology indicate simultaneous azimuthal symmetry and axial uniformity along the entire 100 cm length of the Z-pinch plasma. Separate control of plasma acceleration and compression has increased the accessible plasma parameters and has generated stable plasmas with radii of 0.3 cm, as measured with a high resolution digital holographic interferometer. Compressing the plasma with higher pinch currents has produced high magnetic fields (8.5 T) and electron temperatures (1 keV) with an electron density of 2×1017 cm−3, while maintaining plasma stability for many Alfvén times (approximately 50 μs). The results suggest that sheared flow stabilization can be applied to extend Z-pinch plasma parameters to high energy densities.

Physics-Based-Adaptive Plasma Model for High-Fidelity Numerical Simulations

“…Some notable examples include the current sheet behavior during geomagnetic reconnection [1,2], plasma photonic crystals [3,4], field-reversed configurations (FRC's) [5][6][7], and Z-pinches [8][9][10]. Reddell [2] showed that the distribution plasma function within the thin sheath boundary was not near Maxwellian, but regions inside the reconnected region or far away from the sheath were still near Maxwellian.…”

Emergent kink stability of a magnetized plasma jet injected into a transverse background magnetic field

“…The effect of sheared flow on current-driven MHD instabilities has previously been investigated theoretically and demonstrated experimentally. [35][36][37][38][39][40][41] Linear MHD calculations show that a sheared axial flow has a stabilizing effect on the kink mode, while a uniform axial flow has no effect on the instability growth. The main prior conclusion is that an axial plasma flow with a linear shear of dV z /dr > 0.1kV A is required for jet stabilization, where k is the axial wave number and V A is the Alfvén velocity.…”

Retrospective of the ARPA-E ALPHA Fusion Program

“…University of Washington/Lawrence Livermore National Lab: Sheared-flow Z-Pinch for Fusion The University of Washington (UW), along with its partner Lawrence Livermore National Laboratory (LLNL), developed a variant of the Z-pinch that exploits sheared flow in the axial direction to mitigate the m=0 and m=1 instabilities that plague Z-pinch plasmas. The concept builds upon prior work in the ZAP and ZAP-HD experiments which demonstrated that a Z-pinch initiated from a high velocity plasma gun experiences a shear flow from r=0 at the center of the Z-pinch axis to r=R at the plasma edge [26], [27] [28]. In those experiments, it was shown that at sufficiently high velocities (typically observed as ~10% of the Alfvén velocity times k, the axial wave number) from the plasma gun, the shear in the plasma was able to suppress the growth of sausage and kink modes in a Z-pinch over a stable period about 700X the expected instability growth time in a non-sheared Z-pinch, at densities of 10 16 -10 17 cm -3 and temperatures of 50-80 eV [27].…”