Azimuthal Clumping Instability in Wire Array Z-pinches*

UM and Sandia have analyzed an azimuthal clumping instability that bunches discrete wires in z-pinches. Highest instability growth rate is pi-mode: wires clumped in pairs. An efficient discrete wire code agrees with analytic theory, showing bunching in sub-100 ns, close to MA z-pinch risetimes. Using Sandia ALEGRA code, wire arrays are simulated utilizing a wedge-shaped sector with reflective boundary conditions. By choosing the wedge sector angle, pi-mode growth of the clumping instability is studied for a given wire number by simulating only a single wire within that wedge. A non-linear analytic theory shows excellent agreement with both ALEGRA and discrete wire codes for high ($\sim $600) and low (8) wire-number arrays. Results are presented of azimuthal clumping experiments on Cornell COBRA.