Development of the axial instability and magnetic field topology in low wire number wire array Z-pinches

Summary form only given. We are investigating the development of the axial instability that occurs on each exploding wire in wire-array Z-pinches. The axial instability is a growing modulation of the size of the coronal plasma around individual wires of the array that results in non-uniform ablation of material from the cold wire core. It has long been known that the wavelength of this modulation is constant late in time and, since it is unique to different materials, it has come to be known as the fundamental mode. In these experiments we have been imaging individual wires with laser shadowgraphy and an XUV framing camera primarily in low wire number, large wire diameter aluminum array. We show also some results from various other arrays for comparison. We document the development of this modulation from the beginning of plasma formation and show its dominant wavelength and amplitude growth as a function of time. The magnetic field topology is also probed using small B-dot probes inside the array. The change from a closed to an open field topology is correlated with the instability growth. Growth of the instability is seen to slow and eventually stop as the magnetic field 1-2 mm inside a wire decreases and changes sign, signifying the change in topology from locally to globally dominated field. Magnetic probe measurements of the field advected inside the array during the ablation phase are used to calculate the amount of current that is associated with the precursor plasma before implosion. With aluminum arrays, the basic scaling is that the rate of field advection towards the axis is roughly proportional to the field outside the array. Thus the precursor current is proportional to driving current and inversely proportional to array radius, and insensitive to wire number, at least for wire spacing of ~1 mm or greater. Further results with aluminum and tungsten arrays and closer wire spacings will be presented.