Wire array z-pinches comprise a cylindrical arrangement of metal wires (typically Al or W) driven by a fast-rising current drive. For most experiments the drive current is 100 kA to several MA and rises to a maximum in around 100ns. Wire array z-pinches are a complex plasma system, and produce a wide range of plasma parameters throughout their evolution. The high voltage from the pulsed power generator (100kV to several MV) initiates the array by rapidly causing electrical breakdown at the wire surface, forming a plasma in a few nanoseconds. The stream densities produced in experiments have ion densities in the range of 1x1014 – 5x1017 cm-3, typically with temperatures 5-15eV and Mach numbers between 3 and 5.
Eventually enough mass has been ablated from the wires that they begin to break, and this triggers the implosion phase.


The magnetic piston snow ploughs up the pre-fill plasma which serves to stabilize the implosion surface. Wire diameters are typically 5 to 50 microns in diameter and are strung between two electrodes. This axial current creates an azimuthal magnetic field which causes the array to collpase onto its axis, creating a high power soft x-ray burst. At this point each wire forms an heterogeneous plamsma structure referred to as the core-corona model. Streams at current of 1 MA are typically collisional on the order of the array diameter (~8 mm), and form shock structures observable in emission images around objects place in their path, such as occurs in nested wire array experiments [5]. On smaller generators, arrays can use as few 8 wires, and larger generators can use several hundred.


The largest pulsed power generator in the world is based at Sandia National Laboratories, and produces 1.8 MJ of x-ray energy at a power of 280 TW, and is a potential driver for both Inertial Confinement Fusion (ICF) and Inertial Fusion Energy (IFE) [1].
Due to the greater volume and higher ionisation (an hence conductivity) of the corona, it is this plasma which carried much of the current and hence is accelerated towards the array axis.
During the ablation phase, which can last for up to 80% of the experiment, the cores remain stationary and replenish the coronal material as it is removed by the Lorentz force [2].The rate at which mass is ablated from the wires typically scales as I2 [3], and this determines the density of both the ablated plasma streams and subsequent structures formed interior to the array as these ‘jets’ converge onto the array axis [4, 5].



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Rubric: Microwave Faraday Cage



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