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Pulsed Power Plasmas |
Laboratory Astrophysics
Wire array z-pinches provide a very flexible plasma generation system and are increasingly used to address astrophysical plasma physics problems. It is possible to create systems analogous to those observed in the universe, and this enables an investigation of theoretical predictions of the behaviour of astrophysical objects in a controlled, highly diagnosed laboratory environment. A brief overview is given below, and a summary of the present capabilities of pulsed power driven wire array in laboratory astrophysics can be found here Simple modifications of the standard cylindrical wire array can be applied to produce dense, highly supersonic plasma jets. In a conical wire arrays the wires are inclined to the axis producing an opening angle typically of ~30 degrees. The ablated plasma is accelerated perpendicular to the wires and collides on axis, where the radial velocity is cancelled by opposing streams but the axial velocity is maintained. This process produces hydrodynamic jets with velocities ~200 km/s at M > 20 with densities ~10^19 cm-3. These jets are designed to experimentally simulate astrophysical outflows such as Herbig-Haro objects (e.g. HH47) associated with newly forming stars (YSOs), and have been shown to scale to astrophysical systems via dimensionless parameters [1].  The long propgation distances of these jets (>20mm) and long lifetimes (>200ns) allows a range of interaction experiments to be performed in an attempt to simulate the bending observed in astrophysical jets due to either a moving course or collisons with other objects (e.g. nebulae). In the laboratory, a plasma cross-wind is created by photoablation of a CH foil located above the wire array which expands across the jet propagation direction. The jets are observed to be deflected on interaction with the plasma cross wind, but maintain thier collimation and develop extensive internal shock struture [2,3]. Such jets are also produced in x-pinch configurations at currents as low as 80 kA [4].  In addition to hydrodynamic jets, a different system can be use to magnetically launch jets from wire arrays. A radial wire array comprises wires strung from a central electrode to a concentric return path in a 'wheel spoke' geometry. The JxB force on the low densiy plasma is now in the axial direction and low density prefill above the array is generated. As the wires closest to the axis begin to break, an expanding magnetic cavity is formed which subsequently launches a clumpy jet structure [5,6]. This again is highy supersonic and propagates into the low density prefill plasma. The variation of the manetic field structure across the palasma create region of both high and low beta plasmas, and the relaxation time of the B-field from the launched jet is comparable to the experiemnt timescale.  
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