Controlling uniformity of gas-puff and wire array implosions with wire current-carrying structures

Abstract

Summary form only given. The magnetic field driving a cylindrically-symmetric Z-pinch implosion equals B=/spl mu//sub 0/I/2/spl pi/r the field produced by an infinite pinch plasma column of the same radius r. In reality, this value of B includes contributions of the finite-length plasma column, as well as those of currents flowing in the anode and cathode, in the return current cage and coax feed. The latter are significant, particularly at the initial stage of the implosion. For conditions characteristic of gas-puff implosions driven by /spl sim/300 ns long current pulses (r=6 cm, anode-cathode gap h=4 cm, return current posts at R/sub b/=8 cm), the current through the pinch column contributes 67% of B in the mid-section, the remaining 33% being equally divided between the contributions of anode/cathode and return current posts/coax. In the vicinity of the cathode and the anode, the contribution of the pinch column drops to about 50%. In the experiments with gas-puff plasma radiation sources, the current-carrying structure is not uniform: the cathode/anode grids consist of radial spokes, the number N of return currents posts is finite. This non-uniformity translates into an azimuthal modulation of the amplitude of the driving magnetic pressure. It is found to be substantial where the distance from the discrete current-carrying structure is much less than the gap between two of its neighboring elements. For instance, relative variation of the driving magnetic field due to a finite number of return current posts equals (4r/R)/sup N/. The current-carrying structure could be designed to help stabilize the implosion by producing axial magnetic field B/sub z/. We discuss the relative merits of the known concept of twisting the return current cage and the new idea of twisting the anode/cathode spokes. If the load itself is a wire array, it can be twisted into a solenoid in such a way that the B/sub z/ field produced inside of it matches the azimuthal magnetic field outside. Then the early-time current in the wires can be made force-free, producing no precursor flow to the axis in the collective magnetic field. The precursor plasma would fill the gaps between the wires, the load thus evolving from a solenoidal wire array to an annular shell. Chances are that the 2D annular load might be better for radiation production (e. g. generate higher X-ray power at stagnation) than a linear wire array, with its inherent variety of 3D phenomena.