The objective of this work is to provide predictability for the threshold of the onset of internal electrical breakdown for helical magnetic flux compression generators (HFCG) to enable higher performance and higher voltage designs and reduce the cost of relying on empirical design methods. The Care'n LLC has used the code CAGEN [1] in conjunction with CALE [2] to investigate the phenomenon of internal electrical breakdown in these devices. The CAGEN was modified to compute the peak electric field strength between the armature and the stator as well as between adjacent wires of the stator. Shock compressed gas within the HFCG plays an essential role so the 2-D hydro-code CALE was necessary in order to get a full prediction of the thresholds. An overview of the modeling and the comparison of experiment are presented. An issue is the role bifurcations play in the electric field breakdown. A new code BSBIF [3] models some of the effects. In order to confirm the modeling, a set of very special HFCGs were fabricated and then fired by Hyperspectral Sciences, Inc. The fabrication details [4], the experimental set-up, and the measurements [5] were entirely the work of HSI.
We will describe the PC based computer program CAGEN in its current state of development. It models the performance of many varieties of Magneto-Cumulative-Generators (MCG) which are energized with High Explosive (HE). CAGEN models helical or coaxial types (in the same generator, ifdesired) which have HE on the interior. Any materials and any HE types may be used. The cylindrical radius of the windings (or outer conductor) and the radius of the armature may vary with axial position. Variable winding width, thickness, shape, and pitch can be represented, and divided windings are allowed. The MHD equations are used to advance the magnetic field into and through the conductors in order to compute resistance, melting, flux loss, pressure and contact effects. The MCG model is treated as part of a lumped circuit, which includes the priming circuit, several different opening &se switches, transformers, peaking circuit, and loads. Several calculations of benchmark published experiments are shown. A typical problem will complete in a few minutes. Graphical input, run control, and results-analysis, is provided by Math&& a CARE" CO. application.
To this date, most (perhaps all) computer simulations and models (including Care'n Co.'s code CAGEN [1]) that are capable of being used to predict the behavior of helical flux compression generators (HFCG) approximate both the helicity of the windings and the bifurcations. The spiral nature of the windings is approximated with loops that lie in a plane perpendicular to the axis of the HFCG. Perhaps the most notable approximation is the assumption that the currents in each of the bifurcate legs are equal. There have been a few experiments that have measured the currents in each "parallel" winding separately, and those measurements have shown that the currents are not equal (none of these experiments have been published). The concern, then, is that the electrical fields within the HFCG will not be represented accurately by those approximations. To examine these short falls, The Care'n LLC has developed a HFCG modeling code that treats the windings in their actual spiral and bifurcated form and, further, allows the currents in each wire to be computed self consistently. This code is called (for the time being) BSBIF standing for Biot-Savart BIFurcation. The details of BSBIF, its implementation, running speeds, and comparisons to CAGEN are presented. No bifurcation issues regarding internal electrical breakdown have been found.
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