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Characterization of current sheet evolution in a pulsed electromagnetic accelerator

Abstract

A detailed experimental characterization of current sheet evolution in a pulsed electromagnetic accelerator is presented based on temporally and spatially resolved measurements of the magnetic field in the device. The dynamics of current sheets generally depart from the ideal snowplow dynamics and are consequently associated with various non-idealities. Magnetic probes (B-dot probes) were used at 432 spatial loca- tions in the device to obtain a description of the magnetic field evolution. The corresponding current sheet evolution was ab- stracted from the magnetic field data and shows a complex structure of the current sheet during the initial part of its evolution. The measurements show the current sheet evolves from a sheet perpendicular to the electrodes to a structure exhibiting a main trunk and a canted branch near the anode. The canted branch evolves into a final canted current sheet front, short circuiting the trunk and leaving it behind in its wake. These observed features are in good agreement with the phenomenological description derived previously from a photographic survey of the discharge. The magnetic field measurements compliment the previous photographic survey as the latter indicates the location of the discharge (through filtered ion emission) but cannot show directly where the cur- rent sheet is located. While the magnetic field measurements can provide an unambiguous picture of the current location, they do not show where all of the plasma is.