Power flow measurements in a small diameter magnetically-insulated transmission line for flash radiography

A self-magnetic-pinch diode (SMP) driven by a pulsed power generator is an attractive radiographic source option for future flash radiography experiments. TW-scale flash radiography systems often must be fielded in confined spaces. This can place challenging constraints on the permitted size for the radiographic diode hardware, as well as on the transmission line that must couple the pulsed power generator to the diode. For conventional SMP diodes at 4-8 MV, driven by the coaxial, magnetically-insulated transmission line (MITL) of an inductive-voltage adder (IVA), the MITL´s vacuum electron flow current is shed to a large (~1 m dia.) outer conductor chamber just before the load using a torus or knob on the inner conductor. The large diameters and electrostatic field shaping are required to suppress new electron flow from developing post-shedding, which can degrade radiographic performance in standard SMP diodes. Space constraints can prohibit use of this large diameter chamber, which makes preventing reestablishment of the vacuum electron flow current very difficult. Because of this constraint, a new approach for an SMP was investigated where electron flow would not be shed before the SMP diode, but instead would be suppressed or harnessed using an advanced power flow configuration. LSP simulations suggest such an approach may be possible at small MITL diameters. Therefore, a constant-impedance (40-Ω Z_flow), tapered MITL was fielded on NRL´s Mercury pulsed power generator to bring the outer conductor from a nominal 38-cm diameter down to 8 cm. The power flow was diagnosed with B-dot monitors along the MITL to measure the impedance and any current losses. The work presented here focuses on these power flow measurements. Outer current losses were minimal through the taper into the load; however, the small diameter MITL operated 10-15% below Mercury´s line-limited impedance. One possible cause for the reduced impedance is localized ion source for- ation at current joints in the outer conductor. Results from LSP simulations and Mercury experiments of the tapered MITL geometry will be presented for both benign and SMP diode loads.