Solid dielectric transmission lines for pulsed power

Abstract

This paper documents recent work developing solid dielectric transmission lines for sub-microsecond, 100 kV class compact pulsed power systems. Polymer-ceramic nanocomposite materials have demonstrated sub-microsecond discharge capability in parallel plate capacitors and transmission lines [1, 2]. With a dielectric constant of approximately 50, the propagation velocity is 2.5 cm/ns, necessitating lines of several meters length to achieve > 100 ns pulse lengths. By folding the line in a fashion analogous to ceramic multilayer capacitors, the physical length of the line can be significantly shorter than the electrical length. We present the results of an experimental effort to develop a folded transmission line using a polymer-ceramic nanocomposite dielectric. The pulse length was somewhat shorter than expected based on a simple calculation using the geometry and the dielectric constant. Fully 3-D electromagnetic calculations were used to examine the role of the edges in curtailing the pulse length. Dielectric breakdown in this device occurred below the electric field threshold demonstrated in the prior work [1]. Improvements in the large scale fabrication of TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> beginning with nanoscale grains have opened the possibility for producing single layer high voltage devices. Given a dielectric constant approaching 140, transmission lines using nano-TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> can be considerably shorter than with other materials. Relatively thick, flat sheets of TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> have been fabricated for testing up to 50 kV. Several transmission lines, employing a serpentine electrode geometry, have been manufactured and tested. Testing up to several 10's of kV has confirmed the operation of the lines according to the design. As expected, the triple point between the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , electrode, and insulating medium has proven difficult to manage for high voltage operation. Several techniques to mitigate the effects of the triple point, including resistive grading at the edges of the electrodes, are discussed. Fully 3-D electromagnetic modeling is used to examine the effects of electrode geometry and composition on the performance of the lines.