30–100kV plastic case capacitors for Ltd's, Marx generators, and other pulse power applications

Plastic case capacitors have been developed that offer compact, economical, low inductance solutions for a variety of high voltage pulse power applications. Both extended foil and tapped foil configurations, combined with rail terminals, have been designed for fast discharge (≤ 1μs pulse width) low inductance and low capacitance units. For the highest rep rates and RMS currents, allpolypropylene dielectrics and extended foils are used, while for lower rep rates, mixed Kraft/polypropylene dielectrics may be utilized, as they offer additional resistance to degradation from voltage reversal. For maximum specific energy, dielectrics based on high density Kraft paper or mixed Kraft and polyester (PET) films were utilized to take advantage of the higher permittivities of these materials. Specific liquid impregnants are used with the different combinations of film and paper materials to achieve the best combined dielectric properties. When inductance is not as critical, a different internal geometry lends itself to the use of standardized winding materials that can satisfy a very wide range of capacitances and voltages used in repetitively pulsed applications such as in PFN modulators for particle accelerators. Such designs have capacitively graded voltage between their axial terminals. For relatively slow pulse (>50 μsec discharge time) applications, self-healing metallized film has been successfully integrated into plastic cases. When not required for utmost performance or undersireable for the application, the liquid dielectric oil has been eliminated, and both it and the thermoplastic case replaced with a molded thermosetting encapsulating and insulating resin. This paper will describe the performance characteristics and different ranges of application of these different types of plastic case capacitors.