Energy Density of Film Capacitors for ETC Gun Applications

In the late 1980s and early 1990s a significant effort was made to improve the energy density of capacitor for the ETC gun application where the pulse width is typically in the millisecond range. At that time, a relatively high energy density capacitor was developed using polyvinylidene fluoride (PVdF) as the dielectric. Relatively little work has been done on this type of capacitor since then. Significant progress has been made with in the use of other dielectric materials using new manufacturing techniques, some of which would most likely work with PVdF dielectric capacitors. This paper is a review of the basic properties of wound film capacitors applied to the ETC gun application along with projections of what would be expected from PVdF capacitors using today's manufacturing techniques. The primary concerns for capacitor used in the ETC gun application are energy density, performance in the application, and cost. The order of importance may very but these are usually the top three concerns. PVdF has an advantage over other dielectric films in that it has a relatively high dielectric constant and a voltage withstand capability equal to polypropylene (PPL). The relative dielectric constant is about 8 for PVdF and 2.1 for PPL. There are a number of disadvantages with the use of PVdF in other applications, not the least of which is the cost. A large capacitor built with PVdF film will cost about 5 times that of a an equivalent capacitor built with PPL. This is the primary reason it is not popular in commercial products where it meets the other requirements of the application.