Development of a high energy density storage capacitor for NIF

The capacitor requirement of the National Ignition Facility (NIF) calls for 85 kJ energy discharge capacitors to be operated at 24 kV DC and 30000 Amps peak current per discharge, with 20000 charge-discharge cycles as the design life. Metallized-Kraft (MK) OPP capacitors (Aerovox type KM) submitted for qualification have exceeded these requirements, but with energy density limited to 0.77 J/cc. Significant cost reductions can be had with the development of an increased energy density capacitor, however, increased energy density is obtained at the cost of life. Results to date for engineering development of a higher energy density capacitor is summarized in this paper. Several electrode-dielectric configurations were evaluated by fabrication and testing of miniature scale, single section, capacitors. From these trials, we selected an alternative approach employing a rough metallized polypropylene (MPP) dielectric system. This rough MPP dielectric-electrode system yields a >10% gain in energy density, which results in a substantial cost reduction over the qualification KM type. Validation of NIF capacitor requirements by comprehensive testing of 1/6-scale full voltage (24 kV DC) capacitors came close to meeting the required shot life. Engineering development is proceeding on a MK high energy density capacitor that can yield the same cost reduction.