Energy conversion efficiency of electrical exploding foil accelerators

We evaluate the energy conversion efficiency of an electrical exploding foil accelerator that accelerates a thin dielectric foil (called the flyer) to more than 1 km/s, which is propelled by electrically exploded bridge material. The effective flyer mass ejected from the accelerator is estimated by impulse measurements obtained using a gravity pendulum as well as by time-resolving flyer velocity measurements obtained using a photonic Doppler velocimetry system. For two different bridge sizes (0.2 and 0.4 mm), the flyer velocity and impulse increase with the input energy at the bridge section. The maximum flyer velocity and impulse, that is, 4.0 km/s and 67 µN s, respectively, are obtained by supplying 0.33 J of input energy. Upon increasing the input energy, the effective flyer mass also increases and exceeds the designed-bridge mass for both bridge sizes. The energy conversion efficiency from input electrical energy to flyer kinetic energy is calculated based on the effective flyer mass, velocity, and input energy. Both bridge sizes show comparable efficiencies: 27% and 30% for 0.2 and 0.4 mm bridges, respectively. The efficiency increases with increasing specific input energy at least up to 15 MJ/kg for the 0.4 mm bridge, whereas the efficiency of the 0.2 mm bridge above 30 MJ/kg decreases. This is owing to the excessively high input energy density in the 0.2 mm bridge, which causes the effective flyer mass to increase by including surrounding materials. These results indicate that the specific input energy should be optimized for obtaining maximum efficiency.