Discharge efficiency of cylindrical storage coils

The influence of eddy currents on the energy transfer from a storage coil to a resistive load in systems with a long charge was studied. The magnetic diffusion equation for a current exponential fall in an infinite slab, as a model problem, was solved analytically. The dependence of the energy loss on the slab characteristics and time constant of the current decay was determined. More complicated cases of cylindrical coils were treated numerically. Both electromagnetic and stress analyses were performed. A jellyroll coil was found to be superior to a pancake coil in terms of discharge efficiency. A jellyroll coil of the Brooks type is able to supply to the load approximately 25% more energy than a pancake coil of identical size, providing both carry the same current. However, the energy density of a pancake coil is approximately 20% higher than that of a jellyroll coil, if the coils are charged up to the limit of their mechanical strength. To calculate the discharge efficiency of a pancake coil, a time-consuming transient analysis could be replaced by a dc or low-frequency ac analysis. Discharge efficiency increases for a faster charge, whereas the total transferred energy decreases, owing to a slight field diffusion into conductors. The experiments with a pancake coil yielded results in close agreement with the theoretical analysis