Pulse properties of large 50-50 NiFe tape cores

Over 400 cores of 1-mil 50-50 NiFe, each core containing 40 or more pounds of material, are used in a linear induction accelerator, which is a part of a controlled thermonuclear research experiment. Each core couples a constant voltage pulse of approximately 10kV lasting 350 ns to an electron beam whose current is in the hundreds of amperes range. To achieve reasonable operation of the accelerator it is essential that the drive power to the cores, both during the forward pulse and reset, be kept to a minimum. Since cores of this size and constant voltage drives of this magnitude are both rarities, potential manufacturers of these cores have been reluctant to guarantee the pulse performance. To overcome this reluctance a study program consisting of both a theoretical analysis and experimental tests was conducted. Specifically, the purpose of the study was to find answers to three key questions. 1) What is the minimum current required in an optimum core? 2) What are the major contributors to deviations from the minimum current? 3) What can be done to insure consistently good cores? Results of the study for cores made of both 1-mil and 1/2-mil tape are discussed; the major findings being that at these drive levels, eddy currents both within individual wraps and between wraps cause the dominant losses. Recently about 200 such cores were obtained. The measured pulse values are compared with the predicted values.