Limitations on the minimum charging time for the field coil of air core compensated pulsed alternators [for EM launcher applications]

Air core compensated pulsed alternators (compulsators) are being developed for a variety of field-based applications in relation with electromagnetic launchers. Since these applications are essentially field portable, minimizing system mass is of great importance. It is also desirable to use a room temperature field coil, since carrying cryogens (such as liquid nitrogen or liquid helium) onboard a vehicle has logistic problems. These requirements have led to the use of a self-excitation scheme using a room temperature field coil. A discussion is presented of the eddy currents induced in the shield of the passive compulsator and the circulating current induced in the compensating winding of a selective passive compulsator during the charge-up of the field coil. It is shown that reducing the charging time of the field coil greatly enhances the efficiency. In some cases the rate of charging may be the determining factor on whether the field coil is room temperature or cryogenic and whether it needs active cooling. There is, however, an optimum charging rate. Increasing the charging rate beyond this limit does not result in proportional benefits