Design of a homopolar generator power supply system for the IGNITEX experiment

Pulsed homopolar generator (HPG) power supplies are well suited for driving the single-turn coils of the Ignitex fusion experiment[1] because they are inherently low voltage DC machines capable of high output currents. Basic operation of an HPG is voltage generation across a conductive disk or drum rotating in 1 steady magnetic field. An electrical load is connected across the generated voltage through two sets of sliding electrical contacts or brushes. As pulsed energy stores, kinetic energy of the disk (rotor) is converted into an electrical pulse by the interaction of the armature current with the excitation field. For the Ignitex experiment, a total of 14.6 GJ are required for the 10 s total pulse length, 12 GJ for the toroidal field excitation and 2.6 GJ for the poloidal field coils. The toroidal field supply is composed of 12, one GJ modules each rated at 12.5 MA output current and an open circuit voltage of 30 V. Five generators are used to drive the poloidal fieldcoils. They have stored energies between 40 and 700 MJ each and output ratings from 8 HA at 60 V to 750 kA at 150 v. Pulse shaping for all the generators is accomplished by actively controlling the HPG excitation field during the discharge. Operation of the poloidal field system uses an underdamped coil/generator circuit to >ring> the total current from 22 to -15.7 MA. The Ignitex HPG power supplies use a combination of iron-core machine technology and relatively new composite energy storage flywheel developments. Ironcore HPG technology provides adequate voltage generation and extensively developed current collection techniques which can be used. For the 1 GJ machines, the brushes operate at a 200 11/s slip speed and a current density of 1.25 kA/cm2. These are below state of the art performance levels for pulsed HPGs. Composite flywheel technology allows the 1 GJ modules to be built compact enough to be placed close to the Ignitex coil system and provides a significant cost savings over steel flywheel energy storage. Flywheels for the generators are fiberglass/epoxy composite rims with 3.75 m outer diameters and 1 m axial lengths.