Investigation of the Homopolar Motor Generator as a Power Supply for Controlled Fusion Experiments

With the impending construction of large controlled fusion feasibility experiments, the need for reliable, inexpensive inertial energy storage to power the confinement magnetic field ls becoming more urgent. To investigate the possibility of using a homopolar machine for this task we have carried out a series of electrical and mechanical tests on a O. 5 MJ unit operating as a pulsed power supply. The machine ls motored up to a design speed of 6000 RPM using a 150 kW SCR power supply, and then, upon disconnecting the supply and connecting a load (short circuit), current pulses of up to 13 kA with a rise time of 25 ms are generated. The peak current and pulse decay time are limited by the armature circuit resistance, RA. This in tum depends primarily on the amount of rotor surface covered by the brushes which, in the ca1e of this machine, ls only 1% leading to a relatively high RA of 4 x 10-4 n. In addition, we observe that up to 80% of the stored energy ls delivered to the load. Motoring efficiencies are considerably less due to friction of the brushes and loading of the thrust bearing. In this connection we are testing an electromagnetic thrust bearing that is based on the principle of restoring the rotor to a position of magnetic equilibrium by differentially exciting field coils on each side of the central plane perpendicular to the axis of rotation. This process involves a photoelectronic device to sense the position of the rotor in order to control the field supplies. Based on results from our small model, we are in the process of constructing a 5 MJ single rotor machine capable of generating 100 kA at 5500 RPM and 45 volts. This model will have substantially higher motoring efficiency and lower armature circuit resistance. Design features of this machine are discussed.