Design of a pulse transformer for the ohmic heating system of a small tokamak

A design methodology of the ohmic heating and plasma current induction system for a small tokamak is presented, capable of inducing in a preionized gas loop an average current of 50 kA over a 20 ms pulse. The design tool consists of a mathematical model that reproduces the transient behavior of a transformer when the primary current is generated by the discharge of a capacitor bank. The model utilizes linked and coupled magnetic fluxes between windings, and includes core saturation effects, as well as the hysteresis effect. A prototype transformer, with known parameters, was used to validate the model. This numerical tool was used in the conceptual design of the ohmic heating system; two combinations of feed circuit and construction parameters were tested, and best results were obtained when the primary solenoid was fed from the discharge of a 540 μF capacitor bank charged to 11.7 kV. The parameters of the solenoid were also determined, taking into consideration size and weight constrains; the best solenoid turned out to be a single-layer, 238-turn, 0.42 m diameter and 1 m long straight solenoid, made with 12 AWG magnet wire. The required steel core has a cross section of 0.1039 m² and a weight of 4 tons.