Circuit Analysis of the Use of Rectifying First Wall to Drive the Plasma Current During the Burn Period of a Tokamak Fusion Reactor
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The possibility of driving a tokamak fusion reactor during the burn with a rectifying first wall and a steadily modulated or slightly decreasing Ohmic Heating (OH) current, is investigated. Computer based electric circuit simulation is performed with the 1975 ORNL EPR as an example. The poloidal field system is modeled as 4 inductively coupled circuits, consisting of the OH coil, shield-decoupling coil, rectifying first wall, and plasma circuits. The plasma resistance is assumed constant since simulation starts after neutral beam heating. The wall is a circular toroidal shell of 1 cm thick AMAX MZC copper at 192 C. The first wall rectifier is ideal. The wall resistance is derived from the de current density. The 15 term system inductance matrix is calculated with the UT CEM tokamak inductance code TINDERB. Voltage equations are solved numerically for input OH currents. A 1 Hz cosinusoidally modulated OH current with wall conduction only on odd half cycles increases the 7.2 MA plasma current by varying amounts as modulation amplitude varies. Waveforms are obtained for plasma current, wall current, OH supply voltage, shield supply voltage, plasma loop voltage, and circuit resistive energy dissipations. Modest OH supply voltages (9 V) and shield supply voltages (226 V) are required to keep the plasma current rise to 0.13%/cycle. Even for steady OH current, the rectifying wall causes a slight plasma current rise due to supply voltage pumping of the wall current. Modest OH current decreases, followed by plateaus, produce a 0.6 Hz sawtooth plasma current waveform with a 0.2% amplitude above the initial value. Peak wall current densities are 8.5 A/cm2. With this scheme small power supplies and decreasing OH currents can provide burn power. At the present level of analysis plasma current burn maintenance with a rectifying first wall appears attractive and feasible.