Thyristor autotapchanger power supply for Alcator C-Mod toroidal field

The large block of power required for a tokamak toroidal confinement field imposes conflicting requirements on the power supply. For any given design current level, a high supply voltage is desirable to minimize parasitic heating in the coils. However, a high supply voltage at a high current may overload the power source. The transformer ratings and costs would also be raised. In the Alcator C-Mod system, a conventional thyristor power supply could not provide a satisfactory compromise between coil heating and generator current limitations. The solution to this dilemma was the use of an autotapchanger thyristor convertor which supplies a high voltage when the load current is low and which gradually reduces the voltage as the load current builds up. Since voltage reduction is accomplished by continuous, subcycle switching of transformer taps, generator current can be maintained within acceptable bounds without sacrificing current rise and fall times. The Alcator C-Mod toroidal field supply is the first to use this circuitry for a tokamak