Cryogenic system for the Alcator C-Mod tokamak

Alcator C-Mod is a compact tokamak (major radius 66 cm), designed to operate at high toroidal magnetic field (up to 9 Tesla), and high plasma current (up to 3 MA). The coils and the tokamak structure must be cooled to liquid nitrogen temperature in order to withstand the pulse-induced mechanical and thermal stresses (e.g. 316 LN steel has higher strength at LN₂ temperature). Operating at low temperatures has also the advantage of reducing the required voltage and power from the power supplies thus significantly reducing their cost. This report will describe the design of the cryogenics delivery system which must remove up to 300 MJ of heat from the tokamak in 20 minutes. Details of the various components (tanks, valves, pump) will be presented along with the cooling scheme of the coils (poloidal field, toroidal field and ohmic heating) and of the structure. We will also report on the PLC-based control system which regulates the cooling efficiency for the various components. This will include details about the extensive instrumentation required for feedback control. The interaction of the cooling system with other tokamak subsystems will also be described