Design of the Alcator DCT tokamak at M.I.T

Alcator DCT is an experimental tokamak proposed to be built at M.I.T. It features extremely long pulses, RF heating and current drive, and an all superconducting magnet system. The toroidal magnets produce a field on-axis of 7 T, permitting current drive at high density and ion heating with existing power supplies. The device is designed to maximize the use of existing facilities at M.I.T. in order to build a machine large enough for simultaneous heating and current drive at low cost. This report concentrates on a design option with 24 circular toroidal field (TF) magnets, which represents the second iteration in the conceptual design of this machine. This design is a modification of the HESTER concept developed by the authors. The DCT design is an advance over the HESTER design, in that it has adequate horizontal port space for human access and for tangential viewing of the plasma at the geometric center. This was achieved by decreasing the number of TF coils from 36 to 24, increasing the magnet bore from 52 to 62 cm and shaving diagonals from noncritical areas of the case in the lead and header region. Recent perceptions of the requirements of the tokamak program in the areas of impurity control and in-vessel component screening indicate that a third significant iteration of the DCT concept is necessary. The Alcator DCT uses pumped limiters for long term impurity control. Doubts about the efficacy of pumped limiters and a desire to concentrate on long-term impurity control issues led to the recommendation that DCT be modified to include expanded boundary and simplified poloidal divertor operation. Early work on these options is described briefly