Preliminary Design of Central Solenoid of SST-2 and Demo

Abstract

Preliminary design of the central solenoid (CS) for SST-2 and Indian DEMO reactor has been initiated. The expected maximum magnetic field calculated by magnet design code MAC at the conductor location is ~13 T. The key role of the CS magnet is for initiating efficient plasma current and maintaining it for longer duration. In order to maximize the CS capability, the higher magnetic field with a greater magnetic flux linkage is required, and this could be possible with a large area of the solenoid. CS consists of a number of modules acting independently, each following an individual current profile leads to a better initial null formation, plasma current build up, longer pulse duration, and good shaping. Hence, the function of CS is to provide the required volt-sec for plasma initiations, sustaining plasma current, and to provide good equilibrium for few hundreds of seconds. For this purpose, it is advantageous to split CS into many modules. The dimensional parameters of various modules, tentative model, volt-sec capability, and allowable ramp rates are the prime requisites for CS design. The total volt-sec requirements for long pulsed plasma, which include plasma initiation, ramp up, flat top, and ramp down, are estimated using plasma properties. In this estimation, inductive flux and resistive flux requirements are estimated using appropriate analytical or empirical formula. In the flat top, only resistive flux is considered. The duration of the flat top is about few hundreds of seconds, and for steady-state operation of the reactor, during the flat top, the noninductive current drive mechanism will fully sustain the plasma current. This methodology is applied to typical ITER-like discharges and compared with ITER CS available volt-sec. The same methodology is used to estimate for SST-2 and Indian DEMO reactor. The radial buildup of inboard side of Tokomaks is obtained through a system code which uses physics and engineering constraints. This provides the available space for the CS coil. It is observed that the negative convertor operation is needed to enhance the available volt-sec. In order to fulfill the plasma physics requirements with the available inner bore area and height, a multimodule CS coil is proposed with Nb3Sn conductor. This work has been carried out for ITER CS to validate the procedure and applied to SST2 and DEMO. The tentative model of CS coil, calculation methodology of magnetic field, expected electromagnetic forces, stored volt-sec, and current ramp-up and ramp-down requirements will be presented in this paper.