Magnetic System for the Ignitex Fusion Ignition Experiment
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Controlled D-T fusion in a tokamak device by ohmic heating alone can be realized with toroidal confinement fields between 15 and 20 T and plasma currents in the 12- to 15-MA range. These conditions are achieved in the IGNITEX concept [1,2) by using a single turn toroidal field coil designed for 20-T operation and a set of five, single turn poloidal field coil pairs located within the plasma bore of the 1.5 m major radius machine (0.5 m minor radius). Total pulse length for the experiment is 10 s, including a 5 s flat-top period. A single-turn configuration for the toroidal field coil was adopted to maximize load-carrying ability and virtually eliminate insulation problems. Peak current for 20-T operation is 150 MA which results in an average inner leg current density of 57 MA/m2, a relatively low value for the field produced. Support of the toroidal field coil includes axial preloading of the inner leg to a 469 MPa compressive stress with an externa 1 hydraulic press structure, a 234 MPa radial compressive preload applied at the top and bottom of the coil by two thermally fitted steel rings, and a 0.8 m diameter compression bar located in the coil central bore. The coil itself is housed in a cryostat for precooling to liquid nitrogen temperature to extend the usable temperature excursion. Time dependent finite element analysis of the single-turn coil indicates that a maximum von Mises stress of 531 MPa will occur in the inner leg region. The analysis also includes time evolution of temperature and current distribution within the coil as well as calculation of energy requirements. Peak temperature after the 10-s pulse will be less than 100°c. A dispersion strengthened copper alloy has been selected as the toroidal coil material for its excel lent combination of yield strength and conductivity, which are 582 MPa and 92% IACS, respectively. The material exhibits a high fatigue limit of 207 MPa. To demonstrate the operation of the single-turn coil at the 20-T level, a 1/6th scaled prototype is proposed using an existing 60 MJ, 9 MA six module hompolar generator (HPG) power supply located at the Center for Electromechanics at The University of Texas at Austin (CEM-UT).