Mechanical design considerations of a spherical torus volumetric neutron source

The mechanical design of a spherical torus based volumetric neutron source (ST VNS) is being studied under the support of a DOE-SBIR funding. A device capable of staged operation from a neutron wall loading of 0.5-5.0 MW/m² has been scoped out, as the physics and engineering design assumptions are raised from modest to aggressive levels. Margins in the design are ensured since operation of the VNS will be adequate at a wall loading of 2 MW/m². The device has a naturally diverted plasma with major radius of 1.07 m, a minor radius of 0.77 m for an aspect ratio of 1.4, an elongation of 3 and triangularity of 0.6. In the neutral beam driven version, the plasma current is 11.1 MA and the toroidal field at the plasma major radius is 2.13 T. The baseline fusion power is 151 MW giving an average neutron wall loading of 2 MW/m² on the outboard side over an accessible area of over 15 m² for blanket testing. The device utilizes a normal Cu conducting bell jar as the return leg of the toroidal field current, a concept developed at the Oak Ridge National Laboratory, The current is carried by an unshielded single-turn center post (CP) made of dispersion strengthened Cu which is cooled by water in a single pass from top to bottom. A special sliding electrical interface between the CP and the bed jar is provided on the upper end to allow for differential expansion and to isolate the CP from tensile and torsional forces from the bell jar. The ohmic heating in the CP is 153 MW at the start of operation and increases to 178 MW after 3 full power years of operation. Over this period the maximum Cu temperature does not exceed 160 C. This report primarily deals with the design of the CP, one of the most challenging issues of a low aspect ratio spherical torus. Maintenance approaches for the CP and the divertor assemblies have been determined and are addressed in the paper