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 WL 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 will be adequate at WL = 2.0 MW/m². The design as given in a companion paper has a major radius = 1.07m, a minor radius = 0.77m for an aspect ratio of 1.4, a plasma current = 11.1 MA, and a toroidal field at the plasma major radius = 2.13 T. The baseline fusion power is 151 MW giving an average wall loading of 2.0 MW/m² on the outboard side over an accessible area of over 15 m² for blanket testing. Two neutral beams delivering 57 MW are needed to drive the steady state plasma current.

The ST-VNS utilizes a normal conducting bell jar as the return leg for the toroidal field current, a concept developed at the Oak Ridge National Laboratory. The current is carried by a single-turn center-post (CP) made of dispersion strengthened (DS) Cu, which is cooled by water in single pass from top to bottom. A water-cooled first wall, constructed from ferritic steel and covered by tiles, protects the CP from the plasma and serves as the inboard limiter. The CP carries a current of 11.4 MA, is 16.4 m in length, and has a radius of 27.5 cm extending +/- 1.4 m from the mid-plane. The radius gradually increases to 59.8 cm in about 0.5 m in length towards the extremities. At the mid-plane the coolant fraction is 30% and goes down to 20% at the extremities.

Ohmic heating is calculated using temperature dependent properties of DS Cu and increases in resistivity due to transmutation are accounted for. The ohmic heating at the start of operation is 112 MW and increases to 130 MW after 3 full power years (FPY), the corresponding water mass flow rate is 410 kg/s and 475 kg/s, respectively. During this time, the maximum Cu temperature does not exceed 200 C.

The CP is designed to bear only magnetic compressive loads produced by the current it carries. The magnetic pressure is estimated to be 27.5 MPa, giving an average Cu compressive stress of 52 MPa. Sliding electrically conductive joints at the extremities eliminate tensile and twisting loads on the CP. Assuming a naturally diverted plasma scrape-off layer that is dominated by pressure driven instabilities, the peak heat flux in the internal limiter is estimated to be less than 1.5 MW/m² and on the outer divertor, around 3 MW/m². Approaches for maintaining discrete divertor plates have been proposed. Replacement of the CP as well as the whole divertor assemblies, breeding blankets and test modules are also simplified, and the generated timelines are reasonable with a minimal impact on the availability of the ST-VNS.

*Work supported by the U.S.DOE, SBIR Grant No. DE-FG03-95ER82098.