Concept Design and Analysis of the Magnet System of the Sustained High Power Density Tokamak Facility

A recent National Academy study recommended a next-step sustained high power density (SHPD) facility for the United States that can be a bridge to a compact fusion pilot plant. A design has been initiated to investigate a possible SHPD non-deuterium-tritium device that builds upon recent low aspect ratio tokamak studies. A 1.2-m device with a 2.4 aspect ratio has been chosen to evaluate physics performance goals within a double-null plasma machine arrangement centered on three component features: high current density toroidal field (TF) and central ohmic heating/poloidal field (PF) coils, liquid metal divertor/first wall systems, and the integration of a limited set of outboard dual-coolant lead lithium test blankets that can be maintained within a vertical maintenance approach.

The underlying initiative of the U.S. program is to promote research and technology advancements that lead to the construction of a compact pilot plant that produces electricity from fusion at the lowest possible capital cost. High-field, higher power density will bring down the size of a fusion device and can lower the cost, but this can only occur with the development of high-current density TF and PF windings, a support system to handle the higher magnetic loads, and a cost structure that is economically viable. High-temperature superconductors (HTSs) offer the potential to meet high-field/high-current requirements and is under development by a number of institutions. Fabrication process improvement would continue to bring down the price if a market for fusion devices were to develop and the production level of HTS cable were to increase; however, this is not happening with any expedience especially related to fusion applications. At present, HTS conductors are an order of magnitude more expensive than low-temperature superconductors (LTSs), making an early application for a near-term SHPD experimental device problematic, especially for a low aspect ratio design that has minimum component space within the device inboard region. In lieu of an HTS-based, high-current density TF winding design, a moderately high-current density winding design (>80 MA/m²) based on cable-in-conduit LTS conductors in a multiwinding pack arrangement sized for low and high field was defined. This paper focuses on the design of the magnet system, with attention given to the analysis performed on the SHPD TF casing structure. The analysis is carried out for various sections: (1) Emag analysis of the SHPD structure, (2) TF casing winding pack evaluation for TF-TF self-load only with slit model and three-dimensional geometry, (3) effect of the shear pin in the TF casing structure, and (4) compression ring effect on the SHPD structure.