Parametric Study on Effect of Friction and Overbanding in Screening Current Stress of LBC Magnet

Magnetic stress is of particular concern for ultra high field magnets, especially, peak hoop stress of those magnets may be locally concentrated by the screening current stress (SCS). For instance, the Little Big Coil (LBC), which was designed, constructed, and operated by a collborate team led by NHMFL, generated the world record magnetic field of 45.5 T, but the conductor in LBC experienced plastic deformation due to excessive SCS. In this paper, to further analyze details of the mechanical stress behavior of LBC magnet, the effect of friction coefficient and overband radial build on stress alleviation is investigated by the parametric study. First, total current density with screening current is calculated by using the H-formulation together with domain homogenization method. Later, the entire magnet structure is modeled by the 2D finite element method to represent preload and axial interactions between each adjacent single pancake (SP), and 3 turns are assumed to be one engineering turn with frictionless radial turn-to-turn contact. While, friction acts between the coil and spacer in a direction that limits the radial displacement. Finally, we discuss the effects of friction coefficient and overband thickness by sweeping those parameters. Our parametric study implies that both friction and overband radial build can reduce peak hoop stress levels of LBC, however, effect of friction is dominant when overbanding is thicker than 0.5 mm.

Automated summary

This paper focuses on the mechanical stress behavior of ultra high field magnets, especially the impact of screening current stress on the conductor. Through a parametric study, it is found that both friction coefficient and overband radial build can reduce peak hoop stress levels of LBC, with friction playing a dominant role when the overband thickness exceeds 0.5 mm.