A helical grid equivalent circuit model for an intra-layer no-insulation coil

The intra-layer no-insulation (LNI) coil, which has the advantages of self-protection and high magnetic field homogeneity, is of great potential in HTS NMR and MRI applications. Compared to the more commonly-used double-pancake coils, the transient characteristics are more complex due to additional axial current. While the present circuit model cannot accurately describe the geometry of the LNI coil and the internal characteristics of each turn of the coil, a helical grid equivalent circuit model with consideration of types of mutual inductances is proposed in this paper. In the circuit model, details of the coil are characterized with subdivided helical surface elements in each turn, and each helical surface element can be equivalent to corresponding circuit parameters. The circuit model is verified by a comparison experiment with an intra-layer insulation coil. And elaborate current distributions in each turn of the LNI coil can be obtained with this circuit model. Analysis results demonstrate that the azimuthal current and the axial current are always symmetrically distributed about the middle plane, and the radial current is always anti-symmetrically distributed about the midplane. The circuit model can be used to simulate the quench electromagnetic characteristics of the LNI coil in the future.