Structural behavior of coaxes in transverse magnetic fields

Abstract

The ALCATOR C-MOD tokamak employs poloidal field (PF) coils inside the toroidal field (TF) coils. C10100 copper coaxial current leads pass radially through an 18 tesla toroidal field to reach the terminals of the ohmic heating (OH) coil. In addition, there are radial and poloidal fields on the order of 2 Tesla near the OH terminals. The 50 kA lead currents interact with those fields to produce 900 kN/m forces which act in opposite directions on the coaxial conductors. The net load is zero on the conductor pair. However, the internal forces and moments cause stresses in the outer conductor tube that depend strongly on the size of the air gap between the tube and the insulation. Analysis indicates a peak stress of 430 MPa for a 0.19 mm air gap. The stress field is complicated by a 90 degree bend to vertical at the terminal block. Internal loads in the bend region lead to bourdon tube behavior, which produces longitudinal bending stresses in the coax. The coax must resist fatigue failure for reversed 50,000 pulses with a Safety Factor of 10. Ironically, this component is considered innocuous, and rarely given such attention, simply because there is no net load on a coax in a transverse field.