Stress and strain analysis of a REBCO high field coil based on the distribution of shielding current

Abstract

There are growing concerns about the stresses created by shielding currents in high field superconducting magnets fabricated from tape conductors leading to reduced performance and lifetime. This paper presents results of stress/strain calculations caused by shielding currents assuming the conductor deformations follow a linear constitutive relation. An anisotropic bulk approximation approach was used to calculate the electromagnetic field distributions in a REBCO high field coil with a stack of pancakes and a large number of turns first, and then the Lorentz force distribution and mechanical response characteristics were studied in the two-dimensional axisymmetric configuration. A new discrete contact mechanical model implemented by the finite element method, which is able to simulate the contact and separation behaviors between adjacent turns during the deformation, was proposed to analyze the distributions of hoop stress, hoop strain, radial stress and radial displacement in the coil. The influences of shielding current on those mechanical responses were obtained by comparing the simulation cases with and without taking shielding current into account. Besides, a continuum bulk mechanical model, which is parallel to the discrete contact mechanical model and treats the pancakes as continuum bulks, was modeled as well in order to understand the influences of different models on the simulation results. Furthermore, we studied the influences of a couple of practical factors (including the -value, ramp rate, and operating mode of the REBCO coil winding) on the shielding current and hoop stress. A couple of novel and important conclusions were found. (1) Neglecting the shielding current behavior would significantly underestimate the maximum local hoop stress in a REBCO high field coil. (2) The continuum bulk mechanical model is not adequate for the stress analysis of dry-wound high field coils, by which unreasonably large tensile radial stresses could be obtained. (3) The highest local hoop stresses at the fully-charged moment and the fully-discharged moment are located in a certain pancake near the end of the coil winding and the end pancake, respectively. (4) Decreasing the -value and the ramp rate of the REBCO coil could be two auxiliary ways to suppress the shielding current and maximum local hoop stress in the coil. (5) For the ramp-and-hold operating mode, the REBCO coil experiences the highest stress level at the moment when it right achieves the goal field. (6) The cycling operation of a REBCO high field coil can cause the tape experiencing alternative positive and negative hoop stresses and this may decrease the fatigue life of the tape and then the life of a magnet.

Screening current effect on the stress and strain distribution in REBCO high-field magnets: experimental verification and numerical analysis

“…These calculations also indicate that, practically, by lowering the value of n, the maximum radial Lorentz force could be reduced, as suggested in Ref. 14.…”

“…Recently, as is indicated by some studies, when taking the screening effect into account, the non-uniformly distributed currents in REBCO tapes may have a significant impact on the stress and strain distribution 5,14 , despite its high tolerance for tensile stress 15 . In particular, in the pioneer experiment reaching 45.5-T with 14.4-T HTS insert, the one-side wrinkle pattern demonstrated in REBCO tapes also suggests the influence of screening currents 16 .…”

Ramping loss and mechanical response in a no-insulation high-temperature superconducting layer-wound coil and intra-layers no-insulation coil

“…The upper boundary of the coil is assumed to be free because of the coil being in compression along with the height during the cooling and charging. Due to the self-supporting turn effect, the inner and outer boundaries of the coil are chosen to be free [39,48,49]. Since the number of layers along the radial direction is relatively less, the prestress during the winding is lower and neglected in the simulation.…”

Development and large volume production of extremely high current density YBa2Cu3O7 superconducting wires for fusion

“…The progress of HTS technology has been especially impressive in the last decade, culminating recently in J E values in excess of 1000 A/mm 2 at 4.2 K in 18–20 T magnetic field 6 , 7 . While this is ample for some applications 8 , higher J E opens new frontiers in magnets for fusion reactors 9 , 10 , particle accelerators 11 – 13 , magnetic resonance imaging 14 , nuclear magnetic resonance spectroscopy 15 , 16 and space detectors 17 . For instance, a minimum engineering current density of 700 A/mm 2 at 20 K, 20 T is essential for the magnet system of the prototype commercial fusion device SPARC 18 , which is being designed and constructed by a collaboration of MIT and Commonwealth Fusion Systems (CFS).…”