Strain measurement on superconductor joints using an external bridge completion technique

Accurate measurement of electromagnetic and thermal stresses in cryogenic environments on superconductors is quite challenging, since the environment involves cryogenics and large time-varying as well as dc transport currents through the superconductor. In off-normal cases, such as the transition of a superconducting state to a normal state, which is often referred to as the 'quench' of the magnet, a high voltage is developed across the superconductor and the strain gages including their signal conditioning electronics are necessarily required to be isolated. Cryogenics brings in a large drifting thermal offset in the measurement, whereas the presence of a magnetic field around the strain gage introduces a magneto-resistance error. An external bridge completion technique has been developed and effectively extended to measuring the stresses on the joint region of the superconductors with noisy environment and space constraints. This paper describes the stress measurement techniques in the space-restricted joint regions of the superconductors and validates the techniques in a repeatable fashion on measurements carried out on the inter-connecting joints of large superconducting magnet winding packs of the steady state superconducting tokamak. This technique is advantageous in situations where in situ full active bridge configuration is not feasible due to practical limitations.