An experimental setup to measure the minimum trigger energy for magnetothermal instability in Nb3Sn Strands

Magnetothermal instability may affect high critical current density Nb 3Sn superconducting strands that can quench even though the transport current is low compared to the critical current with important implications in the design of next generation superconducting magnets. The instability is initiated by a small perturbation energy which is considerably lower than the minimum quench energy (MQE). At CERN, a new experimental setup was developed to measure the smallest perturbation energy [minimum trigger energy (MTE)] which is able to trigger the magnetothermal instability in superconducting Nb 3Sn-strands. The setup is based on Q-switched laser technology which is able to provide a localized perturbation in nanosecond time scale. Using this technique the energy deposition into the strand is well defined and reliable. The laser is located outside the cryostat at room temperature. The beam is guided from room temperature on to the superconducting strand by using a UV-enhanced fused silica fiber. The strand is mounted on a VAMAS barrel. A part of the beam's energy is absorbed into the strand acting as the trigger energy for the magnetothermal instability. In this paper the experimental setup and the calibration of the absorbed energy is presented.