Development of superconducting voice coil motor of a cold chopper for MICHI

A cold chopper is a key device for next generation mid-infrared instruments such as TMT/MICHI. It should achieve fast and accurate position switching with a large chopping throw at cryogenic temperature. To satisfy the requirements, voice coil motors using superconducting MgB 2 wire have been developed. We have made a first prototype of the VCM and carried out its performance measurements such as a transition temperature, transfer functions, and power dissipation in the laboratory. The results are almost consistent with the expectations and the calculations, but some show significant inconsistency. We have also made a next prototype which is small to fit the size of the MICHI chopper. This will be installed to a developing mid-infrared instrument MIMIZUKU and used for actual observations.