Temperature and electric field distribution measurement inside of the LHD helical conductor

The KISO-32 superconductor applied to the pool-cooled Helical Coils of the Large Helical Device (LHD) consists of 15 NbTi/Cu superconducting strands and a pure aluminum stabilizer with a CuNi clad, both embedded in a copper housing. To study quench development phenomena in such a complicated structure, we prepared a special sample coil. The coil was fabricated in Kyushu University and tested in NIFS. Tiny thermocouples and electrodes were inserted into the conductor´s body in several cross-sections. Resistive heaters initiated a normal zone, and internal temperature distributions as well as electric fields during quench, i.e., during a normal zone propagation and recovery were measured directly. We observed a considerable temperature gradient between the inner and outer parts of the superconductor. The inner electric field distribution was recorded too. Oscillation phenomena were observed with an appearance and disappearance of a normal zone in some areas of the conductor. In the paper, we present the experimental details and some of the results obtained. The mechanism of the oscillation phenomena as well as the reason for the temperature gradient are considered.