Speaker
Description
A CNRS-CEA French collaboration has developed and built a new modular hybrid magnet at LNCMI-Grenoble to provide intense magnetic fields and fluxes. It was designed to reach in its main configuration at least 43 T in a 34 mm warm bore aperture with 24 MW of electrical power. This can be achieved by combining water cooled resistive inserts, including Cu-alloy polyhelix (25.5 T) and Bitter coils (9 T), with a large bore superconducting coil (1100 mm) based on a new specifically developed Nb-Ti/Cu Rutherford cable-on-conduit conductor (RCOCC) cooled down to 1.8 K by pressurized superfluid He. During the commissioning phase, the superconducting coil alone reached the nominal field of 8.5 T eleven times in total without quench. During the combined tests with Bitter coils, an unexpected quench appeared at 17.43 T very close to the nominal field of this configuration equal to 17.5 T. It was not far from a stagnant quench, difficult to detect, and also known as a “silent magnet killer” with a slow propagation velocity of about 10 cm/s. Both magnet protection systems, i.e. the main and the redundant ones, reacted as expected and the hot-spot temperature was limited to about 85 K validating their efficiency in one of the worth case scenarios. For the combined tests between superconducting and polyhelix coils, a magnetic field of 34 T was reached, but during the current ramp-down, the power converter of the superconducting coil overheated and lost communication with the PLC causing a fast energy discharge. No damage to the superconducting coil neither resistive inserts were detected. Investigations revealed a misfunctioning of the power converter with one of the protection crowbars that was switched on before reaching the triggering voltage threshold. A few components of the power converter were changed and mitigation actions were implemented to avoid the occurrence of such a problem, before continuing the commissioning tests. Finally, the Grenoble hybrid magnet reached 42 T, as a first step. Focus will be given to the problems encountered and solved during this successful initial commissioning phase, as well as the potential to achieve higher magnetic fields.
