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 is achieved by combining resistive inserts, including Cu-alloy polyhelix (25.5 T) and Bitter coils (9 T), with a large bore superconducting coil (1100 mm) generating a nominal magnetic field of 8.5 T. The latter is made of a total length of about 9 km of a specifically developed Nb-Ti/Cu Rutherford cable-on-conduit conductor (RCOCC) with strict control of AC losses. This has been achieved from in-house soft-soldering of the Rutherford cable onto a hollow rectangular Cu-Ag stabilizer, which is cooled to 1.8 K by a pressurized superfluid helium bath at 1200 hPa. The superconducting coil consists of 37 double-pancake windings stacked and connected in series. These are housed within a helium vessel, which is connected via a cryoline to an external cryogenic satellite ensuring that all valves are positioned in a stray magnetic field lower than 10 mT. A cylindrical eddy-current shield in Cu, reinforced by austenitic steel and cooled down to 35 K, was inserted in between resistive and superconducting coils to lower the field variation seen by the RCOCC in case of resistive insert trips and therefore preventing quenches. The Grenoble hybrid magnet requires additional equipment such as a 5000 m3/h pumping unit, a dedicated He liquefaction plant of 150 l/h, power converters of 7500 A/±15 V for the superconducting coil and 12 + 18 MW for the resistive inserts delivering separately up to about 32 kA each. Two fully redundant Magnet Safety Systems (MSSs) have been specially developed for the quench detection and energy extraction of the superconducting coil, as well as dedicated control-command and acquisition systems.
Throughout 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 MSSs 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.
