Quench Protection for the MICE Cooling Channel Coupling Magnet

Abstract

The MICE coupling coil is fabricated from Nb-Ti, which has high quench propagation velocities within the coil in all directions compared to coils fabricated with other superconductors such as niobium tin. The time for the MICE coupling coil to become fully normal through normal region propagation in the coil is shorter than the time needed for a safe quench (as defined by a hot-spot temperature that is less than 300 K). A MICE coupling coil quench was simulated using a code written at the Institute of Cryogenics and Superconductive Technology (ICST) at the Harbin Institute of Technology (HIT). This code simulates quench back from the mandrel as well as normal region propagation within the coil. The simulations included sub-division of the coil. Each sub-division has a back to back diodes and resistor across the coil. Current flows in the resistor when there is enough voltage across the coil to cause current to flow through the diodes in the forward direction. The effects of the number of coil sub-divisions and the value of the resistor across the subdivision on the quench were calculated with and without quench back. Sub-division of the coupling coil reduces the peak voltage to ground, the layer-to-layer voltage and the magnet hot-spot temperature. Quench back reduces the magnet hot-spot temperature, but the peak voltage to ground and layer-to-layer voltage are increased, because the magnet quenches faster. The resistance across the coil sub-division affects both the hot-spot temperature and the peak voltage to ground.

The Role of Quench-Back in the Passive Quench Protection of Uncoupled Solenoids in Series With and Without Coil Sub-Division

“…A semi-empirical quench model considering both the sub-division and quench-back was developed to study the quench process of the MICE solenoids [1]. For the focusing magnets, the quench is initiated at the high field point in coil C1 and starts to expand in the three directions with velocities v φ (propagation along the winding), v r (radial propagation) and v z (axial propagation).…”

“…The shunt resistors across the sub-division can balance the overvoltage in the coil, so the voltage to ground in the coil can be limited and reduced [8]. Previous papers have discussed the quench process, the role of quench-back from the mandrel, and magnet sub-division in reducing the hot-spot temperature and the peak coil voltages to ground [1], [2], and [8]. This paper discusses the role of quench-back in the passive quench protection of uncoupled solenoids in series with and without coil sub-division.…”

Progress on the Design and Fabrication of the MICE Focusing Magnets

“…The propagation velocities in all directions will be increased about six percent [7]. The hot spot temperature will increase, but the conductor will probably decrease the quench-back time for the coils, once the normal region has been formed [8], [9]. Table I shows the basic design parameters for focusing magnet as the magnet vendor design dictates.…”

Progress on Design and Construction of a MuCool Coupling Solenoid Magnet

“…Each coil sub-division will have a diode and resistor across it.. Quench back from the coil mandrel and the banding in combination with the coil sub-division ensures that the hot spot temperature in the coil will be no higher than 120 K [7].…”