The Role of Quench-Back in the Passive Quench Protection of Long Solenoids With Coil Sub-Division

Abstract

Abstract-This paper describes how a passive quench protection system can be applied to long superconducting solenoid magnets. When a solenoid coil is long compared to its thickness, the magnet quench process will be dominated by the time needed for quench propagation along the magnet length. Quench-back will permit a long magnet to quench more rapidly in a passive way. Quenchback from a conductive (low resistivity) mandrel is essential for spreading the quench along the length of a magnet. The mandrel must be inductively coupled to the magnet circuit that is being quenched. Current induced in the mandrel by di/dt in the magnet produces heat in the mandrel, which in turn causes the superconducting coil wound on the mandrel to quench. Sub-division is often employed to reduce the voltages to ground within the coil. This paper explores when it is possible for quench-back to be employed for passive quench protection. The role of sub-division of the coil is discussed for long magnets. Index Terms-Magnet Quench-back, Coil Sub-division. INTRODUCTION uench-back is a widely used passive quench protection method that enhances the rate of propagation of the normal zone by using a coupled secondary circuit. As quenching proceeds, the secondary will heat up. If it is in good thermal contact with the magnet winding, it will initiate further quenching-effectively increasing quench propagation velocity and therefore reducing the hot spot temperature. It is particularly effective in a situation where the normal zone hits boundaries rather early in the quenching process and thereafter spreads very slowly in the other dimension [1][2][3].Sub-division provides an alternative path for some of the current by connecting shunt resistors across sections of the magnet, and the shunt resistor can absorb some of the stored energy in the coil. So the hot spot temperature of the coil can This paper describes a passive quench protection system that may be applied to a long superconducting solenoid with sub-division. The role of quench-back in the quench protection of such a solenoid is discussed. I. PASSIVE QUENCH PROTECTION OF A LONG SOLENOID A. Long Solenoid ParametersThe solenoid discussed in this paper is a spectrometer magnet used in the MICE project, shown in Fig. 1 [5]. This magnet consists of two match coils M1 and M2 and three-coil spectrometer section (E1, C, and E2). M1 and M2 are used to match the spectrometer section with the MICE cooling channel. The three-coil section produces a uniform magnetic field (±0.3 percent) within a region that is 1000 mm long and 300 mm in diameter. The uniform field section contains a fiveplane scintillating fiber detector for muon beam emittance analysis. The magnet has an Nb-Ti superconductor with a copper to superconductor ratio of four. The insulated conductor dimensions are 1.65 by 1.00 mm. All five coils are wound on a single 6061-T6-aluminum mandrel. The cryostat consists of 4.2 K helium vessel, an 80 K thermal shield, cold mass supports for 500 kN and a 300 K vacuum vessel. The spectrometer se...

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

“…Mandrels of 6061-T6-Al and 304-stainless steel were used to explore the role mandrel material on quench-back [2]. The focusing magnet was run in the flip mode at 250 A.…”

“…Quench back can cause a small increase in the voltage to ground in the coil, because the coil section resistance grows more rapidly. If the resistivity of the mandrel is lower (say 1100-O aluminum) the peak voltage will be lower, because the mandrel absorbs more magnetic energy [2]. …”

“…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.…”

The Results of Tests of the MICE Spectrometer Solenoids

“…Thus the three-coil spectrometer section has a total of four sub-divisions. The four sub-divisions reduce the voltage to ground and protect the three-coil tracker section of the magnet in the event of a lead failure [7]. The hot spot temperature (particularly in the long center coil) is reduced by magnet quench back from the aluminum mandrel.…”

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.…”