Status of the MQXFB Nb₃Sn Quadrupoles for the HL-LHC

Abstract

The cold powering test of the first two prototypes of the MQXFB quadrupoles (MQXFBP1, now disassembled, and MQXFBP2), the Nb3Sn inner triplet magnets to be installed in the HL-LHC, has validated many features of the design, such as field quality and quench protection, but has found performance limitations. In fact, both magnets showed a similar phenomenology, characterized by reproducible quenches in the straight part inner layer pole turn, with absence of training and limiting the performance at 93% (MQXFBP1) and 98% (MQXFBP2) of the nominal current at 1.9 K, required for HL-LHC operation at 7 TeV. Microstructural inspections of the quenching section of the limiting coil in MQXFBP1 have identified fractured Nb3Sn sub-elements in strands located at one specific position of the inner layer pole turn, allowing to determine the precise origin of the performance limitation. In this paper we outline the strategy that has been defined to address the possible sources of performance limitation, namely coil manufacturing, magnet assembly and integration in the cold mass.

Status and Challenges of the Interaction Region Magnets for HL-LHC

“…8) involving 4 thermal cycles, 7 training quenches and 43 provoked quenches at nominal current. At the end of the test, the magnet reached the target current of 16.53 kA at 1.9 K and the nominal current of 16.23 kA at 4.5 K, showing no degradation [5]. Note that as for all the MQXFA magnets tested so far, results refer to a vertical test without the stainless steel shell which provides the LHe containment [5].…”

“…The stainless steel shell integration around the magnet was validated in MQXFS1 and MQXFS7 [5]. A low preload experiment has been done on MQXFS6: the coil has been prestressed at a level corresponding to 50% of electromagnetic forces at nominal current, rather than 100% as in the baseline: the magnet reached ultimate current, but lost about 1 kA of margin, Fig.…”

“…6). Quench location was in the center of the magnet, inner layer pole turns, with a 20 m/s quench velocity indicating a local damage [5,17]. MQXFBP1 was disassembled and the metallography of the limiting coil showed presence of broken filaments in the strand at the corner of the pole inner turn (see Fig.…”

“…MQXFBP1 was disassembled and the metallography of the limiting coil showed presence of broken filaments in the strand at the corner of the pole inner turn (see Fig. 7), with several breakages along the magnet axis, close to the transition of the Ti poles (see [5,16] for a deeper discussion).…”

“…The extrapolation of this quench provides a estimate of 2 K temperature margin. An outlook of the origins of this limitation and of the corrective actions is discussed in section II.F and in [5].…”