Quench dynamics in MgB₂Rutherford cables

Abstract

The generation and propagation of quench induced by a local heat disturbance or by overcurrents in MgB 2 Rutherford cables have been studied experimentally. The analysed cable is composed of 12 strands of monocore MgB 2 /Nb/Cu10Ni wire and has a transposition length of about 27 mm. Measurements of intra-and inter-strand voltages have been performed to analyse the superconducting-to-normal transition behaviour of these cables during quench. In case of external hot-spots, two different time-dynamic regimes have been observed, a slow stage for the formation of the minimum propagation zone (MPZ), and a fast dynamics once the quench is triggered and propagates to the rest of the cable. Significant local variations of the quench propagation velocity across the strands around the MPZ have been observed, but with average quench propagation velocities closely correlated with the predictions given by one-dimensionalgeometry models. For quench induced by overcurrents (i.e. with applied currents higher than the critical current) the nucleation of many normal zones distributed within the cable, which overlap during quench propagation, gives a distinctive and faster quench dynamics.

MgB2 cables made of thin wires manufactured by IMD process

“…The behaviour of the cables analysed in this study under heat disturbances differs substantially from the results obtained in similarly manufactured MgB2 Rutherford cables, but made with wires processed differently (PIT, in-situ reaction, Cu-Ni alloy sheath and Nb barrier [18]. Unlike for the R-c cables analyzed here, in the Rutherford cables made Cu-Ni sheathed PIT wires, the quench was found to trigger at the same time in all strands (those directly heated and those underneath), and the average quench propagation velocities estimated experimentally showed a close correlation with the prediction given by Wilson and Dresner modes, although with significant local variations of the quench propagation velocity near the hot-spot.…”

“…Quench measurements were performed for the flat cables Rc-B of 2.86 mm × 0.86 mm (w × d), total length of about 11 cm (≈8 cm between current contacts) using a similar setup and procedure as described in [17]. The cable was in vacuum, cooled by conduction from both current contacts and the energy was deposited into the cable by passing current pulses of variable duration, t p , in a heater of resistance 120 Ω, which was glued on one of the sides of the cable and overlays five strands.…”

AC losses of Rutherford MgB2 cables made by powder-in-tube and internal magnesium diffusion processes

“…On the other hand, cabling of not-reacted strands does not cause degradation of transport currents, but reduces AC losses effectively [9]. Recently, a few Rutherford MgB 2 cables assembled from strands manufactured by powder-in-tube (PIT) and also by internal magnesium diffusion (IMD) processes have been successfully manufactured and characterized at low temperatures [10][11][12][13]. Rutherford MgB 2 cables are not suitable for high field accelerator magnets, but are more promising for low field applications at temperatures around 20 K where minimized AC losses are also required (e.g.…”

“Mirror” magnet experiment for quench studies, quench analysis techniques development and in support of superconducting magnet modeling

“…The experiment proposed relies partially on instrumenting individual strands with voltage taps. This was successfully done at Fermilab in past [32] and the technology is also implemented elsewhere [6], [7], [8]. However, the configuration of the voltage taps requires access to the narrow cable side instrumenting many strands at close proximity.…”

“…Although we have good understanding of main characteristics of current redistribution it is still unclear what exactly the role of splices is, leading to competitive assumptions (boundary conditions) [4] needed to analyze magnet data or design magnets. One of the problems is that related tests done so far are mostly on cables alone [4], [5], [6], [7], [8] and not on operating magnets and this is especially true for Nb3Sn based data. Studying all aspects of a quench in real magnets is non-trivial and although analysis tools and simulations are ever improving [4], [9], [10], [11], [12], [13], [14], [15] experiments are still unavoidable in probing fine effects during magnet operation and transients.…”