Measurement and Modeling of Initial Quench Development in Nb₃Sn Accelerator Magnets

Accelerator magnets are equipped with voltage taps and, on the test bench, with so-called quench antenna's to monitor the transient effects occurring during a magnet quench. Proper identification and localization of a quench origin is vital for understanding performance issues in Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn accelerator magnets. In this paper, we describe the physical phenomena that occur during the first few milliseconds of a developing quench and how they affect the signals as intercepted with the diagnostic tools. A better understanding of these phenomena allows for better resolution on determination of the quench start location. Measurements from Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn accelerator magnets are compared with a 3D thermal-electric PEEC-FEM model of a Rutherford cable. The voltage measured over the cable shows an accelerating build-up attributed to the transverse quench propagation in the cable cross-section, which is dominated by inductive effects that results in an avalanche of quenching strands. A slow-down in the voltage build-up then indicates the point at which all strands in the cable cross-section have quenched. This phase of the quench involves a significant current redistribution that creates a magnetic dipole moment picked up by a quench antenna. The harmonic quench antenna used in this work is used to reconstruct the location, magnitude, and direction of this dipole moment, which strongly depends on the start location of the quench in the cable cross-section, on the inter-strand contact resistances and on the magneto-resistance of the copper. It is shown how the quench start location in the cable cross-section can be determined from the time integral of the reconstructed dipole moment.