Modelling V-I measurements and characterizing performance degradation in 11 T and MQXF magnets
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Abstract:
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In recent years, several new accelerator magnets employing Nb3Sn technology for the High Luminosity Large Hadron Collider (HL-LHC) were tested at the test facility at SM18. These include a number of series magnets for the 11 T dipole program as well as a long prototype MQXF magnet. While conducting these tests, certain magnets exhibited reduced quench and powering performance. Besides anomalous quench behaviour and abnormal temperature and ramp rate dependencies, V-I measurements showed voltages unexpected for superconducting magnets during ramps and current plateaus at 1.9 K and 4.5 K.
The leading hypothesis for the cause of this behaviour is the presence of an inhomogeneous defect in the Rutherford cable. Current redistribution takes place around the defect through a diffusion process, which leads to a decaying voltage over the cable. In the past year, I have studied this phenomenon using a model implemented with the THEA software. In this presentation, I will describe the general behaviour of the current redistribution around such a defect. Good qualitative agreement is found between the model and the anomalous behaviour of the HL-LHC magnets. The characterization of measurements could lead to a better understanding of conductor damage and lead to changed acceptance criteria for Nb3Sn magnets.
