Stability of superconducting Rutherford cables for accelerator magnets

The stability of superconducting magnets has a high priority for particle accelerators, since the operational time and operational collision energy depend strongly on it. Local heat dissipation due to beam loss and conductor movement is inevitable, causing local hot spots in the conductor, possibly leading to magnet quench. In stability against local and transient energy deposition, the cable is the most important unit to investigate. Most superconducting accelerator magnets are wound from Rutherford cables with a flat cable layout, consisting of twisted strands.

The mechanisms of normal zone propagation in Rutherford cables have been described in detail with experimental and modeling data. The onset of a local normal zone forces current to redistribute in adjacent neighboring superconducting strands, reducing the longitudinal normal zone propagation. Transversal normal zone propagation in adjacent and crossing strands is caused by the redistribution of current and by heat exchange.

The mechanism of normal zone propagation depends on thermodynamics and electrodynamics. For the first time the criteria for the minimum quench energy of superconducting cables is described in detail. The Minimum Quench Energy is investigated for a large range of heat deposition volumes and the for the existing LHC dipole magnets and for the future SIS 300 quadrupole magnets. Possible enhancements that improve the level of stability located at the thin edge, which has the lowest minimum quench energy, are examined by calculations and discussed.