Protection of HTS coils in the limit of zero quench propagation velocity

The observed low quench propagation velocity in HTS superconductors is a problem for the internal protection of magnet coils, which depends on the propagation velocity for the spread of the normal zone. In principle, a distribution of protection heaters can be used to give a large effective propagation velocity, but such a system raises the question of the amount of power and voltage required by the heaters to protect a coil. Here, an analysis is made for the ideal limiting case of zero propagation velocity. In this case, the heated zone adjacent to the heaters is a well defined normal volume. The analysis addresses two issues: determination of the normal volume required for the protection of the coil, and the power and voltage requirements of the heaters. The analysis is applied to two types of HTS coils, including low field coils operating at high temperature of the type that may be used for gyrotron applications, and high field insert coils operating at low temperature. For the example coils studied, the heater voltages are found to be moderate and the analysis indicates that protection by densely distributed heaters is a practical means of protection of HTS coils.