Enthalpy stability criterion for magnets cooled with superfluid helium II

A criterion is presented which describes the limits to local heat pulses sustained by a magnet cooled in superfluid helium II. The magnet will remain stable for a length of time defined by the enthalpy of the helium between bath temperature and the maximum superfluid temperature (2.17 K). For a bath temperature of 1.8 K, this enthalpy is 300 mJ/cu cm. For long pulses, the volume of helium that absorbs the heat can be considered to approach the contents of the entire dewar. For short heat pulses, there is a limited region into which the heat diffuses. This diffusion process can be defined using experimental results on transient heat transfer. A model is presented to describe how to generalize the thermal diffusivity in superfluid helium II. Application of this criterion to specific magnet designs is discussed.