Supercritical helium expansivity effects on magnet protection systems

Abstract

The recent commissioning of the 36 T Series-Connected Hybrid Magnet at the National High Magnetic Field Laboratory has uncovered an undesirable consequence of using super critical helium for cooling of superconductors. This recent dry magnet cooling technology greatly reduces the LHe requirements of the magnet and may be utilized more frequently in the future. Because the supercritical helium does not complete a phase transition with the addition of heat, the fluid can quickly expand. The increased expansivity of supercritical helium over liquid helium for a given heat load is capable of creating larger pressure waves. The impact pressure for compressible helium gas flow is combined with the speed of sound for ideal gases to determine if a high-speed pressure wave is sufficient to explain the premature failure of the burst discs in the over-pressurization protection system. The addition of an impact pressure can explain, under the right conditions, a reduction of up to 40% in the burst disc pressure rating. The Grüneisen parameter is shown to relate to the expansivity and values for the parameter are given in the supercritical range. A proposal to reroute the pipe near the burst disc is made to mitigate the effects of the impact pressure.