Towards a better understanding of the physics of the two-volume model of accelerator magnet quench thermohydraulics

The LHC currently under construction at CERN will make intensive use of high-field, twin aperture superconducting magnets operating in static baths of pressurized helium II at 1.9 K and at about 100 kPa. As long as magnet construction guarantees pressure homogeneity after a resistive transition by allowing sufficient radial venting, the quench thermohydraulics can be depicted by a mathematical model based on the assumed helium split between two hypothetical volumes of confined and bulk helium. The first phase of the process is dominated by fast adiabatic compression of bulk helium driven by the expanding confined helium while in the second phase the whole amount of the helium undergoes isochoric heating. We make the first conjecture that the confined helium is not solely composed from the helium filling the cable porosities, but also from a thin helium layer surrounding the coil. The second conjecture is that the onset of the isochoric heating phase is a result of fast heat transfer via the magnet collars to the bulk helium, still resting in a superfluid state, quantitatively overtaking in importance the adiabatic compression route as a means of energy transfer. Our hypothesis of the transition mechanism from adiabatic compression of the bulk helium to isochoric heating has been confirmed numerically and has been experimentally justified with String 2 measurements.