Stability and Control of Supercritical Helium Flow in the LHC Circuits

The circulating particle beams of the Large Hadron Collider (LHC) will induce dynamic heat loads into the cryogenic system. Beam screens, maintained at a temperature between 5 K and 20 K by weakly supercritical helium -in order to avoid-two phase flow- are inserted inside the magnet cold bore to intercept most of these heat loads. Evidence has been presented in experimental and theoretical work that the main type of dynamic instability in long channels is that caused by the propagation of density waves due to multiple regenerative feedback. Oscillations are typically observed in circuits operating with low flow rate and/or high energy input. The study of the system behaviour under different operating cases permits assessment of the time constant of the system as well as its temperature-control parameters. A part of this work also concerns the study of flow stability in the other LHC cryogenic circuits working with supercritical helium.