Improvement of Magnet and Cavities cooling at Heavy Ion or Rare Isotopes Accelerators due to Application of sub-cooled Superfluid Helium

Abstract

Several modern Heavy Ion or Rare Iosotopes Accelerator uses superconducting (sc) cavities for an acceleration of the beam particles. Operation of sc magnets with higher magnetic fields and improved field gradients also leads to a better beam focusing (also called “beam optics”) and improves the overall accelerator compactness. In many accelerators, like XFEL, SNS, LCLS-II, HIE-ISOLDE, ATLAS, ISAC-II/ARIEL, SPIRAL-2, FLASH, Jlab-FEL/ERL, the operation is limited either by cooling capability of sc cavities or refrigeration capacities. In most of the cases, increasing of refrigerator capacities is a challenging task due to available free space or investments, and, therefore, improvement of cavity cooling due to increasing of a quality factor (Q 0) or heat transfer at cavity surfaces is considered. Application of a sub-cooled superfluid (sf) helium gives several advantages, like higher heat flux densities, longer time for onset of a film boiling regime and shorter recovery time, reduced Kapitza resistances, etc. In the present paper, application of sub-cooled superfluid helium for cooling of cavities with half and quarter-wave resonators, which are typically applied at Heavy Ion and Rare Isotopes Accelerators or sometimes at electron and proton ones, is considered. In order to limit the present discussion, its application to FRIB-style cryomodules is discussed in details.