Apparatus for pre-cooling cryogenically cooled apparatus housed within a cryogen vessel ( 26 ), comprising a first closed-loop cooling circuit ( 30 ) containing heat transfer fluid, a circulator ( 32 ) for causing the heat transfer fluid to circulate around the closed circuit and a heat extractor ( 34 ) arranged to extracts heat from the heat transfer fluid, wherein the circuit carries heat transfer fluid into and from an interior volume of the cryogen vessel ( 26 ).