A cryogenic cooling apparatus comprising:- a supply gas line and a return gas line adapted to be coupled to a compressor when in use; a coupling element in gaseous communication with the supply and return gas lines, the coupling element being adapted in use to supply gas to a mechanical refrigerator , the pressure of said supplied gas being modulated by the coupling element in a cyclical manner; a sensing system adapted to monitor the operational state of the mechanical refrigerator when in use; and, a control system adapted to modulate the frequency of the cyclical gas pressure supplied by the coupling element in accordance with the monitored operational state; wherein the mechanical refrigerator comprises: a first cooled stage and a second cooled stage , the second cooled stage being adapted to be coupled thermally with target apparatus to be cooled; characterised in that it further comprises: a selectively coupleable thermal link for thermally coupling the first cooled stage of the mechanical refrigerator to the second cooled stage in dependence upon the operational state of the mechanical refrigerator, wherein the thermal link is a heat pipe having a first part coupled thermally to the first stage of the mechanical refrigerator and a second part coupled thermally to the second stage of the mechanical refrigerator, the heat pipe being adapted to contain a condensable gaseous coolant when in use; the apparatus being adapted in use to be operated in a first cooling mode in which the temperature of the cooled member causes the coolant within the second part of the heat pipe to be gaseous and the temperature of the first stage causes the coolant in the first part to condense, whereby the cooled member is cooled by the movement of the condensed liquid from the first part to the second part of the heat pipe; wherein the control system is further adapted to control the environment in the heat pipe when the apparatus is in the first cooling mode so as to ensure that the gaseous coolant is able to condense; and wherein control system is adapted to modulate the frequency of the cyclical gas pressure in order to control the environment in the heat pipe. a selectively coupleable thermal link for thermally coupling the first cooled stage of the mechanical refrigerator to the second cooled stage in dependence upon the operational state of the mechanical refrigerator, wherein the thermal link is a heat pipe having a first part coupled thermally to the first stage of the mechanical refrigerator and a second part coupled thermally to the second stage of the mechanical refrigerator, the heat pipe being adapted to contain a condensable gaseous coolant when in use; the apparatus being adapted in use to be operated in a first cooling mode in which the temperature of the cooled member causes the coolant within the second part of the heat pipe to be gaseous and the temperature of the first stage causes the coolant in the first part to condense, whereby the cooled member is cooled by the movement of the condensed liquid from the first part to the second part of the heat pipe; wherein the control system is further adapted to control the environment in the heat pipe when the apparatus is in the first cooling mode so as to ensure that the gaseous coolant is able to condense; and wherein control system is adapted to modulate the frequency of the cyclical gas pressure in order to control the environment in the heat pipe. <EOS>