Cryogenic cooling system

A method of operating a cryogenic cooling system, the cryogenic cooling system comprising a mechanical refrigerator having a first cooled stage and a second cooled stage, the second cooled stage being configured to reach a lower base temperature than the first cooled stage;

• a heat pipe having a first part coupled thermally to the second cooled stage and a second part coupled thermally to a target assembly, the heat pipe containing a condensable gaseous coolant, a heat switch assembly comprising one or more gas gap heat switches, the heat switch assembly having a first end coupled thermally to the second cooled stage of the mechanical refrigerator and a second end coupled thermally to the target assembly;
• the method comprising the following steps:
• i) causing the target assembly to adopt a temperature sufficient to ensure the coolant within the second part of the heat pipe is in the gaseous phase;
• ii) operating the mechanical refrigerator to cause the second cooled stage to adopt a temperature which causes the coolant within the first part of the heat pipe to condense;
• iii) cooling the target assembly during a heat pipe cooling mode in which the temperature of the second cooled stage is lower than the first cooled stage by causing the movement of the condensed liquid coolant from the first part of the heat pipe to the second part of the heat pipe;
• and
• iv) cooling the target assembly during a gas gap cooling mode in which the temperature of the second cooled stage causes the coolant within the heat pipe to freeze, and wherein further cooling of the target assembly is applied by the second cooled stage via the one or more gas gap heat switches.