Cryocooling of optics is common on insertion device beamlines of 3rd generation synchrotron sources where high powers (wigglers) or power densities (undulators) can cause exceptional distortion, resulting in degradation of the beam.
When a monochromator crystal is heated, thermal gradients are set up across the crystal which cause it to distort, affecting the optical performance. These effects can be minimised by optimising the cooling of the crystal. Cryogenic cooling of monochromator optics has advantages over simple water cooling, as the ratio of the coefficient of thermal expansion, α, to thermal conductivity, k, for silicon, is smaller at liquid nitrogen temperature than at room temperature. Distortions of the crystal due to thermal gradients are therefore reduced if the crystal is cooled to around 100K.
The FMB Oxford Cryocooler is a closed loop liquid nitrogen cooler, which provides pure liquid nitrogen at a controlled and very stable pressure for cooling monochromator optics. Nitrogen vapour has been eliminated from the circulating liquid, so that there are no vibrations due to boiling liquid in the cooling circuit.
The single-phase liquid nitrogen is produced by immersing a Copper coil carrying high pressure liquid nitrogen in a liquid nitrogen bath. The high pressure raises the boiling point of the liquid nitrogen circuit whilst the liquid nitrogen bath provides sufficient cooling to dissipate the heat load from the optical components and internal heat leaks.
The closed loop circuit has a maximum design pressure of 10 bar absolute and the main liquid nitrogen bath operates at 1-2 psi above atmospheric pressure. The liquid level in this bath is continually topped up to replace the liquid boiled away, an autofilling system is provided as part of the system; this may be connected to a local distribution system or to a dewar.
|Maximum recommended continuous cooling load||2.5 kW|
|Coolant temperature||78 – 90 K|
|Flow rate||Controllable up to 30 litres/minute (1800 litres/hour) with 2.5 kW cooling achievable at 11 litres/minute (650 litres/hour)|
|Variable pump speed||20 - 90 Hz|
|Liquid Nitrogen consumption||1500 litres per day at full power (continuous operation).
Less than 50 litres/day when idle
|Control system physical assembly||Floor standing rack on castors / wall mounted or as a separate unit for mounting into customers 19” rack system|
|Control system||Automated PLC control|
|Control system SCADA||Rack mounted|
|Operator interface||19" touch panel OPI|
(support feet to top of burst disc)
|2100 mm (max)|
|Cryocooler vessel diameter||720 mm|
|Working space diameter||1200 mm|