Closed cycle ³He evaporation coolers

He-10 Wet coolers

 

Standard Three-Stage ³He Refrigerator

 

‘Bolocam’ style CRC10.

 

 

Main Features:

• Ultrahead temperature below 250mK.
• Intermediate head capable of buffering 100 ?W at 400mK for 24 hours.
• Up to 48 hours cold time between cycles (no load).
• Rapid cool-down and cycling.
• No pumping on main bath required in order to cycle the cooler.
• No moving parts.
• Compact and self-contained.
• ‘Plug-and-play’. Simple bolt-down interfacing to existing cryostats.
• ‘Fly-by wire’. Simple and reliable to operate.
• Can be automated using packages such as ‘Labview’.
• Diode thermometry to pumps, heat switches and heat exchanger supplied.
• All housekeeping wiring carried to a 37-pin micro-D connector.
• Some customisations possible.
• Indicative PRICE: Standard unit, no modifications: typically around £25,000

Note: Units are pressurized to over 90 bar at room temperature.

 

STANDARD THREE-STAGE HELIUM-3 COOLER

 

TYPE ‘BOLOCAM He-10’.

The three-stage ‘He-10’ type ³He evaporation refrigerator is capable of reaching a base temperature of around 220mK for up to 48 hours continuously (no load specification) when operating from a 4.2K bath, and may be recycled indefinitely. Under typical load conditions it will maintain a working temperature of about 250 mK for 24 hours. Recycling is performed entirely electrically (there are no mechanical valves or heat switches) and may be accomplished in less than 2 hours, and with practise in a little over 1 hour.

 

The ultra-cold head (uppermost structure in picture) is designed to support only a few µW of direct load, as the intermediate head and heat exchanger provide two points at which parasitic loads may be buffered away. The intermediate head (large cylinder to right and below the ultra-head) will run at about 380 mK under 100 µW of applied load. The heat exchanger (below the intermediate head) extracts enthalpy from the cold gas flowing through it, and will run at between 1 and 2K depending upon the load applied to the intermediate head, with a larger load on the head leading to a lower temperature at the heat exchanger, due to the greater volume of cold gas flowing through it. A significant load may be sunk to the heat exchanger, and for optimal performance, any detector wiring or experimental support structures (e.g. a cold table supporting a detector array) should be thermally sunk to both the heat exchanger and the intermediate head, before finally sinking the detector array (or other very low temperature component) to the ultra-cold head.

 

The cooler is a compact and self-contained unit that can be installed into any ⁴He cryostat with a large enough cold workspace. Overall dimensions are 19 cm (7.5”) tall, with a base footprint contained within a 10 x 20 cm (4” x 8”) rectangle. Standard mechanical interfacing is by a 1⁄2” grid of holes drilled to UNC4-40 clearance, though a metric grid is also possible.

 

Diode thermometry to the three cryopumps and two gas-gap heat switches, and to the heat exchanger, is included. Germanium or other low temperature resistance thermometry for the two cold heads is not included, however. We generally ask the customer to purchase two suitable and calibrated sensors, and have them shipped directly to Chase Research, as this is more cost efficient. We recommend either Germanium or ‘Cernox’ (RuO2) sensors from Lakeshore Cryotronics or equivalent (e.g. RO600 sensors from Scientific Instruments Inc.).

 

External connections for control and instrumentation are all electrical, and all ‘housekeeping’ wiring is taken to a 37-pin micro-D connector as standard. There are no valves or external plumbing, and because of the internal ⁴He reservoir it is not necessary to pump on the cryostat main bath in order to cycle the unit. Anyone familiar with 4He techniques can operate a He-10 type unit.

 

Recycling is performed with the main bath at 4.2K. To achieve condensation, electrical power is initially applied to the cryopump heater elements (up to 2W each). Once the ⁴He has condensed, the 4He cryopump heater is turned off, and the corresponding gas-gap switch is turned on. The temperature of the ³He cryopump is stabilized at around 45K by reducing the power input, while the temperature of the cold heads fall to about 1.1K, condensing and cooling the ³He, until the 4He runs out. At this point the power to the 3He pumps is turned off and the corresponding heat switch is turned on. The heads then cool rapidly to the operating temperature. Recycling will consume between 1 and 2 litres of L⁴He, and takes less than 2 hours. Once the condensation phase is over the ⁴He is all adsorbed into the cryopump, and the ⁴He pump switch may be turned off. Full documentation and data from a commissioning test run are included with the instruction manual that is supplied with each unit.

 

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