Main Features.
•
Designed to run from a mechanical cooler (Cooling power > 500
mW @ 4K).
•
Will also work in a ‘wet’ dewar.
• Ultrahead temperature below 250mK.
• Intermediate head capable of buffering 60 microW at 400mK for
24 hours.
• Up to 48 hours cold time between cycles (no load).
• Rapid cool-down and cycling.
• No moving parts.
• Compact and self-contained.
• ‘Plug-and-play’. Simple bolt-down interfacing
support cooler.
• ‘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: in the region of £25,000
Note: Units are pressurized to over 90 bar at room temperature.
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 mechanical cooler head,
or L4He 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 60 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. Overall dimensions
are 24 cm (9.45”) tall, with a base footprint
contained within a 14 cm (5.51”) circle. Standard mechanical
interfacing is by a 6 clearance holes (UNC # 8) on a 5” pitch
circle, with an additional array of clearance holes
for thermal sinking to the cooler head. Alternative
interfacing
schemes
(e.g. a metric pattern) are also possible.
Diode thermometry
to the three cryopumps and three gas-gap heat switches,
and to the heat exchanger
and
main plate,
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 Cryogenics Ltd., as this is more cost efficient.
We recommend either Germanium or ‘Cernox’ sensors
from Lakeshore Cryotronics.
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 for the
support cryogenics
to attain temperatures below 4K in order to cycle the
unit.
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
50K by reducing the power input, while the temperature
of
the cold heads
fall to about 1.5K, 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 takes less than 2 hours when the He-10 unit
is run from a mechanical cooler with more than about
500 mW
of cooling power. 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.
