Lake Shore Model 370 AC Resistance Bridge User’s Manual
2.3.1
Dilution Refrigerator Terminology
Dilution refrigerators remain the workhorse of low temperature research down to approximately 5
–
10 mK. So many
variations of factory and home built units exist that it is unlikely that any two are identical. Figure 2-1 illustrates only the
most basic features.
Helium Dewar: Vacuum insulated Dewar large
enough to contain the following
components. Sometimes includes
a nitrogen outer jacket to improve
efficiency or superconducting
magnet coil when needed.
Figure 2-1. Dilution Refrigerator Block Diagram
4
He
3
He Dilute
Helium
Dewar
Vacuum
Insulation
4
He
Reservoir
Vacuum
Chamber
Vacuum
Space
1 K Pot
Still
Heat
Exchangers
Mixing
Chamber
Sample
Holder
3
He
3
He Dilute
Dilution_Block.eps
4
He Reservoir: Reservoir of liquid
4
He at
atmospheric pressure that
provides bulk cooling down to 4.2
K. Pre-cools gas entering the
vacuum space and acts as a
radiation shield for refrigeration
components.
Vacuum Space: Insulates refrigeration
components and provides a clear
workspace.
1 K Pot (
4
He Evaporator): Pumped reservoir of
4
He
maintained at 1 K. Used in the
process of condensing
3
He rich
gas returning from pumping
system. Also used as a heat sink
for measurement leads
. Good
location for a temperature sensor
for cool down monitoring and
troubleshooting
.
Still Heat Exchanger: Chamber used to evaporate
3
He out of a dilute
3
He mix (liquid
4
He containing some
3
He—
sometimes called mash). It is the
first stage below 1 K and is also
used to liquefy returning
3
He.
The
still requires a few milliwatts of
heat from a resistive heater to
sustain evaporation. The Still
Output can drive the still heater.
Good location for a diagnostic
temperature sensor.
Heat Exchangers: One or more heat exchangers serve to cool returning
3
He rich
liquid to near the temperature of the mixing chamber. Also used to heat sink measurement leads.
Mixing Chamber: Chamber where cooling action takes place. Holds both
3
He rich liquid and dilute
3
He separated by a
phase boundary. When
3
He is depleted from the mix by the still, more crosses the phase boundary to
maintain equilibrium. In doing so it is expanded into (diluted by) the
4
He creating cooling of the
chamber
. Best location for temperature control sensor and resistive heater. The heater output can
control temperature by driving the heater under closed loop PID control or open loop control.
Load (Sample Holder): End of the mixing chamber where experiments can take place. May either be part of or attached
to the mixing chamber and it is often designed for a unique purpose.
Requires a separate temperature
sensor if its temperature is different from the mixing chamber
.
2-2
Theory of Operation
