Lake Shore 370, Руководство пользователя

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Lake Shore Model 370 AC Resistance Bridge User’s Manual
2.3.2
3
He Cryostat
3
He Cryostats are the least complex systems capable of reaching temperatures down to 300 mK. Even at such relatively
warm temperatures the benefits of a well designed resistance bridge must be considered when selecting instrumentation.
Figure 2-2 illustrates the basic features.
Helium Dewar: Vacuum insulated Dewar large
enough to contain the following
components. Sometimes
includes a nitrogen outer jacket
to improve efficiency.
4
He
Helium
Dewar
Vacuum
Insulation
4
He
Reservoir
Radiation
Shield
Vacuum
Space
1 K Pot
Heat
Exchangers
3
He
Evaporator
Sample
Holder
3
He
Vacuum
Chamber
Cryostat_Block.eps
Figure 2-2.
3
HE Cryostat Block Diagram
4
He Reservoir: Reservoir of liquid
4
He at
atmospheric pressure that
provides bulk cooling down to
4.2 K. Pre-cools gasses 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
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
.
Heat Exchangers: One or more heat exchangers
serve to cool returning
3
He
liquid to near the temperature of
the
3
He evaporator. Also used
to heat sink measurement leads.
3
He Evaporator: Chamber where cooling action takes place. Liquid
3
He is pumped under vacuum to reduce its boiling
point to approximately 300 mK.
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
3
He evaporator where experiments can take place. May either be part of or attached to
the evaporator and is often designed for a unique purpose.
Requires a separate temperature sensor if
its temperature is different from the
3
He evaporator.
Theory of Operation 2-3