HODACA can use the liquid-helium-free, sorption-pumped ³He refrigerator manufactured by Niki-Glass Co., Ltd.
The nominal operating range is approximately 0.3–100 K. Below approximately 3 K, the sample temperature is controlled mainly by adjusting the temperature of the ³He sorption pump (sorb).
Because several operating parameters are specific to this refrigerator, the practical settings used at HODACA are summarized below.
For the original manuals and the sample-temperature–sorb-temperature conversion chart, see the ISSP Neutron Science Laboratory Sample Environments page.
The refrigerator uses two Cryo-Con 44C temperature controllers.
Channel A — 2nd Stage
Temperature of the second stage of the refrigerator.
Channel B — Sorb
Temperature of the ³He sorption pump.
Channel C — Heat Switch
Temperature associated with the heat switch.
Channel D — ³He Pot
Temperature of the ³He pot.
The control loops are:
Loop 1 — Sorb
Loop 2 — Heat Switch
Channel A — Sample
The control loop is:
Loop 1 — Sample
Do not change thermometer calibration curves or other low-level controller settings unless instructed by the instrument staff.
The ³He refrigerator cools the sample by evaporative cooling of liquid ³He in the ³He pot.
The sorb acts as the ³He sorption pump.
When the sorb is heated to approximately 50 K, adsorbed ³He gas is released from the sorb.
When the sorb heater is switched off and the sorb is cooled to a few kelvin, the sorb adsorbs ³He gas and reduces the vapor pressure above the liquid ³He in the pot. This causes the liquid ³He to evaporate and cools the ³He pot toward the base temperature.
During normal base-temperature operation, the sorb temperature is typically only a few kelvin.
The heat switch controls the thermal connection between the sorb and the 2nd Stage.
A heat-switch set point of approximately 30 K provides a strong thermal connection and is used when cooling the sorb.
When the heat-switch heater is turned off and its temperature decreases, the thermal connection becomes much weaker. Empirically, the connection is largely suppressed below approximately 12 K.
For temperature control below 3 K, a set point of approximately 16 K is normally used. This provides sufficient thermal coupling to stabilize the sorb temperature without strongly heating the 2nd Stage.
Cooling the refrigerator from room temperature to the base-temperature region requires substantial preparation time.
Initial evacuation typically requires approximately 30 minutes.
Under favorable conditions, cooling to the lowest-temperature region takes approximately 17 hours. In practice, the cool-down frequently takes longer.
For this reason, the sample should normally be installed and the refrigerator started approximately 24 hours before the planned measurement at the lowest temperature.
A typical achievable base temperature is approximately 0.3 K. In recent HODACA operation, temperatures around 0.32–0.34 K have been obtained.
Before starting an automatic initial cool-down or automatic ³He recondensation, check the refrigerator settings carefully.
Confirm that all four mode switches on the ³He control unit are set to:
AUTO
Also confirm the following temperature-controller settings:
Controller 1, Loop 1 — Sorb: 50 K
Controller 1, Loop 2 — Heat Switch: 30 K
Controller 2, Loop 1 — Sample: 0.01 K
These values are important for the automatic sequence.
The ³He control unit switches the sorb and heat-switch heaters according to the sequence, but it does not automatically correct inappropriate temperature set points.
The sorb and heat-switch set points are often changed during measurements at temperatures above the base temperature. Therefore, always restore the standard values before starting an automatic cool-down or recondensation.
Automatic condensation or recondensation does not always complete correctly. If necessary, the ³He can be condensed manually.
Manual operation should be performed by experienced users or in consultation with the instrument staff.
Turn the heat-switch heater off and allow the heat-switch temperature to decrease below approximately 12 K.
This substantially reduces the thermal connection between the sorb and the 2nd Stage.
Set the sorb temperature to:
Sorb: 50 K
Heating the sorb releases the ³He that has been adsorbed by the sorption pump.
Keep the heat switch thermally open and allow the 2nd Stage to cool.
Wait until the 2nd Stage temperature is below approximately 2.8 K.
Typical minimum temperatures of the 2nd Stage are approximately 2.4 K; values around 2.39–2.43 K have been observed during normal operation.
Once the 2nd Stage has reached the low-temperature region, maintain this condition for approximately 30 minutes as a practical guideline for condensing the ³He.
After the ³He has been condensed, turn the sorb heater off.
Then turn the heat switch on and set:
Heat Switch: 30 K
This thermally connects the sorb to the 2nd Stage and cools the sorb.
As the sorb cools, it adsorbs ³He vapor and reduces the vapor pressure above the liquid ³He. Evaporation of the liquid ³He then cools the ³He pot toward its base temperature.
Immediately after the heat switch is turned on, the 2nd Stage and ³He-pot temperatures may temporarily increase because heat stored in the sorb is transferred to the cold stages.
This temporary temperature increase is normal. The temperatures should decrease again as the sorb cools.
Below approximately 3 K, the sample temperature is controlled primarily by adjusting the sorb temperature.
Use approximately the following configuration:
Heat Switch: ON, set to 16 K
Sorb heater: ON
Sorb set point: approximately 14–44 K, depending on the required sample temperature
Pulse tube: AUTO or ON
Solenoid valve: AUTO or OFF
The sorb set point controls the ³He vapor pressure and therefore the temperature of the ³He pot and sample.
Use the sample-temperature–sorb-temperature conversion chart on the ISSP Sample Environments page to select an appropriate sorb temperature.
The heat-switch set point is reduced from the 30 K used during sorb cooling to approximately 16 K during temperature regulation. This maintains relatively weak thermal coupling to the 2nd Stage while allowing the sorb temperature to be controlled.
Using a strong heat-switch connection while heating the sorb to high temperatures, for example around 40 K, would introduce unnecessary heat into the 2nd Stage.
Above approximately 3 K, control the temperature using the sample heater rather than by controlling the ³He vapor pressure alone.
Use approximately the following configuration:
Heat Switch: ON, set to 16 K
Sorb: 44 K
Set the desired temperature using Controller 2, Loop 1 — Sample.
For example, for a measurement at 20 K, set the sample-control loop to approximately 20 K.
The sample heater then regulates the sample temperature directly.
First warm the refrigerator to approximately 40 K using the same procedure as for the 3–40 K range.
For operation above approximately 40 K:
Solenoid valve: ON / OPEN
Continue controlling the temperature using Controller 2, Loop 1 — Sample.
Opening the solenoid valve is important in this temperature range to prevent excessive ³He pressure as the refrigerator is warmed.
The ³He-pot thermometer and sample thermometer are RuO₂ resistance thermometers optimized for low-temperature operation.
Their readings should not be regarded as accurate absolute thermometry in the high-temperature region, particularly above approximately 40 K.
The sample-control loop can still be used operationally, but the displayed RuO₂ temperature should be interpreted with caution at high temperature.
Before starting an automatic recondensation after a temperature-dependent measurement, always recheck the standard settings:
Sorb: 50 K
Heat Switch: 30 K
Sample: 0.01 K
All four ³He control switches: AUTO
These settings may have been changed during measurements at intermediate or high temperatures.
If they are not restored, the automatic sequence may not operate correctly.
The cooling and condensation times given above are empirical values and may vary depending on the sample, sample holder, refrigerator condition, and thermal load.
If automatic cooling or recondensation does not proceed as expected, do not repeatedly restart the automatic sequence without checking the sorb, heat-switch, 2nd-stage, and ³He-pot temperatures.
Manual condensation may be used when necessary, but users unfamiliar with the refrigerator should contact the HODACA instrument staff.