Sub-Kelvin Coolers Recycling

Pump Configuration

Of the three sub-Kelvin stages in MUSCAT two—the 1-K and 450-mK stages—are cooled by continuous sorption coolers. In total there are six sorption pumps used to cool these stages with two He4 pumps used to cool the 1-K stage and two sets of He4-He3 paired pumps to cool the 450-mK stage. For each stage, the pumps are divided into two subsystems (refereed to as A and B). This subsystems are recycled sequentially to provide continuous cooling. The following figures present a highly-simplified schematic of the configuration of each of these coolers.

CC4 Sorption Cooler configuration

CC7 Sorption Cooler configuration

Input Parameters

The recycling of the coolers is controlled by a state machine scripted in LabView. This state machine has two sets of input parameters each contained in a human-readable .ini file.

Hardware Configuration Parameters

The first set of parameters is the physical channel IDs of the various heaters and temperature sensors. This is purely dependent on hardware configuration and present no possibility for optimisation and so is not further discussed here.

Cycle Optimisation Parameters

The second set of parameters defines values like the set temperature each pump is heated to along with the voltage used for heating and the value of the exit condition for each stage of the cycle. The complete set of parameters is listed below

Section	Parameter	Description	Default Value	Unit	Valid Range
CC7	He4APumpSetT	Temperature to heat He4 A pump to condense He4	37.0	Kelvin	Min: 0
CC7	He4APumpVHeat	Voltage used to heat He4 A pump up to He4APumpSetT	24.0	Volts	Min: 0, Max: 24
CC7	He4APumpVHold	Voltage used to hold He4 A pump at approximately He4APumpSetT during condensation	3.5	Volts	Min: 0, Max: 24
CC7	He4AHSVOn	Voltage to turn on (close) heat switch for He4 A pump	3.5	Volts	Min: 0, Max: 5
CC7	He4AHSVOff	Voltage to apply while heat switch for He4 A pump is off (open)	0	Volts	Min: 0, Max: 5
CC7	He3APumpSetT	Temperature to heat He3 A pump to condense He3	35.0	Kelvin	Min: 0
CC7	He3APumpVHeat	Voltage used to heat He3 A pump up to He3APumpSetT	24.0	Volts	Min: 0, Max: 24
CC7	He3APumpVHold	Voltage used to hold He3 A pump at approximately He3APumpSetT during condensation	3.5	Volts	Min: 0, Max: 24
CC7	He3AHSVOn	Voltage to turn on (close) heat switch for He3 A pump	3.5	Volts	Min: 0, Max: 5
CC7	He3AHSVOff	Voltage to apply while heat switch for He3 A pump is off (open)	0	Volts	Min: 0, Max: 5
CC7	He3ASoftStartV	Voltage used to preheat He3 A pump during heating of He4 A pump	0	Volts	Min: 0, Max: 24
CC7	He4BPumpSetT	Temperature to heat He4 B pump to condense He4	37.0	Kelvin	Min: 0
CC7	He4BPumpVHeat	Voltage used to heat He4 B pump up to He4APumpSetT	24.0	Volts	Min: 0, Max: 24
CC7	He4BPumpVHold	Voltage used to hold He4 B pump at approximately He4APumpSetT during condensation	3.5	Volts	Min: 0, Max: 24
CC7	He4BHSVOn	Voltage to turn on (close) heat switch for He4 B pump	3.5	Volts	Min: 0, Max: 5
CC7	He4BHSVOff	Voltage to apply while heat switch for He4 B pump is off (open)	0	Volts	Min: 0, Max: 5
CC7	He3BPumpSetT	Temperature to heat He3 B pump to condense He3	35.0	Kelvin	Min: 0
CC7	He3BPumpVHeat	Voltage used to heat He3 B pump up to He3APumpSetT	24.0	Volts	Min: 0, Max: 24
CC7	He3BPumpVHold	Voltage used to hold He3 B pump at approximately He3APumpSetT during condensation	5.0	Volts	Min: 0, Max: 24
CC7	He3BHSVOn	Voltage to turn on (close) heat switch for He3 B pump	3.5	Volts	Min: 0, Max: 5
CC7	He3BHSVOff	Voltage to apply while heat switch for He3 B pump is off (open)	0	Volts	Min: 0, Max: 5
CC7	He3BSoftStartV	Voltage used to preheat He3 B pump during heating of He4 B pump	0	Volts	Min: 0, Max: 24
CC7	He4CondTemp	Temperature below which we consider He4 to condense in the system	4.2	Kelvin	Min: 0
CC7	He4CondTime	Time to wait for He4 to condense	480	Seconds	Min: 0
CC7	He3CondTemp	Temperature below which we consider He3 to condense in the system	3.1	Kelvin	Min: 0
CC7	He3CondTime	Time to wait for He3 to condense	480	Seconds	Min: 0
CC7	HSOffBelow	Temperature below which a heat switch is considered to be off (open)	15.0	Kelvin	Min: 0
CC7	TimeBetweenCycles	Time between the end of a A/B subsystem cycle and the start of the B/A cycle	480	Seconds	Min: 0
CC7	He3TimeOut	Fall-back parameter - Maximum time to wait after pumping He4 for the head of a He3 pump to cool below He3CondTemp before manually advancing the cycle	2700	Seconds	Min: 0
CC4	He4APumpSetT	Temperature to heat He4 A pump to condense He4	47.0	Kelvin	Min: 0
CC4	He4APumpVHeat	Voltage used to heat He4 A pump up to He4APumpSetT	24.0	Volts	Min: 0, Max: 24
CC4	He4APumpVHold	Voltage used to hold He4 A pump at approximately He4APumpSetT during condensation	4.5	Volts	Min: 0, Max: 24
CC4	He4AHSVOn	Voltage to turn on (close) heat switch for He4 A pump	5.0	Volts	Min: 0, Max: 5
CC4	He4AHSVOff	Voltage to apply while heat switch for He4 A pump is off (open)	0	Volts	Min: 0, Max: 5
CC4	He4BPumpSetT	Temperature to heat He4 A pump to condense He4	47.0	Kelvin	Min: 0
CC4	He4BPumpVHeat	Voltage used to heat He4 A pump up to He4BPumpSetT	24.0	Volts	Min: 0, Max: 24
CC4	He4BPumpVHold	Voltage used to hold He4 A pump at approximately He4BPumpSetT during condensation	5.0	Volts	Min: 0, Max: 24
CC4	He4BHSVOn	Voltage to turn on (close) heat switch for He4 A pump	5.0	Volts	Min: 0, Max: 5
CC4	He4BHSVOff	Voltage to apply while heat switch for He4 A pump is off (open)	0	Volts	Min: 0, Max: 5
CC4	HSOffBelow	Temperature below which a heat switch is considered to be off (open)	15.0	Kelvin	Min: 0
CC4	TimeAfterCC7BeforeCC4	Time to wait after finishing the CC7 subsystem cycle before finishing the CC4 subsystem cycle	0	Seconds	Min: 0
MD	StillVOn	Voltage applied to still heater to circulate He3	1.8	Volts	Min: 0, Max: 2
MD	StartStillBelowT	Temperature of still condensor (450-mK stage) below which to start circulating He3	0.6	Kelvin	Min: 0

Example Files

Example configuration files, based on the currently used parameters are available for download here.

• Hardware Parameters
• Cycle Parameters

Cycle State Flow

The following presents a basic overview of the states used in the state-machine to recycle the coolers in MUSCAT cryostat.

info

The following steps assume that the pulse tube is running and system has cooled to 4 K. These preliminary steps are controlled and reported by the MUSCAT Cryogenics Control and Logging Software but are note detailed here.

0. Start with all HSs CC*He**HSVOn
1. Set CC4 A HS heater CC4He4AHSVOff, go to 2
2. Set CC7 A He3 and He4 HS heaters to CC7He3AHSVOff and CC7He4AHSVOff, go to 3
3. if CC4 He4A HS is < CC4HSOffBelow go to 4; else go to 3
4. Apply CC4He4APumpVHeat to CC4 He4 A pump heater, go to 5
5. if CC7 He4 A HS AND CC7 He3 A HS < CC7HSOffBelow go to 6, else go to 5
6. Apply CC7He4APumpVHeat to CC7 He4 A pump heater, apply CC7He3ASoftStartV to CC7 He3 A pump heater, go to 7
7. if CC7 He4 A pump > CC7He4APumpSetT go to 8 (once only); if CC4 He4 A pump > CC4He4APumpSetT go to 9 (once only); if CC7 He4 A pump > CC7He4APumpSetT AND CC4 He4 A pump > CC4He4APumpSetT go to 10; else go to 7
8. Apply CC7He4APumpVHold to CC7 He4 A pump heater, go to 7
9. Apply CC4He4APumpVHold to CC4 He4 A pump heater, go to 7
10. Apply CC7He3APumpVHeat to CC7 He3 A pump heater, go to 11
11. if CC7 He3 A pump > CC7He3APumpSetT go to 12; else go to 11
12. Apply CC7He3APumpVHold, go to 13
13. if CC7 He4 A head < CC7He4CondTemp go to 14; else go to 13
14. wait CC7He4CondTime then go to 15.
15. Set CC7 He4 A pump heater to 0, apply CC7He4AHSVOn to CC7 He4A HS heater, go to 16
16. if CC7 He3 A head < CC7He3CondTemp go to 17; else if time at this state > CC7He3TimeOut go to 17; else go to 16
17. wait CC7He3CondTime, then go to 18
18. Set CC7 He3 A pump heater to 0, apply CC7He3AHSVOn to CC7 He3A HS heater, go to 19
19. wait CC4TimeAfterCC7BeforeCC4, then go to 20
20. Set CC4 He4 A pump heater to 0, apply CC4He4AHSVOn to CC4 He4A HS heater, go to 21
21. wait CC7TimeBetweenCycles, then go to 22
22. Set CC4 B HS heater to CC4He4BHSVOff, go to 23
23. Set CC7 B He3 and He4 HS heaters to CC7He3BHSVOff and CC7He4BHSVOff, go to 24
24. if CC4 He4 B HS < CC4HSOffBelow, go to 25; else go to 24
25. Apply CC4He4BPumpVHeat to CC4 He4 B pump heater, go to 26
26. if CC7 He4 B HS AND CC7 He3 B HS < CC7HSOffBelow go to 27, else go to 26
27. Apply CC7He4BPumpVHeat to CC7 He4 B pump heater, apply CC7He3BSoftStartV to CC7 He3 A pump heater, go to 28
28. if CC7 He4 B pump > CC7He4BPumpSetT go to 29 (once only); if CC4 He4 B pump > CC4He4BPumpSetT go to 30 (once only), if CC4 He4 B pump > CC4He4BPumpSetT AND CC7 He4 B pump > CC7He4BPumpSetT go to 31, else go to 28
29. Apply CC7He4BPumpVHold to CC7 He4 B pump heater, go to 28
30. Apply CC4He4BPumpVHold to CC4 He4 B pump heater, go to 28
31. Apply CC7He3BPumpVHeat to CC7 He3 B pump heater, go to 32
32. if CC7 He3 B pump > CC7He3BPumpSetT go to 33, else go to 32
33. Apply CC7He3BPumpVHold to CC7 He3 B pump heater, go to 34
34. if CC7 He4 B pump head < CC7He4CondTemp go to 35, else go to 34
35. wait CC7He4CondTime, then go to 36.
36. Set CC7 He4 B pump heater to 0, apply CC7He4BHSVOn to CC7 He4 B HS, go to 37
37. if CC7 He3 B head < CC7He3CondTemp go to 38; else if time at this state > CC7He3TimeOut go to 38; else go to 37
38. wait CC7He3CondTime, then go to 39
39. Set CC7 He3 B pump heater to 0, apply CC7He3BHSVOn to CC7 He3B HS heater, go to 40
40. wait CC4TimeAfterCC7BeforeCC4, then go to 41
41. Set CC4 He4 B pump heater to 0, apply CC4CC7He4BHSVOn to CC4 He4B HS heater, go to 42
42. wait CC7TimeBetweenCycles, then go to 1

Miniature Dilution Refrigerator

The cycle described above does not consider the operation of the miniature dilution refrigerator used to cool the detectors from 450 mK to 120 mK. A dilution refrigerator )either standard or miniature) is inherently continuous and thus does not require recycling in the same way that a sorption-cooler based system does. The miniature dilution refrigerator simply requires a static thermal power to be applied to its still (also known as the evaporator). In theory this power could be applied at all times including during the cooldown from 300 K, however the reality of doing this would be that cooldown of the 450-mK stage from 4 K to its base temperature would take longer were this done. Instead the state machine used to cycle the continuous sorption coolers monitors the cooldown of the 450-mK stage and when appropriate applies the thermal load to the still. To do this the state machine is modified as follows:

• A flag variable called FocalPlaneReady is added with a default value of False
• If at Step 39 above the temperature of the miniature dilution refrigerator's mixing chamber (the detector stage of MUSCAT) is above MDStartStillBelowT, FocalPlaneReady is set to False, the still heater is switched off (0 V applied) and the state machine advances to Step 40
• If at Step 39 above the temperature of the miniature dilution refrigerator's mixing chamber is below MDStartStillBelowT AND FocalPlaneReady is False, FocalPlaneReady is set to True, the still heater is left off (with 0 V applied) and the state machine moves on to Step 40
• If at Step 39 above the temperature of the miniature dilution refrigerator's mixing chamber is below MDStartStillBelowT AND FocalPlaneReady is True (that is to say the above condition was met on the previous B sub-system cycle), the still heater is set to MDStillVOn and the state machine moves on to Step 40