1. Removal of 3 He from 4 He (R&D) David Haase, Franklin Dubose, Travis McCaw and Paul Huffman North Carolina State University

2. Reminder about heat budget (5/2006)

3. Outline Removal of 3 He by differential evaporation Design of test apparatus Testing of dilution refrigerator and parts Next steps

4. Overview - 4 He Purification for nEDM Polarized 3 He is used as co-magnetometer and for signal detection. The duration of a measurement cycle is set by the lifetime of the neutron and the depolarization time of the 3 He. Polarized 3 He must be injected and removed from the 15 liter target volume at the beginning and end of each measurement cycle. The operational goal of the 4 He purifier is to reduce the 3 He concentration in the target fluid from 10 -10 to 10 -12 in less than 250 (1000?) seconds.

5. 4 He Purification Options Phonon wind Removal of 4 He via a porous plug (superleak) Differential purification Most effective for T ~ 1.1 K How to remove the concentrated 3 He? Very slow flow rate Effectiveness shown only to X 3 = 10 -9 Works in principle Implementation details?

8. Differential Evaporation

9. Volume of target ~ 15 liters Removal n 3 ~ 5 x 10 -8 moles/cycle n 4 ~ 1 x 10 -2 moles/cycle n 4 from film burner ~ 10 -8 moles/cycle Capacity of charcoal ~ 4 moles/kg

12. Evaporative Purifier

13. Evaporation Chamber Superfluid Film Flow Heater (0.8 K) 0.4 K 4.2 K

14. Charcoal Adsorber 50 gm of 6-14 mesh activated charcoal Epoxied to both surfaces of copper thermal disk Held by copper mesh cover

15. Low Temperature Valve Swagelok BW Series welded body to bellows seal (SS-4BW-V51, 4.1 mm orifice) MDC rotary motion feed through directly into vacuum can with room temperature feed through Heat sink shaft at 4.2, 1.5 and 0.7 K

16. Lifter Volume MDC 6 inch travel standard linear motion feedthrough - high precision scale Heat sink shaft at 4.2, 1.5 and 0.7 K 1/16 OD tube from volume to valve Capacitance level gauge inside the lifter volume Separate thermometer and heater on volume Direct thermal link to mixing chamber

17. Capacitive liquid level detectors Parallel plate capacitors Patterned circuit board with slots Kapton spacers ~ 0.004 in thick C ~ 300 pF,  liquid = 1.05 GR 1615-A capacitance bridge @ 3kHz, resolution 0.1% 

18. RGA and 3 He- 4 He leak detector performance What level of analysis are we trying to reach? Tests with calibrated 3 He and 4 He leaks –Leybold L200 leak detector seems unreliable and provides only qualitative data –SRS RGA 100 tests Mass 3 and mass 4 backgrounds –Options to remove backgrounds –Mass spectrometry

19. RGA Test Setup

20. RGA Measurement

22. Evaporation cycle for test setup Close low temperature valve Cool down all parts Fill transfer volume while pumping on adsorber volume Lower transfer volume Open valve Raise transfer volume to move liquid to evaporator Lower transfer volume to move liquid out of evaporator Heat adsorber and collect gas for analysis. Measure temperatures and heat loads to other components

23. Testing of SHE 430 Dilution Refrigerator Built 1981, last operated at TUNL in summer, 2000 System has been installed at NCSU since Fall, 2007 New dewar and vacuum can and alterations to pumps and pumping lines New aluminum vacuum can 38 in high x 17.5 ID with 25 in high space below mixing chamber Progress slowed by weld and indium seal leaks in the vacuum can –Parts of the can were re-welded 2-3 times –New transition collar fabricated to seal DR stainless steel flange to top of vacuum can

24. Vacuum Can

26. Testing of SHE 430 Dilution Refrigerator Three cooldowns in March Optimized LHe transfers and vacuum system, checked all pumps and electronics March 25-31 final tests –Calibrated four resistance thermometers against germanium standard (1.0 - 0.1 K) –Ran dilution refrigerator for extended period, measured flow rates and cooling powers –Fridge is running with a smaller pumping system - limits cooling powers

27. Testing of SHE 430 Dilution Refrigerator

28. Optimised LN and LHe precooling process Fixed transfer tubes, etc. LHe transfer and calibration of level sensors Measured gas volumes needed for exchange gas Measured 3 He flow rates Measured dewar LHe boiloff rates Calibrated four resistance thermometers below 1K Calibrated storage volumes and pump volumes Confirmed operating parameters for still heater and for pumping pressures for still and coldplate Checked out all electronic systems

29. Next steps Complete assembly/ vacuum checking Mechanical tests of valves and transfer volume Mount level sensors, heaters, thermometers Assemble vacuum can and lift dewar Liquid nitrogen tests Cool to 4.2 K Tests at operating temperatures

30. Testing of Evaporative Purifier Verification of operating cycle - fill, pump, empty, regenerate Investigation of temperature dependence of purification times Performance of the purifier must be inferred by measuring X 3 for gas desorbed from the charcoal What can be done to minimize the heat to dilution refrigerator from the regeneration of the charcoal adsorber?

31. How far will you have proceeded by June 30? Most of the parts of the evaporator are completed They are being fitted into place in the dilution refrigerator First run of evaporator in June

32. Summary Overview of purification by evaporation Design of the evaporator Measurement of 3 He/ 4 He with RGA Refurbishment and testing of SHE DR Plans for next steps

33. Refurbishment and test of dilution refrigerator Operation Cooling power of refrigerator Helium boiloff rates Calibration of resistance thermometers Exchange gas and pumping times Measurement of flow rates

34. Calibration of thermometers Operation Cooling power of refrigerator Helium boiloff rates Calibration of resistance thermometers Exchange gas and pumping times Measurement of flow rates

35. Dilution Refrigerator Cooling Power Operation Cooling power of refrigerator Helium boiloff rates Calibration of resistance thermometers Exchange gas and pumping times Measurement of flow rates

36. Outline of the purification process Removal of 3 He to the separation volume Removal of the 3 He from the liquid Time frame Amount to be removed

37. Testing of SHE 430 Dilution Refrigerator Calibration of thermometers