1 Cryogenic Design and the 4 He Evaporative Purifier David G. Haase, et al., North Carolina State University

2 Overview Cryogenic engineering design tasks –Cryovessel –Helium liquefier –Dilution refrigerator –Helium insulation volume Evaporative 4 He purifier

3 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 to in less than 250 seconds.

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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 = Works in principle Implementation details?

6 Differential Evaporation

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9 Schematic of Evaporator

10 Superfluid Film Flow Heater (0.8 K) 0.4 K 4.2 K

11 The numbers Volume of target ~ 15 liters Removal n 3 ~ 5 x moles/cycle n 4 ~ 1 x moles/cycle n 4 from film burner ~ moles/cycle Capacity of charcoal ~ 4 moles/kg

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14 Testing 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? A prototype is being prepared for testing at NC State

15 Cryogenic Systems Cryovessel (vacuum system, intermediate and 4.2 K shields and entrainment volume) (Boissevain, et al.) Helium liquefier and associated parts (Boissevain, Ihloff, et al.) Dilution refrigerator (Janis Research) Helium insulation volume (Eischen, Smeltzer and Angell)

16 Cryovessel Principal components –Outer vacuum container –Intermediate temperature shields –4.2 K liquid helium entrainment volume and shields Design is stable and no particular design issues Safety concerns –Pressure vessel certification ($$$) –Pressure release designs

17 Liquefier Knoxville cryogenics workshop (Ihloff) Visit to JLAB (Ihloff and Boissevain) Turbine liquefier is highly recommended for reliability and maintenance and required for the anticipated boiloff Additional equipment such as gas purifier and storage tanks Cost issues

18 Dilution Refrigerator Engineering Study Propose the cooling power, flow rate, geometry, and liquid helium boil-off rates for the dil-fridge; Design to show how the dil-fridge would fit inside the cryovessel liquid helium entrainment volume; Design of the heat exchangers for the optimal cooling of liquid helium in experimental volume;

19 Dilution Refrigerator Engineering Study Specification of physical size and siting requirements, electrical and cooling connections and ancillary equipment or supplies needed for operation; Specifications of materials and constructions techniques required for cryogenic operation, minimal magnetic interference to the experiment and minimal neutron activation. Estimate of cost and time needed for procurement and construction of the device.

20 Dilution Refrigerator Required performance specifications –T operation : K –Cooling power at 0.5 K: 80 milliwatts

21 Dilution Refrigerator One configuration –Pumping station EH4200 Roots pump EH1200 Roots pump (1 or 2) IGH100L dry pump

22 Dilution Refrigerator

23 Dilution Refrigerator

24 Dilution Refrigerator Outcomes to this point The target cooling power is achievable Two pumping stacks are preferred Total LHe boiloff rates require the larger liquefier

25 Helium Insulation Volume Design and stress analysis is being conducted Consultant has NASA experience with composite cryogenic rocket fuel tanks Issues –Vacuum seals –Penetrations in end cap –Testing of smaller tanks, end caps and seals

26 Helium Insulation Volume

27 Measurement of the 3 He concentration Accelerator mass spectrometer Lifetime of neutrons Analysis of adsorbed gas from purifier Mobility of negative ions (?)

28 Summary Cryovessel, et al., engineering design tasks –On schedule –Technical and monetary concerns Evaporative purifier –Work in progress Would someone find a way to measure 3 He concentration?