1 NEDM 4 He Purifier and Other Cryogenic Issues David G. Haase North Carolina State University Update on Cryovessel Engineering Evaporative Purification NC State Students: Travis McCaw (REU) Franklin Dubose Kevin Rasch

2 Cryovessel Engineering (WBS 1.3) Cryovessel - Helium Liquefier - J. Boissevain and E. Ihloff Dilution Refrigerator(s) - call for engineering consultants in preparation –Janis Cryogenics, Leiden, Quantum Technology Liquid 4 He Insulation Volume - –S. Smeltzer (private consultant with NASA experience) –J. Eischen (NCSU Mechanical and Aerospace Engineering)

3 Overview - 4 He Purification The operational goal of the 4 He purifier is to reduce the 3 He concentration in the target fluid from to in roughly seconds.

4 Major Technical Assumptions The purification will take place via differential evaporation using the greater vapor pressure of 3 He at lower temperatures. T ~ 0.2 K K Adsorption vacuum pump Minimize heat load to the system Effects of 4 He film flow must be negated

5 Alternatives Superfluid plug –Operating temperature, purification rate, purification level Phonon wind purification –Operating temperature, purification rate, purification level

6 R&D Questions Can the 3 He concentration be reduced in the 4 He by evaporation? At what levels of dilution? Can we measure the dilution level? What heat load does this produce? Which is the preferred mode of fixing the 4 He film? Can a reliable, robust apparatus and cycle be developed?

7 Evaporative 4 He Purification - R&D Task Installation and testing of dilution refrigerator Construction of room temperature gas handling system Design and construction of evaporator, displacer and valve Testing of 4 He purification system

8 Schematic of Evaporator Test

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10 Fill system with liquid Close isolation valve,raise plunger, fill plunger volume with liquid Raise liquid into evaporator Open isolation valve, lower plunger Liquid level rises into evaporator Evaporate Remove liquid from evaporator Raise plunger, lower liquid level, close valve Regenerate adsorber Pump gas from heat switch Heat adsorber and pump off desorbed gas 4 He Purifier Operation Cycle

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12 X 3 = X 3 = X 3 =

13 Parameters 4 He in target ~ 13 l ~ 470 moles 3 He to be removed each cycle ~ 4.5 x moles If n 4 ~ (10 to 1000)n 3 then n total = 5 x10 -5 mol Time for evaporation ~ 200 seconds Pumping rate dn/dt = (5 x mol) / (200 sec) = 2.5 x mol/sec. Vapor pressure of liquid << 1 Pa

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15 Film burner ~ 3 mW Adsorber ~ 0.35 mW Container walls < 0.2 mW

16 Film burner ~ 0 Adsorber to 4.2 K ~ mW Container walls < 0.2 mW Adsorber to target liquid through gas > 100 mW (!)

17 Heat of Adsorption Initial condition: –P = Pa, T =4.2 K –Q = 10 mol/kg, H = 15 kJ/kg (of charcoal) Final condition: –P = 100 Pa, T = 15 K –Q = 5 mol/kg, H = 9 kJ/kg Q net = 5 mol/kg, H net = 6 kJ/kg H net /mol = 1200 J/mol (of 3 He) For a 50 gm adsorber –Q tot = 0.25 mol, H tot = 350 J

18 Implementation of Evaporator at NC State SHE 530 Dilution Refrigerator Flow rate ~ 600  moles/sec Built 1981, last operated July, 2000 Moved to NCSU from TUNL Installation Construction of new support, dewar and vacuum can Alterations to pumping system Awaiting vacuum can to proceed to initial cooldown and operation testing

19 Schematic of test apparatus

20 Moved SHE 530 DR and system from TUNL to NCSU Acquired LHe dewar Set up DR support structure Tested cryostat electronics Vacuum can in shop Gas handling system Design of evaporator/plunger Progress to date

21 Moved SHE 530 DR and system from TUNL to NCSU Acquired LHe dewar Set up DR support structure Testing dewar electronics Preliminary design of vacuum can and evaporator Progress to date

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23 Vacuum can Mixing chamber Plunger volume Evaporator Valve

24 External Gas Handling System

25 Prototype Adsorber 50 gm of activated charcoal pellets 25 cm diameter Adsorb at 4.2 K Regenerate at 15 K and 100 Pa 3 He- 4 He capacity ~ 0.25 mol Adsorption enthalpy ~ 350 J Time between regeneration ~ days (n He = 10  mol/measurement cycle)

26 Design of Evaporator Gas heat switch Heated flange Heated tube Cooled baffle Adsorber plate Input from valve Support rods

27 Possible approach to reduce heat flow during regeneration of adsorber

28 Issues/Questions Efficiency of film burners –P 3 /P 4 is greater at low temperatures –Have not calculated effect of imperfect film burner Heat flow during regeneration –Large possible heat flow during regeneration –Insulating stopper a possible solution –Regeneration may be seldom needed

29 Next steps Complete installation and test SHE 530 refrigerator with new dewar and vacuum can Complete and submit designs to NCSU shop Complete gas handling system Install evaporator and plunger Measure purification, heat flows, operation