4 Cryogenic Design of the Liquid Helium Experiment “Critical Dynamics in Microgravity”

W. A. S. T. P. Duncan”*

] University of New Mexico, Albuquerque, New Mexico, USA

2 Sandia National Laboratories, Albuquerque, New Mexico,

to measure the static properties of systems near

II) - normal fluid (He 1) interface in helium under

critical

processes.

- interface may be positioned by adjusting (or regulating) the

temperature of the superfluid component.In the low heat flux limit of our experiments this superfluid component is isothermal.In Earth orbit, theory predicts the interface should be stabilized by the heat

q

flux [2]; however once in Earth orbit, these experiments do not depend upon this predicted stabilization in

order to obtain data.

The superfluid transition in pure liquid

sample isotope, which may be

purified to a few parts in

homogeneous. According to the Psi Theory [5], superfluid helium can be described by a two-component

order parameter, often referred to as the

varies with pressure (-113 bar/K near saturated vapor pressure [6]),

in Earth orbit in late 1992 [7]. Another heat capacity

experiment, the Confined Helium

paper documents the cryogenic apparatus and procedures which are currently used in the prototype instrument to obtain data on Earth.

The LTRF platform has already flown twice in Earth orbit and is scheduled to fly once again prior to the launch of

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with the interface forming at the bottom

when the interface reaches a sidewall

effects are predicted to be more than an order of magnitude larger than the predicted bulk nonlinear helium properties [9] under study. To make at least a

from either cell endplate. A reduced temperature of

may be realistically maintained in our experiment. At this reduced temperature,

rule mandates that the measurements be made at least 2.2 mm from the

O), it is necessary to measure the actual thermal profile near and through the

-

[7,8]. For Q -0.1

the pressure-induced variation in

thermal gradient in a cell of approximately 7 mm that convection is produced in the normal fluid on Earth [ 19].

these experiments in values of Q.

the 1 interface has been predicted to vary as Q“x, with x -0.5 [2,9]. Under gravity, however, this

variation cannot be directly measured since gravity reduces the initial width of the

K refrigerator and is not regulated, Four RF

The final stage, before the experimental platform, is the shield stage. The shield stage surrounds the experimental platform to provide isolation from

bridge or a paramagnetic salt thermometer (PST).

has heaters attached to both its top and bottom end plates. The top heater

works in conjunction with either the

SQUID signals are brought through the superconducting SQUID housings along rigid stainless steel coaxial lines that are themselves enclosed in stainless steel sheaths to form a SQUID bias ground from

the low-level signal ground of the cryostat. The

the two inner conductors are separated and each is fed through its own glass-

5

in the temperature A superconducting pick-up

coil, wound around the salt, maintains a constant magnetic

by an

of PSTS have been cryostat. Both are

wires which provides for thermal contact, The sensing element is placed inside a sapphire post which is

attached to a high purity

in the powder for thermal contact. A

thermometers are placed inside superconducting

keep the flux tubes cooled and superconducting through the launch. a 3He exchange gas must be introduced into the cryostat’s vacuum space. Up to two days, during which no data can be collected, will be required to

pump this exchange gas out to space.Changing the geometry of the

built and read out with an inductive bridge [21]. Work

thermometer with a SQUID readout. The sensing element of the

of CAD with an inner diameter of 1.3 cm, an outer

1266 epoxy blank of

approximately the same size is then epoxied to the to supply the constant magnetic field for

the CAB, approximately 1000 turns of

located within 200

located about 5 mm from the cooler

not exist while in

the liquid helium [12, 13, 14], at the low heat flux levels intended for this experiment, the

acoustic noise, which would otherwise

propagate down to the cell from warmer regions. The use of a

pumping out the hydraulic volume and the valve actuating line before data collection. For space flight,

having both the cell till line and the valve actuating line evacuated before launch provides a measure of

security against any possible venting of helium into the guard

needle and

shield stage when the

of-balance signal from a GRT bridge to a Linear Research LR-130 proportional-integral-differential

130 in turn provides current to a 5

on the stage to be regulated. This technique allows

less than 5 Higher resolution temperature

control is achieved by using the signal from a

magnetometer. The RF SQUID’S signal is first

range of the

resolution. portion of the signal is digitized by a

National Instruments 16-bit data acquisition board.

10

V. THERMAL MODELING

Thermal Model: Probe

raised to above 10 K to recharge the PST flux tubes without disrupting the prediction was confirmed

Notice that this heat conduction may be countered by adjusting the temperature of, and hence the heat This adjustment permits a heat balance at the sidewall thermometer stage, node 8, which is being used in tile data data may

Earth orbit, this adjustment of the SQUID stage temperature will allow for compensation of cosmic-ray

heating of the sidewall platform used in data acquisition. throughout the orbit, and

certainly will vary with any altitude or attitude changes throughout the mission. Typically, the variation in

the radial heat flux from the cell sidewall platform is about per 1

thermal model are updated as more data are obtained.This thermal model also allows the impact of

different materials selections to be evaluated before the flight instrument is developed.

Thermal Model: Experimental Cell

‘l’he thermal performance of the experimental cell has been simulated to optimize the sidewall

design, and to investigate the cell’s ability to resolve the subtle thermal profiles associated with the

nonlinear region very near the interface.In all the thermal simulations discussed here, the normal fluid

helium bulk thermal conductivity + where the

= (122.2

conductivity is taken as its theoretical value [9] of K

where reduced temperature is defined relative to

taken to be 0.01

between all metal surfaces and the liquid helium is taken to be

constant at 0.4

ing is displayed in Figure 6. here the He

= 50

that the radial sidewall.

near the center O) of the cell, The

helium temperature near the center of the temperature variation is noticed, is taken to

6. For a radial heat flux of zero, A is zero. For a given is weak and

readily correctable, making the ultimate data analysis in this

platforms, the abrupt change in the sidewall’s effective thermal conductivity in the vicinity of the platform creates a radial component of the heat flux which perturbs the otherwise through the cell. This radial

heat flux integrated over the entire cell must equal zero since no sources or sinks are present on the platforms.The effect of the

Notice and that it

varies

and not on the

platforms while a constant heat flux passes I

corrections for the variation of A with

he most significant effect was the increase of noise levels on the ground [20]; this noise

Some of this increase resulted from

the unhappy circumstance that the cosmic ray events occurred

of the

increase can be reduced to an insignificant level if two improvements are made [26]; improve the thermal

contact of the

Both of these measures are planned for the low temperature m

away from the SAA, and 5

flight experiments to get the

best temperature data, Therefore,

The thermal conductivity of this

and

These measures should lead to

is expected to have calorimeter,

so, with the reduced liquid helium sample volume, the radiation heating will result in a warming of the sample and cell approximately equal to that seen in the

less than they did LPE. Very little of the cell mass is located at the warm end of the DYNAMX cell, so radiation heating will be on the order ofpicowatts of the DYNAMX cell mass

is at the cold end, where the temperature is controlled by a servo loop that applies about 50 microwatt to maintain the temperature. A few nanowatts of radiation heating will have little effect at this location. The

SAA passages will cause significant heating only for the lowest, approximately 50 pW, heat currents

applied to the warm end of the cell. With careful planning low heat current data can

be obtained in orbits which do not encounter the SAA.

[1] J. A. Physics 107B: 331 (1981).

[2] A. 50:433 (1983); 5S: 309 (1 984), and references therein.

[3] Available from

[4] P. C. and P. V.

[5] V. L.

Rev. 171:275 (1968),

[7] Principal Investigator: J. Lipa, Stanford University.

[8] Confined I Principal Investigator: J. Lipa, Stanford University.

[9] and V. Dohm, 67:3404 (1991); B87: 229 (1992).

[10] and V. Dohm,

Rev.

and V. Steinberg, 58:337 (1 987);

Jpn. J. Rev.

13] M. Dingus, and H. Meyer, J. Low Temp. J. Tuttle,

and 11. Meyer, J. Low 79:9 (1990); D. Murphy and

T. C. P. J.

‘1’, C. P. Bull. Am.

and V. Rev.

and R. V. Duncan, 61:846

Thesis, Stanford University (1990), unpublished.

Physics B 194-196:597 (1994);

FIGURE CAPTIONS

Table 1: Variation of A with

PSI’ with its shield, packed powder PST, and

cell

band represents a change of approximately 10

=

300

Vapor

Thermal Shield

,.-’. . .

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