Diffusive Transport of 3He in the EDM Experiment

Steve Lamoreaux

Basic Idea

• Superfluid Helium at sufficiently low “looks like” a perfect mechanical vacuum

• We are working with low 3He density so 3He-3He collisions will not hinder diffusive/ballistic flow

• Flow in pipes: Knudsen’s formula

2225

cm

sec

3

42731059.1

dd

l

T

MW

What W do we need?

• We have two cells, 10000 cm3 volume. We need an emptying time constant of about 50 seconds (4 time constants to get to 98% clean)

• So W=1/(200 cm3/sec)

• m=2.2 m3

• For 50 cm length, T=0.3 K, d=2.3 cm

• For 100 cm length, d=2.9 cm

What is the time constant to empty the cell through a 2 cm diameter hole?

vAV /4For v= 50 m/s, A=3.14 cm2, V=10000 cm3

= 2.5 sec

Or, for 10000 sec, A=10-3 cm2

This is a 1 micron gap around a 6 cm circumference

Overall Scheme

Exp. Cells

Helium Level

SelectionValve

Collection Volume

charcoal

Cell valve

The Pressurization Problem

• What if we have to run with the superfluid pressurized to get a high dielectric strength?

• Then we can’t have a free liquid surface

• Possible to have the ‘main bath’ pressurized, the cells at low pressure?

Gradient Requirements

Holding Cells:

See hreq.pdf for details

Charcoal Conveyor Belt

Purificationvolume

Charcoal coated“conveyor belt”periodically movescharcoal to hot regionfor cleaning

Electrical Detection of Scintillations

5000 ion pairs???per capture

I+

I-

About 1 pA if collectedin 1 msec; 1 fA/rtHz ampprovides s/n of 25

Ion Mobility in SFHe

~ 2.0 K

E > 104 V/cm ionsdetach from vortices