Continuous Cryogenic He-3 Adsorption Pump
Harry Charalambous, David Schuster
8/22/14
Abstract
A dilution refrigerator uses the mixing of He-3 with He-4 to
achieve multiple temperature levels including millikelvin range
temperatures. One major drawback of current models is the need to
cycle the He-3 out of the cryostat to room temperature using an
external pump in order to flow continuously. We propose an
activated carbon adsorption pump system to allow for pumping of
He-3 without the gas leaving the low temperature confines of the
dilution refrigerator. While activated charcoal adsorption pumps
already exist ours would be able to run indefinitely. In order to
work continuously one sorption pump adsorbs helium at 4.2K, a
valve is sealed, and then is heated to 25K to desorb the bulk of the
helium while the other is cooled back down to 4.2K using a gasgap heat switch. Such a system has few moving parts and allows
for potentially faster cool down time. The sorption pumps have
been built, but more work needs to be done to test their ability to
achieve continuous cooling using He-4 to 1K. Following this He-3
testing to a full dilution refrigerator should be done.
Introduction
A cryostat is any device that allows samples or other devices to be
cooled to low temperatures. A dilution refrigerator is a type of cryostat
that uses an endothermic movement of He-3 from an He-3 fluid to a
mostly He-4 mixture to achieve cooling power.
Modern dilution refrigerators are often operated in conjunction with
pulse tube refrigerators. The procedure of a pulse tube refrigerator is
summarized below (Figure 1).1
1
National Institute of Standards and Technology. Development of the Pulse
Tube Refrigerator as an Efficient and Reliable Cryocooler. Web.
http://www.cryogenics.nist.gov/Papers/Institute_of_Refrig.pdf (Aug. 11 2014),
22.
Figure 1. Pulse tube refrigerator schematic diagram. T0 is the
temperature before precooling, Th is the temperature after compression,
and Tc is the temperature after expansion.
He-4 is pre cooled using a regenerator before moving through the cold
end of the pulse tube. Once in the pulse tube a piston compresses the gas
which is heated by the compression and flows through an orifice into the
reservoir, passing through a heat exchanger at the warm end of the pulse
tube. The piston then reverses expanding and cooling the gas in the
pulse tube in an adiabatic process. Finally the gas in the reservoir moves
back through the orifice, pushing the colder gas in the pulse tube to the
cold end. At the cold end a heat exchanger allows the gas to absorb heat
from thermally connected objects.2
This process, coupled with a dilution refrigerator allows for 50K and 4K
stages that may be used to cool samples or to precool He-3 before it
enters the dilution system. Once past the 4K stage the He-3 continues
through a further cooling process through multiple stages (Figure 2).3
2
National Institute of Standards and Technology. Development of the Pulse
Tube Refrigerator as an Efficient and Reliable Cryocooler. Web.
http://www.cryogenics.nist.gov/Papers/Institute_of_Refrig.pdf (Aug. 11 2014),
5-6.
3
Fred M. Ellis, Cryostat. Wesleyan University,
http://fellis.web.wesleyan.edu/research/cryostat/cryostat.html (Aug. 12, 2014).
Figure 2. Dilution refrigerator schematic after pulse tube cooling.
The gas is cooled to approximately 1.2K by a 1K pot of liquid He-4.
Then it flows through an impedance where it is in contact and cooled by
the still stage at approximately 700mK. At this point the gas flows down
through a counter flow heat exchanger and enters into the mixing
chamber.
The mixing chamber contains He-3 and He-4 in two phases. The gas
enters the He-3 rich phase which is almost entirely composed of He-3.
Then it flows through a phase boundary into the He-3 poor phase which
contains He-3 at about 6.6% concentration with He-4.4
This flow through the phase boundary is an endothermic process which
provides the cooling power of the dilution refrigerator
, (1)
where
is the flow rate of the He-3 gas,
is the enthalpy of the
is the enthalpy of the He-3 rich phase.5 The
dilute phase, and
mixing chamber is attached to the lowest stage for samples cooled to
approximately 10mK.
He-3 leaves of the dilute phase and flows up through the counterflow
heat exchanger where it absorbs heat with the aforementioned downward
flowing He-3. A plate may be connected here to provide a 100mK stage.
The gas enters into the still which is composed of superfluid He-4. At
low enough temperature He-4 has a very low vapor pressure so the vapor
pressure of the still is almost entirely He-3. Thus a pump working on the
still will pump out primarily He-3 which can then be cycled back into the
system. The endothermic evaporation of He-3 in the still maintains the
700mK temperature stage.
An inefficiency results at the last stage in the cycle when the He-3 gas is
pumped back to the start of the cycle. Currently used dilution
refrigerators mostly use external turbine pumps that work at room
temperature. Thus the He-3 must exit the cryostat, be thermally exposed
to room temperature, and then be pumped into the cryostat. The turbine
pumping system has many moving parts, is expensive, and has a slow
cool down time.
We propose replacing this external pump with an internal sorption pump
using activated carbon as the adsorbent because of its very high internal
surface area. This system would have few moving parts, would cost less,
and potentially provide for quicker cool down time. While activated
carbon sorption pumps are not a new concept and are often used already
there are no known dilution refrigerators with sorption pumps that work
continuously. Instead the activated carbon gradually becomes saturated
with helium until the dilution refrigerator stops providing enough cooling
power.
As a solution we designed a dual sorption pump system with one pump
regenerating while the other is adsorbing helium. The sorption pumps
have been built and the next steps are to test their seals at low
temperatures as well as to test for continuous cooling power to 1K using
4
Frank Pobell. Matter and Methods at Low Temperatures. (Revised/Expanded
ed. Berlin: Springer, 2007), 149-160.
5
Frank Pobell. Matter and Methods at Low Temperatures. (Revised/Expanded
ed. Berlin: Springer, 2007), 165-166.
He-4. If successful the system will be tested in a full dilution refrigerator
using He-3.
Experimental Setup and Procedure
The pump is sealed using a ball valve attached to a rod. The rod has a
neodymium magnet and a stainless steel 316 spring attached on one end
with a surrounding solenoid composed of superconducting niobium wire
wrapped around a Teflon sleeve. When the solenoid is on the magnet is
attracted towards the center, pushing the spring and ball valve to achieve
the seal (Figure 3a). When the solenoid is off, the spring moves back to
rest position which opens the valve (Figure 3b). Thus the default
position is for the helium to have free flow in case of malfunction.
Inside the pump is a chamber providing room for the activated carbon.
Figure 3. (a) Sorption pump is in the open configuration. He-3 flows in
from the still and is adsorbed onto the surface of the charcoal. Capillary
attached to the top provides impedance to flow. (b) Sorption pump is in
the closed configuration. Pump may now be heated for desorption of
He-3. Capillary connection is the only escape for gas due to the ball
valve seal.
The pump will be initially open and thermally connected to the 4K stage.
He-3, with a much higher vapor pressure than He-4, flows into the pump
where it is adsorbed onto the surface of the activated carbon. After a
predetermined period the valve is then be sealed and the pump will be
heated to 25K. The vast majority of the helium desorbs by 25K and will
flow through the capillary to restart the cycle. As the pump is cooled
down gradually using a gas-gap heat switch the other pump is adsorbing
carbon. Thus the system is continuously working.
Results
The two sorption pumps with connections to 1K pots have been
completed as well as the 50K, 4K, and Still plates for the initial tests
(Figure 4).
Figure 4. Completed sorption pump.
Future Work
The pumps will be tested with He-4 initially due to the high cost of He-3.
On the still are 1K, 101 copper pots attached to the pumps with 316
stainless steel to hold the He-4 before entering the pump. If the pumps
do not leak and the thermometry indicates that the temperature lowers
when the He-4 is allowed to cycle through the pumps then He-3 will be
tested in the same way. If the cooling power is sufficient and continuous
then the pumps can be tested with a full dilution refrigerator.