U.S. patent application number 14/086697 was filed with the patent office on 2014-05-22 for systems and methods for cryogenic refrigeration.
This patent application is currently assigned to D-Wave Systems Inc.. The applicant listed for this patent is D-Wave Systems Inc.. Invention is credited to Richard G. Harris, Jacob Craig Petroff.
Application Number | 20140137571 14/086697 |
Document ID | / |
Family ID | 50726652 |
Filed Date | 2014-05-22 |
United States Patent
Application |
20140137571 |
Kind Code |
A1 |
Petroff; Jacob Craig ; et
al. |
May 22, 2014 |
SYSTEMS AND METHODS FOR CRYOGENIC REFRIGERATION
Abstract
Systems and methods for improving the performance of dilution
refrigeration systems are described. Filters and traps employed in
the helium circuit of a dilution refrigerator may be modified to
improve performance. Some traps may be designed to harness
cryocondensation as opposed to cryoadsorption. A cryocondensation
trap employs a cryocondensation surface having a high thermal
conductivity and a high specific heat with a binding energy that
preferably matches at least one contaminant but does not match
helium. Multiple traps may be coupled in series in the helium
circuit, with each trap designed to trap a specific contaminant or
set of contaminants. Both cryocondensation and cryoadsorption may
be exploited among multiple traps.
Inventors: |
Petroff; Jacob Craig;
(Burnaby, CA) ; Harris; Richard G.; (North
Vancouver, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
D-Wave Systems Inc. |
Burnaby |
|
CA |
|
|
Assignee: |
D-Wave Systems Inc.
Burnaby
CA
|
Family ID: |
50726652 |
Appl. No.: |
14/086697 |
Filed: |
November 21, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61729250 |
Nov 21, 2012 |
|
|
|
61736456 |
Dec 12, 2012 |
|
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Current U.S.
Class: |
62/6 ;
62/55.5 |
Current CPC
Class: |
F25B 9/145 20130101;
B01D 8/00 20130101; F25B 9/10 20130101; F25B 9/12 20130101; F25D
19/006 20130101 |
Class at
Publication: |
62/6 ;
62/55.5 |
International
Class: |
B01D 8/00 20060101
B01D008/00 |
Claims
1. A dilution refrigeration system comprising: a dilution
refrigerator including a helium circuit; a secondary cold source
that in operation provides cooling power at temperatures between
about 4K and about 77K; and at least a first trap coupled in series
with the helium circuit, wherein the first trap comprises a first
trap volume and at least one cryocondensation surface inside the
first trap volume, and wherein the at least one cryocondensation
surface is thermally coupled to the secondary cold source.
2. The dilution refrigeration system of claim 1 wherein the first
trap volume includes multiple cryocondensation surfaces to provide
a large surface area in the first trap volume.
3. The dilution refrigeration system of claim 1 wherein the
secondary cold source includes a pulse tube cryocooler.
4. The dilution refrigeration system of claim 1, further
comprising: a second trap coupled in series with the helium
circuit, wherein the second trap comprises a second trap volume and
an adsorptive material inside the second trap volume.
5. The dilution refrigeration system of claim 4 wherein the second
trap is thermally coupled to the secondary cold surface.
6. The dilution refrigeration system of claim 4, further
comprising: a tertiary cold source that in operation provides
cooling power at temperatures between about 4K and about 77K,
wherein the second trap is thermally coupled to the tertiary cold
source.
7. The dilution refrigeration system of claim 6 wherein the
tertiary cold source includes a pulse tube cryocooler.
8. The dilution refrigeration system of claim 6 wherein the
tertiary cold source includes a bath of liquid cryogen.
9. The dilution refrigeration system of claim 1 wherein the
dilution refrigerator is thermally coupled to the secondary cold
source.
10. A cryogenic trapping system comprising: a cryoadsorption trap
including an adsorption material and a plurality of thermalization
surfaces interposed throughout the adsorption material, wherein the
thermalization surfaces form a circuitous route through the
adsorption material through which gas flows; and a cryocondensation
trap coupled in series with the cryoadsorption trap, wherein the
cryocondensation trap includes a length of tubing that is at least
partially filled with a high surface area cryocondensation
material.
11. The cryogenic trapping system of claim 10 wherein the
cryoadsorption trap and the cryocondensation trap are both
thermally coupled to a first temperature stage in a cryogenic
refrigeration system.
12. The cryogenic trapping system of claim 11, further comprising:
at least two parallel trapping subsystems both coupled in series
with the cryoadsorption trap, wherein the at least two parallel
trapping subsystems are each controllably thermally coupleable to a
second temperature stage in the cryogenic refrigeration system via
a respective heat switch, and wherein the second temperature stage
of the cryogenic refrigeration system is warmer than the first
temperature stage of the cryogenic refrigeration system.
13. The cryogenic trapping system of claim 12 wherein the at least
two parallel trapping subsystems each employ at least one of
cryoadsorption and/or cryocondensation.
14. The cryogenic trapping system of claim 10 further comprising at
least one gauge connected in series with the cryogenic trapping
system, the at least one gauge selected from a group consisting of:
flowmeters, pressure gauges, and contaminant sensors.
Description
BACKGROUND
[0001] 1. Field
[0002] The present systems and methods generally relate to
cryogenic refrigeration technology, and particularly relate to
cryogenic trapping systems and methods for removing contaminants
from cryogen circuits.
[0003] 2. Description of the Related Art
[0004] Refrigeration
[0005] Temperature is a property that can have a great impact on
the state and evolution of a physical system. For instance,
environments of extreme heat can cause even the strongest and most
solid materials to melt away or disperse as gas. Likewise, a system
that is cooled to cryogenic temperatures may enter into a regime
where physical properties and behavior differ substantially from
what is observed at room temperature. In many technologies, it can
be advantageous to operate in this cryogenic regime and harness the
physical behaviors that are realized in the realm of cold. The
various embodiments of the systems, methods and apparatus described
herein may be used to provide and maintain the cryogenic
environments necessary to take advantage of the physics at cold
temperatures.
[0006] Throughout this specification and the appended claims, the
term "cryogenic" is used to refer to the temperature range of 0 to
about 93K. A variety of technologies may be implemented to produce
an environment with cryogenic temperature, though a commonly used
device that is known in the art is the dilution refrigerator.
Dilution refrigerators can even be used to achieve extreme
cryogenic temperatures below 50 mK. In the operation of a typical
dilution refrigerator, the apparatus itself requires a background
temperature of about 4K. In order to provide this background
cooling, the apparatus may be, e.g., immersed in an evaporating
bath of liquid helium-4 (".sup.4He") or, e.g., coupled to another
type of refrigeration device, such as a pulse-tube cryocooler. The
dilution refrigerator apparatus may comprise a series of heat
exchangers and chambers that allow the temperature to be lowered
further to a point where a mixture of .sup.3He and .sup.4He
separates into two distinct phases and pure .sup.3He can move into
a mixture of .sup.3He and .sup.4He in a process analogous to
evaporation, providing cooling and allowing a temperature of around
10 mK to be achieved. Full details on this dilution effect and the
operation of typical dilution refrigerators may be found in F.
Pobell, Matter and Methods at Low Temperatures, Springer-Verlag
Second Edition, 1996, pp. 120-156.
[0007] In most dilution refrigerator designs, mechanical pumps and
compressors, and an external gas-handling system, are used to
circulate .sup.3He such that it is warmed from the lowest
temperature in the fridge up above cryogenic temperatures and
towards room temperature before it is returned to the low
temperature. The pumps and compressors used are large, expensive,
noisy, in need of periodic maintenance, and they inevitably add
contaminants, such as air (i.e., nitrogen, oxygen, carbon dioxide,
argon, etc.) to the helium. These contaminants typically have
higher freezing points than the helium and so may solidify in the
helium fluid channels, creating blockages. Such blockages may plug
fine capillaries in the dilution refrigerator, causing problems
with reliability. Plugging often requires a complete warm-up of a
dilution refrigerator in order to remove the contaminants and
restore the fridge to normal operations. The procedure of warming
and subsequently cooling back down to operating temperatures can
take several days. Filters and cold traps can be used to reduce the
frequency of plugging by removing contaminants from the helium, but
current filters and traps used in the art are of limited
effectiveness. Thus, plugging due to contaminants remains a serious
technical challenge in cryogenic refrigeration technology affecting
fridge performance, and there remains a need in the art for
improved systems and methods for contaminant filtering and/or
trapping in cryogenic refrigeration systems.
BRIEF SUMMARY
[0008] A dilution refrigeration system may be summarized as
including: a dilution refrigerator including a helium circuit; a
secondary cold source that in operation provides cooling power at
temperatures between about 4K and about 77K; and at least a first
trap coupled in series with the helium circuit, wherein the first
trap comprises a first trap volume and at least one
cryocondensation surface inside the first trap volume, and wherein
the at least one cryocondensation surface is thermally coupled to
the secondary cold source.
[0009] The first trap volume may include multiple cryocondensation
surfaces to provide a large surface area in the first trap volume.
The secondary cold source may include a pulse tube cryocooler. The
at least one cryocondensation surface may be formed of a material
having a low thermal conductivity, the low thermal conductivity not
less than a lowest of a respective thermal conductivity of a
material selected from a group consisting of: steel, stainless
steel, Monel.RTM., and titanium alloy. Each of the multiple
cryocondensation surfaces may be formed of a material having a high
thermal conductivity, the high thermal conductivity not less than a
highest of a respective thermal conductivity of a material selected
from a group consisting of: copper, silver sinter, brass, bronze
and aluminum. The dilution refrigeration system may further
include: a second trap coupled in series with the helium circuit,
wherein the second trap comprises a second trap volume and an
adsorptive material inside the second trap volume. The second trap
may be thermally coupled to the secondary cold surface. The
dilution refrigeration system may further include: a tertiary cold
source that in operation provides cooling power at temperatures
between about 4K and about 77K, wherein the second trap is
thermally coupled to the tertiary cold source. The tertiary cold
source may include a pulse tube cryocooler. The tertiary cold
source may include a bath of liquid cryogen. The dilution
refrigerator may be thermally coupled to the secondary cold
source.
[0010] A cryogenic trapping system may be summarized as including:
a cryoadsorption trap including an adsorption material and a
plurality of thermalization surfaces interposed throughout the
adsorption material, wherein the thermalization surfaces form a
circuitous route through the adsorption material through which gas
flows; and a cryocondensation trap coupled in series with the
cryoadsorption trap, wherein the cryocondensation trap includes a
length of tubing that is at least partially filled with a high
surface area cryocondensation material.
[0011] The cryoadsorption trap and the cryocondensation trap may
both be thermally coupled to a first temperature stage in a
cryogenic refrigeration system. The cryogenic trapping system may
further include: at least two parallel trapping subsystems both
coupled in series with the cryoadsorption trap, wherein the at
least two parallel trapping subsystems are each controllably
thermally coupleable to a second temperature stage in the cryogenic
refrigeration system via a respective heat switch, and wherein the
second temperature stage of the cryogenic refrigeration system is
warmer than the first temperature stage of the cryogenic
refrigeration system. The at least two parallel trapping subsystems
may each employ at least one of cryoadsorption and/or
cryocondensation. The condensation material may be formed of a
material having a high thermal conductivity, the high thermal
conductivity not less than a lowest of a respective thermal
conductivity of a material selected from a group consisting of:
copper, silver sinter, brass, bronze, and aluminum. The length of
tubing of the cryocondensation trap may be formed of a material
having a low thermal conductivity, the low thermal conductivity not
more than a highest of a respective thermal conductivity of a
material selected from a group consisting of: steel, stainless
steel, Monel.RTM., and titanium alloy. The cryogenic trapping
system may further include at least one gauge connected in series
with the cryogenic trapping system, the at least one gauge selected
from a group consisting of: flowmeters, pressure gauges, and
contaminant sensors.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0012] In the drawings, identical reference numbers identify
similar elements or acts. The sizes and relative positions of
elements in the drawings are not necessarily drawn to scale. For
example, the shapes of various elements and angles are not drawn to
scale, and some of these elements are arbitrarily enlarged and
positioned to improve drawing legibility. Further, the particular
shapes of the elements as drawn are not intended to convey any
information regarding the actual shape of the particular elements,
and have been solely selected for ease of recognition in the
drawings.
[0013] FIG. 1 is a schematic diagram of an exemplary dilution
refrigeration system employing improved trap designs in accordance
with the present systems and methods.
[0014] FIG. 2 is a schematic diagram of a cryogenic trapping system
in accordance with the present systems and methods.
DETAILED DESCRIPTION
[0015] In the following description, some specific details are
included to provide a thorough understanding of various disclosed
embodiments. One skilled in the relevant art, however, will
recognize that embodiments may be practiced without one or more of
these specific details, or with other methods, components,
materials, etc. In other instances, well-known structures
associated with refrigeration systems, such as heat exchangers,
impedances, and control systems including microprocessors, heat
switches, drive circuitry and nontransitory computer- or
processor-readable media such as nonvolatile memory for instance
read only memory (ROM), electronically erasable programmable ROM
(EEPROM) or FLASH memory, etc., or volatile memory for instance
static or dynamic random access memory (ROM) have not been shown or
described in detail to avoid unnecessarily obscuring descriptions
of the embodiments of the present systems and methods.
[0016] Unless the context requires otherwise, throughout the
specification and claims which follow, the word "comprise" and
variations thereof, such as, "comprises" and "comprising" are to be
construed in an open, inclusive sense, that is as "including, but
not limited to."
[0017] Reference throughout this specification to "one embodiment,"
or "an embodiment," or "another embodiment" means that a particular
referent feature, structure, or characteristic described in
connection with the embodiment is included in at least one
embodiment. Thus, the appearances of the phrases "in one
embodiment," or "in an embodiment," or "another embodiment" in
various places throughout this specification are not necessarily
all referring to the same embodiment. Furthermore, the particular
features, structures, or characteristics may be combined in any
suitable manner in one or more embodiments.
[0018] It should be noted that, as used in this specification and
the appended claims, the singular forms "a," "an," and "the"
include plural referents unless the content clearly dictates
otherwise. Thus, for example, reference to a problem-solving system
including "a refrigeration system" includes a single refrigeration
system, or two or more refrigeration systems. It should also be
noted that the term "or" is generally employed in its sense
including "and/or" unless the content clearly dictates
otherwise.
[0019] The headings provided herein are for convenience only and do
not interpret the scope or meaning of the embodiments.
[0020] The various embodiments described herein provide systems and
methods for improving the performance of cryogenic refrigeration
systems. More specifically, the various embodiments described
herein provide systems and methods for improved filtering/trapping
of contaminants in the helium circuit of a dilution
refrigerator.
[0021] Most dilution refrigeration systems available today are
susceptible to plugging in the helium circuit caused by the
freezing out of contaminants that have permeated into the helium
itself. For example, a small leak in a pump or portion of tubing in
the helium circuit may allow the ingress of air into the helium
circuit, and the components of this air may freeze at a temperature
at which the helium remains a gas or liquid. The frozen air may
adhere to the inner walls of the tubing that forms the helium
circuit and plug the circuit. Such plugging will affect, and may
completely disrupt, the operation of the dilution refrigerator.
[0022] In some applications, it may be desirable for a dilution
refrigerator to be capable of continuous operations for on the
order of years. For example, in applications of superconducting
computing (such as superconducting quantum computation) where the
computer processor is cooled by a dilution refrigerator, it may be
desirable for the computer processor to remain cold (i.e.,
operational) for on the order of years. Current dilution
refrigeration systems will typically experience a plugging event on
shorter timescales (i.e., on the order of days, weeks, or months)
and are not well-suited to providing continuous operation for on
the order of years. Current dilution refrigeration systems rely on
filters or "cold traps" to remove contaminants from the helium in
the helium circuit.
[0023] Most cold traps available today employ cryoadsorption to
essentially extract contaminants from the helium as it flows
through the cold trap. Systems and methods for cryoadsorptive cold
trapping are known in the art and would be understood by a person
of skill in the art. In brief, a cryoadsorptive cold trap comprises
a large volume (i.e., the "trap") having an input port and an
output port. Tubing of the helium circuit is connected to these
ports such that the internal volume of the trap is part of the
helium circuit. The trap is cooled to a cryogenic temperature,
typically by immersion in a liquid cryogen such as liquid nitrogen.
The trap is at least partially filled with a cryoadsorptive
material, such as charcoal, activated charcoal, or zeolite. When
the cryoadsorptive material is cooled to a sufficiently cold
temperature (by thermal coupling to, e.g., the liquid cryogen bath;
"sufficiently cold" depends on the specific material being
employed) the cryoadsorptive material will adsorb certain
substances from its environment. The cryoadsorptive material may be
thought of as a sort of "sponge" that soaks up certain materials in
its environment and allows other materials to pass through. Whether
any given material will be "soaked up" or "pass through" the
cryoadsorptive material depends, at least in part, on the
temperature of the cryoadsorptive material. Helium can typically
only be significantly adsorbed at very cold temperatures (i.e.,
colder than most other substances), thus, a cryoadsorptive material
may be cooled to a temperature at which it does not adsorb helium
itself but does adsorb contaminants that may be present in the
helium. This is the basis for most modern cold traps.
[0024] There are many potential sources of performance degradation
in cryoadsorptive cold traps. For example, the liquid nitrogen bath
employed to cool the trap may continually boil away nitrogen,
causing the level of liquid nitrogen surrounding the trap to fall
and reductions/variations in the cooling of the cryoadsorptive
material. Cold traps of this form require regular replenishment of
liquid nitrogen. Furthermore, many cryoadsorptive materials (such
as charcoal) are very poor thermal conductors and not easily
thermalized to the temperature of the liquid nitrogen immersion
bath. Thus, even if a trap is immersed in liquid nitrogen, there is
no guarantee that the cryoadsorptive material within the trap is
sufficiently cooled to provide the desired trapping performance.
Furthermore, cryoadsorptive cold traps are not designed to
accommodate phase changes of contaminants therein. Solidification
of contaminants of cold surface of cryoadsorptive material can
influence the flow of helium through the trap and can provide "low
resistance channels" through which contaminants can flow without
being adsorbed.
[0025] A further limitation of modern dilution refrigerator/cold
trap designs is that they are typically designed as "one trap for
all contaminants."
[0026] In accordance with the present systems and methods, the
performance of a cold trap may be improved by harnessing the
effects of cryocondensation as opposed to cryoadsorption and/or by
implementing multiple "contaminant-specific" traps each at a
specific temperature in the helium circuit.
[0027] Cryocondensation is a physical phenomenon whereby molecules
of gas encounter a very cold surface and freeze to it.
Cryocondensation is, in effect, a mechanism by which blockages and
plugging by contaminants may occur within the helium circuit of a
dilution refrigerator (as described above). In accordance with the
present systems and methods, this mechanism may be used to
deliberately trap contaminants in dedicated regions of the helium
circuit such that the contaminants do not form blockages that plug
the circuit.
[0028] A "cryocondensation trap" may be similar to a cryoadsorption
trap in that it employs a trapping volume connected in series with
the helium circuit via an input port and an output port. However, a
cryocondensation trap may not employ a cryoadsorptive material to
adsorb contaminants. Instead, a cryocondensation trap may employ a
material having a high thermal conductivity and a high specific
heat, such as a metal (e.g., copper, stainless steel, silver
sinter, brass, bronze, aluminum, etc.) or other material, such as
alumina silicate, clay, glass wool, etc. For example, the trapping
volume may be formed of such a material and the inner walls of the
trapping volume may function as cryocondensation surfaces. In
practice, it may be advantageous to provide a large surface area of
cryocondensation material. To this end, the inner surface of the
trap volume may be rifled, corrugated, textured, finned, etc. or
alternatively the inner volume of the trap may include a sintered
metal, a screen, a mesh, a wool, or other "perforated" formation
that provides a high contact surface area for the helium. Ideally,
the cryocondensation material will have a binding energy that
matches the contaminant(s) to be trapped but that does not match
helium so as to minimize the trapping of helium. The residency of
molecules on the cryocondensation surface(s) should be long, e.g.,
on the order of years.
[0029] As described previously, certain contaminants may
cryocondense and/or cryoadsorb at a first temperature range and
other contaminants may cryocondense and/or cryoadsorb at a second
(different) temperature range. For example, water, carbon dioxide,
and most hydrocarbons may cryocondense/cryoadsorb at around 77K.
Thus, in accordance with the present systems and methods, it may be
advantageous to implement a first cold trap at or below about 77K.
This first cold trap may employ cryoadsorption or cryocondensation,
or a combination of cryoadsorption and cryocondensation. However,
nitrogen, oxygen, and argon may cryoadsorb/cryocondense at around
20K. Thus, in accordance with the present systems and methods, it
may be advantageous to implement a second cold trap at or below
about 20K. This second cold trap may employ cryoadsorption or
cryocondensation, or a combination of cryoadsorption and
cryocondensation. Here it is noted that a single cold trap at 77K
may not be sufficient to trap nitrogen, oxygen and argon, whereas a
single cold trap at 20K may be forced to trap too many contaminants
and quickly become plugged with water, carbon dioxide,
hydrocarbons, nitrogen, oxygen, and argon. In general, a trap that
is designed to trap a large number/volume of contaminants must
employ a correspondingly large trapping volume to prevent becoming
plugged. Furthermore, neon and hydrogen may cryoadsorb/cryocondense
at or below about 5K. Thus, in accordance with the present systems
and methods, it may be advantageous to implement a third cold trap
at or below about 5K. This third cold trap may employ
cryoadsorption or cryocondensation, or a combination of
cryoadsorption and cryocondensation. Particular care must be taken
for a cold trap operating at .about.5K to minimize trapping of
helium. For example, a cold trap operating at .about.5K may employ
simple structure, such as a copper tube, to minimize trapping of
helium.
[0030] In accordance with the present systems and methods, each
respective cold trap in a multi-cold trap system may be
individually cooled to operation temperature. Some cold traps, such
as zeolite adsorption traps for trapping water, may be operated at
room temperature. A 77K trap for trapping, e.g., water, carbon
dioxide, and most hydrocarbons may be cooled by immersion in liquid
nitrogen, or it may be cooled by thermal coupling to a 77K stage of
a cryocooler, such as a pulse tube cryocooler. A 20K trap may for
trapping, e.g., nitrogen, oxygen, and argon may be cooled by, e.g.,
thermal coupling to a 20K stage of a cryocooler, such as a pulse
tube cryocooler. A 5K cold trap for trapping, e.g., hydrogen and
neon, may be cooled by, e.g., thermal coupling to a 5K stage of a
cryocooler such as a pulse tube cryocooler. In some
implementations, at least one cold trap may be thermally coupled to
a cryocooler that is dedicated to the cooling of cold traps. For
example, any or all of the 77K trap, the 20K trap, and the 5K trap
described above (or any other trap at any other temperature) may be
thermally coupled to the corresponding temperature stage of a
single pulse tube cryocooler. In implementations where the dilution
refrigerator is itself cooled by a pulse tube cryocooler (i.e., for
a "pulse tube dilution refrigerator) the same pulse tube that is
used to cool the dilution refrigerator may be used to cool any or
all of the 77K trap, the 20K, trap, the 5K trap described above, or
any other trap at any other temperature. Some implementations may
employ at least two pulse tubes, with a first pulse tube used to
cool the dilution refrigerator and a second pulse tube used to cool
at least one cold trap. If desirable, the first pulse tube may also
be used to cool at least one cold trap.
[0031] FIG. 1 is a schematic diagram of an exemplary dilution
refrigeration system 100 employing improved trap designs 110-130 in
accordance with the present systems and methods. Dilution
refrigeration system 100 includes dilution refrigerator 101, which
is background cooled by a pulse tube cryocooler 102 (thermal
coupling between dilution refrigerator 101 and the cold head of
pulse tube 102 is not shown in the Figure to reduce clutter).
Dilution refrigeration system 100 also includes helium fluid
circuit 103 (comprising fluid channels and tubing for helium flow)
which passes through a series of three traps: absorptive trap 110,
cryocondensation cold trap 120, and cryocondensation cold trap 130.
As illustrated, absorptive trap 110 is operated at room
temperature; however, in alternative systems absorptive cold trap
110 may be cooled by immersion in a liquid cryogen bath (e.g.,
liquid nitrogen) or by thermal coupling to a pulse tube cryocooler,
such as pulse tube 102 or a second pulse tube separate from pulse
tube 102. Adsorptive trap 110 may include, e.g., zeolite or
activated charcoal for trapping a specific species or set of
species of contaminants, such as water and/or hydrocarbons.
Adsorptive trap 110 may employ a desiccant material or a molecular
sieve. Absorptive trap 110 is coupled in series with
cryocondensation trap 120 in helium circuit 103. Cryocondensation
trap 120 is thermally coupled to pulse tube 102 and is formed by a
segment of tubing in helium circuit 103 that is serpentine (to
enhance surface area) and formed of a material having high thermal
conductivity and high specific heat (such as, e.g., copper).
Cryocondensation trap 120 may be thermally coupled to pulse tube
102 at a particular temperature (e.g., 20K) that specifically
cryocondenses a certain species or set of species of contaminants
(e.g., nitrogen, oxygen, and argon). Trap 120 may also cryocondense
contaminants that have made it through trap 110, such as water
and/or hydrocarbons. Cryocondensation trap 120 is coupled in series
with cryocondensation trap 130 in helium circuit 103.
Cryocondensation trap 130 is thermally coupled to pulse tube 102
and is formed by a segment of tubing that has a finned or
corrugated inner surface (to enhance surface area) and formed of a
material having high thermal conductivity and high specific heat
(such as, e.g., metal or clay). Cryocondensation trap 130 may be
thermally coupled to pulse tube 102 at a particular temperature
(e.g., 5K) that specifically cryocondenses a certain species or set
of species of contaminants (e.g., hydrogen and neon). Trap 130 may
also cryocondense contaminants that have made it through trap 110
and 120, such as water, hydrocarbons, nitrogen, oxygen, and/or
argon. In some implementations, it may be advantageous for the
helium circuit 103 to return to a higher temperature in between
serial traps 120 and 130. Such may allow by-pass valves, etc., to
be inserted in the helium circuit so that, e.g., a plugged trap may
be isolated and potentially even serviced without warming up the
entire fridge.
[0032] In some implementations, it may be desirable to couple at
least two traps to the same temperature so as to provide redundant
trapping mechanisms for contaminants that may be trapped at that
temperature.
[0033] Dilution refrigeration system 100 also include vacuum can
104 which contains dilution refrigerator 101, pulse tune 102, and
traps 120 and 130. Since traps 120 and 130 are contained within the
vacuum can 104 that houses dilution refrigerator 101, traps 120 and
130 may be referred to as "internal cold traps" in system 100.
Since trap 110 is located outside of vacuum can 104, trap 110 may
be referred to as an "external trap."
[0034] A further aspect of adsorption traps is that the "sponging"
type mechanism by which they operate inevitably results in the
adsorptive material becoming saturated such that it can no longer
adsorbs contaminants. When this happens, the trap ceases to remove
further contaminants from the helium and the fridge may become
plugged. To avoid such plugging, adsorptive traps need to be
regenerated. Regeneration of an adsorptive trap involves heating
the adsorptive material until the contaminants are released.
Accordingly, adsorptive traps typically include a vent port through
which contaminants may be released during regeneration. The
timeframe within which an adsorption trap needs to be regenerated
(i.e., the regeneration cycle of the trap) depends on many factors,
including the adsorptive material used, the size of the trap, and
the type and quantity of contaminants being adsorbed. In accordance
with the present systems and methods, if the regeneration cycle of
an adsorptive trap is less than the desired cooling cycle of the
dilution refrigeration system, then multiple adsorptive traps can
be coupled in parallel in the helium circuit such that at least one
adsorptive trap is active at all times while the other trap(s)
is/are regenerating. In such instances, each trap in a set of
parallel traps may each be operated at the same temperature, and if
that temperature is colder than room temperature then each trap in
a set of parallel traps may be thermally coupleable to the same
cold source (e.g., the same pulse tube cryocooler) via a respective
thermal switch such that any actively adsorbing trap is thermally
coupled to the cold source and any regenerating trap is thermally
decoupled from the cold source.
[0035] In general, adsorber regeneration is improved as the
regeneration temperature is increased. However, if an adsorber had
adsorbed hydrocarbons it may be preferable to regenerate only at a
temperature that is sufficient to release other contaminants (e.g.,
water) and not the hydrocarbons themselves, since releasing
hydrocarbons may require regenerating at a temperature that is so
high that is may damage other trap components (or the adsorber
material itself). An adsorption trap that has become saturated with
hydrocarbons may be "regenerated" by replacing the adsorption
material rather than by baking the trap.
[0036] A further limitation of modern cold trap designs is that
they provide little or no mechanism for clearly monitoring the
performance of the trap. Most dilution refrigerator diagnostics are
based on the condensing pressure in the fridge, which provides
indirect information about helium flow and overall fridge health.
In accordance with the present systems and methods, the performance
of cold trap systems may be enhanced by building active gauges into
the trap itself, and/or into the helium circuit immediately before
and after the trap. Examples of the types of gauges that may be
employed include flowmeters, pressure gauges, and contaminant
sensors such as water sensors, nitrogen sensors, and the like. It
is also advantageous to automate the operation of traps as much as
possible by providing, e.g., remotely controlled sensors, gauges,
and/or valves and switches, etc., as needed.
[0037] The present systems and methods provide individual cold trap
designs and systems of multiple cold traps coupled in series and/or
parallel in order to enhance the removal of contaminants from the
helium circuit in a dilution refrigerator and extend the operating
time of the dilution refrigerator. However, similar effects may be
achieved by redesigning portions of the helium circuit that are
prone to blocking (e.g., narrow tubes, right angles or otherwise
unfavorable geometries, etc.) and/or by minimizing air/contaminant
ingress into the helium circuit. An example of how to minimize air
ingress into the helium circuit is to replace all elastomer
seals/gaskets (e.g., O-rings, etc.) with metal seals (e.g., solder)
or other seal types having lower permeability.
[0038] In some implementations, higher temperature traps may be
thermally coupled to the regenerator of a pulse tube to take
advantage of available cooling power that is otherwise unused.
[0039] Most adsorption traps typically seen in the art employ a
single mass of adsorption material contained in a large reservoir
volume. This inevitably results in the formation of preferential
flow paths through the adsorption material such that only a
fraction of the adsorptive surface is actually encountered by the
flowing gas (e.g., helium). Also, since adsorptive materials (e.g.,
charcoal) are typically not good thermal conductors (more like
insulators), a large mass of adsorption material typically does not
thermalize well throughout.
[0040] In accordance with the present systems and methods, it may
be advantageous to design an adsorptive trap so that the flowing
gas is forced to encounter more of the adsorptive material (i.e.,
as much as possible of the adsorptive material, or at least more
than what is achieved in conventional traps). It may also be
advantageous to ensure that more of the adsorptive material (again,
as much as possible of the adsorptive material, or at least more
than what is achieved in conventional traps) is thermalized to the
desired temperature.
[0041] FIG. 2 is a schematic diagram of a cryogenic trapping system
200 in accordance with the present systems and methods. Cryogenic
trapping system 200 includes cryoadsorptive trap 210, in which the
large reservoir volume of typical adsorption traps is replaced by a
circuitous route (either using a single serpentine tube or by
creating staggered openings as illustrated). The circuitous route
(a portion of which is indicated by the arrows in trap 210)
increases the effective surface area of the adsorptive material
211, meaning that the flowing gas encounters more adsorptive
material and there is better coupling between the adsorptive
material and thermalization surfaces, e.g., thermalization surface
212. The flowing gas must pass over/through a greater proportion of
adsorbing material 211 than in typical single-mass traps, and
adsorptive material 211 is also generally more effective because it
is uniformly colder. The circuitous route also serves to increase
the residency time in the trapping volume of adsorption trap 210.
As a result, there is better coupling between flowing gas and
adsorptive material 211 because of at least: i) the increased
surface area (i.e., effective quantity of) of adsorptive material
211; ii) adsorptive material 211 is better thermalized (via
thermalization surfaces 212) and so adsorbs more efficiently; and
iii) the circuitous route of the flow path forces the gas to pass
through/explore more of the reservoir volume, thus keeping the gas
in proximity to adsorptive material 211 for a longer time. Any or
all of these features can be exploited in alternative trap designs,
where the design shown in trap 210 of FIG. 2 is just an example of
one design in accordance with the present systems and methods.
[0042] Cryogenic trapping system 200 also includes cryocondensation
trap 220 that has a high surface area or condensation material 221.
Condensation trap 220 effectively replaces a portion of tubing that
is normally present in dilution refrigeration systems. The tubing
that is normally used is this part of the helium circuit is
typically very narrow in diameter and it only takes a relatively
small amount of contaminant to form a blockage therein.
Condensation trap 220, this narrow tubing is replaced with large
diameter tubing 222 that is at least partially filled with high
surface area condensation material 221 such as metal/glass
mesh/foam. Tubing 222 may be thermalized to one specific
temperature, or have dedicated sections thermalized to specific
temperatures, or be thermalized to multiple temperatures to
establish a temperature gradient over the trapping surface.
Providing trapping surfaces at multiple temperatures (or over a
gradient of temperatures) may help ensure multiple contaminant
material are captured. The combination of increased volume in
tubing 222 and added condensation surface area (221) within the
tubing may be designed to have a low net effect on the impedance of
this tubing section in the fridge.
[0043] Cryoadsorption trap 210 and cryocondensation trap 220 of
cryogenic trapping system 200 may be used to form a "fine" trap at
or below the 1st pulse tube stage and down to the 2nd pulse tube
stage in a PTDR (both are illustrated as being thermalized to the
first stage of pulse tube 240 in FIG. 2). A fine" trap may operate
a very cold temperatures where contaminants such as Ne and H may be
trapped. Such contaminants represent a relatively low percentage of
contaminants in the flowing gas. A "coarser" trapping subsystem may
be used to remove higher-percentage contaminants from the flowing
gas at higher temperature. For example, cryogenic trapping system
200 also includes coarse trapping subsystem 230 in which at least
two parallel trapping subsystems 231, 232 are coupled to a higher
temperature point in the fridge, such as the 1st pulse tube stage.
The two parallel trapping subsystems 231, 232 may be similar to
and/or include portions of trap 210 and/or trap 220, and/or may
employ other cryoadsorption and/or cryocondensation trapping
designs. Parallel trapping subsystems 231,232 may be configured so
that one may be active while the other regenerates, and their
relative roles may be controlled by switches (e.g., heat switches
251 and 252 that respectively provide controllable thermal coupling
between trapping subsystems 231, 232 and the 1.sup.st stage of
pulse tube 240) and valves (e.g., valves 261 and 262). The
operation of parallel cryogenic devices (wherein one device is
active and thermally coupled to a cold source while the other
device regenerates and is thermally decoupled from the cold
surface) would be understood be one of skill in the art. Parallel
trapping subsystems 231 and 232 each provide "coarse" filtering of
the flowing gas to remove, e.g., H.sub.2O, CO.sub.2, N.sub.2,
O.sub.2, oils, etc. that can be cryoadsorbed/cryocondensed at
relatively warmer temperatures (i.e., warmer than the temperatures
at which Ne and H cryoadsorb/cryocondense) so that these more
prevalent contaminants do not make it through to the "fine" traps
below (i.e., traps 210 and 220). While one of parallel traps 231,
232 is operating, the other may be regenerated (by valving off from
the system via the corresponding one of valves 261, 262; decoupling
from the cooling source via the corresponding one of heat switches
251, 252; and applying heat to the regenerating trap) and the
contaminants that were trapped may be vented out of the system via
a vent port (not shown to reduce clutter).
[0044] The above description of illustrated embodiments, including
what is described in the Abstract, is not intended to be exhaustive
or to limit the embodiments to the precise forms disclosed.
Although specific embodiments of and examples are described herein
for illustrative purposes, various equivalent modifications can be
made without departing from the spirit and scope of the disclosure,
as will be recognized by those skilled in the relevant art. The
teachings provided herein of the various embodiments can be applied
to other methods of quantum computation, not necessarily the
exemplary methods for quantum computation generally described
above.
[0045] The various embodiments described above can be combined to
provide further embodiments. All of the U.S. patents, U.S. patent
application publications, U.S. patent applications, International
(PCT) patent applications referred to in this specification and/or
listed in the Application Data Sheet are incorporated herein by
reference, in their entirety including U.S. No. 61/729,250 filed
Nov. 21, 2012 and U.S. No. 61/736,456 filed Dec. 12, 2012. Aspects
of the embodiments can be modified, if necessary, to employ
systems, circuits and concepts of the various patents, applications
and publications to provide yet further embodiments.
[0046] These and other changes can be made to the embodiments in
light of the above-detailed description. In general, in the
following claims, the terms used should not be construed to limit
the claims to the specific embodiments disclosed in the
specification and the claims, but should be construed to include
all possible embodiments along with the full scope of equivalents
to which such claims are entitled. Accordingly, the claims are not
limited by the disclosure.
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