U.S. patent application number 11/255031 was filed with the patent office on 2006-05-18 for apparatus for cryosubstitution or low-temperature substitution.
This patent application is currently assigned to Leica Mikrosysteme GmbH. Invention is credited to Anton Lang, Reinhard Lihl, Paul Wurzinger.
Application Number | 20060101832 11/255031 |
Document ID | / |
Family ID | 36313637 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060101832 |
Kind Code |
A1 |
Wurzinger; Paul ; et
al. |
May 18, 2006 |
Apparatus for cryosubstitution or low-temperature substitution
Abstract
An apparatus for cryosubstitution or low-temperature
substitution is disclosed. The apparatus encompasses a Dewar vessel
(1) that is embodied with a neck. A chamber (5) for reception of at
least one specimen is inserted in the neck (5.sub.3). The chamber
(5) is embodied with a heavy base (5.sub.1) that is connected to a
first thermal conduction rod (7). A platform (8) is provided at the
end of the first thermal conduction rod (7) facing away from the
base (5.sub.1). An insulator (12) is provided above the platform
(8) of the first thermal conduction rod (7).
Inventors: |
Wurzinger; Paul;
(Deutsch-Wagram, AT) ; Lihl; Reinhard; (Vienna,
AT) ; Lang; Anton; (Vienna, AT) |
Correspondence
Address: |
Robert P. Simpson, Esq.;Simpson & Simpson, PLLC
5555 Main Street
Williamsville
NY
14221-5406
US
|
Assignee: |
Leica Mikrosysteme GmbH
Vienna
AT
|
Family ID: |
36313637 |
Appl. No.: |
11/255031 |
Filed: |
October 19, 2005 |
Current U.S.
Class: |
62/51.1 |
Current CPC
Class: |
A01N 1/02 20130101; F25D
19/006 20130101; A01N 1/0257 20130101; F25B 2700/04 20130101; F25D
3/105 20130101 |
Class at
Publication: |
062/051.1 |
International
Class: |
F25B 19/00 20060101
F25B019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2004 |
DE |
102004055148.0-13 |
Claims
1. An apparatus for cryosubstitution or low-temperature
substitution comprises: a Dewar vessel that is embodied with a neck
and is filled with a liquid coolant, a chamber for reception of at
least one specimen wherein the chamber is inserted in the neck, a
heavy base is provided to the chamber wherein the heavy base of the
chamber is connected to a first thermal conduction rod that is
connected, at the end facing away from the base to a platform; and
the thermal conduction rod is equipped, above the platform, with an
insulator.
2. The apparatus according to claim 1, wherein the liquid coolant
is liquid nitrogen.
3. The apparatus according to claim 1, wherein the first thermal
conduction rod and the platform are embodied integrally.
4. The apparatus according to claim 1, wherein a second thermal
conduction rod is connected to the first thermal conduction
rod.
5. The apparatus according to claim 4 wherein the second thermal
conduction rod is connected at one end to a platform; the end of
the second thermal conduction rod located opposite to the platform
is connected to the platform of the first thermal conduction rod;
and the platform is directed toward a receptacle embodied at the
base of the Dewar vessel.
6. The apparatus according to claim 5, wherein the platform of the
second thermal conduction rod is in contact with the
receptacle.
7. The apparatus according to claim 5, wherein the platform of the
second thermal conduction rod is pot-shaped and fits around the
receptacle.
8. The apparatus according to claim 4, wherein the second thermal
conduction rod is surrounded, above the platform, by an
insulator.
9. The apparatus according to claim 1, wherein at least one heating
element and at least one temperature sensor are recessed into the
base of the chamber; and the heating element and temperature sensor
are connected to an electronic control system.
10. The apparatus according to claim 9, wherein the base of the
chamber is connectable to an annular plate; the at least one
heating element is recessed into this plate, the at least one
temperature sensor being recessed into the base of the chamber or
into the annular plate.
11. The apparatus according to claim 9, wherein the temperature
sensor is embodied as a thermocouple or as a resistance temperature
sensor; and its temperature signal serves as a controlled variable
for the electronic control system in order to control the chamber
temperature.
12. The apparatus according to claim 1, wherein a further heating
element, which preferably is electrically operated, is immersed in
the liquid nitrogen in order to vaporize additional liquid nitrogen
so that the cold gas cools the platform of the first cooling
rod.
13. The apparatus according to claim 1, wherein at least one sensor
is provided which ascertains the fill level of the liquid nitrogen
in the Dewar vessel.
14. The apparatus according to claim 13, wherein the at least one
sensor is a temperature sensor that is connected to the electronic
control system in order to measure the fill level.
15. The apparatus according to claim 14, wherein several
temperature sensors are provided which are arranged along the first
and the second thermal conduction rod.
16. The apparatus according to claim 5, wherein the liquid coolant
is liquid nitrogen and at least one of the insulators of the first
or the second thermal conduction rod is embodied as a tube that is
immovably adhesively bonded to the corresponding platform so that
the thermal conduction rod is insulated with respect to the liquid
nitrogen.
17. The apparatus according to claim 16, wherein a pump is provided
which pumps the liquid nitrogen into at least one of the tubes that
surround the first and/or the second thermal conduction rod.
18. The apparatus according to claim 17, wherein the pump is
embodied as a membrane pump having a membrane and ball valves.
19. The apparatus according to claim 18, wherein the pump is
divided, so that the membrane is arranged outside the Dewar vessel
and the ball valves provided in a valve head are arranged inside
the Dewar vessel.
20. The apparatus according to claim 1, wherein the insulator that
is not embodied as a tube is an insulating foam.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of the German patent
application 10 2004 055 148.0, filed on Nov. 16, 2004 which is
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The invention concerns an apparatus for cryosubstitution or
low-temperature substitution. The invention concerns in particular
an apparatus for cryosubstitution or low-temperature substitution
in which the apparatus encompasses a Dewar vessel. The Dewar vessel
is embodied with a neck, and is filled with a liquid coolant. A
chamber for reception of a specimen is inserted in the neck, this
chamber being embodied with a heavy base.
BACKGROUND OF THE INVENTION
[0003] Patent application WO 94/05995 discloses an apparatus for
dewatering and/or embedding of preferably frozen specimens. The
apparatus encompasses a Dewar vessel filled with liquid nitrogen
and a metallic element, anchored to the base of the Dewar vessel,
which is made of highly thermally conductive material. The metallic
element possesses at its upper end, in the attachment region of the
Dewar neck, a cover having a metallic cooling surface. The cooling
surface is connected to the complementarily embodied lower contact
surfaces of the thermostatically heated substitution (PLT)
containers and of the lower part of a freeze-drying chamber, in
such a way that good thermal conduction between the corresponding
surfaces is ensured. The metallic element is embodied in the form
of a thermally conductive tube. The highly thermally conductive
tube requires a large wall thickness for the necessary high thermal
conduction. It therefore possesses a large mass, and is anchored to
a base element for support. A cryosubstitution unit is operated,
depending on the process, in a very wide temperature range from
-140.degree. C. to +70.degree. C. As described in WO 94/05995, a
highly thermally conductive coupling to the cooling surface at low
temperatures is necessary. For the high temperatures, however, it
brings about a large heat flow from the chamber into the Dewar
vessel, and therefore high nitrogen consumption. Different coupling
tubes are therefore used for different temperature ranges. This
method thus has the disadvantage, however, that the coupling tubes
must be exchanged by the user in accordance with the process
temperature that is set. On the one hand this can result in
operator errors, and on the other hand the automatic control system
must be assisted by manual interventions for optimum functionality,
which is disadvantageous for an automatic control system.
[0004] A Dewar vessel according to the existing art is depicted in
the JEOL brochure. The cooling apparatus substantially comprises a
chamber or holding apparatus for the specimens, which is lowered
into the cold nitrogen gas in a Dewar vessel filled with liquid
nitrogen. The holding apparatus hangs from a mounting element that
can be adjusted for the desired lowering depth using a locking
element. Cooling is accomplished in this case by direct heat
exchange with the gas or with the liquid nitrogen. In the case of
the existing art depicted in FIG. 1, only relatively high
temperatures can be attained as long as the specimen is sitting at
a depth in the Dewar vessel that is accessible for manipulation
(e.g. replacement of the substitution medium). The process
temperature can be reduced by further lowering into the Dewar
vessel, but the specimens must always be lifted up for
manipulations. As a result, contamination with moisture from the
ambient air can more easily occur.
[0005] The brochure for the Leica EM AFS discloses a unit according
to the existing art. A Dewar vessel is filled with liquid nitrogen,
the Dewar neck having a chamber that can be brought to a specific
temperature. The temperature range extends from -140.degree. C. to
+65.degree. C. The desired temperature is set via a control circuit
and built-in heating elements. Level sensors for the liquid
nitrogen are additionally mounted in the Dewar vessel, and indicate
to the user the fill level of liquid nitrogen in the Dewar
vessel.
SUMMARY OF THE INVENTION
[0006] It is therefore the object of the present invention to
create an apparatus for cryosubstitution or low-temperature
substitution that can establish all temperatures in the temperature
range from -140.degree. C. to +70.degree. C. without intervention
by a user, and with no need for the user to modify elements of the
Dewar vessel.
[0007] The aforesaid object is achieved by an apparatus for
cryosubstitution or low-temperature substitution that comprises: a
Dewar vessel that is embodied with a neck and is filled with a
liquid coolant, a chamber for reception of at least one specimen
wherein the chamber is inserted in the neck, a heavy base is
provided to the chamber wherein the heavy base of the chamber is
connected to a first thermal conduction rod that is connected, at
the end facing away from the base to a platform; and the thermal
conduction rod is equipped, above the platform, with an
insulator.
[0008] The apparatus for cryosubstitution or low-temperature
substitution has the advantage that it encompasses a Dewar vessel
that is embodied with a neck. The Dewar vessel is filled with a
liquid coolant that preferably is liquid nitrogen. A chamber for
the reception of at least one specimen is inserted in the neck of
the Dewar vessel. The chamber is pot-shaped and possesses a heavy
base. The base of the chamber is connected to a thermal conduction
rod that is connected, at the end facing away from the base, to a
platform. The thermal conduction rod is equipped, above the
platform, with an insulator. It is particularly advantageous if the
thermal conduction rod and the platform are embodied integrally. In
addition, the first thermal conduction rod can be connected to a
second thermal conduction rod, the second thermal conduction rod
possessing a platform at one end. The second thermal conduction rod
is connected, with its end located opposite to the platform, to the
first thermal conduction rod. The platform of the second thermal
conduction rod is directed toward a receptacle embodied at the base
of the Dewar vessel.
[0009] In a preferred embodiment, the platform of the second
thermal conduction rod is in contact with the receptacle. It has
proven particularly advantageous, for the stability of the
arrangement of the first and the second thermal conduction rod in
the Dewar vessel, if the platform of the second thermal conduction
rod is pot-shaped and fits around the receptacle. The second
cooling rod is likewise equipped, above the platform, with an
insulator.
[0010] At least one heating element and at least one temperature
sensor are recessed into the base of the chamber, the heating
element and temperature sensor being connected to an electronic
control system.
[0011] The base of the chamber can likewise be connected to an
annular plate, the at least one heating element being recessed into
this plate, and the at least one temperature sensor being recessed
into the base of the chamber or into the annular plate. The
temperature sensor is embodied as a thermocouple or as a resistance
temperature sensor. The temperature signal of the temperature
sensor serves as a controlled variable for the electronic control
system in order to control the chamber temperature.
[0012] It is also advantageous if a further heating element, which
preferably is electronically operated, is immersed in the liquid
nitrogen. Liquid nitrogen is additionally vaporized during
operation by way of this further heating element, so that the cold
gas cools the platform of the first cooling rod.
[0013] At least one sensor is provided in order to ascertain the
liquid level of the liquid nitrogen in the Dewar vessel. At least
one sensor is a temperature sensor that is connected to the
electronic control system in order to measure the fill level. It is
particularly advantageous if several temperature sensors are
arranged along the first and the second thermal conduction rod in
order thereby to ascertain the fill level of the liquid nitrogen in
the Dewar vessel.
[0014] It is additionally advantageous if at least one of the
insulators of the first or the second thermal conduction rod is
embodied as a tube. This tube is immovably, adhesively bonded to
the corresponding platform, so that the thermal conduction rod is
insulated with respect to the liquid nitrogen. A pump is provided
which pumps the liquid nitrogen into at least one of the tubes that
surround the first and/or the second thermal conduction rod. The
pump can be embodied as a membrane pump having a ball valve.
[0015] Further advantages and advantageous embodiments of the
invention may be inferred from the dependent claims, and are the
subject matter of the Figures below and the descriptions
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] In the individual drawings:
[0017] FIG. 1 is a cross section through a Dewar vessel according
to the existing art;
[0018] FIG. 2 is a cross section through a Dewar vessel according
to a first embodiment of the invention;
[0019] FIG. 3 is a cross section through a Dewar vessel according
to a second embodiment of the invention;
[0020] FIG. 4 is a cross section through a Dewar vessel according
to a third embodiment of the invention; and
[0021] FIG. 5 is a cross section through a Dewar vessel according
to a fourth embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1 is a cross section through a Dewar vessel according
to the existing art. Dewar vessel 1 comprises substantially an
outer container 1.sub.1 and an inner container 1.sub.2. Inner
container 1.sub.2 is insulated with respect to outer container
1.sub.1. Inner container 1.sub.2 holds liquid nitrogen for cooling.
A neck 1.sub.3 of Dewar vessel 1 extends from inner container
1.sub.2 to outer container 1.sub.1. A chamber 5 can be introduced
into inner container 1.sub.2. Specimens 2 can be inserted into
chamber 5 so that they are exposed to cryosubstitution and
low-temperature substitution. Chamber 5 is connected to a mounting
element 5.sub.1 that projects out of neck 1.sub.3 of Dewar vessel
1. The level of chamber 5 above liquid nitrogen 3 can be adjusted.
A locking element 60 is provided in order to immobilize the
position of chamber 5 above the level of liquid nitrogen 3. A
desired temperature can thus be set by lowering or raising chamber
5. Cooling is accomplished in this case by direct heat exchange
with the gas or with liquid nitrogen 3.
[0023] FIG. 2 is a cross section through a Dewar vessel 1 according
to a first embodiment of the invention. In the description that
follows, identical reference characters are used for identical
elements. The cooling apparatus shown in FIG. 2 serves for
cryosubstitution or low-temperature substitution of biological
and/or other water-containing specimens. As already mentioned in
the description referring to the existing art, the Dewar vessel
encompasses an inner container 1.sub.2 and an outer container
1.sub.1. The inner container is filled with a liquid coolant that
preferably is liquid nitrogen 3. A chamber 5 is inserted into neck
1.sub.3 of Dewar vessel 1. Chamber 5 is pot-shaped and possesses a
heavy base 5.sub.1. Chamber 5 is open toward the top and can be
closed off with a cover 6 for insulation with respect to ambient
temperature. Chamber 5 serves to receive multiple specimen
containers 2 in which specimens 30 for
cryosubstitution/low-temperature substitution are located. A first
thermal conduction rod 7 is joined to base 5.sub.1 of chamber 5. A
platform 8 is provided at the end of first thermal conduction rod 7
facing away from base 5.sub.1 of chamber 5. Platform 8 can be
detachably joined to first thermal conduction rod 7. It is
furthermore conceivable for first thermal conduction rod 7 and
platform 8 to be embodied integrally. Above platform 8, first
thermal conduction rod 7 is surrounded by an insulator 12.
Insulator 12 serves to insulate first thermal conduction rod 7
against liquid nitrogen 3 or cold nitrogen gas 3.sub.1. As a result
of insulator 12, the heat flux for cooling chamber 5 and base
5.sub.1 is directed principally through platform 8. The cooling
capacity can therefore advantageously be determined by modifying
the geometrical dimensions or by selecting a suitable material for
first thermal conduction rod 7. The temperature in chamber 5 can be
regulated by the operation of at least one heating element 14. Also
provided is at least one temperature sensor 15 that is used for
temperature measurement. Temperature sensor 15 can be embodied as a
thermocouple or resistance temperature sensor. The temperature
signal is used as feedback for an electronic control system 16 that
controls the temperature of chamber 5 by adapting the heating
output of heating element 14. The length of first thermal
conduction rod 7 is advantageously selected so that platform 8 is
immersed in liquid nitrogen 3 only above a certain fill level. With
a high fill level, platform 8 is immersed in liquid nitrogen 3, and
chamber 5 is coupled via first thermal conduction rod 7 directly to
liquid nitrogen 3. With a low fill level, platform 8 interacts with
cold nitrogen gas 3.sub.1. Cold nitrogen gas 3.sub.1 is heated by
the heat flux from chamber 5 into inner container 1.sub.2 of Dewar
vessel 1. By convection and by interaction with the walls of inner
container 1.sub.2, this heat is fed back into liquid nitrogen 3 and
results in an increase in the evaporation rate. A temperature
equilibrium that is largely independent of the present fill level
of liquid nitrogen 3 in inner container 1.sub.2 thus develops
between platform 8 and chamber 5. It is self-evident that the
thermal coupling between chamber 5 and liquid nitrogen 3 is much
greater at a high fill level than at a low fill level. Lower
temperatures can therefore be achieved in chamber 5 with a high
fill level. On the other hand, the liquid nitrogen consumption is
lower with a low fill level.
[0024] This arrangement is advantageous in that in standard
substitution processes, the lowest process temperatures
(-90.degree. C. and lower) are required at the beginning of the
processes. The temperature is raised in the course of the
substitution processes. Because liquid nitrogen 3 is also consumed
during the process, the cooling output achievable by way of first
thermal conduction rod 7 and platform 8 reflects the temperature
profile of the substitution process. At the same time, insulator 12
limits the coupling to liquid nitrogen 3 even with a high fill
level. Even in this situation, therefore, high temperatures can be
established without going beyond reasonable limits for liquid
nitrogen consumption and for the necessary heating output of
heating element 14.
[0025] FIG. 3 is a cross section through a Dewar vessel 1 according
to a second embodiment of the invention. Here a second thermal
conduction rod 9 is attached to first thermal conduction rod 7.
Second thermal conduction rod 9 once again possesses a platform 10
at one end. Second thermal conduction rod 9 is in contact, at the
end opposite platform 10 of second thermal conduction rod 9, with
platform 8 of first thermal conduction rod 7. First thermal
conduction rod 7 and second thermal conduction rod 9 are detachably
joined to one another. Inner container 1.sub.2 of Dewar vessel 1
possesses a receptacle 11 with which platform 10 of second thermal
conduction rod 9 is in contact. Advantageously, platform 10 of
second thermal conduction rod 9 is pot-shaped and is dimensioned so
that it fits around receptacle 11. This arrangement ensures secure
support in Dewar vessel 1 of the system made up of first thermal
conduction rod 7 and second thermal conduction rod 9. Like first
thermal conduction rod 7, second thermal conduction rod 9 is also
surrounded by an insulator 13 above platform 10. Insulator 13 thus
insulates second thermal conduction rod 9 against liquid nitrogen
3. Insulators 12 and 13 around first thermal conduction rod 7 and
second thermal conduction rod 9 are made from a suitable foam. It
is also conceivable for insulators 12 and 13 to be constituted by
tubes that are adhesively bonded to platforms 8 and 10, so that the
respective thermal conduction rod 7 and 9 is insulated against
liquid nitrogen 3. With the combination of first thermal conduction
rod 7 and second thermal conduction rod 9, platform 8 of the first
thermal conduction rod is positioned so that it is immersed in
liquid nitrogen 3 when the latter's fill level is high. The cooling
behavior in this case is identical to the cooling behavior as
described in the embodiment according to FIG. 2. Platform 10 of
second cooling rod 9 is in contact with receptacle 11 and is thus
positioned close to the bottom of inner container 1.sub.2 of Dewar
vessel 1. With a fill level below platform 8 of first thermal
conduction rod 7, second thermal conduction rod 9 thus represents a
permanent coupling between platform 8 of first thermal conduction
rod 7 and liquid nitrogen 3. This coupling to liquid nitrogen 3
acts in addition to the coupling via cold nitrogen gas 3.sub.1. The
intensity of the coupling can therefore easily be adapted to the
particular desired cooling parameters by way of the geometrical
dimensions or the thermal conductivity of second thermal conduction
rod 9.
[0026] The configuration of platform 10 of second thermal
conduction rod 9 is also advantageous. Because this platform 10 is
pot-shaped, a lateral support of first and second thermal
conduction rods 7 and 9 against receptacle 11 is therefore
achieved. This thus represents an immobilization against lateral
loads, especially during transport of the device. Because thermal
conduction rods 7 and 9 possess a relatively small mass, they need
not be fixedly joined to receptacle 11 in order to perform this
supporting function. The entire structure is therefore insensitive
to tolerances in the installation surface of the Dewar vessel. The
relatively small mass of thermal conduction rods 7 and 9 also
offers the advantage that much less liquid nitrogen is consumed for
cooling the apparatus when it is first filled.
[0027] FIG. 4 is a cross section through a Dewar vessel 1 according
to a third embodiment of the invention, depicting a further
capability for cooling chamber 5. In addition to heating elements
14 provided in base 5.sub.1 of chamber 5, an additional, preferably
electrically operated, heating element 17 is immersed in liquid
nitrogen 3. The heating of heating element 17 thus evaporates
additional liquid nitrogen 3, and the resulting cold nitrogen gas
3.sub.1 cools platform 8 of first thermal conduction rod 7 and
therefore also chamber 5. This cooling method is very easy to
control externally, and can therefore be used for more intense
short-term cooling. Electronic control system 16 provides the
corresponding open- and closed-loop control of the chamber
temperature. A further advantageous embodiment of the present
invention is also depicted in FIG. 4. At least one sensor 18 is
provided for measuring the fill level of liquid nitrogen 3 in inner
container 12. The at least one sensor 18 is configured in such a
way that it detects wetting by liquid nitrogen 18. Preferably,
several sensors 18 are arranged along first thermal conduction rod
7 and second thermal conduction rod 9. The present fill level is
therefore ascertained by way of these sensors 18, and on the one
hand can be used as a basis for the control system and on the other
hand can be made available as information to the user. The
measurement is once again advantageously made by an electronic
system that is integrated into electronic control system 16.
[0028] FIG. 5 is a cross section through the Dewar vessel according
to a fourth embodiment of the invention, some elements of the
apparatus having been omitted for reasons of clarity. Insulators 12
and 13 around first thermal conduction rod 7 and second thermal
conduction rod 9, respectively, are embodied here as tubes 20. Tube
20 around first thermal conduction rod 7 is immovably adhesively
bonded to platform 8, so that the liquid nitrogen is kept away from
first thermal conduction rod 7 with the exception of platform 8.
Second thermal conduction rod 9 is likewise surrounded by a tube 20
that is immovably adhesively bonded to platform 10 of second
thermal conduction rod 9. Liquid nitrogen 3 is likewise kept away
from second thermal conduction rod 9 with the exception of platform
10. The embodiment described in FIG. 5 creates the capability of
filling tube 20 around first thermal conduction rod 7 or second
thermal conduction rod 9 with liquid nitrogen 3, thus making
possible a strong coupling between liquid nitrogen 3 and chamber 5
regardless of the fill level of liquid nitrogen 3 in Dewar vessel
1. With this arrangement, a very low temperature for chamber 5 or a
very high cooling rate can be achieved. In the depiction of FIG. 5,
the tube around second cooling rod 9 is filled. It is self-evident,
however, that the tube around first cooling rod 7 can also be
filled. It is likewise conceivable for an insulator that is not
provided for filling with liquid nitrogen to be embodied, in the
form of an insulating foam, around first thermal conduction rod 7
or second thermal conduction rod 9. Filling of tube 20 around first
cooling rod 7 or second cooling rod 9 is accomplished with a pump
21 that is embodied as a membrane pump. It is likewise advantageous
if pump 21 is divided, so that the pressure fluctuations generated
by membranes positioned outside Dewar vessel 1 are transferred by
way of a hose or tube to the valve head positioned in the liquid
nitrogen. As a result of the directional effect of the valve head,
the pressure fluctuations are converted into a pumping motion. The
embodiment depicted in FIG. 5 also shows a further possibility for
arranging the at least one heating element 14 and the at least one
temperature sensor 15. For this purpose, base 5.sub.1 of chamber 5
is connectable to an annular plate 25. The at least one heating
element 14 is recessed into this annular plate 25. The at least one
temperature sensor 15 can be recessed into base 5.sub.1 of the
chamber or into annular plate 25.
* * * * *