U.S. patent number 3,668,453 [Application Number 05/051,706] was granted by the patent office on 1972-06-06 for electrical switch device having a fed liquid-metal cathode and a non-intercepting anode.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Ronald C. Knechtli, Kenneth T. Lian.
United States Patent |
3,668,453 |
Lian , et al. |
June 6, 1972 |
ELECTRICAL SWITCH DEVICE HAVING A FED LIQUID-METAL CATHODE AND A
NON-INTERCEPTING ANODE
Abstract
The electrical switch device has an envelope in which is mounted
a liquid-metal cathode, an anode, and a condenser. The cathode is
capable of very high electron-to-atom emission ratio. A desirable
value for the electron-to-atom emission ratio is on the order of
100 or more and is attainable by means of a cathode such as
disclosed in U.S. Pat. No. 3,475,636, when used in the switch
device. The condenser has a very much larger area than the exposed
liquid metal area on the cathode, and it is kept at a low enough
temperature to efficiently condense the liquid-metal vapor emitted
by the cathode. With mercury used as the liquid metal, the
condenser temperature is kept substantially below 0.degree. C.,
preferably at about -35.degree. C. which is just above the melting
point of mercury. When arcing occurs from the liquid metal, a
plasma jet of electrons, ions, and neutral particles is emitted
from the arc spot. The anode is mounted between the cathode and the
condenser, and it is positioned at the edge of the plasma jet to
capture the major portion of the electron flow for electrical
conduction. Most of the ions and neutral particles as well as a
sufficient number of electrons to preserve space-charge and current
neutrality, pass the anode in the plasma jet and are captured on
the condenser. The combination of the high electron-to-atom
emission ratio of the cathode with the large, low-temperature
condenser results in an equilibrium background pressure (i.e.,
pressure outside the plasma jet) of at least as low as
10.sup.-.sup.3 Torr during arcing, and lower than 10.sup.-.sup.4
Torr during non-arcing periods. These low pressures are obtained by
maintaining the condenser in the range of low temperatures defined
above. This low background pressure, in turn, permits the
essentially unperturbed propagation of the plasma jet between the
cathode and the surfaces upon which it impinges, i.e., condenser
and anode. Such a discharge mode is commonly referred to as a
"vacuum arc." The fact that the plasma jet is emitted only during
arcing, and that the pressure within the space surrounding this jet
is kept low, results in the ability to hold off electric fields up
to 50 kV per centimeter between anode and cathode immediately after
cessation of arcing.
Inventors: |
Lian; Kenneth T. (Thousand
Oaks, CA), Knechtli; Ronald C. (Woodland Hills, CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
21972875 |
Appl.
No.: |
05/051,706 |
Filed: |
July 1, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
720692 |
Apr 11, 1968 |
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Current U.S.
Class: |
313/7; 313/167;
313/34; 313/173 |
Current CPC
Class: |
H01J
13/04 (20130101); H01J 2893/0089 (20130101) |
Current International
Class: |
H01J
13/00 (20060101); H01J 13/04 (20060101); H01j
001/10 (); H01j 007/16 () |
Field of
Search: |
;313/7,29,33,34,163,167,173 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Demeo; Palmer C.
Parent Case Text
CROSS REFERENCE
This application is a continuation-in-part of U.S. Pat. application
Ser. No. 720,692, filed Apr. 11, 1968, now abandoned. This
application is related to U.S. Pat. application Ser. No. 51,868,
filed July 2, 1970.
Claims
1. An electrical switch device, said electrical switch device
comprising:
a vessel;
cathode means within said vessel for issuing electrons, ions, and
neutral particles, said cathode means having a high arc voltage
material wall;
means for feeding a low arc voltage metal in other than the solid
state to a position adjacent said wall so that low arc voltage
metal is on said wall;
anode means within said vessel for receiving electrons from said
cathode means;
condenser means having a condensing surface within said vessel for
receiving and condensing ions and neutrals, said condensing surface
of said condenser means (a) having a sufficiently large area with
respect to the low arc voltage metal on said wall of said cathode
means, (b) being positioned with respect to said cathode means and
anode means and (c) being maintained at a sufficiently low
temperature to cause the interior of said vessel to be maintained
at a sufficiently low pressure, and (d) said cathode means, anode
means, and condenser means being operated for causing an electric
discharge between said cathode means and said anode means in a
vacuum arc mode wherein an arc spot is formed on the low arc
voltage metal on said wall and a plasma cone is ejected from the
arc spot to issue directly toward said condenser means, the plasma
cone containing electrons, ions and neutral particles so that said
condenser means condensing substantially all of the neutral
particles and ions emitted from the arc spot so that said condenser
means maintains the background pressure within said vessel
sufficiently low that the atmosphere in said vessel does not
substantially interfere with the plasma cone so that discharge in a
vacuum arc mode is maintained;
said anode means being positioned between said cathode means and
said condenser means and having an anode wall defining an opening
therethrough, said anode wall being positioned substantially
tangent to the plasma cone to minimally perturb the ions and
neutral particles in the plasma cone and
2. The electrical switch device of claim 1 wherein said condenser
means is maintained below 0.degree. C. to maintain a pressure
within said vessel outside of the plasma cone at least as low as
10.sup..sup.-3 Torr during
3. The switch tube of claim 1 wherein said high arc voltage
material wall in said cathode is substantially conical, said
substantially conical cathode wall defining an axis, said anode
means being positioned substantially coaxially with respect to said
axis, said anode wall being substantially conical and being
positioned adjacent the plasma cone
4. The electrical switch device of claim 3 wherein said anode means
has an annular body having an interior frusto-conical surface, said
interior surface of said anode body having its apex substantially
at said high arc
5. An electrical switch device comprising:
a vessel;
condenser means having a condensation surface within said vessel
for condensing ions and neutral particles in said vessel to
maintain a sufficiently low background pressure within said vessel
during arcing that the atmosphere in said vessel does not
substantially interfere with arcing so that a conical plasma jet
containing electrons, ions, and neutral particles is formed, so
that electric arc discharge in a vacuum arc mode can be maintained,
said condensation surface being positioned in the path of the
conical plasma jet;
cathode means within said vessel for carrying a low arc voltage
metal in sufficiently small area to minimize evaporation of low arc
voltage metal from said cathode means into the atmosphere within
said vessel to permit said condenser means to maintain the
background pressure within said vessel during arcing sufficiently
low so that the plasma jet is an arc discharge in a vacuum arc
mode;
anode means within said vessel for interacting with said cathode
means by collecting electrons from the plasma jet so that an
electric arc discharge in a vacuum arc mode is maintained from the
low arc voltage metal on said cathode means to said anode means,
said anode means being formed with a conical anode wall defining an
opening therethrough, with said anode wall positioned substantially
tangent to the conical plasma jet so as to minimally perturb the
ions and neutrals in the plasma cone and so as to
6. The electrical switch device of claim 5 wherein:
said condenser means has a sufficiently large area with respect to
the low arc voltage metal area on said cathode means and being
positioned with respect to said cathode means and said anode means
for maintenance of the background pressure within said vessel
during arcing outside of the conical plasma jet, at least as low as
10.sup..sup.-3 Torr so that the arc occurs in the vacuum arc mode
and the atmosphere in said vessel does not
7. The switch device of claim 6 wherein said condenser means has
sufficiently large condenser surface area with respect to the low
arc voltage metal on said cathode means and is maintained at least
as low as about 263.degree. K. so that pressure within said vessel
outside of the conical plasma jet is not higher than about
10.sup..sup.-3 Torr during arcing and not higher than about
10.sup..sup.-4 Torr during non-arcing.
8. The electrical switch device of claim 7 wherein said cathode
means is operated so that the ratio of electrons to atoms within
said conical
9. The switching device of claim 5 wherein said surface on said
anode means comprises a truncated hollow cone with its apex
substantially at the lower
10. The process of switching an electric current comprising the
steps of:
maintaining a vessel in which is positioned a condenser surface of
sufficient size to dominate the vapor equilibrium in said vessel at
a reduced pressure by maintaining the condenser surface at a
reduced temperature;
feeding liquid metal to a cathode in the vessel for conducting
electric current by arcing;
cooling the condenser surface to a sufficiently reduced temperature
so that arcing which occurs between the cathode and an anode in the
vessel is in a vacuum arc mode;
discharging an arc between the anode and cathode which produces a
plasma jet containing electrons, ions and atoms with the ratio of
electrons to atoms being at least 30-1; and
directing the plasma jet through the anode toward the condenser so
that the anode lies substantially tangent to the plasma jet so that
the anode is directly electrically coupled with the plasma jet for
electron flow from the plasma jet to the anode.
Description
BACKGROUND OF THE INVENTION
This invention is in the field of mercury arc electrical devices
where an arc extends from a cathode to an anode, to permit electron
current flow therebetween.
Prior art devices which permit rectification and inversion by means
of a mercury arc are well known. These prior devices employ a large
mercury pool against which the arc strikes. However, the prior
devices are very limited in the amount of forward and reverse
voltage standoff between the anode and cathode because of the
presence of the large mercury pool. The pressure within the tube is
fairly high, because of the evaporation from the mercury pool, even
though the mercury pool temperature is kept as low as is possible
consistent with arcing. The relatively high mercury vapor pressure
in such tubes, when non-conducting, limits the permissible peak
forward and reverse voltage as well as the recovery and
de-ionization rates. An example of such structures is shown in
Steenbeck U.S. Pat. No. 2,205,231. To overcome this limitation,
state of the art high voltage mercury tubes are provided with
grading electrodes. These grading electrodes lead to another
limitation: the current which can pass to one anode through such a
set of grading electrodes is limited to such extent that for higher
currents, a number of parallel anodes and sets of grading
electrodes are required. These limitations thus lead to the need
for complex multi-anode tubes, with current dividing transformers
to divide the current uniformly between parallel anodes, and
grading electrodes with attendant voltage dividers.
The voltage holdoff properties of the conventional mercury pool
liquid cathode devices are determined by a tradeoff between the
desired voltage holdoff, and the peak current, the voltage drop
across the arc and voltage recovery rates. These conflicting
requirements do not permit the device to be designed for high
voltage holdoff and high current without the complex grading
electrodes and the multiple anodes mentioned above.
SUMMARY OF THE INVENTION
In order to aid in the understanding of this invention, it can be
stated in essentially summary form that it is directed to an
electrical switch device having a force fed liquid-metal cathode,
an anode and a condenser positioned within an enclosure. The
cathode has metal in other than a solid state fed thereto to
maintain a small pool or film of liquid metal for electrical
arcing. The condenser maintains the background pressure in the
vessel below 10.sup.-.sup.3 Torr during arcing so that arcing
occurs in the vacuum arc mode wherein neutral particles, electrons
and ions are expelled in a fairly well defined plasma cone from the
arc spot, at an electron-to-atom emission ratio on the order of 100
to 1. The condenser is positioned in the path of the plasma jet
cone to capture essentially the entire metal atom and ion efflux
jetting from the cathode spot. The anode is positioned intermediate
the cathode and condenser at the edge of the plasma jet cone to
capture the electron flow for electrical conduction between the
cathode and anode. In order to maintain the vacuum arc mode of
discharge, and in order to maintain high holdoff of electric fields
during nonarcing, when mercury is used as a liquid metal, the
condenser is maintained at a temperature below 0.degree. C,
preferably as low as about -35.degree. C. so as to provide an
envelope pressure of as low as about 5 .times. 10.sup..sup.-6 Torr
during nonarcing.
It is thus an object of this invention to provide an electrical
high voltage, high current, single gap switch device which has a
small liquid-metal cathode emitting a substantially conical plasma
jet and an anode structure positioned with respect to the cathode
so that the anode does not directly intercept the conical plasma
jet emitted from the cathode. It is further object to provide an
enclosed switch device with the background pressure maintained
therein at a sufficiently low level, such as 10.sup..sup.-3 Torr or
less during arcing, that it does not interfere with the plasma jet
cone so that the plasma jet cone is not distorted and an anode
structure can be positioned with respect to the plasma jet cone at
the boundary of the plasma jet to prevent substantial interference
with the jet cone. It is another object of this invention to
provide an enclosed switch tube having an anode structure, a
cathode and a condenser and arranged so that the condenser,
optionally together with vacuum pumping devices, maintains the
pressure within the tube at a sufficiently low level that the
pressure in the tube does not interfere with the flow of neutrals
and ions from arc spots, so an anode structure can be positioned
with respect to the plasma jet cone and at the boundary of the
plasma jet cone issuing from the arc spot. It is a further object
of this invention to provide a liquid-metal cathode which is fed
with non-solid metal so that a small area of liquid metal is
provided for arcing activity which area defines the position of
arcing activity and thus defines the apex of the plasma cone
issuing from the liquid-metal area to accurately define the plasma
cone position and thus permit anode positioning with respect to the
plasma cone. It is a further object of this invention to provide a
liquid-metal cathode which is fed with non-solid metal so that a
small area of liquid metal is provided for arcing activity. Other
objects and advantages of this invention will become apparent from
a study of the following portion of the specification, the claims
and the attached drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of an electrical switch device having a
liquid-metal cathode and having an anode shaped and positioned in
accordance with this invention.
FIG. 2 is a vertical section therethrough.
FIG. 3 is an enlarged, partial vertical section showing a portion
of the cathode and a portion of its associated anode.
DESCRIPTION
The electrical switch device of this invention is generally
indicated at 10 in FIGS. 1 and 2. The electrical switch device has
a cathode 12, an anode 14 and a condenser 16 positioned within
vessel 18. The vessel is closed so that a low pressure can be
maintained interiorly thereof. Vacuum pump connection 20 is
optionally provided for the purpose of aiding maintenance of the
low pressure necessary for proper operation of the switch
device.
It is essential that the cathode 12 be the type of cathode taught
in U.S. Pat. No. 3,475,636. Any one of the cathode embodiments
illustrated in that patent can be employed, or any other specific
cathode configuration falling within the teachings of that patent
is useful herein.
Cathode 12 is illustrated in more detail in FIG. 3 wherein cathode
body 22 has liquid-metal passage 24 therein. Passage 24 terminates
in flow restriction 26 which can either take the form of the porous
plug shown, or of a narrow capillary flow control tube or any other
flow impedance. Liquid metal is made to pass through passage 24 and
through flow restriction 26. One method of accomplishing this is by
use of liquid-metal pump 28 which delivers liquid metal through
line 30. An example of such a pump is shown in H. J. King, U.S.
Pat. No. 3,444,816. If recirculation is required, the metal can be
drawn from condenser 16. In such a case electrical isolator 32 is
inserted in line 30. Said isolator 32 can be of the type taught in
H. J. King et al, U.S. Pat. No. 3,443,570. An evaporative type
transport system or a suitably arranged gravity feed system could
optionally be used to transfer the liquid metal from the vessel 18
to the electrical isolator 32. In some cases, no heating is
required to maintain the metal in its liquid state as it is
delivered to passage 24, but in other cases, heating may be
necessary to maintain the liquid state. The question of whether or
not heating is employed is dependent upon the choice of the metal,
and the ambient conditions.
In the case of mercury, the specified condenser temperature is kept
substantially below 0.degree. C. In the case of mercury, a
preferred condenser temperature is about -35.degree. C., this being
just above the melting point of mercury and permitting the
maintenance of a background pressure as low as 5 .times.
10.sup..sup.-6 Torr during non-arcing conditions. The cathode is
cooled as taught in U.S. Pat. No. 3,475,636 so as to keep its
evaporation from the liquid metal into the background atmosphere
negligible during non-arcing conditions. Such evaporation is said
to be negligible when the pressure of the background atmosphere is
approximately equal to the vapor pressure of the liquid metal at
the condenser temperature, this vapor pressure being approximately
5 .times. 10.sup..sup.-6 Torr for mercury at a temperature of
-35.degree. C. Of course, during arcing the cathode heats up with
the result that the pool of mercury is above that temperature.
However, the pool size is sufficiently small in comparison to the
size of the condenser that during arcing the background pressure is
maintained at at least as low as 10.sup..sup.-3 Torr. When heating
is necessary for metals other than mercury to maintain the liquid
state, such heating can be applied to the pump, the isolator and
the connecting line 30. Additionally, heating or cooling of the
cathode body 22 may be necessary to maintain the cathode at proper
operating temperature, depending on the choice of liquid metal, the
cathode current, heat losses to the atmosphere, and the like.
The front of cathode 12, in front of flow restriction 26, has
pool-retaining walls 34. These pool-retaining walls are preferably
conical, and are preferably formed with the total included angle of
the apex of the cone of approximately 60.degree. or more. The apex
of the cone is positioned at the face of flow restriction 26. In
the downward direction in FIG. 3, below the conical area, the walls
are curved outward to present a substantially planar front face 36.
For further details of the construction and operation, attention is
drawn to W. O. Eckhardt U.S. Pat. No. 3,475,636, the entire
disclosure of which is incorporated herein by this reference.
As is seen in FIG. 2, cathode 12 is insulated from the anode 14 by
means of insulator 38. Insulator 38 is sealed to both the anode and
cathode to provide closure of the interior of the device 10 from
the external atmosphere. The front face 36 is formed on the front
of a skirt 40 which extends outwardly and up into the interior of
the space enclosed by insulator 38. This skirt helps to prevent
deposition of liquid-metal particles on the insulator adjacent the
cathode and protects the junction between insulator 38 and cathode
12 from high electric fields.
Anode structure 14 is annular and is substantially coaxial with the
axial center line of cathode 12. The interior surface 42 of anode
14 is generally in the form of a truncated cone, smoothed into
faired out with rounded edges at top and bottom. The interior
surface 42 is arranged with approximately a 40.degree. to
70.degree. total included conical angle, similarly to the angle of
the plasma cone emitted from the cathode as described below. The
interior surface 42 lies adjacent to the plasma cone for good
electrical coupling. This relates the interior surface 42 of the
anode and the outside of the jet formed from the liquid-metal pool
positioned between pool-retaining walls 34. The skirt 40, anode 14
and insulator 38 are maintained at a temperature such that metal
vapor will not condense on these surfaces.
Anode 14 is mounted upon insulative support ring 44 which is
secured to anode 14 on its upper side, and is secured to the body
of vessel 18 on its lower side. Connector 46 extends outwardly from
anode 14 to permit electrical connection thereto. Heat exchanger 47
controls anode temperature. Skirt 48 extends upward from anode 14
into the space on the interior of insulator 38, and substantially
parallel to skirt 40, to protect the junction between insulator 38
and the anode 14 from high electric fields and sputtered
deposition. Anode structure 14 may comprise an anode structure
having plurality of electrically separate electrodes for special
electrical connection such as polyphase rectification.
Condenser 16 is built up of a plurality of thin truncated conical
shells or fins which are coaxially arranged with cathode 12.
Condenser 16 can be mounted on legs in the bottom of vessel 18, or
by any other conventional, convenient support structure.
Alternatively, it can also consist of a single cup-shaped container
or of a container in the form of a closed box with an appropriate
opening to permit entrance and capture of the plasma jet past the
anode. Again, the total included cone angle is approximately
60.degree. to 70.degree. so that the shells are arranged edgewise
to material flow. Conical shells 50 are mounted on top of shells 52
which are cylindrical tubes. The shells 50 and 52 are cooled by
appropriate cooling means such as circulating coolant which flows
through jacket 54 through connections 56 and 58. The shells are
maintained at a suitably low temperature to condense metal vapor
into the liquid or optionally the solid state and permit it to
gravitationally discharge out of the bottom of vessel 18. Drainage
openings at the bottoms of shells 58 permit the condensed liquid
metal to drain so that it moves through line 60 to liquid-metal
pump 28 or any other appropriate recirculating means or other
appropriate means of disposition.
The term "liquid metal" is used to define those metals which are
liquid at or somewhat above room temperature. While called liquid
metal, it is not necessarily in the liquid state when fed to the
pool-keeping surfaces. Mercury is a convenient liquid metal because
it is normally liquid at room temperatures. Additionally, cesium,
lithium, and gallium are also examples of suitable materials to act
as the liquid metal. If necessary, the liquid-metal circulating
system can be heated to maintain the liquidity of the liquid metal,
as is discussed above.
Cathode 12 and anode 14 are preferably of refractory metal. When
mercury is employed as the liquid metal, molybdenum serves as a
suitable material for the anode and cathode. Liquid metal is pumped
through inlet pipe 30 to cathode 12, to form a small pool 76, see
FIG. 3, which is retained between walls 34. Alternatively,
liquid-metal vapor is fed to the pool-keeping walls, whereon it
transiently condenses.
U.S. Pat. No. 3,475,636 discloses the cathode in more detail. Any
one of the cathodes disclosed in that Eckhardt patent is useful
herein. The pool-keeping walls are indicated at 34, and the liquid
metal is delivered thereto. When a liquid pool is employed, it
rests in the cone between the pool-keeping walls and is as small a
pool as possible, consistent with reliable arcing.
When liquid metal vapor is fed through passage 24, see FIG. 3, flow
restriction 26 is normally not necessary, the passageway itself
providing the flow control pressure drop. The passageway 24 can be
quite small in diameter, and enter directly into the apex of the
cone defined by pool-keeping walls 34. When liquid metal vapor is
fed, the pool-keeping walls 34 are maintained at a temperature
where transient condensation occurs. This is a temperature within a
few degrees above equilibrium temperature so that vapor molecules
can stick to the surface for a short time, but cannot build up into
a layer which is multimolecule thick. In such a case there is no
"pool" which comprises a drop of liquid mercury, but instead,
molecules of mercury upon the surface of walls 34 supply the liquid
metal for arcing. Under the circumstances, these walls 34 can be
larger in physical size, although the amount of mercury transiently
condensed thereon is preferably far less than the surface coverage.
This method of feeding liquid metal to a cathode is disclosed in
more detail in U.S. Pat. application Ser. No. 720,694, filed Apr.
11, 1968, entitled "Vapor Feeding of Liquid Metal Cathodes" by
Wilfried O. Eckhardt, now U.S. Pat. No. 3,538,375, granted Nov. 3,
1970, the entire disclosure of which is incorporated herein by this
reference.
The pressure within vessel 18 is maintained sufficiently low that
when arcing occurs, it occurs in the vacuum arc mode. The vacuum
arc mode is broadly defined as an arc having electrons, positive
ions and neutrals supplied in a plasma jet by arc spots within a
vessel having a background pressure sufficiently low that it does
not substantially affect the trajectories of the atoms and ions in
this plasma jet. In the vacuum arc mode there must be negligible
permanent gas (outside of the arcing material) present in the arc.
Thus, when the arc becomes extinguished, the pressure in the arc
space returns to a sufficiently low value to provide high electric
field holdoff. To maintain a vacuum arc mode of operation, the
vessel must not contain large areas of liquid metal or other
material available for evaporation into the atmosphere of the
vessel.
The background pressure in the vessel during non-arcing and during
arcing is sufficiently low that the mean free path of the gas
molecules or atoms of the background gas is large compared with the
greatest dimension of the arc. The vacuum arc is therefore
dependent for the atmosphere in which it burns on the emission of
metal vapor and plasma from its cathode spots in the form of a
plasma jet. This plasma jet being essentially electrically neutral
because of the presence of a sufficient number of the positive ions
to substantially neutralize the electronic space charge, the
discharge runs at a low arc voltage.
Current between the plasma jet and the anode is carried by the
plasma electrons reaching the anode. Current between cathode and
plasma jet is believed to be carried both by electrons emitted from
the cathode and by ions falling back from the cathodes spots to the
cathode. Neutral metal vapor in the efflux from the cathode
condenses on the condenser, as well as ions reaching the condenser
from the plasma jet.
Conditions in the vacuum arc plasma are characterized largely by
the fact that the vacuum arc depends for its plasma on the metal
vapor emitted from its own cathode spots, and that this plasma and
metal vapor are emitted from the region of the cathode spots in the
form of a jet, called here the "plasma jet." It is by these
characteristics that the vacuum arc differs most markedly from the
more common low pressure arc.
The pressure within vessel 18 is maintained sufficiently low that
when arcing occurs, it occurs in the vacuum arc mode. As discussed
above, the vacuum arc mode is broadly defined as having electrons,
positive ions and neutrals supplied by the arc spots, the
background pressure within the vessel being sufficiently low that
it does not substantially affect the trajectories of the atoms and
ions emitted from the arc spot. More complete discussion of the
vacuum arc and of the arcing voltage metals is found in the
Proceedings of the Institute of Electrical Engineers, Vol. 110, No.
4, Apr. 1963, pages 793-802. In this specification, arc voltage and
arcing voltage are interchangeably used. To provide the vacuum arc
conditions described above, the pressure in the background volume
outside of the plasma jet should not exceed about 10.sup..sup.-3
Torr. In order to keep the pressure at 10.sup..sup.-3 Torr or less
in the background volume outside of the plasma jet during arcing, a
condenser temperature of about -10.degree. C. or less is necessary
when mercury is used as the liquid metal. A preferred condenser
temperature for mercury is about -35.degree. C., which corresponds
to just-liquid mercury on the condenser surface, and permits to
attain a pressure as low as 5 .times. 10.sup..sup.-6 Torr during
non-arcing.
The arc is initiated by any convenient means, including those well
known in the art, such as auxiliary electrode igniters,
semiconductor igniters, and the like. Alternatively, a laser
igniter directed onto the liquid-metal surface is suitable, but
schematically illustrated is igniter 86 which emits a puff of
plasma into the space between the anode and the cathode to initiate
arcing. Plasma puffers are well known. One is described in detail
in an article by Winston H. Bostick, entitled "Plasma Motors," at
pages 169 through 178 in the proceedings of the "Conference on
Extremely High Temperatures," edited by Fischer and Mansur and
published by Wiley, 1958. Other suitable igniters are disclosed in
"Gaseous Conductors" by James D. Cobine, Dover Publications, New
York, 1941, particularly at pages 421-426. Once the arc is
initiated, a plasma jet is emitted from the cathode. This plasma
jet contains electrons, ions and neutral particles. The jet issues
from the arc spot on the liquid metal and issues forth in a solid
cone having a cone angle of about 60.degree. to 70.degree.. The
interior surface 42 of anode 14 is positioned in such relationship
with the plasma cone that it does not substantially interfere with
the progress of ions and neutral particles to the condenser. The
condenser rapidly captures these particles, so that the background
pressure inside of vessel 18 remains low. Furthermore, the small
liquid metal area adjacent to wall 34 is sufficiently small and
maintained at sufficiently low temperature that the evaporation
therefrom does not adversely affect the pressure inside the vessel,
this pressure being maintained sufficiently low that vacuum arc
conditions are maintained, that there is no substantial
interference with the high velocity of the plasma jet, and that low
enough pressure can be maintained to prevent breakdown. Electrons
are extracted from the plasma cone and captured on anode 42 to thus
cause current conduction.
The advantage of vacuum arc operation as defined above, is that
when arcing ceases, the high velocity jet of particles from the arc
spot is rapidly captured on the condenser so that the space between
the anode and the cathode very quickly is returned to vacuum
conditions wherein the vacuum has high insulative value. This
permits rapid application of reverse voltage without conduction, at
a rate of voltage rise up to 10 kilovolts per microsecond or even
more. This makes the electrical switch device 10 of great utility
for high voltage rectification, controlled rectification, and
inversion, particularly at high currents.
The cessation of arcing can occur in operation by the change in
polarity applied to the terminals. Additionally, cessation of
arcing can be made to occur by stopping the flow of liquid metal to
the liquid-metal pool 76. Thus, the device is useful for dc
switching, for when it is desired that current be stopped, the
liquid-metal flow is stopped to starve the pool and thus cause
cessation of arcing. The material of the cathode around the pool is
of such high arc voltage that the arc is extinguished rather than
transferred to this material. To maintain stable current flow in
the device under the desired condition of high electron-to-atom
emission ratio at the cathode, it is desirable to operate at
approximately constant electron-to-atom emission ratio. To this
effect, the feeding of the liquid metal must be proportional to the
arc current. When the average current is constant, the liquid metal
can be fed at a constant rate. Additionally, the electrical
switching device 10 can act as an overcurrent fault protector. When
an adequate amount of liquid metal is fed in an amount proportional
to normal current, up to a predetermined maximum feed rate of
liquid metal is fed to the cathode to supply normal current needs;
when a fault occurs which draws a larger amount of current, the
liquid-metal pool rapidly becomes exhausted because of its small
volume. When the pool is exhausted, again arcing stops so that
excessively high fault currents can be interrupted. Of course,
modulated feeding of the liquid metal produces controlled forced
current interruption when the current starves the pool.
This invention having been described in its preferred embodiment,
it is clear that it is susceptible to numerous modifications and
embodiments within the ability of those skilled in the art and
without the exercise of the inventive faculty.
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