U.S. patent number 3,638,061 [Application Number 05/055,124] was granted by the patent office on 1972-01-25 for magnetically controlled crossed-field interrupter and switch tube with pressure control for long duration pules.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Ronald C. Knechtli, Michael A. Lutz.
United States Patent |
3,638,061 |
Lutz , et al. |
January 25, 1972 |
MAGNETICALLY CONTROLLED CROSSED-FIELD INTERRUPTER AND SWITCH TUBE
WITH PRESSURE CONTROL FOR LONG DURATION PULES
Abstract
In the absence of appropriate means for controlling the gas
pressure, the time during which a crossed-field electrical switch
can conduct is limited by the rapid pumping action of the
discharge. The present invention discloses a crossed-field
discharge switch in which pressure control is accomplished by
selection of gas and electrode materials to minimize gas losses,
and means is provided for adding additional gas, if needed. This
structure permits a crossed-field electrical switch device to
conduct for reasonable lengths of time without off-switching due to
gas losses.
Inventors: |
Lutz; Michael A. (Los Angeles,
CA), Knechtli; Ronald C. (Woodland Hills, CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
21995766 |
Appl.
No.: |
05/055,124 |
Filed: |
July 15, 1970 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
664722 |
Aug 31, 1967 |
|
|
|
|
Current U.S.
Class: |
313/161;
313/231.01; 313/311; 313/643 |
Current CPC
Class: |
H01J
17/26 (20130101); H01J 17/14 (20130101) |
Current International
Class: |
H01J
17/26 (20060101); H01J 17/02 (20060101); H01J
17/14 (20060101); H01j 007/16 (); H01j
007/20 () |
Field of
Search: |
;313/231,161,311,224
;331/86 ;315/39.51,111 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kominski; John
Parent Case Text
CROSS-REFERENCE
This application is a continuation-in-part of U.S. Pat. application
Ser. No. 664,722, filed Aug. 31, 1967, entitled "Magnetically
Controlled Crossed-Field Interrupter and Switch Tube."
Claims
What is claimed is:
1. A switch device of the gas discharge type comprising:
a first tubular electrode, a second tubular electrode insulatingly
mounted with respect to said first tubular electrode and mounted
within said first tubular electrode in spaced-apart relationship,
defining an annular gas discharge region in the interelectrode
space;
means for maintaining helium gas in said annular gas discharge
region below the critical pressure required for low-pressure
electric discharge between the electrodes at the voltage to be
applied across said electrodes;
means for producing a magnetic field in said annular gas discharge
region above a critical value required to provide sufficient
electron trapping to sustain current between said electrodes when
said voltage is applied, the invention comprising:
said gas being helium and at least the one of said electrodes
acting as a cathode in the electric discharge being made of
material selected from the group consisting of hafnium, tantalum,
tungsten and rhenium for minimizing gas loss due to chemisorption
and burial.
2. The device of claim 1 wherein the helium gas pressure is about
0.05 Torr and the interelectrode spacing is less than 5
centimeters.
3. The device of claim 2 wherein said electrodes are positioned
within an envelope that is totally sealed.
4. The device of claim 1 wherein said electrodes are positioned
within an envelope that is totally sealed.
5. The device of claim 1 wherein gas supply means is connected to
said interelectrode space to supply helium to said interelectrode
space to aid in preventing off-switching due to gas losses from
said interelectrode space due to electric discharge in the
interelectrode space.
6. The device of claim 5 wherein said gas supply means comprises a
helium reservoir and a helium gas supply tube connected between
said reservoir and said interelectrode space, first and second
valves in said tube defining a gas volume therebetween,
valve-operating means connected to said first and second valves for
alternately opening said first and second valves so that the volume
between said first and second valves is alternately connected to
said helium reservoir and to said interelectrode space so that gas
is discharged from said reservoir to said interelectrode space upon
operation of said valve-operating means.
7. The switch device of claim 6 wherein a pressure sensor is
connected to said interelectrode space, said pressure sensor being
connected to said valve-operating means so that upon detection of
low pressure in said interelectrode space, said valve-operating
means is operated to discharge helium into said interelectrode
space.
8. The switch device of claim 6 wherein a coulomb meter is
connected to one of said electrodes, said coulomb meter being
connected to said valve-operating means so that, upon passage of a
predetermined charge through said switch device, said coulomb meter
actuates said valve-operating means so that said gas supply means
discharges helium gas into said interelectrode space.
Description
BACKGROUND
This invention is directed to crossed-field electrical switch
devices and particularly for gas pressure control therein.
The prior art includes C. G. Smith U.S. Pat. No. 1,714,405; J. L.
Stratton U.S. Pat. No. 2,352,231; G. Boucher et al. U.S. Pat. No.
3,215,893; G. J. Boucher U.S. Pat. No. 3,215,939; and K. Wasa et
al. U.S. Pat. No. 3,405,300. These patents teach that a device
having spaced electrodes and a low-pressure gas therebetween may be
electrically switched by changing the magnetic field strength. By
this means, the device is made electrically conductive or
nonconductive.
None of the prior art patents has any means for controlling the gas
pressure. The Smith and Stratton patents specifically show totally
enclosed tubes and do not discuss either electrode materials or the
gas fill. Boucher et al. teaches a vessel having a pumpout stem for
initially evacuating the enclosure and thereafter filling the space
with a gas.
Neither the Boucher nor Wasa et al. patents has any substantial
teaching as to the closure of the vessel. Both call for a gastight
enclosure or envelope, but do not have any further teaching. This
implies that they each have a totally enclosed envelope, but the
teaching is not clear.
Whenever current flows through a switch device as described in
these patents, sputtering (erosion under by ion bombardment) of the
cathode occurs. There is also a smaller amount of anode sputtering.
The eroded metal deposits on all adjacent surfaces. This deposited
metal forms a film which is highly chemically active, as in a
sublimation pump. This film getters reactive gases with very high
efficiency.
These prior art patents variously teach the use of hydrogen, neon,
nitrogen, argon, Freon and ethyl alcohol as the gas material.
Furthermore, they teach the employment of molybdenum, titanium,
tantalum, copper, nonmagnetic nonoxidizable steel and platinum as
suitable electrode materials. Choosing a popular example of
hydrogen as the gas and 304 stainless steel as the electrode
material and starting with a gas pressure at such a level that a
high-current, low-voltage gas discharge can be obtained, a
switching device of the dimensions used in the exemplary embodiment
below will off-switch by gas depletion after a charge of less than
1 coulomb has passed therethrough. This amounts to undesired
off-switching (by gas depletion rather than by magnetic field
change) after 1 millisecond, when passing its rated current of
1,000 amperes. Thus, the device is unsuitable for use as a
switching device when conductive periods exceed 1 millisecond. Even
if gas were supplied to the device, gas would have to be supplied
at the high rate of 50 Torr liters/sec. and carefully controlled
for pressure level or the device would be unsatisfactory.
SUMMARY
In order to aid in the understanding of this invention, it can be
stated in essentially summary form that it is directed to a
crossed-field electric switch device which has pressure control so
that the gas pressure therein is managed such as to provide
electrical conduction for reasonable lengths of time and
predictably manage switching by controlling the applied magnetic
field. Pressure maintenance is accomplished by selecting the
cathode material from the group consisting of hafnium, tantalum,
tungsten and rhenium to result in low-sputter yield cathode
material and employing helium as the gas to eliminate gettering and
provide fast deionization. Furthermore, for those devices which
must conduct an appreciable charge of electricity, a gas supply
means to the enclosure is provided. This gas supply means can be
responsive to the pressure in the switch device or can be
responsive to the amount of charge passed through the device; the
amount of charge being substantially directly proportional to the
amount of gas cleaned up.
Accordingly, it is an object of this invention to provide a
crossed-field electric switch device including means to control the
pressure therein. It is a further object to provide pressure
control means which includes the elimination of reactive gases
combining with freshly sputtered surfaces by employing inert helium
as the gas in the crossed-field electric switch device. It is a
further object to minimize physical burial of gas during the
sputtering activity by selecting the cathode and possibly the anode
materials such that sputtering is minimized. It is another object
to provide a combination of electrode material and gas which
minimizes gas cleanup during electrical conductivity of the
electrical switch device.
It is still another object to provide means attached to the
electrical switch device which supplies additional gas as needed to
make up for losses of gas. It is still another object to
alternatively measure the pressure or measure the charge through
the electrical switch device to determine when additional gas is
required and appropriately activate the gas supply means. Another
object is to provide long life by minimizing sputtering, for
sputtering coats surfaces and, when too much sputtering has
occurred, material flakes off and may destroy the insulating
properties of the tube. It is a further object to employ helium as
the fill gas, for helium is so light it sputters the quoted metals
less than any other inert gas. As another object to make the
voltage hold-off strength of a single tube as high as possible,
helium is used because of its superior Paschen curve which is
better than all other gases. 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view, with parts broken away and partly
shown in section, of the crossed-field electrical switch device of
this invention.
FIG. 2 shows a gas supply means for connection to the electrical
switch device.
FIG. 3 illustrates a means for actuating the gas supply means.
FIG. 4 illustrates the Paschen curve for hydrogen and helium.
DESCRIPTION
An embodiment of the crossed-field electric switch device is shown
in FIG. 1. The device can operate either to turn a circuit on or
off and, thus, its description as being a switch device includes
the function of interrupting circuit current.
The device 2 comprises a cylindrical cold cathode 4, which in this
embodiment also forms the cylindrical wall of the interrupter
envelope 6, and a cylindrical anode 10. The cylindrical electrodes
4 and 10 are coaxial and together define a gas-filled
interelectrode space 8. The envelope 6 is closed at each end by end
plates 12 and 14. Alternately, it is understood that both the
cathode 4 and the anode 10 could be surrounded by a separate
vacuumtight envelope 6, this envelope being not necessarily
identical with the cathode 4. It is furthermore understood that,
when the vacuum tight envelope 6 is physically distinct from the
outer electrode shown here as cathode 4, the latter can be slit in
a substantially longitudinal manner to prevent the induction of
eddy currents in the outer electrode 4 when the applied magnetic
field is rapidly charged for switching purposes. Such eddy
currents, if permitted to flow, result in a delay between an
applied change in current in the solenoid generating the magnetic
field, and the change of this magnetic field in the interelectrode
space. By making the envelope 6 out of electrically insulating or
relatively high-electrical resistivity material, and by providing
said slit in the outer electrode 4, said switching delay can be
substantially eliminated.
The top end plate 12 is closed, while the bottom end plate 14
carries a support post 16. The support post passes through the
bottom metal closure plate 14, but is electrically insulated
therefrom. The support post 16 thus physically supports the bottom
plate 14 and the structure supported thereon, and it physically
supports anode 10 by physical connection thereto. Support post 16
also acts as an electrical connection to anode 10. Tube 18 is
connected to the interior space. Tube 18 is flanged off by flange
19 and can have a pressure gauge 20 connected to tube 18 by means
of a tee.
A water-cooling coil 22 can optionally spirally encircle the outer
wall of the envelope 6. A magnetic field coil 24 is mounted
externally of the envelope 6 for use in applying a magnetic field
(indicated schematically by the dotted lines 26) having at least
one component perpendicular to the electric field (indicated
schematically by the arrows 28) between the anode 10 and cathode 4.
The inlet, outlet, pump, and heat exchanger for the water-cooling
coil 22 have been omitted from the drawing in the interest of
clarity. The electrical lead 30 to the anode extends through the
plate 14 and the anode mount 32. The cathode lead 34 is attached to
the metal plate 14.
Moderately large cathode surface areas are required to carry
sizable currents at current densities small enough (<25
A/cm..sup.2) to avoid arcing because an arc is not extinguished by
off-switching the magnetic field.
The maximum voltage hold-off capability of the apparatus of this
invention can be determined by the use of a Paschen curve. One such
curve is shown in FIG. 4 for both hydrogen and helium. Each curve
predicts the breakdown voltage for a pair of electrodes immersed in
a specific kind of gas, the electrodes being separated by a
distance "d," and the gas being at a pressure "p," (and with no
magnetic field in the interelectrode space). The apparatus of this
invention operates to the left of the Paschen minimum. That is, if
it is desired that the interrupter or switch tube of this invention
be able to hold off 25 kv., the product "pd" must be made less than
about 0.8, if helium, for example, is the ambient gas. If the
electrodes are parallel and spaced-apart 2 cm. or less, the maximum
pressure which can be used is about 0.25 Torr, if breakdown in the
absence of the magnetic field is to be avoided.
Consider now the device 2, with no voltage applied across the
electrodes, but with an applied magnetic field having a magnitude
above a critical value, which may fairly uniformly occupy the
entire interelectrode space, and having at least one component
extending parallel to the electrodes. A voltage applied now will
drive an electron in a cycloidal path with a drift perpendicular to
both the electric and magnetic fields. A suitable geometry will
allow this motion to continue unhindered until the electron makes a
collision. The electrons are then said to be "trapped" under the
influence of the crossed electric and magnetic fields. The magnetic
field has prevented an electron from going directly from the
cathode to the anode and has, therefore, considerably increased the
length of the electron trajectory. The increase in path length is
such that the average electron makes at least one collision with
the gas in the interelectrode space before it is captured by the
anode. The collision produces more electrons so that avalanche is
quickly established. With respect to ionization, this is equivalent
to an increase in pressure. Hence, a device of suitable design at a
pressure below the critical pressure can be made to conduct with a
modest ( 300 volts) discharge conduction drop by the application of
a magnetic field of suitable orientation and magnitude.
Now consider that such a gas discharge device is operating in the
above manner and the magnetic field is suddenly switched below the
critical magnitude required for conduction. At this time, the
electrons in the interelectrode space are no longer trapped and,
because of the low gas pressure in the device, the electrons are
drawn directly to the anode in substantially collisionless
trajectories (the positive ions are essentially unaffected by the
magnetic field because of their large masses). This leads to
recovery rates (i.e., of the insulating strength) which are very
fast; for example, on the order of 1 kv. per microsecond.
The design criteria for a 25 kv. sealed-off switching device
described in FIG. 1 are the following: select a suitable pressure
at which a high-current, low-voltage crossed-field discharge can be
maintained. This is typically 0.05 Torr for either hydrogen or
helium, this pressure being relatively independent of gas type.
From the Paschen curve for the gas selected, determine the maximum
interelectrode space d which will keep the pd product to the left
of the curve at the desired maximum hold-off voltage. From FIG. 4,
this pd value for a 25 kv. tube is 0.26 Torr-cm. for hydrogen and
0.80 Torr-cm. for helium. Since p has been selected independently
to be 0.05 Torr for both gases, this means d must be less than 5.2
cm. for hydrogen and d must be less than 16 cm. for helium. There
is another restriction on d; namely, d must exceed some minimum
spacing to avoid field emission between cathode and anode when the
tube is holding off the full voltage (magnetic field off). The rule
is to allow about 50 kv./cm. for well-arced surfaces. Hence, the
minimum d for a 25 kv. tube is one-half cm. independent of the type
of fill gas. It is, thus, clear that a value of d somewhere between
these two limits will provide successful operation and that there
is no conflict between the lower and upper limits. As the desired
hold-off voltage is raised, however, such a conflict can occur.
Much above 100 kv., for example, the minimum d requirement meets
the maximum d requirement and further voltage hold-off in a single
gap becomes impossible. It is at this high voltage that the
improved Paschen curve for helium is advantageous over hydrogen
(and all other gases) in that the maximum d for helium is greater
than the maximum d for any other gas, enabling the highest voltage
hold-off capability to be found in a helium filled tube.
When a switching device of the dimensions given by this example is
employed to pass current, two principal phenomena result in the
removal of the gas from free activity in the device. The first
phenomenon is chemisorption. Sputtering erosion of the cathode, and
to a lesser extent the anode, under ion bombardment causes
deposition of the eroded metal on all adjacent surfaces. This
deposited metal forms a film which is highly chemically active. The
use of refractory metals for low-sputter yield and greater arc
resistance is incompatible with reactive gases from a chemisorption
point of view.
In addition to the loss of reactive gas from the device by chemical
reaction at the freshly sputter-deposited surfaces, the sputter
deposition of cathode material on adjacent surfaces buries
considerable amounts of gas, if the deposited layers are being
simultaneously bombarded by energetic ions or neutrals. This is
strictly a physical phenomenon and is unrelated to the chemical
reactivity between the gas and the sputtered material, but is
directly related only to the discharge intensity, the amount of
material being sputter-deposited, and the potential of the surfaces
on which the sputter-deposited material lands. To minimize this
effect, the electrode material is selected to have a minimum
sputtering rate.
In accordance with this invention, helium is employed as a
nonreactive gas fill and a cathode (and most desirably, also an
anode) material (selected for low-sputtering rate) from the group
consisting of hafnium, tantalum, tungsten, and rhenium is employed.
In the exemplary switch device described above, employing a helium
gas fill at 0.05 Torr and tantalum as an electrode material, a
charge of 250 coulombs can be passed before the pressure decreases
to an extent that the switching device uncontrollably off-switches
due to pressure loss. This corresponds to conduction for one-fourth
seconds at 1,000 amperes. This is sufficiently long to be useful as
a switching device. Thus, in accordance with this invention, it is
critical that helium be employed as the fill gas in the switch
device and at least the cathode electrode, and preferably both
electrodes be made from a material selected from the group
consisting of hafnium, tantalum, tungsten and rhenium.
The switch device is described as having a cathode electrode 6 and
anode electrode 10, strictly for convenience of description. With
appropriate shaping of the magnetic field, either electrode can
operate as the cathode, depending on applied polarity. Furthermore,
the device can be employed in AC-circuits, with reversing
polarity.
FIG. 1 illustrates the switch device 2 as being totally closed off,
with the blank flange 19 closing the pipe 18 by which the switching
device is initially evacuated and refilled to the proper initial
helium pressure. For longer life operation, a gas supply is
employed. FIG. 2 illustrates reservoir 34 which contains helium
under pressure and which is connected by tube 36 to flange 38. This
flange is connected to tube 18, instead of the closure flange 19.
Valves 40 and 42 are electrically operable and are normally closed.
They are serially mounted in tube 36 and define a known volume in
the tube therebetween.
When valve 40 is open, the tube 36 between the valves is charged to
the pressure of the reservoir 34. Thereupon, when valve 40 is
closed, followed by the opening of valve 42, the pressurized volume
of the space between the valves is discharged through tube 18 into
the interelectrode space. By this means, a known amount of helium
gas can be quickly discharged into the switch device. Valve control
unit 44 controls the sequence of valve operation.
The actuation of the valve control unit 44 to discharge helium into
the switch device 2 can be accomplished by detecting the pressure
by pressure sensor 20 and actuating valve control unit 44 when the
pressure decreases to a predetermined value. By this means, gas
resupply is accomplished.
Another way of determining when gas resupply is required is by the
employment of a coulomb meter 46, see FIG. 3. The coulomb meter is
connected to the terminals 48 and 50 in FIG. 1 in place of the
shunt illustrated therebetween. The coulomb meter measures the
charge which is passed through the switch device and, when the
amount of charge passed reaches a predetermined amount, a
connection between the coulomb meter and the valve control unit 44
actuates the valve control unit to deliver gas to the switch
device.
These resupply means are operable, when helium is employed as the
fill gas and at least the cathode electrode is made of material
selected from the group consisting of hafnium, tantalum, tungsten
and rhenium, because of the relatively slow rate of gas loss. If
hydrogen is used as the fill gas and 304 stainless steel is used as
the cathode material, a gas supply means, as illustrated in FIG. 2,
would not be sufficiently fast to overcome the gas loss. Similarly,
titanium hydride ribbon and sponge would not be able to supply
hydrogen at an adequate rate to maintain a sufficient pressure
level for maintenance of conductivity under the conditions
described above.
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.
* * * * *