Gas discharge closing switch with unitary ceramic housing

A gas discharge closing switch, such as a thyratron, has a one-piece ceramic housing containing an anode, a cathode, and a control electrode. The anode and cathode form fluid-tight seals with opposite ends of the housing. The control electrode is mounted entirely within the housing, and, in one embodiment, is affixed to an interior wall of the housing. The housing preferably supports the anode, the cathode and the control electrode, and maintains electrical isolation between them.

BACKGROUND OF THE INVENTION 
The present invention relates to a gas discharge closing switch and, more 
particularly, to a thyratron having a unitary ceramic housing. 
Gas discharge closing switches, such as thyratrons, are used for rapid 
switching of high voltage, high current signals with low power 
consumption. A typical thyratron has an anode connected to high voltage 
and a cathode held at ground potential. A control electrode or "grid" is 
placed between the anode and the cathode. Upon application of a positive 
control pulse, the control electrode closes the switch by drawing 
electrons from the cathode to transform gas within a housing or "envelope" 
of the device into a dense, conducting plasma. 
Thyratrons generally fall into two classes, depending on whether their 
housings are made of glass or ceramic material. Although glass thyratrons 
are suitable in many applications, ceramic is preferred where a device is 
subjected to substantial external forces. For example, ceramic thyratrons, 
often referred to as metal/ceramic structures, are used in environments of 
high acceleration (up to approximately 100 G's) and high vibrational 
forces (up to 11 G's). 
The housings of ceramic thyratrons are typically made from at least two 
separate ceramic elements, i.e., an upper element between the anode and 
the control electrode and a lower element between the control electrode 
and the cathode. The anode is affixed to the top edge of the upper ceramic 
element and the control electrode is affixed to the bottom edge of the 
same element. The control electrode is also typically affixed to the top 
edge of the lower ceramic element and the cathode is affixed to the bottom 
edge of the lower ceramic element. Each of these attachments must form a 
fluid-tight or "vacuum" seal in order to maintain the required gaseous 
environment within the housing. When assembled, the three major electrodes 
and the two ceramic elements form a stack, alternating between electrodes 
and ceramic elements. The complexity of this arrangement leads to a 
variety of difficulties and expenses in manufacturing, however. 
Because a portion of the control electrode of a traditional ceramic 
thyratron is exposed to the air at a location between the anode and the 
cathode, there is a possibility of arcing from the control electrode to 
the anode. For this reason, it is necessary to provide a relatively large 
spacing between the points where the anode and the control electrode 
structures exit the housing. However, the optimal distance between the 
anode and the control electrode within the device is generally much 
smaller than that required to avoid arcing outside. It is therefore 
necessary to use "deeply drawn" anode and control electrode cups in order 
to satisfy both of these requirements. Such cups must be drawn two or 
three times during their manufacture to achieve the required depth, adding 
significantly to the cost of the device. 
All three major electrodes of traditional ceramic thyratrons must also be 
affixed to the upper and lower ceramic elements in a way that creates a 
fluid-tight seal. The anode is brazed to the top of the upper ceramic 
element, the control electrode is brazed to both the bottom of the upper 
ceramic element and the top of the lower ceramic element, and the cathode 
is brazed to the bottom of the lower ceramic element, for a total of four 
vacuum-tight seals. Unfortunately, each braze increases the likelihood 
that the overall vacuum seal of the housing will fail. Therefore, it is 
desirable to decrease the number of individual seals, if possible, in 
order to increase the reliability of the thyratron. 
For a thyratron to operate efficiently and reliably, it is also important 
that the electric field within the device be as uniform as possible. To 
facilitate this, and to avoid concentrations of the field along electrode 
edges, the anode and the control electrode must be maintained in precise 
axial alignment. In the manufacture of traditional thyratrons, all 
electrodes are aligned relative to the housing through the use of brazing 
fixtures which are extremely expensive. 
In addition, all current flow of a thyratron in the conducting state passes 
through the control electrode, causing a significant amount of heat to be 
generated in that region. Much of this heat can be removed by conduction 
from an existing thyratron along a flange of the control electrode which 
extends outwardly through the ceramic housing. In fact, the heat generated 
in metal/ceramic thyratrons is so intense that designers have heretofore 
considered it essential to conduct it away in this manner. Unfortunately, 
this requires that the ceramic housing be separated into two or more 
parts, significantly increasing the cost of the device. 
Therefore, it is desirable in many applications to provide a metal/ceramic 
thyratron design which is simple and less expensive than prior models, yet 
provides equal or better performance. 
SUMMARY OF THE INVENTION 
The present invention provides an advantageous gas discharge closing switch 
having a housing which contains a control electrode and is formed of a 
single ceramic element. Because the control electrode does not penetrate 
the housing, two of the troublesome and expensive seals required in prior 
devices are eliminated. Thus, the number of vacuum brazes is reduced by 
fifty percent from that of a traditional two-piece ceramic thyratron. 
Arcing to the anode through the air outside the switch is also avoided 
because the control electrode is disposed entirely within a unitary 
ceramic housing. This eliminates the need for deep draw electrode cups. In 
addition, applicants have discovered that the switch of the present 
invention does not overheat even though the control electrode is 
completely encapsulated. 
In a preferred embodiment, the control electrode is dimensioned to closely 
engage the inner surface of the ceramic housing, causing it to expand 
against that surface and thereby align itself with the housing when heated 
to brazing temperatures. Hence, the number of required brazing fixtures is 
reduced from three in a traditional ceramic thyratron (one for each 
electrode) to two in a switch configured according to the present 
invention. 
Accordingly, a thyratron constructed according to the present invention 
includes: a unitary ceramic housing for maintaining a gaseous discharge, 
the housing having open upper and lower ends; an anode structure forming a 
fluid-tight seal with the upper end of the housing; a cathode structure 
forming a fluid-tight seal with the lower end of the housing for 
maintaining a gaseous environment therein; and a control electrode 
structure disposed within the housing between the anode structure and the 
cathode structure. In a preferred form, the control electrode structure is 
disposed entirely within the housing between the upper and lower ends 
thereof. In another preferred form, the unitary ceramic housing supports 
the anode, the control electrode and the cathode, and simultaneously 
maintains electrical isolation between them. In still another form, the 
anode, the control electrode and the cathode are mutually parallel and 
coaxial, and the control electrode is affixed to a step defined by the 
inner surface of the housing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a thyratron or other gas discharge closing switch 10 
constructed in accordance with the present invention has an anode 
structure 12, a control electrode structure, or "grid", 14, and a cathode 
structure 16, all of which are supported relative to a one-piece 
("unitary") ceramic housing 18. The control electrode structure 14 is 
preferably located entirely within the ceramic housing 18 between the 
anode structure 12 and the cathode structure 16, as illustrated in FIG. 1, 
and does not penetrate the housing. This configuration avoids the cost and 
reliability issues inherent in multiple ceramic housing elements and in 
vacuum seals between a control electrode structure and a ceramic housing. 
It also eliminates the need for deeply drawn anode and control electrode 
cups. 
In the illustrated embodiment, the housing 18 is substantially cylindrical 
and has an interior surface 20 with a step 22 which serves as a transition 
between a first interior surface portion 24 and a second interior surface 
portion 26 thereof. The step 22 supports a bottom edge 28 of the control 
electrode structure 14 to locate the control electrode structure within 
the ceramic housing. 
Referring now to FIG. 2, the step 22 includes a substantially 
radially-directed segment 30 of the interior surface 20 which extends from 
the first interior surface portion 24 to the second interior surface 
portion 26 and defines an interior angle 32 with the first surface portion 
24. This angle, which is preferably ninety (90) degrees, receives the 
bottom edge 28 of the control electrode structure. 
In the disclosed embodiment, the bottom edge 28 of the control electrode 
structure has two flattened surface segments 34 for bonding to the first 
interior surface portion 24 and the radial segment 30 of the housing. 
Bonding is preferably accomplished by brazing to appropriate metallized 
coatings 36 on the housing surface. When the control electrode structure 
is made of copper, the metallized coatings 36 may, for example, be formed 
by firing a moly-manganese mixture into the surface of the ceramic housing 
18 and later plating nickel over the impregnated region. This connects the 
control electrode structure 14 securely to the ceramic housing along two 
substantially perpendicular surfaces, creating a bond secure enough to 
withstand high external forces. Advantageously, the control electrode 
structure expands sufficiently during the brazing process to force itself 
against the interior surface 20 and thereby align itself with the axis of 
the housing. Thus, no special jigging fixture of any type is required to 
achieve accurate alignment of the control electrode. 
Because the control electrode does not extend outside the ceramic housing 
18 and contact the air, it is not necessary to separate the edges of the 
control electrode structure 14 and the anode structure 12 by a great 
distance. The step 22 can therefore be placed at any convenient height 
within the housing, permitting shallowly drawn metal cups to be used for 
the control electrode structure 14 and the anode structure 12. In this 
context, "shallowly drawn" means that each cup can be formed from a single 
piece of stock in a single drawing operation, as distinguished from prior 
ceramic thyratrons in which anode and control electrodes require two or 
more drawing steps. For copper stock having an initial thickness of 0.036 
inches (0.9 mm), such cups have a height less than one inch (2.54 cm), and 
preferably no more than one-half inch (1.27 cm). 
Referring again to FIG. 1, the anode structure 12 may have an anode cup 38 
with a horizontal anode plate 40 at its lower end. The anode cup, which is 
preferably made of copper, has an upper flange 42 brazed or otherwise 
affixed directly to an open upper end 44 of the housing 18 to form a 
fluid-tight seal. An external jigging fixture is preferably used in the 
brazing operation to assure accurate axial alignment of the anode 
structure 12. 
The cathode structure 16 is made up of a cathode 46 and a cathode heat 
shield 48, both supported within the unitary ceramic housing 18 on a 
cathode base plate 50. The cathode base plate 50 is preferably made of a 
suitable conductor, such as copper, and has a flange 52 for mounting of 
the thyratron 10. The cathode base plate 50 is bonded directly to a lower 
end 54 of the ceramic housing, preferably by brazing, to provide a 
fluid-tight seal at that location. This process can be performed without a 
high precision jigging fixture, though, because axial alignment of the 
cathode structure 16 is much less critical than that of the anode 
structure 12 and the control electrode structure 14. The cathode structure 
16 is also provided with a plurality of fluid-tight bushings 56 extending 
through its base plate 50 to connect the interior of the housing 18 to the 
outside world. Electrical connection to the control electrode structure 14 
is preferably made by an insulated lead 58 extending through one of the 
bushings 56. 
The one-piece ceramic housing 18 is filled with a suitable plasma-forming 
gas, such as hydrogen, and is then sealed off from the atmosphere. A 
suitable gas reservoir 60 of conventional design is provided within the 
housing 18 to maintain the gas pressure at a preselected optimal level. In 
addition, a tube 62 extends through the cathode base plate 50 for 
evacuation and back-filling of the device during the manufacturing 
process. 
The unique construction of the thyratron 10, including its one-piece 
ceramic housing 18, simplifies the manufacturing process by reducing the 
number of fluid-tight brazes or other bonding operations that must be 
performed. Because the control electrode 16 is located entirely within the 
housing, it need not be connected to the housing in a fluid-tight manner. 
It is necessary only that the bond between the flattened surface segments 
34 of the control electrode and the metallized coatings 36 of the housing 
be mechanically sound. Likewise, manufacture of the ceramic housing is 
simplified because only its exterior surface and the counterbored first 
interior surface portion 24 must be machined to close tolerances. The 
second interior surface portion 26, which is smaller in diameter than the 
first, can be left in "as fired" condition with no ill effects. In 
addition, as noted above, the anode structure and the control electrode 
structure need not be deep drawn. All of the foregoing features combine to 
render the structure of the closure switch 10 significantly less expensive 
to manufacture than prior ceramic closure switches without adversely 
affecting performance or reliability. 
In operation, a high positive voltage is applied to the anode structure 12 
and the cathode structure 16 is grounded. The control electrode structure 
14 is either grounded or maintained at a small negative potential to repel 
electrons emitted by the cathode structure 16 in the "open" condition of 
the switch. Substantially all of the voltage across the switch 10 is 
therefore present between the anode structure 12 and the control electrode 
structure 14 in the open condition, but breakdown does not occur because 
of the absence of free carriers and the small spacing between these 
components. When a positive pulse is applied to the control electrode 
structure 14, electrons are drawn from the cathode structure 16, which is 
preferably coated with a thermionic coating and heated to a temperature of 
approximately 800.degree. C., to ionize the gas within the housing 18 and 
create a plasma of highly energized gas species. As the electrons and 
other charge carriers travel through the gas, they collide with gas 
molecules and set up an avalanche ionization process which results in a 
dense conducting plasma throughout the interior of the housing 18. 
The thyratron 10 returns to its nonconducting state only when the anode 
voltage is removed for a time sufficient to allow the charged particles of 
the plasma to recombine. This period is known as the "recovery time" of 
the device. After the recovery period, the grid potential returns to its 
original (typically negative) value and a positive voltage can be applied 
to the anode structure 12 without conduction taking place. The thyratron 
10 is then ready to fire in response to the next positive control pulse. 
While certain specific embodiments have been disclosed as typical, the 
invention is not limited to these particular forms, but rather is 
applicable broadly to all such variations as fall within the scope of the 
appended claims.