Information
-
Patent Grant
-
6664720
-
Patent Number
6,664,720
-
Date Filed
Monday, April 23, 200123 years ago
-
Date Issued
Tuesday, December 16, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Lowe Hauptman Gilman & Berner, LLP
-
CPC
-
US Classifications
Field of Search
US
- 313 293
- 313 257
- 313 447
- 313 456
- 313 270
- 313 451
- 315 537
- 315 390
-
International Classifications
-
Abstract
The invention relates to linear beam amplification devices having an electron emitting cathode and an RF modulated grid closely spaced therefrom, and more particularly, to a novel support structure for the grid that accommodates thermal expansion while maintaining an optimum grid-to-cathode spacing.
Description
FIELD OF THE INVENTION
The present invention relates to cathode-grid assemblies for linear beam microwave vacuum tube devices having an electron emitting cathode and a microwave modulated grid closely spaced therefrom, and more particularly, to such an assembly including a support structure for the grid, wherein the support structure accommodates differential thermal expansion of a cathode assembly and the grid while maintaining an optimum grid-to-cathode spacing.
BACKGROUND OF THE INVENTION
It is well known in the art to utilize a linear beam microwave vacuum tube device, such as a klystron or traveling wave tube amplifier, to generate or amplify high frequency, microwave RE energy. Such devices generally include an electron emitting cathode, an anode spaced therefrom, and a grid positioned in an inter-electrode region between the cathode and the anode. Grid to cathode spacing is directly related to the performance and longevity of the linear beam device. A problem that has long existed in the art is that during initial heat up, the grid to cathode spacing changes as the cathode is heated, thereby causing performance and reliability problems.
Prior solutions to this problem suggested a grid support structure that is closely connected to the cathode button. These solutions however required complicated mechanical means to deal with the different radial thermal expansion of cathode and grid. In order to electrically insulate the cathode and the grid a plurality of ceramic members was needed to connect the grid to the cathode button. These ceramic members create a plurality of difficulties because the ceramic members are mechanically stressed from the expansion difference. Thus, it would be very desirable to provide a cathode support structure for a linear beam device that maintains a proper spacing between the cathode and grid across the operating temperature range of the device. It would be further desirable to provide such a grid support structure which is formed of a one-piece ceramic. Further, some cases are known where the cathode support cylinder has changed its shape over time due to thermal stress by many heat cycles. In a grided tube with a grid support independent from the cathode button this would cause the cathode to short out with the grid or at least change the initial cathode grid spacing. In both cases the tube will fail early.
SUMMARY OF THE INVENTION
In accordance with one aspect a grid support structure maintains a proper grid-to-cathode spacing across an operating temperature range of the linear beam device.
Another aspect of the present invention also provides a cathode grid connection that allows the grid to follow all cathode movements.
In one aspect of the present invention a linear beam device has an axially centered cathode and an anode spaced therefrom. The anode and cathode are operable to form and accelerate an electron beam. The linear beam device includes an axially centered grid positioned between the cathode and the anode. The grid is operable to accept a high frequency control signal to density modulate the electron beam. A grid support is in contact with the cathode and the grid and keeps the spacing between the cathode and the grid constant, while electrically insulating them.
It is another aspect of the present invention to provide a linear beam device having a cathode and an anode. A linear beam device includes a grid positioned at a predetermined distance from the cathode between the cathode and the anode. The grid is operable to accept a high frequency control signal to density modulate a beam. A grid support supporting the grid which is operable to maintain the predetermined distance between the cathode and the grid throughout the operating temperature range of the linear beam device.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein:
FIG. 1
is a side cross-sectional view of a temperature compensated gun according to a preferred embodiment of the present invention;
FIG. 2
is an enlarged cross-sectional view of the cathode-grid assembly of the gun of
FIG. 1
; and
FIG. 3
is a side cross-sectional view of the grid support of FIGS.
1
and
2
.
DETAILED DESCRIPTION OF THE INVENTION
The present invention satisfies the need for a grid support structure for a linear beam microwave vacuum tube device that maintains a proper spacing between the cathode and grid across the operating temperature range of the device. It should be understood that although terms such as “above” and “below” are used herein, these terms are to be interpreted in the relative sense as the linear beam device or temperature compensated gun is usable in any orientation.
Referring first to
FIG. 1
, the temperature compensated gun of a linear beam device, generally indicated at
10
, is illustrated according to the present invention. Because the gun operates conventionally, and the arrangement of the gun is known to one of ordinary skill, other than the inventive grid support structure of the present invention, the gun and the components illustrated in
FIG. 1
will only be described briefly and generally.
As illustrated in
FIG. 1
, linear beam microwave vacuum tube device
10
includes a temperature compensated gun, i.e., cathode-grid, assembly, generally indicated at
12
, a heater assembly
14
, a cathode assembly
16
, a planar anode-pole flange
18
connected to an anode-drift tube
20
, an input ceramic
22
, a focus ring
24
, a grid connection
26
and a cathode support connection
28
. The heater assembly
14
extends into the cathode assembly
16
without touching it. The anode includes a central aperture, and by applying a high voltage potential between the cathode
40
and the anode-pole flange
18
, electrons can be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture. Gun
12
is particularly useful in one class of linear beam microwave vacuum tube devices, referred to as an inductive output tube (IOT) which includes a grid
30
disposed in the inter-electrode region between the cathode
40
and the anode
20
. The electron beam can thus be density modulated by applying an RF signal to the grid
30
relative to a cathode
40
. As the density modulated beam is accelerated to the anode and propagates across a gap provided downstream within the TOT, RF fields are induced into a cavity coupled to the gap. The RF fields can then be extracted from the cavity in the form of a high power, modulated RF signal. An example of an TOT is disclosed by U.S. Pat. No. 5,650,751 to R. S. Symons, entitled “INDUCTIVE OUTPUT TUBE WITH MULTISTAGE DEPRESSED COLLECTOR ELECTRODES PROVIDING A NEAR-CONSTANT EFFICIENCY,” the subject matter of which is incorporated in the entirety by reference herein.
A grid support structure is illustrated in
FIGS. 1-3
, in which the linear beam microwave vacuum tube device
10
includes the axially centered grid
30
disposed in close proximity to the cathode
40
. To permit high RF voltage and high RF gain, it is desirable to space the grid
30
close to the cathode
40
surface. The grid support structure prevents, during start-up, the cathode
40
from moving toward the grid
30
. If the cathode
40
moves toward the grid
30
, then: 1) a change in perveance occurs during heat-up; 2) there is a possibility to short out the cathode and the grid; and 3) there is a variance in perveance. More particularly, the axially centered grid
30
is operable to accept a microwave high frequency control signal to density modulate an electron beam emitted by the cathode
40
. The grid
30
comprises a central active portion
34
and a peripheral portion or grid flange
36
with the peripheral portion comprising a plurality of evenly spaced mounting holes. The grid
30
is comprised of pyrolytic graphite material. The cathode
40
comprises a concave electron emitting surface
42
and the active portion
34
of the grid comprises a concave shape that corresponds with the emitting surface
42
. The concave electron emitting surface
42
and the grid
30
are concentric spheres, having the same center so that the grid
30
and emitting surface
42
are generally parallel to each other. The grid
30
is secured in place by a grid support structure (described below). The grid flange
36
is flat and lies in a plane that is substantially normal to the axis of the electron beam emitted by the cathode
40
.
The cathode assembly
16
is bolted to a cylindrical lower support
44
which in turn is connected to an upper support
46
. The lower support
44
has a plurality of threaded bolt holes
48
and is connected to a cathode flange
51
through corresponding bolt holes
55
in the cathode flange
51
. The cathode flange
51
has an annular recess
53
which receives one end
54
of a cylindrical molybdenum cylinder
56
. The end
54
of the molybdenum cylinder
56
is brazed to the recess
53
of the cathode flange
51
. An opposite end
57
of the molybdenum cylinder
56
is brazed to the cathode
40
. Since it is desirable to space the grid
30
closely to the cathode
40
surface, the grid
30
must be capable of withstanding very high operating temperatures. In view of these demanding operating conditions, it is known to use pyrolytic graphite material for the grid
30
due to its high dimensional stability and heat resistance. The pyrolytic graphite grid
30
may be made very thin, with a pattern of openings formed therein, such as by conventional laser trimming techniques, to permit passage of the electron beam therethrough. Because of the low coefficient of expansion of the pyrolytic graphite, heating of the grid
30
(by direct thermal radiation from the cathode
40
and by dissipation of RE drive power applied between the cathode
40
and grid
30
) does not cause the grid
30
to expand into the cathode
40
and short circuiting of these two elements does not occur. As a result, the grid
30
can be positioned very close to the cathode
40
surface
42
, permitting high RE drive voltage and high gain. Nevertheless, a practical limitation on the efficiency of such linear beam devices has been the difficulty of supporting the cathode
40
in a proper position relative to the grid
30
.
Heater assembly comprises an insulated flange package
62
connected to two posts (one has heat shields). Posts are connected to a heating element
64
. The flange package is bolted to a heater connection
60
(upper flange) and a “ground” connection
66
(lower flange) which is at cathode potential. The heating element
64
is spaced from the cathode
40
. The grid
30
is mechanically connected through the grid support
114
to the cathode
40
and moves together with the cathode
40
as the cathode assembly expands.
As previously mentioned, in a linear beam device, such as a microwave electron beam vacuum tube with a gun, driven with RF applied to a grid, the spacing between the cathode
40
and the grid
30
must be precisely maintained because the spacing is in the range between 0.005 and 0.010 inches; this spacing is necessary to make the tube work at microwave frequencies, e.g., close to 1 GHZ, i.e., the grid to cathode spacing is a fraction of a wavelength of the tube operating frequency.
In operation, when the tube operation is started the cathode
40
is heated and expands towards the grid
30
. As depicted in
FIG. 1
, for example, the molybdenum cylinder
56
expands when the heating elements
64
are energized. Because the cathode
40
is rigidly connected to molybdenum cylinder
56
during a transient heat up condition, the grid cathode spacing would change if the cathode
40
were to move toward the grid
30
. If such movement is not prevented, the heating would cause a change in the cathode
40
to grid spacing if the grid support structure is not closely connected directly to the cathode
40
. The change in spacing would disadvantageously cause:
(1) A change in perveance during heat up. Applying constant beam and grid voltage the beam current would change during the first 15 to 20 minutes of operation after applying heater voltage. For many tube applications this long waiting time to get stable operation is unacceptable so that the only other solution is to constantly preheat the cathode (=stand by). This causes a constant evaporation of barium from the cathode
40
and limits the lifetime of the gun
10
. In many applications it would be desirable to reduce the total heat up time to less than five minutes.
(2) A possibility to short out the cathode
40
and the grid
30
. Especially in applications where the cathode
40
temperature is variable due to a variable heater voltage, cathode
40
might expand into the grid
30
to cause a short circuit between them. This will immediately damage both cathode
40
and the grid
30
and must be avoided. Tubes with tungsten dispenser type cathodes can usually be recovered from weak emission by overheating the cathode for the regeneration of barium on its surface. In the case of a tube with a grid, however, overheating might cause the cathode
40
to expand more than the gun was designed for and short out with the grid. This means that the useful tool of overheating the cathode cannot be used for a grided electron beam tube with small cathode to grid spacing.
(3) A variation in perveance depending on the cathode
40
temperature. As described with regard to the change in perveance during heat up, the expansion of the cathode
40
would decrease the spacing between cathode and grid. In many applications it is desirable to vary the cathode heating during the lifetime of the tube to optimize the barium production of the cathode and by this stabilize and secure the emission. Within the first couple hundred hours of operation the cathode should be heated slightly more to stabilize the barium production. Once the barium production is stable enough the cathode can be operated at lower temperature to evaporate less barium. This increases the lifetime of the cathode. When the tube reaches the end of its lifetime many operation hours can be added by increasing the cathode temperature to activate more barium. This procedure is well known for television klystrons and many other electron beam tubes. However, it is difficult or impossible to apply this procedure to a grided tube if the spacing between cathode and grid depends on the cathode temperature. So it is desirable to have a grided gun with constant cathode to grid spacing.
The electrical and mechanical connections of the grid
30
to cathode
40
via grid support structure
114
are illustrated in detail in
FIG. 2. A
copper foil
90
is disposed between a grid connection support
80
and the focus ring
24
. The thin copper foil
90
is used to provide electrical contact to the grid
30
through the grid connection support
26
(
FIG. 1
) and the grid connection support
80
. The copper foil
90
also has a plurality of evenly-spaced holes aligned with holes
84
of the grid connection support
80
. Tightening of the bolts
91
holding the focus ring
24
to the holes
84
in the grid connection support
80
compresses the copper foil
90
so that the foil conforms to support
80
and ring
24
. During high temperature “bake-out” of the linear beam device
10
, the copper foil
90
softens to reduce internal stress. The copper foil
90
has a portion
92
which extends inwardly and which has a plurality of substantially evenly spaced holes
94
. The foil is bolted together by bolts
96
with the grid flange
36
and the grid support
114
through corresponding bolt holes. The copper foil
90
provides for expansion and is flexible and has a fold or stepped portion
97
to provide for cathode
40
movement. For better heat transfer, the copper foil
90
can be constructed from a plurality of foils. An inner portion
98
of the copper foil
90
is positioned radially inwardly from bolts
96
and is clamped between a grid cover ring
110
and a flange
120
of grid support
114
together with the grid flange
36
. Disposed below and adjacent to a lower surface
106
of the stepped portion
97
is an upper surface
112
of the grid flange
36
of the grid
30
. The grid cover ring
110
is positioned below a lower surface
112
of the grid flange
36
. The grid cover ring
110
is made of a glassy carbon. The grid cover ring
110
could be left out if the grid flange
36
is thick enough to distribute the bolt
96
force evenly enough to get good contact between the grid flange
36
and the copper foil
90
. Also, instead of the glassy carbon, one could use small segments of stainless steel or any other metal or ceramic. Glassy carbon was chosen because it has the same expansion coefficient as the grid
30
and the grid support
114
while it is less expensive than PBN or pyrolytic graphite. The grid cover ring
110
is an annular member having a plurality of bolt holes matching the holes of the grid flange and grid support. The bolts
96
tighten the grid support
114
, the copper foil
90
, the grid flange
36
and the grid cover ring
110
together.
As depicted in
FIGS. 2 and 3
, the grid support
114
has an outwardly extending flange portion
120
, an intermediate vertically extending portion
122
and an inwardly extending lip
124
which together form a cup-like structure. Four (or more) circumferentially spaced and inwardly extending slots
126
are cut in the inwardly extending lip
124
and partially into the vertically extending portion
122
to provide flexibility in the grid support
114
. The cathode
40
has an outer button portion
86
which has an inwardly extending annular groove
88
which receives the lip
124
of the grid support
114
.
The grid support
114
is a one-piece ceramic structure to support the grid
30
and directly connect it to the cathode
40
. The grid support
114
is made from a pyrolytic boron nitride (PBN) ceramic. The grid support
114
has a cup shape with its bottom removed and has a thin slotted wall that is flexible enough to be clipped to the cathode
40
like a spring. The grid support
114
can also be brazed to the outside diameter of the cathode
40
. The slots
126
of the grid support
114
also cause the expanding cathode
40
to only bend the remaining tab formed sections of the cylindrical part of the grid support
114
, to prevent substantial stressing of the flange shaped portion. The material provides a minimal heat transfer characteristic so the grid
30
is not additionally heated by conduction. The flexibility and other mechanical properties of PBN are fairly stable up to 2000° C., so that grid support
114
does not substantially change size as a result of operation of device
10
. The machinable ceramic is machined to very small tolerances so no structure is necessary to align support
114
axially and radially to the cathode
40
. The ceramic of support
114
provides a non-moving, non-expanding mounting platform for the grid
30
that keeps the cathode
40
to grid
30
spacing stable at all temperatures. The surface of vertically extending portion
122
of the grid support
114
facing the grid
30
forms a mounting platform and is shaped as a flange. The flange
120
has a plurality of holes
128
through which the bolts
96
extend. The grid
30
is made of pyrolytic graphite which has nearly the same expansion coefficient as PBN which is used to form the ceramic support
114
. Therefore, the grid-ceramic connection remains unstressed at all operating temperatures of device
10
. A glassy carbon flange
110
on top of the grid flange
36
provides distribution of the clamping force. The glassy carbon flange could also be formed of thin stainless steel flange sections.
The grid
301
cathode
40
spacing can be adjusted by choosing the right number of shims between the grid rim
36
and ceramic flange
120
, i.e., the number of foils
90
between rim
36
and flange
120
determines the spacing between grid
30
and cathode
40
. The axial alignment is provided by the holes in the grid rim that are large enough to allow for adjustment before tightening the screws.
During operation of the linear beam device
10
, the pyrolytic graphite material of the grid
30
experiences slight thermal expansion. The cathode
40
on the other hand exhibits some thermal expansion in both the axial and radial directions. The material composition of the grid support
114
and the grid
30
and the grid cover ring
110
are selected to have similar coefficients of expansion and thus expand and contract at a uniform rate. As the cathode
40
expands in the radial direction, the grid support
114
flexes outwardly. Thermal expansion in the axial direction is basically caused by the molybdenum cylinder
56
. The axial expansion of cylinder
56
moves the cathode
40
together with the grid support
114
and the grid
30
and leaves the cathode
40
to grid
30
spacing basically constant. The only portion of cathode
40
that expands into the grid
30
is the part of the cathode
40
between the grid
30
and the inwardly extending annular groove
88
which is very small and causes only an acceptable variation in spacing.
It will be readily seen by one of ordinary skill in the art that the present invention fulfills all of the objects set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.
Claims
- 1. A cathode-grid assembly for a linear beam microwave tube, the assembly and a beam adapted to be derived by the assembly as a result of operation of the tube having a coincident longitudinal axis, the cathode-grid assembly comprising a cathode assembly that expands radially and axially relative to said axis as a result of operation of the tube, a grid made of material that does not expand substantially as a result of operation of the tube, and an electrical insulating structure that is flexible relative to the cathode assembly and made of material that does not expand substantially as a result of operation of the tube, the structure mechanically connecting the grid and cathode assembly together so that the grid is spaced in the direction of the longitudinal axis from an electron emitting surface of the cathode by a fraction of an operating wavelength of the tube, the structure being connected only to peripheral portions of the grid, the material and construction of the structure being such as to maintain the spacing, in the direction of the longitudinal axis, between the grid and the cathode electron emitting surface substantially the same during operation of the tube,wherein the structure is made of pyrolytic boron nitride, wherein the structure includes: (a) a thin wall extending in the direction of and spaced from the longitudinal axis, (b) a first flange extending radially inward from the wall, and (c) a second flange extending radially outward from the wall, the first flange having an end remote from the wall fixedly connected to a cylindrical surface of the cathode assembly, the second flange being fixedly connected to the grid.
- 2. The assembly of claim 1 in combination with the microwave tube.
- 3. The assembly of claim 1 wherein the structure is carried by the cathode assembly and carries the grid.
- 4. The assembly of claim 1 further including a metal foil between the structure and the grid, the metal foil having a thickness in the direction of the axis.
- 5. The assembly of claim 1 further including a plurality of stacked metal foils between the structure and the grid, each of the metal foils having a thickness extending in the direction of the longitudinal axis.
- 6. The assembly of claim 1 wherein the first flange includes plural radially extending slots connected to corresponding slots in the wall.
- 7. The assembly of claim 6 wherein the structure is carried by the cathode assembly and carries the grid.
- 8. The assembly of claim 7 in combination with the microwave tube.
- 9. A cathode-grid assembly for a linear beam microwave tube, the assembly and a beam adapted to be derived by the assembly as a result of operation of the tube having a coincident longitudinal axis, the cathode-grid assembly comprising a cathode assembly that expands radially and axially relative to said axis as a result of operation of the tube, a grid made of material that does not expand substantially as a result of operation of the tube, and an electrical insulating structure that is flexible relative to the cathode assembly and made of material that does not expand substantially as a result of operation of the tube, the structure mechanically connecting the grid and cathode assembly together so that the grid is spaced in the direction of the longitudinal axis from an electron emitting surface of the cathode by a fraction of an operating wavelength of the tube, the structure being connected only to peripheral portions of the grid, the material and construction of the structure being such as to maintain the spacing, in the direction of the longitudinal axis, between the grid and the cathode electron emitting surface substantially the same during operation of the tube,wherein the structure is made of flexible ceramic material and includes: (a) a thin wall extending in the direction of and spaced from the longitudinal axis, (b) a first flange extending radially inward from the wall, and (c) a second flange extending radially outward from the wall, the first flange having an end remote from the wall fixedly connected to a cylindrical surface of the cathode assembly, the second flange being fixedly connected to the grid.
- 10. The assembly of claim 9 further including a metal foil between opposed surfaces of the second flange and the grid, the metal foil having a thickness extending in the direction of the longitudinal axis.
- 11. The assembly of claim 9 further including a plurality of stacked metal foils between opposed surfaces of the second flange and the grid, each of the metal foils having a thickness extending in the direction of the longitudinal axis.
- 12. The assembly of claim 9 wherein the first flange includes plural radially extending slots connected to corresponding slots in the wall.
- 13. The assembly of claim 12 wherein the structure is carried by the cathode assembly and carries the grid.
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