U.S. patent number 3,891,884 [Application Number 05/425,435] was granted by the patent office on 1975-06-24 for electron discharge device having electron multipactor suppression coating on window body.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Lawrence H. Tisdale.
United States Patent |
3,891,884 |
Tisdale |
June 24, 1975 |
Electron discharge device having electron multipactor suppression
coating on window body
Abstract
A dielectric body which is permeable to electromagnetic wave
energy of a material selected from the group consisting of alumina
and beryllia ceramics is provided with a coating of a
semiconducting oxide to substantially suppress electron
multipactoring. The exemplary materials include semiconducting
oxides of silicon and transition metals including copper, cobalt,
chromium, iron, manganese and nickel. Thicknesses averaging 1,000
Angstrom units have resulted in substantial increases in the power
handling ability of electromagnetic devices employing such
dielectric bodies.
Inventors: |
Tisdale; Lawrence H.
(Wakefield, MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
26951571 |
Appl.
No.: |
05/425,435 |
Filed: |
December 17, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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266020 |
Jun 26, 1972 |
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Current U.S.
Class: |
313/107;
333/99MP; 333/248; 333/252 |
Current CPC
Class: |
H01P
1/08 (20130101); H01J 23/36 (20130101) |
Current International
Class: |
H01J
23/00 (20060101); H01J 23/36 (20060101); H01P
1/08 (20060101); H01j 043/28 (); H01p 001/08 () |
Field of
Search: |
;333/98P,99MP
;313/107 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Preist, D. H. "Multipactor Effects & Their Prevention in
High-Power Microwave Tubes," Microwave, Jr., 10-1963, pp.
55-60..
|
Primary Examiner: Smith; Alfred E.
Assistant Examiner: Punter; Wm. H.
Attorney, Agent or Firm: Rost; Edgar O. Pannone; Joseph D.
Murphy; Harold A.
Parent Case Text
This is a division of application Ser. No. 266,020 filed June 26,
1972, now abandoned.
Claims
I claim:
1. An electron discharge device comprising:
an evacuated envelope having an access opening for propagating
electromagnetic energy;
a body of a dielectric material sealing said access opening;
a coating of a semiconducting oxide material of a transition metal
having a thickness averaging about 1,000 Angstrom units on the
surface of said dielectric body exposed to the vacuum to suppress
electron multipactoring.
2. The device according to claim 1 wherein said dielectric material
is selected from the group including alumina and beryllia and said
coating material is selected from the group including silicon,
chromium, cobalt, copper, iron, manganese and nickel.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to means for suppressing electron multipactor
on the surface of dielectric bodies.
2. Description of the Prior Art
Bodies of dielectric materials selected from the group including
alumina and beryllia have been utilized as windows in the microwave
art for the transmission of electromagnetic energy and to provide
vacuum seals in high power electron discharge devices due to the
ability to withstand thermal shock up to predetermined levels.
Magnetrons, klystrons, crossed field amplifiers, as well as
oscillators, are exemplary electron discharge devices employing
such dielectric bodies. At high output power levels the dielectric
windows dissipate substantial thermal energy with forced fluid
cooling without puncture or fracture which destroys the vacuum
condition. A limitation, however, has been imposed on the
state-of-the-art with regard to such dielectric ceramic materials
in that they typically have secondary electron emission coefficient
values of 3 or higher and when bombardment by free electrons within
the vacuum envelope, become subjected to destructive heating due to
electron multipactoring and the build-up of electric fields on the
surfaces. Electron multipactor phenomena and the resultant window
failures during high power operation have been described in an
article entitled "Some High-Power Window Failures" by J. R. M.
Vaughan, IRE Transactions on Electron Devices, July 1961, pps.
302-308. Window failures due to cracking as well as punctures is
stated to be a result of electrical arcs and thermal shock.
Attempts to solve the foregoing problem include the deposition of
such materials as titanium, carbon, metal carbides and nitrides to
provide a surface on the dielectric body having a secondary
electron emission coefficient value of substantially unity. Such
metallic type conduction materials deposited on the surface of the
dielectric bodies typically decrease the resistivity to values less
than 10.sup.8 ohms per square unit area. Examples of such prior art
efforts may be found in U.S. Pat. No. 3,252,034, issued May 17,
1966, to D. H. Preist et al and U.S. Pat. No. 3,330,707, issued
July 11, 1967, to L. Reed. Typically, such layers must be
discontinuous and are approximately 100 Angstrom units or less in
thickness in order to avoid ohmic losses. The requirement for the
extremely thin film due to the low electrical resistivity is at
conflict with the need for a sufficient thickness to absorb the
primary electrons and substantially suppress the escape of
secondary electrons generated at the dielectric body surface. The
method utilized in the deposition of the foregoing enumerated
materials for such relatively thin films is difficult to control.
With ever increasing power levels of applicable electron discharge
devices operating in the electromagnetic wave energy spectrum,
electron multipactoring is a continuing problem limiting advance of
the art.
SUMMARY OF THE INVENTION
In accordance with the present invention, a dielectric body
permeable to electromagnetic energy selected from a group including
alumina and beryllia is coated with an oxide of a semiconducting
material to effectively suppress secondary electron emission. The
selected semiconducting oxides are principally of silicon or any of
the transition metals including manganese, chromium, cobalt,
copper, iron and nickel, which demonstrate stable characteristics
after high bake-out temperature conditions of, for example,
600.degree.C or higher utilized in the evacuating of applicable
devices. The term "semiconducting" is interpreted for the purposes
of the present invention to refer to a solid material whose
electrical conductivity is between that of a conductor and that of
an insulator. Further, the term "microwave" defines that portion of
the electromagnetic energy spectrum having wavelengths in the order
of 1 meter to 1 millimeter and frequencies in excess of 300 MHz.
The coating of semiconducting oxide material may be deposited on
the dielectric body material by one of the following
techniques:
a. evaporation of the metal in low pressure oxygen;
b. evaporation of the metal in high vacuum followed by controlled
oxidation of the film;
c. reactive sputtering of the metal in an atmosphere containing
oxygen;
d. sputtering of the metal followed by controlled oxidation of the
metal film;
e. RF sputtering from a target composed of the desired oxide.
Coating thicknesses averaging 1,000 Angstrom units of the selected
materials have been found to substantially suppress electron
multipactoring and permit operation at much higher power levels,
typically over one megawatt peak and above 10 kilowatts average.
The secondary electron emission coefficient characteristics of the
materials is typically lower than the titanium suboxide coatings
utilized in the prior art. The utilization of the substantially
thicker coatings also results in a visible evidence capability not
permissible with coatings having thicknesses averaging only 100
Angstroms or less to simplify process control as well as quality
assurance measurements. In exemplary embodiments of the invention
to be hereinafter described a two-fold increase in power handling
ability by substantial reduction of thermal energy generated in the
dielectric body material by electron bombardment was observed for
the semiconducting oxide coated windows as compared to the prior
art coated windows. This has resulted in an increase of power
handling capabilities of the applicable devices by at least a
factor of two.
BRIEF DESCRIPTION OF THE DRAWINGS
Details of the invention will be readily understood after
consideration of the following description of an illustrative
embodiment and reference to the accompanying drawings, wherein:
FIG. 1 is a cross-sectional view of a microwave window assembly for
high power microwave devices with the view taken along the line
1--1 in FIG. 2;
FIG. 2 is an isometric view of a high power crossed field amplifier
embodying the invention; and
FIG. 3 is a graph of the results of thermal dissipation of
dielectric bodies uncoated, coated in accordance with the prior art
and coated in accordance with the invention with relation to
average power transmitted.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2 of the drawings, an exemplary device for
the amplification of electromagnetic energy is shown. The crossed
field amplifier 10 has typical operating characteristics for pulsed
type operation of 3 megawatts peak power when driven by an RF
signal of 550 kilowatts. Such devices are conventionally utilized
in a frequency band of from 2,900 to 3,100 MHz. The average power
output for such devices for operation at this band as well as
L-band is typically 10 kilowatts or higher. The device shown is of
the integral magnet type with the magnetic fields provided by
substantially U-shaped magnet members 12 and 14. A vacuum sealed
metallic envelope 16 houses the internal components including the
slow wave anode structure and cold cathode. A metal-to-ceramic
cathode assembly 18 provides for the application of anode-cathode
electrical voltages, typically in the range of from 40 to 50
kilovolts. The electromagnetic energy is coupled to the anode
structure by input and output rectangular access waveguide sections
19 and 20 which are sealed at the outer ends by energy permeable
window assemblies 22 and 24. The high powers handled by such
devices require forced fluid cooling coupled through conduit means
26 and 28 in each of the window assemblies.
A representative high power handling window assembly is shown in
detail in FIG. 1. Such assemblies conventionally employ dielectric
window members which are permeable to electromagnetic energy of a
high thermal shock resistance material such as any of the materials
in the group including alumina and beryllia ceramics. Such window
members are typically of a circular configuration and dielectric
window member 30 of the desired composition is shown sealed within
a circular metallic waveguide section 32 by any of the known
metallizing and brazing techniques. A hollow passageway 34 provides
for circulation of a cooling fluid introduced through tubular
adapters 36 and 38 internally threaded as at 40 and 42 to receive
the threaded ends of the conduits 26 and 28. The circular waveguide
32 is provided at one end with a circular flange 44 for brazing the
assembly to the rectangular waveguide sections or in many devices
the window assembly is affixed directly to the metallic tube
envelope to enclose an access opening. The opposing end of the
circular waveguide body is provided with a thicker circular
waveguide mounting flange 46 of the type conventionally used in
electromagnetic transmission systems for coupling the energy to or
from the device. In accordance with the teachings of the invention,
a relatively thick film coating 48 having a thickness averaging
about 1,000 Angstroms is deposited on at least one surface of the
dielectric window body 30 of an oxide of a semiconducting material.
Transition metals selected from the group including chromium,
cobalt, copper, iron, manganese and nickel, (atomic numbers 24-29),
as well as oxides of other semiconductor materials, such as
silicon, have demonstrated successful performance in exemplary
embodiments of the invention.
The semiconducting oxide coatings of manganese and chromium (MnO
and Cr.sub.2 O.sub.3) have shown in the results plotted in FIG. 3 a
two-fold increase in the amount of thermal energy dissipation
measured by conventional calorimetric techniques. An uncoated
dielectric body shown by curve 50 will dissipate only approximately
80 watts at 10 kilowatts which would be well off the graph shown. A
prior art titanium oxide dielectric body is represented by curve 52
and thermal energy dissipation of 40 watts was measured at the 10
kw average power level. Dielectric bodies coated in accordance with
a semiconducting oxide are represented by curve 54. Such coated
dielectric bodies have demonstrated a thermal energy dissipation
capability of approximately 22 watts at the average power level of
10 kw. This almost two-fold increase in the thermal energy
dissipated permits substantially a two-fold increase in the energy
handling capability of an applicable device. The thicker coatings
of the semiconducting oxide materials have measured resistivities
typically in the range of about 10.sup.6 ohms per square unit area
which has no adverse effects on the electromagnetic energy
propagation characteristics or arcing. The thicker coating is
believed to substantially prevent the penetration and bombardment
by primary electrons in the presence of intense RF fields leading
to high secondary electron emission with the accompanying rise in
thermal energy.
A varied number of methods are possible in the practice of the
invention for the provision of the multipactor suppressing coating
on the surface of the dielectric body member. A number of these
methods have previously been enumerated and only one exemplary
embodiment will, therefore, be described. In the case of chromium
semiconducting oxide and maganese oxide coating RF sputtering from
target members provided the control necessary for the deposition of
the coating thicknesses in accordance with the invention. The
window assembly including waveguide 32 and window 30, after
cleaning by conventional techniques, is then mounted in an RF
sputtering system which is evacuated to a pressure of less than 2
.times. 10.sup.-.sup.6 torr. High purity argon is introduced into
the system and the pressure adjusted to approximately 5-6
millitorr. Because of the size and shape of the dielectric bodies
(3-5 inches) a screen at anode potential is disposed between the
target source and dielectric body to provide a uniform RF field and
insure uniform sputtering. An RF power level of 300 watts is
employed for the chromium while a power level of 110 watts is
employed for the manganese oxide. The sputtering times for the
1,000 Angstrom coatings is determined on the basis of
interferometer measurement of the coating thickness in
premanufacturing experiments.
With the RF sputtering process the material released from the
target at cathode potential upon bombardment by the argon gas ions
is deposited on the dielectric body (anode) forming the window
member of the assembly. Due to the fact that the target material is
electrically nonconductive, only RF fields can be utilized. In the
case of a manganese oxide coating a secondary electron emission
coefficient value of approximately 1.46 was measured which is lower
than the prior art titanium suboxide coatings having higher values
ranging between 1.54 and 1.88. The thicker coatings are less
critical to produce in order to avoid conduction losses. The
thicker coatings also reduce the overall cost of fabrication and
have an ancillary benefit in that any errors resulting from
confusion between coated and uncoated windows is substantially
reduced by the visual evidence of the thicker coatings.
In addition to the foregoing high power electron discharge devices,
the multipactor suppressing coating may be applied to other
dielectric bodies where high RF and DC electric fields are present.
An example of such an additional application would be in the field
of high voltage stand-off structures where the effective surface
electric field strength leading to damaging arcs as well as
multipactoring is reduced by the deposition of a semiconducting
oxide coating on the surfaces where such fields are likely to
occur. Other applications, variations and modifications will be
evident to those skilled in the art. It is intended, therefore,
that the foregoing description of the invention and illustrative
embodiments be considered broadly and not in a limiting sense.
* * * * *