Electric lamp and discharge devices – Electrode and shield structures – Cathodes containing and/or coated with electron emissive...
Reexamination Certificate
1999-07-15
2002-07-16
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
Cathodes containing and/or coated with electron emissive...
C313S495000, C313S310000, C313S311000, C313S351000
Reexamination Certificate
active
06420822
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electron emitting devices, and more particularly, to an electron emitting cathode that operates based upon the triple-junction effect.
2. Description of Related Art
Electron emitting cathodes are used in a variety of devices ranging from cathode ray tubes for display purposes to sophisticated amplifiers used in communication and radar systems for amplifying radio frequency (RF) or microwave electromagnetic signals. For example, it is well known in the art to utilize an electron emitting cathode within a traveling wave tube (TWT), klystron, or other microwave device. In these devices, electrons originating from the electron emitting cathode are focused into a beam and caused to propagate through a tunnel or a drift tube generally containing a RF interaction structure. A RF wave is made to propagate through the interaction structure so that it can interact with the electron beam that gives up energy to the propagating RF wave. Thus, the device may be used as an amplifier for increasing the power of a microwave signal. At the end of its travel, the electron beam is deposited within a collector or electron beam dump, which effectively captures the remaining energy of the spent electron beam. The electron beam may be focused by magnetic or electrostatic fields in the interaction structure of the device to prevent the electron beam from expanding due to space-charge forces and to permit it to effectively travel from the electron gun to the collector without current lost in an undesirable fashion to the interaction structure.
The electron emitting cathode may include some form of heater, such as an internal heater disposed below the cathode surface, that raises the temperature of the cathode surface to a level sufficient for thermionic electron emission to occur. Alternatively, the cathode may be made to produce electrons without the aid of a heater, such as for a cold-cathode gas tube where the electrons are produced by bombardment of the cathode by ions and/or by the action of a localized high electric field. When the voltage potential of an anode spaced from the cathode is made positive with respect to the cathode, electrons are drawn from the cathode surface and caused to move toward the anode.
In addition to the desired electron emission from the cathode in vacuum electron beam devices, there is often undesirable emission from negative electrodes of the devices. In a typical vacuum electron beam device, such as the TWT or klystron noted above, a significant weak point from an electrical breakdown perspective is the interface between a metal, an insulator, and a vacuum. This interface is referred to as a “triple junction” (i.e., metal-insulator-vacuum) and is illustrated in FIG.
1
. The triple junction has been positively identified as a source of field emission electrons in vacuum electron beam devices. The inventor first encountered enhanced field emission from a triple junction in the early 1960's when attempting to build a very large, high power klystron.
FIG. 2
shows a portion of the prior art klystron that uses an insulator
10
that is cylindrical in shape, approximately twelve inches in diameter and eight inches long, and made of alumina ceramic. The insulator
10
has a negative electrode
12
at one end and a positive electrode
14
at the other end and is immersed in a magnetic field
16
with symmetry about an axis
18
of the insulator
10
. The insulator
10
is brazed to the negative and positive electrodes
12
and
14
, respectively, after metalizing the ends of the ceramic using the molybdenum-manganese process commonly used to make vacuum-tight brazes between a ceramic and a metal. The magnetic field
16
is stronger at the positive electrode
14
than at the negative electrode
12
so that electrons
20
following a trajectory from a triple junction at the negative electrode
12
, upon hitting the positive electrode
14
, impinge on a circle having a diameter that is smaller than the diameter of the insulator
10
. The insulator
10
was intended to hold off 200-300 kilovolts (kV) DC, but at voltages of approximately 150 kV to 200 kV, an electronic discharge was found to occur between the positive and negative electrodes
12
,
14
. The power was such that melting would occur at the above-referenced smaller circle on the positive electrode
14
and the electronic discharge would develop into a full-fledged arc.
The personnel at the Lawrence Berkeley Laboratory at the University of California were having similar problems, but with lucite cylinders located between essentially flat metal plates that had been glued to the ends of the lucite cylinder with acrylic cement. Using pinholes to collimate the beam of electrons, they had also discovered that the electrons were coming from the junction between the lucite cylinder and the negative electrode, in other words, at the triple junction. A solution that developed in the electron device community and the physics community was to place short metal cylinders with rounded edges inside and outside the insulating cylinder in such a way that the contact between the insulator and the metal is shielded from electric fields. This solution was applied to the klystron of
FIG. 2
, as shown in
FIG. 3
, with the placement of short metal cylinders
22
,
24
that act to shield the triple junction from the electric fields. This solved the arcing problem.
Over the years, various theories have been proposed as to the cause of the electron emissions near the triple junction. One such theory is stated by H. Craig Miller, in a paper entitled “Surface Flashover of Insulators,” presented at the Workshop On Transient Induced Insulator Flashover In Vacuum (Aug. 24-25 , 1988), sponsored by the Microwave and Pulsed Power Thrust Area of the Lawrence Livermore National Laboratory (CONF-8808171). The Miller paper dealt at length with the initiation of flashover near the triple junction at the negative end of insulators. Miller appeared to support the idea that the enhanced emission of electrons from the triple junction was due to a crack between the insulator and the metal, which produced high electric fields at the surface of the metal. For example, when the insulator is mechanically held in place, a crack would exist at the union between the insulator and the metal. Nevertheless, this theory was refuted by others with experience with brazed ceramics, which generally had no cracks at the union.
Another theory is that an electric field concentration caused by the edge of the metalizing is the source of the problem. For example, during the process of metalizing the surface of the insulator and brazing it to the metal, a fillet of braze material on the surface of the insulator unavoidably forms. This theory in turn is contradicted by the experience with lucite insulators that have no fillet of braze material.
In summary, it is known that electron emission does occur at the triple junction, but no hypothesis that fully explains the triple junction effect has been proposed. It would be very advantageous to avoid the undesired consequences of triple junctions and to provide a cathode that utilizes the triple-junction effect to achieve desired electron emission. The triple-junction cathode would be able to provide electron emissions, such as for an electron gun in an electron beam device, display devices or other devices utilizing emitted electrons in their operation.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an electron emitter is provided that is based upon the triple-junction phenomena. The electron emitter is based on the hypothesis that, even with the plain parallel equipotentials and parallel electric field lines that would exist between two plain parallel metal plates separated by a cylindrical dielectric insulator, the electric displacement vector and consequently, the surface charge under the ends of the insulator, will be higher than the surface charge outside of the region contacted by the insulator. In
Berck Ken
O'Melveny & Myers LLP
Patel Vip
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