1. Field of the Invention
The present invention relates to an improved electron gun, and more particularly, to an electron gun assembly utilizing precision ground ceramic cylinders for precise alignment of the electron gun elements in order to simplify assembly and improve reliability.
2. Description of Related Art
It is well-known in the art to utilize a linear beam device within a traveling wave tube (TWT), klystron, or other microwave device. In such a linear beam device, an electron beam originating from an electron gun is caused to propagate through a tunnel or a drift tube generally containing an RF interaction structure. An RF wave can be made to propagate through the interaction structure so that it can interact with the electron beam which 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 beam dump which effectively captures the remaining energy of the spent electron beam. The beam may be focused by magnetic or electrostatic fields in the interaction structure of the device to prevent its expansion due to space-charge forces and permit it to effectively travel from the electron gun to the collector without undesired energy loss to the interaction structure.
The electron gun which forms the electron beam typically comprises a cathode and an anode. The cathode includes 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. When the potential of the anode is made positive with respect to the cathode, electrons are drawn from the cathode surface and caused to move toward the anode. The geometry and placement of the cathode and the anode define the shape and position of equipotential contour lines of an electric field within the inter-electrode region between the cathode and anode. The equipotential contour lines define the electron flow pattern, since electrons tend to follow a path generally perpendicular to the equipotential contour lines. The flow of electrons then passes from the electron gun to the interaction region of the device. An electron gun of this type is known as a Pierce gun.
In a Pierce gun, the cathode and another electrode, such as the anode or a grid, are often disposed within a single assembly, though the anode may be disposed external to the electron gun. Electrically insulating materials are utilized within the electron gun assembly to maintain the electric field potential between the anode and the cathode, and to prevent electrical breakdown or arcing within the assembly. Since the positioning of the anode with respect to the cathode is critical to the determination of the electric field shape and position of the equipotential contour lines, the cathode and anode elements are generally fixed in position within the electron gun, such as by brazing. This way, the electron beam focusing characteristics will remain substantially unchanged during operation of the electron gun within a microwave device.
A significant drawback of this type of electron gun assembly is in the difficulty of its manufacture. Once the electron gun is assembled together, its focusing characteristics cannot be varied without disassembling the electron gun components, often requiring the breaking of permanent braze joints. The brazed construction does not allow for variation of the component spacing after assembly. As a result, it is difficult to customize electron guns for unique focusing characteristics.
Accordingly, an electron gun having improved assembly characteristics and which avoids the alignment problems of the prior art would be very desirable.