1. Field of the Invention
The present invention relates generally to workpiece dies and more particularly to dies which are directed to eccentric workpieces.
2. Description of the Related Art
Traveling-wave tubes (TWTs) are capable of amplifying RF signals over a considerable frequency range, e.g., 2-100 GHz. They can achieve large signal gains over broad bandwidths, e.g., &gt;10%. They generally include a microwave slow-wave circuit and a surrounding magnetic beam-focusing structure which are both positioned between an electron gun and a collector.
In operation, a beam of electrons is launched from the electron gun into the slow-wave circuit and is guided through that circuit by the focusing structure. At the same time, an RF signal moves along the slow-wave circuit from a signal input port to a signal output port. The slow-wave circuit causes the phase velocity (axial velocity of the signal's phase front) of the RF signal to approximate the velocity of the beam's electrons. As a result, the beams are velocity-modulated into bunches which overtake and interact with the slower RF phase wave. In this process, kinetic energy is transferred from the electrons to the RF signal. After their passage through the slow-wave structure, the beam's electrons strike the collector where their remaining kinetic energy is dissipated in the form of heat.
In TWTs that are directed to applications in which power is more important than size and weight, the magnetic-focusing structure typically includes a solenoid which is wrapped around the slow-wave circuit. For ether applications, the bulky solenoid (and its power supply) are usually replaced by a periodic permanent magnet (PPM) which is formed with a series of annular, permanent magnets that are arranged so that adjacent magnets have opposite polarity.
Although there are many types of slow-wave circuits, the majority of TWTs employ a helix which is generally formed from a metal wire. In an exemplary helix TWT, the helix is supported within a barrel assembly by radially-directed support rods. The barrel assembly includes alternating segments of annular spacers and annular magnetic pole pieces. The pole pieces are made of a material, e.g., high purity iron, that efficiently channels the magnetic field to the beam region. The alternating pole pieces and spacers are brazed together to form external, annular focusing cells around an internal bore. Each of the focusing cells is configured to receive one of the annular permanent magnets. The brazed assembly must be capable of maintaining a high vacuum in the bore. Accordingly, the spacers are preferably made from a material (e.g., Monel--a nickel, copper alloy) that is nonmagnetic, can be easily brazed and has a thermal coefficient close to that of iron.
The support rods position and electrically isolate the helix within the bore of the barrel assembly. Lossy sections are typically positioned along the slow-wave circuit to enhance the TWT stability by absorbing internal power reflections. These sections are often realized with a thin carbon coating which is carried on the support rods. Consequently, the support rods are also referred to as attenuator rods.
Because of large helix voltages, the support rods must have a high dielectric strength. In addition, a considerable amount of heat is generated in the helix and, for successful TWT operation, this heat must be removed from the helix. Therefore, the support rods must also provide a good thermal conduction path. Accordingly, they are generally formed of ceramic and are tightly compressed between the helix and the bore of the barrel assembly. If this compression fails to obtain a good thermal conduction path between the support rods and the barrel assembly, the consequent heat buildup can cause degraded TWT performance or even TWT failure. Helix slow-wave circuits, support rods and magnetic focusing structures are described in numerous references, e.g., Hansen, James W., editor, TWT/TWTA Handbook, Hughes Aircraft Company, Electron Dynamics Division, Torrance, Calif., pp. 46-55
Several assembly methods have been developed for positioning the helix and support rods within the barrel assembly's bore. In a "heat shrink" method, the helix and support rods are joined with an adhesive, e.g., acrylic polymer, and inserted into the barrel assembly while it is at an elevated temperature. The barrel assembly is then allowed to cool to normal temperature and the consequent contraction places the support rods in compression. The adhesive is generally removed with solvents.
In a "cold stuff" assembly method, the support rods and helix are contained within a split sleeve of a strong, rigid metal, e.g., molybdenum, and physically forced (stuffed) into the bore of the barrel assembly. In a "coining" method, the support rods and helix are positioned within a copper sleeve which is then axially deformed to place the support rods into compression. The magnets and pole pieces are then assembled over the tube. In a "triangulation" method, the support rods and helix are positioned within a stainless steel sleeve which is radially deformed to place the support rods in compression. In contrast with the coining process, the deformation of the sleeve is circumferentially located between the support rods. The deformed appearance of the sleeve leads to the process name.
Therefore, TWT assembly is generally facilitated by the ability to apply radial force to the barrel assembly so as to obtain a good conductance path through the support rods. This ability can also be used as a supplemental fabrication technique at an interim assembly state. After partial assembly, TWTs are typically tested electrically to ascertain the sufficiency of the thermal path through the support rods. A drop in power at the output port immediately after the start of amplification, generally indicates a deficient thermal path. Pressure application which deforms the barrel assembly's spacers is often successful in correcting the thermal path problem.
Conventional fabrication techniques, e.g., reaming or honing, can obtain a smooth concentric bore. In contrast, the outer diameter of the spacers is relatively eccentric for several reasons, e.g., variations in spacer outer diameter, spacer misallignment prior to brazing and surface brazing remnants. When rigid structures, e.g., elongate bars, have been used to transmit radially deforming force, a low success rate has been experienced in improving the thermal path along the support rods. The low success rate has been attributed to the inability of the rigid structures to accommodate the outer diameter eccentricity of the spacers.