1. Field of the Invention
This invention generally relates to pulse generators for generating high-voltage pulses and, more particularly, to vector inversion generators that operate on the principle of combining inverted electric field vectors with static electric field vectors.
2. Related Art
Various high-voltage sources are available to generate high-voltage pulses that are utilized for the operation of high-voltage devices, such as, pulse radar, X-ray machines and the like. A conventional vector inversion generator (VIG) is one such high-voltage source suitable for supplying high-voltage pulses to a variety of high-voltage devices. The VIG is structured similar to a two-layer spiral wound capacitor and functions as two parallel plate transmission lines sharing a common conductor. The VIG has a capacitance that receives a charge from a voltage source connected to a terminal pair of the VIG and after the capacitor is fully charged, the voltage source is replaced by a short circuit. The short circuit causes, after a time delay, a high-voltage pulse to appear on an output terminal pair of the VIG. The plates of the capacitor function as two transmission lines with the first transmission line (often referred to as the active transmission line) reflecting a time varying electrical field and combining the resulting inverted electric field vector with a relatively slowly decaying electric field on the second transmission line (often referred to as the static transmission line).
Conventional vector inversion generators are made by winding several layers of a VIG fabric (“fabric”) about a circular core in a manner similar to winding ribbon on a cylinder. The circular core may have a variety of diameters and lengths, and the fabric is typically comprised of alternate layers of foil-like conductors such as copper or aluminum and dielectrics such as Mylar or Teflon. The foil-like conductor is preferably shim stock, a type of foil that has consistent dimensions and electrical characteristics. A variety of dielectrics may be used and such dielectrics typically have a relative dielectric constant (permittivity) greater than two.
Most conventional VIGs use a fabric comprised of two alternating layers of conductor and dielectric. Each layer of dielectric is typically comprised of multiple sheets of dielectric material to minimize the chance of breakdown due to pin holes that sometimes occur in a dielectric material. The fabric is usually wound about a cylindrical core two to ten or more times. The diameter of the cylindrical core is usually significantly greater than the total thickness of all the layers of the wound fabric. The fabric has a first end, the inside end of the spiral, and a second end, the outside end of the spiral. Two transmission lines are formed by the wound fabric, and the transmission lines have a common conductor. FIGS. 1A and 1B illustrate the wound fabric and the two transmission lines. The dielectric layers between the conductors, as shown in FIG. 1, form a capacitor that is capable of storing energy in an electric field in accordance with the well-known capacitance energy storage theory.
When a source voltage is applied to a conductor pair at the first end of the conventional VIG, the capacitance of the generator starts to charge and will reach full charge in accordance with the time constants of the charging circuit. Once the generator's capacitor is fully charged, the conventional VIG has energy stored in electric fields of the dielectric materials. The capacitance of the capacitor may be determined using well-known capacitance relations. The amount of energy stored, Estored, may be determined by using the electrical relationship,
      E    stored    =                    1        2            ·              C        T              ⁢                  V        2            .      
After the capacitor is fully charged by the source voltage, the source voltage is effectively replaced by a short circuit. The short circuit causes a time varying electric field to propagate from the origin of the short circuit, the first end of the VIG, to the second end or far end of the VIG in accordance with transmission line theory. A reflected electrical field wave, from the far end, then returns toward the first end, inverting the polarity of the initial electric field vector. The return wave, when added to the relatively static electrical field of the second transmission line, provides a voltage that, ideally, is 2N times the source voltage. Because the voltage increases by a factor of 2N, the value for the new equivalent capacitance is equal to the previous capacitance divided by (2N)2 thereby satisfying the conservation of energy principle.
Much of the current research pertaining to conventional VIGs is directed towards properties of materials and the diameter to turns ratios of such generators. For example, the diameter of the spiral, the number of turns of fabric, the dielectric values and thickness of the dielectrics, the conductivity and thickness of the conductors, and the number of layers (2 or more) are some of the design parameters. The voltage efficiency and energy efficiency are important consideration when selecting values for the design parameters. Designers must also consider limitations due to corona discharge, which occurs when voltages get too high.
Among the shortcomings associated with the conventional VIG is their size and weight. Because a large diameter is often desirable, to improve efficiency, for the conventional VIG, such a device may be undesirably large for certain applications. For some applications of the conventional VIG, it may be necessary or desirable to place the VIG in an enclosure containing non-conducting oil or potting material, thereby improving performance, but with the disadvantage of sometimes making such a device unacceptably heavy. Further, it is impractical to change the electrical characteristics of a conventional VIG after it has been fabricated. For example, when a VIG is fabricated, the number of fabric turns, the characteristics of the conductors and dielectrics, and the dimensions are fixed making such parameters essentially permanent, thereby making changes impractical. In order to change the parameters and characteristics of a conventional VIG, it is necessary to fabricate a new device.