Modern trends in High Power Microwave (HPM) sources for a variety of applications are directed to increasing radiated power and efficiency as well as to increasing energy density (energy per unit volume). Transmission line pulse generators with photoconductive switches could achieve some of the best results.
High powered and high energy density transmission line pulse generators imply low and very low (mOhm range) characteristic impedances of transmission lines by using thin dielectrics. This low range of characteristic impedances frequently causes problems when coupling high powered and high energy density transmission line pulse generators with radiating impedances. In this scenario, the total value of impedance transformation may increase by, for example, 1:1000.
FIG. 1(a) illustrates a schematic of a well-known bipolar pulse generator. In this generator, with a closing switch, the matched load impedance is equal to the characteristic impedance of the transmission line, and voltage on the load is equal to ½ of the charged voltage of the transmission lines. The ideal waveform on the load is shown in FIG. 1(b).
A more complicated circuit shown in FIG. 2, also based on a closing switch. It provides a higher load impedance and a load voltage equal to ¾ of the voltage of the charged transmission line. Further increasing the load impedance with respect to the lowest characteristic impedance of the transmission line, and higher load voltage has been achieved in a relatively complicated structure shown in FIG. 3. For all of these circuits to achieve impedance for effective radiation, an additional high ratio step-up impedance transformer must be used as shown in FIG. 4.
Impedance transformers used to perform impedance transformations starting from low impedances may have especially low efficiency and large sizes. Therefore, the transformers themselves can defeat some of the advantages of high powered and high energy density transmission line generators. Moreover, problems presently exist with creating impedance transformers that increase impedance by large ratios.
For unipolar pulses, stepped transmission line pulse generators have been used by S. Darlington and provide impedance transformation depending on the number of charged, stepped transmission lines. Increased impedance transformation is achieved by applying increased (different) charging voltage on transmission lines and using extra switches. Such a design is complicated.
Bipolar pulses from high power sources provide two or more times higher radiation efficiency, due to differences in frequency spectra, compared to unipolar pulses. However, Bipolar pulse generators with electrical energy storage in charged transmission lines conventionally have not provided enough impedance transformations based on charged stepped transmission lines. Accordingly, for high efficiency of bipolar generation systems with conventional, for example exponential, transformers, the total size has conventionally been large and the designs complicated or impractical. Accordingly, there remains a need for a bipolar pulse generator solution based on Only voltage charged transmission lines that is capable of implementing high impedance transformation ratios (≧100) starting from tenths of mOhm. There is a further need for a bipolar pulse generator that is capable of being implemented in a simple structure. There is a further need for a high efficiency design that has a relatively low total size. There is a further need for such a system that allows simple access by fibers to a closing photoconductive switch that actuates the bipolar pulse generator. There is still a further need for a bipolar pulse generator that provides current multiplication in a simple structure.
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