Sensor and SinudationNotes Design Considerations for Ultra-Wideband, High-Voltage Baluns

We consider here how to build a balun for f%st-nsetime pulses at high vokage. We are concerned with signals with risetimes on the order of 100-200 ps, with high peak voItages and peak powers. We propose the simplest method is a coaxial unzipper. We consider dielectric strength and maximum coaxial radius allowable while still maintaining the risetime. Also considered are methods to reduce the coupling to the common mode. FmaUy, we caIculate the peak electric field and characteristic impedance of the coaxird unzipper at various point along its length using a two-dimensional Finite Element code. . . . L Introduction * Many high-voltage impulse generators provide a high-voltage, fkst-risetime pulse into a a coaxial geometry. ASthe voltages increase and risetimes decrease, it will be necessary to develop the essential circuit components such systems require. A balun for such a system is a particularly challenging component to develop, due to competing demands for high dielectric strength of materials and small dimensions, in order to preseme the risetime. We consider here the design principles required for baluns with voltages in the range of megavolts, powers in the range of tens of gigawatts to one hundred gigawatts, and risetimes in the range of 100-200 ps. We begin by providing a rationale for keeping the d~erential-mode impedance low, while keeping the common-mode impedance high, Next, we consider how to choose a feed impedance that keeps the electric field Iow for a given outer diameter of a cable, The dielectric properties of materials are then considered. We also provide a simple model for estimating the risetime that can be maintained through a given brdun. Finally, we provide an exampIe of numerical calculations, which would allow one to maintain a constant impedance through a coaxial unzipper, while keeping the peak field low. Let us begin now with a simple expkation of why such a baiun is necessary in the first place.