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Systems emitting ultra-wideband high power microwave (HP/UWB) pulses are developed for military and civilian applications. HP/UWB pulses typically have durations on the order of nanoseconds, rise times of picoseconds and amplitudes around 100 kV m(-1). This article reviews current research on biological effects from HP/UWB exposure. The different references were classified according to endpoints (cardiovascular system, central nervous system, behavior, genotoxicity, teratology …). The article also reviews the aspects of mechanisms of interactions and tissue damage as well as the numerical work that has been done for studying HP/UWB pulse propagation and pulse energy deposition inside biological tissues. The mechanisms proposed are the molecular conformation change, the modification of chemical reaction rates, membrane excitation and breakdown and direct electrical forces on cells or cell constituents, and the energy deposition. As regards the penetration of biological matter and the deposited energy, mainly computations were published. They have shown that the EM field inside the biological matter is strongly modified compared to the incident EM field and that the energy absorption for HP/UWB pulses occurs in the same way as for continuous waves. However, the energy carried by a HP/UWB pulse is very low and the deposited energy is low. The number of published studies dealing with the biological effects is small and only a few pointed out slight effects. It should be further noted that the animal populations used in the studies were not always large, the statistical analyses not always relevant and the teams involved in this research rather limited in number.

The need for repetitive high-power microwave systems, for instance within the scope of convoy protection, requires the availability of compact, repetitive pulsed-power generators. The development and the first experimental results of an upgraded balanced ISL Marx generator for future repetitive operations at pulse repetition frequencies in the order of 100 Hz are introduced. A key objective is to keep the fundamental modular coaxial concept by reason of its scalability and compactness. Simulation models were developed under the PSpice software package in order to investigate the charging and discharging phases of the Marx generator and also to determine the design criteria for repetitive operations. The charging resistors were replaced by ultracompact inductors for rapid charging, being able to withstand pulsed voltages up to 60 kV and pulsed currents up to 1.4 kA during the discharge phase. An improved type of strontium-titanate high-voltage ceramic capacitor was successfully tested experimentally up to 70 kV. Each new elementary stage of the Marx generator consists of eight 1.1 nF sectors of a cylinder capacitors mounted in parallel, two charging inductors of about 17.8 µH and two halves of spherical spark gaps. The pressurized self-triggered gas switches are arranged along the axis for fast consecutive breakdown thanks to the UV radiation emitted during the breakdown in each gap. SimulationThe ISL ultracompact Marx generator using charging resistors, an in-depth description can be found here [1], represents the starting point of the development of the repetitive Marx generator.The modeling of the charging phase of a resistive Marx generator shows that the charging time can be described by t ch (N, RC) = aN 2 + bN + c + dRC + e, the formula's coefficients being given by the particular characteristics of the applied power supplies. This slightly differs from the empirical relation [2], although the results are comparable. The calculations show that the resistance values necessary for repetitive operations are unrealistic, for example about 225 Ω for a pulse repetition frequency (PRF) of 100 Hz, which is too low to prevent the discharging of the capacitor bank and consequently causes a significant loss of energy during the discharge mode. Previous experiments with the ISL Marx generator already showed massive problems at values of about 1 kΩ. The last stages did not trigger reliably. This is also due to another problem related to resistive charging, i.e. the single stages are charged asynchronously. Therefore, replacing the resistive elements of the Marx ladder network by charging inductors is imperative for repetitive operation at pulse repetition frequencies of about 100 Hz. * corresponding author; e-mail: rainer.bischoff@isl.eu Figure 1a shows the PSpice simulation scheme for the discharge phase of a balanced 7-stage Marx generator, which also considers the stray capacitors. The results (964)

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