Study On Long Pulse PFN-Marx Generator Based On Pulse Transformers

High power, high repetitive rate, long pulse duration and miniaturization are the development trends of high power pulse technology. In order to obtain a pulsed power source with long pulse duration, the technique of PFN-Marx is proposed. In this thesis, the discharge characteristic of PFN is analyzed. The components of the PFN-Marx generator, such as the charging system, multistage PFNs, gas switches and the load resistor are well designed. Based on the design, a compact bipolar PFN-Marx generator based on pulse transformers is constructed, and the technique of PFN-Marx is proved to be feasible for the generation of square waveform with long pulse duration by experimental results. The detailed contents include the followings:1. Discharge characteristic of PFNThe circuit theory was used to analyze the ladder PFN. The expressions of pulse width, characteristic impedance, output voltage and natural frequencies to the inductance, capacitance of a single LC unit and sections of LC units were obtained, and the factors influenced the rise time and fall time of the formed load voltage pulse were analyzed. Besides, the voltage across the capacitors and the current through the inductors were simulated by PSpice software, and the discharge model of PFN was presented.2. Research of pulse transformer(1) Two sets of pulse transformers with thyristor were accomplished, and iron-based amorphous material was utilized as transformer core. The transformer parameters were as follows, the primary and secondary capacitance is 940 μF and 30 nF respectively, and the turns ratio of the primary and secondary winding is 2:270. In the air, the maximum voltage on load capacitor can be as high as 100 kV, and the step-up ratio could reach 160.(2) Magnetic core saturation phenomenon in the experiment was analyzed. It was found that, the thyristor was shut off when the current near zero in the primary loop, and the voltage waveform on the secondary capacitor alternating positive and negative changed with the influence of the magnetic core saturation. The waveform envelope was similar to the discharge waveform in the resistance and capacitance loop. Besides, it could also be found that the area formed by oscillation waveform and time axis(that is, volt-second) decreased slightly during the development of every single discharge process, and the volt-second increased slightly with the increasing of charging voltage.(3) A simplified circuit model was established for the oscillation process due to the magnetic core saturation. The model could preliminarily make a specification of the reason and the development of oscillation in the secondary loop, and the simulation analysis was consistent with the experimental results.3. Research of gas gap switchesThe principle for switch choosing was presented, and the electrostatic field simulation of switch was carried out. The electric field reached its peak value at the edge of the cathode surface, which indicated the breakdown path could be more easily developed here. A gas gap switch unit was designed, and the experiment of breakdown strength test was carried out, and the breakdown strength of gas gap switch was obtained. It could be found that a higher gas pressure and over-voltage could reduce the jitter of the gas switch.4. Research of bipolar PFN-Marx generator(1) The discharge characteristic of single stage PFN with ceramic capacitor was discussed. It was found that, with the increasing of the sections of LC unit, the pulse duration could be increased. The increasing of the LC sections had little effect on the rise time of output pulse, while the fall time would be increased significantly. Larger inductance of a LC unit could also lead to longer pulse duration. Besides, with the decreasing of connection wire inductance, the rise time could be reduced.(2) The circuit of PFN-Marx was discussed by PSpice simulation, and the issues of the uniformity of charging voltage on every PFN, the over-voltage on every switch and the rise time of output waveform were analyzed in detail. It was found that, the charging voltage difference on every PFN which was caused by the connective charging inductors could be neglected. Besides, the over-voltage on every switch with inductors in parallel type was much higher than the over-voltage on them with inductors in series type connection. Additionally, with the increasing of the connection wire inductance and load loop inductance, the rise time was increased significantly.(3) A compact bipolar 6-stage PFN-Marx generator based on pulse transformers was constructed. When the charging voltage on the PFNs was 35 kV, the matched resistive load of 48 ? could deliver a high-voltage pulse with an amplitude of 100 kV. The full width at half maximum of the load pulse was 173 ns, and its rise time was less than 15 ns. The technique of PFN-Marx was proved to be feasible for the generation of square waveform with long pulse duration by experimental results.