A TLT Based Multiple-Switch Pulsed Plasma Source

Gas switch is usually used for high power nanosecond pulse generation, and multiple switches can be employed in series or in parallel for high voltage or large current pulse generation, respec-tively. The classical Marx generator uses multiple switches in series for voltage adding, and the switches can auto-synchronize themselves by over voltage. However, when switches are in a par-allel connection, an external triggering system is usually required for synchronization. TLT (Trans-mission Line Transformer) based multiple-switch technique can realize a auto-synchronization of switches in a parallel connection. This feature makes it very competitive for industrial applica-tion that requires high repetition rate and continuous operation. Meanwhile, TLT shows a good performance in matching a plasma reactor and has a high energy efficiency. Based on these advan-tages, TLT based multiple-switch technique is prospective for environmental applications such as air cleaning or water treatment.A GW multiple-switch pulsed plasma source is developed based on this technique. The sys-tem mainly consist of a pulsed power generator with 20 switches and a 20 stage TLT, a resonant transformer with multiple windings for charging, and a D-I(Differential-Integral) probe for HV nanosecond pulse measurement. All of these apparatuses are introduced in a way of principle, de-sign and experimental results. Finally, an application of water treatment is carried. The contents and conclusions of this dissertation are summarised as follow:1) The pulsed power generator is constructed with 20 switches and a 20 stage TLT. Each stage consists of a 5.6nF capacitor bank, a three electrodes spark gap switch, and a TLT made by 5 coaxial cables. All the capacitor banks, spark gap switches, and input side of the TLT are integrated in one compact unit, to minimize the stray inductance and achieve a better performance. The auto-synchronization of 20 switches has been realized for the first time. The synchronization process takes several tens of nanosecond. When each stage TLT is connected in series to a matched resistive load,20 independent and identical pulses are generated simultaneously, with a risetime of about 15ns, a width of about 70ns, a peak voltage up to 200kV, respectively. The time interval between each pulses at the rising edge is less than 2ns.2) Based on a model of secondary mode impedance, a numerical method is established to calculate the over voltage distribution in the synchronization process, which can be applied for any number of switches. Indicated by the calculated results, the over voltage induced by a closed switch mainly drops on the two adjacent stages which are still open. This means even for a system with a large number of switches, any one of the switches’closing can effctively trigger the chain reaction of synchronization.3) In a D-I measuring system, the differential end is usually integrated with the electrode being tested at a fixed position. In this work, a standalone type of differential end is designed. Both the differential end and the integrator have a compact coaxial structure. Finally the probe can measure a high voltage pulse up to 200kV, with a bandwidth of 49.5kHz to 40.6MHz at an accuracy of 95%. Ahd the responses of 3 individual probes have no obvious difference.4) The resonant transformer is constructed with 4 magnetic cores and 8 winding pairs, with a energy transfer capability of 300J per shot. All the 8 primary windings are connected in series, resulting an equal energy injection in each winding pairs or magnetic core. Meanwhile, all the 8 secondary windings are in a parallel connection, resulting an equal current distribution by 8 high voltage diodes. Such a design can overcome the two shortcomings when a conventional single core transformer is applied for large energy transfer per shot:firstly large size of core is required which may exceed the specifications available in market, secondly large current through HV diode increases the risk in operation. The theoretical analysis indicates that the permeability differences of each cores will not cause a negative influence in the current sharing of each windings. The experimental results show that the current is well shared by 8 diodes uniformly. The current ratio of each diode ranges from 11.7% to 13%. The energy efficiency of the transformer is about 94%.5) A 4-switch TLT generator is applied to a surface discharge plasma reactor for water treat-ment. The energy efficiency between the generator and reactor ranges from 63% to 73%. In the decolorization of indigo carmine, the energy yield for this system ranges from 1.60g/kWh to 2.06g/kWh. The maximum value appears when the energy per pulse is about 2J. A typical hydro-gen peroxide yield is 1.02g/kWh. For e-coli disinfection, a 5.6 order reduction of cell density can be obtained while the energy consumption is 6J/mL.