Klaus-Dieter Rohde

High-efficiency high-voltage pulse generator based on a fast recovery pseudospark switch

Experiments are reported to demonstrate the ability of pseudospark switches for extremely fast recovery after forward conduction at an anode voltage of up to 25 kV and peak anode currents of 1 to 2 kA. At a pulse duration of around 5 /spl mu/s and at a rate of current rise of up to 1 kA//spl mu/s, the reverse current is blocked at current zero transition due to an extremely fast recovery rate. The maximum achievable rate of rise of anode voltage after current zero is above 50 kV//spl mu/s; the recovery time of the switch, as measured from the end of the anode current pulse to >80% of anode voltage recovery, is of the order of 0.3 /spl mu/s. Initial experiments toward reprate applications were successful in a burst mode operation, at a circuit-limited pulse repetition rate of up to 96 pps.

Large area pulsed corona discharge in water for disinfection and pollution control

To investigate the efficiency of submerged pulse corona (SPC) discharges in water we built a laboratory scale, parallel-plate reactor that is part of a closed loop water circulation system. A pulsed voltage is applied across the electrodes. One of the electrodes is coated with a porous ceramic layer to create local field enhancements to initiate corona discharges. For energization of the SPC reactor a pulse generator was developed which is based on a capacitor discharge initiated by a semiconductor switch. A pulse transformer, followed by two magnetic pulse compression stages, produces voltage pulses with amplitudes of up to 30 kV at a pulse width of 0.3 ¿s. Simulation of the circuit behavior leads to good agreement with voltage and current measurements. Details of the pulse generator and first experimental results concerning the efficiency of radical production are presented. Depending on the conductivity of the water to be treated, pulse currents of > 600 A at a voltage of 20 kV to > 30 kV are obtained for electrode sizes of around 50 cm2. The efficiency of the radical production is measured in terms of the hydrogen peroxide (H2O2) concentration, which is formed by recombination of hydroxyl radicals (OH.) at sufficiently high concentrations downstream of the plasma reactor. At pulse repetition rates of 20 to 100 Hz, H2O2 concentrations of several mg/l are produced, at efficiencies in the range of up to ¿1 g/kWh.

All-Solid-State Power Modulator for Pulsed Corona Plasma Reactors

Atmospheric pressure pulsed corona plasma (PCP) reactors of wire-plate design offer novel solutions to environmental issues and to a number of industrial processes. Emerging applications include indoor air sterilization and odor removal in air conditioning systems, chemical synthesis in non-thermal plasmas, and plasma reforming of gaseous fuels. We previously reported on experimental investigations of a laboratory size, wire-plate plasma reactor for pulsed corona treatment of gas flows. Operation with gas flow, at pulse repetition frequencies of between 10 pps and 200 pps, has been achieved at pulse voltage amplitudes of between 10 and >30 kV, at pulse durations of around 0.3 mus (FWHM). High efficiencies of up to 70 g/kWh have been reported using an all-solid-state pulse generator. In this work, we report on the development of all-solid-state power modulators for use with nonlinear loads like pulse corona plasmas. The pulse generators are based on a fast thyristor switch discharging pulse capacitors, a pulse step-up transformer, and one or two stages of magnetic pulse compression. At pulse repetition rates of up to 200 pps, amplitudes of > 30 kV into a resistive-capacitive load (1 kOmega200 pF) have been achieved, at risetimes of about 80 ns and a pulse width of 0.3 mus. The pulse generator is insensitive to load variations, in particular to sparking in the reactor. An advanced generator version uses two magnetic pulse compression stages resulting in even shorter rise times. The modulator and its performance concerning experimental results will be described in detail when driving a pulsed corona reactor.

A three-stage inductive voltage adder for industrial applications

We report on an Inductive Voltage Adder (IVA) [1] development designated for industrial applications like electroporation, environmental applications, etc. The IVA described [2]-[4] is a three-stage demonstrator which shows the feasibility of using conventional high power IGBT semiconductor switch modules instead of spark gaps or arrays of low-power IGBTs. The pulse generator described produces pulses with peak voltages and currents of up to 12kV and 6kA, respectively, at a pulse duration of typically 1μs FWHM (full width at half maximum). The IVA is tested in single pulse mode and at low repetition rates due to load and power supply constrictions. Each IVA stage is designed as a radial transmission line fed by several parallel pulse modules (“bricks”) which contain the electrical components, like capacitors, inductors, and switches. The combined power of an individual stage is added to the preceding stages in a section called “transformer”. The transformer matches the electric and magnetic fields and is realized as a combination of radial and coaxial transmission lines. Each stage of the IVA is matched to the next stage and is connected in series with a coaxial transmission line. The mechanical dimensions of the three-stage IVA demonstrator are: outer diameter 820 mm, outer diameter of the coaxial transmission line 210 mm, height of the IVA 352 mm. The I VA geometry, in particular the most critical parts radial transmission line and transformer section, is simulated by a transient electromagnetic field solver to analyze the reflections and transmission coefficients of the device.

Pulsed Corona in Water: Pulse Generation and Applications

Summary form only given. Removal of organic pollution from water is of increasing concern for drinking water as well as for many industrial processes. Application of pulsed corona discharges is a known possibility to attack that problem by producing highly active radicals in-situ, without using additional chemistry. To investigate the efficiency of submerged pulsed corona discharges in water we have built a laboratory scale, parallel-plate, flow-type reactor that is part of a closed loop water circulation system. A pulsed voltage is applied across the electrodes. One of the electrodes is coated with a porous ceramic layer in order to create local field enhancements to initiate corona discharges. For energisation of the plasma reactor a pulse generator has been developed which is based on a capacitor discharge initiated by a semiconductor switch. A pulse transformer, followed by two magnetic pulse compression stages, produces voltage pulses with amplitudes of up to 37 kV at a pulse width of 0.3 mus. Simulation of the circuit behavior leads to good agreement with voltage and current measurements. Details of the pulse generator and first experimental results concerning the efficiency of radicals production are presented. Depending on the conductivity of the water to be treated, pulse currents of > 600 A at a voltage of 37 kV are obtained for electrode sizes of around 50 cm2. The efficiency of the radical production is measured in terms of the hydrogen peroxide (H2O2) concentration, which is formed by recombination of hydroxyl radicals (OH) at sufficiently high concentrations downstream of the plasma reactor. At pulse repetition rates of 20 to 100 Hz, H2O2 concentrations of up to several mg/l are produced. Production efficiencies have been measured to be in the range of up to ap1 g/kWh.

The thermohydraulic generator-a pulse power source of intense pressure pulses

Intense pressure pulses, often focused to form strong shockwaves, are used for a variety of medical and industrial applications. In this contribution, a new principle, the thermohydraulic generation of intense acoustic pulses, is reported, which promises a virtually unlimited lifetime. The energy efficiency is comparable to that of commercial shockwave sources based on the magnetodynamic principle. The underlying physics is that of the generation of strong thermoelastic waves in electrically conducting media, in particular, in electrolytes, by direct ohmic heating with intense current pulses. A simplified model is discussed to describe the pressure amplitudes and the influence of the thermoacoustic properties of the electrolyte. The source is scalable over a large range, and is arbitrarily adaptable in its shape. Large-area plane waves with amplitudes of up to 5 MPa have been achieved, as well as self-focusing geometry with peak pressures of over 50 MPa.

The thermohydraulic generator: a pulse power source of intense pressure pulses

Intense pressure pulses, often focussed to form strong shockwaves, are used for a variety of medical and industrial applications. In this contribution, a new principle-the thermohydraulic generation of strong pressure (sound) pulses-is reported which promises a considerably extended lifetime. The energy efficiency is comparable to that of commercial shockwave sources based on the magnetodynamic principle. The underlying physics is that of the generation of strong thermoelastic waves in electrically conducting media, in particular in electrolytes, by direct ohmic heating with intense current pulses. A simplified model is discussed to describe the pressure amplitudes and the influence of the thermoacoustic properties of the electrolyte. The source is scalable over a large range, and is arbitrarily adaptable in its shape. Large-area plane waves with amplitudes of up to 5 MPa have been achieved, as well as a self-focusing geometry with peak pressures of over 70 MPa.

High average power high voltage modulator using a dual pulse transformer circuit

A novel modulator concept is presented using two step-up transformers in series, with an intermediate pulse compression stage. This concept has advantages in terms of minimizing the insulation requirements and core sizes in each stage, as well as minimizing stray capacitances and inductances, respectively. Hence, although this scheme requires an additional component (the second pulse transformer), the overall setup can be optimized in terms of core sizes, insulation requirements, and energy efficiency as compared to standard circuits. Thus, it is possible to start from a comparatively low primary stage voltage of only 1 kV, using standard IGBT switches and off-the-shelf capacitors, in order to achieve pulse amplitudes of over 50 kV at pulse widths of the order of 150 ns, at pulse repetition rates of the order of kHz.

Undrodynamic instability of exploding wires

Exploding wires are used for a variety of pulsed power applications, like soft x-ray generation in imploding liners, as opening switches in high voltage generation, for thin film coating and nanopowder production, and as protective fuses in high current applications. In many cases, the hydrodynamic instability of the wire prior to the complete deposition of the electrical energy of the driving pulse leads to detrimental effects, very often adversely influencing the process. In this work, we report on the direct visual observation of the onset of magneto-hydrodynamic (MHD) instabilities of tin (Sn) wires exposed to 100 MW/cm2 pulses of up to several milliseconds duration. A high-speed, high resolution video camera is used to visualize the dynamical behavior of the wire under such a load. Numerical simulation of the electrical circuit, and a simple one-dimensional model are used to explain the observed behavior. The theory shows a satisfactory agreement with the experiment, not only qualitatively but also on a quantitative scale, in particular in regard of the onset of the MHD instability. The preferred instability is m=0 mode, leading to a premature pinch-off of the wire at multiple locations almost simultaneously

Environmentally advantageous industrial processes based on pulsed power technology

The application of intense pressure pulses on two industrially interesting applications, i.e. noncontact, ultra-pure milling/deagglomeration of powders, end noncontact removal of SMD parts from printed circuit boards, is presented, including threshold process parameters. The grinding efficiency of powders (in terms of decreasing particle mean diameter per unit energy consumed) increases strongly above a threshold pressure, indicating that a further decrease of the measured specific energy consumption of this process is feasible. Application of a series of shocks to cemented or soldered SMD connections results in crack formation and propagation within the bond region, and eventually to the removal of the SMD part itself. Processing is done in a liquid environment (i.e., degassed water, silicon oil or hydrocarbons like alcohols), and can be used for either parts recycling, PCB repair, or process control in terms of online bond quality evaluation. Initial results are given concerning threshold parameters for soldered connections. This process opens up a vast field of applications, in particular in the area of rapid on-line testing and quality control in a variety of industrial processes.