A new High Voltage pulse generator for generating plasma and Plasma Activated Water

For his PhD research, Jeroen van Oorschot dedicated his efforts to develop a high-voltage pulse source tailored for plasma research. This innovative source excels at generating pulses characterized by a swift rise time and diverse pulse shapes. Jeroen's inquiry delves into the impact of the pulse shape on plasma generation, particularly in the context of producing plasma-activated water. This investigation is an integral component of a broader project aimed at creating disinfectants and fertilizers exclusively utilizing water, air, and electricity. The overarching goal is to establish a sustainable method that substitutes environmentally polluting chemical processes with eco-friendly electricity.

This work presents the development of a new High Voltage (HV) pulse generator and its use for generating plasma and Plasma Activated Water (PAW). Plasmas have been widely studied and used in industrial and environmental applications. Such applications include air purification, water purification, biological tissue sterilization, material surface modification and PAW generation. PAW is water treated with plasma: the plasma produces free radicals, and these interact with the water molecules. When generated using plasma in ambient air, water rich in oxygen and nitrogen species (i.e., nitrates) can be produced, which is, among others, useful for disinfection and as fertilizer in agriculture. For different applications, a different PAW composition is optimal, hence the need to adjust the PAW properties.

Steering the HV pulse waveform

Many factors influence the PAW composition, and in this work, it was aimed to control the PAW composition by steering the HV pulse waveform used to generate the plasma. The pulse waveform includes many features, like differences in the pulse width, rise-time, and amplitude, but also custom shapes. To produce these flexible, arbitrary-waveform, HV pulses to study PAW generation, a new HV pulse generator was developed. This generator is a solid-state Impedance-Matched Marx Generator (IMG), a design which allows for all the mentioned pulse properties. It is similar to a conventional (solid-state) Marx generator, but as we use an internal structure based on a transmission-line the inductance is kept low and the output pulse can be of short rise-time. We prove the validity of this concept in theory, simulations, and measurements. The implemented generator consists of twenty stages in series: each providing at most 1 kV of the output voltage at up to 500 A. The output voltage will be the sum of all turned-on stages at a given time. By delaying the turn-on of some stages, flexible output pulse shapes can be generated. The IMG structure allows for very short rise-times (in the nanosecond range, in this generator 5–10 ns), while the solid-state Marx topology allows for arbitrary waveform pulses with high repetition-rate (10 kHz). Next to the main circuit, we developed HV-sensors to measure the produced pulses, a charger system for fast recharging and a control system to accurately time the switching of each stage. The developed system provides ample opportunities for (future) research on plasma generation, discharge control and other pulsed-power applications.

Relation between radical yield and applied waveform

Next, a plasma-water reactor was developed, where plasma is generated using the HV pulses from the IMG in ambient air, from a needle to a water surface. The water is continuously sampled and measured, allowing the follow-up of water properties over time as the water changes to PAW. pH, ORP, electric conductivity, and the Ultra Violet (UV) absorption spectra are measured to quantify the species present. Several experiments on PAW-generation are performed, each with a different HV pulsed waveform to generate the plasma. A short pulse rise-time increases the initial current peak of the discharge, while long pulses (>100ns) allow for a larger current later in the pulse. Nitrate is produced up to 2.6 mmol/l at an energy cost of 220 MJ/mol, and hydrogen peroxide is produced up to 0.2 mmol/l at 200 MJ/mol after 30 min treatment of 500 ml water. Species production is related to the total accumulated energy, and it is found that hydrogen peroxide is always produced but more efficient at lower energy density, while for nitrate, the relation is less clear. Although some relations between radical yield and applied waveform are found, more research is needed to draw solid conclusions.

This research was subsidized by NWO as OTP Plasma for plants no. 17183. The research was done in collaboration with the company VitalFluid B.V. and Laan Tulips B.V.

Title of PhD thesis: Flexible, solid-state, nanosecond pulsed power: for plasma-activated water generation. Supervisors: Prof. Guus Pemen and Tom Huiskamp.