A gas discharge is one of the processes that create a plasma, a gas with a good fraction of molecules ionized. In the simplest case, it is formed by applying a potential difference (of a few 100 V to a few kV) between two electrodes in a gas at sub-atmospheric pressure.
The experiments of discharges in gases that started in the 18th century became the basis of the discovery of X-rays and the important discovery of cathode rays made by Hittorf (1824–1914) and later by J J Thomson of the discovery of electrons.
Until 1855, the gas discharges were confined to the vacuum tubes. Theodose du Monsel discovered that a plasma discharge can be generated at atmospheric pressure if the electrode face is covered by a dielectric sheet . In the early 2000s, it was realized that the use of repetitive nanoseconds high voltage pulses energized electrons without considerably increasing the gas temperature provided that the duration of the pulses is less than the critical time for the development of glow-to-arc transition . Atmospheric pressure glow discharge plasma was realized in the 1990s by Roth et. al at the University of Tennessee . This and the discovery of microdischarges led to the development of atmospheric pressure plasma jets. This radically changed the ambit of plasmas and it began to invade strange territories of biology and medicine.
In India, gas discharges were used primarily as a light source for spectroscopy or as an ion source for accelerators. What happens inside the discharge tube from the perspective of plasma physics began to be investigated in the toroidal pinch experiments in TIFR in the 1950s and my experiments on the interaction between microwaves and RF plasmas with time-varying properties at Aligarh Muslim University in the 1960s. The exploitation of the rich plasma physics phenomena happened only after the founding of Plasma Physics activities at PRL and later at the Institute for Plasma Research.
I learned that this was the first Indian conference on gas discharges although the West has a great tradition of holding such conferences. The 23rd International Conference on Gas Discharges and their Applications (GD2023), will be held in Mecklenburg-Western Pomerania, Germany, from 10th to 15th September 2023. The International Conference on Phenomena in Ionized Gases (ICPIG) held since 1953 every two years has been a forum for the discussion of nearly all fields of plasma science, covering modelling and experiments, from the fundamentals of elementary processes, basic data and discharge physics to applications.
The conference was very well organized by Prof. Suraj Sinha of the Pondicherry University. The venue was the Conference Hall in the University Guest House. We stayed at the guest house. The talks and the poster sessions melded beautifully with the coffee breaks in the cafeteria nearby. The topics presented in the meeting at Pondicherry substantially reflected the present excitement with atmospheric non-equilibrium plasmas. The groups that presented such data included those from the Institute for Plasma Research, IIT Jodhpur, IIT Delhi, Pondicherry University etc.
The meeting had a sentimental value to me. My former student, Subroto Mukherjee, now a senior faculty member at IPR was here. He had worked with me studying the transient behaviour of strong ion sheaths and got his PhD in 1995. Prof Suraj Sinha, who organized the meeting at Pondicherry had worked with Mukherjee on the Effect of Plasma Process parameters on Plasma Nitriding during the period 2001–2006. Another of Mukherjee’s students, Satyanand Kar, worked on Plasma Response to Transient High Voltage Pulses and obtained his degree in 2011. Prince Alex, Sinha’s student, worked on Alex worked on Anodic Double Layer Experiments to Study its nonlinear evolution. It was quite satisfying to see four academic generations talking about plasma physics together.
Surajs’s laboratory is brimming with projects and PhD students exploring various aspects of plasma physics. Their work is oriented towards, plasma-metal junctions, and gas discharges. He has an ambitious target to develop a compact fusion engine comparable to a 1600cc diesel engine.
I gave a talk on non-equilibrium plasma (NEP) sources and their emerging applications. In NEP, electron-impact excitation of atoms and molecules to higher electronic and vibrational states, and breaking of chemical bonds create radicals in large numbers. The higher electron temperature and non-equilibrium conditions favour thermodynamically unfavourable reactions to proceed. The medium enhances the rate of chemical reactions and the production of chemicals at relatively low gas temperatures. Liberating the NEP from the vacuum was a great advancement for the enhanced throughput, scaling up of the operation, economic advantage and extensions to new regimes.
The ambient pressure NEP has certain unique features, not seen in low-pressure glow discharges. The high collisionality encountered in ambient pressure plasmas leads to increased gas heating and enhances the tendency to develop spatio-temporal instabilities. the EEDF depends not only on the applied electric field but also on the space charge and the evolving plasma chemistry. Ion mean free paths at ambient pressure are in the micron range and hence hundreds of times smaller than typical sheath widths. High collisionality prevents the collision-less transition of ions across the sheath and ions reaching the target do so with very little energy.
Decarbonising the chemical industry would have a significant impact on global carbon dioxide emissions. Electrifying the industry using renewable sources is the first step toward reducing the carbon footprint of the chemical industry. However deep decarbonisation would depend on converting conventional chemical processes with their plasma equivalents.
Nitrogen fixation is the sustaining mechanism of the nitrogen cycle in nature and is also an important global industrial activity . Ammonia manufacturing hogs about 2 per cent of the world’s energy and makes 1 per cent of its CO2. It emits more than 300 million tons of carbon dioxide.
The Haber–Bosch (H-B) process, where one molecule of Nitrogen combines with three molecules of Hydrogen dominates ammonia synthesis. The process demands high temperatures and pressures, an ultra-pure Hydrogen feed, and large-scale operation to achieve economic viability. The energy cost of hydrogen production in the commercial Haber–Bosch process is primarily from the energy-intensive methane reforming process at 800–1000 °C.
A modified Haber-Bosch process can enable a second ammonia revolution in a carbon-free economy by using renewable energy to replace the CO2-intensive methane-fed process with hydrogen produced via water splitting drastically reducing CO2 emissions (78%, 0.38 tCO2 tNH3−1) . New plasma-based Ammonia production methods are also being developed.
Ammonia synthesis using a low-temperature plasma normally requires a catalyst to improve adsorption, reaction activity, and production selectivity. The reactor produces nitrogen oxides in a non-thermal plasma bubble column reactor to generate NOx intermediaries at the low specific energy consumption of 3.8 kW h mol−1. A high rate of ammonia production (2.67 mmol gcat.−1 h−1) has been demonstrated with a low activation energy of dissociative adsorption of N2 on the catalyst surface, paving a new way for supplementing the Haber–Bosch process .
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