An Electrical Corona
(From the eBook: Experiments with Plasmas)
I had a conviction that a student working for his Ph D degree should earn it the hard way, right from building his experimental device, making it work, planning and doing experiments and writing papers using the results. I believed that this was the surest way for him to be an independent scientist and from what my former students have evolved into, this belief is quite vindicated.
I also believed that the experiment should not be so heavily instrumented that the hands-on feeling essential for fundamental research should not be lost. Another pet conviction was that he should find unconventional problems to work on so that the results would have the chance of being novel. Finally, the experiment should be fun; playing with the plasma state as it were!
The Corona
Electrical coronas form when electric fields intensified at a sharp point causes the ionization of air. On nights, one can observe coronas on high voltage transmission lines. The name derives from the crown-like appearance. Corona discharge is used in various applications since long, especially where charging of dust is required, e.g., in electrostatic precipitators, photocopying and formation of ozone. Moreover, in the last decade or so, corona discharge has also shown its potential in treatment of gaseous pollutants, e.g., in simultaneous removal of NO2, SO2, CO2 and soot from flue gas and automobile exhaust.
The presence of dust particles is ubiquitous in the corona. An experiment work initiated by Deepak Gupta shows that the corona current pulse gets occasionally modified with an added second spike when dust particles are present close to the needle. We speculated that the modification is due to multiple avalanches in a single corona current pulse when dust particles are present very near a high voltage active electrode.
The Experiment
The experiment required generation of nanosecond high voltage pulses, with fast rise time and a good flat top. We built a pulse-forming line (PFL) for this. A PFL has the capability of providing a flat top rectangular pulse with fast rise time. A rotating spark gap switch acted as the fast-closing switch providing repetitive pulse chain. With a charging voltage of 10 kV, we could generate 20 kV across a matched load and a maximum of 40 kV across an open load with a rise time of ~ 10 ns.
To understand the physical processes behind the generation of these modified pulses, Deepak developed an extensive array of novel diagnostics. Diagnostics for corona discharge is no mean task because of the small dimensions, fast time scales and non-uniform profiles of micro-discharges. He developed a novel, non-intrusive capacitive probe diagnostic for the measurement of spatio-temporal development of the charge density profile of a corona discharge from which the axial electric field and velocity of space charge wave front were estimated. Deepak showed that the space charge in discharge is related to the experimental measurements, done at many points outside the discharge volume, by a Fredholm integral equation of first kind, which does not have a direct analytical solution. Even the numerical solution is ill conditioned in that a small change in data due to inherent random experimental errors can result in a large change in solution. Deepak used an effective and involved regularization technique for numerically solving this equation, to obtain the charge density profiles. This innovative diagnostic is a major invention by Deepak to accurately measure the space and time history of not only corona but also any fast-developing discharge.
To quantify the interaction of charged dust particles with discharge, Deepak simultaneously measured the charge and location of dust particles by photographing the trajectories of particles at the instant of occurrence of modified pulse. He derived a relation to estimate the charge on the dust from the deflected dust trajectory, under the influence of electrical, gravitational and gas drag force. Deepak invented a triggering method to capture and record the elusive modified pulses in a train of unmodified pulses, which is also published in Review of Scientific Instrument Journal. This simple and yet powerful technique developed by Deepak helped to systematically study the interaction of dust with discharge and provided the quantitative measurements to compare the experimental results with the computer simulation, which Deepak also did extensively during his Ph.D. The axial electric field due to space charge is also estimated by considering the discharge to be of finite radius and with uniformly distributed charge density along the radial direction.
Deepak’s Simulation
Deepak complemented the experimental work with a numerical simulation, which solves time dependent continuity equations for electrons, positive ions, and negative ions simultaneously along with Poisson equation in a nonuniform numerical grid. Dust charging is modelled using Orbit Motion Limited theory including the effects of negative ions. This study helps in understand the complex physics of corona discharge, which is well known for providing the experimental diagnostics challenges. This study explains the formation of step on leading edge of corona current pulses in the presence of dust particles (for which no consistent theory exists) for the first time. Results explain experimentally observed modified current pulses in presence of dust particles. The charge on the dust for modified pulses is close to that obtained experimentally. Simulation with SF6 gas is also carried out to understand the contribution of space charge in defining the modified current pulse shapes. This study gave an entirely new insight to corona discharges and their interaction to dust. Study also highlighted the exciting possibilities to use the dust to improve the efficiency of corona discharge and plasma chemistry for the application of gaseous pollutants treatment and processing of powdered material.
Deepak complemented the experimental work with a numerical simulation of a needle to plane negative corona discharge, which solves one-dimensional time-dependent continuity equations for electrons, positive ions, and negative ions simultaneously along with a three-dimensional Poisson equation in a nonuniform numerical grid. He modelled dust charging using the Orbit Motion Limited theory including the effects of electrons, positive ions, and negative ions. The time-dependent dust charge equation is solved simultaneously with other discharge equations. This study explained the formation of the ‘step on leading edge’ of corona current pulses in the presence of dust particles (for which no consistent theory exists) for the first time. Results explain experimentally observed modified current pulses in presence of dust particles. The charge on the dust for modified pulses is close to that obtained experimentally. Simulation with SF6 gas is also carried out to understand the contribution of space charge in defining the modified current pulse shapes.
Experimental and numerical study done by Deepak is first of its kind that provided a detailed insight into the physics of the interaction of dust particles with micro-discharges. The study is useful for all applications of high-pressure non-equilibrium plasmas where presence of dust is unavoidable, including many industrial plasmas and pollution treatment applications. His dexterity with the numerical simulation technique got him a post-doctoral invitation to the Universite Paul Sabatier, Toulouse, where he did computational modelling of the starting phase of fluorescent lamps for optimizing their operation and lifetime. This work is important for the development of energy efficient lamps for lighting and UV disinfestation of water.
Conclusions
These activities, where students could pursue small experiments and get trained in experimental work, performed an important function in both Physical Research Laboratory and the Institute for Plasma Research. Many second-generation Plasma Physicists in India were trained through basic research programmes. They formed the human resource base necessary for carrying out the present and future fusion research programmes.
Deepak K Gupta and P I John, Design, and construction of double-Blumlein HV pulse power supply, Sadhana, Vol. 26, Part 5, October 2001, pp. 475–484. © Printed in India
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