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Taming the Lightning: A Variety of Plasmas

For a theoretical plasma physicist, plasma is an abstraction without a persona. However, for an experimentalist, each plasma has an identity, blood and bone and character. An RF plasma is distinctly different from a DC plasma.

During a recent, long flight to Delhi en route to Aligarh Muslim University where my encounters with plasmas began as a PhD student, I began to think of all plasma avatars that I have encountered.

The thesis work required a plasma with time-varying density. My choice to realise this was to excite the plasma with a high-power amplitude-modulated RF generator. The ‘grandfather’ of experimental plasma physicists, Dr K. A George from TIFR taught me how to build the RF generator. I made a capacitive discharge with large copper plates bent with the contours of the glass discharge tube. I did not know about the impedance matching for transferring full power to the discharge. Even without it, the plasma was quite bright. When the modulation frequency was very low, it started flickering. I measured the plasma density and temperature with double probes inserted into the glass tube.

A critical requirement of a project to simulate the conditions of the Electrojet region of the ionosphere I was entrusted with in the Physical Research Laboratory was to have a plasma region with a variable density gradient. The original plan was to create a plasma by photoionisation of Caesium vapour by Ultraviolet light from Mercury discharge lamps. Such a source would have uniform plasma production within the volume because of the transparency of the plasma to the ultraviolet light. However, I realised that this source would have a diffusion-controlled density gradient which would not be externally variable. Therefore, we discarded this approach in favour of a more elegant method of using two coaxial annular plasma columns using RF discharge in an axial magnetic field. The relative densities in the axial columns could control the radial density gradient. The critical requirement of a radial electric field was realised by having the plasma terminate on concentric metal rings on which a variable potential gradient was imposed. We could inset Lamngmuir probes to measure the density fluctuations representing the plasma instabilities spinning around azimuthally.

By this time, we got interested in ion-acoustic waves and solitons, triggered by a visit by Igor Alexeff from the University of Tennessee. In plasmas with electron temperature exceeding the ion temperature, a compressional pulse evolves into an ion-acoustic soliton, travelling with a speed greater than the ion-acoustic velocity and width inversely proportional to the square root of the amplitude. For these experiments, we built a plasma device with large dimensions to study waves without the boundary effects. The plasma was produced by a large number of filaments heated by a DC power source. The filaments, biased negative relative to the chamber, produced a low current arc supported by the thermionic emission from the filaments. The result was a well-behaved, uniform plasma filling the chamber by infusion of the plasma made at the radial boundary.

To produce fast-moving plasma streams for an experiment on plasma-neutral gas interaction, we decided to experiment with coaxial plasma guns powered by an energy storage capacitor bank. The discharge of the capacitor banks with homemade spark gaps made the experiment rather noisy. The experimental device produced fast-moving plasma streams from a coaxial plasma gun impinged on a neutral gas cloud formed by the release of gas into a vacuum through a fast-opening gas valve.

For the experiments on injecting an intense 30 kA electron beam into a preformed plasma, we developed a gas-injected washer plasma gun. The washer stack made of brass formed a distributed cathode with a grounded anode at the exit. A seven-stage capacitor pulse-forming network with a pulse width of 7.6 μs excited the plasma stream.

The long-awaited tryst with fusion plasmas began with the commissioning of the ADITYA tokamak in 1989. The heart of the tokamak was the Ohmic Transformer, an Inductive Energy Storage system that stores magnetic energy. The disruption of the inductor current provides the high voltage pulse necessary to create the toroidal voltage loop to produce the plasma and drive a high plasma current. We built a multistage capacitor bank to energize the Ohmic transformer. A combination of capacitors charged to different voltages is switched sequentially with ignitrons to realize an initial high loop voltage surge followed by a lower sustaining loop voltage. In the International Conference in Plasma Physics, held in Delhi in 1989, we could declare that ADITYA was operational after a seven-year effort.

An electrical corona is a low-density plasma which forms when electric fields intensify at a sharp point and cause the ionization of air. On nights, one can observe coronas on high-voltage electrical lines. The crown-like appearance gave it the name.

This plasma source demanded a fairly complex instrumentation in the shape of a repetitive nanosecond pulse generator for driving the corona discharge pulses. A practical way of getting high-voltage rectangular pulses of a width of less than one microsecond is with the help of a pulse-forming line (PFL). A PFL has the capability of providing a flat-top rectangular pulse with a fast rise time. We built a coaxial double-Blumlein high-voltage pulse generator with a sufficiently fast rise time and good voltage gain. The generator is of very low cost and is compact and easy to make. A fast closing switch is necessary for achieving fast rise time and rotating spark gap switches turned out to be the most appropriate.

My forays into plasma-based industrial applications allowed me to experiment with a variety of plasmas formed for the specific purpose of the application. These included abnormal glow discharges in Nitrogen-Hydrogen mixtures for plasma nitriding and very low-pressure plasma formed by an anode with an extremely constricted area.

A thermal plasma formed by driving a discharge in a flowing Argon gas between a hot tungsten cathode and three consuming graphite anodes formed very large plumes for applications like Zircon reforming and alumina spherodization. This was our first experience with hot thermal plasmas. The consumable anodes requiring no water cooling made the torch very energy efficient. A similar configuration was used later for building a plasma pyrolysis system.

A microwave plasma torch had a quartz tube inserted into a waveguide through which high-power microwave radiation propagated. A spark would initiate a brilliant thermal plasma in the high-speed glas flow through the glass tube.

We jumped into ambient pressure non-equilibrium plasmas when work on surface texturing Angora rabbit wool fibres demanded cold plasma jets. These jets are driven by repetitive pulse trains exciting a gas flow through narrow glass tubes. Such jets were also used to create plasma-activated water by creating Reactive Oxygen and Nitrogen Species by immersing the jet in the water.

Plasma is star stuff. When I explained this to a member of the Jaipur royal family, who came on a visit to the laboratory, he insisted that I make him a plasma globule which he could keep in his mini-temple at home. He, being a Suryavanshi thought it only appropriate that he had a sample of the Sun at home.

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