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Experiments with Plasmas

Updated: Oct 11, 2022

There are two ways of learning new things. In one, you read up everything (prior art in the patent patois) on the field before doing anything. Then, in a second approach, you plunge into the new thing and learn as you struggle. This is the empirical, seat-of-the-pants approach to learning. Being somewhat impulsive, I have been a practitioner of the second school of learning. Chance made me an experimental plasma physicist. Going from University to University in search of an opportunity to work for a Ph, D, I ended up at the Aligarh Muslim University. I had not realised at that time that the Physics department had lost its earlier glory as a seat of physics research. The work in cosmic rays led by Prof. P. S. Gill, Nuclear Physics by H S Hans and spectroscopy by Prof. Putcha Venkateswara had ended. Prof. Rais Ahmed, the head of the department who had worked on speech recognition in England, was trying to rebuild the research base of the department. He was aware of the work at Harwell on fusion and Oxford on ionised gases and induced me to take a risk in starting experimental work in plasma physics. The department had no prior art or faculty members established in this area. Prof. D. C. Sarkar, who was suggested as my guide, had worked in the Varian Laboratories in the US in plasma physics. Without thinking of the consequences of my choice, I rashly agreed to his suggestion to set up an experiment. Looking back, I believe that while I had stepped into an extremely risky situation, I had the ideal conditions to become an independent experimentalist since there was no one to tell me what to do. The topic of the thesis was an experiment to simulate the Luxembourg effect in which the powerful Radio Luxembourg modulated the ionospheric plasma such that weak European stations became gratuitous carriers of Radio Luxembourg. The Luxembourg Effect was first documented by Prof Bernard Tellegen (1). One night, Tellegen was in the Netherlands, listening to a station transmitting from Beromunster, Switzerland, on 652 kHz. In the background of the Swiss signal, he could hear the audio of Radio Luxembourg, which usually broadcast on 252 kHz. He was far away from each station that neither station’s signal would have been strong enough to overload his receiver. The two signals seemed to be mixing somehow through a phenomenon of cross modulation between two radio waves, one of which is strong, passing through the same part of the ionosphere, a plasma region in the upper atmosphere. In my M.Sc course, I had not heard of Plasma Physics. So, having no prior knowledge, I set up a plan to read the essential books and papers in plasma physics. The department had Physical Review Letters, Review of Scientific Instruments, Physica and Nuovo Cimento. The last two journals had articles from European Plasma Physics Laboratories. Though Nuovo Cimento was essentially dedicated to particle physics, the journal reported occasional experimental work in plasma physics. The textbooks were W. B. Thompson’s An Introduction to Plasma Physics and Von Engles’s Ionized Gases, both excellent for a basic understanding. I learned from the research journals that nonlinear interaction between electromagnetic waves was a topical and important subject. Learning experimental techniques was the hard part. I had to learn vacuum techniques, how to build high-power RF circuits, how to modulate a carrier wave, how to use Langmuir double probes to measure the plasma density, how Klystrons operate, how microwaves propagate through waveguides, how to make waveguide components like horns and couplers. The department had a 14.5 MeV neutron generator driven by a DC accelerator which used an RF plasma source as the proton source. The source was my first introduction to laboratory plasma. My friend Rajeshwari Prasad Mathur, working on the accelerator, would tell me about the problems he faced with the plasma source. My Radio Luxembourg was a high-power Radio-frequency source that would be used to form the plasma and, because of the amplitude modulation, would produce a periodical variation of the plasma parameters like density and electron temperature. Dr K. A. George from the Tata Institute for Fundamental Research advised me on how to build the RF equipment necessary for the work. An old Amateur radio handbook in the Department library was my guide. I designed and built a simple push-pull circuit using World War II vacuum tubes foraged from the Electrical Engineering department. The tubes had no data sheets, and I generated the current-voltage characteristics experimentally. Dr George also supplied me with a glass discharge tube with a Langmuir double probe in his TIFR workshop. The modulated RF discharge plasma with time-varying parameters was the medium through which an X-band microwave signal travelled, and its amplitude picked up the modulation. The microwave source, transmission lines and power supply were scraped together from Prof. Putcha Venkateswarlu’s laboratory, who had left by this time to join the new IIT at Kanpur. My research scholar friends, Subhas Chandra, Yogendra Kumar and Rajeshwari Prasad Mathur, were very helpful in helping me chart the unfamiliar environment in the Physics Department. I measured the modulation transferred to the microwaves while they went through the RF plasma. Prof Sodha, who used to visit the department from the IIT Delhi, helped me to work out a simple theory to calculate the modulation transfer using the measured plasma parameters like thee density etc. The comparison yielded a reasonable agreement, except for a bump in the measured modulation at the low-frequency end. The results were published in Radio Science (2). I justified the excess modulation due to acoustic resonances in the low-pressure gas. Although this created some controversy in the thesis defence, I could defend it successfully. I had Uno Ingard’s paper (3) on the generation of sound waves because of neutral gas heating in an RF discharge as my supporting work. Later on, I proved the validity of this interpretation by observing neutral acoustic waves in modulated RF discharges and publishing a paper on this in the Journal of the Phys. Soc. Japan (4). This was a lesson in the importance of the supremacy of observation in experimental work and standing by one’s convictions. My thesis work taught me everything from glassblowing to machining, and I got Ph D degree in 1969. Looking back with the sophistication gained through five decades of making and manipulating plasmas, I find my first experiment crude and unsophisticated. I did not extract as much information from the experiment as I could have. Had I measured the phase modulation of the microwaves, I could have extracted the density of the plasma and its modulation and cross checked the density data from the Langmuir probe. The generation of acoustic waves and its resonance at low frequency should have been studied more extensively. On the other hand, now I realize the huge risk I had undertaken in volunteering to build the experiment with such limited resources. The sum of it all was that by setting up such an experiment, I learned how to do plasma physics experiments and gained a great boost of self-confidence. I was mentally ready to do greater things. My dream those days was to build a Q-machine without really appreciating the complexity of such a device. This remained a dream until I collaborated in starting a project in the Institute for Plasma Research to create a thermally ionised plasma confined with surface magnetic fields much later. Within a few years, I left Aligarh to join the Physical Research Laboratory in Ahmedabad and join the ambitious programme to revive experimental plasma physics in India after the closure of the TIFR efforts of the 60s. 1. 2. P I John and D.C. Sarkar: Amplitude modulation of microwaves propagated through a periodically varying plasma, Radio Science 5, 101, 1970 3. Uno Ingard, Acoustic Wave Generation and Amplification in a Plasma, Phys. Rev. 145, 41 — Published 6 May 1966 4. P I John and A S Gupta: Observation of neutral acoustic waves in modulated RF discharges. Jour. Phys. Soc. Japan, 31, 1851, 1971

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