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Plasma Physics at PRL

My first exposure to “Plasma Physics” in the context of a possible research career happened when Professor Rais Ahmed mentioned the work going on in the Harwell laboratories on thermonuclear Fusion. I was being interviewed for a research position at the Physics Department at the Aligarh Muslim University, where he was the head of the Department. However, he warned that there was no Laboratory in plasma physics and that I would have to start from scratch.

Plasma is the state of matter existing at extremely high temperatures for the uninitiated. Most of the matter in the universe exists as Plasma: the sun, stars and interstellar space are Plasma. Lightning streak and the welding arcs are plasmas. Plasma is created in the laboratory in electrical discharges.

The thesis work led to a Ph. D degree in 1969. I worked as a lecturer in the Department for another three years and started several experiments on RF plasma–microwave interaction. A persistent dream was to build a Q-machine, the rage in Princeton with their experiments on Universal Drift instabilities.

While visiting the Department, Prof. Bimla Buti from the Physical Research Laboratory (PRL) asked me whether I would like to join PRL, which was planning to start an experimental programme in plasma physics. The institute, founded by Vikram Sarabhai in 1947, was the birthplace of India’s space research, both fundamental and applied. ISRO worked out of PRL in its initial years. So naturally, I jumped at the opportunity.

The first modern Plasma Physics Laboratory in India planned at PRL had plans to pursue a research programme oriented towards simulating space plasma phenomena. I joined PRL in 1972 and was assigned the responsibility of setting up the programme. PRL was very different from Aligarh; substantial funds, lots of discussion on planning the experiments, excellent library and workshop etc. The Plasma Theory group consisting of Bimla Buti, Predhiman Kaw, Ram Varma, R. V. Pratap, A. C. Das and Abhijit Sen was very active and helpful in consultations. Y. C. Saxena was an early collaborator.

Prof. Satyaprakash’s rocket experiments from PRL had observed instabilities in the Equatorial electrojet region, which carried a current due to electrons drifting across the magnetic field in the Earth’s ionosphere. The first experiment was designed to simulate ionospheric conditions in which these instabilities occur. We set up a device with two coaxial annular plasma columns using RF discharge in an axial magnetic field and imposed a radial electric field using biased endplates. The radial density gradient could be controlled by the relative densities in the axial columns. The azimuthal drift of electrons would drive both low frequency and high-frequency instabilities. Farley-Buneman instability is triggered when electrons stream with a velocity exceeding the ion sound speed. The experimentally obtained dispersion relation differed significantly from linear theory predictions for strong electric fields. The first publication was on the nature of the spectrum of high frequency instability. More publications followed on the nonlinear aspects of the cross-field instabilities.

With the success of the first experiment, a more sophisticated experiment on the confinement of single particles in a non-adiabatic magnetic mirror was attempted by Dhiraj Bora, Saxena and myself. The motivation was theoretical work by Prof. Ram Varma, which attributed the non-adiabatic loss of particles from a mirror trap to tunnelling from the adiabatic potential well by particles of energy lower than the maximum height of the potential barrier. It predicted the decay of the number of particles from the trap with multiple lifetimes. An ultra-high vacuum chamber with multiple water-cooled solenoids forming asymmetric mirrors was set up. Electrons were injected into this from a thermionic injector. An electrode beyond a mirror throat collected electrons leaving the trap. The low electron density ruled out collective behaviour. The experimental results conformed to theoretical predictions in some essential aspects:

  • The existence of more than one decay time
  • Their dependence on the magnetic field gradient and particle energy

The slope of lifetime values versus magnetic field at different pitch angles, radical positions, and particle densities agreed with theoretical predictions.

An interest in ion-acoustic waves was 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. As the soliton properties result from a balance between nonlinearity and dispersion, we examined its behaviour in inhomogeneous plasmas. When solitons are launched into a negative density gradient, the amplitude decreases, and the velocity increases as the pulse propagates. Several experiments on modification of soliton propagation by the ponderomotive force produced by RF, a reflection of solitons in sharp density gradients etc. followed. This phase highlights the novel observation of rarefaction solitons and their “fissioning” while propagating away from the launcher.

Alfven had proposed that when Plasma and neutral gas in relative motion across a magnetic field interact, rapid ionization of the neutral gas will happen when the velocity exceeds a critical value. He had used this as a basis to explain the formation of the solar system. A theory developed at PRL had proposed the existence of a threshold velocity determined by the kinetic energy of the ion species for the interaction to happen. 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. The experiment by S. K. Mattoo and Venkata Ramani confirmed the critical velocity phenomenon while showing the absence of the threshold velocity. 

Thus the plan to establish an experimental programme in basic Plasma physics-oriented towards the simulation of space plasma phenomena got off to a good start. However, there was an unstated purpose in choosing each experiment to acquire the skills necessary for fusion research. My next venture in experiments with intense electron beams was to meet that objective.

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