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Intense Electron Beams

A talk by Prof Igor Alexeff of the University of Tennessee on the work in the Soviet Union by Leonid Rudakov and others on inertial fusion using intense electron beams triggered my interest in intense electron beams. Electron beams have interesting physical properties, even at moderate beam energies and currents, say a few hundred keV and tens of thousands of amperes. The associated space charge is intense, producing self-electric fields of the order of MV/cm. The self-magnetic fields are strong enough to turn the beam trajectories into complex shapes or even reflex the beam electrons. When injected into a plasma, the electric field gets cancelled by the expulsion of plasma electrons. The rising front of the self-magnetic field drives return currents. When such beams pass through plasmas, fascinating effects involving the interaction of beam fields with plasmas will happen.

The essential elements of sub-microsecond pulse power technology are high energy density capacitors, high-pressure spark gaps, Marx generators and transmission lines for pulse formation, etc. This field was born in the early 1960s at Aldermaston, UK, and we had to catch up with more than a decade’s accumulated knowledge. We experimented with every aspect, including making energy storage capacitors by rolling polyester sheets sandwiched with aluminium foils. A Marx generator erected on a wooden frame with exposed spark gaps was the learning tool.

Alexeff suggested writing a proposal for investigating the interaction between intense electron beams and plasmas for funding by US National Science Foundation. I submitted this with Alexeff and Charles Wharton of Cornell University as collaborators. NSF sent an expert to look at our capabilities and, based on his report, reviewed the proposal. They said that while the proposal’s objectives were sound, they doubted whether Indians would be able to master the Intense Pulsed Electron Beam technology. I thought that the best way to respond to this rebuff was to demonstrate our capability independently and get money to start this work from PRL. Denial regimes are the best triggers for indigenous capacity building.

K. K. Jain had joined as a student. My collaborators were Dr Punitha Velu and Dr Prabhakar Rao, who had a PhD from Oxford with experience in high voltage techniques. We finally put together a 20 stage Marx bank using Maxwell capacitors housed in an oil-filled tank. A dielectric surface flash triggered the spark gap switches. The Marx output charged a 100-nS water pulse forming line, which switches through an overvolted water switch into a graphite cathode generating a 300 kV 30 kA 100 nanosecond annular beams. This is nominally a beam carrying Gigawatt power. We described this work in a paper in Sadhana.
Pulse power technology was a classified field, and hence even friends in US could not advise us if we had problems. For example, the Perspex flange in the water switch often used to crack, and we realized it was due to the shock generated during the switch firing. When I asked US expert Magne Christianson how to solve this problem, he admitted that he had the answer but could not help us since it was classified. So, we had to solve such problems from the first principles.

By the time we were ready with the beam, laminar beam plasma interaction studies had been exhausted, and we decided to get into the novel territory of rotating electron beams. We performed interesting experiments on rotating electron beams for plasma heating, which formed Jain’s thesis. He observed effects like excitation of a cross-field return current layer after the beam exits the plasma, generation of magnetosonic waves by the return current layer, and heating by magnetosonic waves. To get higher energy, longer duration beam, the Marx generator was fired directly into the diode. As a result, the beam return currents were so high as to reverse the original mirror magnetic field. This results in forming a field-reversed configuration commonly known as a compact torus.

The device also led to another thesis by Vijay Shankar on the self-field effects and the effects of charge neutralization on the dynamics of intense beam propagation through non-adiabatic cusp fields. He studied the dynamics leading to the conversion of the laminar beam into a rotating beam by the action in the cusp region and the effects of charge neutralization and beam self field on the conversion.

Prof. Wharton revisited us and sent a report to NSF highlighting what we had done independently in pulse power development. This time NSF got convinced that we had the requisite expertise to deal with pulsed power systems and suggested putting in a new proposal. NSF funded our proposal for an experiment on understanding what happens when a high current beam is injected into a toroidal system, a hollow cylinder bent into a ring. Thus, the first toroidal device in IPR in which Chenna Reddy started experiments on injection and stacking of high current electron beams produced by a graphite cathode driven by a Pulse forming line charged by a Tesla transformer. We also had enough money to build a new plasma physics laboratory in PRL. The toroidal plasma device evolved into BETA, an acronym for Basic Experiments in Toroidal Assembly.

In 1982, we persuaded the Government of India to start an indigenous programme in Fusion Research. As a result, the Plasma physics Programme under the Department of Science and Technology’s project on Intensification of Research in High Priority Areas began operating that year. This evolved into the Institute for Plasma Research, and we moved into a new campus by 1984.

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