ADITYA: Reflections on Building India’s First Tokamak
Fusion reactions make the stars shine and give the hydrogen bomb its terrible destructive power. Fusion research started in the 1950s when scientists realized that controlled thermonuclear fusion reactions in the laboratory would open a path to unlimited and safe nuclear energy. As a result, many countries began building fusion devices, magnetic bottles to keep the hot plasma confined. A type of magnetic bottle, called the Tokamak, invented by the Russians, successfully reached the extreme temperature and other conditions necessary for fusion.
In India, there were early efforts in high-temperature plasma research at the Tata Institute for Fundamental Research, abandoned in the 60s. However, Vikram Sarabhai picked up the threads again when he assembled a group in PRL in the early 1970s. The group also acquired engineering expertise. The plan was to establish an experimental programme in plasma physics, oriented towards the simulation of space plasma phenomena. However, there was an unstated purpose of eventually acquiring the skills necessary for fusion research.
In 1982, the Department of Science & Technology, realizing the importance of starting an indigenous fusion research programme, established a Plasma Physics Programme in PRL under its “Intensification of Research in High Priority Programmes”. PPP grew into the Institute for Plasma Research (IPR) in 1986. Within three decades, India acquired an international presence in Plasma Physics and its diverse applications. For example, it is a partner in the International Thermonuclear Experimental Reactor (ITER) project, a device to prove fusion’s viability as an energy source. Many students learn fundamental and applied plasma physics and theory and experiments. All this began with the Tokamak ADITYA, and I have been fortunate to have been on this journey.
The Aditya concept developed under two conflicting demands. Being the first Tokamak to be built indigenously with no direct help from experts, it had to be reasonably straightforward in engineering terms. On the other hand, a contrary view was that being a late entrant to the field of tokamaks, Aditya had to be complex enough to be capable of doing exciting experiments. This dichotomy led to various conflicts on the scale and complexity of the machine.
The project team for building the machine had the following constitution: Y. C. Saxena and Dhiraj Bora (magnets and structure), N. Venkata Ramani (Vacuum System), S. K. Mattoo (Diagnostics) and me in charge of Power Systems. Predhiman Kaw, an internationally known plasma physicist and the Director of IPR, led us through many tutorials to initiate us into the physics of tokamaks. I had seen Versator tokamak at MIT at close hand thanks to an invitation of my colleague Prof. Abhijit Sen to visit him in Boston. Versator, with its picture frame coils and a capacitor bank, looked to be quite doable.
To acquire a first-hand assessment of the engineering support we could get in India, we visited BARC Central Workshop, BHEL at Hyderabad and Bhopal, L&T, IBP Vacuum Division, Kamani Copper in Baroda, and many other places. Discussions with L&T convinced us that a torus formed out of four welded quadrants was a feasible engineering concept. The Vacuum Group in BARC also thought that this concept made sense.
There was no history of large aperture high current wound magnets in India. So, we thought magnets formed out of brazed copper plates would be a sound concept in magnet design.
Predhiman used to say that the Princeton Plasma Physics Laboratory, where he had worked, could do great plasma experiments because they had the best power supply in the world. So, as project leader for Aditya Pulsed Power System (APPS), I ensured to meet Aditya’s peak power demands of 500 MVA with an average demand of 50–60 MVA.
We could draw Pulsed power from capacitor banks, and we had some competence in this. However, the Capacitor Bank for Aditya’s total capacity turned out too large to be viable. Procuring Energy Storage Capacitors also was problematic.
Princeton and most other Fusion Labs had Flywheel Generators where mechanical energy is stored in massive flywheels and extracted in high-power, short-duration pulses. We started to think in that direction. Unfortunately, importing such machines was not possible due to the 1974 US sanctions. Predhiman and I visited BHEL Bhopal to explore the feasibility of the indigenous development of such devices. We were discouraged by the projected timescales.
What was left was the power grid. The idea of directly tapping the electricity grid to draw a large quantity of power transiently to energize the tokamak magnets and drive the plasma current was radical in 1982. The Joint European Torus (JET) in the UK was the only Tokamak currently using Grid Pulsing. Our power supplier was the Ahmedabad Electricity Company. Their total power capacity was inadequate to meet our demand.
We knew that the Gujarat Electricity Grid was powerful even those days. So, we started talking to them. Pulsing the GEB grid to extract 50 MVA (enough to power a small town) for a few seconds was an idea that made GEB very uncomfortable. Tata Consulting Engineers did extensive grid impact simulations to convince GEB of our sanity. What finally convinced them was the promise of massive tariffs from the 50MVA peak power demand! GEB agreed to lay a 132 kV line from Ranasan to the IPR site at Bhat, for which we had to pay.
The heart of the APPS 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 chose AEG, a German Company, to supply the APPS. Their design stood out for the overall simplicity. Charles L Neumeyer from Princeton helped us in the final decision-making.
We had a design review of Aditya at the Texas University in Austin organized by Swadesh Mahajan. Experts listened to our design presentations. The general comment was that the design was sound but that we were being very ambitious in our first machine.
The ADITYA subsystems were engineered by 1987, and we did the machine assembly the following year. The Assembly and Commissioning team used to work from early morning to midnight with an erection team for almost a year before the machine got assembled.
ADITYA was a system with electrically active components like magnetic field coils distributed over large volumes. We decided to do an impulse test on the coil systems to assess their vulnerability to electrical failure. The first attempt produced a shower of sparks all over the machine. The faults took almost a month to identify and seal electrically until the machine became breakdown proof.
At last, by early 1989, the machine was ready for commissioning. Unfortunately, that was when the primary 50 MVA transformer failed due to an accidental short circuit of the secondary distribution system. The transformer had to be sent back to Bangalore for repair. The repair was to take almost a year. We were heartbroken.
The delay was too long and unacceptable. So we found a plan B. Sathyanarayana, Yogesh Saxena, Harshad Pujara, K. K. Jain and I decided to build 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. We had never built a capacitor bank of this complexity before. But the system worked beautifully. A capacitor bank thus generated the first plasma in Aditya, a concept we had abandoned favouring a more versatile Grid driven power system. In the Internal Conference in Plasma Physics, held in Delhi in 1989, we could declare that ADITYA was operational after a seven-year effort.
Capacitor Bank discharges were a quick way to learn plasma control. We became experts at producing high quality, repeatable discharges quickly. APPs came into full-fledged operation in a year, and we went on to regular Grid driven power shots without much problem. Later, we strengthened the 50MVA Transformer by adding an external inductance to make it sturdier by increasing the impedance.
I believe that designing, building, and operating ADITYA was a great challenge and an excellent opportunity for learning and developing teamwork. Among its many unique achievements, it has India’s most advanced ultra-high vacuum system with a large deployment of turbo mechanic pumps. ADITYA has India’s largest pulsed power system and the first inductive energy storage system. It is the only power consumer tapping two different power grids, and the only power consumer who measures its power consumption.
