Atomic energy can be released in two ways – either by breaking up heavy nuclei, like uranium, into smaller fragments, releasing a whole lot of energy in the process, or by fusing together light nuclei like hydrogen to form heavier stable nuclei and high-energy neutrons which carry a lot of energy that can be harnessed. The former process is nuclear fission, and it is what happens in established nuclear reactors around the world. The second route is nuclear fusion, and this is the way stars generate energy. In our Sun, for example, hydrogen is being converted into helium, releasing huge amounts of energy.
Nuclear fusion is a clean and green route to producing energy, as it does not involve any remnant radioactive waste products. Fusion reactions power hydrogen bombs. However, so far, fusion devices that show a net energy gain have not been demonstrated in labs.
An experiment at the U.S. National Ignition Facility (NIF), within the Lawrence Livermore National Laboratory, Livermore, California, comes close to demonstrating this. In this lab, using laser beams, tiny pellets of deuterium and tritium (heavier isotopes of hydrogen) have been fused to form helium and release energy that very nearly matches the amount of energy input using the lasers.
The NIF has been trying to achieve this for nearly a decade. Now, the experiment has produced a yield that almost equals the laser energy input. To be functional, a reactor has to produce an output that is at least tens of times the input energy.
A tiny pellet of the fuel (deuterium and tritium) is placed in a cylidrical thumbnail-sized vessel, known as a hohlraum that has holes on both faces. A total of 192 laser beams are directed through the holes to strike the walls of the hohlraum. This causes the hohlraum to emit x-rays which, in turn, impinge on the pellet and compress it. The heated core of the pellet reaches 100 million degrees temperature which starts the fusion reactions. Further, the pellet has to “ignite” and only then can it reach the stage of becoming a microbomb – a deuterium-tritium fusion reactor – and release energy that can be tapped.
The present success is the result of many careful and painstaking efforts in fine tuning the laser pulses, playing with the shapes of the hohlraum and the design of the pellet — through a series of experiments and computer simulations.
Laser facility
The laser facility itself occupies a large area, equal to nearly three cricket fields, and the lasers can deliver up to 500 terawatts of power using its 192 individual laser beams. This is focused into the openings in the hohlraum which contains the pellet measuring some 2-3 mm.
“The amount of laser energy used in these experiments is quite modest, 1.9 megajoule (MJ). This is approximately equal to the energy it takes to heat a large pot (8 litres) of water by 100 degrees Celsius. The amount of fusion energy produced in these experiments was approximately 1.3 MJ which is now for the first time comparable to amount of laser energy input,” says Arthur Kazdan Pak, Stagnation Science Team lead, Lawrence Livermore National Laboratory, in an email to The Hindu . This is the first time, in a controlled laboratory setting, that an inertial fusion system ( another name for a laser driven fusion system) has produced nearly as much energy was supplied to initiate the reaction. Dr Kazdan Pak further explains that only a fraction of the total laser energy is actually coupled to the fusion capsule target: “If we do the energy accounting we estimate that the fusion energy production is approximately 5 times the amount of energy coupled from the laser to target.”
Tremendous progress
“A megajoule sounds like a lot, but it is just enough energy to boil a pot of water.To make a fusion reactor, hundreds of pellet implosions have to happen per second and means have to be found to extract the neutron energy as heat and produce electricity. This [experiment] is far from that stage, but the researchers have made tremendous progress in the last decade,” says P. I John, Former Meghnad Saha Chair Professor, Institute for Plasma Research, Gandhinagar, and an expert in thermonuclear fusion.
Several steps remain before a viable nuclear fusion reactor can be realised. Ignition, or energy break-even must be achieved. Many laser pulses must be made to act per second to increase the net yield to a sufficiently high value. Then the technology to convert the neutron energy into electricity has to be developed.
Meanwhile, Dr Kazdan Pak makes a mind-blowing comparison: “The fusion energy produced is released in an incredibly short amount of time, approximately, 90 picoseconds producing close to 15 petawatts of power. This is approximately equivalent to some recent estimates of the total world power consumption, however the experiment only produces this power for an incredibly short period of time, whereas power is consumed continuously across the world.”
