A wave of elation swept the scientific community when the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California announced in December that they had obtained an energy of 3.15 million joules (MJ), up 50% from the 2.05 MJ put of laser energy into the fusion capsule. Nuclear. This is a leap in an effort that spans over six decades and marks a turning point in laser fusion research, although there are still miles to go before we can harness fusion energy to power a power plant to meet human needs.
Two types of nuclear reactions produce energy – fission, which is traditionally used in nuclear power plants around the world, and fusion, which is how, for example, the sun generates energy.
For decades, nuclear fusion has been an abundant and safer source of energy. The basic idea is to fuse two light nuclei—usually deuterium and tritium—both isotopes of hydrogen. When the cores are forced to fuse by an external drive, their internal energy that can be harnessed is released. The powerful external engine ionizes deuterium and tritium, heating them into a dense, hot plasma. Most of the visible universe lies in this hot plasma state. The star’s gravitational collapse compresses its hydrogen gas to a high density and, at the same time, heats it to millions of degrees Celsius, causing fusion reactions in a chain. Unlike nuclear fission—which produces energy by breaking up a heavy nucleus (usually uranium) into lighter radioactive components that cannot be disposed of safely—fusion is seen as a safe process, with a much greater supply of fuel because light elements are more abundant and safer than uranium. .
However, confinement of hot plasma is very challenging. There are now two ways to achieve this, both involving complex engineering and many first-of-its-kind developments. In the first case, the hot plasma is trapped with sufficient density in a toroidal container called a tokamak, which contains a strong magnetic field created by the current-carrying coils. The Russians invented many tokamak that operate around the world, including the Plasma Research Institute in Gandhinagar. The largest effort in this direction, in which India is an important partner, is a multinational project called the International Thermonuclear Experimental Reactor (ITER), which is under construction in Cadarache, France.
The other way the current is excited is where deuterium and tritium are held in a tiny capsule, a few millimeters long, and irradiated by multiple, focused high-energy laser pulses in flashes of a few nanoseconds (a nanosecond is a billionth of a second). The laser compresses, ionizes, and heats deuterium and tritium to high temperatures so quickly that the nuclei fuse before the gas ball explodes. We pick it up before disassembling it or on inertia.
The idea of laser fusion was proposed shortly after Theodore Maiman’s invention of the laser in 1960. A big boost was given by quantitative estimations made since 1972, also at LLNL. Why then the road is long and difficult?
Simply put, the obstacle was the complex behavior of the plasma, which is fraught with instability that prevents the desired temperature and density from being achieved. The primary requirement is the high level of symmetry required in the fusion capsule irradiation, which is a difficult condition to achieve. Sophisticated physical models and high-level computer simulations are used to predict this behavior, but plasmas continue to challenge us, revealing new aspects each time. Research in this field has produced a wealth of understanding of these dynamics that are important to many other branches of science, including astrophysics, materials physics, and studies of planetary interiors.
Laser fusion has been pursued for decades, but NIF was a leap of faith, begun in early 1995 and commissioned in 2009 for more than $2 billion. An engineering and technical marvel, it occupies three football fields and 10 floors and delivers 192 nanosecond-duration laser beams, totaling 2 megajoules of energy. NIF was expected to achieve the current feat as early as 2010, the laser’s 50th anniversary, but challenges from fusion plasmas canceled the plans. Still, it is a tribute to the scientists, engineers, and decision-makers at NIF that the program has persisted and encouraged during this uncertain period, and it includes lessons for science planners and policymakers about the importance of supporting quality research, even when there are no immediate answers. In sight.
What has changed in the past ten years? Target designs have been greatly improved, and spatial and temporal laser profiles have been continuously improved. In this sense, the December 13th Declaration epitomizes the triumph of the never-ending effort for more than a decade.
This breakthrough heralds a major shift, as we have for the first time gained energy from fusion. But we have a long way to go before we can make the laser fusion reactors that power a power plant. The laser facility itself consumed more than 300mJ to produce the required 2mJ laser pulses. The wall-plug efficiency (the efficiency with which the system converts electrical energy into light energy) is still small. For the power plant, we will have to produce much more than all the energy we take from the existing power grid. Second, the NIF laser pulses were fired at a very low rate (400 shots in all of 2017). At this rate, the factory can only run in batches – unacceptable for practical use. Third, the sophistication and cost of laser systems and target manufacturing facilities make this a huge process compared to conventional power plant or alternative energy sources such as solar and wind. We need improvements, some dramatic, on all of these fronts. The road ahead is long and unknown, but we have crossed an important juncture.
G. Ravindra Kumar Distinguished Professor of Nuclear and Atomic Physics, TIFR, Recipient of the 2003 Shanti Swarup Bhatnagar Award and the 2015 Infosys Award Views expressed are personal