A Burst of Starfire
“Scientists have come tantalizingly close to reproducing the power of the sun — albeit only in a speck of hydrogen for a fraction of a second”: Kenneth Chang, writing in the New York Times August 17.
“A ‘Wright Brothers moment’ in nuclear fusion”: CNBC website.
“National Ignition Facility heralds ‘significant step’ towards fusion break-even target”: Michael Banks, editor of Physics World.
“National Ignition Facility experiment puts researchers at the threshold of fusion ignition “: The Lawrence Livermore National Laboratory, August 8.
“With the explosive new result, laser-powered fusion effort nears ‘ignition’. US lab stands on the threshold of key nuclear fusion goal”: Daniel Clery of Science Magazine.
“US lab stands on the threshold of key nuclear fusion goal”: Paul Rincon on BBC website.
“U.S. National Ignition Facility makes headway in nuclear fusion experiment”: Hindu’s Shubhashri Desikan, 22nd August.
This burst of journalistic exuberance was prompted by a recent experiment conducted at the National Ignition Facility in the Lawrence Livermore National Laboratory in Livermore, US.
The experiment was as conceptually simple as it was technologically complex. 192 beams of laser light, carrying a quadrillion (one followed by 15 zeros) watts of power, are focused onto the inner walls of a gold shell about the size of a pencil eraser. The vaporization of the gold generates X-rays that fall on a millimetre-sized pellet at the centre of the shell. The pellet is made of frozen Deuterium and Tritium, isotopes of Hydrogen and coated with diamond film. The diamond surface absorbs energy from the laser and ablates outwards. The resulting shockwave compresses the pellet creating a hotspot with a temperature of a hundred million degrees Kelvin. At this temperature, Deuterium and Tritium nuclei fuse to form Helium and release high energy neutrons.
The burst — essentially a millimetre-sized star — lasted only 100 trillionths of a second. However, the significance of the result is that it was for the first time that a self-sustained sequence of fusion reactions was produced in an inertial fusion experiment in the laboratory. Energetic particles and heat emanating from the hot spot at the core of the pellet heated the neighbouring material and created more fusion reactions that spread outwards.
The National Ignition Facility is a manifestation of many extremes. The laser complex fills a building with an area close to three football fields. The lasers are focused onto a target chamber which is a metre diameter. At the centre of the target, chamber sits the Hohlraum which is the size of a pencil sharpener. At the centre of the Hohlraum is the pellet of millimetre dimensions. The Laser delivers 10 quadrillion watts of fusion power for 100 trillionths of a second.
Because the capsule absorbs only a portion of the total laser energy focused on it, the reactions produced more energy than directly went into igniting them. By that consideration, the fusion reactions produced about five times as much energy as was absorbed by the target.
NIF itself cannot serve as a model for a future power plant. The net electrical efficiency of NIF (UV laser energy out divided by the energy required to pump the lasers from an external source) is less than one per cent. The present laser system can operate typically once a day. A laser fusion power plant would need to be substantially more efficient and should be able to fire several times per second to be sustainable. In addition, it should be able to convert the energy carried by the energetic neutrons into heat and electricity. These are formidable challenges to be overcome before we can conceptualize a working reactor based on laser fusion.
In contrast to the Laser fusion’s approach of compressing and igniting solid pellets, the doughnut-shaped Magnetic Confinement Fusion devices called tokamaks to confine low-density plasma in a magnetic trap and heat it to 100 million degrees to produce self-sustaining fusion reactions. In the late 1990s, the Joint European Torus experiment in England was able to generate 16 million watts of fusion power for a brief moment, going about 70 per cent of the way to producing as much power as it consumed. This is the approach being followed in the ITER (International Thermonuclear Experimental Reactor) tokamak being built in Caderache in France by a seven-nation consortium including India. The device is expected to be commissioned in 2025.
Thermonuclear fusion reactions power our Sun and the stars. The physics of nuclear fusion started becoming clear only in the 1920s when British astrophysicist Arthur Eddington suggested that stars burn bright due to the energy released from the fusion of hydrogen to form helium. The hydrogen bomb was the first realization of fusion on earth. A less destructive approach was proposed in 1950 by Soviet scientists Andrei Sakharov and Igor Tamm. This was Tokamak, a type of magnetic trap to confine the hot plasma for fusion. Subsequently, the tokamak became the most efficient concept. Academician Evgeny Velikhov sold the idea of international cooperation in building a fusion reactor to General Secretary Gorbachev, who in turn sold it to President Bush. This was how ITER got its life.
The attempts to replicate the hydrogen bomb at a micro-miniaturized level started when John Nuckolls published the concept of inertial confinement fusion. The laser, introduced in the same year, appears to be a suitable “driver”. With the development of powerful lasers in the 1960s, experiments led by Nikolay Basov in the Soviet Union and John Nuckolls at the Lawrence Livermore National Laboratory started. The first experiment at LLNL consisted of 12 beams of a ruby laser called 4-pi, aimed at the centre of a spherical vacuum vessel. The larger Shiva laser with 10 kJ of laser light succeeded 4-pi, and in turn, that was replaced by the Nova laser, which delivered 40 kJ.
The recent NIF experiment was built on several advances gained from insights developed over the last several years by the NIF team. They include new diagnostics; target fabrication improvements in the hohlraum, capsule shell and fill tube; improved laser precision; and design changes to increase the energy coupled to the implosion and the compression of the implosion.
It is anybody’s guess as to when fusion reactors would start supplying us with electrical power. Mid-century is a reasonable date. Its pursuit symbolises the eternal creative urge of the human mind. In that pursuit, brilliant and novel concepts were created, complex technologies were developed and harnessed, simulations that emulate reality with absolute fidelity were created and nations have knit together collaborations transcending ideological and political differences. Given that mankind’s civilizational journey has been the realization of impossible dreams, the dream of harnessing the Starfire on earth would be achieved one day in the future.