A new source of power on which the Sun will never set

Published Date: 25 July 2009
By Dorothy H Crawford
THE Sun, created 4.6 billion years ago by the Big Bang, is Earth's essential life-giving energy source. Plants require solar energy for photosynthesis, and this chemical reaction produces the oxygen necessary for animal respiration.
Although the Sun is losing heat all the time, fortunately, it will continue to light our skies for another five billion years. However, as it cools, it will swell into a red giant, swallowing its surrounding planets, and then collapse into a white dwarf. At present, the temperature at the outer surface of the Sun is 6,000C, heated by energy generated in its core where its furnace reaches an incredible 15,000,000C. So how does this immense production of energy occur? The Sun is a vast nuclear reactor, generating energy by nuclear fusion. The intense heat at its core, combined with gravitational pressure of more than 300 billion times Earth's atmospheric pressure, causes hydrogen nuclei (protons) to fuse together to form helium nuclei (alpha particles). This reaction creates energy that radiates from the surface of the Sun, providing heat and light to Earth. In a single second, the Sun pumps out an astonishing 500 million tons of energy – 100 million times the world's total annual requirements. So, the question is: could we tailor nuclear fusion to produce energy for domestic consumption? This was first posed in the 1950s, but then it seemed impossible to mimic the extreme conditions of temperature and pressure needed to cause the implosion that sets the reaction going. The breakthrough came in the 1970s with the invention of laser power, which could deliver the heat required to start the reaction. "Inertial confinement fusion", also called "laser fusion", became feasible, with the promise of generating enough energy to meet our ever-increasing needs. A one-to-one mixture of deuterium and tritium, both heavy forms of hydrogen, is the best fuel, or target, and high energy lasers act as the heat source, or driver. The driver evaporates the outer layer of the target, causing it to implode inwards. This compresses the target's core to a tiny ball of plasma, with a density of about 100 times that of lead and a temperature of 100,000,000C. When these conditions are achieved, thermonuclear fusion occurs, producing alpha particles and an excess of energy. Fusion, where light atoms are fused together, is the opposite of fission, where heavy atoms are split apart. Fission is used to generate energy in the conventional, but controversial, atomic power stations, but this also produces radioactive waste with a half-life of hundreds of thousands of years. Radioactive waste is not a problem with laser fusion, and neither is the atmospheric pollution and greenhouse gas production inherent in burning fossil fuels. So fusion could potentially provide a clean, safe, inexhaustible source of energy.Since the 1970s, the concept of domestic energy from laser fusion has progressed from a distant dream to near reality. With the manufacture of increasingly powerful lasers, a controlled implosion can now be achieved. However, the system was still very expensive compared to conventional fuels, and it was not until the invention of "fast ignition", using one laser to heat the target and a second to compress it, that commercial viability became a real possibility. The idea would be to harness the energy generated, either by using it to boil water or to split water into hydrogen and oxygen. There is still a long way to go, but clearly the pay-off would be enormous. Most of the early work on nuclear fusion was pioneered at the National Ignition Facility in California, where the world's largest and highest energy lasers were commissioned earlier this year. With the promise of commercial energy production in store, a new European project, HiPER (High Power laser Energy Research), is focusing on fast fusion and aims to demonstrate the feasibility of a power-plant reactor based on this technology. These are exciting times: can physicists recreate conditions in the Sun's core to give us an entirely new and safe source of electricity? Watch this space! •
Dorothy Crawford is professor of medical microbiology and assistant principal for public understanding of medicine at the University of Edinburgh.