Technology : One step nearer the Sun

2019-02-27 11:14:06

By Roland Pease AFTER four decades working with temperatures of up to 100 million °C, scientists have finally proved that the fuel in nuclear fusion reactors can actually heat itself up. Experiments at the Tokamak Fusion Test Reactor (TFTR) at Princeton University show that some of the energy released during fusion reactions in plasma goes back into the plasma, supplying heat that helps to sustain the reaction. The goal of fusion researchers is to reproduce the reactions that power the Sun, where nuclei of light elements fuse to release large quantities of heat. Most fusion research is carried out in tokamaks—doughnut-shaped vessels in which gases are heated to form plasmas contained by powerful magnetic fields. Until a few years ago, researchers avoided experimenting with the fuel mixture that will eventually be used in commercial reactors: an equal mix of the hydrogen isotopes deuterium and tritium. Fusing deuterium and tritium releases neutrons that irradiate the apparatus, and because the researchers were interested mainly in investigating the hot plasma, they wanted to avoid having to deal with the additional problem of radiation. But by 1993, having completed the initial experimental programme, the Princeton researchers were ready to begin testing real fusion fuel in the TFTR. They have tried a variety of plasma conditions, achieving fusion powers of up to 10.7 megawatts. It is not enough, however, for a tokamak merely to release energy. Like a candle flame, it must also heat up the plasma if it is to go on burning. In the case of the deuterium-tritium reactions at the TFTR, energetic alpha particles produced in the fusion reactions must remain within the plasma, heating the mixture by giving up their energy to electrons. Since fusion research began in the 1950s, it has been accepted that the alpha particles would heat the plasma. But this had never been demonstrated, and there were even suspicions that fusion might destabilise the plasma. Now, in an extensive analysis of trials run in TFTR since 1993, Gary Taylor and his colleagues have shown that the alpha particles generate heat without causing instability (Physical Review Letters vol 76, p 2722). They found that an equal mix of deuterium and tritium raises the temperature of the electrons in the plasma by 5 to 7 million °C. This is only a marginal increase compared with the base temperature of 90 million °C, and certainly nowhere near high enough for “ignition”, when the fusion reactions become self-sustaining. In the past year, the Princeton researchers have been trying a new configuration of magnetic fields that could double the power output of the TFTR. Some theorists believe that the new configuration could generate conditions in the plasma’s core that might even approach ignition. But these experiments come at a time when cutbacks in US government support for tokamak fusion are restricting the TFTR to working for only five or six months in the year. “We are only just beginning to do real fusion research,” says Taylor. “Before, they called it fusion research, but it was actually only plasma physics.” More on these topics: