Ktwo weeks ago, physicists at the Laurence Livermore National Laboratory in California announced an important advance on the way to the fusion of atomic nuclei by means of laser bombardment. This so-called inertial fusion is one of the two main approaches pursued to make the nuclear fusion usable for energy production. Nuclear fusion processes make the Sun and most stars shine. If they could be controlled technically on Earth, we would have a clean, climate-neutral and practically inexhaustible source of energy.
This afternoon, the other approach to achieving this goal had its big moment: nuclear fusion by so-called magnetic confinement. Scientists from the European experimental reactor JET (Joint European Torus) in Culham in the English county of Oxfordshire presented results from experiments carried out last year, to which they had been working for more than a decade. In their reactor, it was possible to ignite a self-sustaining fusion reaction, which produced a total of 59 megajoules of energy for about five seconds. In doing so, JET broke its own record set in 1997, when 22 megajoules of fusion energy were released in five seconds.
The fusion performance of the new test on the JET from 2021 (red) compared to a result from 1997 (black)
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Picture: Eurofusion consortium
Now 59 megajoules is the energy content of about 1.7 liters of gasoline. However, they were released by fusion reactions in an amount of fuel used of only 200 micrograms of a mixture of the hydrogen isotopes deuterium and tritium – also called heavy or superheavy hydrogen. In terms of fuel quantity, the fusion of deuterium and tritium to helium is more than six million times more productive than the combustion of natural gas and more than four million times more efficient than the nuclear fission of enriched uranium, according to the Eurofusion consortium, which coordinates JET’s scientific program.
And yet this new success of fusion research is still a long way from something that could be used to operate an economical power plant. Because the 59 megajoules within five seconds – i.e. a generated power of roughly 10 megawatts – only came out because the hydrogen isotopes were present as a plasma that was more than a hundred million degrees hot – the temperature in the core of the Sun only brings it to 15.6 million degrees. In order to heat the fuel in this way, 33 megawatts of heating power were required. The ratio of generated and inserted energy, the so-called Q plasma value, was therefore slightly above 0.3 and thus less than one.
The thing about the Q-value
However, in order to assess how close a fusion experiment is to a power plant, the energy generated must be related to the total energy expended. Not only the heating energy has to be taken into account, but also the operation of the huge magnetic coils, whose fields keep the plasma away from the inner walls of the reactor vessel – up to the energy expenditure for the separation of the heavy water from normal water. This ratio, the actual Q value, is not always useful to estimate in scientific fusion experiments, but it is still at least one order of magnitude lower than the Q plasma value. According to the Eurofusion consortium, a “break even”, i.e. a fusion reactor that really generates more energy than has to be put into it, can only be expected from a Q plasma value of 10 – a factor of 30 above what has currently been achieved.