New hope for hot fusion as clusters make an impact

时间:2019-03-04 04:12:10166网络整理admin

CHEMISTS at Brookhaven National Laboratory in Upton, New York, have devised a new and energy-efficient way to achieve nuclear fusion. They fire clusters of heavy water molecules at a solid target that contains deuterium. Other fusion scientists, wary of the publicity which has surrounded recent claims for so-called ‘cold fusion’, are remaining cautious about the new results, describing them as ‘interesting’. Many have expressed doubt that the method will prove to be a short cut to power production. However, the new results come from a well-known laboratory, and have already been published in Physical Review Letters, so their credibility is greater than that of the claims for cold fusion. Robert Beuhler, Gerhart Friedlander and Lewis Friedman confirmed that fusion had taken place by detecting protons with energies up to 325 kiloelectronvolts. ‘What is radically new,’ says Friedlander, ‘is being able to get (deuterium-deuterium reactions) with deuterons carrying only about 300 electronvolts of kinetic energy.’ Like another approach called inertial confinement fusion, the technique pioneered at Brookhaven briefly produces a tiny, hot and dense plasma where fusion occurs. The reaction rate so far is low, so not much energy emerges. Only a few fusions occur per billion clusters fired at a target of titanium deuteride. The experiments achieve less than one-billionth of the level needed for ‘breakeven’, where the energy output matches consumption. However, the impacts of the clusters appear to transfer energy to the plasma much more efficiently than the ones that cause inertial fusion. Molecular clusters are clumps of atoms that are bonded together chemically. The atoms share a single electric charge, which the Brookhaven group began studying 15 years ago. Like other charged particles, the clusters can be accelerated. In the early 1980s, Alfred Mashke, a physicist then at Brookhaven, proposed accelerating clusters or heavy ions to drive inertial fusion, but that idea was dropped. In the recent experiments, the researchers accelerated clusters of 25 to 1300 heavy water molecules to the point where they carried energies of 200 to 325 kiloelectronvolts, or a few hundred electronvolts per deuterium atom. The fundamental difference between cluster impact fusion and inertial confinement is in energy transfer. In inertial fusion, a high-energy pulse of laser light or charged particles heats a target, which implodes. Fusion occurs in the centre of the target where pressures and temperatures peak. In cluster fusion, both the clusters and the target compress and heat up. Although each atom in the cluster carries only a modest amount of kinetic energy, they combine in the cluster to reach high densities of current and energy. Individual ions cannot be focused to such high densities because their charges repel each other. In inertial fusion, much of the energy of implosion heats electrons, not atoms. ‘You want to bypass this nightmare of coupling electronic and vibrational energy,’ says Friedman. Clusters seem to do this because they strike the target at only about 200 kilometres per second, which Friedlander says causes the energy generated to heat atoms, not electrons. Beuhler and his colleagues report initial results in Physical Review Letters (18 September). After submitting the paper, they found that deuterated targets of polyethylene yielded 30 per cent more fusions than in the reported experiments. They plan further tests with a stronger accelerator and larger clusters,