CCFE tests neutral beam injector based on negative ions

Firing beams of high-speed neutral atoms into the plasma is one of the main heating methods used in tokamaks. Just like billiard balls, the atoms of the beam transfer their energy to the slower plasma ions during the process of collision. Traditionally, fusion machines use neutral beam systems with positive ions that are accelerated and neutralised before they enter the plasma. However, a lot of energy is lost during the neutralisation process. This loss increases rapidly with the rising beam energy. A power plant requires a one megaelectronvolt beam to operate, and for this the neutralisation efficiency would be as low as 2%. It seems nearly impossible to attach an electron to a beam particle that is moving at some ten thousand kilometres per second. Causing the particle to lose electrons is much easier – the neutralisation efficiency for a negative ion beam is 58% at the same energy level. ITER and the demonstration power plant DEMO will therefore use negative ion neutral beam systems. Europe is in charge of providing the ITER system and a test bed is under construction at Consorzio RFX, Italy.

CCFE’s Small Negative Ion Facility (SNIF) started operation in June and its main role is to look for ways of improving techniques for negative ion beam systems beyond ITER. Despite being a small-scale test bed, SNIF is able to emulate the negative ion beam production process used in tokamaks. A relatively cold plasma (10,000 °C) is formed to produce negative hydrogen ions; a beam of ions is then directed through a set of accelerating grids to speed the particles up. Instead of being neutralised and directed into a tokamak, the ion beam hits a copper target plate containing diagnostic sensors which allow physicists at SNIF to analyse the shape and profile of the beam. These results can then be scaled up to predict performance in the large high-power tokamaks of the future.

Materials for ion sources

One of the key areas SNIF will investigate is materials for ion sources on post-ITER machines. A coating on the walls of the source material reacts with the incoming ions and atoms, giving up electrons to produce negative ions to flow into the heating beam. ITER will use caesium as an ion source material, but for fusion power plants, other candidates are also being considered. These should be capable of producing beams without the problems that are posed by the highly reactive caesium. SNIF will test alternative materials, such as boron-doped diamond samples.

Built at a low cost, and primarily using spare parts from previous systems, SNIF can also be switched over to act as a positive ion system, giving it sufficient flexibility to research other areas of neutral beam development and materials testing. Jamie Zacks, a Lead Physicist at SNIF, said: “For the first time in fifteen years, CCFE now has its own negative ion test bed, instead of borrowing facilities from other fusion labs. This, combined with its size, gives us much more control and flexibility over experiments and allows us to open up new collaborations with partners. So far SNIF is performing very well but there is much development still to do.” Elizabeth Surrey, CCFE’s Technology Programme Leader, added: “SNIF is going to make major contributions to European power plant studies under EFDA but also to CCFE’s own technology projects. Its flexibility will complement the larger negative ion facilities at IPP and RFX that are developing the ITER neutral beam technology and will enable us to progress towards DEMO faster.“

Nick Holloway, CCFE