Popular science presentation

The most promising nuclear reaction to realise fusion power here on earth is the one between deuterium and tritium, since the probability of fusion between them is comparatively high at lower temperature than for other similar reactions, already at 100 million degrees. There is also a larger excess of energy from each reaction of D+T than for example D+D.

Fusion gives several million times more energy per gram fuel than chemical fuels such as coal and oil. Therefore, very small amounts of fuel would be consumed in a fusion reactor. 10 grams deuterium and 15 grams tritium contain enough energy for a life consumption of energy for an average Swede!

Although tritium is not found naturally on earth, it can be produced from the metal lithium. Therefore, an imagined fusion reactor includes lithium as part of the fuel. Both lithium and deuterium are abundant in ocean water.

The reaction between deuterium and tritium gives as a result a helium nucleus (also called an alpha-particle) and a neutron, and the released energy becomes kinetic energy of these particles. The “waste” from such a reaction is also helium, which is a noble gas and therefore not poisonous. It is not radioactive either, and do not contribute to the greenhouse effect.

Fusion on Earth

To make two atomic nuclei merge in a fusion process it is demanded that they get close enough to each other to feel the strong nuclear interaction. This interaction has a very short range. At the same time, the atomic nuclei have an electric charge which makes them repel each other, so it is hard to get them close enough. The electromagnetic interaction becomes a barrier, called the Coulomb-barrier. To get past the barrier, the nuclei have to approach each other at a speed of at least 1000 km/s!

It is of course possible to use a particle accelerator to merge atomic nuclei, but then one has to use much more energy than it is possible to extract from the reaction. This is thus not something that works as an energy source. Instead, the solution is to create a very hot state, where the fuel is a plasma – the electrons of the atoms have been stripped away from the atomic nuclei – and where the thermic movements are large enough for the ions to get past the Coulomb-barrier.

The Challenge of Constructing a Fusion Reactor

For the reactor to be able to produce more energy than it consumes, the fuel has to be heated by the nuclear reactions themselves, instead that extra energy is needed from outside. The fuel then burns, and the process is called thermonuclear.

The great challenge with the realisation of thermonuclear fusion power on earth is the extreme temperatures needed to ignite a fusion reactor, 100 million degrees Celsius, or more. What to contain such hot fuel? There are no materials which can withstand even near such high temperatures.

One solution of the problem is to suspend the fuel in a magnetic field. This can be done because the fuel at these temperatures is an electrically conducting plasma. A fusion plasma thus consists of atomic nuclei and electrons, which are fixed in a magnetic field. What has proven to work best so far is a swimming ring shaped magnetic field (torus, this shape is called in the language of mathematics). Reactors built like this are called tokamaks.

There are a number of research reactors based on this principle. The largest fusion reactor today is Joint European Torus, JET, which lies right outside Oxford in England.

At JET, fusion with deuterium and tritium has been studied, but more often a plasma with just deuterium is used. Tritium has to be present as a part of the fuel mix in a future energy producing reactor, but there are certain complications with the use of this. Firstly, tritium is today manufactured in heavy water reactors and is extremely expensive. Secondly, tritium is radioactive and special precautions are required during the handling.

In future fusion reactors, the idea is that tritium for the fuel will be produced in reactions with the neutrons produced in the fusion. A fusion reactor would thus partially produce its own fuel.

The next step towards commercial fusion is a new research facility, ITER, which will be ready in south France in 2018. The goal with ITER is to show that it is possible to produce more energy than is needed to heat the plasma. On the other hand there is no plan to produce electricity with ITER, this will be the next step in the development. Maybe we will get there in the 2030s.

The Role of the Neutrons

In the reaction between deuterium and tritium, the released energy is shared between a helium nucleus and a neutron.

In a future fusion power plant the idea is that the neutrons will be caught by water in the reactor wall, which is heated and can power a steam turbine that generates electricity. The helium nuclei remain in the reactor and keep the fuel hot.

Since the neutrons are electrically neutral they escape from the plasma and can pass through the reactor wall, and carry information directly from the reactions. The amount of neutrons is a direct measure of the number of fusion reactions, and the neutrons’ energy may tell us something about the energy of the fuel ions before the reaction. This can be measured with instruments placed outside the reactor, and which thus not have to be affected by the extremely high temperature and the strong magnetic fields.

In particular we search for what we call fast ions, ions which move much faster than the main part of the particles of the plasma.

The heating of a fusion plasma is done by slowing down fast ions in the plasma by colliding with other particles and transferring some of the kinetic energy to them. In a burning plasma, the fast ions are the helium nuclei formed in the DT-reactions, but in the research reactions of today, fast ions are primary created with the help of external heating systems.

It is very important that the fast ions can be trapped in the magnetic field, otherwise the plasma cools and the fusion reactions slow down. The research is at present much about how fast ions are affected by various phenomena in the plasma, and the neutron measurements give important information on this.

Our Instruments

The fusion diagnostics group at the Division of Applied Nuclear Physics at Uppsala University is developing instruments used in the fusion experiments JET and MAST. We also take part of prestudies of instruments for ITER.

In the instrument MPR (Magnetic Proton Recoil spectrometer) the neutrons that come out of the reactor pass through a layer of plastic, which contains a lot of hydrogen. When a neutron collides with a hydrogen nucleus, i.e. a proton, that proton may be pushed away. The protons which are pushed out in the same direction as the neutron was moving receive the entire neutron’s kinetic energy, and that energy can be measured by letting the proton pass through a magnetic field. Since the proton is electrically charged, its motion will be deflected in the magnetic field, to various degrees depending on how fast it moves. We measure where the protons end up after the deflection, and in this way we may see which different energies the neutrons had.

The instrument TOFOR is something called a time-of-flight spectrometer, which measures the time of flight of neutrons between two detectors. The detectors sense when a neutron collides with an atomic nucleus. The time from when the neutron collides with an atomic nucleus in the first detector to when it gives such a signal in the other gives us its speed, and thereby the energy.

The group has also developed a prototype of a neutron camera installed at MAST. The principle of this neutron camera is that it can catch neutrons from the plasma along various sightlines, and thus create an image of the distribution of fast ions in the experiment.

(The text is an adaptation of the abstract in Swedish in the thesis Diagnosing Fuel Ions in Fusion Plasmas using Neutron Emission Spectroscopy by Carl Hellesen.)