Q: How does fusion work?
A: Fusion releases energy when the nuclei of two forms of Hydrogen (in our case, called Deuterium and Tritium) are collided together at such high velocities that they stick together or fuse. Shortly after this, they break apart, forming a neutron and a Helium nuclei and some net energy (mostly contained in the neutron). In a fusion powerplant, the neutrons will be used to heat water and drive a steam turbine. The fusion reaction is maintained as the Helium nuclei will help keep the remaining Deuterium and Tritium nuclei hot enough to keep fusing together.

Q: Tell me about the ways to heat plasmas
A: In magnetically confined plasmas that we study at Culham, there are three main ways of heating the plasma. The first is to use the strong electric current that is generated in the plasma (to help its stability and control) - this heats the plasma just like the current in a wire heats the wire. This current is very large (3-4MA on JET) and can heat the plasma to very high temperatures. However, to access the temperatures required for fusion to occur (which has been observed on JET) other methods are required. Powerful microwaves, injected in different frequency ranges (MHZ to GHz) can (under the right plasma conditions) give up their energy to the electrons or ions in the plasma and heat it up. Another method is to use very powerful neutral beams - these are beams of highly accelerated neutral atoms which are injected into the plasma and give up their energy as they undergo collisions with the background plasma ions.

Q: Could you please specify the temperature required for a Fission (not Fusion) reaction of Deuterium and Tritium?
A: One cannot easily see how Deuterium and Tritium (which are very light nuclei) can undergo fission (which is the splitting apart of heavy nuclei to release energy). The fusion of these nuclei (i.e. their joining together to release energy) - observed in the EFDA-JET tokamak device here at Culham requires temperatures in the region of 100million degrees C.

Q: Incredibly high temperatures and pressures to produce "random" collisions resulting in fusion seems difficult to sustain, and to have limitations to develop to produce heavier products. Has there been any/much research into different (possibly more controlled) methods of bringing nuclei together in fusion? For example, head on ion beam collision?
A: You are right that the fusion collisions in the plasma in a tokamak are essentially random, although due to the high plasma ion temperatures, these collision are quite frequent. The big challenge is to maintain the high temperature in these plasmas for a long enough confinement time (i.e. the time the ions stay within the magnetically confined plasma) so that sustained fusion is observed. JET has seen plasmas lasting for 10-20s in equilibrium, maintaining very high plasma temperatures and, indeed has seen some fusion occurring. The next step device, ITER, which is 2-3 times bigger than JET, will see much more fusion power and there is confidence from what has been learned on JET that ITER and future powerplants will work effectively.

The Collision of two beams as you describe cannot produce more fusion energy than the energy required to produce the beams. When the beams collide, most of the energy is dissipated and only a negligible number of beam particles would happen to have head-on collisions and fuse. The majority of particles would just get deflected and decelerated. Therefore, high temperatures must be achieved as part of a thermal equilibrium, where particles collide from all directions, not just the direction of the beam.

Q: I read somewhere that the energy released by the tritium-deuterium fusion reaction is given by the strong force. Could you explain this more clearly?
A: The forces at work in the fusion process include two main important ones. The powerful Coulomb electrostatic force acts over long distances (compared to the size of the nuclei) and prevents the nuclei coming close enough to fuse (as they are both positively charged), unless their energy (temperature) is high enough to overcome this. This why the plasma temperature needs to be high (in a tokamak) for fusion to occur. However, the STRONG force is the very short range force that holds the protons and neutrons together in the nuclei. When fusion occurs these forces are rearranged (in going from a Deuterium and Tritium nuclei to He and a neutron) - ending up with a lower potential energy (or mass) and a release of energy. To summarise, the Coulomb force is a long distance, inter-nuclei force and the strong force is a short distance force acting only within the nuclei.

Q: At what point, during the fusion process, do the helium nuclei stop adding to plasma heating and become an impurity to be removed?
A: About 20% of the net energy gained from the fusion reaction is carried by the Helium ion - the remaining 80% is carried by the neutron which, in a powerplant, will leave the confining magnetic field, penetrate and heat a surrounding blanket, heating water to make steam and drive a turbine. The net energy gained by the Helium ions will remain within the plasma and, through collisions with the Deuterium and Tritium fuel ions, will be transferred to these fuel ions, maintaining the high temperature required for the reaction to occur. This effect has been observed and verified on JET (the only existing device to observe fusion power being produced from a magnetically confined plasma). When the Helium ions have slowed down (through these collisions) and no longer can heat the plasma, they essentially become an impurity to be removed - in fusion research this Helium is known as Helium ash. The removal of this Helium has been one of the major challenges facing the realisation of fusion energy from devices like JET. The basic approach is to form a D shaped plasma which touches the bottom of the vessel in a so-called divertor structure. The helium ions will naturally (like all ions) move to the edge of the plasma, where a powerful flow will transport them to the divertor - where they will be pumped away.

Q: How are only the impurities (particularly helium ash) removed, when they are all charged? Wouldn't any sort of flow also take some of the plasma into the divertor?
A: The crucial point to understand is that there is no perfect confinement of plasma, that all plasma components (including the fuel ie deuterium and tritium) diffuse across the magnetic field, though their transport across the magnetic field is much much slower than transport along the field. You can imagine it as a minor leakage of the magnetic trap.

That is, the divertor actually is receiving flow of all plasma components, ie fuel, helium ash, and impurities. The trick is that at the same time we keep supplying fuel into the vessel, and the fresh fuel (even when ionised) diffuses to the plasma centre. Additionally, we can supply some extra fuel to the plasma centre directly via pellet injection and neutral beam heating.

However, some experimental regimes do confine high Z nuclei (impurities) better than low Z (fuel). We try to avoid these regimes and rather try to achieve the opposite situation.

Another important point: When a He nucleus is born in a fusion reaction, it is very energetic ie very fast. Fast particles diffuse across the magnetic field more rapidly than slower plasma particles (with thermal distribution of velocities). As a matter of fact, one of the challenges is to confine the fast Helium particles well enough in order to allow them to lose their kinetic energy in collisions with incoming fuel particles, that is: to let Helium heat up the plasma. Only when He loses its kinetic energy, it is desirable that it diffuses out of the plasma in order not to dilute the fuel.

A burning plasma is the final goal, when the energy of helium from fusion sustains the extreme temperatures required for fusion to occur. To date, we have only achieved and sustained these temperatures via external heating.

Q: How do fission and fusion reactions compare?
A: In fission, energy is gained by splitting apart heavy atoms (Uranium) and using this excess energy to boil water to drive a steam generator, thus producing electricity. Experiments such as JET are still researching how to use the energy gained from nuclear fusion reactions - where light Hydrogen-like nuclei are fused together, producing an excess of energy. This is reproducing what is happening in the Sun. Here, a hot gas (or plasma) of Hydrogen-like nuclei is formed, held in place using powerful magnetic fields, and heated until fusion starts to occur - indeed, JET has achieved temperatures where fusion products have been observed. It is hoped that fusion powerplants (similar but larger than JET) will be producing electricity in 40 years or so.

Q: How is it that both fission and fusion produce power? If splitting a large atom into two smaller atoms releases energy, it seems that combining two smaller atoms into one larger atom would require energy, not release it.
A: Actually, the "energetic valley" of the atomic structure has the deepest part for atomic nuclei around the size of iron (with 26 protons in the nucleus). That is, one can release energy either by splitting very large nuclei (like uranium with 92 protons) to get smaller products, or fusing very light nuclei (like hydrogen with just one proton) to get bigger products. In both cases the reaction shifts the size of the atoms involved towards iron, that is towards lower energies in the “valley”. The energy gain is released in the form of kinetic energy of products (which usually converts to heat - chaotic motion in the medium).

Why is it that mid-sized nuclei have an optimum structure from the energetic point of view? A simple "liquid drop model" of the nucleus gives an answer: Fusion of two small nuclei is energetically advantageous, because the joint nucleus has a smaller surface area than the two original nuclei - the same principle applies for droplets (e.g. of mercury). Indeed, the short-distance chemical forces between molecules of a liquid are similar to the short-distance "strong force" that keeps nuclear particles together. However, when a nucleus is too big, the long-distance Coulomb repulsion between protons sums up and becomes too strong. That is why very large nuclei (transuraniums) are unstable. For nuclei bigger than iron the overall energy loss due to mutual repulsion is more important than the energy gain due to smaller surface.

More about the liquid drop model