Posted on: 12th August 2013
We all know that fusion happens when nuclei of atoms collide – and first you have to undress the atoms, by peeling off their electrons, the process of forming a plasma. But what happens to those electrons – do they just disappear?
The simple discussions of fusion only mention the ions – hydrogen isotopes, deuterium and tritium, colliding to form helium. But for every ion in a fusion plasma, there is at least one electron – in the case of some impurities, many more (although the picture above perhaps overdoes this). And these electrons do not discreetly withdraw from the plasma so as not to inconvenience the fusion process – they hang around and get in the way.
“We often view electrons as a bit of a nuisance,” laughs Dr Sean Conroy, who works on the neutron and gamma diagnostics at JET. “But they do have their uses. We actually couldn’t have a plasma without them – they make the plasma neutral. A plasma of ions only wouldn’t work!”
Electrons weigh approximately 1800 times less than a proton, although unlike the “hundreds and thousands” sprinkles pictured above they are extremely small – smaller than we can measure with today’s technology. (They are also not multicoloured!)
Because they are so much lighter electrons behave very differently to ions in the plasma. For example at a given temperature the speed of a particle depends inversely on the square root of its mass; this means the electrons in the plasma will be moving about 40 times faster than the ions. And even more surprising, sometimes the temperature of the electrons is quite different from the ions.
“A lot of our heating systems heat the electrons first,” explains Dr Emilia Solano, plasma physicist in EFDA. “For example, when we inject fast ions into the plasma with the neutral beam injection system, they are travelling at speeds close to the electrons’ speed, so they “see” each other and interact strongly. If the density is high the electrons then share their energy with the ions, through collisions. But at low density, they don’t.”
This can lead to one of a plasma physicist’s greatest fears, a bunch of fast moving electrons, zooming around the torus. They pick up energy from the electric field generated by the tokamak’s central coil – “ohmic heating” – and as they speed up they interact less and less with the ions. This can lead to a runaway process: small beams of electrons can form, with energies up to a thousand times more than the other the plasma particles.
Preventing runaway electrons is a good reason to keep the density up; but too dense, it turns out, is problematic as well. With increased density comes lots of collisions – great for fusion when the collisions are between ions – but collisions between ions and electrons create an energy leak from the plasma. This is because of a process called bremsstrahlung – a German word meaning “braking radiation”. Although the relatively heavy ions are barely moved by the collisions, the light electrons “bounce” off at more acute angles. The laws of electromagnetism dictate that, because electrons are charged, this change of direction will create light – bremsstrahlung. If the density increases, the number of collisions grows and generates a significant amount of radiation which carries energy away from the plasma. Experienced operators note this effect as a kind of brown tone developing in the normally clear core of plasma – a sign the density has got too high.
Despite these negatives there is a big upside to electrons, says Dr Solano. “Ions are actually really hard to measure: their density, the distribution profile and temperature… these are all much easier to measure for electrons. From these measurements we can calculate the ion parameters, we can’t measure them directly.”
Knowing how hot and dense the plasma is arguably the most important thing for fusion – so we should be thankful electrons are not too discreet, after all; despite their negative charge they do make a positive contribution to fusion physics.