A: What does the tokamak plasma-wall interaction look like?
Hello and welcome to the second larger chapter in my Guide to Fusion. I am sorry I havent been posting a lot lately, but I was super busy at work. I was pretty much working from 7 am to 9 pm as I had lots of meetings and conferences so I didnt really find much time to do my beloved animations and make trully quality posts.
But now I should have more free time so I can start where I left of. I left of by describing the different ways of harnessing fusion energy right here on Earth. The content until now can be summed up in a few simple statements.
- Tokamaks are the most efficient way of harnessing fusion energy right now. Stellarators could be the best solution in a few generations.
- One of the most pressing matters that must be adressed is the physics of the plasma-wall interaction.
The whole second chapter of the Guide to Fusion will be focused specifically on this topic. Luckily this is also my field of expertice. Lets begin.
Pure surface processes
All of the processes begin with a particle from the plasma hitting the wall. So we must first discuss how the identity of the particle and its energy affects the dominant processes.
But first we must mention the process that you can see in the gif to the right, as it is possible no matter the energy of the incoming particle or its mass. This is called reflection. Although the process is quite complicated if we try to interpret it physicaly rigourosly we can simplify it by simply making a reflection coefficient R(E, m, θ) which is dependant on the mass of the impinging particle, its energy and its impact angle.
In general terms it can be stated that the reflection coefficient increases with increasing energy, increases with increasing mass. Also it goes to 1 when the impacting angle goes nears π/2.
When a particle recoils from the surface a whole host of other processes can occur with it depending on the identity of the impacting and wall particle. The most important such process is erosion which you can see in the left figure. There are two kinds of erosion. One is called physical sputtering and the second is called chemically assisted physical sputtering.
Both are very similar as they start with an impacting particle transfering a lot of energy to a surface atom. If enough energy is transfered either directly through nuclei-nuclei interaction or with nuclei-electron cloud interaction then the surface atom can be knocked out of its place. This is the case with physical sputtering - it is quite simple. But when the impinging particle can form strong chemical bonds with the wall material like in the case of hydrogen isotopes hitting the carbon limiters (see previous post) then the threshold for physical sputtering decreases and the net erosion is greated than in pure physical sputtering. This is what we call chemically assisted physical sputtering.
Another process is also shown in the previous figure. So-called re-deposition. This is due to atoms that are knocked-out of the wall heating up as they enter the plasma. The heating up makes them lose electrons which makes them ionized and charged. This charge forces them to get caught by the magnetic field which forces them back onto the wall. In the case of W this happens very quickly and very near the erosion zone. This means that the wall has some self-regenerating ability.
In the case of C and they very quickly form hydro-carbons in the plasma because of the high chemical reactivity. This bonds are extremely strong to break which means that the thing that is re-deposited back onto the wall is in fact not pure C but CH complexes. This is called co-deposition and was one of the reasons C is no longer considered as a candidate for a wall material. This co-deposition would drive tritium retention through the roof in a tokamak reactor.
Mixed bulk-surface interaction
Now lets move on to more complex interactions. The first is shown in the left figure. Here a very heavy and energetic ion penetrates directly into the bulk. It transfers a lot of its kinetic energy onto the lattice atoms and knock them out via phonon interactions usually. This leaves a so-called vacancy behind. I will tell you in the future why they are extremely important, so keep them in mind. The lattice atom knocked out now has a lot of kinetic energy and it travels through the material doing additioanl damage to the lattice in so called cascades which are not shown in the gif.
The last process I want to talk about is only really relevant for hydrogen isotopes. Here the hydrogen isotopes become trapped on the surface or in the bulk of the wall material.
If a hydrogen isotope has about 1 eV of energy or larger than it can freely ignore the surface and penetrate into the bulk of the material. There it can diffuse throgh the lattice, but that is a story for a another time.
Another possibility is that the hydrogen isotope has a kinetic energy which is smaller than eV. In that case it reflects from the surface a couple of times interacting with the electron cloud of the material. In this manner it slowly gives away all of its kinetic energy and it becomes trapped on the surface of the material. There it sits for a long time, eventually either escaping into the plasma or into the bulk of the material.
Today we have learned something about the various plasma-wall interaction possible inside a tokamak reactor. With this knowledge we will be able to investigate why some materials are better than others in the next post and later how hydrogen isotopes specifically interact with the wall material, which is one of the biggest things to understand in a fusion reactor.
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