# A: What does the tokamak plasma-wall interaction look like?

in #stemq5 years ago (edited)

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.

1. Tokamaks are the most efficient way of harnessing fusion energy right now. Stellarators could be the best solution in a few generations.
2. 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.

# Conclusion

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.

StemQ Notice: This post was originally submitted on StemQ.io, a Q&A application for STEM subjects powered by the Steem blockchain.

Sort:

This is a very informative post to understand what happens inside the tokamak plasma chamber. Thank you for your sharing. I have little question here:

• 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.*

Is this self-regenerating ability a wanted thing for us? And can we control it by changing some kind of process parameters? As I understand from the text it is a kind of preventing way against the erosion?! Am I right?

This is definitly wanted. First if the material atoms find themselves in the core plasma they radiate geat through photond ehich means that thr plasma will get cooled.

The second reason is that this just as you have said reduces the overall erosion.

Thank you for the answer. I am going to be following on your new posts about these subjects.

Well I am very glad you like my writing.

Hi, @maticpecovnik!

You just got a 1.52% upvote from SteemPlus!
To get higher upvotes, earn more SteemPlus Points (SPP). On your Steemit wallet, check your SPP balance and click on "How to earn SPP?" to find out all the ways to earn.
If you're not using SteemPlus yet, please check our last posts in here to see the many ways in which SteemPlus can improve your Steem experience on Steemit and Busy.

5 years ago (edited)

Great gifs! and great explanation

Why you are in the "plasma mood", what about the cold plasma, we had some fun with that, treating biological stuff (bacteria, plants, seeds, cells...)

Well I am not really that familiar with cold plasma, but I can tell you it is not as cold as you might think.

Yes, it's stove-like hot :D

This post has been voted on by the SteemSTEM curation team and voting trail in collaboration with @utopian-io and @curie.

If you appreciate the work we are doing then consider voting all three projects for witness by selecting stem.witness, utopian-io and curie!

For additional information please join us on the SteemSTEM discord and to get to know the rest of the community!

Congratulations @maticpecovnik! You have completed the following achievement on the Steem blockchain and have been rewarded with new badge(s) :

 You made more than 1500 upvotes. Your next target is to reach 1750 upvotes.

If you no longer want to receive notifications, reply to this comment with the word `STOP`

To support your work, I also upvoted your post!

Support SteemitBoard's project! Vote for its witness and get one more award!

Congratulations @maticpecovnik! You have completed the following achievement on the Steem blockchain and have been rewarded with new badge(s) :

 You received more than 7000 upvotes. Your next target is to reach 8000 upvotes.

If you no longer want to receive notifications, reply to this comment with the word `STOP`

Do not miss the last post from @steemitboard:

 Meet the Steemians Contest - The results, the winners and the prizes

Support SteemitBoard's project! Vote for its witness and get one more award!

#### Hi @maticpecovnik!

Your post was upvoted by @steem-ua, new Steem dApp, using UserAuthority for algorithmic post curation!
Your UA account score is currently 2.903 which ranks you at #11301 across all Steem accounts.
Your rank has improved 371 places in the last three days (old rank 11672).

In our last Algorithmic Curation Round, consisting of 249 contributions, your post is ranked at #46.

##### Evaluation of your UA score:
• Only a few people are following you, try to convince more people with good work.
• The readers appreciate your great work!
• Good user engagement!

Feel free to join our @steem-ua Discord server

#### Hi @maticpecovnik!

Your post was upvoted by Utopian.io in cooperation with @steemstem - supporting knowledge, innovation and technological advancement on the Steem Blockchain.

#### Contribute to Open Source with utopian.io

Learn how to contribute on our website and join the new open source economy.

Want to chat? Join the Utopian Community on Discord https://discord.gg/h52nFrV

Congratulations,
you just received a 12.45% upvote from @steemhq - Community Bot!

Wanna join and receive free upvotes yourself?
Vote for `steemhq.witness` on Steemit or directly on SteemConnect and join the Community Witness.

This service was brought to you by SteemHQ.com

Congratulations @maticpecovnik! You have completed the following achievement on the Steem blockchain and have been rewarded with new badge(s) :

 You received more than 500 as payout for your posts. Your next target is to reach a total payout of 1000

If you no longer want to receive notifications, reply to this comment with the word `STOP`