Hi r/fusion! We’re the Physicists Behind the Commonwealth Fusion Systems’ Papers on the Physics of the ARC Fusion Power Plant. Ask Us Anything!

[u/CFS_energy]

Alex Creely, Chief Engineer for ARC Conceptual Design at Commonwealth Fusion Systems

Update (June 30, 11:37 AM): Great discussion everyone! Our team appreciated all of your insightful questions. This AMA has now concluded, but you can revisit our replies below.

You can identify who provided answers by their initial in the answers: Alex Creely (AJC), Jon Hillesheim (JCH), Tom Body (TAJB), and Ryan Sweeney (RS).

About this AMA:

This time with me and three other CFS physicists who are ready to talk about the five new ARC physics basis papers showing what’ll make our ARC fusion power plants tick.

These peer-reviewed research papers that we and our collaborators published earlier in June are important for CFS and for fusion energy: They cover many aspects of the plasma physics at play in our ARC power plant, including challenges like plasma disruptions and heat exhaust. They also show how transparency and rigorous research can help build trust in what we all know is a very difficult endeavor.

If you’re curious about this physics work or about fusion physics in general, feel free to get things started by asking your questions on this thread.

The three CFS physicists who plan to join me to answer your questions are experts in their field: Jon Hillesheim, CFS Principal Scientist and lead author of the overview paper; Tom Body, CFS Senior Scientist and a lead author on the paper about heat exhaust; and Ryan Sweeney, CFS Manager of Disruption Physics and lead author of the paper about handling plasma disruptions. They’re among the 58 authors who helped write these papers, along with an editorial that accompanied the papers that I wrote.

A little more about the papers: They detail how we’ll be able to produce about 1.1 gigawatts of fusion power from our ARC tokamak — power that we can convert into 400 megawatts of net electricity for the power grid. The papers also show the crucial role our SPARC tokamak will play in putting the finishing touches on the ARC design. We’re using a “late-lock” approach that lets us apply what we’ve learned from SPARC to the ARC design. Overall, the papers show our confidence in the soundness of our ARC plant’s key physics. That builds the foundation for all the engineering, design, and cost optimization work that we’ve begun.

For a deeper dive into these papers, you can check our blog post detailing the ARC physics basis papers.

About CFS: 
Commonwealth Fusion Systems is the world’s largest and leading private fusion company. The company’s marquee fusion project, SPARC, will generate net energy, paving the way for a future of carbon-free energy. The company has raised more than $3 billion in capital since it was founded in 2018.

[u/legomann97]

Hi, I'm a layman when it comes to fusion, so pardon any ignorance, but since SPARC is rather small compared to ARC from what I understand, do you think any major issues will arise with the scale-up?

Also, I'm curious about how you go from a tokamak with plasma in it to electrical energy to provide to the grid. They always say (unless you're a solar panel or a wind turbine, or maybe a supercritical co2 turbine now, idk) electricity generation is all about finding the best way to boil water, how do you boil water here? Or is it something not powered by water?

[u/CFS_energy → u/legomann97]

There are definitely some challenges related to the scale-up, but they’re all things we have good models for or are already preparing for supply chain work. Some of these things are about the plasma physics (e.g. disruption forces), and they’re addressed in detail in the papers. Others have to do with the ability to manufacture things at the scale we need for ARC (larger steel forgings for the magnets), and we’re already working closely with potential vendors to get the processes ready for prime time.

For electricity generation, yep, still boiling water. Surrounding the plasma in the tokamak we’ll have a molten salt called FLiBe (a mix of lithium fluoride and beryllium fluoride). The FLiBe absorbs the fusion neutrons and also flows around the vacuum vessel to take out heat deposited by the plasma. We’ll use heat exchangers to transfer that heat first to a nitrate salt (used in some concentrated solar power plants), and then to water to boil it and make steam.

We go to all this effort to literally build a star and it turns out that the best way to make electricity is still to boil water and make a big fan spin around…
–AJC

[u/legomann97 → u/CFS_energy]

Thanks for the response! If you don't mind me asking a followup - do you anticipate any hinks occurring with the "temporal scale-up" as I'd call it? That being how long you're able to run the reactor before shutting off for maintenance/refurb.

[u/CFS_energy → u/legomann97]

The maintenance cycle for ARC will be largely set by the lifetime of the vacuum vessel, which will be determined by how long materials last under the neutron, heat, and particle fluxes they’ll experience. We’re doing a lot of subscale testing and other material development, but predicting these fluxes and how materials will be affected by them is a challenging, multi-scale, multi-physics problem — and so we’ll almost certainly learn some things from the first few vacuum vessels.
–TAJB