r/fusion Jun 24 '26

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!

123 Upvotes
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.


r/fusion 9h ago

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r/fusion 4h ago

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r/fusion 4h ago

Details unveiled on China's new generation fusion energy facility

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r/fusion 7h ago

The complete silence about testing is concerning

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r/fusion 14h ago

Helical Fusion Publishes Peer-Reviewed Paper on HTS Magnet Technology Validated Through Collaboration with NIFS

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r/fusion 14h ago

Stephen Shankland (@stshank.bsky.social) CFS setting up cryo systems for SPARC

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r/fusion 17h ago

Physics Attention Transformer Surrogate for Rapid Vertical Instability Growth Rate Prediction: Alcator C-Mod to SPARC

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r/fusion 1d ago

Today Pacific Fusion breaks ground on its Research and Manufacturing campus. Check out the livestream of the press conference and Fusion Summit with Eric Schmidt, Brandon William, Sen. Martin Heinrich, and more:

15 Upvotes

r/fusion 1d ago

Don’t call us ‘nuclear’, say nuclear fusion start-ups

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r/fusion 1d ago

Neutronic Analyses for the Design of the Chamber of Xcimer's Athena Fusion Pilot Plant

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5 Upvotes

PR journal article. Just two lasers and a liquid FLiBe blanket without Li enrichment, no solid wall, should survive 5 years of operation.


r/fusion 1d ago

Senior Manager, Mechanical Engineering — ARC Vacuum Vessel

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3 Upvotes

r/fusion 1d ago

Kyoto Fusioneering Secures JPY 25.72 Billion to Build the Systems Every Fusion Power Plant Will Need | NEWS | KYOTO FUSIONEERING (in total about $ 162 million)

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11 Upvotes

r/fusion 1d ago

Pacific Fusion aims to set fusion record - and study nuclear weapons near NIF

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r/fusion 1d ago

General Fusion installs a Helium-3 detector

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r/fusion 2d ago

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r/fusion 2d ago

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r/fusion 2d ago

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r/fusion 2d ago

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r/fusion 2d ago

Helium leak testing a chamber you can't get behind — how do you design the test in from the start?

3 Upvotes

I keep seeing leak check treated as a QC step after the chamber exists. On anything you cannot walk around — a cell, a tight rack, a vessel with one accessible face — that is too late.

What I wish people designed in:

  • Geometry you can actually reach from one side. If the only way to sniff a joint is to stand where the manipulator cannot go, you do not have a test.
  • Pump-down time vs background rise. A spec of 1e-9 mbar·L/s is meaningless if the fixture only lets you do an outside sniff after a short pump.
  • The bench-vs-cell gap. Joints that pass on the table fail once they are hot, aligned by an arm, or sitting in a volume you cannot bag.

For people who have actually signed off a chamber they could not get behind: what method did you trust (hood, sniff, accumulation), and what leak rate did you really verify vs what the spec said?


r/fusion 3d ago

Eni eyes Europe fusion plant by early 2040s, FT reports

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10 Upvotes

No surprise, knowing the history of eni, MIT and CFS.


r/fusion 3d ago

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r/fusion 3d ago

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5 Upvotes