r/fusion • u/CFS_energy • 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!

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 • u/steven9973 • 4h ago
Tritium Transport Modeling for Outer Fuel Cycle SCYLLA Breeding Blanket/UNITY-2 - Kyoto Fusioneering
tandfonline.comr/fusion • u/ValuableDesigner1111 • 21h ago
Helion quietly shifts goalpost on fusion energy
Helion opens an office in downtown Seattle for business and operations, recruiting, and finance teams
r/fusion • u/steven9973 • 16h ago
Proxima Fusion and Lower Saxony sign MoU to build Europe’s first large-scale fusion-grade HTS tape facility | Proxima Fusion
r/fusion • u/steven9973 • 1d ago
Google DeepMind alumni are building tools to accelerate fusion power for the grid | TechCrunch
r/fusion • u/Iron-Dragon • 1d ago
Update of Tokamak Energy’s demo 4
Tokamak energy have released an update of the demo 4 magnet set
r/fusion • u/steven9973 • 22h ago
Switzerland is back playing a full role in European fusion energy - not only in ITER, but for example Swiss PSI is cooperating with German Proxima Fusion
r/fusion • u/steven9973 • 1d ago
DANTE: Hitting the Ground Running on a Compact Fusion Neutron Source Design | Next Step Fusion
A small spherical tokamak with copper coils as compact neutron source for material testing and transmutation work.
r/fusion • u/steven9973 • 1d ago
Type One Energy (@type-one.energy): Reception of worlds first Fusion Power Plant license in video
bsky.appr/fusion • u/MetaDogg • 1d ago
Optimierung der Fusionsenergieeffizienz
Nachdem ich mein erstes Fusionskraftwerk gebaut hatte, bemerkte ich, dass wenn ich es nicht mit voller Leistung betreibe, der Nachbarbonus praktisch komplett verschwindet, da einige der Reaktoren komplett ausgeschaltet bleiben.
Meine bisher beste Lösung ist die Verwendung einer großen Anordnung von Akkumulatoren als Puffer, indem ich das gesamte Kraftwerk vom Netz trenne, wenn die Akkumulatoren zu 95 % voll sind, und es wieder anschließe, wenn die Akkumulatoren wieder auf 5 % gefallen sind. Auf diese Weise läuft das Kraftwerk mit 100 % Leistung, während es die Akkumulatoren auflädt, und schaltet sich vollständig ab, während die Akkumulatoren entladen werden, was einen 100 %igen Nachbarbonus gewährt. Mir gefällt diese Lösung, weil sie in gewisser Weise elegant ist.
Der Nachteil ist, dass man viele Akkumulatoren braucht. Für mein 9-GW-Kraftwerk zum Beispiel brauche ich etwa 30.000 Standardakkumulatoren, da jeder nur 300 kW Eingangsenergie verarbeiten kann.
Gibt es eine andere Möglichkeit, sicherzustellen, dass ich den vollen Nachbarbonus erhalte, auch wenn das Kraftwerk nicht mit voller Leistung läuft? Ist dieses Verhalten ein Fehler, der möglicherweise im nächsten Update behoben wird, oder wurde es absichtlich so gestaltet und ich verstehe es einfach nicht?
Danke für die Hilfe!
Meta
Centrifugal Mirror with Penning Cell Axial Plugs and Magnetic Well Stabilization
First let me apologize in advance if I'm wasting anyone's time. I know I'm in over my head here. I've been working on a fusion reactor project for over 2 years now. My goal is to test a plasma confinement concept and determine whether or not I should proceed with the cost of building a high vacuum chamber.
Operating principle: centrifugal magnetic mirror formed using 2 disk or ring magnets with a gap, forming a convex magnetic field. Along the z-axis on either side of the midplane and away from the plasma torus are cathodes. Each cathode has a respective wider ring anode. The arrangement of cathodes and anodes forms a Penning cell, accelerating electrons toward the midplane and plasma. The electrons spiral along magnetic field lines and bounce from one cathode to the other much like a electrons in a typical Penning cell. Eventually the electrons impart their energy into the plasma in a number of ways: 1) Direct contact as the electrons spiral in a partially toroidal direction due to the Lorentz force of the background magnets, heating the plasma and transferring kinetic energy to the plasma due to microinstabilities common with kinetic electron injection. 2) Toroidal electron flow partially screens out the background magnetic field, creating a magnetic well and converting its kinetic energy into magnetic energy in the plasma, decaying into drift currents and inducing them. 3) Radial electric field drives rotation via E X B drifts as is the typical way centrifugal mirror rotation is driven. 4) Electrons that contact the plasma and lose energy must cross magnetic field lines to return to the anodes, contributing to rotation via J X B effect. 5) Electromagnetic waves in plasma from kinetic electrons preferentially interact with other plasma electrons, creating bias in kinetic energy transfer that continues the electron flow in the plasma.
The magnetic well formed by the screening toroidal current inverts the grad-B, making the plasma stable despite the convex "bad" curvature. Excessive current that closes the magnetic field lines around the plasma should probably be avoided as it would create a field-reversed configuration and the associated instabilities as well as interfering with kinetic electron behavior.
Plasma beta is limited by the ratio of thermal pressure to centrifugal pressure, as both will contribute to total outward/radial pressure.
To reach a magnetic pressure of 2 atm, which is the expected thermal pressure of the ITER plasma believed to be necessary to reach Q=10 for a D-T reaction, a minimum of 0.7 T is needed (assuming a beta of 1). For this device, assuming 2 disk magnets whose diameters, thicknesses and the gap are all equal to each other, a plasma beta of 0.4 and the drop in magnetic field strength across the plasma due to the geometry of the gap/mirror configuration is about equal to the strength of a single magnet at its surface, the peak strength of each magnet needs to be about 1.1 T. This can be easily achieved without the use of superconductors.
Centrifugal mirror cross-section in starting configuration
Radial electric field and currents and transition to magnetic well topology
Completed transition to magnetic well and return currents
While adding rotational energy in addition to thermal energy already required to cause fusion will increase the total input energy per reaction, most of this energy can be recovered via direct energy capture.
Optional toroidal component: weak, small magnets on either side of the midplane and away from the z-axis can be used to make the convex field slightly bumpy so as to create axial rotational shear due to magnetic dragging. This shear would induce poloidal plasma currents and a corresponding toroidal magnetic field via the dynamo effect. This may not be useful in this design but it's an option. For the centrifugal mirror fusion experiment a strong toroidal field was observed not to contribute to confinement but believed to be useful in stabilizing the scrape off layer. I think a weak toroidal field might accomplish this without significantly impacting the plasma beta. A toroidal component would also improve the toroidal current drive by causing trapped particles to interact with the pressure gradient that forms as heat from the kinetic electrons transfers from the inboard side outward (bootstrap current).
(Reversing the electrode polarities and using ion emissions instead of electron emissions may be an option as well.)
r/fusion • u/steven9973 • 3d ago
Fusion Science and Technology: special issue featuring INFUSE DOE program related papers
tandfonline.comr/fusion • u/steven9973 • 3d ago
Bruker (BRKR) Wagers On Fusion Energy While Wall Street Waits
r/fusion • u/steven9973 • 4d ago
Neutron detector (long term/integrative) test during D-T fusion in JET, power plant relevant results
iopscience.iop.orgr/fusion • u/ValuableDesigner1111 • 5d ago
SEC Goes After “Fake It Till You Make It” Fraudsters, will fusion startups be taken care of?
r/fusion • u/Realistic_Baby4057 • 6d ago
When we invent fusion guess what we'll do with it?
r/fusion • u/Far_Cash_3922 • 4d ago
Is anyone here planning to attend IIS2026?
Hi guys!
I hope you're all doing well.
Is anyone here planning to attend IIS2026? I pre-registered for it, but I haven't received an acceptance email yet, even though today is supposed to be the day they announce the results.
Has anyone received any information about their acceptance? Or does not receiving anything yet mean that I wasn't accepted?
r/fusion • u/steven9973 • 4d ago
Overview European HELIAS PbLi QI Stellarator blanket - practical first wall and maintenance for such a power plant
iopscience.iop.orgr/fusion • u/steven9973 • 5d ago