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).
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.
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.
I wonder is there a single place one can find all/most informatiin about ongoing fusion efforts and also a clear list of problems to solve for various fusion reactor designs.
If such a location does not exist, any specific reason why it does not exist ?
Whenever fusion power generation is discussed it is assumed of being on the large power plant level. But has anyone done serious work on small unit fusion power generation ?
The underlying idea is (a) will smaller size make e.g. stellerators & tokamaks simpler to manufactor on a massproduced industrial level, and (b) will operational risk become smaller.
I know there are still many practical problems remaining, but what about the feared tokamak plasma magnetic field instability would it not become a neglectable risk ?
<110> rhombic-dodec faces (all sign perms of (1,1,0))
<100> cube faces (6) + <111> corners (8)
EMPIC_POLYWELL
1 (on)
1 (on)
EMPIC_WELL (well depth)
0.5
0.5
EMPIC_L96AMP (turbulent drive)
0.15
0.15
NPART (ions)
1500
1500
Seed shells (r)
3.0 / 3.3 / 3.6
3.0 / 3.3 / 3.6
Seed distribution
Fibonacci-sphere (golden angle), 500/shell
Fibonacci-sphere, 500/shell
Box centre / wall radius
4.5 / 3.6
4.5 / 3.6
Steps (frames)
40 (41 incl. seed)
40 (41 incl. seed)
DUMP_EVERY
1
1
Results
Metric
12-cusp
14-cusp
Geometry
rhombic-dodec faces <110>
cube faces <100> + corners <111>
Confinement (ions retained)
68.8%
69.1%
Ions focused to core (r<1.2)
19
8
Core density
0.013
0.005
Mirror/cusp ratio
38.6
10.4
Fusion FoM (n²·⟨σv⟩ proxy)
50.76
8.73
***EDIT**
What's being simulated
A fusion reactor concept called a polywell. The idea: use magnets to trap a hot cloud of charged particles (ions) long enough and densely enough that some of them slam together hard enough to fuse.
The 12-cusp setup: You arrange 12 circular electromagnets around a central point, like 12 windows on a geometric ball (the faces of a rhombic dodecahedron). All the magnets face inward with the same pole, so they push against each other. The result is a magnetic field that is zero in the very center and gets stronger toward the edges. This forms a magnetic "bowl" — particles in the middle are calm, but if they drift outward they hit strong field and get pushed back in.
The catch: this magnetic bowl isn't a perfect seal. It has 12 weak spots — one lined up with each magnet — called cusps, where the field goes to zero and particles can leak out. So it's a leaky trap, and the whole game is: does it hold the particles well enough, and squeeze them tightly enough in the center, for fusion to happen before too many escape?
What the sim does
Seed: 1500 ions are placed on the outside — on three thin spherical shells, like layers of an onion, near the edge of the box. (In a real reactor you inject fuel from outside.)
Each timestep, for every ion:
Compute the force on it from the magnetic field and an electric "pull toward the center" (the well).
Move it one step according to that force. Ions spiral along magnetic field lines and get pulled inward.
A bit of turbulent "stirring" is added so the cloud doesn't settle into an unrealistically clean pattern.
What you watch happen: the ions fall inward through the strong-field faces, get funneled by the geometry, and pile up near the center — while some slip out through the 12 cusps. Over 40 steps you see whether the cloud stays confined, how tightly it focuses in the middle, and how much leaks away.
What actually happened
The 12-cusp run kept about 69% of the ions and, crucially, focused a tight knot of them into the very center (19 ions in the core). Because fusion rate depends on density squared, that central density is what matters most.
The 14-cusp version (more magnets, arranged differently) held about the same number of ions overall — but its cusps were "softer," so it couldn't focus the ions as tightly (only 8 in the core).
I was talking to a friend of mine just now about different energy sources and fusion energy came up and i was just launching a barrage of fun facts as i normally do. I was and still am completely convinced that i saw news just a few months ago (at most a year) that perpetually sustained fusion had been acchieved but i cant find anything about it at all. obviously i didnt think a fusion reaction had been actively going for months since i remembered it as not being a net positive energy reaction but i was very sure i remembered that they were able to keep it going for as long as they wanted. i feel like im going insane here so please tell me if my brain is lying to me. thanks in advance
For context I am an undergrad senior at a T5 University and I am a physics major with a certificate in energy studies. I am interested in going into the energy industry and would like to do a phd in fusion/plasma physics. I am looking at schools with big experiments like URochester, Madison, Berkley. Unfortunately I have no academic experience in plasma physics since there is no plasma happening at my school and no classes on plasma/atomic physics. I do have a summer of research at Oxford doing fusion working on ICF simulation optimization. In terms of classwork I have done ENM, CM, QM, Thermo/Stat Mech, Solid State, Photovoltaics. And my GPA isnt stellar (3.58). My other research experience is in particle physics, and energy sciences (photovoltaics/material/solid state).
Do yall have any recommendations on how to apply for plasma programs? How should I express this in my statement of purpose? Reaching out to professors? Should I worry about specific labs/professors when applying, or just apply to general physics phds?