How This Was Built

The physics in this book is solved. But a solved physics is not a company, a company is not a machine, and none of it builds itself. Behind every equation in these pages is a person who checked it, an argument that took years to win, and a great deal that had nothing to do with plasma at all. This chapter is about that — the people who gave the work its shape, and the honest fact that assembling them was harder than any calculation in the book. Part I told you who these people are and why the century's streams converge in them; this chapter tells the lived version — how it actually happened, email by unanswered email.

I came to fusion by an unlikely road, and I have never tried to hide it. I am not a physicist by training. What I had, at the start, was a conviction inherited from my family, a habit of building things learned in industry, and a willingness to send my work to people far more expert than I was and ask them, plainly, whether it held. Much of what follows began exactly that way — as unsolicited email into inboxes I had no claim on.

The twins who answered

Some of the most consequential relationships of my life began as emails I wasn't sure anyone would answer. In the suspended year of 2020, with the world shut indoors, I started mailing mathematical proofs of fusion-energy designs to Carl and Bob Weggel — twin brothers who between them carry more than a century of building the magnets that make fusion possible, both veterans of MIT's Francis Bitter Magnet Laboratory, where the Alcator tokamaks were born. Carl designed the Alcator series; one of those machines still holds a Guinness world record for the strongest steady-state magnetic field ever produced. I knew neither of them. I sent the work anyway.

They answered in character, and the division has held ever since: Carl replied on the design, Bob on the mathematics. Bob is the quieter twin — he can sit through a whole meeting and say almost nothing — but he reads everything and proves everything, and every equation in this book passed under his eye before it reached the reader. The field-coil design he handed Kronos is good enough that Brookhaven National Laboratory, where he spent years building record-setting magnets, still asks regularly to collaborate with us.

Carl is our chief scientist, and he became my teacher in fusion. Most of what I know of this field that did not come from a textbook, I owe to him — the physics as it is actually practiced by someone who has spent fifty years building magnets that either hold their field or fail with nothing in between. He taught with a rigor that never condescended: he expected me to follow the argument all the way down, and gave me the room to get there on my own.

The shape of the whole thing

If Carl taught me the physics and Bob proved the mathematics, Dr. Gerald Kulcinski gave the program its shape. He has been with Kronos almost from the start, and he is one of the great figures in the history of this field — to my mind one of the finest scientific minds of his generation. He ran the Fusion Technology Institute at the University of Wisconsin–Madison for decades; he is a member of the National Academy of Engineering; and in his own laboratory he achieved the first steady-state fusion using helium-3 fuel in an inertial-electrostatic device — the low-neutron pathway most of the field had quietly written off. For half a century he has argued that the cleanest fusion fuel, helium-3, is waiting for us on the surface of the Moon. That argument is his life's work. In a real sense, it is also ours.

Much of what I understand about how fusion actually came to be, I learned from him directly — not from the papers but from a man who was in the room for a great deal of it. To take the history of this field firsthand, from one of its pioneers, is a privilege I still don't quite have words for.

And it is Kulcinski, more than anyone, who is the reason this book describes two machines instead of one. Left to my own instincts I might have chased a single reactor. He — with Dr. David Hammer of Cornell, one of the deans of pulsed-power fusion, and Dr. Siegfried Glenzer, carrying the ignition campaign's hard-won realism, pressing in the same direction — kept turning the problem until the answer separated cleanly in two: a breeder that makes the fuel, and a burner that turns it into firm, clean power. The neutrons a fusion machine throws off, he taught me, are not only a hazard to be shielded against; they are a product — a way to breed tritium, to make helium-3, to manufacture the isotopes the world runs short of. The architecture at the center of this book — build the fuel first, then the machine that burns it — was his insight before it was ever our design.

The field's memory, and a debt older than the company

Dr. Steven O. Dean carries sixty years of the fusion program in his memory — he ran the Confinement Systems Division at the AEC and the DOE, and co-founded Fusion Power Associates. But he means something more particular to me than his résumé. Decades ago he worked alongside my great-uncle, P.K. Iyengar, helping to stand up India's atomic program — the same great-uncle who first told me, when I was seven, that fusion was fifty years away. That the man who helped my family's generation build one nuclear program is now, at the far end of his career, helping me build this one, is a symmetry I did not plan and will never take for granted.

There is a strand of this work that runs on computation, and it has a patron I still find it faintly unreal to name. Dr. Jack Dongarra joined us at the end of 2025. He has been a hero of mine since childhood — my car's license plates read MATLAB — because the numerical libraries he built, LINPACK and LAPACK, are the bedrock the world's scientific computing quietly runs on. In 2021 he won the Turing Award, computing's highest honor. That the man who wrote the software beneath everyone else's software now helps design the machine at the center of this book is one of the stranger privileges of getting to build it. He is one of many — the plasma theorists, the materials scientists, the magnet engineers, the naval and defense veterans named in The People Behind the Work — who, for reasons I am still grateful for, said yes.

The part that was not easy

I will not pretend the road was smooth. Building anything real means gathering people, and gathering people means that some of them leave, and a few turn on you. There were partings I did not choose and did not want. There was, once, a threat against my safety serious enough that I have had to think carefully about what belongs in a book and what belongs only in a courtroom. I have chosen, here, to keep the names and the details out of these pages — not because they don't matter, but because this is a book about a machine, and some accountings are better kept where they can be answered properly.

I mention it at all only so that no one mistakes this for an easy story. It was not. The physics, in the end, was the tractable part. The harder work was the human one: convincing people who had every reason to be skeptical, holding a company together through the years when it was only a promise, and continuing when continuing was expensive in ways that had nothing to do with money. I am the third link in a chain that has spent three generations near the atom, and if I have learned one thing from the people in this chapter, it is that the work is worth doing patiently, and worth doing right — even, especially, when it is hard.