Moscow, early 1950s: Igor Tamm and Andrei Sakharov propose a toroidal magnetic bottle to hold a fusion plasma off the walls — the tokamak. Whether it ever ignites is not a matter of temperature alone: it is a threshold on the triple product n·T·τ — the Lawson criterion. This panel is runnable: set density, temperature and confinement time, and it computes n·T·τ against the D–T ignition threshold — and refuses to call a sub-threshold plasma ignited.
source The tokamak (тороидальная камера с магнитными катушками, “toroidal chamber with magnetic coils”) — Tamm & Sakharov, USSR, early 1950s; still the leading approach to controlled fusion (ITER is a tokamak). Room: THE MACHINE. Rendered, not quoted.
In the early 1950s at the Kurchatov Institute, Igor Tamm and his student Andrei Sakharov worked out a machine to confine a hot plasma with magnetic fields wound into a torus — a doughnut. The name “tokamak” is a Russian acronym: toroidal kamera, magnitnye katushki — toroidal chamber, magnetic coils.
Fusion needs the plasma at tens of millions of degrees — hotter than any wall can touch. The tokamak’s answer: never let it touch. Hold it in a magnetic cage and let it burn in vacuum.
A charged particle spirals along a magnetic field line but is hard to push across one. Wrap the field into a closed torus and the plasma has no end to leak out of. A helical twist (toroidal + poloidal field) cancels the drifts that would otherwise fling particles to the wall.
So the torus is a bottle with no lid and no floor. But it is a leaky bottle: heat still escapes across the field. How long it holds is the energy-confinement time τ — the third number the engine needs.
The Soviet T-3 tokamak (1968) reported electron temperatures so high that a British team was flown in to check — and confirmed it. That result made the tokamak the world standard.
The line runs T-3 (1968) → JET (Europe, record fusion energy) → ITER (under construction, France) — every one of them a tokamak. Seventy years on, Tamm and Sakharov’s torus is still the leading bet for controlled fusion.
Three numbers decide a D–T burn: density n (ions per m³), temperature T (keV), and energy-confinement time τ (s). The engine forms the triple product P = n·T·τ and compares it to the ignition threshold.
Ignition threshold set at the optimum T ≈ 15 keV: n·T·τ must exceed ~3×10²¹ keV·s/m³ approx.
▮ triple product P = n·T·τ ║ ignition threshold ~3×10²¹ (log scale)
triple product — keV·s/m³ · threshold 3.0×10²¹ · margin —
fusion power (∝ n²) — W/m³ loss power (∝ n·T/τ) — W/m³ approx
THE LAWSON CRITERION. WALL A plasma ignites (self-heating carries the burn, Q→∞) iff n·T·τ ≥ threshold. Fusion power grows as n²; losses grow as n·T/τ. Ignition is when the balance tips — and no amount of one number alone buys it.
this plasma: —
| n,T,τ | n·T·τ | claim | physics | agree |
|---|
The claim (“ignited”) is checked against the threshold on the actual triple product: ignited only if n·T·τ ≥ ~3×10²¹. Claim and physics must agree on every row.
THE ADVERSARY. “Just make it hotter.” But push T too high with the same n and τ and you can still fall short of the triple product — and hotter plasmas are harder to confine. Temperature is one factor of three, not the whole.
“Fusion just needs a hotter plasma.” Cut. Temperature alone is not enough; ignition is a threshold on the triple product n·T·τ. A blazing but thin or leaky plasma still does not ignite.
“The tokamak was a Cold-War dead end.” Cut. T-3 → JET → ITER — it is still the leading fusion machine on Earth.
“Break-even means ignition.” Cut. Break-even (Q=1) is far below ignition (Q→∞); the Lawson threshold here is the ignition bar, higher still.
The red move: take a plasma below the threshold (n·T·τ < ~3×10²¹) and claim it ignited — drop the Lawson check so a leaky, sub-threshold plasma gets stamped burning.
Drop the bar and the claim lies about the sub-threshold rows: losses exceed fusion power (Q<1) yet it calls them ignited. The witness compares claim to physics and turns red. The attack is real and it is caught.