The Spacetime Metric
Section 2Fusion engineering

Aneutronic fuel and the compact machines that suit it

University to national lab. · 2 min read

What it proposes

Burn proton and boron-11 instead of deuterium and tritium. The reaction gives three helium nuclei and 8.7 million electronvolts with no neutrons in the main channel, so the products come back as charged particles you can convert straight into current instead of neutrons you must shield against and whose damage limits the life of the machine. Aneutronic fuel also changes what size of machine makes sense — small, clean and fast to iterate rather than gigawatt-scale.

Who it is forPlasma engineers · Power-electronics designers · Detector builders

Before any machine, the fuel makes demands. Proton and boron-11 ask for a great deal more ion energy than the easier fuels, and that energy is paid for in radiation the fuel's own charge drives up. Walk the four demands below, each from a paper this card already cites, and the size and kind of machine that answers them stops being a preference.

The proton–boron ladder

Proton and boron-11 give three helium nuclei, 8.7 million electronvolts and no neutrons in the main channel. That prize is bought with four demands the fuel makes before anything else, and each one narrows the kind of machine that can answer it. Step through the rungs: how hot the ions must be, what that heat costs in radiation, what the three-way product has to reach, and which machines have made the density and the field. What the sheets state as a ratio rather than a magnitude is drawn as a ratio and said so, on the face of the figure.

The four demands, in order

How energetic do the ions have to be?

One demand of proton–boron-11 fuel, drawn on a logarithmic axisA horizontal logarithmic axis of mean confined ion energy from 1 to 1000 kiloelectronvolts. A shaded band opens at 100 kiloelectronvolts and runs to the right: the energies proton–boron-11 burning requires. Inside it sit four published marks — a ten-shot mean of 125, the Large Helical Device's 160-kilovolt beams, FF-1's record mean of 240 with its uncertainty bar, and the 300-kiloelectronvolt worked example from the magnetic-field calculation. Below the axis a bracket carries the thirty-fold temperature ratio to deuterium–tritium, drawn as a bracket because the sheet gives the ratio and neither temperature.
Not drawn, and why. Thirty times the deuterium–tritium temperature. The paper gives the ratio and neither temperature, so the ratio is drawn as a bracket and no second fuel is placed on this axis. Deuterium–tritium appears because this paper uses it as its own reference point — the candidate fuel with the lowest operating temperature — and for no other reason. Read the papers these figures come fromNothing here moves on its own. The drawing changes only when you choose a different rung.

Proton–boron-11 burning needs mean ion energies well above 100 keV, and Magee and colleagues put the temperature a thermonuclear reactor on this fuel would need at thirty times that of deuterium–tritium — which they name as the candidate fuel with the lowest operating temperature, and use as their reference point. Machines have reached this band: LPPFusion's FF-1 recorded a mean confined ion energy of 240 keV, and the Large Helical Device made its protons with hydrogen beams at 160 kilovolts. The energy is not the obstacle it was.

  1. 1. What the reaction asks for: mean ion energies well above 100 keV
  2. 2. FF-1, ten-shot mean, 125 keV
  3. 3. Large Helical Device, three hydrogen beams at 160 kilovolts
  4. 4. FF-1 record mean confined ion energy, 240 ± 20 keV
  5. 5. The worked example in the magnetic-field calculation, 300 keV

The machines the demands select

The first axis is a mean confined ion energy and not a temperature; the two are different measurements and this figure never writes one as the other. Where the third rung's product uses a mean ion energy in place of a temperature, that step is the source's own and is argued in the source.

This fuel is harder than deuterium–tritium and its own proponents say so in the papers drawn here. Magee and colleagues put it exactly: the proton–boron path trades downstream engineering challenges for present-day physics challenges. What is bought with those challenges is a reaction whose products come back as charged particles you can convert directly, in a machine small enough to iterate on.

Every figure above, and the sheet it comes from

Why the library suggests it

Put the machines this card names on the plane the whole field trades across — ion density one way, confinement time the other, and each placement an order of magnitude rather than a record.

Every fusion machine, on one chart

Every machine in this chapter is answering the same question in a different way: hold matter dense enough, energetic enough, for long enough that nuclei fuse. Because those three factors trade against each other, the families spread out along a diagonal — a magnetic trap holds a thin plasma for a second, an inertial driver holds an enormously dense one for a fraction of a nanosecond, and a pinch sits between them. Pick a family, then pick a machine, and read what it squeezes and for how long.

Confinement approach
Machine
Ion density drawn against confinement time for twelve fusion machinesEleven numbered markers on logarithmic axes, with a twelfth machine named beside them rather than placed,, density running left to right and confinement time bottom to top. The machines fall along a diagonal band: magnetic traps at low density and long times in the upper left, pinches and focus devices in the middle, inertial drivers at enormous density and vanishingly short times in the lower right. A sloping line marks the density times confinement time that deuterium-tritium fuel has to reach.

Beside the axis: none of these three numbers published

  • 3. Century, Zap Energy
The sloping line is the density times confinement time a deuterium-tritium plasma has to reach at the temperature that bar assumes. Machines above and to the right of it have the product; the whole art of the field is getting there with a machine that survives.Both axes are logarithmic and every placement is an order of magnitude — the regime a family works in, not a record shot and not a citation. The chart compares architectures. It is not a scoreboard, and moving one machine a little along either axis would not change what it teaches.One machine here is named beside the axis rather than drawn on it. Its own paper publishes no ion density, no plasma temperature and no confinement time, so there is no honest place for it on these axes. What it does publish is written out in full beside it. That empty slot is the lesson: a machine can be real, working and well reported and still have no position on this chart.Nothing here moves on its own. The chart changes only when you choose a family or a machine.

Selected machine

Sheared-flow Z-pinch, FuZE

What this machine squeezes: A sheared axial flow keeps the column quiet, and neutrons come out steadily while it lasts.

Ion density, particles per cubic metre:
1023
Held for:
10-5 s
Ion energy:
1.8 keV — a bulk ion temperature
Triple product, density times energy times confinement time:
3.5 decades below the deuterium-tritium bar

Its source sheet publishes the density and confinement figures that locate it on this chart. Its energy value is shown separately. A position reports a published regime, not a performance or net-gain claim. Read the source sheet for the experiment it describes.

Open the source sheet for this machine

TAE Technologies and Japan's National Institute for Fusion Science produced the first proton-boron-11 fusion inside a magnetically confined plasma, dropping boron powder into the edge of the Large Helical Device and firing three 2 MW hydrogen beams at 160 kilovolts; a silicon detector out of the plasma's direct line of sight counted about 150,000 fusion products a second with boron present and under a thousand without (First measurements of p11B fusion in a magnetically confined plasma, 2023). Their own conclusion names the lever this site cares about: the rate can be pushed by engineering the particle distribution rather than only by raising temperature. LPPFusion's dense plasma focus reports a mean confined ion energy of 240 kiloelectronvolts — inside the range proton-boron-11 needs — from a plasmoid only tens of microns across holding a large share of the machine's whole energy, with the products emerging as an ion beam one way and an electron beam the other so current is induced directly in a coil (Focus Fusion: Overview of Progress Towards p-B11 Fusion with the Dense Plasma Focus, 2023). The work order for the first decaborane shots, with the instruments already installed, is Preparations for pB11 tests in the FF-2B dense plasma focus (2024). On the compact side, Princeton's field-reversed configuration holds its plasma inside closed field lines the plasma largely makes for itself, in a tube four to eight metres long, heated by an odd-parity rotating magnetic field; PFRC-2 has heated electrons past 500 electronvolts and held plasmas for 300 milliseconds (The Princeton Field-Reversed Configuration for Compact Nuclear Fusion Power Plants, 2023), and Samuel Cohen makes the case for that whole size class to the committee that sets the decade's priorities in FESAC Decadal Study: Clean, multi-purpose fusion power from small field-reversed configuration reactors (2024).

The experiment or build

The near-term rung is direct conversion, not ignition. Take an existing machine that makes charged fusion products at any rate at all, and instrument the collection: put a coil or a set of biased grids in the product path and measure the electrical energy recovered per fusion event. The settling measurement is joules of electricity out per joule of fusion product energy, on a real machine, with the collection efficiency stated separately from the fusion yield. That number is the whole economic argument for aneutronic fuel and it is almost never published.

Research context

Proton-boron-11 fusion has been measured in a magnetically confined plasma and in an accelerator, the compact machines exist as hardware, and no aneutronic machine anywhere has reached net gain.

Take it up

The measurement that settles it
The settling measurement is joules of electricity out per joule of fusion product energy, on a real machine, with the collection efficiency stated separately from the fusion yield.
What it costs to start
University to national lab.
The engineer it grows
The direct-conversion problem in particular is an electrical engineering problem hiding inside a plasma physics field.

What it rests on

Where it sits in the curriculum

Fusion Machines: Pinches, Focus Devices and Inertial Drivers · Exotic Vacuum Objects, Plasmoids, and the Orbs · The Reference Documents and the Institutional Record