Historical research guide · 1034 · 2004

Aneutronic Fusion in a Degenerate Plasma

S. Son · N. J. Fisch

In one minute

S. Son and N. J. Fisch of the Princeton Plasma Physics Laboratory ask what happens to clean, neutron-free fusion fuel when you squeeze it until its electrons stop behaving like a gas. Compress hard enough and the electrons fill every low-energy slot available to them — the plasma becomes Fermi-degenerate — and a passing ion can no longer scatter off them freely, because the states it would knock them into are already taken. Son and Fisch show that this blocking makes the plasma leak far less energy than the classical formulas predict: less drag on the fusion products, less bremsstrahlung X-ray loss. That changes the arithmetic of proton–boron-11 and deuterium–helium-3 burning. They find a window — density above a hundred thousand grams per cubic centimetre, ions near 200 keV, electrons near 27 keV — in which the fuel keeps itself burning. They are candid about what stands in the way: compressing fuel that far, and lighting a hot spot inside it.

Connection

Chapter 12's argument is that aneutronic fuel is the one worth having, because it hands back charged particles you can convert directly instead of neutrons you have to shield against — and the standard objection is that proton–boron-11 radiates away more than it makes. This paper answers that objection on its own ground: in a dense enough plasma the loss channels themselves change, and the sum comes out positive. It is also the cleanest published statement of a self-heating dense plasma ball, which is the physics chapter 9 keeps meeting in other forms.

From the source

In a fully degenerate plasma, when an ion moves more slowly than the electron Fermi velocity, the electronic stopping power becomes almost independent of the electron density and simply proportional to the ion velocity, because many electron transitions are forbidden — the collisions happen with the fastest electrons rather than the thermal ones.Abstract; Section 2, Equations 1 and 2; Appendix A, Equations 19 and 20
With the boron-to-proton ratio optimised at 0.3 and an ion temperature of 200 keV, proton–boron-11 fuel reaches a self-burning regime once the electron density exceeds 6.69 times ten to the twenty-eighth per cubic centimetre; balancing the ion-to-electron energy drain against bremsstrahlung then gives an electron temperature of about 27 keV, at a density slightly above 3.8 times ten to the fifth grams per cubic centimetre.Section 3.1, Equations 3, 5 and 6
The second condition for burning — that the fusion products be stopped by ions rather than electrons — is satisfied automatically in this regime: at the threshold density 85 percent of the alpha particle energy goes to the ions, and at twice that density 92 percent does.Section 3.1, Equation 4
Deuterium–helium-3 can also reach a self-sustaining burn at a density of three times ten to the fifth grams per cubic centimetre, because nuclear elastic collisions hand the 14.7 MeV proton's energy mainly to helium-3 rather than to electrons; about 70 percent of the fusion energy then goes to the ions and the electron temperature settles near 32 keV against an ion temperature of 70 keV.Section 3.2, Equations 7 and 8
Rider's standard argument against aneutronic fuel — that the recirculating power needed to hold ions hot and electrons cool exceeds the fusion power — rests on two-body effects scaling with the square of the density; in an ultra-dense plasma the stopping power is not proportional to the electron density at all, so that assumption breaks and the negative conclusion does not follow.Section 5, Discussion, first paragraph
At a pellet radius of ten microns the calculated energy gain is about 18 for proton–boron-11 and about 15 for deuterium–helium-3, short of the 200 usually required of a reactor; creating a hot spot for fast ignition — possibly a small deuterium–tritium pellet inside the aneutronic fuel — is the named route to raising it.Section 4, Reactor Prospects; Section 6, Summary

The way in

https://doi.org/10.2172/835895

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

Princeton Plasma Physics Laboratory report PPPL-4003 (UC-70), September 2004, prepared for the U.S. Department of Energy under Contract DE-AC02-76CH03073 and distributed by the Office of Scientific and Technical Information. A Department of Energy laboratory report is a work of the US Government and is in the public domain, so the full text is reproduced here. The library’s first fetch returned no text; the seventeen-page PDF was downloaded from osti.gov/servlets/purl/835895 and extracted in reading order. The same work was published as Physics Letters A volume 329, issue 1-2, pages 76 to 82, August 2004 — that journal version is under Elsevier’s terms and is not the copy reproduced here. The paper is equation-dense; the formulas are given as named results and stated in words rather than re-typeset, and the mathematical appendix is summarised. Publisher records carry the authors only as S. Son and N. J. Fisch, so the initials are left as the record has them.

How to cite it

S. Son, N. J. Fisch (2004) Aneutronic Fusion in a Degenerate Plasma doi:10.2172/835895

Where it sits in the curriculum

Fusion machines: pinches, focus devices and inertial drivers