Low Fuel Convergence Path to Direct-Drive Fusion Ignition
Kim Molvig · Mark J. Schmitt · B. J. Albright · E. S. Dodd · N. M. Hoffman · G. H. McCall · S. D. Ramsey
Abstract and summary · read the original at the source
In one page
Laser fusion has spent forty years trying to squeeze a small ball of fuel very hard and very fast, and the National Ignition Facility’s campaign underperformed. Kim Molvig and his Los Alamos colleagues propose the opposite bargain. Their Revolver capsule is three nested metal shells with gas cushions between them and a few tens of micrograms of liquid deuterium-tritium at the centre. The laser drives the outer shell gently, each shell hits the next like a billiard ball, and the fuel is lit while the innermost shell is still moving at nearly full speed — upstream of the moment everything stops. Because the heavy shell traps the radiation, the usual rule inverts: slower implosions want more squeeze, not less, and a faster implosion is not automatically better. They give the complete recipe as formulas — shell masses, radii, laser pulse — and the number they trade for is not gain but margin. High gain is abandoned on purpose. What they want is an ignition you can actually reach and then measure.
Why it matters hereChapter 12 keeps the ledger of fusion routes, and this is a complete engineering proposal on the shelf — shell by shell, joule by joule, with a first shot specified on an existing machine. Chapter 9 gets the physics of a fuel plasma held and lit inside a heavy metal shell rather than by a magnetic field, which is the other way humans have found to make matter burn.
What it claims
01A Revolver capsule is three concentric spherical shells — a beryllium ablator, a drive shell and a heavy metal pusher — with buffer gas between them and tens of micrograms of cryogenic liquid deuterium-tritium at the centre; the implosion is mechanical, dominated by the metal shells, and each shell collision converges by less than three, with fuel convergence around nine.Abstract; Fig. 1 and the main-features list, p. 255003-2
Designed, not yet built02Because the dense metal pusher traps the radiation, implosion velocity and convergence turn out to be inversely related — the opposite of the traditional inertial-fusion principle that high velocity is a virtue. For a fuel convergence of ten and an ignition temperature of 2.5 keV the implosion velocity is 19 cm per microsecond, and the simulations show that a faster implosion is not always better.Eqs. 2 and 3 with the paragraph following, p. 255003-2
Published and peer-reviewed03Pusher mass buys ignition margin. The design parameter is the ratio of mean shell velocity at ignition to peak implosion velocity; setting it at 0.9 with a convergence of ten gives a pusher-to-fuel mass ratio near 80 and a pusher kinetic energy about five times the fuel internal energy. That unspent energy is the price paid for igniting upstream with almost no deceleration and burning a large fraction of the fuel.Eq. 4 and the paragraph following it, p. 255003-2 to 255003-3
Designed, not yet built04Upstream ignition is given an unambiguous definition — alpha heating reaching one quarter of the hydrodynamic heating rate while both are still positive — and solving that condition with Bosch-Hale reactivity for the 25 microgram baseline returns an ignition temperature of 2.46 keV, which justifies the 2.5 keV long assumed in this line of work. At ignition the pusher has been compressed to around 2000 grams per cubic centimetre and has spent about a fifth of its kinetic energy.p. 255003-3, the ignition-condition and ignition-temperature paragraphs
Published and peer-reviewed05The drive is a directly driven beryllium ablator of 3 mm radius and 50 micrometres thickness taking 24 TW for 2 ns and then 320 TW for 4.5 ns out of the facility’s 1.5 MJ, accelerating 70 percent of the 10.3 mg ablator to 139 kJ — a hydrodynamic efficiency of 9.3 percent at a surface intensity of 2.8 times ten to the fourteenth watts per square centimetre, which is below the thresholds for laser plasma instability, cross-beam energy transfer and two-plasmon decay, and removes the need for laser zooming.p. 255003-4, drive paragraph; Abstract
Designed, not yet built06Rayleigh-Taylor penetration fractions computed in the fully developed self-similar limit come out at 0.5 for the ablator, 0.57 for the drive shell and 0.4 for the pusher, so every shell stays intact through its acceleration — whereas existing single-shell ignition capsules typically score above two on the same measure and depend on ablative stabilisation instead. The open item is drive uniformity: net convergence is 86, and multidimensional simulations were under way to find what symmetry the partitioned implosion actually demands.Eq. 8 and the paragraphs on hydrodynamic instability and low-order asymmetry, p. 255003-4 to 255003-5
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Read it · abstract
Abstract
A new class of inertial fusion capsules is presented that combines multishell targets with laser direct drive at low intensity (2.8×10^(14) W/cm^(2)) to achieve robust ignition. The targets consist of three concentric, heavy, metal shells, enclosing a volume of tens of μg of liquid deuterium-tritium fuel. Ignition is designed to occur well "upstream" from stagnation, with minimal pusher deceleration to mitigate interface Rayleigh-Taylor growth. Laser intensities below thresholds for laser plasma instability and cross beam energy transfer facilitate high hydrodynamic efficiency (∼10%).
Kim Molvig, Mark J. Schmitt, B. J. Albright, E. S. Dodd, N. M. Hoffman, G. H. McCall and S. D. Ramsey, Low Fuel Convergence Path to Direct-Drive Fusion Ignition, Physical Review Letters 116, 255003 (2016). The work was carried out at Los Alamos National Laboratory.
(Abstract only — see the rights note above. On this site, Badziak’s survey of laser nuclear fusion, its status and its prospects is at /library/stm-f585d315d5, Mourou and colleagues on relativistic laser-matter interaction and fast ignition is at /library/stm-3a51ad6c6a, and Sefkow and colleagues’ design of magnetized liner inertial fusion experiments on the Z facility — the magnetic cousin of this metal-shell approach — is at /library/stm-57d0076dfb.)
The way in
https://doi.org/10.1103/physrevlett.116.255003Published as Physical Review Letters 116, 255003 (2016), copyright American Physical Society. The article carries no United States Government or open-licence statement — the only such line is the acknowledgement that the work was performed under the auspices of the U.S. Department of Energy by Los Alamos National Laboratory under contract DE-AC52-06NA25396 — so the sheet is abstract-only and no text beyond the published abstract is reproduced. The five-page article was read in full on 2026-09-08 from the APS full-text endpoint for this DOI (harvest.aps.org, the fulltext path, which returns the article PDF where the bare article path answers 401), and every claim below is located to a page, equation or figure of it. The abstract as deposited flattens the exponents in the laser intensity and the microgram symbol; it is left exactly as deposited, and the claims restate the same numbers in words. Kim Molvig is given with Los Alamos National Laboratory and Massachusetts Institute of Technology affiliations in the byline; the remaining six authors are Los Alamos. Given names beyond the first two authors are printed as initials by the journal and are left that way.
How to cite it
Kim Molvig, Mark J. Schmitt, B. J. Albright, E. S. Dodd, N. M. Hoffman, G. H. McCall, S. D. Ramsey (2016) Low Fuel Convergence Path to Direct-Drive Fusion Ignition. doi:10.1103/physrevlett.116.255003
Where it sits in the curriculum
Lattice confinement fusionPlasmoids, charge clusters and the orbs