Historical research guide · 1075 · 2016
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
In one minute
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.
Connection
Chapter 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.
From the source
A 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
Because 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
Pusher 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
Upstream 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
The 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
Rayleigh-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
The way in
https://doi.org/10.1103/physrevlett.116.255003How 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