Inertial confinement fusion, or ICF, is one of the two “mainstream” approaches to harnessing nuclear fusion in the laboratory. As its name would imply, it involves dumping energy into nuclear material, commonly consisting of heavy isotopes of hydrogen, so fast that its own inertia hold it in place long enough for significant thermonuclear fusion to occur. “Fast” means times on the order of billionths of a second. There are, in turn, two main approaches to supplying the necessary energy; direct drive and indirect drive. In direct drive the “target” of fuel material is hit directly by laser or some other type of energetic beams. In indirect drive, the target is mounted inside of a “can,” referred to as a “hohlraum.” The beams are aimed through holes in the hohlraum at the inner walls. There they are absorbed, producing x-rays, which supply the actual energy to the target.
To date, the only approach used at the biggest ICF experimental facility in the world, the National Ignition Facility, or NIF, at Lawrence Livermore National Laboratory (LLNL), has been indirect drive. So far, it has failed to achieve the goal implied by the facility’s name – ignition – defined as more fusion energy out than laser energy in. A lot of very complex physics goes on inside those cans, and the big computer codes used to predict the outcome of the experiments didn’t include enough of it to be right. They predicted ignition, but LLNL missed it by over a factor of 10. That doesn’t necessarily mean that the indirect drive approach will never work. However, the prospects of that happening are becoming increasingly dim.
Enter direct drive. It has always been the preferred approach at the Naval Research Laboratory and the Laboratory for Laser Energetics (LLE) at the University of Rochester, the latter home of the second biggest laser fusion facility in the world, OMEGA. They lost the debate to the guys at LLNL as the NIF was being built, but still managed to keep a crack open for themselves, in the form of polar direct drive. It would have been too difficult and expensive to configure the NIF beams so that they would be ideal for indirect drive, but could then be moved into a perfectly symmetric arrangement for direct drive. However, by carefully tailoring the length and power in each of the 192 laser beams, and delicately adjusting the thickness of the target at different locations, it is still theoretically possible to get a symmetric implosion. That is the idea behind polar direct drive.
With indirect drive on the ropes, there are signs that direct drive may finally have its turn. One such sign was the recent appearance in the prestigious journal, Physics of Plasmas, of a paper entitled Direct-drive inertial confinement fusion: A review. At the moment it is listed as the “most read” of all the articles to appear in this month’s issue, a feat that is probably beyond the ability of non-experts. The article is more than 100 pages long, and contains no less than 912 references to work by other scientists. However, look at the list of authors. They include familiar direct drive stalwarts like Bob McCrory, John Sethian, and Dave Meyerhofer. However, one can tell which way the wind is blowing by looking at some of the other names. They include some that haven’t been connected so closely with direct drive in the past. Notable among them is Bill Kruer, a star in the ICF business who specializes in theoretical plasma physics, but who works at LLNL, home turf for the indirect drive approach.
Will direct drive ignition experiments happen on the NIF? Not only science, but politics is involved, and not just on Capitol Hill. Money is a factor, as operating the NIF isn’t cheap. There has always been a give and take, or tug of war, if you will, between the weapons guys and the fusion energy guys. It must be kept in mind that the NIF was built primarily to serve the former, and they have not historically always been full of enthusiasm for ignition experiments. There is enough energy in the NIF beams to create conditions sufficiently close to those that occur in nuclear weapons without it. Finally, many in the indirect drive camp are far from being ready to throw in the towel.
In spite of that, some tantalizing signs of a change in direction are starting to turn up. Of course, the “usual suspects” at NRL and LLE continue to publish direct drive papers, but a paper was also just published in the journal High Energy Density Physics entitled, A direct-drive exploding-pusher implosion as the first step in development of a monoenergetic charged-particle backlighting platform at the National Ignition Facility. An exploding pusher target is basically a little glass shell filled with fusion fuel, usually in gaseous form. For various reasons, such targets are incapable of reaching ignition/breakeven. However, they were the type of target used in the first experiments to demonstrate significant fusion via laser implosion at the now defunct KMS Fusion, Inc., back in 1974. According to the paper, all of the NIF’s 192 beams were used to implode such a target, and they were, in fact, tuned for polar direct drive. However, they were “dumbed down” to deliver only a little over 43 kilojoules to the target, only a bit more than two percent of the design limit of 1.8 megajoules! Intriguingly enough, that happens to be just about the same energy that can be delivered by OMEGA. The target was filled with a mixture of deuterium (hydrogen with an extra neutron), and helium 3. Fusion of those two elements produces a highly energetic proton at 14.7 MeV. According to the paper copious amounts of these mono-energetic protons were detected. Ostensibly, the idea was to use the protons as a “backlighter.” In other words, they would be used merely as a diagnostic, shining through some other target to record its behavior at very high densities. That all sounds a bit odd to me. If all 192 beams are used for the backlighter, what’s left to hit the target that’s supposed to be backlighted? My guess is that the real goal here was to try out polar direct drive for later attempts at direct drive ignition.
All I can say is, stay tuned. The guys at General Atomics down in San Diego who make the targets for NIF may already be working on a serious direct drive ignition target for all I know. Regardless, I hope the guys at LLNL manage to pull a rabbit out of their hat and get ignition one way or another. Those “usual suspects” among the authors I mentioned have all been at it for decades now, and are starting to get decidedly long in the tooth. It would be nice if they could finally reach the goal they’ve been chasing for so long before they finally fade out of the picture. Meanwhile, I can but echo the words of Edgar Allan Poe:
Over the Mountains
Of the Moon,
Down the Valley of the Shadow,
Ride, boldly ride,
The shade replied —
If you seek for El Dorado.
8 thoughts on “Fusion Update: The Turn of Direct Drive”
In my novel, “On Deception Watch”, the direct drive method with deuterium and helium-3 are used. The US and Cnhina join forces to control the supply of He-3 found on the moon and together form the super corporation, World Federation, that ushers in a new international governing paradigm. It was published six years ago, ahead of its time, as futurist novels should be.
It’s a good read. David H. Spielberg
3d bioprinting = Immortality = go to stars
3D Bioprinting…will arrive in time for us the Fantastic Voyage towards the Immortal Future?…
Are there aliens in Moon occult face?…perhaps they could be go to/come from faraway in straight line, for no to be detected from Earth… (TV) Apollo XI astronaut said by radio: “they are here…on aside crater…watching us”… Why not go again to Moon from half century ago…
…interstellar travel constant acceleration (Sun-Deneb: 1000g)… Earth…the 2 ships that will go formation flying for mutual assisting if there are problems…indestructible structures made of Hexapentas material, awaiting in airport the arrival of passengers… Day 1: zero-speed… THE SHIPS TAKEOFF►… navigation computer places on screen the spacecraft in the center of sphere…spherical\tridimensional\spatial Heading: Deneb… Antimatter rocket engines…ON… Here we go…goooooo!…1g…10g…100g…constant acceleration cruise: 1000g (9.8 kms/sec²)… Inside the living areas (the same as going submerged in water: constant acceleration downwards…less…constant thrust, constant acceleration, from water upwards)…the gravitational transformers, perfectly synchronized with the acceleration, running: 1000g constant acceleration toward the floor ↓↓(motors)↓↓…less…999g constant acceleration toward the ceiling ↑↑(gravitational transformers)↑ = 1g constant acceleration toward the floor↓… 8.5 hours: light-speed = 1c…the fusion reactor as an artificial sun illuminating the immense Vital Support Gardens to lowering, from their comfortable apartments, cheerful passage to the pool…the electromagnetic shield anti-radiation…antigravity fields generator run forward, working: light objects away from the path of the ship…and trajectory ship away from the heavy objects…superluminal-speed > 1c… 42.5 hours: reaches hyperluminal-speed = 5c… Day 508: Half Journey…1000 light years…high hyperluminal-speed = 1435.39c… OFF engines…a few minutes of weightlessness during maneuver…the ship rotates 180º around its axis…motors ON again and… ◄STARTS TO BRAKE… Day 1017 (2.79 years): End Path party…2000 light years…zero-speed… The forever young passage of the 1st Immortal Generation (3D Bioprinting…Telomerase…modified Biological Timers…) disembarks at destination: an extra-stellar planet which came errant to orbit of Deneb giant.
…interstellar travel not acceleration constant (graphene: planet errant)… every bit a Terraformed planet errant into a bubble graphene how diamond transparent…with fusion reactors which will give the light and heat how an artificial sun, encircling an antigravitational field Tesla/Brown “electrogravitics” prevents objects collision against the “blue-arch of heaven”… Who needs already a star…
CO2 + UV laser → C + O2… 3d bioprinting = Immortality = go to stars ((typewrite: interstellar travel constant acceleration))
(2a)…interstellar travel not acceleration constant (laser)… a light´s beam that rotates is not a light´s beam that rotates… if it would be 1 only beam even the infinitesimal centrifugal force from gyration perhaps would break the beam launching tangentially its photons at straight line, as the stone of a sling loop that breaks. The arriving light (radiation) from a gyratory pulsar star is something seemed, the same than gyratory laser pointer from horizon to horizon 180º in 1 second, each beam that exits from a gyratory source is an independent light´s ray with limited length that only goes away in straight line: source on, beam starts; source turns, that beam finishes, is off, and starts-on another beam in the new direction… Photon, its mass “is believed” that is zero, but is the Photons Mass impacting against the future Space-Sail that produces the Transfer of Linear Momentum (mass*speed), the impulse… Photon, “relativistic” mass M = E/c²… constant h=6.626*10^-34 joules-sec… v, frequency e.g. red light=4*10^14 Hz…that photon energy E = h*v; E = (6.626*10*-34) * (4*10^14); E = 2.6504*10^-19 joules * (6.24*10^18 eV)=1.6538 eV…c=3*10^8 m/s, that photon MASS M = E/c²; M = (2.6504*10^-19) / (9*10^16); M = 2.944889*10^-36 kg… that Photon “relativistic” Mass ~29 ten-sextillionth of kg ______ 4 semicircular-Screens, if it would be 1 only beam, infinitesimal centrifugal Force ((N=kg*m/s², we do reference to g-Earth in any Universe place, for that also on the Moon kilograms force=Newtons/9.8)) in kgf (kp) from that Photon, according to huge centrifugal G (kgf=kg*number of g´s)… radius Earth→screens, 1: 95493 km (96,105,971g)=2.8*10^-28 kgf (~28 hundred-thousand-quadrillionth of kgf)… 2: Moon 384,403 km (386,870,541g)=1.1*10^-27 kgf (~11 ten-thousand-quadrillionth of kgf)… 3: Sun, 1.5*10^8 km (150,962,873,002g)=4.4*10^-25 kgf (~44 hundred-quadrillionth of kgf)… 4: Andromeda, 1.9*10^19 km (913,135,316,143,368g)=2.7*10^-21 kgf (~27 ten-thousand-trillionth of kgf)… When source turns, each individual laser mark on the semicircular-screen (L=3.14*r) has zero speed, do not moves, arrives and it vanishes, such as a light-bulbs row that they go being on and off one after another, from the first to the last hyperluminal speed, but is Not the speed of a mobile because there is Not any mobile.