Laser sets records for neutron yield, laser energy

Nov 08, 2010
Installed and aligned symcap target inside its protective cover.

(PhysOrg.com) -- The National Nuclear Security Administration's National Ignition Facility (NIF) has set world records for neutron yield and laser energy delivered from laser-driven capsules to an inertial confinement fusion (ICF) target. NIF researchers will report on these and other recent experimental results this week at the annual meeting of the American Physical Society Division of Plasma Physics in Chicago.

The neutron yield record was set on Sunday, Oct. 31, when the NIF team fired 121 kilojoules of ultraviolet laser light into a glass target filled with deuterium and (DT) gas. The shot produced approximately 3 x 1014 (300 trillion) neutrons, the highest neutron yield to date by an inertial confinement fusion facility. Neutrons are produced when the nuclei of deuterium and tritium (isotopes of hydrogen) fuse, creating a helium nucleus and releasing a high-energy neutron.

On Tuesday, Nov. 2, the team fired 1.3 megajoules of ultraviolet light into a cryogenically cooled cylinder called a hohlraum containing a surrogate fusion target known as a symmetry capsule, or symcap. This was the highest-energy laser shot and was the first test of hohlraum temperature and capsule symmetry under conditions designed to produce and energy gain. Preliminary analysis indicated that the hohlraum absorbed nearly 90 percent of the laser power and reached a peak radiation temperature of 300 electron volts (about six million degrees Fahrenheit) -- making this the highest X-ray drive energy ever achieved in an indirect drive ignition target.

The experiments followed closely on the heels of NIF's first integrated ignition experiment on Sept. 29, which demonstrated the integration of the complex systems required for an ignition campaign including a target physics design, the laser, target fabrication, cryogenic fuel layering and target positioning, target diagnostics, control and data systems, and tritium handling and personnel and environmental protection systems. In that shot, one megajoule of energy was fired into a cryogenically layered capsule filled with a mixture of tritium, hydrogen and (THD), tailored to enable the most comprehensive physics results.
"The results of all of these experiments are extremely encouraging," said NIF Director Ed Moses, "and they give us great confidence that we will be able to achieve ignition conditions in deuterium-tritium fusion targets."

NIF, the world's largest and highest-energy laser system, is located at Lawrence Livermore National Laboratory (LLNL) in California. NIF researchers are currently conducting a series of "tuning" shots to determine the optimal target design and laser parameters for high-energy ignition experiments with fusion fuel in the coming months.

When NIF's 192 powerful lasers fire, more than one million joules of ultraviolet energy are focused into the ends of the pencil-eraser-sized hohlraum, a technique known as "indirect drive." The laser irradiation generates a uniform bath of X-rays inside the hohlraum that causes the hydrogen fuel in the target capsule to implode symmetrically, resulting in a controlled thermonuclear fusion reaction. The reaction happens so quickly, in just a few billionths of a second, that the fuel's inertia prevents it from blowing apart before fusion "burn" spreads through the capsule -- hence the term inertial confinement fusion. In ignition experiments, more energy will be released than the amount of laser energy required to initiate the reaction, a condition known as energy gain. NIF researchers expect to achieve a self-sustaining fusion burn reaction with energy gain within the next two years.

The experimental program to achieve fusion and energy gain, known as the National Ignition Campaign, is a partnership among LLNL, Los Alamos National Laboratory, the Laboratory for Energetics at the University of Rochester, General Atomics of San Diego, Calif., Sandia National Laboratories, Massachusetts Institute of Technology, and other national and international partners.
In the Oct. 31 "neutron calibration" shot, NIF's lasers were fired directly onto a DT-filled glass target, as opposed to the indirect-drive geometry used in NIF's TD and THD experiments. The purpose of the shot was to calibrate and test the performance of NIF's extensive neutron diagnostic equipment.

The capsule used in the Nov. 2 symcap experiment has the same two-millimeter outer diameter doped shell as an ignition capsule, but replaces the DT fuel layer with an equivalent mass of material from the outer shell to mimic the capsule's hydrodynamic behavior. Achieving a highly symmetrical compression of the fuel capsule is a key requirement for NIF to achieve its goal of fusion ignition.

NIF, a project of the U.S. Department of Energy's National Nuclear Security Administration (NNSA) was built as a part of NNSA's program to ensure the safety, security and effectiveness of the nation's nuclear weapons stockpile without underground testing. With NIF, scientists will be able to evaluate key scientific assumptions in current computer models, obtain previously unavailable data on how materials behave at temperatures and pressures like those in the center of a star, and help validate NNSA's supercomputer simulations by comparing code predictions against observations from laboratory experiments.

Because of its groundbreaking advances in technology, NIF also has the potential to produce breakthroughs in fields beyond stockpile stewardship. NIF experiments have been conducted in support of Department of Defense X-ray testing activities, and NIF experiments in nuclear forensics and other national security areas are planned for the next several years. NIF also will provide unprecedented capabilities in fundamental science, allowing scientists to understand, for example, the makeup of stars in the universe and planets both within and outside our solar system. NIF will also help advance fusion energy technology, which could be an element of making the United States energy independent.

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realist
3 / 5 (1) Nov 09, 2010
"300 trillion neutrons": At 18Mev per fusion reaction, that works out to about .8 kiloJoule total. And it only took "121 kilojoules of ultraviolet laser light" to generate it. So it seems after ten years and billions of dollars they are creeping up to 1% efficiency. But how much energy went into the laser:50 maybe 100 times the 120 kJ figure. Also, how much do these gold-plated hohlraums cost? I assume they are consumed with each shot.
NotAsleep
not rated yet Nov 09, 2010
Realist, do you suggest we stop all research that hasn't turned a profit in under ten years?
realist
4 / 5 (1) Nov 09, 2010
General answer: no, for NIF yes. NIF tries to induce fusion in a 2mg DT pellet,like a tiny H-bomb but the fusion yield of an H-bomb is limited to about a hundred times the fission primary(that was true of the first bomb; remains true after 50+years R&D and there is a simple reason for that). NIF uses flash-lamp induced UV laser. What would be the efficiency of that?: 2% maybe?.--Article doesn't say. That brings up another issue with NIF: the PR they put out is often disingenuous; it borders on dishonesty. They tell you the laser output energy but not the input energy. They talk of "300 trillion neutrons", not the puny energy output. They use the word Megajoule a lot-(A Mega Joule is about a nickel's worth of electricity) but they won't tell you what the hohlarum costs. They laser, however is impressive.
migmigmig
not rated yet Nov 09, 2010
What parts of "not intended to break unity" and "built for helping prove nuclear weapons simulation accuracy" do you luddites not understand?
realist
not rated yet Nov 10, 2010
NIF's goal is to break unity:"We are well on our way to achieving what we set out to do---controlled nuclear fusion and energy gain for the first time ever in a laboratory." (Edward Moses quoted in Wired Magazine 05.04.09
http://www.wired....ry_nif).
As for "...nuclear weapons simulation accuracy",
What's the point of that? Don't we already know how to build nuclar weapons in any size, configuration that any semi-sane person would want?

Eikka
1 / 5 (1) Nov 13, 2010
Let's say they break unity 10 times over with a starting energy of 1 megajoules per shot, so they get 10 MJ energy in every shot, of which about 2/3:rds is lost when converted to electricity. One of those golden hohlraums would be worth 1 kWh of electricity in the grid.

This is really just designing a really really big cannon to shoot a fly. To run a plant that outputs 1 GW, you'd have to destroy a million golden pellets every hour. There has to be a more wholesale solution to producing energy, and I'm not at all convinced that the system would work simply by building even bigger lasers and bigger gold pellets, because you're talking about blowing up small fusion bombs in a sequence. Even 10 MJ is the same as blowing up 2 kg of TNT.

If you don't release that energy slowly over time, the whole thing will just tear itself apart.
Eikka
1 / 5 (1) Nov 13, 2010
If you don't release that energy gradually over time, over a great mass of matter to dampen it, the whole thing will just tear itself apart.
Shootist
1 / 5 (1) Nov 14, 2010
It's been said before, Bussard's Polywell, for the win.