Laser Fusion and Exawatt Lasers

Oct 01, 2009

( -- In the recent past, producing lasers with terawatt (a trillion watts) beams was impressive. Now petawatt (a thousand trillion watts, or 10^15 watts) lasers are the forefront of laser research. Some labs are even undertaking work toward achieving exawatt (10^18 watts) levels.

Todd Ditmire at the University of Texas currently produces petawatt power through a process of chirping, in which a short light pulse (150 femtoseconds in duration) is stretched out in time. This longer pulse is amplified to higher energy and then re-compressed to its shorter duration, thus providing a modest amount of energy, 190 joules in a very tiny bundle.

Ditmire claims that his petawatt device has the highest power of any system now operating, even the one at the National Ignition Facility at the Lawrence Livermore National Lab, owing to the very short pulse-compression he and his colleagues use.

The main research use for the Texas Petawatt Laser, as it is called, has been to produce thermonuclear fusion; the laser light strikes a target where fusion of light nuclei occurs, releasing neutrons into the vicinity. These neutrons can themselves be used for doing research. The first results of this fusion experiment will be presented at this meeting. Other applications include the study of hot dense plasmas at pressures billions of time higher than atmospheric pressure and the creation of conditions for accelerating electrons to energies of billions of electron-volts.

Another figure of merit for a laser, in addition to power, is power density. The Texas device is capable of producing densities exceeding 10^21 watts per square centimeter. At this level many novel interactions might become possible.

To get to exawatt powers, Ditmire hopes to combine largely-existing laser technology and his already-tested 100-femtosecond pulses with new laser glass materials that would allow amplification up to energies of 100 kilo-joules. Ditmire’s current energy level, approximately 100 joules, is typical of laser labs at or near the petawatt level, such as those in Oxford, England, Osaka, Japan and Rochester, N.Y. With support from the government and the research community, building an exawatt laser might take 10 years to achieve, Ditmire estimates.

Scientists will present their paper "The Texas Petawatt Laser and Technology Development toward an Exawatt Laser" at the Optical Society’s Annual Meeting, Frontiers in Optics, on Tuesday, Oct. 13. The meeting takes place at the Fairmont San Jose Hotel and the Sainte Claire Hotel in San Jose, Calif.

Provided by Optical Society of America (news : web)

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User comments : 10

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1 / 5 (3) Oct 01, 2009
Picture a laser pulse so short- 1 photon thick, a dinner plate of coherent particles of exawatt density.
Oct 01, 2009
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Oct 01, 2009
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1 / 5 (2) Oct 02, 2009
Petawatt Lasers could send a lightsail to our nearest star, with 1kg payload, in a few years.
1 / 5 (1) Oct 02, 2009
Petawatt Lasers could send a lightsail to our nearest star, with 1kg payload, in a few years.

Any idea on what material would be light enough to make the sail -and- withstand that kind of power density?
3 / 5 (3) Oct 02, 2009
As for withstanding the power density, launch the ship at partial power. As it recedes, increase the power, staying below the melting point of the sail. If the idea is to provide power at distances up to a light year, you don't need the full power at close range.

A bigger problem is that the current petawatt lasers, as in the article, are actually very low power devices, compressing that power into short pulses. For a launching laser you need continuous power, which is still a long ways off.
3.8 / 5 (4) Oct 03, 2009
Based on what is known today, it is unlikely that National Ignition Facility (NIF) will produce practical amounts of fusion energy.
not rated yet Oct 04, 2009
power intensity, not density

waves, not motes

Laser-sail propulsion is daft. Anyone who knows Coulomb's law would know better to use a matter beam, not laser.
1 / 5 (1) Oct 04, 2009
Whats the tightest, fastest, matter beam you know?
Matter beam projectors and laser cannons can be placed at strategic points along the space probe trajectory to save power.
Oct 04, 2009
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Oct 05, 2009
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1 / 5 (1) Oct 05, 2009
There are two problems with matter beams, although they would work.

The first is that it takes more energy to accelerate mass to a given energy than a photon. The most efficient possible reaction drive is a solar sail, as reflecting the photons gives twice the momentum transfer of absorbing/emitting them. Second best is a photon drive, which is basically a VERY large laser. Might as well leave the laser, and its power supply, at home. On the other hand, if it's easier to accelerate enough mass to a given energy than to convert the power required to coherent photons, the particle beam would have the advantage. Efficiency is less important than being able to build the thing!

The second problem with particle beams is more basic. It's very hard to accelerate a neutral particle. On the other hand, charged particles repel each other, causing the beam to spread. They would also be deflected by natural magnetic and electrical fields, making aiming difficult.
1 / 5 (1) Oct 05, 2009
(ran out of characters)

One could accelerate both protons and electrons separately and combine them to produce a neutral hydrogen beam, but that would seem to be rather complicated.

It might still be easier and cheaper than building a launching laser, though. Remember, that laser is, by definition, a photon drive. As Robert Forward pointed out, it will have to be in orbit around a planet-mass object, or the beam will push IT out of the system!
Oct 05, 2009
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1 / 5 (1) Oct 05, 2009
There is matter - the lightsail. The photon transfers momentum to the sail, then, when it's reflected, the sail gains more momentum. That's why the lightsail is more efficient than any ship-based drive.

Yes, using the Sun itself for a lightsail power source in the outer system wouldn't be practical, although it would work with a large enough sail. But for the inner system, out to Mars, the idea is very practical, even with just sunlight. The ships would have very low acceleration, but the "fuel" is free, and they can accelerate as long as they like.

The Messenger mission to Mercury is using the idea now, even though they didn't design the probe for solar sailing. They've save a small but measurable amount of fuel by using the reflective solar panels as "sails", and avoided some midcourse correction burns.

As for reaction mass, where are you going to get the matter for your particle beam? And how much extra energy will it take to transport and process it?
Oct 06, 2009
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