June 29, 2011

Laser enabled megawatt class fusion propulsion

IEEE Spectrum - John J. Chapman, a NASA engineer, is proposing nuclear fusion propulsion for space satellites and space probes. He made a presentation at the IEEE Symposium on Fusion Engineering in Chicago.

In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10^18 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.

Advanced Fusion Reactors for Space Propulsion and Power Systems
J. J. Chapman, Engineering Division, NASA, Hampton, VA, United States

The first layer is a 5- to 10-µm-thick sheet of conductive metal foil. It responds to the teravolt-per-meter electric field created by the laser pulse by "acting as a de facto proton accelerator," says Chapman. The electric field releases a shower of highly energetic electrons from the foil, leaving behind a tremendous net positive charge. The result is a massive self-repulsive force between the protons that causes the metal material to explode. The explosion accelerates protons in the direction of the target’s second layer, a film of boron-11

Each pulse of the laser should generate roughly 100 000 particles, making the method tremendously efficient, says Chapman. And according to his calculations, improvements in short-pulse laser systems could make this form of thruster more than 40 times as efficient as even the best of today’s ionic propulsion systems that push spacecraft around. Even at 50 percent efficiency, burning off 40 milligrams of the boron fuel would deliver a gigajoule of energy. The amount of power depends on the laser pulse rate. The motor could generate 1 megawatt per second if the pulses are frequent enough to start reactions that consume that amount of boron in 1000 seconds. (According to Chapman, using this aneutronic fusion technique with helium-3 isotopes would yield roughly 60 percent more energy per unit mass. But boron is a more attractive fuel source because it is abundant on Earth and helium-3 is scarce.)

Another big advantage of fusion space propulsion, Chapman claims, is that some of the energy can be converted into electricity to power a spacecraft’s onboard control systems. "A traveling wave tube—basically an inverse klystron—captures most of the particles’ flux kinetic energy and efficiently converts it into electrical energy," says Chapman. The process, he says, is 60 to 70 percent efficient.

This collection of ideas is still a long way from being a practical device. For example, losses from the alpha particles striking the walls of the exhaust nozzle or each other lower the net power output. Figuring out how to control the particles’ path is an important consideration.

In recent years the methodology proposed for conversion of light elements into energy via fusion has made steady progress. Scientific studies and engineering efforts in advanced fusion systems designs have introduced some new concepts with unique aspects including consideration of Aneutronic fuels. The plant parameters for harnessing aneutronic fusion appear more exigent than those required for the conventional fusion fuel cycle. However aneutronic fusion propulsion plants for Space deployment will ultimately offer the possibility of enhanced performance from nuclear gain as compared to existing ionic engines as well as providing a clean solution to Planetary Protection considerations and requirements. Proton triggered Boron 11 fuel (p-B11) will produce abundant ion kinetic energy for In-Space vectored thrust. Thus energetic alpha particles “exhaust “ momentum can be used directly to produce high ISP thrust and also offer possibility of power conversion into electricity. p- B11 is an advanced fusion plant fuel with well understood reaction kinematics but will require some new conceptual thinking as to the most effective implementation.

Recent Progress on Laser Plasma Accelerators and Applications for Compact High-Quality Particle Beam and Radiation Sources (2010, 5 pages)

Berkeley Lab Targets 1 Meter Long 10 GeV laser based Table top Particle Accelerator for 2013 However, this is only going to shoot once per second and not 75 million times per second.

Completing BELLA will require a 1-Hz, 1-PW laser — the highest average power (40 W) petawatt-class laser in the world.

The European x-ray free electron laser (European XFEL) is targeting 30 thousand pulses per second The 3.4 km long tunnel for the European XFEL housing the superconducting linear accelerator and photon beamlines will run 6 to 38 m underground.

Richard Dell Jr and George Miley are working on nuclear fusion for space propulsion

Richard Dell Jr was interviewed by Sander Olson of Nextbigfuture. Richard Dell Jr believes that he and his team have developed a method for generating fusion power which is appropriate for providing the propulsion for exploring and colonizing space. He is confident that this approach could lead to spacecraft capable of flying to the moon and landing on it, and returning to earth using a single craft without jettisoning any stages or equipment. This technology could also be used to send humans to mars in only 2 months. His company, which is still largely in stealth mode, plans on generating short term revenue by selling more efficient satellite maneuvering thrusters to the satellite industry. Mr. Dell is confident that breakeven fusion power generation will be demonstrated within the next 3 years.

Richard Dell Jr is working with George Miley who has a long history of technical achievements with nuclear fusion for space propulsion and energy

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