Sandia LabNews

A step toward more efficient fusion


New Sandia fusion proposal marries Z’s electric pulse with laser preheating and magnetically contained plasma; increased efficiency sought over other concepts

Roger Vesey (1684, right) explains to Ogden Jones of Lawrence Livermore National Laboratory the progress made on integrated target designs for pulsed power-driven liners containing magnetized and preheated fuel.

An efficient way to harvest energy from a staccato, nanosecond burst of controlled fusion reactions was proposed in two Sandia technical papers and a number of related technical posters presented at the sixth international conference on Inertial Fusion Science and Applications (IFSA 2009), held two weeks ago in San Francisco.

The new ecumenical Sandia approach combines the Laboratories’ staple of extremely powerful, fast pulses of electricity with laser preheating and fuel magnetization, ordinarily thought to be the provenance of plasma-confining tokamaks and most recently ITER, the huge international thermonuclear experimental reactor under construction in southern France.

Research led by Steve Slutz (1684) suggests firing a burst of laser energy to preheat a deuterium-tritium plasma initially contained by a tiny metal tube (technically called a liner) that has been magnetized by external field coils. Almost immediately, a 26-megaamp electrical pulse from Sandia’s Z machine would further energize the plasma and amplify the magnetic field within the fuel. The resultant magnetic fields would confine and compress the heated plasma so that its isotopes fuse. They also are expected to keep alpha particles, composed of two protons and two neutrons, from leaving the mix so that their energy heats the plasma for further fusion. (This form of magnetic confinement would only be for nanoseconds rather than minutes, as ITER hopes to do.)

Idea minimizes power losses

“The basic idea of magnetizing fuel in an inertial fusion liner was first proposed decades ago,” says Steve, “but the combination of a laser to preheat the fuel and the power of Z were unavailable until recently.”

Sandia supercomputer simulations have indicated that the method could achieve efficiencies greater than 3 percent from wall-plug electricity to input into the plasma.

The method minimizes power losses inevitable when converting energy from one form to another with the aim of eventually producing fusion. Today’s approaches to inertial fusion involve many such conversions, including NIF’s use of electricity to power lasers, and then convert infrared light to ultraviolet; Z’s use of electricity to ionize wire arrays to form a plasma that then collapses to produce X-rays; or in the kinetic energy lost in both NIF and Z when X-rays ablate matter from a fusion capsule’s surface to drive, rocket-like, the remaining surface inward to fuse atoms.

It’s a question of economy, says Steve.

Consider that if the 0.15 megajoules of laser energy expected to enter a target capsule releases a fusion output of 10 to 20 megajoules, there would be dancing in the streets by physicists (figuratively speaking) at the achievement of this high yield. The capsule would have released roughly 100 times more energy than was put into it. But to the lay person, since the beams originate from a 400-megajoule capacitor bank, the process would seem an overall loss: 400 megajoules in, 20 out. The problem occurs because only 0.04 percent of the initial energy enters the target (0.15 divided by 400).

The idea of Steve, Mark Herrmann (1680), and their team is to minimize the number and types of power conversions to achieve higher efficiencies. They would use lessons learned from laser and pulsed power efforts. The result ideally would be efficient enough to produce a net overall power gain.

That said, the IFSA conference was devoted overwhelmingly to examining methods that might produce fusion itself rather than in improving the efficiency of the overall process.

Particularly notable was the impressive effort toward fusion made by the researchers at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. (Other national labs, including Sandia, played a role in building sensors and other parts of the machine.) NIF management has proposed a high-yield fusion shot by the end of 2010.

“After 49 years, all the elements of ignition are in place,” NIF director Ed Moses told the assembled audience of 400 scientists.

Researchers from around the world presented ideas on modifying inertial fusion targets’ density, size, shape, temperature, and the velocity and frequency of light or ions striking them.

“It’s right that there’s so much money and attention given to lasers and NIF,” says Steve. “NIF is clearly the next step. There’s no reason for DOE or anyone else to put in big bucks to create a more efficient process for inertial confinement until we prove that it works at all.”

That, he says, would be putting the cart before the horse. "First NIF must show that high-yield inertial confinement fusion is possible. It’s very important that NIF succeed because it will show that it’s possible to compress a pellet using inertial confinement methods."

When it does, he says, scientists, engineers, and technicians at Z may have the next step half-completed, using a magnetically insulated, preheated plasma that would be much more efficient.

Inertial confinement typically applies a burst of energy — whether generated by electricity, light, or heavy or light ions — to compress a pellet filled with deuterium and tritium ions until they fuse. If the proper conditions are met, the pellet will ignite — achieve ignition, in the parlance of the trade — to release more energy than it absorbs, roughly following Albert Einstein’s famous equation, E=mc2.

Magnetic confinement, used by tokamaks and in particular by the upcoming ITER machine, creates a standing plasma confined by magnetic fields over relatively long periods of time – perhaps as much as 15 minutes.

The Sandia group proposes to use a sleeve (visually, a tube) less than a centimeter long, closed at both ends to maintain a deuterium-tritium (DT) mix. A single laser’s pulse, two Helmholtz coils, and Z’s enormous amperage should do the trick of producing more efficient fusion. Implosions would occur at 100-300 nanoseconds rather than the 10-30 microseconds proposed in earlier scenarios involving inertial fusion using magnetized fuel. Higher fuel densities will aid in trapping alpha particles. A sufficiently thick sleeve wall should maintain its integrity until the implosion is complete.

Peering into the maelstrom

In another paper presented with a separate poster at the IFSA conference, Kyle Peterson (1684) and Dan Sinars (1683) used a crystal as though it was sunglasses to transmit only a few selected frequencies to see into the maelstrom of Z when it fires. The enhanced view showed that sausage-like Rayleigh-Taylor instabilities predicted by LASNEX code were accurate; thus the code’s veracity under these circumstances was established. A logical extension of the work is that the validated code may be used with provisional confidence to aid in design of the cylinder’s dimensions, materials, electrical current requirements, firing time, and pulse- shaping in creating a system that mitigates instabilities past the point of energy generation.

A third poster by Ryan McBride, Mike Cuneo (both 1683), Christopher Jennings (1641), and Eduardo Waisman (1683) showed that a hollow metal torus that acts as a 2:1 current transformer when inserted in the transmission line of Z could amplify the current delivered to the liner. This should increase Z’s electrical current flow to target from 26 megaamps to 40 megaamps, providing additional experimental capability.

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