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March 20, 2012

Nuclear fusion simulation shows high-gain energy output of 100 to 1000 times input current

High-gain nuclear fusion could be achieved in a preheated cylindrical container immersed in strong magnetic fields, according to a series of computer simulations performed at Sandia National Laboratories.

The simulations show the release of output energy that was, remarkably, many times greater than the energy fed into the container’s liner. The method appears to be 50 times more efficient than using X-rays — a previous favorite at Sandia — to drive implosions of targeted materials to create fusion conditions.

In the simulations, the output demonstrated was 100 times that of a 60 million amperes (MA) input current. The output rose steeply as the current increased: 1,000 times input was achieved from an incoming pulse of 70 MA.

Since Sandia’s Z machine can bring a maximum of only 26 MA to bear upon a target, the researchers would be happy with a proof-of-principle result called scientific break-even, in which the amount of energy leaving the target equals the amount of energy put into the deuterium-tritium fuel.


Prototype assembly of MagLIF system - the top and bottom coils enclose the lit target. (Photo by Derek Lamppa)
Z machine

Fusion Power’s Road Not Yet Taken (Inertial confinement fusion)

With ignition as a starting gun they could have a pilot plant—dubbed LIFE, for Laser Inertial Fusion Energy running in 12 years.

Instead of NIF’s single giant laser split into 192 beamlets, LIFE would have twice as many beamlets, each produced by a replaceable 8-kilojoule (kJ) laser unit. The laser units, each housed in a box big enough to accommodate a torpedo or two, would be built in a factory and delivered to the plant ready to use. If one or two of them failed, they could be replaced without stopping the power plant.

The key technology here is the pulsed current source. Sandia has a huge device called the Z machine, which stores up enormous amounts of electrical energy and then produces intense current pulses. Researchers use these pulses—up to 27 mega-amps (MA) for 100 nanoseconds—to produce x-rays and for other experiments. Although the Z machine can be used to test the feasibility of doing fusion with such pulsed power, Sandia’s Cuneo says a new machine able to generate 60 MA will be needed to really put the theory to the test.

As with other drivers, the key challenge is repetition rate. Researchers at Sandia are testing a new technology called linear transformer drivers (LTDs). They have a
couple of LTD modules rigged up, and each is producing 1-MA pulses at a rate of 1 every 10 seconds.



Another way to make ICF easier, says Los Alamos’s Wurden, is to poach some ideas from magnetic fusion. They are experimenting with a technique called magneto-inertial fusion (MIF, sometimes known as magnetized target fusion), which uses a magnetic field to help contain the plasma of deuterium and tritium in the target and stop heat from escaping. As a result, the driver does not need to be as strong or as fast. “You can use drivers that are 20 to 30 years old, on the $50-[million]-to-100-million scale,” Wurden says.

In experiments at Los Alamos, researchers make a target from a metal can (roughly the size of a tall beer can) filled with plasma and apply a magnetic field of about 2 to 3 tesla to hold the plasma in the middle of the can. They use an explosive to compress the can to around 1 centimeter across in less than 20 microseconds. This is a pretty sedate compression by ICF standards, but the magnetic field inside the can gets boosted 100 times, to as much as 300 tesla—a field strength “so huge that it is not known in this corner of the galaxy,” Wurden says. At the moment, the Los Alamos team is refining the compression technique, but its explosive driver is not practical for energy production.

The MIF technique heats the fusion fuel (deuterium-tritium) by compression as in normal inertial fusion, but uses a magnetic field to suppress heat loss during implosion. The magnetic field acts like a kind of shower curtain to prevent charged particles like electrons and alpha particles from leaving the party early and draining energy from the reaction.

The simulated process relies upon a single, relatively low-powered laser to preheat a deuterium-tritium gas mixture that sits within a small liner.

At the top and bottom of the liner are two slightly larger coils that, when electrically powered, create a joined vertical magnetic field that penetrates into the liner, reducing energy loss from charged particles attempting to escape through the liner’s walls.

An extremely strong magnetic field is created on the surface of the liner by a separate, very powerful electrical current, generated by a pulsed power accelerator such as Z. The force of this huge magnetic field pushes the liner inward to a fraction of its original diameter. It also compresses the magnetic field emanating from the coils. The combination is powerful enough to force atoms of gaseous fuel into intimate contact with each other, fusing them.

Heat released from that reaction raised the gaseous fuel’s temperature high enough to ignite a layer of frozen and therefore denser deuterium-tritium fuel coating the inside of the liner. The heat transfer is similar to the way kindling heats a log: when the log ignites, the real heat — here high-yield fusion from ignited frozen fuel — commences.

Tests of physical equipment necessary to validate the computer simulations are already under way at Z, and a laboratory result is expected by late 2013, said Sandia engineer Dean Rovang.

Portions of the design are slated to receive their first tests in March and continue into early winter. Sandia has performed preliminary tests of the coils.

Potential problems involve controlling instabilities in the liner and in the magnetic field that might prevent the fuel from constricting evenly, an essential condition for a useful implosion. Even isolating the factors contributing to this hundred-nanosecond-long compression event, in order to adjust them, will be challenging.

Portions of the design are slated to receive their first tests in March and continue into early winter. Sandia has performed preliminary tests of the coils.

Potential problems involve controlling instabilities in the liner and in the magnetic field that might prevent the fuel from constricting evenly, an essential condition for a useful implosion. Even isolating the factors contributing to this hundred-nanosecond-long compression event, in order to adjust them, will be challenging.


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