A sequence of false color images generated from a numerical simulation show a MagLIF liner as it is heated by a laser in preparation for an implosion.
On the last Friday before Christmas in 2010, fusion physicist Steve Slutz was in his office at Sandia National Laboratories’ Albuquerque, N.M, location, doing what he’s been doing for the past 34 years: crunching the numbers to get fusion to work on Earth.
This time around, he was calculating the potential fusion energy output from a large-scale version of a new fusion process he and his colleagues had proposed. He’d already shown that magnetized liner inertial fusion, or MagLIF, could theoretically produce enough fusion energy to make it useful for weapons-effects simulations.
Still, if he could show that MagLIF had potential as a large-scale fusion energy source, that could be the added push it needed to get the green light for experiments. That was the problem. Most earlier research by others had dismissed the possibility.
Slutz wanted to see for himself. For MagLIF to go big, he knew he needed a gain in energy of at least 50 times that needed to kick-start the reaction. Hunched over his keyboard, making the calculations, he realized it was possible – with the right technology. He went to a colleague in a neighboring office – one of the few people left in the building – to share this physics good news.
“Go home quick before you figure out what’s wrong with it,” the colleague joked.
Instead, Slutz returned to his calculations and found that, given the right circumstances, MagLIF could produce an energy gain of 1,000.
At which point there was no one at Sandia left with whom to share the news.
Today dozens of Sandia researchers are at work testing MagLIF, seeing if what Slutz’s computer simulations show is indeed a path to fusion that could one day actually power your Christmas lights.
“The message is that MagLIF is a really interesting path because in principle you could have really high gain,” Slutz says. “If this thing works at all.”
Fusion grand slam
Every day you’re bathed in the light and heat of fusion energy. The sun’s massive gravitational self-hug and the resulting sizzling temperatures fuse hydrogen nuclei, releasing vast amounts of energy in the process.
Yet achieving controlled fusion on Earth has proven to be a 60-years-and-counting grand challenge.
The trouble is that controlled fusion is the equivalent of the ultimate baseball bases-loaded grand slam. To make it work, researchers have to take a little bit of star fuel, then heat it, hold it and get it to heat itself for long enough to extract the fusion energy. All this with a fuel that’s heated to a plasma hotter than the interior of the sun – more than hot enough to melt any terrestrial material.There’ve been two primary technical routes to achieving fusion.
The first has been to heat the fuel and contain it for extended periods not with a vessel but with magnetic fields (which don’t melt). This technique, called magnetic confinement, is the one used in Tokamak [1]-type experimental fusion reactors.
The second approach, inertial confinement fusion, opts for speed over containment. It heats a small amount of dense fuel super-quickly, usually with a laser, with the goal of extracting energy before the heated fuel expands and cools. This approach is being tested at Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility, using the world’s most powerful laser source.
To date, neither approach has produced the fusion Holy Grail of break-even – producing an amount of energy equal to that put into the system to spark fusion.
A third way to fusion
MagLIF is a newcomer, third-way approach that combines the magnetism of magnetic confinement fusion and the rapid heating of inertial confinement fusion.
“MagLIF is a new regime and it’s driven entirely by the fact that we’ve got a certain kind of machine at Sandia,” says Slutz, whose extensive MagLIF computational simulations are the prompt and guide for current experiments.
That machine is Z, the world’s most powerful pulsed-power accelerator. For several nanoseconds Z can zap a target with more energy than that being produced elsewhere on the entire Earth.
But with MagLIF, the Z machine is used not for its radiation, as is usually the case, but for the massive magnetic currents it generates.
“MagLIF involves two magnetic fields,” explains Slutz, to both heat the fuel and contain that heat.
The first case, as demonstrated in simulations on Sandia’s Red Sky and Unity supercomputers, applies a magnetic current along the length of a pencil eraser-sized cylindrical fuel capsule, or liner. The beryllium-sided fuel liner contains a mix of gaseous deuterium [2] fuel at about air density.
“The fuel is confined by the walls of a tube,” Slutz explains. “What the magnetic field does is lower the loss of heat out of that plasma. It doesn’t actually physically hold the fuel there.”
Then, in lock-step timing with a laser preheating the fuel, the Z machine delivers the fusion coup de grace, a Z pinch: a hundred-nanoseconds-quick, 25 million-ampere magnetic pulse that acts like a Vise-Grip to implode the capsule. The implosion heats the fuel to three times the temperature of the solar core.
In detailed two-dimensional simulations using Lasnex and Hydra, two codes developed and maintained by LLNL’s George Zimmerman and Marty Marinak, MagLIF appears to be a highly energy-efficient fusion possibility. After Slutz’s pre-Christmas revelation, Sandia officials have decided the approach is worth putting to the experimental test.
Still, after three decades of working to get fusion fired up on Earth, Slutz mixes large doses of patience and caution with his optimism.
“We’re not going to have break-even (energy output) in 2013 – not even in our wildest dreams do we think that’s going to happen,” Slutz says of the ongoing MagLIF experiments. “What we’ve talked about is the much more modest idea that we could perhaps get the same fusion yield out of the fuel as the energy that we put into the fuel.”