Inspired by the space physics behind solar flares¹ and the aurora²（极光）, a team of researchers from the University of Michigan and Princeton has uncovered a new kind of magnetic behavior that could help make nuclear fusion³ reactions easier to start. Fusion is widely considered the ultimate goal of nuclear energy. While fission⁴ leaves behind radioactive waste that must be stored safely, fusion generates helium, a harmless element that is becoming scarce. Just 250 kilograms of fusion fuel can match the energy production of 2.7 million tons of coal.

 

Unfortunately, it is very difficult to get a fusion reaction going.

 

"We have to compress the fuel to a temperature and density⁵ similar to the core of a star," said Alexander Thomas, assistant professor of nuclear engineering and radiological sciences.

 

Once those conditions are reached, the hydrogen fuel begins to fuse into helium. This is how young stars burn, compressed by their own gravity.

 

On Earth, it takes so much power to push the fuel atoms together that researchers end up putting in more energy than they get out. But by understanding a newly discovered magnetic phenomenon, the team suggests that the ignition of nuclear fusion could be made more efficient.

 

Two methods dominate for confining the fuel, made of hydrogen atoms with extra neutrons⁶, so that fusion can begin. Magnetic confinement⁷ fusion uses magnetic fields to trap the fuel in a magnetic 'bottle,' and inertial confinement fusion heats the surface of the fuel pellet until it blows off in a way that causes the remaining pellet（小球） to implode⁸. The team explored an aspect of the latter method through computer simulations.

 

"One of the concerns with nuclear fusion is to squeeze this very spherical⁹ fuel pellet perfectly¹⁰ into a very small spherical pellet," said Archis Joglekar, a doctoral student in nuclear engineering and radiological sciences.

 

To avoid pushing the ball of fuel into an irregular shape that won't ignite, the fuel must be exposed to uniform heat that will cause its surface layer to evaporate all at once. As this layer pushes off at high speed, it applies equal pressure to all sides of the pellet and causes it to shrink to one thousandth of its original volume. When that happens, the fuel begins to fuse.

 

Joglekar calls even heating "the biggest concern in terms of achieving inertial confinement fusion."

 

The heat comes from about 200 laser beams hitting the inside of a hollow metal cylinder¹¹ with the fuel pellet sitting at its heart. The trouble is that the light energy from the laser is converted to heat in the metal by way of electrons, and the electrons can get trapped in magnetic fields created by the laser spots.

 

When the laser light hits the metal, it turns some of the surface metal into plasma¹², or a soup of electrons and free atomic nuclei¹³. The laser and the heat drive the electrons to move in a way that sets up a magnetic field circling the laser spot.

 

The magnetic field acts as a boundary for the electrons -- they can't cross it. But until now, researchers didn't know that the hot electrons, in an effort to get to cooler areas, are able to push the magnetic fence outward.

 

The team showed that the flow of hot electrons could drive the magnetic fields around neighboring laser spots together, causing them to join up. Instead of forming a barrier between the laser spots, the joined fields open a channel between them.

 

"Now there's a clear path for the electrons to move into what would otherwise be the cold region," Joglekar said.

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