March 22, 2023

# The magnetic field heats the fusion

Physics 15, 169

A magnetic field can dramatically increase the performance of a large-scale fusion experiment that could lead to a future clean energy source.

Nuclear fusion could provide a clean energy source, but one of the technological challenges is keeping the fuel at a high enough temperature for a long enough time. In a technique called inertial confinement fusion (ICF) – where lasers initiate the nuclear reaction – a magnetic field has been shown to enhance heating. Now researchers have shown that a magnetic field can also help in a large-scale experiment with a more complicated design that produces significantly more energy. [1]. The applied field increased the fuel temperature by 40% and tripled the efficiency of the fusion reaction. The work provides a step towards increasing the robustness and energy production of the fusion reaction and provides the first proof-of-concept for magnetization-assisted fusion in a large-scale experiment.

In the simplest version of the ICF, synchronized laser pulses strike a capsule filled with cold hydrogen, causing it to implode. The implosion heats the fuel and creates a patch of hot plasma (see Point of View: Fusion Increases Heat). The “hot spot” serves as a spark that initiates combustion throughout the fuel, causing a self-sustaining fusion reaction that releases energy. However, these implosions may not generate significant fusion energy if the fuel pellet has small imperfections on its surface or if the lasers are not perfectly synchronized. But if the fuel could be heated to higher temperatures than was possible in recent experiments, there would be more room for error, which could dampen sensitivity to such detail.

In 2012, researchers at the OMEGA Laser Facility at the University of Rochester in New York demonstrated that a magnetic field dramatically alters the heat flow in laser-heated fuel. This field, in effect, provides insulation around the hottest region of the fuel, providing a means to improve the heating and possibly the efficiency of the reaction. “It’s like a thick polystyrene sleeve that keeps your coffee hot without burning your hand,” says John Moody of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California. In the presence of a magnetic field, the electrons in the plasma are forced to follow helical paths along the magnetic field lines, thus colliding with each other less often. This behavior slows the flow of heat to the cooler surrounding fuel and provides additional heat in the hot spot.

LLNL researchers used computer simulations to investigate the potential performance benefits of magnetization for the NIF, the world’s largest ICF experiment and the one that has come closest to the goal of producing more energy than she doesn’t consume it. OMEGA’s results proved the basic concept, but they could not be applied directly to NIF, because NIF uses a design called indirect drive, in which laser pulses heat a hollow gold cylinder so much that it glows. to X-rays. This radiation in turn illuminates and heats the fuel capsule, located inside the cylinder, and causes the capsule to implode.

Exposing a gold cylinder to a strong magnetic field would generate electric currents in its walls that would destroy it (see Trend: Boosting Inertial-Confinement-Fusion Yield with Magnetized Fuel). To circumvent this problem, Moody and his colleagues experimented with alloys to create a metal cylinder with low electrical conductivity. They discovered that an alloy of gold and tantalum (AuTa4) could tolerate the high magnetic field.

The NIF team conducted experiments using a cylinder made from this alloy with a fuel capsule filled with pure deuterium, a form of hydrogen. They applied a 26 Tesla magnetic field by passing a current through a coil of wire wrapped around the cylinder, just before turning on the lasers. Compared to experiments without a magnetic field, the temperature of the laser-generated hot spot increased by 40%. The energy production, measured by counting the number of neutrons produced during fusion, was multiplied by 3. According to Pascal Loiseau, plasma physicist at the French Alternative Energies and Atomic Energy Commission (CEA), these results are “remarkable” and constitute a proof of concept for magnetic assistance at the NIF.

To reduce equipment risk and reduce infrastructure expense, the NIF team simplified the setup for these early experiments. They reduced the laser power, kept the fuel at room temperature and used deuterium alone. In future higher power experiments that use two forms of hydrogen (deuterium and tritium), Moody anticipates a second effect that will increase performance. High energy particles generated during nuclear reactions will be trapped by the field lines. These charged particles will spend more time depositing energy in the hot spot, providing more heat before escaping.

–Rachel Berkowitz

Rachel Berkowitz is the corresponding editor for physics review based in Vancouver, Canada.

## References

1. JD Moody et al.“Increased Ion Temperature and Neutron Yield Observed in Magnetized Indirect Drive Motors
${\text{D}}_{2}$

– implosions of filled capsules on the National Ignition Facility”, Phys. Rev. Lett. 129195002 (2022).

## Areas

Plasma physicsEnergy research

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