Scientists confirm thermonuclear fusion in Z-pinch sheared-flow device

In results that could help advance another “viable path” for fusion energy, research led by physicists at Lawrence Livermore National Laboratory (LLNL) has demonstrated the presence of neutrons produced through thermonuclear reactions from the Z-pinch lump-flow device.

Researchers have used advanced computer modeling techniques and diagnostic instrumentation honed at LLNL to solve a decades-old problem of distinguishing neutrons they produce thermonuclear reactions of those resulting from the ion-beam-induced instability of the plasma in the inertial magnetic fusion system.

While the team’s previous research showed that neutrons measured from Z-pinch-mounted flow-sheared devices were “compatible with thermonuclear production, we haven’t fully demonstrated that yet,” said LLNL physicist Drew Higginson, one of the research co-authors recently published in Plasma physics.

“This is direct evidence that thermonuclear fusion produces these neutrons and not the ions from beam instability,” said Higginson, principal investigator on the Portable and Adaptive Neutron Diagnostics (PANDA) team that conducts research under the Department of Energy’s Energy Advanced Research Projects Agency- Energy Cooperation Agreement (ARPA-E). “It has not been shown that they will have an energy gain, but it is a promising result indicating that they are on a positive path.”

LLNL physicist James Mitrani was lead author of the research paper, which shows how the broad scope of in vitro research benefits the larger fusion community beyond major advances made by LLNL’s National Ignition Foundation (NIF), the world’s most active laser system.

“The research focused only on this single device, but the general techniques and concepts are applicable to a lot of fusion devices in the mesomagnetic inertia fusion system,” Mitrani said. He noted that the system operates in the area between laser fusion facilities, such as the NIF and the Omega Laser Facility at the University of Rochester, and fusion devices that confine plasmas to a purely magnetic system, such as ITER (a multinational project in southern France), SPARC (under construction near Boston). ) or other Tokamak devices.

Since August, the NIF has caused a stir throughout the global scientific community because its self-confinement fusion (ICF) experiment yielded 1.35 megajoules (MJ) of energy. The achievement brought the researchers to the ignition threshold — defined by the National Academy of Sciences and the National Nuclear Security Administration when a NIF explosion produces more fusion energy than the amount of laser energy delivered to the target. This snapshot precedes the progress LLNL researchers have made in achieving the state of burnt plasma in lab experiments.

Fusion is the energy source found in the sun, stars, and thermonuclear weapons. NIF’s ICF experiments focus 192 laser beams on a small target to compress and heat partially frozen hydrogen isotopes inside a fuel capsule, creating an implosion that replicates the pressure and temperature conditions found only in the cores of stars and giant planets and in the explosion of nuclear weapons. Z-shaped compression machines accomplish fusion by using a strong magnetic field to confine and “compress” the plasma.

The Z-pinch concept is a relatively simple design that has been around as a theoretical model since the 1930s. But Higginson noted that it has a long history of “terrible instability” that has impeded the ability to generate the conditions needed for net fusion energy gains.

In the 1990s, LLNL scientists began working with University of Washington (UW) researchers to push another promising path toward ignition, the stable Z-pinch concept of sheared flow. Instead of the powerful clamping magnets used in other Z-pinch devices, flux-sheared Z-pinch devices use a pulsed electric current to generate a magnetic field flowing through a column of plasma to reduce fusion instability.

“The problem with instability is that it doesn’t create a viable route to energy production, whereas thermonuclear fusion does,” Higginson said. “Diagnosing this difference has always been challenging, particularly in the case of a Z-shaped strain.”

In 2015, LLNL and UW researchers were awarded a $5.28 million ARPA-E collaborative agreement to test disk stabilization physics at higher energies and pressure currents under the university’s Fusion Z-Pinch Experiment (FuZE) project.

Under a subsequent collaborative agreement for the ARPA-E “Power Team,” the LLNL researchers focused on diagnostics that measure neutron emissions generated during the fusion process, including the spatial locations and temporal profiles of those emissions. Combining the plasma diagnostic expertise of national laboratories with the rapid operation of private companies builds on each of their individual strengths and is a key objective of the ARPA-E Integration Capacity Team Program.

With FuZE cylinder radius narrowing to increase pressurewill also cause dips in the plasma that will generate much stronger magnetic fields that will cause plasma To tweak inward more in certain areas than in others. Like the pinched ends of the famous tubular ground beef, the instability of the unwanted “sausage” would create beams of faster ions that produce neutrons that could be confused with the desirable neutrons produced by a thermonuclear.

LLNL researchers placed two flashing plastic detectors outside the device to measure the effects of neutrons Because they appeared in a few microseconds from different points and angles outside the Z-pinch chamber.

“We showed that the energies of the emitted neutrons were equal at different points around this device, which indicates the presence of thermonuclear energy. fusion feedback,” said Matrani.

The analysis involved creating graphs of the neutron pulses detected by the polishers and comparing them using methods such as computerized Monte Carlo simulations that examine all possible outcomes.

Diagnostics isn’t new, Higginson said, but “the idea of ​​using graphs of individual neutron pulse energies to measure anisotropy — the difference in energies when you look in different directions — is a new technology and is something we’ve thought about, developed and implemented here. Plus, we’ve been working with the University of California at Berkeley, which has helped us advance the modeling ability to resolve uncertainties in measurements and fully understand the data we see. We don’t just look through data.”

The research paper, “Thermal Neutron Emission from Z-pinch-Shear Flow,” was published in November and published from a Mitrani invited talk presented at the American Physical Society’s Annual Meeting – Division of Plasma Physics in 2020.

He was joined by Mitrani and Higginson’s colleague Harry MacLean in the LLNL League; Joshua Brown and Thibault Laplace of the University of California, Berkeley; Bethany Goldblum of the University of California, Berkeley and Lawrence Berkeley National Laboratory; and Elliot Clavo, Zach Draper, Eleanor Forbes, Ray Goolingo, Brian Nelson, Uri Shumlak, Anton Stepanov, Tobin Weber and Yu Zhang of the University of Washington.

The search resulted from a privately funded startup in Seattle called Zap Energy in 2017.

The search continues under new grants, with more detailed measurements taken by 16 detectors while Zap Energy continues its experiments.

“We want to get involved because we don’t know what surprises might come,” Higginson said. “It can turn out that when you go to a higher current, all of a sudden you start to push the instability back. We want to be able to show that as the current goes up it’s possible to maintain high quality and stability.”