Jefferson Lab plays pivotal role in improving spin polarization technologies for harnessing abundant clean power
NEWPORT NEWS, VA – Long considered the ultimate source of clean energy, nuclear fusion promises abundant electrical power without greenhouse gas emissions or long-lasting radioactive waste. The process has fueled the core of the sun for more than four billion years – with billons more to go. Now, the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility is joining the global pursuit of harnessing that reaction.
Twice in the past year, scientists at DOE’s Lawrence Livermore National Laboratory in California have announced a net energy gain created by combining two or more atomic nuclei. These breakthroughs were decades in the making, and the applied technology has a long way to go be near a commercially viable scale.
That’s why Jefferson Lab and a team of science partners from across the U.S. are working to improve the hardware – and the nuclear fuel. The project has just been awarded funding by DOE’s Fusion Energy Sciences Office.
A rare collaboration
Jefferson Lab is on the ground level of a multi-year, combined research venture to measure spin-polarized fusion (SPF). If achieved, SPF will lower the requirements for the ignition of a burning plasma in a fusion reactor.
Atomic nuclei have a behavior akin to a spinning top. Low temperatures and high magnetic fields can be used to “polarize” all the nuclei in a fuel pellet by forcing them to all spin in the same direction. When polarized, particles spin parallel to the surrounding magnetic field. Aligned this way in a fusion reactor, deuterium (heavy hydrogen) and helium have a 50% greater chance of fusing. In a large-scale power reactor this should produce 75% more power production, according to recent fusion simulations.
What scientists don’t know, however, is just how long that spin alignment can last. The fuel can be polarized near absolute zero, but the key question is how long that alignment will survive in the hundred-million-degree plasma of a fusion reactor. Only tens of seconds are needed to garner the power boost, and theory predicts the spin alignment should outlast that. But a plasma is the most unstable form of matter, and an experimental test is essential. That is the goal of Jefferson Lab’s research partnership.
Staff Scientist Xiangdong Wei will lead Jefferson Lab’s segment of the effort, which includes researchers from the University of Virginia in Charlottesville; Oak Ridge National Laboratory in Tennessee; University of California, Irvine; and General Atomics’ DIII-D National Fusion Facility in San Diego.
“This project really benefits from combining two science offices within DOE that normally have little overlap – nuclear physics and fusion,” Wei said, adding that cross-office collaborations are rare. “A project has to be interesting enough for both, and this is one of them.”
The first legs of the combined study involve acquiring and optimizing nuclear fuel – mainly isotopes of hydrogen and helium. The next phases include designing and building devices to store the fuel and align the particle spins. The team then will develop a system to deliver the fuel to a fusion device and, finally, find ways to measure the lifetime of that polarization.
“If you can lower the requirements for fusion, the costs are a lot cheaper,” Wei said. “That’s why people really want to do this.”
Meet Xiangdong Wei
The SPF project isn’t Wei’s first foray into fusion research.
The experimental nuclear physicist, who joined Jefferson Lab in 2008 and has co-authored more than 130 scientific papers, got his start in the fusion field in the early 1990s as a graduate student at Syracuse University in New York. There, he helped create a polarized target for a fusion device at the University of Rochester’s Laboratory for Laser Energetics.
Those experiments were conducted on a fusion device in which lasers confine – or rather, squeeze – the nuclear target. The same sort of device was used in the Lawrence Livermore breakthroughs.
Meanwhile, the SPF project involves an experiment that uses magnetic fields to achieve the same goal.
“My advisor got a similar kind of random idea,” Wei said, referring to the late Syracuse physics icon Arnold Honig. “He was a target physics expert. He wanted to use a target in all different kinds of applications.”
Wei later earned his Ph.D. in polarized target physics from Syracuse in 1994. Previously, he studied astrophysics at Peking University and taught physics in China. In 1998, he joined Brookhaven National Lab as an assistant physicist and was subsequently promoted to physicist. Wei helped to develop SPHICE target at Brookhaven National Lab and HDice target at Jefferson Lab.
At Jefferson Lab, Wei specializes in low-temperature physics and polarized targets – a skill set vital to the joint SPF project. He started preparing an SPF experiment for the DIII-D Tokamak fusion facility under the leadership of by Dr. Andrew Sandorfi, a senior scientist who recently retired from Jefferson Lab but who continues to work on this SPF project as part of the University of Virginia team.
Jefferson Lab’s role
The SPF study will require fuel pellets that can maintain the spin of polarized heavy-hydrogen particles. The material best suited for this is lithium-deuteride, or LiD. Jefferson Lab will be responsible for acquiring the LiD and fabricating solid pellets from it.
Next, Wei and his team will build a cryostat – a device that can maintain cold temperatures while the LiD pellets are “dosed” with radiation from an electron beam to prepare them for polarization. Jefferson Lab is well-suited for this, with world-leading cryogenics experts on-site and several machines capable of delivering a beam of electrons.
“We’re using very low-energy electron beams, running through this material, making little imperfect centers inside,” Wei said. “This material normally is non-polarizable. It requires that you put in some impurity or what they call a paramagnetic center – in which the un-paired electrons can be polarized by the high (magnetic) field and low temperature. The electron polarization can be simultaneously transferred to D by shining microwaves, with the right frequency and correct power, on the LiD pellets.”
The team also will develop a system for cold-storing this “dosed” LiD. Finally, Jefferson Lab will design a device that can get most of the particles spinning in the same direction before the pellets are transferred to the device's fuel-delivery system.
More pieces of the fusion project
UVa’s segment is led by Associate Professor G. Wilson Miller, who has a goal of creating highly polarized helium gas and compressing it into small fusion capsules for tokamak injection. UVa already has a helium polarizer but will need to upgrade it. Ultimately, the helium will discharge into the tokamak and fuse with the hydrogen isotopes contained in the LiD pellets.
Scientists and engineers at Oak Ridge, led by Staff Scientist Larry Baylor, will design and construct the pellet injector. The facility already specializes in building these types of delivery systems, which can send the fuel to the fusion device at high speeds using compressed gas – while operating at temperatures nearly as cold as deep space and using magnetic coils to maintain the particles’ parallel spin.
Finally, Physics Professor William Heidbrink, the project’s Point of Contact with DOE, will lead researchers at UC Irvine to create detectors that can measure the lifetime of the particles’ polarization by monitoring the particles that are produced in the fusion reactions.
All this research and development will eventually be combined, and the final experiments will be conducted at the DIII-D National Fusion Facility, operated by General Atomics in San Diego.
“It’s all about how to get the tiny match to start,” Wei said, “and have it strong enough to keep burning.”
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By Matt Cahill
Contact: Kandice Carter, Jefferson Lab Communications Office, 757-269-7263, firstname.lastname@example.org