Superconductive Coatings May Float NIF Targets

Magnetic levitation is now, and it’s here. It’s been done with trains and even frogs; now it’s being attempted with tiny NIF fusion fuel capsules.

The concept behind an ambitious NIF target fabrication program is to levitate inertial confinement fusion (ICF) capsules inside hohlraums, harnessing the physics of superconducting levitation.

In ICF experiments, researchers have been striving to minimize the impact on implosion performance from capsule support hardware like tents, fibers, and fuel fill-tubes—micron-sized though they are. Reducing perturbations from these engineering features, they anticipate, will help maximize the neutron yield from NIF implosions (see "Perfecting the Spider’s Art to Support NIF Targets" and "Working to Tame Disturbances in the NIF Force").

For one possible solution, a team of LLNL scientists led by physicist Sergei Kucheyev has been developing superconducting coatings on capsules to enable magnetic levitation—because, as with all technology, "magnets make it better."

"Levitation is an ideal solution to get rid of the undesirable perturbations from the capsule support hardware," said Kucheyev. "The levitation approach offers a completely interference-free solution."

The project is a collaboration involving the Laboratory’s NIF & Photon Science, Physical and Life Sciences (PLS) and Engineering directorates and General Atomics. Along with Kucheyev from PLS, team members working on targets in NIF & PS include Scott McCall, L. Bimo Bayu Aji, Alex Baker, and Elis Stavrou (PLS), David Steich (Engineering), and John Bae (General Atomics). Their work has been supported by the Laboratory Directed Research and Development program (LDRD).

The capsule is coated with a thin (about 100-nanometer) superconducting layer and supported by an upward Lorenz force generated by magnetic coils outside the hohlraum.

In order to entirely eliminate perturbation-causing mechanical support, capsules are coated with a thin film of a superconductor called magnesium diboride (MgB₂), currently the most promising material. The superconducting coating can be less than 100 nanometers (billionths of a meter) thick. The levitating force is produced by a magnetic field generated by coils positioned outside the metallic hohlraum. The magnetic field passes through the hohlraum virtually unobstructed before impinging on and flowing around the capsule itself.

The levitation of capsules might at first sound exotic, Kucheyev acknowledges. After a year working on this project, he is aware that the topic of levitation makes some people smile, as it tends to bring up associations with deep meditation and conjurers like "the great Harry Kellar."

But the concept of magnetic levitation has scientific grounding, he notes, and has in fact been demonstrated many times. There are several commercial Maglev trains, for example, and the ITER tokamak, the large fusion project in France, uses superconductors to generate very high magnetic fields to contain and control superheated plasma.

"The levitation idea by itself is not new," Kucheyev said. "It has been under consideration for ICF targets for more than 40 years. The challenge is to achieve stable levitation of capsules without wrecking their exquisite spherical symmetry."

The team has had some initial success so far with depositing superconducting films of magnesium diboride. At first their approach faced setbacks, the worst being the number of films that outright refused to superconduct, and others that blistered and resembled rusted flaking metal rather than the desired mirror-like surfaces.

They are finding the growth of MgB₂ on polycrystalline or amorphous capsules to be particularly challenging, in what Kucheyev calls "an inherently challenging non-epitaxial and non-planar growth regime." The issue was understanding how to control roughness, stress, and superconducting properties of MgB₂ films.

They recently succeeded in making high-quality superconducting MgB₂ films at about 100-nanometer thickness, with the results resembling a professionally smooth paint job.

Finding a Sweet Spot

Not surprisingly, they found the thinner the superconducting film, the harder it was to make. "With sufficient effort," Kucheyev said, "we did end up finding a sweet spot."

Nor does the complexity of boron chemistry make the film fabrication process any easier. "Thin boron films, which are the precursor in the fabrication of MgB₂, were actually gradually disappearing during storage in air!" Kucheyev said. This was unexpected, because boron is a high-melting-point material with a low vapor pressure, and the boron surface is well known for its high resistance to corrosion.

After discovering that sufficiently thin boron films were perishable, the team tried a variety of methods to keep them from deteriorating. They used freezing, baking, and vacuum sealing. In the end, they found that a relatively simple high-temperature bake in an inert atmosphere can stabilize boron films, making them resistant to corrosion and evaporation.

"We are now focused on what appears to be the most critical technological step toward a successful magnetic levitation of ICF capsules—the fabrication of conformal superconducting coatings," Kucheyev said. "It’s all about understanding the film growth and processing at this stage. After the demonstration of this critical step, we will move to the next stage involving engineering and building levitation trap magnets."

"In that stage," added Steich, "we will wrestle with the remaining unknowns such as how the thin film MgB₂ material responds to high Lorentz forces and high current densities, as well as magnetic flux pinning details. While we are very encouraged by the results thus far, there is still more work ahead of us."

The first laboratory experiments to demonstrate levitating capsules are scheduled for next year.