Linus (fusion experiment)

The Linus program[a] was an experimental fusion power project developed by the United States Naval Research Laboratory (NRL) starting in 1971.

A suitable fusion fuel, heated to several thousand degrees to form it into a plasma, is injected into the center of the cavity.

In the Linus concept, the reactor chamber consists of a drum filled with a liquid metal liner, typically molten lead-lithium.

The energy released by these reactions, in the case of the typical deuterium-tritium (D-T) fuel, is mostly in the form of high-energy neutrons about 14.1 MeV.

This allows the system to run continually, limited generally by the ability to clear out the results of the last reaction and generate and inject new fuel plasma, on a timescale of a few seconds.

[6] The Linus effort ultimately traces its history to a discussion between Ramy Shanny of the United States Naval Research Laboratory (NRL) and Evgeny Velikhov of the Kurchatov Institute.

Shanny, believing Velikhov was saying spinning would address Rayleigh-Taylor problems, performed the calculations and found that it did indeed stabilize these instabilities.

This was built on Suzy II using a plastic liner inside a steel drum, filled with sodium-potassium alloy (NaK) at its eutectic ratio (22% Na, 78% K) which is a liquid at room temperature.

As the liner began to expand again when the compression current was turned off, it once again caused a heavy fluid to move into a lighter one, and the R-T instabilities reappeared.

This caused the liner to break up into droplets, which, due to their high mass and velocity, impacted the container randomly with the entire embodied energy.

This precludes the use of electromagnetic drivers as in Suzy, and attention turned to using a mechanical piston driving material from a reservoir into the main chamber.

This led to the Linus-0 design, which consisted of a 48 inches (1,200 mm) diameter steel rotor surrounded by a gas cylinder that was pressurized to 5,000 pounds per square inch (34,000 kPa) using a series of small high-explosive DATB (C6H5N5O6) charges, also known as the polymer-bonded explosive PBXN,[10] chosen for its high melting point, low particulate matter, and compatibly low cost.

[9] Linus-0 proved to be slow to build due to the only machine shop large enough to make the rotor being busy with other tasks, and the device was not completed until 1978, shortly before the program closed down.

[8] The initial proposals for the Linus designs were based on the cylindrical collapse of the liner with a continuous plasma inside.

There was some concern about bad curvature at the ends of the cylinder, which can lead to the interchange instability that operates much faster than the speed of sound.

Using an FRC inside the machine would provide natural confinement at the ends of the cylinder, preventing the plasma from escaping.

But as they appeared to represent a significant advance in the state of the art, potentially making a successful fusion system even without the implosion, NRLs interest quickly changed to the underlying physics of the FRC.

Time wasn't allocated to recover from delays or unexpected challenges, and the machines were eventually disassembled and placed in storage.

[12] The Linus project encountered several engineering problems which limited its performance and thus its attractiveness as an approach to commercial fusion power.

These issues included performance of the plasma preparation and injection method, the ability to achieve reversible compression–expansion cycles, problems with magnetic flux diffusion into the liner material, and the ability to remove the vaporized liner material from the cavity between cycles (within a duration of about 1 s) which was not accomplished.

This condition could quench the fusion reaction by reducing compression efficiency, and by injecting liner material (vaporized lead and lithium) contaminants into the plasma.