The Linus program was an experimental fusion power project developed by the United States Naval Research Laboratory (NRL) starting in 1971. The goal of the project was to produce a controlled fusion reaction by compressing plasma inside a metal liner. The basic concept is today known as magnetized target fusion. The reactor design was based on the mechanical compression of a molten metal liner. A chamber would be filled with molten metal and rotated along one axis, creating a cylindrical cavity in the center. A suitable fusion fuel, heated to several thousand degrees to form it into a plasma, is injected into the center of the cavity. The metal is then rapidly collapsed, and due to the conservation of magnetic flux within the metal, the plasma is confined within the resulting collapsing shell and is itself collapsed. The adiabatic process would raise the temperature and density of the trapped plasma to fusion conditions. The use of a liquid metal liner has many advantages over previous Soviet experiments that imploded cylindrical solid metal liners to achieve high-energy-density fusion. The liquid metal liner provided the benefits of recovering the heat energy of the reaction, absorbing neutrons, transferring kinetic energy, and replacing the plasma-facing wall during each cycle. Added benefits of a liquid liner include greatly simplified servicing of the reactor, reducing radioactivity, protecting the permanent sections of the reactor from neutron damage, and reducing the danger from flying debris. The concept was revived in the 2000s as the basis for the General Fusion design, currently being built in Canada.
Conceptual design In the Linus concept, the reactor chamber consists of a drum filled with a liquid metal liner, typically molten lead-lithium. The drum is spun, creating centrifugal force which causes the liquid to be forced onto the inside wall of the container. There is only enough liquid metal to fill perhaps 20% of the total volume, so a large open area in the middle forms during rotation. For operation, a system, typically consisting of pistons, is used to drive additional liquid metal into the drum. This causes the entire liner to be forced inward. In experimental systems, this provided about ten-to-one compression. The extra metal is then removed again by releasing the pistons, causing the compression to reverse and the metal reach the original position at the outside of the drum. To create fusion, a fusion-fuel plasma is injected into the cavity before the piston stroke. Because of magnetic interactions in the metal, the plasma in the cavity is forced inward as well. This compression causes the plasma temperature to increase through the adiabatic process, raising it to fusion-relevant temperatures and pressures, around 100 million K and 1017 ions per cm3. At these temperatures and pressures, the rate of fusion, according to the fusion triple product, is very rapid and completes before the mechanical compression reverses. 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. These are captured in the liquid metal, raising its temperature. Some of the neutrons will interact with the lithium in the liner, undergoing a nuclear reaction that produces new tritium. In a functioning reactor, the energy would then be extracted using a steam generator as is the case in conventional heat driven power plants, while the tritium would be extracted through a variety of chemical processes. A key advantage of the Linus concept is that the compression cycle is reversible, in contrast to other concepts that use thin solid metal shells that can only be used once. 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. Additionally, systems using non-rotating shells are subject to the Rayleigh-Taylor instability and have proven extremely difficult to stabilize. The rotation of the liquid in Linus suppresses these instabilities. Finally, the metal protects the rest of the reactor from the neutron flux, which is a major problem in other designs.
History 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. The basic idea of super-high magnetic fields as a path to fusion had been considered as early as the 1950s by Andrei Sakharov, who proposed imploding metal liners to produce the required field. The concept was not picked up until the 1960s, when Velikhov began small-scale experiments. It was realized that the cost of the metal liners would likely be higher than the value of the electricity they would produce, the "kopeck problem", and they considered the idea of using a liquid metal liner instead. Shanny asked about how such a system would be stabilized against Rayleigh-Taylor issues. Velikhov misunderstood the question, thinking he was asking how it would be stabilized against gravity within the drum. He replied that they would spin it. Shanny, believing Velikhov was saying spinning would address Rayleigh-Taylor problems, performed the calculations and found that it did indeed stabilize these instabilities. The Linus program was born.
Suzy I To gain experience with the concept, NRL initially built liner imploders. The first experimental device was Suzy, constructed in 1971 under the direction of D.C. dePackh. The system used solid metal liners, like the Soviet experiments and many later devices. The liner was driven inward through the theta pinch process, using a 50 kJ capacitor bank.
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