LINUS (fusion experiment)

A technician looks through the center of the NRL Linus-0 reactor

The LINUS program was an experimental fusion power project developed by the United States Naval Research Laboratory (NRL) in 1972.^[1] The goal of the project was to produce a controlled fusion reaction by compressing plasma inside metal liners. The basic concept is today known as magnetized target fusion.

The reactor design was based on the mechanical compression of magnetic flux (and therefore plasma) inside a molten metal liner. A chamber was filled with molten metal and rotated along one axis. This spinning motion created a cylindrical cavity into which plasma was injected. Once the plasma was contained within the cavity, the liquid metal wall was rapidly compressed, which would raise the temperature and density of the trapped plasma to fusion conditions.

The use of a liquid metal liner has many of the advantages over previous experiments that imploded cylindrical 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.^[2] Additional benefits of a liquid liner include greatly simplified servicing of the reactor, reducing radioactivity, and protecting the permanent sections of the reactor from neutron damage.^[3] The danger from flying shrapnel was also mitigated with the use of liquid liners.

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, plasma is injected into a molten lead-lithium liner. The liner is then imploded mechanically, using high pressure helium pistons. The imploding liner acts to compress the magnetically-confined plasma adiabatically to fusion temperature and relatively high density (10¹⁷ ions per cm³).^[4] In the subsequent expansion the plasma energy and the fusion energy carried by trapped alpha particles is directly recovered by the liquid metal, making the mechanical cycle self-sustaining. The implosion cycle would be repeated every few seconds. The LINUS reactor can thus be regarded as a fusion engine, except that there is no shaft output; all the energy appears as heat.^[4]

The liquid metal acts as both a compression mechanism and heat transfer mechanism, allowing the energy from the fusion reaction to be captured as heat.^[4] LINUS researchers anticipated that a lithium liner could also be used to breed tritium fuel for the power plant, and would protect the machine from high-energy neutrons by acting as a regenerative first wall.^[4]

Experiments

Several experimental machines were constructed throughout the LINUS project, to gather data and demonstrate various aspects of the system concept.

SUZY II

To obtain detailed information about the behavior of the inner surface of solid and liquid metal liners during the final moments of an implosion, and experiment called SUZY II was built at NRL. The experiment was used to compress various metal liners from an initial diameter of 20–30 cm to a final diameter of about 1 cm using magnetic fields. An overall compression ratio of 28:1 was achieved.^[5]

One goal of SUZY II was to demonstrate the use of electromagnetic driving techniques to achieve liner implosions. The central feature of SUZY II was a bank of capacitors charged to 60 kV, which was able to quickly deliver 540 kJ of energy to be used for generating large magnetic fields. Pressures greater than 20 kpsi were achieved during the implosions. SUZY II was named after its predecessor, SUZY I, a 50 kJ capacitor bank.^[5]

LINUS-0

To study the hydrodynamic behavior and magnetic flux compression at the target energy density regimes, a device called LINUS-0 was constructed in 1978. The experiment involved a 30 cm rotating cylindrical chamber filled with molten metal or water. A large number of pistons (16 or 32)^[5] were attached to the chamber, and were in contact with the rotating liquid. During the experiment, all pistons were simultaneously propelled to drive the liquid radially inward. The pistons in the LINUS-0 experiment were driven by the high-explosive agent DATB (C₆H₅N₅O₆), also known as PBXN,^[6] chosen for its high melting point, low particulate matter, and compatibly low cost.

The experimental parameters for LINUS-0 required the cylindrical chamber to rotate at 5000 RPM, which was accomplished with a 454 Chevrolet V8 engine. All pistons were required to fire within 50 μs of each other. During data collection, LINUS-0 was fired as often as three times per day.^[4]

HELIUS

A similar machine, called HELIUS, was constructed to demonstrate magnetic flux compression. HELIUS was a half-scale version of LINUS-0,^[5] and was designed to use liquid sodium and potassium in the liner chamber. In practice, the use of water was sufficient for the hydrodynamic studies.^[7] In the HELIUS experiment, the liquid sodium-potassium liners were imploded using high-pressure Helium (120 atm) to drive mechanical pistons.^[5]

Fate of project

Experiments on LINUS-0 and HELIUS were largely unsuccessful due in part to delays incurred in the design, fabrication, and assembly phases. Time wasn't allocated to recover from delays or unexpected challenges, and the machines were eventually disassembled and placed in storage.^[8]

The LINUS project encountered several engineering problems which limited its performance and therefore its attractiveness as an approach to commercial fusion power. These issues included performance of the plasma preparation and injection technique, the ability to achieve reversible compression/expansion cycles, and problems with magnetic flux diffusion into the liner material. Also, the ability to remove the vaporized liner material from the cavity between cycles (within a duration of about 1 s) was not accomplished. There were also shortcomings with the design of the inner mechanism which pumped the liquid-metal liner.^[9]^[10]

One major problem that LINUS encountered was related to hydrodynamic instabilities in the liquid liner. If the liquid wasn't properly compressed, it could result in instabilities in the plasma called RT instabilities. This condition could destroy the fusion reaction by injecting liner material (vaporized lead and lithium) into the plasma. This problem diminished the efficiency of the fusion reactions, and could even cause damage to the reactor.^[3] Synchronizing the timing of the compression system was not possible with the technology of the time, and the proposed design was canceled.^[11]