Project fate
The initial proposals for the Linus designs were based on the cylindrical collapse of the liner with a continuous plasma inside. This arrangement meant there was nothing to confine the plasma from being squirted out the ends of the imploding cylinder of metal. This was not necessarily a problem; both the liner and the plasma would move at the speed of sound, but because the speed of sound in the metal is much higher than in the plasma, most of the plasma would not have time to move before it had already completed the reaction. 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. The magnitude of this effect, if was present at all, was not explored.
The disadvantage of this approach was that some plasma did escape, and that amount increased as the speed of the implosion decreased. To get a reasonable reaction rate, driver energies on the order of 75 to 100 MJ were required. While this was not impossible to achieve, it still represented a significant capital cost to build such a storage system, and the resulting high-energy and high-speed implosion represented an engineering challenge.
Linus was being developed while another fusion concept was first emerging, the field-reversed configuration, or FRC. This is essentially a smoke ring of plasma that is naturally stable until it cools. Using an FRC inside the machine would provide natural confinement at the ends of the cylinder, preventing the plasma from escaping. This would significantly reduce the required implosion energy, and thus lower the size and cost of the machine as a whole.
At the time, FRCs were very new technology. 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. Experiments on Linus-0 and Helius were relatively brief 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.
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. Shortcomings also occurred with the design of the inner mechanism which pumped the liquid-metal liner.
Another major problem encountered involved hydrodynamic instabilities in the liquid liner. If the liquid was imprecisely compressed, the plasma boundaries could undergo Rayleigh–Taylor instability. This condition could quench the fusion reaction by reducing compression efficiency, and by injecting liner material (vaporized lead and lithium) contaminants into the plasma. Both effects reduce the efficiency of fusion reactions. Strong instability could even cause damage to a reactor. Synchronizing the timing of the compression system was not possible with the technology of the time, and the proposed design was canceled.
