Magnetized Target Fusion
Magnetized target fusion is a hybrid between magnetic fusion and inertial confinement fusion. Whereas magnetic fusion creates confinement without compression and inertial confinement fusion creates compression without any sustained confinement, magnetized target fusion creates a relatively low-temperature and low-density magnetically confined plasma and then compresses it to thermonuclear conditions.
Magnetized target fusion allows for a slower compression and a lower peak density than inertial confinement fusion since heat cannot escape through the magnetic field as quickly as it does through the open conditions of an imploding shell. Furthermore, the magnetic containment only needs to exist for microseconds as opposed to the several seconds of magnetic fusion since the compressed plasma density is one million times greater than in a tokamak machine.
This combination of a slower compression rate and shorter containment time avoids the need for massive superconducting magnets or extremely high power lasers, allowing for a simpler and cheaper fusion system that uses less power.
Magnetized target fusion was first developed by the US Naval Research Lab in Washington DC in the late 1970s. Their design was nicknamed “LINUS” and was intended to generate a magnetically trapped toroid of hot deuterium-tritium
plasma within a rotating cylinder of liquid lead. Fast-acting steam-operated pistons would drive the liquid lead to compress the plasma by a factor of about 1,000, thereby creating thermonuclear temperatures and very high pressures within the plasma for a few milliseconds. The resulting fusion would release neutrons that would heat up the lead through direct impact and would breed tritium through reaction with lithium diffused through the lead. The hot lead would pass through a heat exchanger to produce steam, which in turn would be used in a turbine to generate electricity.
Why LINUS was never built
The LINUS design proved impossible to build using the technology of the time. Plasma toroids would last for about a microsecond, but the action of
multiple air pistons could compress the liquid lead no faster than a matter of milliseconds, and was therefore too slow. It was also not possible to synchronize the impact of the fast-moving air pistons accurately enough with the electronics and servo controls of the day.
Ongoing Magnetized Target Fusion work
Over the decades, the Los Alamos National Laboratory in the US has continued to work on magnetized target fusion concepts. Their approach involves injecting a plasma toroid into an aluminum tube and then discharging a large bank of capacitors into the tube. The current in the tube induces a magnetic field that implodes the tube at high speed, compressing the plasma to thermonuclear conditions in the process.
The difficulties with this approach include the amount and cost of the electrical power supplies and energy needed to implode the tube, and the fact that the tube and electrical cable connections are destroyed during each pulse. Furthermore, neutrons from the fusion reaction damage the surrounding chamber and equipment.
These difficulties notwithstanding, Los Alamos National Laboratory has a wealth of knowledge and understanding regarding magnetized target fusion, and has made significant progress with respect to plasma densities and the numerical simulation of compressed plasmas.