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Inductively Driven Liner Compression

The fusion based on inductively driven liner compression program studies a unique third path to nuclear fusion energy using magneto-inertial confinement fusion concept, which attempts to fix the shortcoming of the more traditional magnetic and inertial confinement fusion concepts. In magneto-inertial fusion, the buffer magnetic field is added to an inertial compression of the fusion plasma, which mixes both magnetic and inertial confinement fusion ideas. The addition of the magnetic field greatly reduces the thermal transport loss compared with pure inertial confinement fusion, allowing it to operate at less extreme plasma conditions, leading to system without the use of inefficient and costly laser array systems. The inertial compression of the fusion plasma, allows for operations at much higher density than traditional magnetic confinement systems, leading to more compact systems.

In the inductively driven liner compression fusion concept, a thin metallic cylindrical shell (liner) is inductively compressed (imploded) on to a Field-Reversed Configuration (FRC) plasmoid at very high velocity (> 2 km/sec). The inductive compression of the liner is achieved by placing the liner inside a pulsed single-turn high field magnet, insulated from the magnet wall. When the magnet is fired, the azimuthal current flowing in the magnet induces a reverse azimuthal current in the liner due to Lenz’s law, causing high magnetic field build up in the region between the inner surface of the magnet and the outer surface of the liner. As a result, a strong radially inward JxB force develops at the liner surface, causing radial implosion of the thin liner.

After the initial acceleration away from the magnet wall, the liner continues to implode due to its inertia. As a result, the magnetic flux inside the liner in compressed, allowing the field inside the liner to increase, since liner behaves more of less as a flux conserver. The FRC is injected into the imploding liners to be captured and subsequently be compressed. The fast implosion of the liner compresses the FRC in the time scale shorter than the FRC lifetime to reach fusion conditions. The liner eventually comes to rest from the compression, and dwells at the peak condition for sufficient time longer than the energy loss time of the FRC, allowing for thermal fusion reactions.

Potential applications of the inductively driven liner fusion concept includes space propulsion as well as terrestrial fusion energy reactor. In particular, for the space application, a NASA Institute for Advanced Concepts program, the Fusion Driven Rocket (FDR) is being investigated by PDL and MSNW (a small business in Redmond, WA), where the fusion thermal energy released from the inductively driven liner fusion is directly converted into both directed (propulsive) energy and electrical energy. In FDR, the fusion energy is directly deposited on to the metallic liner, causing it to heat and vaporize. The resultant hot metallic liner and fusion product mixture is exhausted through a magnetically insulated nozzle to achieve thrust.

Collaborative research between PDL and MSNW LLC has demonstrated the ability to inductively implode liners successfully, with peak internal field reaching megagauss (100 T) conditions. Aluminum liners with radii from 6 to over 40 cm have been successfully imploded with speeds reaching 2 km/sec. The merged formation of the FRC and the magnetic compression in the compression chamber was also successfully demonstrated, with observed lifetimes in excess of what is required for the inductively driven liner fusion concept. The most important next step of the combined simultaneous operation of the FRC merging and liner compression still remains to be tested. Current research effort at PDL includes theoretical and numerical investigation into the liner and FRC dynamics to understand the underling physics of the inductively driven liner compression of the FRC to optimize future experimental designs to achieve fusion gain conditions.