|Fig. 1: Schematic of a basic magnetic mirror machine. (Source: Wikimedia Commons)|
As one of the two major competing techniques for achieving sustained fusion reactions, magnetic confinement fusion continues to receive generous funding and attention. In these reactors, the atoms that make up the fuel are in a plasma state of matter, and hence can be confined by magnetic fields. Hence, a natural solution would be to confine the plasma in a solenoid. However, there remains the problem of the ends of the solenoid, as atoms would readily leak and terminate confinement. Until now, numerous methods have been researched to solve this problem, including the tokamak, wherein the magnetic field lines are folded on themselves by designing the reactor to be toroidal. Preceding the tokamak, the original idea was to simply add two "magnetic mirrors" on the ends of a solenoid. For various reasons, the idea was replaced by the tokamak and other more advanced proposals.  However, the magnetic mirror has made a comeback in a recent proposal by Lockheed Martin's Skunk Works team.
The magnetic mirror effect (Fig. 1) occurs when a charged particle enters a high density magnetic field from a low density magnetic field. Thus, the magnets used must be capable of producing a highly non-uniform magnetic field. As a charged particle moves through a magnetic field, it experiences a Lorentz force that causes it to move in a helical path. When the particle enters a region of higher density magnetic field, the combination of the radial component of the fields and the azimuthal motion of the particle results in a force pointed opposite the gradient, in the direction of the lower magnetic field. In this way, a particle can be reflected. 
The technology was utilized by the Lawrence Livermore National Laboratory, which built a test reactor that cost approximately 372 million dollars and was completed in 1986. The reactor was never opened citing budget concerns, and it was closed on the day construction completed. In its testing it was able to achieve confinement for one second, and may very well have paved the way towards the more practical attempts being made by teams such as Skunk Works today. 
Skunk Works has proposed a compact fusion reactor the size of a jet engine based on magnetic mirror confinement that can generate 100 MW. The reactor is based on the old idea of cusp confinement, where particles that try to escape are rounded back into the reactor through carefully constructed magnetic fields. Cusp confinement was investigated decades ago, but fell out of favor due to particles escaping through gaps in the fields. Skunk Works plans to encapsulate a cusp confinement reactor inside a magnetic mirror device, such that any particles that leak through the gaps in the cusps are reflected back into the active region. Simultaneously, the reactor will utilize recirculation to counter particle losses from cusp confinement. 
Lockheed's claims have been met with great skepticism. The original press release was lacking in details and provided no experimental results to support their claims. Some scientists in the field primarily cannot envision how Skunk Works managed to reach a breakthrough on technology that had been rigorously studied decades ago. For example, Professor Steven Cowley, director of the Culham Centre for Fusion Energy, has remarked "The proof is the pudding in science. I'm surprised that a company like this would make this kind of announcement without announcing any results".  Ultimately, many like Cowley believe the entire project to be no more than publicity for a team that no longer shakes the world like they did when they were pioneering groundbreaking projects like the SR-71 Blackbird. Time will tell whether the claims they've made will come good.
© Yousif Kelaita. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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