In our last post we introduced two “types” of confinement that are different to magnetic confinement. Today we return to magnetic confinement fusion and the various designs within that.
One of the earliest designs in magnetic confinement fusion is the magnetic mirror. Very simply, a magnetic mirror is a bottle shaped magnetic field. A normal bottle has a neck and a bottom; in the magnetic mirror, the bottom is replaced by a second neck, so that a magnetic mirror is like a bottle with two necks and no bottom.
As you probably already guessed, a bottle with two necks (and thus two openings) would not be very good at keeping a plasma in. The scientists that first came up with the idea were hoping that they could make the openings at either end so tight that they could minimise the losses and thus achieve good confinement.
The idea had enough sway for the Mirror Fusion Test Facility (MFTF) to be designed, paid for and built in the USA as part of public fusion research. The MFTF was finished in 1986 for a reputed total cost of $372 million, and then promptly mothballed, without firing a single experimental pulse.
Today, the magnetic mirror is widely considered to be an unlikely candidate to achieve commercial fusion. Research into this idea is still active (e.g. in Russia), but it absorbs a very small amount of the money spent worldwide on fusion.
More on other designs in our subsequent posts.