Nuclear fusion has long been considered the “holy grail” of energy research. It represents a nearly limitless source of energy that is clean, safe and self-sustaining. Ever since its existence was first theorized in the 1920s by English physicist Arthur Eddington, nuclear fusion has captured the imaginations of scientists and science-fiction writers alike.
Fusion, at its core, is a simple concept. Take two hydrogen isotopes and smash them together with overwhelming force. The two atoms overcome their natural repulsion and fuse, yielding a reaction that produces an enormous amount of energy.
But a big payoff requires an equally large investment, and for decades we have wrestled with the problem of energizing and holding on to the hydrogen fuel as it reaches temperatures in excess of 150 million degrees Fahrenheit. To date, the most successful fusion experiments have succeeded in heating plasma to over 900 million degrees Fahrenheit, and held onto a plasma for three and a half minutes, although not at the same time, and with different reactors.
Reliably reaching the break-even point is a twofold problem: getting the reaction started and keeping it going. In order to generate power from a fusion reaction, you must first inject it with sufficient energy to catalyze nuclear fusion at a meaningful rate. Once you have crossed this line, the burning plasma must then be contained securely lest it become unstable, causing the reaction to fizzle.
To solve the issue of containment, most devices use powerful magnetic fields to suspend the plasma in midair to prevent the scorching temperatures from melting the reactor walls. Looking something like a giant doughnut, these “magnetic containment devices” house a ring of plasma bound by magnetism where fusion will begin to occur if a high enough temperature is achieved. Russian physicists first proposed the design in the 1950s, although it would be decades before they actually achieved fusion with them.
To create a truly stable plasma with such a device, two magnetic fields are required: one that wraps around the plasma and one that follows it in the direction of the ring. There are currently two types of magnetic confinement devices in use: the tokamak and the stellarator.
The differences between the two are relatively small, but they could be instrumental in determining their future success. The main disparity in their design arises from how they generate the poloidal magnetic field — the one that wraps around the plasma. Tokamaks generate the field by running a current through the plasma itself, while stellarators use magnets on the outside of the device to create a helix-shaped field that wraps around the plasma.
According to Hutch Neilson of the Princeton Plasma Physics Laboratory, stellarators are considered more stable overall, but are more difficult to build and suffer from a lack of research. Tokamaks, on the other hand, are much better understood and easier to build, although they have some inherent instability issues.
At the moment, there is no clear winner in the race between the two, as neither appears to be close to the “holy grail.” So, due to lack of a victor, researchers are building both.
“There is a lack of a solution at this time, so looking at two very realistic and promising configurations for closing that gap is the responsible thing to do,” says Neilson.
NOTE: Above information has been taken from wikipedia and/or official websites of topics.