Our sun powers itself with burning plasma, radiating enough energy to warm the planets and light up the solar system.
For 50 years, scientists have been trying to harness the process and create self-sustaining fusion reactions. Thanks to UW-Madison researchers at the Helically Symmetric eXperiment (HSX), they are now one step closer.
Professor David Anderson and scientists Joe Talmadge and Steven Anderson have been hard at work, building a stellarator in Engineering Hall. Unlike other stellarators or tokamaks, the two most common devices used for controlling hot plasma, this one combines elements from both. Magnetic coils of a unique design allow better containment of plasma with lower energy cost.
""It's not the biggest breakthrough,"" Talmadge said, ""But what [running the stellarator] does is demonstrate the concept works.""
The search for sustainable power makes this advance all the more welcome, since the world has a practically unlimited supply of hydrogen, the primary fuel for fusion reactions. Scientists hope fusion will provide energy to the world without the radioactive waste characteristic of nuclear fission.
Fusion power still faces many hurdles. Unlike fission, which produces energy when radioactive atoms are broken apart, fusion requires a lot of power to get started.
""The basic idea is to combine isotopes of hydrogen to make helium,"" Talmadge said. ""The electrons are stripped off and you force the hydrogen ions together.""
Fusion works by superheating plasma until the particles crash into each other with enough force to fuse together, instead of bouncing off. Ideally, energy from the reaction feeds back into the plasma to keep the whole thing going. But this requires an efficient design to contain the plasma—otherwise the energy just dribbles out. Without a good plasma containment system, the reaction sputters and dies.
Talmadge said the difficulty is in containing the hot gas. The sun manages it with a large gravitational force, which is not an option for scientists on earth.
Instead, they use magnets. Electric wires coil around a metal, doughnut-shaped tube and create a magnetic field that holds the super-hot ions away from the walls.
""It's like beads on a wire,"" Talmadge said. He explained the ionized particles like to stay on the magnetic field.
Stellarators actually have two magnetic fields, one that follows the ""long way"" around the tube, and one that wraps around the tube ""the short way."" Both types are necessary to contain the plasma, but combining them has drawbacks. Namely, ""bumpiness,"" when energy gets lost in uneven spaces created by the two different magnetic fields.
The breakthrough answer, the HSX researchers found, lay in the shape of the coils.
""We simplify the field by making the coils complex,"" Talmadge said. Instead of flat loops, the new coils appear warped, as though they were repeatedly run over by a bus. But inside the stellarator, the path the particles follow along the magnetic field looks almost straight.
As a result, the plasma loses less energy, is better contained, and can get hotter. Ultimately, it will reach the burning point, where it puts out more energy than it needs to keep the reaction going.
To be clear, the HSX lab does not actually run fusion reactions.
""You need a bigger and stronger magnetic field with enough power to be self-sustaining,"" Steven Anderson said. ""Our machine was never designed to do that.""
""It's like if the Wright brothers went from flying their little plane for a year to a 747,"" David Anderson said. The HSX machine runs for a twentieth of a second at a time, which is not enough time for much build up of heat or helium by-product.
However, other fusion experiments, designed on the same principles as the HSX stellarator, are larger. Some have even hit the breakeven point—meaning the energy pumped in equals the energy produced. And some of the biggest projects being built use the helical coils of the HSX stellarator.
""A lot of experiments are breathing a sigh of relief,"" Steven Anderson said.
Now that they have smoothed out the problem of bumpy magnetic fields, the HSX scientists are eager to attack the next hurdle. Turbulence, David Anderson said, happens when particles collide and ""fall off"" the magnetic field. It is similar to how a candle flame dances from the heat. But turbulence in plasma is only partly understood.
""Sometimes the plasma sets up its own effect,"" Talmadge said. ""Almost like a living body, it develops its own motion to damp down the turbulence.""
