|Fig. 1: Tokamak Magnetic Fields. (Source: Wikimedia Commons)|
Fusion reactors work by forcing hydrogen atoms close together to create helium, which generates energy. A major class of fusion reactors are magnetic confinement reactors, which use extremely powerful magnetic fields to force hydrogen atoms close together in hopes of creating fusion. A simple example would be a solenoid, which is just a coil of wire, which creates a magnetic field around some plasma. However, since it is not possible to build an infinitely long solenoid, the solenoid can be twisted it into a doughnut shape to keep the plasma confined. These types of reactors are known as tokamaks.
The problem with tokamaks is that due to the doughnut shape, magnetic fields are denser on the inside of the donut than on the outside as seen in Fig. 1. This can cause plasma to leak out of confinement, leading to decreased fusion performance or even damaging the reactor. There are solutions, but they prevent the tokamak from operating continuously, since they need a changing field.
Another approach is the stellarator, which twists the doughnut along the inner axis of the doughnut as seen in Fig. 2. This means that some parts of the inside of the doughnut are now flipped on the outside, preventing the concentration of magnetic fields on the inside. Since no other fields are now necessary, the stellarator can operator continuously. One form of the stellarator is the heliotron, which twists the confinement regions into a helix. The largest heliotron in the world is the Large Helical Device in Japan.
The goals of the Large Helical Device is to understand the problems heliotron nuclear fusion reactors face at scale and what changes would be necessary to build a commercial scale fusion reactor. In the future, they hope to increase the temperature of the plasma as well as the operating time of the reactor. 
|Fig. 2: Stellarator Magnetic Fields. (Source: Wikimedia Commons)|
Currently they use several techniques. The first is neutral beam injection. Here, neutral particles are injected into the confinement chamber and collide with the plasma. The magnetic fields in the chamber keep the neutral particles in the plasma and the neutral particles transfer their energy to the plasma.  These beams are can be injected tangential to the flow of plasma, which also helps increase the overall speed of the plasma. Other techniques they use are electron cyclotron resonance heating and ion cyclotron radio frequency, which bombard the plasma with radio waves to heat it. 
In the future they plan to increase the density of the plasma by directly injecting frozen hydrogen pellets and researching possible optimizations for the injection of hydrogen pellets. They plan to also investigate supercooling the superconductors used to create the magnetic fields.  This way they increase the strength of the magnetic fields. In this way, they hope to understand and overcome the challenges of helical reactors paving the way for commercial fusion reactors.
© Qingping He. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purpose only. All other rights, including commercial rights, are reserved to the author.
 M. Fujiwara et al., "Status of Large Helical Device Project," Fusion Eng. Des. 26, 547 (1995).
 D. G. McAlees and R. W. Conn, "Heating of a Large CTR-Tokamak By Neutral-Beam Injection," Nucl. Fusion 14, 419 (1974).