through a coil spread along the vacuum chamber. The frequency is chosen to be similar to the natural frequency of the particles around the magnetic lines of force, the cyclotron frequency. This causes the particles in the area to gain energy, which causes them to orbit in a wider radius. Since other particles are orbiting their own lines nearby, at a macroscopic level, this change in energy appears as an increase in pressure. According to the ideal gas law, this results in an increase in temperature. Like ohmic heating, this process also becomes less efficient as the temperature increases, but is still capable of creating very high temperatures. When the frequency is deliberately set close to that of the ion circulation, this is known as ion-cyclotron resonance heating, although this term was not widely used at the time. Inherent problems Work on the then-new tokamak concept in the early 1970s, notably by Tihiro Ohkawa at General Atomics, suggested that toroids with smaller aspect ratios and non-circular plasmas would have much-improved performance. The aspect ratio is the comparison of the radius of the device as a whole to the radius of the cross-section of the vacuum tube. An ideal reactor would have no hole in the center, minimizing the aspect ratio. The modern spherical tokamak takes this to its practical limit, reducing the center hole to a single metal post, elongating the cross-section of the tubing vertically, producing an overall shape that is nearly spherical and has a ratio less than 2. The MAST device in the UK, among the most powerful of these designs, has a ratio of 1.3. Stellarators generally require complex magnets to generate the desired field. In early examples, this was often in the form of several different sets of magnets, and while modern designs combine these together, the designs that result often require a considerable working volume. As a result, stellarators require a fair amount of working room in the center of the torus, and as a result, they also have relatively large aspect ratios. For instance, W7-X has an aspect ratio of 10, which leads to a very large overall size. There are some new layouts that aim to reduce the aspect ratio, but these remain untested and the reduction is still nowhere near the level seen in modern tokamaks. In a production design, the magnets would need to be protected from the 14.1 MeV