Editing Stellarator (section)

=== Previous work ===
In 1934, [[Mark Oliphant]], [[Paul Harteck]] and [[Ernest Rutherford]] were the first to achieve fusion on Earth, using a [[particle accelerator]] to shoot [[deuterium]] nuclei into a metal foil containing [[deuterium]], [[lithium]] or other elements.<ref>{{cite journal |journal=Nature |first1= Mark |last1=Oliphant |first2= Paul |last2=Harteck |first3= Ernest |last3=Rutherford |title= Transmutation Effects observed with Heavy Hydrogen |volume=133 |date= 17 March 1934 |issue= 3359 |page=413 |doi=10.1038/133413a0|bibcode= 1934Natur.133..413O |s2cid= 4078529 |doi-access=free }}</ref> These experiments allowed them to measure the [[nuclear cross section]] of various reactions of fusion between nuclei, and determined that the [[tritium]]–deuterium reaction occurred at a lower energy than any other fuel, peaking at about 100,000&nbsp;[[electronvolt]]s (100&nbsp;keV).{{sfn|McCracken|Stott|2012|p=35}}{{efn|Extensive studies in the 1970s lowered this slightly to about 70&nbsp;keV.}}

100&nbsp;keV corresponds to a temperature of about a billion [[kelvin]]s. Due to the [[Maxwell–Boltzmann statistics]], a bulk gas at a much lower temperature will still contain some particles at these much higher energies. Because the fusion reactions release so much energy, even a small number of these reactions can release enough energy to keep the gas at the required temperature. In 1944, [[Enrico Fermi]] demonstrated that this would occur at a bulk temperature of about 50&nbsp;million&nbsp;Celsius, still very hot but within the range of existing experimental systems. The key problem was ''confining'' such a plasma; no material container could withstand those temperatures. But because plasmas are electrically conductive, they are subject to electric and magnetic fields which provide a number of solutions.{{sfn|Stix|1998|p=3}}

In a magnetic field, the electrons and nuclei of the plasma circle the magnetic lines of force. One way to provide some confinement would be to place a tube of fuel inside the open core of a [[solenoid]]. A solenoid creates magnetic lines running down its center, and fuel would be held away from the walls by orbiting these lines of force. But such an arrangement does not confine the plasma along the length of the tube. The obvious solution is to bend the tube around into a torus (donut) shape, so that any one line forms a circle, and the particles can circle forever.{{sfn|Bromberg|1982|p=16}}

However, this solution does not actually work. For purely geometric reasons, the magnets ringing the torus are closer together on the inside curve, inside the "donut hole". Fermi noted this would cause the electrons to drift away from the nuclei, eventually causing them to separate and cause large voltages to develop. The resulting electric field would cause the plasma ring inside the torus to expand until it hit the walls of the reactor.{{sfn|Bromberg|1982|p=16}}