Editing Tokamak (section)

=== Advanced tokamaks ===
In the early 1970s, studies at Princeton into the use of high-power superconducting magnets in future tokamak designs examined the layout of the magnets. They noticed that the arrangement of the main toroidal coils meant that there was significantly more tension between the magnets on the inside of the curvature where they were closer together. Considering this, they noted that the tensional forces within the magnets would be evened out if they were shaped like a D, rather than an O. This became known as the "Princeton D-coil".<ref>{{cite tech report |url=https://www.osti.gov/servlets/purl/5233082 |first1=W.H. |last1=Gray |first2=W.C.T. |last2=Stoddart |first3=J.E. |last3=Akin |publisher=Oak Ridge National Laboratory |date=1977 |title=Bending free toroidal shells for tokamak fusion reactors}}</ref>

This was not the first time this sort of arrangement had been considered, although for entirely different reasons. The safety factor varies across the axis of the machine; for purely geometrical reasons, it is always smaller at the inside edge of the plasma closest to the machine's center because the long axis is shorter there. That means that a machine with an average ''q'' = 2 might still be less than 1 in certain areas. In the 1970s, it was suggested that one way to counteract this and produce a design with a higher average ''q'' would be to shape the magnetic fields so that the plasma only filled the outer half of the torus, shaped like a D or C when viewed end-on, instead of the normal circular cross section.

One of the first machines to incorporate a D-shaped plasma was the [[Joint European Torus|JET]], which began its design work in 1973. This decision was made both for theoretical reasons as well as practical; because the force is larger on the inside edge of the torus, there is a large net force pressing inward on the entire reactor. The D-shape also had the advantage of reducing the net force, as well as making the supported inside edge flatter so it was easier to support.{{sfn|Wesson|1999|p=22}} Code exploring the general layout noticed that a non-circular shape would slowly drift vertically, which led to the addition of an active feedback system to hold it in the center.{{sfn|Wesson|1999|p=26}} Once JET had selected this layout, the [[General Atomics]] Doublet III team redesigned that machine into the D-IIID with a D-shaped cross-section, and it was selected for the Japanese [[JT-60]] design as well. This layout has been largely universal since then.

One problem seen in all fusion reactors is that the presence of heavier elements causes energy to be lost at an increased rate, cooling the plasma. During the very earliest development of fusion power, a solution to this problem was found, the ''[[divertor]]'', essentially a large [[mass spectrometer]] that would cause the heavier elements to be flung out of the reactor. This was initially part of the [[stellarator]] designs, where it is easy to integrate into the magnetic windings. However, designing a divertor for a tokamak proved to be a very difficult design problem.

Another problem seen in all fusion designs is the heat load that the plasma places on the wall of the confinement vessel. There are materials that can handle this load, but they are generally undesirable and expensive [[heavy metals]]. When such materials are sputtered in collisions with hot ions, their atoms mix with the fuel and rapidly cool it. A solution used on most tokamak designs is the ''limiter'', a small ring of light metal that projected into the chamber so that the plasma would hit it before hitting the walls. This eroded the limiter and caused its atoms to mix with the fuel, but these lighter materials cause less disruption than the wall materials.

When reactors moved to the D-shaped plasmas it was quickly noted that the escaping particle flux of the plasma could be shaped as well. Over time, this led to the idea of using the fields to create an internal divertor that flings the heavier elements out of the fuel, typically towards the bottom of the reactor. There, a pool of liquid [[lithium]] metal is used as a sort of limiter; the particles hit it and are rapidly cooled, remaining in the lithium. This internal pool is much easier to cool, due to its location, and although some lithium atoms are released into the plasma, its very low mass makes it a much smaller problem than even the lightest metals used previously.

As machines began to explore this newly [[plasma shaping|shaped plasma]], they noticed that certain arrangements of the fields and plasma parameters would sometimes enter what is now known as the [[high-confinement mode]], or H-mode, which operated stably at higher temperatures and pressures. Operating in the H-mode, which can also be seen in stellarators, is now a major design goal of the tokamak design.

Finally, it was noted that when the plasma had a non-uniform density it  would give rise to internal electrical currents. This is known as the ''[[bootstrap current]]''. This allows a properly designed reactor to generate some of the internal current needed to twist the magnetic field lines without having to supply it from an external source. This has a number of advantages, and modern designs all attempt to generate as much of their total current through the bootstrap process as possible.

By the early 1990s, the combination of these features and others collectively gave rise to the "advanced tokamak" concept. This forms the basis of modern research, including ITER.