stellarator
A stellarator confines Plasma (physics), plasma using external magnets. Scientists aim to use stellarators to generate fusion power. It is one of many types of magnetic confinement fusion devices. The name "stellarator" refers to stars because ...
(a
magnetic confinement fusion
Magnetic confinement fusion (MCF) is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma (physics), plasma. Magnetic confinement is one of two major branches of controlled fusi ...
reactor) that satisfies the property of
omnigeneity
Omnigeneity (sometimes also called omnigenity) is a property of a magnetic field inside a magnetic confinement fusion reactor. Such a magnetic field is called omnigenous if the path a single particle takes does not drift radially inwards or outw ...
, avoids the potentially hazardous toroidal
bootstrap current In a toroidal fusion power device, a plasma is confined within a donut-shaped cylinder. If the gas pressure of the plasma varies across the radius of the cylinder, a self-generated current will spontaneously arise within the plasma, due to collisio ...
, and has minimal
neoclassical transport
In Plasma (physics), plasma physics and magnetic confinement fusion, neoclassical transport or neoclassical diffusion is a theoretical description of collisional Transport phenomena, transport in toroidal plasmas, usually found in Tokamak, tokama ...
in the collisionless regime.
Wendelstein 7-X
The Wendelstein 7-X (abbreviated W7-X) reactor is an experimental stellarator built in Greifswald, Germany, by the Max Planck Institute for Plasma Physics (IPP), and completed in October 2015.quasi-symmetric fields, exactly QI fields on
flux surface
In magnetic confinement fusion, a flux surface is a surface on which magnetic field lines lie. Since the magnetic field is divergence-free (and magnetic nulls are undesirable), the Poincare-Hopf theorem implies that such a surface must be either ...
s cannot be expressed analytically. However, it has been shown that nearly-exact QI can be extremely well approximated through
mathematical optimization
Mathematical optimization (alternatively spelled ''optimisation'') or mathematical programming is the selection of a best element, with regard to some criteria, from some set of available alternatives. It is generally divided into two subfiel ...
, and that the resulting fields enjoy the aforementioned properties.
In a QI field, level curves of the magnetic field strength on a
flux surface
In magnetic confinement fusion, a flux surface is a surface on which magnetic field lines lie. Since the magnetic field is divergence-free (and magnetic nulls are undesirable), the Poincare-Hopf theorem implies that such a surface must be either ...
close poloidally (the short way around the torus), and not toroidally (the long way around), causing the stellarator to resemble a series of linked
magnetic mirror
A magnetic mirror, also known as a magnetic trap or sometimes as a pyrotron, is a type of magnetic confinement fusion device used in fusion power to trap high temperature Plasma (physics), plasma using magnetic fields. The mirror was one of the e ...