Interchange instability

The interchange instability, also known as the Kruskal–Schwarzschild instability or flute instability, is a type of <a href="/facts/Plasma_instability/QmzpEW7B">plasma instability</a> seen in <a href="/facts/Magnetic_fusion_energy/ThThVDpr">magnetic fusion energy</a> that is driven by the gradients in the <a href="/facts/Magnetic_pressure/KRhUy12h">magnetic pressure</a> in areas where the confining <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> is curved.
The name of the instability refers to the action of the plasma changing position with the <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> lines (i.e. an interchange of the lines of force in space) without significant disturbance to the geometry of the external field. The instability causes <a href="/facts/Fluting_(architecture)/VTH5tbDE">flute-like structures</a> to appear on the surface of the plasma, hence it is also referred to as the flute instability. The interchange instability is a key issue in the field of <a href="/facts/Fusion_energy/jyETSzm4">fusion energy</a>, where magnetic fields are used to confine a <a href="/facts/Plasma_(physics)/1d3btMVU">plasma</a> in a volume surrounded by the field.
The basic concept was first noted in a 1954 paper by <a href="/facts/Martin_David_Kruskal/XJDpvkrC">Martin David Kruskal</a> and <a href="/facts/Martin_Schwarzschild/F2aAHSBf">Martin Schwarzschild</a>, who demonstrated that a situation similar to the <a href="/facts/Rayleigh%E2%80%93Taylor_instability/hmdtnnAc">Rayleigh–Taylor instability</a> in classic fluids existed in magnetically confined <a href="/facts/Plasma_(physics)/1d3btMVU">plasmas</a>. The problem can occur anywhere where the magnetic field is concave with the plasma on the inside of the curve. <a href="/facts/Edward_Teller/9g3Mr9Mp">Edward Teller</a> gave a talk on the issue at a meeting later that year, pointing out that it appeared to be an issue in most of the fusion devices being studied at that time. He used the analogy of <a href="/facts/Rubber_band/PMSbEPEp">rubber bands</a> on the outside of a blob of jelly; there is a natural tendency for the bands to snap together and eject the jelly from the center.
Most machines of that era suffered from other instabilities that were far more powerful, and whether or not the interchange instability was taking place could not be confirmed. This was finally demonstrated beyond doubt by a Soviet <a href="/facts/Magnetic_mirror/j2BymN1Q">magnetic mirror</a> machine during an international meeting in 1961. When the US delegation stated they were not seeing this problem in their mirrors, it was pointed out they were making an error in the use of their instrumentation. When that was considered, it was clear the US experiments were also being affected by the same problem. This led to a series of new mirror designs, as well as modifications to other designs like the <a href="/facts/Stellarator/HXSJF0Cz">stellarator</a> to add negative curvature. These had cusp-shaped fields so that the plasma was contained within convex fields, the so-called "magnetic well" configuration.
In modern designs, the interchange instability is suppressed by the complex shaping of the fields. In the <a href="/facts/Tokamak/kDtb8Bsw">tokamak</a> design there are still areas of "bad curvature", but particles within the plasma spend only a short time in those areas before being circulated to an area of "good curvature". Modern stellarators use similar configurations, differing from tokamaks largely in how that shaping is created.

Interchange instability open-in-new

Interchange instability