Interchange instability

When the particle's original linear motion is superimposed on this transverse force, its resulting path through space is a helix, or corkscrew shape.

In this case, the plasma will orbit the lines running down the center of the chamber and be prevented from moving outward towards the walls.

[6] One of the two effects, which became known as the kink instability, was already being seen in early z-pinch experiments and occurred slowly enough to be captured on movie film.

[citation needed] The other instability noted in the paper considered an infinite sheet of plasma held up against gravity by a magnetic field.

It suggested there would be behaviour similar to that in classical physics when one heavy fluid is supported by a lighter one, which leads to the Rayleigh–Taylor instability.

[citation needed] In October 1954 a meeting of the still-secret Project Sherwood researchers was held at Princeton University's Gun Club building.

Edward Teller brought up the topic of this instability and noted that two of the major designs being considered, the stellarator and the magnetic mirror, both had large areas of such curvature and thus should be expected to be inherently unstable.

This exchange of position appeared to be identical to the mirror case in particular, where the plasma naturally wanted to expand while the magnetic fields had an internal tension.

[citation needed] As the magnitude of the problem became clear, the meeting turned to the question of whether or not there was any arrangement that was naturally stable.

Nevertheless, as Amasa Bishop noted; The suggestion of the picket-fence concept did little, however, to dispel the atmosphere of gloom which settled over the conference, toward its end.

However, if the plasma is displaced, the non-uniform nature of the field means the ion's larger orbital radius takes them outside the confinement area while the electrons remain inside.

The same effect occurs in any reactor design where the plasma is within a field of sufficient curvature, which includes the outside curve of toroidal machines like the tokamak and stellarator.

As this process is highly non-linear, it tends to occur in isolated areas, giving rise to the flute-like expansions as opposed to mass movement of the plasma as a whole.

Although never fully realized, the idea of controlled thermonuclear fusion motivated many to explore and research novel configurations in plasma physics.

Instabilities plagued early designs of artificial plasma confinement devices and were quickly studied partly as a means to inhibit the effects.

In 1958, Bernstein derived an energy principle that rigorously proved that the change in potential must be greater than zero for a system to be stable.

In 1959, Thomas Gold attempted to use the concept of interchange motion to explain the circulation of plasma around the Earth, using data from Pioneer III published by James Van Allen.

[9] Gold also coined the term “magnetosphere” to describe “the region above the ionosphere in which the magnetic field of the Earth has a dominant control over the motions of gas and fast charged particles.” Marshall Rosenthal and Conrad Longmire described in their 1957 paper how a flux tube in a planetary magnetic field accumulates charge because of opposing movement of the ions and electrons in the background plasma.

Since Kruskal and Schwarzschild's papers a tremendous amount of theoretical work has been accomplished that handle multi-dimensional configurations, varying boundary conditions and complicated geometries.

Acting opposite to the gravity in the simple model, the centrifugal force moves the plasma outward where the ripple-like perturbations (sometimes called “flute” instabilities) occur on the boundary.

Without gravity or an inertial force, interchange instabilities can still occur if the plasma is in a curved magnetic field.

Physically, this means that if the field lines are toward the region of higher plasma density then the system is susceptible to interchange motions.

The energy method is similar to the simpler approach outlined above where is found for any arbitrary perturbation in order to maintain the condition .

The recording of these events in the magnetospheres of Earth, Jupiter and Saturn are the main tool for the interpretation and analysis of interchange motion.

[10] The injections occur predominantly in the night-time hemisphere, being associated with the depolarization of the neutral sheet configuration in the tail regions of the magnetosphere.

The interchange instability also has been found to have a limiting factor on the night side plasmapause thickness [Wolf et al. 1990].

The first evidence of this behavior was published by Thorne et al. in which they discovered “anomalous plasma signatures” in the Io torus of Jupiter's magnetosphere.

[11] Using the data from the spacecraft Galileo's energetic particle detector (EPD), the study looked at one specific event.

In Thorne et al. they concluded that these events had a density differential of at least a factor of 2, a spatial scale of km and an inward velocity of about km/s.

However, it was found that Jovian injections can occur at all local time positions and therefore can't be directly related to the situation in Earth's magnetosphere.