The word fishbone refers to the shape of bursts in magnetic field when plotted as a function of time. Fishbones are rapid bursts of magnetohydrodynamic activity sometimes observed when neutral beam heating is used in tokamaks.
The niche of plasma physics that fishbones belong to is known as magnetohydrodynamics (MHD). MHD tackles the challenges of plasma physics by using a fluid flow model for the charged particles moving under the influence of a magnetic field.
Inside a tokamak the magnetic field is made up of two parts: the toroidal field, which guides the charged particles around the torus vessel – a circle in the horizontal plane – and the poloidal field, which, with its vertical orientation, adds a twist to the magnetic field. The amount of twist is not the same everywhere; in the hot core of the plasma, where the particles are moving fastest, the magnetic field is much more twisted than in the cooler outer layers. As these different layers interact the physics becomes extremely complicated.
In the case of fishbones fast particles on a particular orbit near the core of the plasma hit a resonant sweet spot, where they complete exactly one poloidal loop for each toroidal circuit. This causes the plasma to become unstable and begin to wobble, a bit like a hula-hoop does, except at tens of thousands of vibrations per second. The magnetic sensors on the outside of the vessel pick this up, but soon the fast particles fly out of the orbit and the wobble dies away, only to begin again a fraction of a second later. These bursts of instabilities are the bones of the fish seen in the traces.
Related to the fishbones are instabilities where the core temperature of the plasma repeatedly builds up only to fall away quickly, giving a pattern known as sawteeth – especially large ones are termed “monster sawteeth”. It turns out sawteeth affect the types of fishbones observed. But that, along with kink and tearing modes, banana and potato orbits – and not forgetting snakes – is a whole different MHD story.