Tokamak sawtooth
A sawtooth is a relaxation that is commonly observed in the core of tokamak plasmas, first reported in 1974.
[1] The relaxations occur quasi-periodically and cause a sudden drop in the temperature and density in the center of the plasma.
A soft-xray pinhole camera pointed toward the plasma core during sawtooth activity will produce a sawtooth-like signal.
Other repeated relaxation oscillations occurring in tokamaks include the edge localized mode (ELM) which effectively limits the pressure gradient at the plasma edge and the fishbone instability which effectively limits the density and pressure of fast particles.
If the amplitude of the current density in the plasma core is high enough so that the central safety factor
The mode amplitude will grow exponentially until it saturates, significantly distorting the equilibrium fields, and enters the nonlinear phase of evolution.
After a relaxation, the flattened temperature and safety factor profiles become peaked again as the core reheats on the energy confinement time scale, and the central safety factor drops below unity again as the current density resistively diffuses back into the core.
One discrepancy is that relaxations caused a much more rapid drop in the central plasma temperature of hot tokamaks than predicted by the resistive reconnection in the Kadomtsev model.
Another discrepancy found was that the central safety factor was observed to be significantly less than unity immediately after some sawtooth crashes.
Two notable explanations for this are incomplete reconnection[4] and rapid rearrangement of flux immediately after a relaxation.
[5] The Wesson model offers an explanation fast sawtooth crashes in hot tokamaks.
[6] Wesson's model describes a sawtooth relaxation based on the non-linear evolution of the quasi-interchange (QI) mode.
[10] In large tokamaks with larger Lundquist numbers, sawtooth relaxations are observed to occur much faster than predicted by the resistive Kadomtsev model.
Large, hot tokamaks with significant populations of fast particles sometimes see so called "giant sawteeth".
drops well below unity during the long period of stabilization, until instability is triggered, and the resulting crash is very large.
