The mechanism of sawtooth precursors and sawtooth relaxations
It is shown that certain specific properties are required from the current density profile in order to arrive at a coherent theoretical explanation of the sawtooth phenomena in tokamaks. Firstly the shear must be small at the q = 1 surface, so that the current density profile has a plateau or shoulder near q = 1. Secondly J' varies abruptly not far away from q = 1, and the outer edge of the plateau in particular must be close to q = 1. These constraints allow the derivative of the otherwise odd logarithmic solution of the MHD equation both to become discontinuous and to yield a jump comparable to the lowest order jump. In consequence, the theory suggests that the inverse of the jump of the logarithmic derivative can now assume either positive or negative values. Current density and density plateaux and shoulders have been observed in TEXTOR discharges. This and other comparisons with experiment, including the acceleration rate of the perturbation, are discussed. Analysis of the particle trajectories in the perturbed magnetic structure yields an amplitude criterion for the onset of collapse that agrees well with observations on TEXTOR. The ontology of the crash phase requires an ambipolar electric field to build up inside the q = 1 surface so that only suprathermal electrons are lost. This enables the calculation of ratios of the density, current density and electron temperature crashes; these are found to agree with the measured ratios. Theory finally suggests that the sawtooth limitation of the central temperature is not unavoidable.
Bibliographic Reference: Article: Physics of Fluids B, Vol. 2 (1990) No. 5, pp. 953-972
Record Number: 199110791 / Last updated on: 1994-12-02
Original language: en
Available languages: en