Mechanisms underlying sawtooth precursors, relaxations and ramp phases
To arrive at a coherent explanation of the sawtooth precursors, two properties are required of the current density profile which invalidate assumptions inherent to the conventional calculation of the jump of the logarithmic derivative of the MHD solution across the q = 1 singular magnetic surface. In the revised toroidal theory, this jump passes through zero at a value of the shear parameter at the q = 1 surface which depends on the values of r(1) and q(0). In the course of the sawtooth ramp, this shear value is approached from below, with the value of the jump being negative, contrary to conventional wisdom, and a dramatic increase of the precursor follows a more moderate one. The magnetic surfaces then lean asymmetrically on the discharge "confining zone" where drift instabilities act as particle and heat sinks. This situation leads to a sheared flow along the field lines which destabilises the Kelvin-Helmholtz mode; the latter, in turn, triggers very rapid cross-field transport. Assuming that the q = 1 surface remains still during the ramp phase, it can be demonstrated that the ratio of the rates of rise, on the magnetic axis, of the current density and temperature is about 0.7. The paper compares theoretical predictions and observations for sawtooth precursors, relaxations and ramp phases.
Bibliographic Reference: Article: Proceedings of the 13th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Vol. 2 (1991) pp. 231-237
Record Number: 199210131 / Last updated on: 1994-12-02
Original language: en
Available languages: en