Global sawtooth instability measured by magnetic coils in the JET tokamakFunded under: FP2-FUSION 10C
This paper describes measurements of the sawtooth instability in JET, in which the instability wave function is shown to extend to the edge, where it is measured using magnetic coils. The numerous magnetic probes in JET allow the time evolution of the (n = 0, 1, 2, 3) toroidal Fourier components to be analysed. The n = 1 magnetic component is found to be similar to the m = 1 soft X-ray centroid motion. This indicates the potential of edge signals for retrieving the poloidal mode spectrum of the q = m / n = 1 surface. The spectrum evolution of the instability is compared for normal sawteeth (NST) and quasi-stabilised monster sawteeth (MST). The spectrum is slowly decreasing with n for NST and all the components belong to one ballooning-like deformation, whereas MST show a large n = 1 kink-like motion with small and independent accompanying higher n modes. Important equilibrium changes are detected during the growth of the instability and the growth rate is much faster than exponential, both facts implying the non-linear nature of the instability growth. Parametric dependence of growth rates, amplitudes and toroidal spectrum shape are studied to characterise the NST and MST instability.
Bibliographic Reference: Report: JET-P(91)43 EN (1991) 58 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199210210 / Last updated on: 1994-12-02
Original language: en
Available languages: en