The scaling of sawtooth parameter and the occurrence of single sawteeth in the start-up phase of TEXTOR
For a theoretical description and a physical understanding of sawtooth oscillations in tokamaks, it is necessary to be familiar with the experimental observations and the empirical scaling relations. Characteristic of sawtooth oscillations is the simultaneous loss of energy and particles from the plasma centre and the redeposition in the outer plasma. Between central crash and redeposition there exists a surface within the plasma, where the sawtooth phase change is zero. This defines the inversion radius. For regular sawteeth, the inversion radius is independent of density at constant current, but it scales as the q=1 radius of the normalised current density profile. The inversion radius is typically 15% smaller than the q=1 radius. However, a relationship between inversion radius and q=1 radius does not exist in the case of single sawteeth, which are observed in the start-up phase of TEXTOR, before the onset of regular sawtooth oscillations. The inversion radius for single sawteeth at early times in the discharge is large and reduces successively until the regular sawteeth start. The linear increase of the sawtooth period with density is a well known experimental scaling law. With boronised walls in TEXTOR, the linearity is only observed at low densities. At higher densities the sawtooth period becomes smaller again.
Bibliographic Reference: Article: 18th European Conference on Controlled Fusion and Plasma Physics, Vol. II (1991) pp. 13-16
Record Number: 199111386 / Last updated on: 1994-12-02
Original language: en
Available languages: en