Sawtooth oscillations were investigated using an electron cyclotron emission imaging diagnostic technique on the HT-7 tokamak. High-order harmonic modes are observed in sawtooth precursors. They cause sharp pressure points, leading to the occurrence of reconnection events at more than one place, and which are not preferential on the low field side of the q ∼ 1 radius at low density. The reconnection proceeds in two stages. In the first stage, a weak reconnection happens which is characterized by a slow emergence of a small amount of heat and particles through the finite openings. Subsequently, in the second stage, most of the remaining heat and particles in the core escape outward. The observations at low density indicate that high-order harmonic modes are responsible for the secondary reconnection stage, which has seldom been mentioned in reconnection model. The characteristics of the modes are then described by investigating a large number of sawteeth oscillations with different densities and qa values. It is found that when the density increases above a threshold at qa = 3.3, the m/n = 1/1 mode amplitude significantly exceeds its harmonics. The threshold decreases as qa increases.
Endogenous and asymmetric magnetic reconnection with associated processes of relevance to fusion burning plasmas