Importance of axi-symmetry of the 1/1 kink mode (2-D or 3-D) in determination of the reconnection time scales in the sawtooth instability

The recent study demonstrated that the “full reconnection model” originally proposed by Kadomtsev for sawtooth instability in tokamak is indeed the correct model [1]. This model was suspected due to the dominantly observed fast reconnection time and measurements of the core current density changes. The measured q0 by MSE for sawtoothing plasmas on KSTAR was ~1.0±0.03, but it was difficult to conclude any outcome, since the change of the q0 (or core current density) is too small compared to the instrumental error to have a definitive proof. Therefore, a supplementary double tearing mode experiment validated that the q0 had to be above ~1.0 after the crash. Another unresolved issue is the fast reconnection time which has been an order of magnitude faster than the prediction of the model. In 2-D experimental observation by ECEI system indicates that the dominant fast reconnection process is due to non-axisymmetry (i.e. 3-D) whereas the rarely observed axisymmetric reconnection cases (i.e. 2-D) have an order of magnitude slow reconnection process. The reconnection time can be controlled by perturbing the q~1 surface with the local current drive (ECH/CD). Also the non-axisymmetric cases have thin current sheet which resembles Petcheck model while the axisymmetric cases are similar to the Sweet-Parker model with wide current sheet.

[1] Y.B. Nam, et al., Validation of the “full reconnection model” of the sawtooth oscillation, to be published in Nuclear Fusion, 2018.