Predictive transport modelling and MHD stability analysis of mixed type I-II ELMy H-mode JET plasmas

Mixed type I-II ELMy H-mode, a mode operation with small, frequent type II edge localized modes (ELMs) interrupted by occasional large type I ELMs, has been observed in various experimental situations. This paper combines two simple models for type I and type II ELMs, respectively (used e.g. in Lönnroth et al 2003 Plasma Phys. Control. Fusion 45 1689) into an improved scheme for modelling of mixed type I-II ELMy H-mode, which has been implemented in the 1.5D core transport code JETTO together with simple schemes for modelling of pure type I and type II ELMy H-modes based on the same ideas. In the ELM modelling, transport during the ELMs is enhanced by edge-localized radially Gaussian-shaped perturbations to the transport coefficients. Type I and type II ELMs are represented by perturbations with different widths and amplitudes and controlled by different stability limits derived from magnetohydrodynamic (MHD) stability analysis. Some justification from theory and numerical analysis is given for the representation of each ELM type. Predictive transport simulations with JETTO demonstrate that the experimental dynamics of mixed type I-II ELMy H-mode can be qualitatively reproduced using the present model. For completeness, the modelling of mixed type I-II ELMy H-mode is compared with reference simulations of pure type I and pure type II ELMy H-mode and the differences, e.g. in confinement are explained. In addition, this paper presents the results of MHD stability analysis of a number of situations experimentally found to be favourable for the occurrence of type II ELMs, namely situations with strong external neutral gas puffing, a quasi-double-null magnetic configuration, high poloidal bgr (ratio of the total pressure to the kinetic pressure) and combinations of high edge safety factor q95 and high triangularity dgr. The results of the analysis of the given scenarios are such that the model used in this paper can explain why these situations can be favourable for mixed type I-II or pure type II ELMy H-mode.