Description
Operation of future tokamak reactors relies on achieving high-confinement scenarios without unacceptably high transient heat fluxes due to type-I ELMs. ELM-free H-modes (such as QH-mode, EDA H-mode, and others) represent a possible solution for avoiding giant ELMs. The transition to H-mode can occur via an intermediate state known as I-phase, characterised by limit-cycle oscillations (LCOs). The power necessary for the transition to H-mode is one of the uncertainties in the design of a fusion reactor. Pilot plant designs envisage operation close to this threshold, making the power threshold a critical design parameter.
We present analysis of the fluctuation dynamics during I-phase and ELM-free H-mode on the spherical tokamak (ST) ST40. The LCOs in I-phase are a feature of predator–prey dynamics between turbulence and sheared flows. We find that bursty LCOs characteristic of I-phase are also observed in many ELM-free H-modes. In both I-phase and H-mode, non-linear coupling between LCOs and turbulence is found using bispectral analyses of magnetic probe signals. The ELM-free H-modes are typically characterised by the presence of a quasi-coherent mode detected in the magnetic probe signals, which is likely responsible for enhanced edge transport. The LCO frequency exhibits dependence on edge electron temperature, q95, and power crossing the separatrix.
The H-mode power threshold studies were performed using I-phase periods with and without the H-mode transition. The power threshold was found to be several factors above predictions of the ITPA scaling [1], similar to findings on other STs. We find that the power threshold is closer to the scaling that account for low-aspect-ratio effects and Zeff [2], but still above the predictions. We present dependencies of the power threshold and compare I-phases with and without the H-mode transition.
References
[1] Martin Y. et al 2008 J. Phys.: Conf. Ser. 123 012033
[2] Takizuka T. 2004 PPCF 46 A227