Exploring the physics of a high-performance H-mode with small ELMs and zero gas puffing in JET-ILW

Recent experiments in JET-ILW have been successfully exploring a high-performance H-mode scenario with no gas dosing at low q95 (Ip = 3 MA, Bt=2.8 T, q95 = 3.2) and low triangularity, with peak neutron rates reaching values of 3.6×1016 s-1. This was enabled by operation at very low gas fueling, which is challenging in JET with the metal wall due the need to control the W influx into the core region. By starting H-mode operation at high density, applying a high level of gas injection early during the NBI heating phase to avoid ELM-free phases, it was possible to reduce the gas puffing to very low levels (? 1021e/s), achieving a high performance, low density regime (called no-gas regime in the rest of the text) with averaged ne ? 3 × 1019 m-3 and Greenwald fraction of ?0.35, amongst the lowest ever achieved in JET-ILW. Operation at such low densities allows decoupling ions from electrons, resulting in higher Ti/Te than what obtained in conventional ELMy H-modes at higher densities and similar heating power.