Speaker
Description
The Thermal Helium Beam (THB) diagnostic provides simultaneous measurements of electron temperature ($T_{e}$) and density ($n_e$) profiles in the plasma edge and scrape-off layer (SOL) using helium line ratios interpreted through a collisional–radiative model (CRM). For the RFP experiment RFX-mod2, a new-generation THB system has been designed and experimentally validated at the TCV tokamak. The diagnostic extends the conventional three-line configuration (667.8, 706.5, 728.1 nm) with a fourth He I emission line at 501.6 nm, enabling direct assessment of radiation re-absorption effects.
This upgrade is essential in the far SOL, where radiation trapping under conditions of high neutral helium density and low $T_{e}$ and $n_{e}$ can significantly affect the line intensities. For the first time in a tokamak, the impact of radiation re-absorption on $T_{e}$ and $n_{e}$ reconstruction has been experimentally quantified, demonstrating that its inclusion in the CRM is necessary for accurate edge profile measurements.
Beyond steady-state reconstruction, the THB enables time-resolved studies of edge dynamics. In Type-I ELMy H-mode plasmas at TCV, coherent inter-ELM fluctuations, mainly associated with density perturbations, were detected and radially localized. The diagnostic also allowed investigations of profile evolution during the ELM cycle and of turbulence-driven transport.
The validated four-line THB thus provides a robust tool for edge kinetic, fluctuation, and turbulence studies. In RFX-mod2, it will support accurate profile characterization and their temporal evolution, as well as poloidal profile reconstructions, contributing to a deeper understanding of edge transport and plasma-wall interaction processes.