In magnetic confinement fusion, we rarely measure physical quantities directly; instead, we measure their indirect signatures -photons, currents, and phase shifts. Bridging the gap between these raw signals and the physical parameters of interest (such as density or temperature) has traditionally relied on the inverse problem, a mathematical reconstruction process often plagued by ambiguity and ill-posedness. This tutorial advocates for a paradigm shift toward forward modeling via Synthetic Diagnostics. By processing simulation data through the same transfer functions as experimental instruments, we can translate theoretical predictions into synthetic signals, enabling a rigorous comparison with experiment. We formalize this approach within the TWINTOK framework, a Digital Twin architecture designed for systematic validation. Through detailed case studies -ranging from the spectroscopic evaluation of divertor detachment to the full-wave modeling of turbulence staircases -we demonstrate how synthetic diagnostics reveal physics that standard inversions often miss, such as the significant contribution of wall reflections to bolometric signals. As the community moves toward ITER and fusion power plants, where diagnostic access will be limited and interpretation challenging, this forward modeling approach provides the essential link to validate our predictive capabilities.
This chapter presents the activity conducted by the ITPA topical group (TG) on Diagnostics over about the last 15 years. Following a general introduction of the ITER Diagnostics led by their measurement roles, the document is organized in several subchapters detailing the design support, research and development activity conducted by each of the specialist working groups (WGs) of the TG. Please note that the magnetic diagnostics were supported at the TG without a specific WG. Their status is included in the general introduction. In the following some highlights of the subchapter's contents are provided. Recent advances in ITER first wall (FW) diagnostics for the measurements of plasma-metallic wall interaction in support of the ITER research plan are reported. An InfraRed imaging Video Bolometer for ITER has been developed and tested on several tokamaks to measure the radiated power loss. A laser-induced breakdown spectroscopy (LIBS) technique which utilizes a pulsed laser beam to ablate locally by Nucl. Fusion 65 (2025) 113001 Review inconsistencies. Physics-based modeling and parameter relationships provide additional information improving the treatment of ill-posed inversion problems. A coherent combination of all kind of available information within a probabilistic framework allows for improved data analysis results. The concept of integrated data analysis (IDA) in the framework of Bayesian probability theory is outlined and contrasted with conventional data analysis. Components of the probabilistic approach are summarized and specific ingredients beneficial for data analysis at fusion devices are discussed. This paper is part of the Special Issue: On the Path to Tokamak Burning Plasma Operation: A collection of papers prepared by the ITPA Topical Physics Groups reviewing progress in the development of the physics basis for burning plasma operation.