Conceptual design of high-dynamic-range spectroscopy for neutral flux measurements in SPARC

We present a conceptual design for High-Dynamic-Range Spectroscopy (HDRS) to measure the penetration of neutrals into the confined plasma of the SPARC tokamak. HDRS detects these penetrating neutrals—hereafter core neutrals—in the far wings of the Balmer-series spectrum, rather than relying on spatial inversion, which is strongly affected by scrape-off layer (SOL) emission. These neutrals undergo charge exchange with high-temperature confined ions, producing significantly greater Doppler broadening than the SOL neutral emission. While this diagnostic concept has been applied to multiple devices, applying it to SPARC will pose unique challenges due to the device’s high density and strong magnetic field. Stark broadening at divertor densities exceeding 1021 m−3 and Zeeman splitting at the ∼12 T magnetic field both broaden the edge emission toward the wavelengths where the core-neutral emission appears, and the metallic first wall can reflect bright divertor light into core-viewing sightlines. To quantify these effects, we developed a framework that couples plasma backgrounds from the scrape-off layer plasma simulation-ITER code, OpenADAS atomic data, and Cherab/Raysect ray tracing and used it to produce line-integrated spectra for illustrative chords and derive preliminary spectrometer requirements. The synthetic spectra identify a Dα core-neutral shoulder visible around 1–3 nm away from the line center, embedded in the Stark tail of the cold edge emission; Dγ, by contrast, remains Stark-dominated and provides an independent constraint on the edge electron density. The resulting spectrometer concept targets a resolving power of 20 000–40 000, a 7.5–15 nm bandwidth, and a 105–106:1 dynamic range.