Molybdenum density profiles in Alcator C-Mod plasmas with implications for tungsten X-ray spectroscopy on SPARC

Understanding high-Z impurity peaking in current devices (Alcator C-Mod and ASDEX) can provide some insight into what might be expected in ITER and SPARC [1,2]. Here we present Molybdenum (Mo, Z=42) density spatial profiles obtained for C-Mod L-, I- and H-mode plasmas from X-ray observations of the neon-like Mo32+ 4D transition [3]. C-Mod Mo profile shapes range from flat to modestly peaked in the core. The observed peaking factors are in good agreement with those calculated by TGLF across all confinement regimes. There is little dependence seen with core electron density gradient, sawtooth amplitude or rotation velocity. Predictions for Tungsten (W, Z=74) peaking in SPARC using TGLF are presented and are in good agreement with previously published results using CGYRO [4]. Low-resolution survey spectrometers are being designed for SPARC to monitor X-ray emission from argon- to neon-like W (W56+ to W64+) and hence potential W accumulation in the core [5]. The projected high neutron flux environment limits the allowable lines-of-sight into the machine. This adds a challenge to do a similar analysis as was done for C-Mod, so we scope inferring W density peaking in SPARC.



Work supported by Commonwealth Fusion Systems RPP031.



[1] D. Fajardo et al., Nucl. Fusion 64, 104001 (2024)

[2] D. Fajardo et al., Plasma Phys. Control. Fusion 67, 015020 (2025)

[3] C. Perks et al., Submitted to Nucl. Fusion (2025)

[4] N.T. Howard et al., Phys. Plasmas 28, 072502 (2021)

[5] C. Perks et al., Rev. Sci. Instrum. 95, 083555 (2024)