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Description
Quantification of shine-through (not absorbed by the plasma) heat flux from neutral beam injection (NBI) is essential for protecting plasma-facing components (PFCs). In this work, we present the direct measurement of heat flux deposition from NBI shine through on the DIII-D first wall using newly implemented, high temperature capable Surface Eroding Thermocouple (SETC) sensors [1]. The SETCs were installed on strategically selected high field side center-post tiles with direct line-of-sight to the 30R neutral beam. These tiles are adjacent to the Low Hybrid Current Drive (LHCD) antenna, a component known to be highly sensitive to localized heat loading. The SETCs provide real-time monitoring of surface temperature and incident heat flux, enabling improved protection of the LHCD antenna during high power beam operation.
During the beam-into-gas shots (no plasma), the SETCs measured peak heat fluxes of 20–40 MW/m², corresponding to surface temperature rises exceeding 300 °C in only 8 milliseconds. In plasma discharges, the measured shine-through heat flux shows a strong dependence on the plasma line-averaged density. Higher density plasmas ionize a larger fraction of the injected neutrals, thereby substantially reducing the shine-through power reaching the wall. For discharges with neutral beams at 75 keV injection energy, the measurements indicate that the plasma density of approximately (2–3) × 10¹³ cm⁻³ is required to reduce the shine-through fraction by 50%. The local heat fluxes measured by SETC were quantitatively compared with the simulated power deposition of neutral beams. The shine through power is calculated by the pencil beam attenuation model [2] and the heat deposition is further mapped to the first-wall on the center post, following the beam footprint associated with the NBI divergence. Furthermore, a one-dimensional slab model is used to estimate the first wall temperature from the projected power, enabling a direct comparison between the simulation with the measurement at the SETC locations. The calculated heat flux footprint and the simulated temperature responses agree with the SETC measurements within 20%.
This work demonstrates that the SETC system is robust and reliable to monitor the neutral beam shine-through power. These measurements provide new insight into beam–wall interactions in high-power scenarios and offer a practical pathway for validating NBI deposition models and guiding future wall-protection designs.
Work supported by US DOE under DE-FC02-04ER54698, DE-SC0023378.
[1] J. Ren et.al. Rev Sci Instrum, 93, 103541 (2022), doi: 10.1063/5.0101719
[2] M. A. Van Zeeland et.al. Plasma Phys. Control. Fusion 52, 045006 (2010) doi: 10.1088/0741-3335/52/4/045006