Speaker
Description
Boron remains a key material for real-time wall conditioning in fusion devices due to its strong gettering properties and its impact on impurity control and hydrogen recycling. Previous LHD experiments have identified boron monohydride (BH) emission as a sensitive indicator of surface processes during boron powder injection, with localized signatures near the divertor and rotational temperatures around 3600 K.
In this study, we extend earlier work by analyzing a new dataset obtained on the Large Helical Device with two magnetic-axis configurations and systematic scans of heating power and plasma density. Spectrally resolved BH emission was recorded with two complementary instruments:
(1) a multi–line-of-sight visible spectrometer used in previous campaigns, enabling spatially resolved characterization of the divertor region; and
(2) a high-resolution echelle spectrometer providing access to BH together with additional species such as H₂ (Fulcher-α), although the latter is still under evaluation.
Preliminary analysis of the visible-range data shows that the BH Q-branch remains strongly localized near the divertor for all operational conditions studied, consistent with surface-associated production mechanisms. Rotational temperatures are obtained by generating a synthetic BH spectrum using tabulated transition coefficients and fitting the calculated Q-branch envelope to the measured spectra. The resulting temperatures are similar to earlier results (≈3.3–3.7 kK), with only weak dependence on magnetic-axis position, density, or heating power. The persistence of these high rotational temperatures suggests that BH formation involves energetic surface-driven chemistry rather than purely sputtering-based processes.
These new results indicate that BH emission retains clear diagnostic sensitivity to wall interaction processes across different magnetic configurations and plasma conditions in LHD. Completion of the echelle-based analysis—particularly including H₂ Fulcher-α rotational temperatures—will further clarify the chemical and thermal environment of boron-containing species in the divertor region.