Enhanced H-mode by Boron Powder Injection

Boron powder injection has, for the first time on DIII-D, enabled enhanced H-mode pedestals with transient ELM-free periods (~200 ms), similar to previous results with lithium injection (e.g., Osborne et al. 2015), doubling the pedestal height and improving global energy confinement by over 60%. This regime was accessed across a moderate q95 window (3.5–4.4) and NBI power range (2.8–4.6 MW), achieving βN up to 2.2. During these phases, the pedestal width expands by approximately 60% compared to EPED predictions. The enhancement in pedestal structure grows with NBI power and remains robust within the q95 window.

Beam Emission Spectroscopy (BES) and Doppler Backscattering (DBS) diagnostics offer new insights into pedestal dynamics during low-Z powder injection, revealing low-frequency Bursty Chirping Modes near the separatrix and a separate 1.5 MHz mode located radially inwards from the separatrix. These observations suggest impurity-driven turbulence plays a key role in reshaping transport and stability boundaries. This result establishes that the benefits of low-Z impurity injection are not limited to lithium, suggesting that a broader class of materials could be effective for pedestal and confinement optimization in ITER and future pilot plants. A better understanding of edge turbulence during Enhanced H-mode provides new opportunities for pedestal control in forthcoming studies.