Effectiveness of wall conditioning by means of boron powder injection in DIII-D plasmas*

We present DIII-D experiments demonstrating effective wall conditioning by boron (B) powder injection into tokamak plasma discharges. In present-day fusion devices, boronization is commonly used to condition plasma-facing components, to allow operation with reduced wall fueling and impurity sources. Typically gaseous boron in the form of hazardous diborane is introduced in glow discharges. The beneficial effects last until the boron layers are eroded, challenging the scalability of the technique to steady state devices with high power and plasma exhaust rates. In a recent DIII-D experiment, increasing amounts of boron were injected during plasma discharges with identical target parameters. Metallic B powder (size < 0.1 mm) was injected to the upper scrape-off layer by a recently installed impurity powder dropper, for time intervals of 1‑3 s, at rates 5-100 mg/s. B powder injection correlated in a step-wise reduction of plasma density, neutral pressure and radiation from impurity radiation (C, N, O), all signs of improved wall conditions. The effectiveness of this technique for enabling access to low collisionality scenarios such as QH-mode will be discussed.