2025 – Pellet dynamics and impurity transport resulting from PFC slag in DIII-D

Pellet dynamics and impurity transport resulting from PFC slag in DIII-D

2025 Research Campaign, Plasma-Material Interactions

Purpose of Experiment

The purpose of this experiment is to investigate the transport dynamics of dust grains and pellets in the scrape-off layer (SOL) and core plasma regions of DIII-D, with a focus on comparing behavior in both L-mode and H-mode discharges. By using a range of materials, including low-Z, mid-Z, and high-Z elements, we aim to simulate the transport of slag (eroded material from plasma-facing components) in reactor-relevant conditions. The experiment will provide key insights into how plasma conditions—particularly edge-localized modes (ELMs), SOL flows, and magnetic field configurations—affect the movement and deposition of these materials. A specific challenge addressed is the difficulty of delivering tungsten impurities inside the separatrix, which we will overcome by using specially designed carbon-shelled pellets with tungsten cores. This research will leverage advanced diagnostics and numerical modeling to reconstruct plasma conditions and predict material behavior, utilizing tools like UEDGE and DUSTT for trajectory simulations. The experiment’s results will help refine current models of dust transport, contributing to better understanding and management of impurity control in fusion reactors. Ultimately, the findings will inform strategies for minimizing the adverse effects of slag accumulation in next-generation fusion devices.

Experimental Approach

The experiment will explore the transport dynamics of dust grains and pellets in both L-mode and H-mode plasmas, all within the Lower Single Null (LSN) configuration of DIII-D. By varying the Neutral Beam Injection (NBI) power between 2.5-2.7 MW for L-mode and 3.5 MW for H-mode, and alternating the toroidal magnetic field (Bt) direction in H-mode shots, we aim to study how plasma conditions and drifts impact the movement of materials. Pellets and dust grains will be injected into the plasma to simulate slag transport, and their behavior will be tracked using advanced diagnostic tools. High-resolution diagnostics, such as Fastcam and IR cameras, will capture 3D trajectories and ablation histories, while other diagnostics will document plasma conditions for UEDGE and SOLPS modeling. The experimental results will be compared with DUSTT simulations to refine models of dust transport. Additionally, if time allows, the impact of varying ELM size and frequency on dust and pellet dynamics will be explored to gain further insights into impurity transport under fusion-relevant conditions.