2025 – Impurity turbulent transport: SOL, pedestal, core predictions in ITER- and FPP-relevant low-grad-n plasmas

Impurity turbulent transport: SOL, pedestal, core predictions in ITER- and FPP-relevant low-grad-n plasmas

2025 Research Campaign, Turbulence and Transport

Purpose of Experiment

Understanding how impurities behave in fusion plasmas is essential for the success of ITER and future fusion power plants. High-Z impurities, such as tungsten (W), can accumulate in the plasma core, increasing radiation losses and degrading performance. This experiment on DIII-D will investigate how turbulent and neoclassical transport mechanisms govern impurity behavior in reactor-relevant conditions. By systematically injecting multiple impurity species, we will assess how impurity transport varies with charge state (Z-scaling) and plasma parameters. The study will also explore pedestal impurity screening to determine how well high-Z impurities are expelled before reaching the core. The experiment takes advantage of DIII-D’s hybrid scenario, featuring small, grassy ELMs and improved confinement. Advanced diagnostics, including charge exchange recombination spectroscopy (CER), bolometry, and Thomson scattering, will provide high-resolution impurity measurements. The data will be compared to state-of-the-art gyrokinetic and neoclassical transport models to refine predictions for ITER. These findings will improve impurity transport models and help develop impurity control strategies for next-generation fusion reactors. The results will be crucial for optimizing plasma scenarios in preparation for DIII-D upcoming transition to a tungsten wall.

Experimental Approach

Step 1: Start from a Reference Shot (200436) (2-4 Shots) We begin by reproducing the reference hybrid scenario (shot 200436), establishing a stable plasma condition for impurity transport studies. The primary objective is to extend the heating duration to 6.0s while ensuring the presence of a stable 3/2 mode. If necessary, ECH heating locations will be adjusted to optimize plasma stability. Early testing of the Laser Blow-Off (LBO) system will be conducted, focusing on measuring Ca18+ transport using CER chords (CT1 and CT2). To ensure proper alignment with diagnostic timing, the MICER integration time will be increased from 5ms to 10ms. Step 2: Suppress Type-I ELMs with RMP (2-3 Shots) With the reference hybrid scenario established, we will apply resonant magnetic perturbations (RMPs) from 3.0s onward to suppress type-I ELMs. This setup will mirror the hybrid discharge 199099, ensuring consistent plasma conditions. An outer gap sweep (+/-15mm at 5Hz, 3.0-6.0s) will be performed to evaluate its effect on edge impurity transport. Initial impurity transport measurements will be conducted using CER at C6+, followed by configuring the diagnostic setup for argon (Ar) CER measurements. Ar gas will be injected via gas puffing (3e20 particles/s at 3.5s and 3.7s) to assess pedestal impurity screening efficiency. Step 3: Power Scan to Assess Ion Temperature Dependence (3-4 Shots) To evaluate the role of ion temperature (Ti) on impurity transport, we will perform a power scan by varying NBI power / beta feedback target from 8MW to 12MW over two shots. The goal is to examine how changes in Ti gradients impact impurity diffusion and peaking. ECH power will remain constant to isolate the effects of Ti variation. Maintaining a consistent torque-to-power ratio, we will investigate impurity transport behavior under different Ti profiles. The Ar puff and outer gap sweep will be retained from the previous step. CaF2 LBO injections at 3.5s, 4.2s, and 4.9s will be introduced to examine charge-state-dependent transport. Background subtraction for impurity diagnostics will be performed by incorporating blips in 30L and 330R beams for CER and MICER measurements.

Interested in a behind-the-scenes look at DIII-D? Join us for a virtual OR in-person tour during Fusion Energy Week (May 5-9)! Sign up for a tour here.

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