Understanding Carbon Sourcing and Transport Originating from the Helicon Antenna Surfaces During High-Power Helicon Discharge in DIII-D Tokamak

TL;DR

This work employs the STRIPE framework to quantify carbon sourcing and transport from RF-induced sheath effects near a high-power helicon antenna in DIII-D. By integrating SOLPS-ITER background plasmas, COMSOL-based sheath potentials, RustBCA sputtering yields, and 3D GITR/GITRm impurity transport, the study compares two DIII-D H-mode discharges with different antenna–plasma gaps and RF powers, revealing that carbon erosion is dominated by self-sputtering at multi-keV sheath energies, with D$^+$ ions contributing a small, but non-negligible, portion of erosion. The large-gap case shows lower total erosion and weaker core penetration, while the small-gap case exhibits higher erosion and greater potential for core impurity transport; however, experimental data indicate no significant rise in core carbon during helicon operation in graphite-wall configurations. The results underscore the need for sheath-aware antenna designs and predictive impurity transport modeling to mitigate PMI risks for future high-$Z$ wall materials in fusion devices, especially under conditions of tight plasma coupling and elevated RF power.

Abstract

The high-power helicon wave system in the DIII-D tokamak introduces new plasma--material interaction (PMI) challenges due to rectified RF sheath potentials forming near antenna structures and surrounding tiles. Using the STRIPE modeling framework-which integrates SOLPS-ITER, COMSOL, RustBCA, and GITR/GITRm-we simulate carbon erosion, re-deposition, and global impurity transport in two H-mode discharges with varying antenna--plasma gaps and RF powers. COMSOL predicts rectified sheath potentials of 1-5 kV, localized near the bottom of the antenna where magnetic field lines intersect at grazing angles. Erosion is dominated by carbon self-sputtering, with RF-accelerated D+ ions contributing up to 1 % of the total erosion flux. GITRm simulations show that in the small-gap case, only ~ 13 % of eroded carbon is re-deposited locally, with 58 % transported into the core. In contrast, the large-gap case exhibits lower total erosion, along with reduced core penetration (~ 35 %) and weaker re-deposition (~ 4 %), consistent with lower collisionality and limited plasma contact. The simulation trends are consistent with experimental observations, which have not shown elevated core impurity levels during helicon operation in the present graphite-wall configuration. However, under certain plasma conditions and magnetic configurations, the helicon antenna may still act as a finite source of net erosion and core-directed impurity transport, potentially influencing the overall core impurity balance. These findings emphasize the need for sheath-aware antenna designs and predictive impurity transport modeling to support future high-power RF systems with high-Z first wall materials in fusion devices.

Figures (24)

• Figure 1: STRIPE workflow for modeling RF-induced erosion and impurity transport originating from RF antenna structures, coupling SOLPS/SolEdge3x, COMSOL, RustBCA, GITR/GITRm, and ColRadPy.
• Figure 2: EFIT reconstructions of poloidal flux surfaces for two DIII-D discharges with different antenna-plasma gaps: #196154 (larger gap $\sim 7$ cm) and #200882 (smaller gap $\sim 4$ cm). The change in LCFS position relative to the helicon antenna modifies the sheath-connected surface area and plasma exposure, affecting RF sheath formation and impurity sourcing.
• Figure 3: Time traces of (a) klystron forward power, $\mathrm P_{forward}$ (HK1KDCFWD) and (b) coupled power, $\mathrm P_{coupled}$ (TWAPWRC) for DIII-D discharges #196154 (black) and #200882 (red). Discharge #196154 operated with higher forward power (550 kW) and a larger antenna–plasma gap, while #200882 exhibited closer plasma contact but lower power (800 kW).
• Figure 4: Time traces of power and carbon impurity signals. (a, c) Total injected power (NBI, ECH, Ohmic, and Helicon) and radiated power for discharges #196154 and #200882, respectively. Green trace shows helicon coupled RF power. (b, d) Corresponding time evolution of line-integrated carbon impurity emission. No significant rise in carbon levels is observed during helicon operation, indicating minimal global impurity response.
• Figure 5: Comparison of SOLPS-ITER simulations and Thomson scattering measurements for DIII-D discharges #196154 and #200882. (a) Electron density $n_e$ and (b) electron temperature $T_e$ for discharge #196154; (c) $n_e$ and (d) $T_e$ for discharge #200882. Experimental data (black circles with error bars) are $\tanh$ fit (orange), while SOLPS-ITER results are shown as blue curves. The profiles show strong consistency across the separatrix and into the SOL in both discharges.