Interstitial and plasma-facing sheath dynamics for volumetrically complex materials

Volumetrically complex materials (VCMs) reduce wall sputtering and secondary‑electron emission by 70–80%, yet their impact on sheath dynamics is unresolved. For high-density plasma, VCMs experience plasma infusion and related 3D interstitial sheaths that change the effective collection area and global non‑ambipolar transport. We analyze these behaviors by biasing carbon‑foam anodes to act as a live impedance probe. A 5 kHz, sinusoidal “tickle’’ superposed on the DC bias produces synchronous RF voltage and current whose amplitude–phase pair yields sheath capacitance C_s, thickness L_s, and complex impedance. In 10^-4 Torr Ar plasma, 10 and 40 PPI foams raise the plasma potential ≥ 20%, elevate local electron temperature T_e ≥ 2eV, and increase near‑anode electron density n_e, relative to a flat plate. Log–log C_s vs V_DC and current-voltage phase deviate from collision‑less Child-Langmuir scaling, implying an ensemble of interstitial micro‑sheaths that form, merge, and reconfigure along intra‑ligament pores, producing a 3-D, non‑uniform impedance landscape. Interrogating this link between porous topology and global non‑ambipolar transport enables a new perspective on plasma wall interactions for a wide range of applications including electric propulsion, fusion, and plasma-processing.