Electric field measurement in a continuous RF plasma using Rydberg-EIT

Non-perturbative measurement of plasma electric fields is essential for modeling sheath dynamics and impurity transport, yet conventional optical diagnostics generally lack the sensitivity required to probe low-pressure (sub-Torr) discharges. We leverage Rydberg Stark shifts for in situ electric-field sensing with ~V/cm sensitivity in an inductively coupled (ICP) rf plasma generated in a few mTorr of Argon. Rubidium tracer vapor seeded into the plasma and interrogated via Electromagnetically Induced Transparency (EIT) enables field measurement within the plasma bulk. In the absence of plasma, the EIT spectra exhibit signatures of both rf electric and magnetic fields caused by RF Zeeman and Rydberg Stark effects, which are comparable inside rf coils (such as ICP drive coils). Upon plasma ignition, RF modulation sidebands are suppressed due to the screening of the rf drive field from the plasma interior. The resulting EIT line shapes, governed by the Holtsmark microfield distribution, are analyzed for quantitative determination of plasma density and collisional dephasing rates as a function of gas pressure and RF drive power. We further report progress toward spatially resolved measurements of sheath electric fields at plasma-dielectric interfaces. The work is expected to find applications in fundamental studies, such as modelling of dusty and magnetized plasmas, as well in technological applications, such as materials processing.