Saturated cavity ringdown spectroscopy at Balmer-α line of atomic hydrogen for estimating sheath electric field in plasma

We applied saturated cavity ringdown spectroscopy (CRDS) to the measurement of the Doppler-free absorption spectrum of the Balmer -a line of atomic hydrogen in an inductively coupled hydrogen plasma. The spectrum was used for estimating the electric field in the sheath region in the vicinity to a biased electrode. The absorption frequency (the absorption coefficient multiplied by the speed of light) and the saturation parameter were estimated by fitting the experimental ringdown curve with the theory reported by Giusfredi and coworkers (Phys. Rev. Lett. 104 (2010) 110801). We detected the Lamb dip corresponding to the 2p2Po3/2 - 3d2D5/2 transition in the absorption spectrum in the field-free condition. We observed the change in the Lamb dip spectrum when we repeated the measurement in the sheath, and we succeeded in estimating the sheath electric field of 220-230 V/cm at a distance of 1.8 mm from the electrode which was biased at -100 V with respect to the ground potential. The experimental results indicate the potential of saturated CRDS for the measurement of sheath electric fields in plasmas.

Automated summary

In this study, saturated cavity ringdown spectroscopy (CRDS) was used to measure the Doppler-free absorption spectrum of atomic hydrogen in an inductively coupled hydrogen plasma. The spectrum was used to estimate the electric field in the sheath region near a biased electrode. The experimental results demonstrate the potential of saturated CRDS for measuring sheath electric fields in plasmas.