Stark spectroscopy at Balmer-αline of atomic hydrogen for measuring sheath electric field in a hydrogen plasma

Abstract

This paper reports a diode laser based system which is applicable to the measurement of electric field in the sheath region of a hydrogen plasma. The electric field is deduced from the Stark spectrum of the Balmer-α line of atomic hydrogen. Saturation spectroscopy with a Doppler-free spectral resolution is adopted to detect the Stark effects of the low energy states. We have demonstrated a detection limit of 10 V/cm, which is a sufficient sensitivity for investigating the structures of the sheath electric fields in low-temperature plasmas. We have discussed the detection limit, the measurement ambiguity, the spatial resolution, and the limitation of the developed method.

Estimation of sheath electric field in inductively coupled hydrogen plasma on the basis of Doppler-broadened absorption spectrum of hydrogen Balmer-α line

“…We have succeeded in detecting weak electric field of ∼ 10 V/cm in the sheath by employing the Balmer-α line of atomic hydrogen. 36) This sensitivity is comparable to that realized by laser-induced fluorescence-dip spectroscopy, [28][29][30][31][32][33][34] where Rydberg states with principal quantum numbers up to n = 40 − 55 are detected. However, the Stark spectroscopy of lower-lying states combined with saturated absorption spectroscopy is problematic in strong electric field (≥ 500 V/cm).…”

“…In a previous work, we reported the measurement of the electric field in the same plasma source by saturated absorption spectroscopy. 36) The electric field at a distance of 0.5 mm from the grounded electrode was approximately 25 V/cm. The electric field increased with the electrode potential monotonically, and they were ∼ 600, ∼ 850, and ∼ 1200 V/cm at electrode potentials of -40, -80, and -120 V, respectively, when the distance between the electrode surface and the measurement position was 0.4 mm.…”

“…The Dopplerbroadened absorption spectrum was measured by standard laser absorption spectroscopy. The conclusion is that the method works in electric fields higher than ∼ 350 V/cm, when the translational temperature of atomic hydrogen is 400-500 K. This sensitivity is worse than those of laser-induced fluorescence-dip spectroscopy [28][29][30][31][32][33][34] and saturated absorption spectroscopy, 35,36) but it is comparable to optogalvanic spectroscopy 24,25) and laser-induced collisional fluores-6/?? cence spectroscopy.…”

White paper on the future of plasma science for optics and glass

“…that occur due to the interaction of plasmas with other materials, and hence to focus on the fingerprints of plasma‐surface interactions . Characterization‐ and control‐related diagnostics are performed by means of Langmuir probes, laser‐based techniques, optical emission spectroscopy (OES), infrared and Fourier transform infrared absorption spectroscopy, mass spectrometry, microwave interferometry, and radio frequency diagnostics . Since low‐temperature, non‐equilibrium, and partially ionized plasmas are characterized by cold electrons (with a mean energy of bulk electrons of a few eV) dedicated methods are necessary for the qualitative/quantitative monitoring of neutral/charged atomic/molecular species and electric fields, over a wide range of pressures, electrode geometries, and gaseous mixtures .…”

Editorial for plasma diagnostics using spectroscopic methods

“…However, the measurement of electric fields in plasma is still difficult, and until recently the methods developed need expensive laser sources and sophisticated experimental skills. Nishiyama et al report on a new method for measuring sheath electric fields in hydrogen plasma [1]. They apply saturation spectroscopy to obtain the Stark spectrum of the Balmeralpha line of atomic hydrogen, and deduce the sheath electric field with a detection limit of 10 V cm −1 .…”