Laser absorption spectroscopy for measurement of He metastable atoms of a microhollow cathode plasma

Abstract

We generated a 0.3-mm-diameter DC, hollow-cathode helium discharge in a gas pressure range of 10-80 kPa. In discharge plasmas, we measured position-dependent laser absorption spectra for helium 2 3 S 1 -2 3 P 0 transition with a spatial resolution of 55 µm. From the results of the analysis of the measured spectra using Voigt functions and including both the Doppler and collision broadening, we produced two-dimensional maps of the metastable 2 3 S 1 atomic densities and gas temperatures of the plasmas. We found that, at all pressures, the gas temperatures were approximately uniform in space with values in the range of 400-1500 K and the 2 3 S 1 atomic densities were >10 19 m %3 . We also found that the two-dimensional density distribution profiles became ring-shaped at high gas pressures, which is qualitatively consistent with the two-dimensional fluid simulation results.

Spatially Resolved Measurements of Metastable Atom Density and Electric Field Strength in a Microhollow Cathode Helium Plasma by Laser Absorption Spectroscopy

“…However, the discharge voltage and the plasma parameters of the current results that are described in Sec. 3 are quantitatively different from those that were produced previously [5,6] even though the discharge setup is the same in both the experiments. This is because the surface conditions of the electrode walls are observed to influence the discharge condition.…”

“…2. We evaluate n 23S from S 1 and the gas temperature T g from Δ D [5]. Here, we assume the absorption length to be 4 mm based on the thickness of the electrodes.…”

“…The main mechanism for sustaining the plasma is the backand-forth motion (pendulum motion) of electrons that are emitted from the cathode wall and ionization by the accelerated electrons in the sheath region [2]. The typical gap voltage values between anode and cathode at cathode-hole diameters of smaller than 1 mm ranges between ∼200 and 300 V [3][4][5][6].…”

“…The plasma parameters in MHCD have been estimated using the emission spectroscopy [3,4], laser absorption spectroscopy [5,6] and numerical simulations [7]. Lazzaroni et al obtained the two-dimensional images of the emission intensities of Ar 5p[3/2] 1 -4s[3/2] 1 (427.2 nm) and Ar + 4p 2P 3/2 -4s 2D 5/2 (427.8 nm) lines in 0.4-mmdiameter argon MHCD plasma in the pressure range of 4 -27 kPa [3].…”

“…Applying laser absorption spectroscopy on the He 2 3 S-2 3 P (1083 nm) transition in 0.3-mm helium MHCD plasma at 10 -80 kPa [5], it was observed that the two-dimensional profile of the helium metastable (2 3 S) became ring-shaped at a high pressure of greater than 60 kPa. Kothnur et al performed a two-dimensional fluid simulation of 0.2-mm helium MHCD plasma at 40 and 133 kPa [7], and the results were qualitatively consistent with our previous experimental results [5]. Furthermore, the 2 1 P atom density, n 21P , and electric field strength, E, were evaluated in 0.3-mm helium MHCD plasma at 10 kPa based on the laser absorption spectra of the 2 1 P-3 1 D [6].…”