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

Abstract

We apply laser absorption spectroscopy to direct-current hollow-cathode helium plasma generated in a 0.3mm-diameter cylindrical hole at gas pressures of 10-80 kPa. The measured transitions are 2 3 S-2 3 P and 2 1 P-3 1 D. We evaluate the helium metastable 2 3 S density and gas temperature using the former transition and the 2 1 P atom density and electric field strength from the latter one. The observed lateral distributions of the 2 3 S and 2 1 P atom densities depend on the gas pressure, which results from the sheath length estimated based on the spatial distribution of the evaluated electric field strength.

Spatially resolved laser absorption spectroscopy on a micro-hollow cathode He plasma

“…At 20-60 kPa, both n 23S and n 21P are locally high in the vicinity of the cathode-wall. In our previous results at the cathode-hole diameter of 0.3 mm, and at 10-80 kPa, 27) it is found that a hole appears in the structure of n 23S and n 21P near the center at high pressures and this may be due to the shrinking sheath length. This is consistent with the present results at the cathode-hole diameter of 1.0 mm.…”

“…The values are roughly a factor 5-10 lower than those of the results at the cathode-hole diameter of 0.3 mm. 22,27) At 5 kPa and 10 kPa, both n 23S and n 21P exhibit peaks at the center. At 20-60 kPa, both n 23S and n 21P are locally high in the vicinity of the cathode-wall.…”

“…34) The sheath thickness is ∼50-100 μm at 40 kPa in the case of the cathode-hole diameter of 0.3 mm. 27) Figure 6 depicts a ratio of the value of n 23S,21P at (x, y) = (0, 431 μm) to that at (x, y) = (0, 0 μm) (defined as R), at 5-60 kPa. At 5−20 kPa, R of n 21P is higher than that of n 23S .…”

“…A 1-mm-thick ceramic insulator plate separates a 2-mm-thick cathode and a 2-mm-thick anode, each of which has a 1.0 mm diameter discharge hole in the center. The discharge system is basically the same as that of a previous study 22,27) except for the diameter of the discharge hole. The voltage between the cathode and the anode is measured to be approximately 250 V (at the gas pressure of 5, 10, and 20 kPa) and 270 V (at 40 and 60 kPa).…”

“…For this reason, to investigate the role of 2 3 S atoms in detail, it is effective to compare the behavior of the 2 3 S atoms with that of the excited atoms which have a short natural lifetime. On the other hand, for experiments of two transitions, it is important to setup the measurement system so that we can conduct it with the identical discharge condition, because the discharge condition of the MHCD plasma changes easily due to the change of the wall condition 27) caused by the plasma damage to the wall. 28) For this reason, we conducted the measurements for the two transitions in sequence with the identical discharge condition 27) to the MHCD helium plasma of 0.3 mm diameter at 10-80 kPa, and discussed the two-dimensional distribution of the 2 3 S or 2 1 P atom density.…”