In situ measurements of the spectral reflectance of metallic mirrors at the H α line in a low density Ar–H plasma

Abstract

The efficient and reliable control and monitoring of the quality of the optical properties of mirrors is an open problem in laboratory plasmas. Until now, the measurement of the reflectance of the first mirrors was based on the methods that require additional light calibration sources. We propose a new technique based on the ratio of the red- and blue-shifted emission signals of the reflected hydrogen atoms which enables the in situ measurement of the spectral reflectance of metallic mirrors in low-density Ar-H or Ar-D plasmas. The spectral reflectance coefficients were measured for C, Al, Ag, Fe, Pd, Ti, Sn, Rh, Mo, and W mirrors installed in the linear magnetized plasma device PSI-2 operating in the pressure range of 0.01-0.1 Pa. The results are obtained for the H line using the emission of fast atoms induced by excitation of H atoms through Ar at a plasma-solid interface by applying a negative potential U = -80, …, -220 V to the mirror. The agreement between the measured and theoretical data of reflectance is found to be within 10% for the investigated materials (except for C). The spectra also allow us to efficiently determine the material of the mirror.

Emission of Fast Hydrogen Atoms in a Low Density Gas Discharge—The Most “Natural” Mirror Laboratory

“…A very good agreement with the theoretical or the measured results in the laboratory was obtained for all targets. The exception showed only C; however, as discussed above, the applied negative potential must be increased for this target, which resulted in a reduction of the ratio of corresponding integrals [2]. For all other targets, the applied potential of −100...−120 V showed the optimal condition for measurements of reflectance.…”

“…Conditions (1), (2), and (4) represent the minimal set of requirements for reflectance measurements in plasmas.…”

“…Using these values, the sheath dimensions were ≈ 0.02 cm, ∆L(H α ) = √ 2E/mτ 3d ≈ 0.22 cm, and λ cx ≥ 10 cm. The conditions (1), (2), and (4) were fulfilled at PSI-2. Moreover, the initially low-and later high-resolution data of the spatial propagation of the emission [20,21] validated the condition (4) at PSI-2 experimentally.…”

“…We point out that this approach did not include the energy loss of atoms by excitation of H atoms with collisions with Ar or the instrumental and fine-structure broadening. The experimentally-found value E m was compared to Equation (3) in Figure 6 of [2]. For instance, for C the value of onset of emission equaled 63 eV, being lower as required for the optimal emission signal.…”

“…the difference in the intensity and/or shape of emission between the red-and the blue-shifted signal is determined by the optical properties of the mirror. Without loss of generality, we postulate that the optical properties of the mirror could be derived using the ratio between the red-and the blue-shifted signal of emission [1,2]. Instead of the timely-resolved measurements for the standard calibrating surface and the mirror outside of the plasma, one performs a simultaneous observation using the Doppler effect during plasma operation.…”