Tunable Diode Laser Absorption Spectroscopy Measurements Research Articles

Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Arm) level 3s23p54s excited at 801.478 nm, in the dense plasma region in front of the magnetron target in a high power impulse magnetron sputtering (HiPIMS) discharge. From the Doppler profile the evolution of the temperature and density was derived during the pulse as well as during the plasma decay, i.e. in the afterglow. It is shown that the Arm density sharply increases at the beginning of the discharge pulse, followed by a severe Arm depletion along with increasing gas temperature around the peak of the HiPIMS discharge current. The strong dynamics of these parameters involve many elementary processes such as electron-impact excitation, electron-impact de-excitation and ionization of Arm, gas rarefaction, electron temperature increase at the end of the pulse and gas diffusion. These phenomena are discussed with respect to several parameters: distance from the target, peak discharge current during the pulse, pulse length, and gas pressure.

TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames

We report the first quantitative and calibration-free in situ C2H2 measurement in a flame environment using direct Tunable Diode Laser Absorption Spectroscopy(TDLAS). Utilizing a fiber-coupled Distributed Feedback diode laser near 1535nm we measured spatially resolved, absolute C2H2 concentration profiles in a laminar non-premixed CH4/air flame supported on a modified Wolfhard–Parker slot burner with N2 purge slots to minimize end flames. We developed a wavelength tuning scheme combining laser temperature and current modulation to record with a single diode laser a mesh of 37 overlapping spectral windows and generate an ∼7nm (30cm−1) wide, high-resolution absorption spectrum centered at 1538nm. Experimental C2H2 spectra in a reference cell showed excellent agreement with simulations using HITRAN2004 data. The enhanced wavelength coverage was needed to establish correct C2H2 line identification and selection in the very congested high temperature flame spectra and led to the P17e line as the only candidate for in situ detection of C2H2 in the flame. We used highly efficient optical disturbance correction algorithms for treating transmission and background emission fluctuations in combination with a multiple Voigt line Levenberg–Marquardt fitting algorithm and Pt/Rh thermocouple measurements to achieve fractional optical resolutions of up to 4×10−5 OD (1σ) in the flame (T up to 2000K). Temperature dependent C2H2 detection limits for the P17e line were 60 to 480ppm. By translating the burner through the laser beam with a DC motor we obtained spatially resolved, absolute C2H2 concentration profiles along the flame sheet with 0.5mm spatial resolution as measured with a knife edge technique. The TDLAS-based, transverse C2H2 concentration profiles without any scaling are in excellent agreement with published mass spectrometric C2H2 data for the same flame supported on a similar burner, thus validating our calibration-free TDLAS measurements.

Absolute production rate measurements of nitric oxide by an atmospheric pressure plasma jet (APPJ)

Tunable diode laser absorption spectroscopy (TDLAS) has been applied to measure the absolute production rate of NO molecules in the gas phase of an atmospheric pressure plasma jet (APPJ) operating at rf (13.56 MHz) in argon with small (up to 1%) admixtures of air. The resulting NO production rates were found to be in the range (0.1–80) × 10−3 sccm or (0.05–35) × 1018 molecules s−1 depending on the experimental conditions. Maximum rates were obtained at 0.2% air. For TDLAS measurements the APPJ was arranged inside an astigmatic multi-pass cell of Herriott type with 100 m absorption length. The insertion into a closed volume differs slightly from the normal, open operation with the jet propagating freely into air. Therefore, the measuring results are compared with optical emission of the open jet to verify equivalent experimental conditions. The dependence of the optical emission of NO (237 nm) on power and gas mixture has been measured. The similar shape of the dependence of absorption and emission signals gives evidence that the comparability of experimental conditions is sufficiently satisfied. It is concluded that the NO production rate of the APPJ in ambient air can be characterized using TDLAS and provides reliable results in spite of differing experimental conditions due to the set-up.

Investigation of Current Density Effect on Water Vapor Concentration Profile along PEMFC Channels by TDLAS

In this article, we applied tunable diode laser absorption spectroscopy (TDLAS) technique to measure variation of water vapor concentration in gas flow channels in an operating polymer electrolyte membrane fuel cell (PEMFC). TDLAS measurement offered an optical remote sensing to detect slight change of water vapor concentration, showing that electrochemical reaction in an operating PEMFC is not uniformly observed. We performed TDLAS measurement in gas flow channels in the anode and the cathode with variation of cell current density. We observed increase of water vapor concentration due to electrochemical reaction along the cathode channel as well as along the anode channel from the inlet, suggesting that generated water was exhausted through the both channels. It was also shown that less electrochemical reaction was observed in the upstream of the channel presumably due to less hydration of the membrane because of dry gas supply.