Session DT3: Optical Diagnostics I

Laser Diagnostics for Spacecraft Propulsion

Over the past several decades, a variety of laser diagnostic techniques have been developed and applied to diagnose spacecraft propulsion devices. Laser diagnostics are inherently non-intrusive, and provide the opportunity to probe properties such as temperature, concentration or number density of plume species, and plume velocities in the harsh environments of combustion and plasma discharges. This presentation provides an overview of laser diagnostic capabilities for spacecraft propulsion devices such as small monopropellant thrusters, arcjets, ion engines and Hall thrusters. Particular emphasis is placed on recent developments for time-resolved ion velocity measurements in Hall thruster plumes. Results are presented for one such diagnostic method, a time-synchronized CW-laser induced fluorescence (LIF) technique based on a sample hold scheme. This method is capable of correlating measured fluorescence excitation lineshapes with high frequency current fluctuations in the plasma discharge of a Hall thruster and is tolerant of natural drifting in the current oscillation frequency [1]. \\[4pt] [1] N. A. MacDonald, M. A. Cappelli, and W. A. Hargus Jr., Review of Scientific Instruments, 83 (2012) 113506.   

Student Award Finalist: Reactive species generated in atmospheric-pressure plasmas with water admixtures for biomedical applications: Absolute measurements and numerical simulations

We investigate the production of atomic oxygen (O), hydroxyl (OH) and atomic hydrogen (H) in an rf atmospheric-pressure plasma operated in helium with water admixtures. These species, and their longer-lived products, are known to influence biological systems. Absolute measurements of species densities are required to develop these plasmas for therapeutics. Accurate determination of radical densities is challenging at elevated pressures in complex gas mixtures due to collisional quenching. We measure radical densities using VUV high-resolution Fourier-transform absorption spectroscopy with synchrotron radiation, UV broadband absorption spectroscopy, and picosecond two-photon absorption laser induced fluorescence (ps-TALIF). These diagnostics are the most suitable techniques allowing direct, absolute and 2-dimensional spatial resolution measurements at atmospheric pressure. Ps-TALIF also enables measurements of the lifetimes of laser-excited states of O and H, providing insight into the chemical kinetics and ambient air diffusion into the plasma jet region. Good agreement has been found between the measurements and a numerical chemical-kinetic simulation.   

Development and Calibration of Electron Density Measurements in Argon Plasma Using Laser Collision-Induced Fluorescence

Laser collision-induced fluorescence (LCIF) is a powerful diagnostic which can be used for making temporally and spatially resolved measurements of electron densities in a plasma discharge. The technique, which involves the measurement of optical emission emanating from higher energy excited states due to the redistribution of the lower energy laser-excited state by collisions with energetic electrons, has been readily employed to study argon discharges. In this work, we report on recent efforts to extend the LCIF technique to argon based plasma systems. Discussion will be offered on the spectroscopic pathway used for the interrogation of argon and discussion will be given on the procedures used to calibration the LCIF diagnostic. Particular emphasis will be placed on the double-pulse excitation of a plasma column that enables near independent control of electron density and electron energy. Anticipated bounds on the range of application of the calibrated transitions will likewise be discussed. Finally, the utility of the LCIF diagnostic will be demonstrated by applying the technique to spatially and temporally varying plasma systems.   

Influence of Molecular Gas Concentration on Measurement of Plasma Electron Density by Saturation Spectroscopy

Recently, we applied saturation spectroscopy to electron density measurement of argon plasmas. In general, the peak height of saturation spectrum or the saturation parameter is a function of the relaxation frequency of related energy levels. In the case of the metastable level of argon in argon plasma, the relaxation frequency is dominated by electron impact quenching, hence we can deduce the electron density from the saturation parameter. On the other hand, in the case of mixture plasma of argon and molecular gas, molecular species also contribute to the relaxation frequency of the metastable level. In this study, we investigated the influence of molecular gas concentration on the electron density measurement. An ICP source was used for producing argon-nitrogen and argon-hydrogen mixture plasmas. The frequency of a tunable diode laser was scanned over the Doppler width of the $\rm 4s[3/2]^o_2 - 4p[3/2]_2$ transition (763.51 nm). We confirmed a linear relationship between the inverse of the saturation parameter and the electron density in the argon-nitrogen mixture plasma. However, the linear relationship was not found in the argon-hydrogen mixture plasma. The breakdown of the linear relationship is caused by the change in the density of molecular hydrogen due to dissociation.   

Plasma Propagation Speed Model For Electron Temperature Investigated Of Ar And N$_{2}$ In Atmospheric Pressure Non-Thermal Indirect-Plasma Jet

Space and time resolved discharge images from an atmospheric pressure non-thermal indirect-plasma jet have been observed by a high-speed single-frame camera to investigate the electron temperatures. The propagation velocity of the indirect Ar and mixture N$_2$ (0 - 5\%) plasmas along the plasma column has been shown to be in the order of 104 m/s, and that corresponds to an ion acoustic velocity in order of 102 m/s. Plasma has been generated by input discharge voltage of 3.0 kV at driving frequency of about 40 kHz. Particularly, there are two kinds of the electron, and it has been presented in atmospheric pressure non-thermal indirect-plasma jet. At slow electron energy, the average electron temperature has been found to be about 0.33 eV for Ar plasma and change to 0.42 eV for mixture Ar/N$_2$ plasma. And fast electron energy, the average electron temperature has been found to be about 1.19 eV for Ar plasma and change to 1.40 eV for mixture Ar/N$_2$ plasma. Implications of the results and directions for further studies are discussed.