An atmospheric plasma jet was generated by ionizing nitrogen ejected from a torch loaded with high voltage. The acquisition of transmission spectra by terahertz time-domain spectroscopy was carried out at three locations at different distances from the plasma torch nozzle. Combined with the theoretical model of terahertz pulse propagation in uniform plasma medium, the average density of plasma was calculated from the phase shift and spectral amplitude ratio measured experimentally. By comparing the measurement results of plasma density with those of Langmuir double-probe for these three different positions, the feasibility of measuring average density of atmospheric plasma with terahertz time-domain spectroscopy was corroborated.
We have used Terahertz time-domain spectroscopy(TDS) to investigate the dielectric properties of water and water solutions of containing different ions. The complex permittivity of NaCl solutions, for different concentrations of ions, was obtained by attenuated total reflection(ATR) method. We have observed that, when increasing o ion concentration, the real part and imaginary part of the permittivity are increased. Furthermore, we also calculate the dielectric constant of ionic aqueous solutions with different concentrations based on molecular dynamics method. The simulation method, based on Newton mechanics, can estimate the dielectric constant of liquid substances without the experimental limitation.
Based on analyzing the condition of transmission through high density plasma and its ionization front, and the theory of detecting plasma with holographic interferometer, diagnostic experiments of ionization front and plasma ionized by high laser electric field has been reported in this paper. The Mach-Zehnder interferometer with the femosecond laser of center wavelength of 800nm and pulse duration of 58fs has been used to get interference images in CCD camera. By Abel inversion, the density map of plasma has been obtained based on the interference image. Evolution pictures of ionization front within the time or propagation distance have been reported and ionization front’s size has been estimated with the help of 4mm diameter of nozzle. One good diagnosis method for ionization front and plasma has been given within these experiments.
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