To research the initiation of breakdown and the earliest stages of plasma formation, analyzing the ignition time and mechanism is necessary. The ignition time of laser-induced air plasmas is numerically simulated on the basis of air breakdown conditions in atmospheric air. This time is on the order of a few nanoseconds, and increases with increasing laser facula radius and pulse width, and decreases with increasing laser wavelength and incident laser energy. In addition, images obtained using a high-speed camera (NX5-X2) indicate that the ignition process of laser-induced air plasmas includes preionization, ionization, significant ionization, plasma shielding , plasma ignition and extinction.
The electron temperature and density of air plasma hold important meanings for researching the process of laser-induced air breakdown plasma. In this paper, a TEM 00 Q Nd:YAG laser with a wavelength of 1064 nm is used. When the laser is focused on the atmosphere, it will produce a plasma flash. The spectrum of the resulting air breakdown plasma is collected using a Avantes-ULS3648 spectrometer with nine separate channels. Changes in electron temperature and density of the plasma at different delay times are studied by spectral analysis. According to the spectrum of a given element with different peak positions, the electron temperature of the plasma can be obtained by employing the intensity comparative method, and the electron density of the plasma can be obtained by invoking the Stark broadening method. It was found that both the temperature and density of the air plasma decrease with increasing of the delay time. Such results maintain a high degree scientific significance for improving the accuracy and precision of on-line measurements in the atmospheric environment.
A method used for precisely measuring the placement of the fast or slow axis of wave plate is presented. In this method, a test wave plate is placed between a polarizer and an analyzer. With the polarizer and analyzer being rotated to different positions, the intensity of the emergent light is measured and one of the optics principal axes of the test wave plate is marked. Then whether this optics principal axis is a fast or slow axis is measured by checking the state of the emergent light polarization. By taking the method of identifying the intensity value near the inflection point, the error caused by directly searching the extreme value of the light intensity can be avoided and the accuracy of determining axis can also be improved significantly. In addition, the source of the determining axis accuracy, which is ±0.1° . is also analyzed in detail. With regard to the method, there are no requirements for the wavelength of the light source or the linearity, undercurrent and isotropy of the photoelectric detector. Above all, it can be applied to determining the axis of wave plate with any phase retardation.