The Diagnosis Of Evolution Of Plasma In Air Switch By Laser Interferometry

Pulsed power technology is developing quickly in recently years which has beenwidely applied in civilian industry and national defense research. Switch plays animportant role in high pulsed power technology due to that it determines the outputcharacteristics of high pulsed power system. Gas switch is widely used in the highpulsed power system. The switch gap produces large of plasma in the process ofdischarge. After discharge, the plasma will recombine and the temperature field of theswitch will recover to normal. It is well known that the analysis of the evolution ofplasma and the recovery of the temperature field has great significance forcomprehending the process of switch and the repetitive running of switch. Also, as anuseful method, laser interferometry measurement with high sensitivity has been used tomeasure the plasma in various pulsed plasma. In this paper, based on the laserinterferometry, a Mach-Zehnder interferometer is designed and employed to measurethe electron density of the plasma in an air switch. The high speed framing camera(HSFC-PRO) is used to capture the interferogram. And the image processing methodsare used to get the information of fringes. And the electron density distributions of theplasma at different times are calculated out according to the fringes shift. Analyze theevolution of the plasma and the influence of temperature field on the environment of theswitch. The main works of the paper shows as follow:1. The breakdown theory of gas switch is described. The computational method tocalculate the parameter of spark channel is described. Also the basic principle of laserinterferometry to measure the density of plasma is analyzed.2. An experimental system to measure the density of plasma in air switch isdesigned. The experimental system consists of a Mach-Zehnder interferometer and ahigh-speed framing camera. In the Mach-Zehnder interferometer a semiconductor laseris used, the wavelength and the power of the laser are532nm and100mW, respectively.The high-speed framing camera can shoot four pictures with different exposure delaytime in one discharging pulse.3. The image processing of interferogram is studied. By researching the feature ofinterferogram, an image processing method is given. Firstly, the interferograms arepretreated by the band-pass filter in Fourier-frequency domains. Secondly, the filteredinterferograms are compressed to2-gray level images. Thirdly, the2-gray level imagesare thinned by Hilditch thinning method. Finally the center of the interference fringes isobtained.4. The electron density distribution of the plasma is obtained by using thenumerical integration method of Inverse Abel transformation according the fringe shift.Then, the evolution of the plasma and the recovery of the temperature field of the switch are analyzed.