The Diagnosis Of Laser-ionizing Air-plasma With Terahertz Pulse

The characterizing of plasma properties by the optical method is an important physical problem,but inherently limited to the spectral region,and spectral regions such as X-rays,visible light and microwaves have been applied for diagnostic purposes of different electron densities.Especially for the new generation Tokamak plasmas such as fusion ignition research experiment(FIRE),the characteristic frequency is located at the terahertz(THz)range,which is far away from the traditional visible and infrared detector range.Therefore,THz-based plasma diagnostics should be performed to meet the need.Furthermore,THz time-domain spectroscopy(THz-TDS)system is the most popular system in THz research,which is the method used in our work,because it records abundant information of different frequencies in every scan,not only on amplitude but also on its phase.We characterize the transmission properties of the focusing-laser-induced filament by using THz time-domain spectroscopy.Significant resonance absorption of THz wave is identified.Our results suggest that the electron density of the filament can be determined by fitting the phase shifts and absorption properties around the absorption.Besides,the polarization and intensity of the filament generated pulse have been varied to study their influences on the resonance absorption.The results suggest no significant polarization dependence on the absorption features.As the laser intensity of the plasma-generated pulse grows in the certain range,the electron density increases linearly.