The Effect Of Plasma Thermal Blockage On Target Under Laser Irradiation

With the development of laser weapon, it can be predicted that the power of laser weapon served in the future may reach to hundreds of million watts (MW). This paper focused on the absorption of beam energy and ablation of C/C composite material from laser, which has wavelength about1.6μm and power at hundreds of MW.Based on the theory of heat conduction, ablation on the surface of material is calculated with numerical method, then temperature and ablation velocity ignoring vapor pressure are obtained. It is shown that the surface begins ablation in few nano seconds with high power laser, where gradient of temperature field also rise abruptly. Ablation velocity rises abruptly at the beginning then gets steady, which may be caused by variation of laser power and accumulation of space temperature field. Besides, for laser power under5X105W/cm2, it is found that the missile spin is an effective anti-laser measure which can delay the initial ablation time.With the Mott-Smith approximate method being used, and simultaneous solution of quality continuous equation, the state of flow of particle which runs away from the surface as well as the ablation velocity is analyzed. Result shows that gas temperature on the surface is higher than usual. Vapor pressure and temperature of the surface increase along with laser power; vapor pressure influence on the ablation velocity and change with a nonlinear rule.At last, according to ionization theory, self-similar expansion model and inverse bremsstrahlung absorption theory, the effect of plasma thermal blockage on target under laser irradiation is analyzed. It is shown that electron density mainly depends on the laser power and duration time. Besides, the expansion rate increases rapidly first and then decreases, at the same time, the density of neutral particle rises, keeps steady then decreases. The process that plasma absorbs laser, which is called thermal blockage effect, weaken the laser ablation of target material clearly. As laser power enhanced, thermal blockage effect becomes more obvious and the equivalent heat has a exponential growth. The laser in1.6μm wavelength absorbs more than which in1.064μm. The surface temperature is affected by the absorption of energy of laser.