Interaction of CO2 laser radiation with dense plasma
The instabilities which occur in the interaction of CO2 laser radiation with a dense plasma have been studied. A TEA CO2 laser provided pulses of up to 30 joules of energy with a duration of 50 nanoseconds. By focussing the radiation on to a plane target a focal spot of about 180 micrometers diameter was formed with a irradiance of 10 to 10 W cm. The scattered radiation was collected by a laser focussing lens and analysed with a grating spectrometer. Linear relationships have been found between the incident irradiance and the back scattered energies. This is in contrast to the results of other workers who have found quadratic relations at high irradiance. The back scattered energies were of the order compared with the incident energy. The spectrum of shows a displacement to longer wavelength which is attributed to stimulated Brillouin scattering, there being an ion acoustic wave which is driven by the laser radiation. A similar displacement occurs in the 2 W spectrum and is attributed to the scattering of plasmons from the ion acoustic wave. The measured displacement is in agreement with a theory due to Silin. Fine structure also exists in the 2W spectrum, a shoulder being found on the red side of the line and this may result from a non-Maxwellian electron temperature distribution. There is also a satellite which is displaced towards longer wavelengths by about 0.1 micrometers from the precise value of 2W Explanations of this feature are offered. The self-generated magnetic field has been measured and its effect on the interactions have been examined. The plasma temperature has been determined by X-ray measurements and compared with the values estimated from the scattered spectrum.