Experimental Studies Of Axial Magnetic Fields And Optical Diagnostic Of Femtosecond Laser Plasmas

Based on Faraday effect, the axial magnetic field in plasmas produced by femtosecond laser pulses was measured, to our knowledge, for the first time. The experiments showed that the polarization plane of the backscattered plasma emission rotated 20 relative to that of incident laser beam in the interaction of 150 fs laser pulse with aluminum plasmas. This gave an average axial field of MageGauss. The peak value of the field was estimated to be up to 1.76 MG if the axial magnetic fields had an exponent spatial distribution, according to J. Briand抯 theory. There existed several hundred MA/cm2 currents, which developed simultaneously with the magnetic fields in femtosecond laser plasmas. It was believed that the axial magnetic field developed from dynamo effect in our experiments. No magnetization information was recorded on the magnetic material since the lifetime of self-generated field was shorter than the relaxation time of magnetic domains probably. Interferometric and shadowgraphic measurements of the evolution of plasma produced by ultrashort laser pulses were carried out using frequency-doubled femtosecond probe beam (2o). The temporal and spatial distribution of electron density and refraction index of plasma was obtained after abelizing the time sequence of interferograms. The filamentation instability and other instabilities in high-density region induced the local density modification. Large-scale toroidal magnetic fields confined plasma expansion in the transverse direction resulting in the formation of plasma jet. The plasma expansion along the target normal scaled as t1~2. The time-space distribution of electron density was calculated by self-similar method.