Intense self-generated magnetic field in the interaction of a femtosecond laser pulse with an underdense plasma

The interaction of femtosecond (80 fs) laser pulses with an underdense gas at moderate laser intensity of 3-5.10(15) W/cm(2) is studied. We report esp ecially on time-resolved measurements of intense self-generated magnetic fi elds above 30 T at the given irradiance using a pump probe device with. a F araday rotation diagnosis on the probe pulse. The induced magnetic field is found far above the one predicted by the inverse Faraday effect. The theor etical mechanism responsible for this strong field production is identified as a self-ponderomotive effect induced by the pump pulse.