Measurements of the inverse Faraday effect from relativistic laser interactions with an underdense plasma

Magnetic fields in excess of 7 MG have been measured with high spatial and temporal precision during interactions of a circularly polarized laser pulse with an underdense helium plasma at intensities up to 1 x 10(19) W cm(-2). The fields, while of the form expected from the inverse Faraday effect for a cold plasma, are much larger than expected, and have a duration approaching that of the high intensity laser pulse (<3 psec). These observations can be explained by particle-in-cell simulations in 3D. The simulations show that the magnetic field is generated by fast electrons which spiral around the axis of the channel created by the laser field.