Intense magnetic field generation by relativistic ponderomotive force in an underdense plasma

It is shown that the relativistic ponderomotive force exerted by an ultra intense laser pulse launched in a plasma could generate a very strong low frequency (with index 1f hereafter) magnetic field B 1f with a maximum value of the order of B 1f / B 1 = ω pe / ω , where ω pe is the electron plasma frequency and ω the laser one, and B 1 the high frequency magnetic field of the laser pulse. We derive the mechanism for this B 1f -field creation in an underdense plasma and show how to diagnose its presence and propose an experimental set-up to achieve the measurement. We also stress the universality of this detection as far as picosecond pulses and the far infrared radiation range (FIR) are concerned. The great influence of this B 1f -field on related important phenomena, such as relativistic self-focusing and wakefield generation by short laser pulses, is briefly underlined.