Large-amplitude radial-plasma-wave generation during high-intensity laser interactions with underdense plasmas

Forward-scattered radiation produced from the interaction of an ultraintense laser pulse (P~2TW) with underdense hydrogen and helium plasmas (nₑ≈10¹⁹cm^-3) is measured. The coherently Thomson scattered satellites of the second harmonic at off-axis scattering angles are observed to be significant, in contrast to observations in the directly forward direction. These results are likely due to the existence of large-amplitude radial plasma waves generated by ponderomotive forces as the laser pulse propagates through the plasma.