Measuring Coulomb Explosion Ions from OMEGA EP Interactions*

Direct laser acceleration (DLA) can generate electron beams with high charge to produce secondary radiation sources. Experiments at the OMEGA EP laser facility were designed to optimize the direct laser acceleration of electrons in an underdense plasma created from a helium gas target. In these experiments, the ponderomotive force expels electrons from the regions of highest laser intensity to form a channel. The charge separation creates a strong transverse electric field that accelerates ions radially through a Coulomb explosion; it is the same radial channel field, along with the electron beam-generated azimuthal magnetic field, that facilitates DLA. Since the channel formation is key to understanding electron acceleration, the accelerated helium ions, measured with a Thomson Parabola Ion Energy (TPIE) spectrometer, provide an interesting complementary measurement for understanding the field strengths inside the channel.