Precursor Ionization Fronts Ahead of Expanding Laser-Plasmas

Fast ionization fronts have been experimentally observed to propagate ahead of a laser‐produced plasma expanding into low‐density background gas. The ionizing wave has a velocity of 0.3‐3×10⁸ cm/sec and a thickness of 1‐10 cm both quantities increasing with radius of propagation. Langmuir probe and microwave diagnostics indicate that the wave augments the initial photoionization density by a factor of $\text{∼3}$ ; potential fluctuations exist within the wave which can accelerate electrons to energies above the ionization threshold. Equations describing a collisional ionizing electron wave are solved in an approximate manner to yield relations which agree with many features of the measured phenomena.