Spreading of a Pseudowave Front

Using analytic, nonlinear techniques the spreading of the leading edge of a pseudowave (here assumed to be a slab of ions injected at hypersonic velocity into a dense background plasma) is computed. It is found that the front spreads due to collective effects with a characteristic velocity of ${\text{3(n}}_{20}{\mathrm{/n}}_{10}{)}^{\text{1/2}}{v}_a$ , where $n_{20}$ is the initial injected ion density, $n_{10}$ is the background ion density, and $v_a$ is the ion‐acoustic velocity. The computation also predicts a finite leading edge velocity of $v_0{\text{+2(n}}_{20}{\mathrm{/n}}_{10}{)}^{\text{1/2}}{v}_a$ , where $v_0$ is the initial velocity of injection. Neither shock steepening nor an accelerating leading edge as in a free expansion of plasma is found.