Parallel electrostatic collisionless shocks in hot-cold ablative mixing plasmas*

Electrostatic collisionless shocks are common in space, astrophysical, and laboratory plasmas. They are known to have great importance, e.g., in facilitating ion acceleration. The most well-studied cases resemble an explosion in which an over-pressured high-density plasma expands into the rarefied plasma. The initial electron temperature is usually uniform or larger in the expanding plasma (downstream of the shock) with uniform cold ions. A contrasting case, which is also ubiquitous in astrophysical and laboratory plasmas, has a cold/dense plasma roughly in pressure balance with a background hot/dilute plasma. Such hot-cold interface is known to trigger a thermal collapse of the nearly collisionless hot background plasma (e.g. tokamak thermal quench), and an ablative mix of the cold ions with the background ions. By employing the first-principles kinetic simulations, we show that an electrostatic collisionless parallel shock is formed in such hot-cold ablative mixing plasmas, even though the downstream plasma pressure is lower than that of the upstream. The shock front is where the cold ions meet the hot ions, which thus plays an essential role in both the thermal collapse and ablative mix. Its speed and width are shown to be affected differently by the upstream and downstream plasma temperatures. More interestingly, an intermittent emission of cold ion beams into the upstream is observed due to the plasma instabilities in the downstream.