Laminar electrostatic shock waves generated by an ion beam

Strong laminar electrostatic shock waves have been experimentally observed when an ion beam is injected into a collisionless plasma. The structure of the shock is qualitatively different from one with a trailing wave train. A density depression follows behind the shock front, and no trailing wave train due to wave dispersion is found. A significant amount of ions reflected from and transmitted through the shock front form a precursor. The critical Mach number above which no shock is formed is found to be 1.5. Numerical simulations reported here reproduce the experimental observations very well. An analysis based on the water-bag model accounts for the observed value of the critical ion-beam velocity which gives the critical Mach number. It also points out that the reflected ions play an essential role in the persistence of the shock.