Ion Acoustic Shocks Formed in a Collisionless Plasma with Negative Ions

Ion acoustic shocks are formed in a plasma with negative ions, where Tₑ≥T₊T_- ( Tₑ, T₊, T_-: temperatures of electrons, positive ions, and negative ions, respectively). Depending on the ratio ε of negative to positive density, a steepening of positive or negative density jumps is observed. For ε<εc ( ≈0.65, critical value), positive density jumps evolve into compressive shocks. For ε>εc, however, negative density jumps evolve into rarefactive shocks. The ratio εc observed is well explained on the basis of Korteweg--de Vries equation.