Experimental study of shock formation in a dusty plasma

If a large amplitude density pulse is launched into a plasma having equal electron and ion temperatures, Te 5Ti , Landau damping prevents the formation of a sharp leading edge ~‘‘shock’’! and the pulse spreads out. If the Landau damping is eliminated, the pulse steepens as it propagates and a shock structure is formed. This has been observed experimentally, for example, by increasing the Te /Ti ratio by cooling the ions through ion-neutral collisions, or by introducing negative ions into the plasma. In the presence of negative ions, ion waves exhibit two modes of propagation: a ‘‘fast’’ mode and a ‘‘slow’’ mode. As the percentage of negative ions is increased, the phase velocity of the fast mode increases, and as a result the number of ions interacting with the wave decreases and the damping is reduced. In the absence of Landau damping, the fluid equations used to describe ion acoustic-like phenomena in plasmas are mathematically similar to the Euler equations for an ideal fluid. The solutions of these equations have the property that any compressive pulse will steepen as it travels. The present experiment investigates the propagation characteristics of large amplitude ion acoustic-like compressional pulses in a dusty plasma. The formation of shock waves in a dusty plasma may be important, for example, in processes occuring in interstellar dust-molecular clouds, supernova explosions, cosmic particle acceleration, and cometary plasmas. The propagation characteristics of small amplitude sinusoidal ion acoustic waves launched into a dusty plasma have already been studied. In those experiments it was found that the presence of negatively charged dust grains increased the phase velocity of the waves and reduced the strength of the ~Landau! damping. Theoretically it has been shown that the presence of a negatively charged dust component modifies the dispersion relation for ion acoustic waves. This modified ion acoustic mode, the so-called dust ion acoustic ~DIA! mode, has a phase velocity given by