Test charge response and correlations in multi-ion plasmas.

The interaction potentials caused by a pair of moving test charges are studied in collisionless unmagnetized multi-ion plasmas, containing the electrons, singly ionized positive ions, and two negative ions. Starting from the Vlasov-Poisson coupled equations, a generalized electrostatic potential is obtained for a pair of test charges moving with constant velocities along the z-axis. Based on the speeds and correlations of test charges, the cases of slow, fast, and resonating test charge response are specifically studied and analyzed. If the test charges move with velocities much slower than the thermal speeds of all plasma species in multi-ion plasmas, then a short-range Debye-Hückel (DH) shielding potential appears to decay exponentially with the distance, whereas the long-range far-field (FF) potential varies as the inverse cube of the distance from test charges. The shielding phenomenon does not occur if the test charges move much faster than the plasma thermal speeds. However, the oscillatory wakefield (WF) potentials only exist behind the test charges when their speeds resonate with the speed of ion-acoustic waves. Numerically, it is revealed that the profiles of DH, FF, and WF potentials are significantly affected by the multi-ion plasma parameters and correlation effects. The results might be helpful for understanding the shielding phenomena, particle-particle and wave-particle interactions in multi-ion laboratory plasmas. [ABSTRACT FROM AUTHOR]