Electrostatic Waves in a Paired Fullerene-Ion Plasma

Three kinds of electrostatic modes are experimentally observed to propagate along magnetic-field lines for the first time in the pair-ion plasma consisting of only positive and negative fullerene ions with an equal mass. It is found that the phase lag between the density fluctuations of positive and negative ions varies from 0 to $\pi$ depending on the frequency for ion acoustic wave and is fixed at $\pi$ for an ion plasma wave. In addition, a new mode with the phase lag about $\pi$ appears in an intermediate-frequency band between the frequency ranges of the acoustic and plasma waves.

Figure 1

(a) Schematic drawing of the experimental setup. (b) Generation property of pair-ion plasma depending on electron-beam energy $E_e$ in region (III). (c) Pair-ion density depending on dc-bias voltage $V_{\mathrm{an}}$ at $E_e=100\mathrm{eV}$.

Figure 2

Dispersion relations for electrostatic waves propagating along $B$-field lines. Solid lines and dots denote results calculated from two-fluid theory and measured experimentally, respectively.

Figure 3

Typical temporal variations of the densities and potential at $\omega /2\pi =$ (a) 0.4, (b) 20, and (c) 35 kHz. The phase of the negative-ion density is in advance of the positive-ion density.

Figure 4

Phase-lag spectrum of positive- and negative-ion density fluctuations.