Collective Modes in Pair Fullerene-Ion Plasma

W. Oohara and R. Hatakeyama
Department of Electronic Engineering, Tohoku University, Sendai 980-8579, Japan
Abstract. Three kinds of electrostatic modes are experimentally observed to propagate along magnetic-field lines in a
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 is fixed at 0 and

The pair-ion plasma source using fullerenes is installed in a grounded vacuum-chamber of 15.7 cm in diameter
and 260 cm in length, as shown schematically in Fig. 1 (a). A uniform magnetic field of B

The excited modes damp in propagating with the frequency dependency. Figure 2 (a) shows typical Lissajous’
curves of Vexc and the normalized positive-saturation current consisting of time-averaged Ī+ and oscillating Ĩ+
components, which are measured at z = 5 cm. Thus, the propagating-mode amplitude does not exactly correspond to
the excited-mode amplitude right after the excitation but the Lissajous’ curves qualitatively indicates the relation
between |Vexc| and the excited-mode amplitude. The normalized amplitude |Ĩ+|/Ī+ is about 0.06 and relatively large at
|Vexc| = 0.5 V for IAW (7 kHz). On the contrary the amplitude of IFW (12 kHz) is small |Ĩ+|/Ī+ ~ 0.005 even though
|Vexc| is same (0.5 V). |Ĩ+|/Ī+ is about 0.006 at |Vexc| = 1.6 V for IPW (36 kHz) and the mode itself is hard to be
excited, because the damping rate of IPW is high, the response speed of the ions is low, and the ions are difficult to
follow the voltage variation. Therefore |Vexc| needs to be small in a low frequency range and be large in a high
frequency range. The modes can be measured in the higher wave number around the boundary frequency of IAW
and IFW by applying suitable |Vexc| depending on the frequency. Moreover, the modes of plane waves are actively
excited by the grid exciter, because the existence of surface waves propagating along the B-field lines in the
boundary region between the plasma and the vacuum are predicted theoretically when the modes are excited by the
cylindrical exciter [19,20]. The dispersion relations (

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