New Aspects on Plasma Wave and Instability Phenomena − Flow Shear, Polarization Reversal, and Pair Ions

R. Hatakeyama, T. Kaneko, W. Oohara, and K. Takahashi
Department of Electronic Engineering, Tohoku University, Sendai 980-8579, Japan
Abstract. Our basic-plasma experiments relating to fusion-oriented and space/astronomy plasmas have yielded deep and unique insight into wave and instability physics in ordinary electron-ion and unusual pair-particle plasmas. Here recent results on flow shear, polarization reversal, and pair ions are presented in the way that underlying physics is shed light on.
Keywords: Flow Shear, Polarization Reversal, Pair Ions
PACS: 52.35.Kt, 52.35.Qz, 52.35.Hr, 52.25.Mq, 52.40.Fd, 52.27.Ep, 52.35.Fp
LOW-FREQUENCY INSTABILITIES MODIFIED BY FLOW SHEARS
Low-frequency electrostatic instabilities modified by sheared plasma flows in magnetized plasmas are one of very important issues in the field of the basic plasma physics. In order to clarify the mechanisms of excitation and suppression of these instabilities, we externally and independently control the parallel and perpendicular flow shears in a basic plasma device using three-segmented electron and ion sources [1-3]. Experiments are performed in the QT-Upgrade machine of Tohoku University as shown in Fig. 1. We attempt to modify a plasma-synthesis method with opposed electron and potassium ion emitters, where both the emitters are concentrically segmented into three sections [4]. When each section of the electron emitter is individually biased (Vee1, Vee2, Vee3), the radially different plasma potential, i.e., radial electric field is generated. This electric field causes E×B flows and flow shears perpendicular to the magnetic-field lines. On the other hand, the parallel ion flow with radially different energy, i.e., the parallel ion flow shear, is generated when each section of the segmented ion emitter is individually biased (Vie1, Vie2, Vie3) at a positive value above the plasma potential that is determined by the bias voltage of the electron emitter. Therefore, these parallel and perpendicular flow shears can be superimposed by controlling the bias voltages of the ion and electron emitters independently. Under our conditions, the plasma density is 108 cm-3, the electron temperature is 0.2 eV, and the ion temperature is almost the same as the electron temperature. A background gas pressure is less than 10-6 Torr.

are fixed to generate no perpendicular shear. Figure 2(a) presents frequency spectra of electron saturation current Ies of a probe and normalized fluctuation amplitudes esesII/~ of the electron saturation current as a function of Vie1 for Vie2 = −0.8 V, Vie3 = 0 V at r = −1.0 cm. Here the position of r = −1.0 cm corresponds to the central shear region between the first and second emitters. When Vie1 is almost same as Vie2, the fluctuation is not excited. Once Vie1 exceeds a certain threshold, the fluctuation with the frequency of about 6.5 kHz is observed to grow as Vie1 becomes large. When Vie1 is more increased, esesII/~ attains to a maximum value and gradually decreases after that. In order to identify the parallel-shear enhanced drift-wave instability, the theoretical growth rate γ as a function of Vie1 is calculated using the kinetic dispersion relation and is plotted in Fig. 2(a) (solid line). The experimental results are in good agreement with the theoretical curves and it turns out that the growth rate changes into positive when Vie1 exceeds the threshold and gradually increases with an increase in Vie1. This means that the ion-frame phase velocity of the fluctuation becomes large due to the presence of the parallel shear, and thus, the effect of the ion Landau damping on the waves is reduced. For larger Vie1, however, the growth rate saturates and gradually decreases with a further increase in Vie1. When the phase velocity exceeds the ion flow velocity, i.e., the relative electron-ion drift velocity in the ion frame, due to the increase in Vie1, the effect of the inverse electron Landau damping in the ion frame is reduced, which leads to the stabilization of the waves [1,2].
When the perpendicular flow shear is superimposed on the parallel flow shear, on the other hand, the drift-wave instability is found to be suppressed by the perpendicular flow velocity shear even in the presence of the parallel shear. We obtain the contour view of the normalized fluctuation amplitude as functions of the parallel (Vie1) and perpendicular (Vee1) flow shears, as shown in Fig. 2(b), where the bias voltages of the other emitters are fixed. Here, horizontal and vertical dotted lines in Fig. 2(b) denote the situations in the absence of the parallel and perpendicular shears, respectively. The drift-wave instability excited by the parallel shear is rapidly suppressed for Vee1 > −2.2 V in any parallel shear strength. For Vee1 < −2.2 V, on the other hand, the drift-wave is hard to be suppressed when the larger parallel shear is generated. Based on these results, the sign of the perpendicular shear is found to be important in modifying the parallel shear enhanced instability.

ELECTROSTATIC WAVES IN PAIR ION PLASMAS
In addition to wave and instability phenomena in the above-mentioned laboratory plasmas, pair plasmas consisting of only positive- and negative-charged particles with an equal mass have been investigated experimentally and theoretically. Pair plasmas represent a new state of matter with unique thermodynamic property drastically different from ordinary electron-ion plasmas. A pair-ion plasma source using fullerenes has been
developed [7]. The generation of a pair-ion plasma, consisting of C60+ and C60− with an equal mass without electrons, is performed using a hollow electron beam in a uniform magnetic field as shown in Fig. 4, where electron-impact ionization and electron attachment play a key role. A magnetic-filtering effect is used for the separation of the electrons and the ions. The plasma density is 107−108 cm-3 and the ion temperature is 0.4−0.5 eV. The plasma and floating potentials are almost 0 V (chamber wall potential), and the static potential structures including sheaths are not formed in the plasma. This is one of the important features in the pair-ion plasma. Some theoretical works have already been presented, which are concerned with linear and nonlinear collective modes in non relativistic pair-plasmas.
A comprehensive two-fluid model has been developed for the collective-mode analysis in electron-positron plasmas, and longitudinal and transverse electrostatic and electromagnetic modes have been studied [8]. The longitudinal collective modes are analogous to those in the ordinary electron-ion plasmas. In our pair-ion plasma, a density modulation is actively excited by a thick annulus in order to investigate collective modes [9,10]. There appear three electrostatic modes propagating along magnetic-field lines: an ion acoustic wave (IAW, ω/2π < 8 kHz), an ion plasma wave (IPW, ω/2π > 32 kHz), and an intermediate frequency wave (IFW, 8 < ω/2π < 32 kHz), as shown in Fig. 5 (circles). IAW and IPW can be predicted in the twofluid theory (solid lines in Fig. 5), but IFW is not predicted and experimentally observed for the first time. The phase difference between the density fluctuations of positive and negative ions are 0 (IAW in low frequency), about π (IFW), and π (IPW). The phase difference in IAW strongly depends on the frequency. Broad discussion with theoretitians has so far been developed on excitation mechanisms of IFW. 00.10.2020

ACKNOWLEDGMENTS
These works were supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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