負イオンを含む多成分プラズマ中の非線形イオン音波

抄録

Propagation of nonlinear ion-acoustic waves in a three-component plasma which consists of electrons and positive and negative ions have been investigated theoretically and experimentally. The Korteweg-de Vries (K-dV) equation is obtained by the reductive perturbation of the plasma taking the ion temperature into consideration. The dispersion term of the K-dV equation is positive for the concentration r of the negative ion considered here. On the other hand, the coefficient of the nonlinear term becomes negative at a critical concentration r_c which depends on the mass ratio of the negative to positive ions and the temperature ratio of electrons to ions. Therefore, positive (electric potential), compressive (positive ion and electron density) solitons can exist when 0<r<r_c and negative and rarefactive solitons can propagate when r>r_c. When r=r_c, the nonlinear term in the K-dV equation disappear so that a higher-order term should be considered, which results in the modified K-dV equation. The modified K-dV equation has also soliton solutions which are irrespective of the sign, i.e., both positive and negative solitons coexist. The K-dV and the modified equations are obtained under the assumption that the amplitude is small. When the amplitude is large, original fluid equations are integrated numerically. The result is that when r_c<r<r_m, positive solitary waves exist if the amplitude is larger than a threshold. Experiments were carried out in a multi-dipole double-plasma device. The inner diameter of the device is 40cm and its total length is 90cm. The device is separated into a driver and a target section with an electrically floating mesh grid. The chamber was evacuated to 1×10^-6Torr with a turbo-molecular pump. Argon and sulfur hexafluoride were introduced into the chamber under continuous pumping. Plasma parameters measured with a probe and a retarding energy analyzer were the electron density=10⁸-10⁹cm^-3, the electron temperature=0.6-1eV and the ion temperature=0.05-0.1eV. Experimental results are compared with the theoretical results described above.