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A three-dimensional self-organization process of a compressible dissipative plasma with a velocity-magnetic field correlation is investigated in detail by means of a variational method and a magnetohydrodynamic simulation. There are two types of relaxation, i.e., fast relaxation in which the cross helicity is not conserved and slow relaxation in which the cross helicity is conserved approximately. In the slow relaxation case the cross helicity consists of two components with opposite sign that have almost the same amplitude in the large wavenumber region. In both cases the system approaches a high correlation state, dependent on the initial condition. These results are consistent with observational data of the solar wind. Selective dissipation of magnetic energy, normal cascade of magnetic energy spectrum, and inverse cascade of magnetic helicity spectrum are observed for the sub-Alfvénic flow case, as was observed previously for the zero-flow case. When the flow velocity is super-Alfvénic, the relaxation process is altered significantly from the zero-flow case. [ABSTRACT FROM AUTHOR]

The splitting of a tokamak with elliptical and bean-shaped cross sections is studied for finite beta plasmas. When the plasma beta exceeds a critical value, an elliptical tokamak is subject to an ideal pressure-driven instability, which deforms the ellipse in such a way that a thinned plasma current sheet is formed around the magnetic axis. As a result, magnetic reconnection is nonlinearly driven and the ellipse is split. The bean-shaped tokamak, however, is stable against a splitting perturbation for limited equilibria that could be numerically constructed. An interesting similarity to the energy relaxation process in a force-free plasma, namely, a two-step evolution (initial occurrence of an ideal magnetohydrodynamic instability and subsequent occurrence of driven reconnection), is discussed. [ABSTRACT FROM AUTHOR]

A three-dimensional self-organization process in a compressible dissipative plasma is investigated in detail by means of a variational method and a magnetohydrodynamic (MHD) simulation. In the closed system where the total energy and total mass are conserved, the compressible MHD plasma relaxes to a maximum entropy state under the constraint that the total magnetic helicity is constant. This state is equivalent to a force-free equilibrium in which the pressure distribution is uniform. Magnetic reconnection converts the magnetic energy into the thermal energy in the vicinity of the reconnection points where the selective dissipation of magnetic energy effectively takes place owing to the generation of magnetic bubbles [R. Horiuchi and T. Sato, Phys. Fluids 29, 1161 (1986) ]. The enchanced thermal energy is carried away all over the system by the fast MHD waves. The swift expansion of the thermal energy leads to a uniform distribution of the pressure profile in the transit time scale of the fast MHD wave. Numerical simulation shows that the system tends to self-organize to a force-free state where the physical quantities are almost exactly the same as those of the zero pressure case. It is also observed that the magnetic energy spectrum cascades toward the high wavenumber region while the helicity spectrum inversely cascades toward the low wavenumber region. [ABSTRACT FROM AUTHOR]

A large-scale particle simulation of an intense relativistic electron beam injection into a longitudinally periodic and magnetized, three-dimensional plasma channel is performed with application to the steady-state current drive of tokamaks. It is found that the electromagnetically beam-induced electric field uniformly decelerates the beam electrons and generates a plasma return current as far as the injection continues. The beam electrons are decelerated also by the scalar potential field especially before the beam path becomes closed. A net longitudinal current, and hence the azimuthal magnetic field, is formed in the vicinity of the beam path after the return current has reached a steady force-balance equilibrium with the electric field and anomalous friction. The net saturation current is independent of the injection beam current and is scaled as I[sub net] ∼ (γ² - 1)[sup ½], where γ = (1 - υ[sup 2, sub b]/c²)[sup -½] and υ[sub b] is the velocity of the electron beam. For γ » 1, a macroscale helical instability develops and prevents the net current from reaching the level given by the aforementioned scaling. This instability is suppressed by application of a strong longitudinal magnetic field, which recovers the net current of the previous scaling. [ABSTRACT FROM AUTHOR]

A three-dimensional self-organization process in an incompressible dissipative plasma is investigated in detail by means of a magnetohydrodynamic simulation. In an initial phase the total magnetic energy rapidly decreases via magnetic reconnection driven by a global ideal unstable mode, while the total magnetic helicity decreases very slowly throughout the simulation run. This selective dissipation is found to be caused by the excitation of nonhelical, small spatial scale structures of the magnetic field (‘‘magnetic bubble’’) in the vicinity of the reconnection points. Magnetic reconnection takes place at minimum points of the magnetic helicity density where the magnetic bubbles are predominantly created. The disappearance of the bubbles leads to a rapid dissipation of the total magnetic energy. While the magnetic helicity is globally conserved during this process, its spectral density is transferred to the low wavenumber region (‘‘inverse cascade’’). After the rapid dissipation of the magnetic energy, the magnetic configuration adjusts itself to a stable equilibrium with a minimum energy without any drastic decrease in the magnetic energy and helicity. The final stable state takes either an axially symmetric configuration or a helically symmetric configuration depending on the initial condition, which is in good agreement with the relaxation theory. [ABSTRACT FROM AUTHOR]

By means of a three-dimensional (3-D) magnetohydrodynamic (MHD) simulation, the stabilization effects of conducting walls on the tilting instability of spheromaks created by the Princeton induction method are extensively studied. The nonlinear development and deformation of the spheromak resulting from global instabilities with toroidal modes n=1 (tilting), 2, and 3 are also studied in detail. Simulation results have shown that while the induction effect of the wall current is insufficient, the line-tying effect is much more powerful for the suppression of the strong tilting instability. For a perfect line-tying stabilization, a certain amount of surface magnetic flux around the spheromak should be tied to the conducting wall. Next, the influence of the shape of the cylindrical vacuum vessel on the stabilization is studied. It is found that the created spheromak assumes a similar figure irrespective of the vessel shape; thus we conclude that controlling the spheromak shape by changing the vacuum vessel is ineffective. Also investigated is the tilting behavior when the magnetic index is changed. The result is not conclusive in the sense that a well-developed oblate spheromak could not be created when the magnetic index was close to 1. Simulation runs in which n=2 and n=3 perturbations are applied, in addition to the tilt mode (n=1), have shown that the spheromak with a large aspect ratio is unstable to all of these modes. Interestingly, it is found that as the n=2 mode develops sufficiently, the spheromak torus is severely folded and twisted; consequently, the folded arms undergo reconnection internally. [ABSTRACT FROM AUTHOR]

The formation of an electrical potential difference between hot and cold plasmas is studied by means of a particle simulation. It is found that a double layer structure is formed on the hotplasma side of the contact surface between the cold and hot plasmas. The potential difference is given approximately by ϕ[SUBDL] ∼ T[SUBeh]/2e, where T[SUBeh] and e are the hot-electron temperature and the electronic charge, respectively. The double layer is accompanied by a negative potential dip on the Iow potential side (cold plasma) of the double layer, the depth of which is ϕ[SUBdip] ∼ 2 T[SUBec]/e, where T[SUBec] is the cold-electron temperature. Interestingly, however, the positive potential difference and the negative potential dip are created independently. [ABSTRACT FROM AUTHOR]

A one-dimensional electrostatic particle simulation model was constructed to investigate an evolution of the parallel electrostatic potential in the presence of a plasma flow along the mirror field. It is shown that a steady-state, large-scale dc potential structure is created as a result of a continuous plasma flow injection, and that the resultant potential difference increases in proportion to the ion bulk flow energy. In the magnetospheric environment, the potential difference is found to be able to exceed 2.5 kV. [ABSTRACT FROM AUTHOR]

The essence of nonlinear driven reconnection, which does not seem to be well understood by the reader as of yet, is described in order to avoid unnecessary misunderstanding of the magnetohydrodynamic serf-organization mechanism proposed by the present authors. [ABSTRACT FROM AUTHOR]