Index of content:
Volume 4, Issue 8, August 1997

Fishbone mode excitation in the ion kinetic regime
View Description Hide DescriptionBy solving the dispersion relation in the ion kinetic regime, it is found that the threshold of the plasma beta value for exciting the ionfishbone mode is lowered. Thus, for most of the presentday tokamaks where the Bussac criterion [Bussac et al., Phys. Rev. Lett. 35, 1638 (1975)] is not satisfied, it will still be possible to excite the ionfishbone mode.

Production of sheared flow during ion cyclotron resonance heating in tokamak plasmas
View Description Hide DescriptionAn approach of producing the poloidal ion rotation by using ion cyclotron resonance heating (ICRH) is presented in core tokamak plasmas. The mechanism employed here is inducing a poloidal density inhomogeneity by rf cyclotron heating and then destabilizing the anomalous Stringer spinup. A criterion for destabilization of the poloidal ion rotation in the presence of a rfwave is given, which depends on the ratio of the characteristic time of inhomogeneous density formation to the ion collision time. The numerical results have shown that the poloidal ion rotation can be destabilized in the present ICRH power level.

An electrostatic magnetohydrodynamics theory for resistiveviscous helical instabilities of arc discharges
View Description Hide DescriptionA simplified linear analysis for resistiveviscous magnetic helical instabilities of arc discharges in a cylindrical plasma is developed. Based on a set of electrostatic magnetohydrodynamic (MHD)equations, resistiveviscous modes with an external axial magnetic field are studied. Explicit analytic results are obtained, from which the growth rate and the stability criterion can be shown, and the electrostatic assumption can be justified. In comparison with the previous channel model calculations, this analytic treatment can provide a simplified model for instability estimates, while avoiding artificial assumptions and misorderings in the energy equation.

Effects of various forces on the distribution of particles at the boundary of a dusty plasma
View Description Hide DescriptionThe distribution and suspension of dust particles under the action of electrostatic, gravitational, iondrag and neutral collision forces are investigated near the boundary of a dusty plasma. It is shown that the competition among the forces results in spatial oscillations (multilayer) of the particle distribution. For submicron grains the iondrag has a significant effect on the grain dynamics while for micrometer sized grains the gravity quickly dominates over other forces. The effect of the neutral gas flux is to enhance or diminish that of the gravity while the effect of the neutral viscosity is to shift the profile toward the wall. Under the force balance, the particles are suspended in a narrow region with sharp boundaries within the sheath.

Electric field spikes formed by electron beam–plasma interaction in plasma density gradients
View Description Hide DescriptionIn the electron beam–plasma interaction at an electric double layer the beam density is much higher than in the classical beam–plasma experiments. The wave propagation takes place along the density gradient that is present at the high potential side of the double layer. Such a case is studied experimentally by injecting the electron beam from a plane cathode, without any grids suppressing the gradient, and by particle simulations. The high frequency field concentrates in a sharp “spike” with a half width of the order of one wavelength. The spike is found to be a standing wave surrounded by regions dominated by propagating waves. It forms at a position where its frequency is close to the local plasma frequency. The spike forms also when the electric field is well below the threshold for modulational instability, and long before a density cavity is formed in the simulations. Particle simulations reveal that, at the spike, there is a backward traveling wave that, when it is strongly damped, accelerates electrons back towards the cathode. In a simulation of a homogeneous plasma without the density gradient no spike is seen, and the wave is purely travelling instead of standing.

The dust charging effect on electrostatic ion waves in a dusty plasma with trapped electrons
View Description Hide DescriptionThe effect of the dust charging and the influence of the ion density and temperature on electrostatic nonlinear ion waves in a dusty plasma having trapped electrons are investigated by numerical calculation. The nonlinear structure of the dust charging is examined, and it is shown that the characteristics of the dust charge number sensitively depend on the electrostatic potential, Mach number, trapped electrontemperature, ion density, and temperature. An increase of the ion temperature decreases the dust charging rate and the propagation speed of ion waves. It turns out that a decrease of the trapped electrontemperature increases the charging rate of dust grains. It is found that the existence of ion waves sensitively depends on the ion to electron density ratio. New findings of variablecharge dust grain particles, ion density, and temperature in a dusty plasma with trapped electrons are predicted.

Direct magnetic field measurement of flow dynamics in magnetized coaxial accelerator channels
View Description Hide DescriptionA miniature magnetic probe array, consisting of ten spatially separated coils, has been used to obtain profile information on the timevarying magnetic field within the 2.54 cm wide flow channel of the Coaxial Plasma Source experiment (CPS1) [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)] at the North Carolina State University. Twodimensional (2D) current profiles within the annular flow channel, which were constructed from the timevarying magnetic field data, reveal several complex features reflecting the influence of gun inductance, the Hall effect, and the applied magnetic field. When an external, electrode linking magnetic field is applied, the evolution of the 2D current profile shows evidence of an ionizing shock front identified by a narrow current sheet propagating through the channel during the first few microseconds of the discharge. The thickness of this current sheet is on the same order as both the collisional meanfree path and the ion electromagnetic skin depth. In this applied field case, the plasma is prevented from advancing ahead of the current sheet by the applied magnetic field, which turns the ions and electrons without collisions. In the absence of an applied field, plasma is able to advance ahead of the current sheet, where it may initiate ionization downstream before the advance of the ionization front.

Ionizationinduced frequency upshift of a highpower microwave interacting with a plasma
View Description Hide DescriptionThe propagation of an intense electromagnetic wave (EMW), of frequency 9 GHz, in an underdense, near critical, inhomogeneous plasma ( ) leads to an upshift of the EMW frequency of a few MHz. The injected EMW with power, and with pulse duration of 1 μs, produces additional ionization of the neutral argon gas through the heating of plasma electrons. The increase of plasma density, which can reach during the EMW pulse several tens of percents in relative value, leads to a modulation in time of the spatial phase of the EMW. Experimental results for the evolution in space and in time of density, electric field, and frequency shift are compared to a simple nonlinear model of EMW propagation in a plasma, which takes into account the additional ionization due to the absorption of the EMW.

Intermittent selforganization: Nonlinear responses of twisting multiple flux tubes
View Description Hide DescriptionA new type of selforganization is presented where a system evolves intermittently and undergoes selfadaptively local maxima and minima of energy state. Numerical study of nonlinear interactions of twisting multiple flux tubes have shown that each flux tube suffers from a helical kink instability, resulting in the formation of a knotted structure. The knotted deformation stimulates reconnection with a neighboring flux tube whereby kinetic and thermal energies are impulsively and markedly released. Repeating reconnection intermittently with surrounding field lines, the whole structure returns to the more or less originally separated flux tubes and thereafter repeats an intermittent and recursive evolution. All these results lead to a working hypothesis that in an open complex nonlinear system where energy is externally and continuously supplied, the system exhibits an intermittent selforganization, selfadapting local maxima and minima of energy state alternatively in temporal evolution. The present work will also forecast this as some essential concept to an energy conversion problem such as solar flares.

Bäcklund transformations and Painlevé analysis: Exact solutions for the nonlinear isothermal magnetostatic atmospheres
View Description Hide DescriptionThe equations of magnetohydrostatic equilibria for a plasma in a gravitational field are investigated analytically. For equilibria with one ignorable spatial coordinate, the equations reduce to a single nonlinear elliptic equation for the magnetic potential μ̃, known as the Grad–Shafranov equation. Specifying the arbitrary functions in the latter equation, one gets the nonlinear elliptic equation. Analytical solutions of the elliptic equation are obtained for the case of a nonlinear isothermal atmosphere in a uniform gravitational field. The solutions are obtained by using the Bäcklund transformations technique and Painlevé analysis, which are adequate for describing parallel filaments of diffuse, magnetized plasma suspended horizontally in equilibrium in a uniform gravitational field.

Passive particle dynamics in a flow exhibiting transition to turbulence
View Description Hide DescriptionThe behavior of a passive particle in a flow that exhibits bifurcations in the transition to a turbulent regime is investigated. The flow considered is a variant of the Charney–Hasegawa–Mima equation. The scalar particle dynamics is considered for different regimes of the main flow. A regime of anomalous diffusion (hypodiffusion) is observed when the field has few harmonics whereas normal diffusion occurs in the strange attractor regime. The analysis of the singular orbit reveals the presence of traps and flights that control the transport.

Compressible magnetohydrodynamic Kelvin–Helmholtz instability with vortex pairing in the twodimensional transverse configuration
View Description Hide DescriptionFor a twodimensional (2D) transverse configuration, where the plasma motion occurs in a 2D plane transverse to the magnetic field, the nonlinear evolution of the magnetohydrodynamic(MHD) Kelvin–Helmholtz (K–H) instability is investigated by means of a 2D MHD simulation for a convective fast magnetosonic Mach number 0.35, which is defined for the total jump of the flow velocity. The compressibility and the nonzero baroclinic vector are shown to violate the conservation of the enstrophy for the 2D MHD transverse configuration and for the 2D fluid motion. After the nonlinear saturation of the linearly fastest growing vortices, the vortices continue to coalesce until no more vortex pairing is allowed, owing to a finite length of the simulation system. The plasma inside the vortex is rarefied strongly by the fast magnetosonic rarefaction and each vortex is associated with an eddy current, which is inertia current in nature. The plasma flow velocity is enhanced at the periphery of the vortex and the net momentum transport and shear relaxation by the instability occur as long as the vortex pairing continues. Anomalous viscosity by the K–H instability increases with the vortex pairing and its increase is due to the growth of subharmonic modes.

Formation of wavefront pattern accompanied by currentdriven electrostatic ioncyclotron instabilities
View Description Hide DescriptionFormation of a wavefront pattern accompanied by an electrostatic ioncyclotron instability driven by electrons drifting along a magnetic field is investigated by twoandhalf dimensional particle simulations. A clear spatial wavefront pattern appears as the ion cyclotron wave grows due to the instability. When the electron stream is uniform in the system, an obliquely intersected stripe wavefront pattern is formed. When the stream has a bellshaped pattern across the magnetic field, a Vshaped stripe wavefront pattern appears. The wave fronts have small angles with the magnetic field lines and propagate from the highstream region to the lowstream region.

Rarefactive ion acoustic soliton excitation using a modulated highfrequency sinusoidal wave in a negative ion plasma
View Description Hide DescriptionExperiments on the excitation of rarefactive ion acoustic solitons using a fine mesh grid in a negative ion plasma are described. The excitation is novel in that a modulated highfrequency sinusoidal wave voltage signal is applied to the grid. An interpretation of the velocity modulation and bunching of freestreaming ions that pass through the grid to which the signal is applied is given.

Magnetohydrodynamic stability of negative central magnetic shear, high pressure toroidal equilibria
View Description Hide DescriptionThe magnetohydrodynamic(MHD) stability of negative central magnetic shear toroidal equilibria with everywhere and is investigated. Here, is the safety factor of the equilibrium magnetic field in a torus with inverse aspect ratio , and is the ratio of the plasma pressure to the pressure in the poloidal magnetic field. At small , the elimination of the resonantsurface in a negative shear equilibrium greatly improves resistive MHD stability as compared to equilibria with a monotonic profile containing a resonantsurface. However, at large , the reversal of the central magnetic shear and the elimination of the resonantsurface does not improve MHD stability. The existence, or nonexistence, of rational magnetic surfaces has no impact on MHD stability when . Altering the current profile, and with it the profile, does not affect MHD stability when is no longer small. Stability is not improved by vertical elongation of the plasma in the poloidal plane. The utilization of an external vertical magnetic field to move the magnetic axis in major radius also does not improve MHD stability.

Description of turbulent transport in tokamaks by invariants
View Description Hide DescriptionIn general, turbulenttransport drives a plasma toward a state of turbulent equipartition, in which Lagrangian invariants are uniformly distributed. Different invariants decay with different rates, and in tokamaks the frozenin law of particles in the poloidal magnetic field survives longer than the corresponding law for the toroidal field, assuming that the trapped particles dominate the turbulenttransport. Therefore, the plasma profiles depend on the safety factor and the condition for convection of trapped particles is that the shear is positive. There are two ways to suppress this convection and thereby enhance confinement. The first one is to reverse the magnetic shear. The energy of typical trapped particles then increases outward instead of inward, which suppresses instabilities. The second method is to eliminate the trapped ions by poloidal rotation, and thereby create a transport barrier.

Plasma transport near the separatrix of a magnetic island
View Description Hide DescriptionThe simplest nontrivial model of transport across a magnetic island chain in the presence of collisionless streaming along the magnetic field is solved by a Wiener–Hopf procedure. The solution found is valid provided the boundary layer about the island separatrix is narrow compared to the island width. The result demonstrates that when this assumption is satisfied the flattened profile region is reduced by the boundary layer width. The calculation is similar to the recent work by Fitzpatrick [Phys. Plasmas 2, 825 (1995)] but is carried out in the collisionless, rather than the collisional, limit of parallel transport, and determines the plasma parameters on the separatrix selfconsistently.

Simultaneous measurement of viscosity and flow velocity in Texas Experimental TokamakUpgrade (TEXTU) edge plasmas by using a ViscoMach probe
View Description Hide DescriptionShear viscosity] and parallel flow velocity of tokamak edge plasma were simultaneously measured in the Texas Experimental TokamakUpgrade (TEXTU)tokamak by using a “ViscoMach” probe with the iteration method (VMPI) [KS. Chung, Nucl. Fusion 34, 1213 (1994)]. The VMPI was composed of two Mach probes measuring two ratios of ion saturation current densities, with the small Mach probe located within the free presheath generated by the large Mach probe (LMP). Radial variations of the normalized shear viscosity, α, and parallel Mach number, for typical discharges were deduced from two measured ratios of ion saturation current densities, and plasma density from ion saturation currents by LMP. Here α varied from 0.7 to 1.3 with an estimated error of and from 0 to 0.2 for where is the limiter radius. Also, α and increased with the radial direction, while crossfield diffusivity was approximately constant. Here is obtained as where a typical Bohm diffusivity is

Studies of transport scaling and reduction under feedback
View Description Hide DescriptionA feedback control system using an ion beam as a remote suppressor has been previously shown to be very effective in suppressing plasma instabilities in the Columbia Linear Machine [G. A. Navratil et al., Plasma Phys. 24, 184 (1982)]. The first experimental measurements for the effect of this feedback system on anomalous particle transport, as determined from the crosscorrelation of density and potential fluctuations is presented. It is shown that feedback reduces transport due to a rotational E×B mode by up to a factor of 3 in this experiment. Also, it was found that particle transport scales linearly with fluctuation amplitude and feedback control does not alter this scaling. Last, the experimentally observed scaling of particle transport does not agree with any theoretical predictions.

On the stabilization of neoclassical magnetohydrodynamic tearing modes using localized current drive or heating
View Description Hide DescriptionThe effectiveness of using localized current drive or heating to suppress the formation and growth of neoclassical magnetohydrodynamic(MHD) tearing modes is addressed. The most efficient way to use an auxiliary current source is to cause current to flow in the same direction as the equilibrium bootstrap current and phase the current relative to the magnetic island such that the current is deposited on the Opoint of the island. Theoretical estimates for the amount of required current to suppress the formation of a large magnetic island is of order a few percent of the equilibrium current. If the suppression is successful, the magnetic island will saturate at a width of order the radial localization width of the current source. Localized heating at the Opoint of the magnetic island can also produce stabilizing effects relative to magnetic island growth. The effects of the driven current or heating can be illustrated by using a phase diagram of the island growth.