bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental study of kinetic effects on resonance cones
Authors: Piel A. , Oelerich G.
Published: March 2003

The fine structure of resonance cones in the lower hybrid range is investigated experimentally. For large antenna distances the shift of the main cone and the interference structure are found to be in agreement with the low-temperature approximation. On the contrary for short distances the measured interference pattern can only be described by the complete solution for the least damped root of the dispersion relation. The retrograde curvature of this branch is demonstrated. The determination of electron density and temperature from resonance cones can now be performed in an extended parameter range.
Island formation and destruction of flux surfaces in three-dimensional MHD equilibria
Authors: Reiman A. , Boozer Allen H.
Published: March 2003

The physics involved in the appearance of islands caused by resonant pressure driven currents in three-dimensional magnetohydrodynamic equilibria is described. Estimates of island widths are obtained by an expansion in β, with an auxiliary expansion about the magnetic axis. The theory is applied to Princeton’s heliac reference design.
Hamiltonian guiding center drift orbit calculation for plasmas of arbitrary cross section
Authors: White R. B. , Chance M. S.
Published: March 2003

A Hamiltonian guiding center drift orbit formalism is developed which permits the efficient calculation of particle trajectories in magnetic field configurations of arbitrary cross section with arbitrary plasma β. The magnetic field is assumed to be a small perturbation from a zero-order ‘‘equilibrium’’ field possessing magnetic surfaces. The equilibrium field, possessing helical or toroidal symmetry, can be modeled analytically or obtained numerically from equilibrium codes. The formalism is used to study trapped particle precession. Finite banana width corrections to the toroidal precession rate are derived, and the bounce averaged trapped particle motion is expressed in Hamiltonian form. Particle drift-pumping associated with the ‘‘fishbone’’ oscillation is investigated. A numerical code based on the formalism is used to study particle orbits in circular and bean-shaped tokamak configurations.
Enhanced particle losses and hollow distribution functions due to rf-induced velocity diffusion
Authors: Anderson D. , Lisak M.
Published: March 2003

An analytical study is made of the effects of ion-cyclotron resonance heating (ICRH) on high-energy particle containment in tokamaks. In particular, it is found that rf-induced velocity diffusion may significantly enhance scrape-off losses and ripple losses. A concomitant inversion of the distribution function of the heated ions is potentially dangerous since the excitation of wave instabilities could lead to a further increased particle loss because of quasilinear diffusion.
Current-driven instabilities of the kinetic shear Alfvén wave: Application to reversed field pinches and spheromaks
Authors: Meyerhofer D. D. , Perkins F. W.
Published: March 2003

The kinetic Alfvén wave is studied in a cylindrical force-free plasma with self-consistent magnetic fields. This equilibrium represents a reversed field pinch or a spheromak. The stability of the wave is found to depend on the ratio of the electron drift velocity to the Alfvén velocity. This ratio varies inversely with the square root of the plasma line density. The critical line density using the Spitzer–Harm electron distribution function is found for reversed field pinches with deuterium plasmas to be approximately 2×1018 and is 5×1017 m−1 in spheromaks with hydrogen plasmas. The critical line density is in reasonable agreement with experimental data for reversed field pinches.
Ballooning precessional instabilities in a single-cell hot-electron plasma
Authors: Tsang Kang , Lee X. S. , Hafizi B. , Antonsen Thomas M.
Published: March 2003

A fourth-order differential eigenequation along a field line for a hot-electron plasma in a single-cell mirror is derived in the large hot-electron precession frequency limit. This eigenequation is investigated analytically in the electromagnetic-flute limit and numerically. Electrostatic ballooning modes can couple with the hot-electron precessional motion and lead to instability if the frequency matching condition is satisfied. The new stability boundary due to this coupling is explored numerically in appropriate parameter space.
Instabilities in magnetically insulated gaps with resistive electrode plasmas
Authors: Chang C. L. , Antonsen Thomas M. , Ott Edward , Drobot Adam T.
Published: March 2003

Collisional plasma layers exist on the electrode surfaces of magnetically insulated gaps. Coupling of the cathode electron sheath with these plasma layers leads to instabilities. These instabilities are of two types: (i) a destabilized negative energy mode on the electron sheath and (ii) a new low-frequency resistive mode. The growth rates and physical picture of these modes suggest that they might be extremely dangerous in practice, leading to sporadic breakdown of magnetic insulation.
Dislocation glide observed in bimodal convection
Authors: Whitehead John A.
Published: March 2003

Both dislocation climb and glide are known to occur in solid-state physics, but only climb has been observed in dislocation defects (pinches) in convection cells. A search of movies of convection has uncovered observations of dislocation glide in the cross rolls in bimodal convection. The climb also exists with the dislocation moving in a direction opposite of the climb previously observed in convection rolls, i.e., toward long wavelength.
The dynamics of a columnar vortex in an imposed strain
Authors: Neu John C.
Published: March 2003

Exact solutions of the time-dependent Euler equations are presented which correspond to uniform columnar vortices with elliptical cross sections undergoing rotation and deformation in a uniform three-dimensional straining flow. In this work, the recent solutions of Kida [J. Phys. Soc. Jpn. 50, 3517 (1981)] are generalized to include the effect of a strain component along the axis of the vortex which results in its stretching. The time-dependent solutions should play a very useful role in modeling time-dependent vortex interactions in more complicated flows such as the mixing layer.
The effect of radius ratio on the stability of Couette flow and Taylor vortex flow
Authors: DiPrima R. C. , Eagles P. M. , Ng B. S.
Published: March 2003

The effect of radius ratio on the stability of Couette flow to axisymmetric and nonaxisymmetric disturbances and the stability of Taylor vortex flow to nonaxisymmetric disturbances is considered. It is assumed that the outer cylinder is at rest. As the ratio η of the radius of the inner cylinder to the radius of the outer cylinder is decreased, it is found that (i) the number of unstable normal modes for Couette flow at a fixed Taylor number decreases rapidly, and (ii) there is a critical value of η equal to about 0.65, below which there is no instability of the Taylor vortex flow for the range of Taylor numbers considered here. Recent experimental observations are summarized.
Nonequilibrium statistical mechanics of two-dimensional turbulence
Authors: Qian J.
Published: March 2003

A complete set of independent real parameters and its dynamic equation are worked out to describe the vorticity dynamics of two-dimensional turbulence. The corresponding Liouville equation is solved by a perturbation method upon the basis of a Langevin–Fokker–Plank Model. The dynamic damping coefficient η of the LFP model is treated as an optimum control parameter to minimize the error of the perturbation solution. Thereby two integral equations, the enstrophy equation and the η equation, are obtained for two unknown functions: the spectrum and the η. The equilibrium spectrum for the inviscid case is obtained as a stationary solution of the enstrophy equation. The nonlocalness of the enstrophy transfer makes the enstrophy equation divergent for a simple power-law spectrum. In order to avoid the divergence problem, a localization factor g is introduced to characterize the actual spectrum. Finally, the localized forms of the two integral equations are numerically solved, leading to the inertial-range spectrum, E(k)=1.82(ln g−1.23)−2/3χ2/3k−3 for g≥10, χ is the dissipation rate of the enstrophy.
Stimulated Brillouin sidescattering with strong ion wave damping
Authors: Speziale Thomas
Published: March 2003

An exact analytic solution is obtained for stimulated Brillouin sidescattering of light by a plasma slab of finite width in the limit of strong ion-acoustic wave damping. Brillouin sidescattering is shown to predominate whenever the lateral extent of the plasma exceeds the length required to produce backscattering. Under certain conditions the sidescattered light can become more intense than the incident light.
Comment on ‘‘Collisional diffusion of a plasma in multipolar and picket fence devices’’ [Phys. Fluids 2
6, 545 (1983)]
Authors: Jones R.
Published: March 2003

A comment is made on Collisional diffusion of a plasma in multipolar and picket fence devices.
Rising bubbles in a two-dimensional tube with surface tension
Authors: Vanden-Broeck Jean-Marc
Published: March 2003

The motion of a two-dimensional bubble rising at a constant velocity U in a tube of width h is considered. The acceleration of gravity g and the surface tension T are included in the dynamic boundary condition. For T=0, the results of Garabedian and Vanden-Broeck show that a solution exists for all values of the Froude number F=U/(gh)1/2 smaller than a critical value Fc∼0.36. In this paper accurate numerical solutions with T≠0 are computed by series truncations. The results show that for each value of T≠0, there exists a countably infinite number of solutions. Each of these solutions corresponds to a different value of F. As T tends to zero, all these solutions approach a unique limiting solution characterized by F=F*∼0.23. Therefore, the degeneracy associated with T=0 is removed by including the effect of surface tension. In addition, the profile corresponding to F=F* is found to be in good agreement with experimental data.
Time-dependent thermal convection in a stably stratified fluid layer heated from below
Authors: Ueda Hiromasa , Komori Satoru , Miyazaki Takeshi , Ozoe Hiroyuki
Published: March 2003

Time-dependent thermal convection resulting when a stably stratified fluid layer is heated from below is analyzed as an initial-value problem of a linear model. The effect of stratification on its development and structure is investigated. The convection is found to develop in three stages. Initially the given disturbance induces oscillatory motion with the corresponding Brunt–Väisälä frequency. In the second stage the stratification works to form secondary cells above the primary cell. This multiple cell pattern, however, changes slowly but periodically with time. Finally, the convection grows with an exponential growth rate, retaining a similar cell pattern. The vertical scale of the primary cell decreases with the stratification level, while the horizontal scale remains constant. The onset time of the manifest convection is predicted to be delayed as the stratification shifts from neutral to stable conditions. Attaining a maximum, it then becomes slightly earlier. Experimental results supporting the model predictions are also presented.
Symmetric and antisymmetric solutions for nonlinear wave modulations in plasmas
Authors: Rao N. Nagesha , Varma Ram K.
Published: March 2003

The existence of symmetric and antisymmetric stationary localized solutions for nonlinear amplitude-modulated Langmuir waves and circularly polarized electromagnetic waves in plasmas is considered. It is shown that the antisymmetric Langmuir solitons exist in the entire range of the Mach number, namely, 0<M<1, whereas antisymmetric electromagnetic solitons exist for all values of the Mach number. The parameter regions for the existence of different types of symmetric soliton solutions are obtained explicitly. It is found that both the density-hump and the density-dip electromagnetic solitons exist for subsonic as well as supersonic values of the Mach number.
Strong electromagnetic pulses in magnetized plasmas
Authors: Rao N. Nagesha , Shukla P. K. , Yu M. Y.
Published: March 2003

Circularly polarized intense electromagnetic waves along an external magnetic field can nonlinearly produce large amplitude field-aligned electrostatic potentials. The relevant two-fluid equations together with Maxwell’s equations (incorporating the departures from the quasineutrality condition) are shown to be integrable when the radiation pressure dominates over the plasma pressure. The parameter regimes for the existence of large amplitude localized solutions in a magnetoplasma are explicitly obtained through an analysis of the energy integral. The small amplitude limits are also discussed.
Nonlinear evolution of the lower-hybrid drift instability
Authors: Brackbill J. U. , Forslund D. W. , Quest K. B. , Winske D.
Published: March 2003

The results of simulations of the lower-hybrid drift instability in a neutral sheet configuration are described. The simulations use an implicit formulation to relax the usual time step limitations and thus extend previous explicit calculations to weaker gradients, larger mass ratios, and long times compared with the linear growth time. The numerical results give the scaling of the saturation level, heating rates, resistivity, and cross-field diffusion and a demonstration by comparison with a fluid electron model that dissipation in the lower-hybrid drift instability is caused by electron kinetic effects.
A physical model of nuclear lightning
Authors: Gardner R. L. , Frese M. H. , Gilbert J. L. , Longmire C. L.
Published: March 2003

A model is presented which is consistent with the observed features of nuclear lightning. The problem is divided into two parts. First, an electrostatic problem is solved, modeling the lightning channel as a perfectly conducting rod, to determine the focusing of fields and currents near the tip of the discharge. Second, a detailed self-consistent model of the air chemistry, heating rates, and current flow patterns is presented. From the second model the growth rate of the tip is shown to be between 0.8×105 and 1.5×105 m/sec consistent with photographic evidence, and the tip remains sharp, as observed, rather than becoming bulbous and dissipating.
Measuring plasma drift velocities in tokamak edge plasmas using probes
Authors: Stangeby P. C.
Published: March 2003

Plasma drift or rotation is caused at the edge of tokamaks by various mechanisms including flow to surfaces and neutral injection. These drift velocities can be measured using electrostatic probes and a theory is presented providing a method of probe analysis to deduce drift velocity, plasma density, and temperature. Initial probe experiments using this probe analysis yield credible values of drift velocity, but comparison with an independent technique, e.g., spectroscopic Doppler shifts, is sought. OFF
Nonlinear saturation of stimulated Raman scattering in a homogeneous plasma
Authors: McKinstrie C. J. , Simon A. , Williams E. A.
Published: March 2003

Nonlinear saturation of the stimulated Raman scattering instability is considered in an infinite homogeneous plasma. The ‘‘incident’’ wave amplitude exceeds its threshold value by the fractional amount Δ(≪1). A single backscattered electromagnetic wave and a single plasma wave grow until time-asymptotic saturation occurs. The squared amplitude and nonlinear frequency shift of each wave are determined by secular-free perturbation theory and are proportional to Δ. The saturation mechanism is identified and the ‘‘reflection coefficient’’ determined. Both saturation amplitudes are shown to be stable to small deviations from their time-asymptotic values.
Tilt and shift mode stability in a spheromak with a flux core
Authors: Finn John M.
Published: March 2003

The stability of spheromak equilibria with a flux core, or reversal coil, is studied by means of an ideal magnetohydrodynamic (MHD) code. Results depend critically upon whether the flux hole region (the current free area just inside the separatrix) is treated as a perfectly conducting plasma or as a vacuum. This indicates that the tilt and shift modes persist as resistive instabilities if they are stable in ideal MHD. Specifically, for nonoptimally shaped equilibria, the flux core must nearly touch the current channel if the flux hole is a vacuum, whereas the core may be slightly outside the separatrix if the flux hole has conducting plasma. A larger margin exists for optimally shaped equilibria.
Conductivity of weakly and highly ionized gases in quasistatic electric fields
Authors: Kunc Joseph A.
Published: March 2003

Simple and accurate expressions for electric conductivity in steady-state weakly and highly ionized plasmas are presented. These expressions give good agreement with exact ‘‘4×4 matrix formulation’’ numerical calculations. The plasmas are assumed to be isotropic, i.e., the applied magnetic field is neglected. The applied electric field is either static (dc) or of low frequency.
A simple model for gas bubble drag reduction
Authors: Legner Hartmut H.
Published: March 2003

A simple stress model has been developed to explain the observations of reduced drag when small gas bubbles are introduced into a turbulent boundary layer. The drag reduction is caused by a combination of density reduction and turbulence modification. The maximum reduction is obtained when the gas volume fraction approaches the bubble packing limit; the medium viscosity also increases markedly in this limit and becomes the important factor in restricting further reduction in drag. The derived analytical expression represents experimental data well.
Onset of oscillations in Rayleigh–Bénard convection: Horizontally unbounded slab
Authors: Edwards Boyd F. , Fetter Alexander L.
Published: March 2003

Rayleigh–Bénard convection in a laterally unbounded classical fluid layer with low Prandtl number 𝒫 (ratio of kinematic viscosity to thermal diffusivity) is reexamined. An amplitude expansion with only a few normal modes yields lateral oscillations of the convective rolls, which are therefore only weakly nonlinear. For free boundary conditions, additional modes (absent for rigid boundaries) lead to long wavelength (‘‘hydrodynamic’’) oscillations, with explicit nonlinear distortions in the velocity and temperature fields. For oscillations with rigid boundaries, the finite critical wavenumbers are approximately independent of 𝒫 for small 𝒫, and the calculated Rayleigh number, frequency, and wavenumber at onset agree well with observations in air. Discrepancies with experiments in dilute superfluid 3He–4He systems with small aspect ratios (ratio of horizontal to vertical dimensions) suggest that lateral boundaries or two-fluid effects play an important role in these systems.
Approximate analysis for resonance of an incompressible shear layer plus edges
Authors: Durbin P. A.
Published: March 2003

A method for approximately analyzing the feedback between downstream and upstream edges in incompressible shear flow is described. The shear flow is modeled by a vortex sheet. Equations for resonance eigenvalues are derived. After the reduction of growth rate by finite shear layer thickness is allowed for, agreement is found between calculated resonances and those that have been observed experimentally.
Collisional tearing in field-reversed configurations
Authors: Hassam A. B.
Published: March 2003

It is shown by employing a more complete Ohm’s law that field-reversed plasma configurations may magnetically tear at a rate proportional to η1/2, where η is the resistivity. Such a growth rate obtains if the current layer width is small enough and the plasma is not too collisional, the rate reverting to the usual η3/5 scaling otherwise. The present theory, however, assumes cold ions, an assumption that would have to be relaxed for most applications.
Ion end losses from an electrostatically plugged cusp
Authors: Parks P. B. , Sleeper A. M.
Published: March 2003

A general expression for the particle end-loss rate from a low-beta cusp with confining electrostatic potentials is derived. The derived loss rate is a function of flux surface, reflecting the breakdown in adiabaticity and hence magnetic containment in the region near the separatrix. In the fully nonadiabatic region, the result reduces to the previously derived expression of Yushmanov. Moving away from the separatrix into the adiabatic region, the loss rate decreases monotonically to a level of about one third the rate of the nonadiabatic region. Our result for the loss rate in the adiabatic region is enhanced over magnetic square-well estimates by a factor of three to four, which we attribute to geometrical effects owing to the curved magnetic field lines, in agreement with other calculations.
Decay instability of an extraordinary em wave into electron Bernstein and electron-acoustic waves in mirror plasmas
Authors: Sharma R. P. , Kumar A.
Published: March 2003

The parametric decay instability of an extraordinary electromagnetic wave into electron Bernstein and electron-acoustic waves in a plasma having ion temperature greater than electron temperature, is considered. Application of the present investigation to mirror devices has also been pointed out. For typical sets of parameters of the TMX-U (tandem-mirror) machine, the growth time of the instability comes out to be ≂0.07 μsec at a pump power flux of ≂10 W/cm2, and the threshold power at a few mW/cm2.
Theory of the rippled field magnetron
Authors: Chang C. L. , Ott Edward , Antonsen Thomas M. , Drobot Adam T.
Published: March 2003

The rippled field magnetron, a device for the generation of high-power, short-wavelength electromagnetic radiation, is studied theoretically. This device features smooth conducting coaxial anode and cathode electrodes, an axial insulating magnetic field, and a periodic azimuthal transverse wiggler magnetic field. The analysis treats the equilibrium in the fluid limit, including self-fields, and is fully self-consistent. Unstable coupling and growth of the positive energy (transverse electric TE) and negative energy (transverse magnetic TM) waves is demonstrated.
Observation of the two-dimensional structure of a stationary and transient double layer in current-carrying magnetized plasma
Authors: Fujita H. , Yagura Shinya , Matsuo Keiji
Published: March 2003

Observation is made of the two-dimensional structure of a stationary and transient double layer which was formed by a locally applied additional discharge in a magnetized plasma. The two-dimensional double layer is V-shaped. A very large negative potential dip of approximately 16 V [≂(3–5)Te/e] on the low potential tail of the layer is transiently created. The formation process of the layer and the dependence of the dip depth on the limiting current passing through the plasma are also discussed. The observed steady state is confirmed by an analysis. OFF OFF
Spectroscopic study of scattered light at around the fundamental wavelength in UV laser-produced plasmas
Authors: Tanaka K. , Goldman L. M. , Seka W. , Short R. W. , Williams E. A.
Published: March 2003

Spectroscopic studies of scattered light from UV laser plasmas near the fundamental wavelength are reported. Three distinct spectral components are observed in these experiments. Doppler-broadened spectra are observed in transmitted light from thin targets and in specularly reflected light from tilted targets. At intensities above 1014 W/cm2, stimulated Brillouin scattered spectra are measured in backscatter for both normal and oblique incidence of the laser. An additional new component is observed in backscattering from targets at normal incidence. This component is broad and blue-shifted, and is consistent with analytical model calculation based on filamentation.
Electrical potentials at probes in supersonic plasma flow
Authors: Kwan J. , Ahlborn B.
Published: March 2003

Pressure and temperature gradients across a bow shock in a (partially) ionized gas induce an electrical potential that can be used to drive a current. The voltage difference between electrodes (probes) mounted upstream and downstream of the shock depends on the shock strength (flow Mach number and oblique shock angle) and on the difference of the electrode work functions. Such a bow shock generator has an internal resistance that is governed by the electron temperature and the ion current density at the colder (upstream) electrode. The bow shock generator voltage (typically one volt) was measured as a function of the extracted current (up to 0.8 A/cm2) for different shock angles and flow Mach numbers. A theoretical model that agrees well with these experiments has been developed.
The spreading of the distribution function of an ion ring under collisional slowdown
Authors: Ruchti C. B. , Lovelace R. V.
Published: March 2003

A study is made of the slowing down of general, self-consistent, large-orbit ion rings due to small-angle Coulomb scattering in a dense, cold plasma. An earlier study was restricted to rings with distribution functions f( 𝒫θ, ℋ ) which were narrowly peaked at 𝒫̄θ and ℋ̄, where 𝒫θ is the single-particle canonical angular momentum and ℋ is the kinetic energy. In the present work, general distribution functions with arbitrary spreads in 𝒫θ and ℋ are treated by representing f( 𝒫θ, ℋ ) as a sum of many delta-function contributions. The self-consistent collisional evolution of the coupled many-delta function system is determined numerically. The reversal factor δ of field-reversed rings is seen to gradually increase initially, as in the earlier study. However, δ(t) decreases rapidly with time for t>t*, where t* is the time required for the lowest-energy particles to slow down to zero energy.
A theoretical study of the thermal electrohydrodynamic induction pump with harmonics and inhomogeneous fluids
Authors: Krein Philip T.
Published: March 2003

A theory of the electrohydrodynamic induction pump is described which includes charge convection, inhomogeneous fluid properties caused by temperature gradients, and harmonics in the electric traveling wave. The case of stable laminar flow in a wide reentrant rectangular channel is presented as an examle. Results are more realistic than earlier models. A limiting synchronous speed appears, as in magnetic induction devices. Velocity profiles become flatter than a parabolic profile as operating conditions of practical significance are reached.
Development of spot-like turbulence in plane channel flow
Authors: Biringen S. , Maestrello L.
Published: March 2003

A numerical simulation of the final stages of transition in plane chnnel flow is presented. Investigation of various flow field quantities provides some new insight into the breakdown stage and the formation of spot-like packets of turbulence.
Observation of microwave generation from a cusptron device
Authors: Namkung Won
Published: March 2003

The first observation of microwave generation at the sixth harmonic of the electron cyclotron frequency using a 17 kV, 0.6–1.0 A, 3 μsec electron beam is reported. This device, which is called a cusptron, produces radiation from the resonant interaction of a nonrelativistic rotating electron beam and a magnetron-type conducting boundary with six vanes by the negative mass instability. The observed radiated power is approximately 500 W at 4.38 GHz.
Large-scale stationary convection in a cylindrical current-carrying plasma
Authors: Gomberoff L. , Hernández M.
Published: March 2003

Nonideal effects such as viscosity and thermal conductivity on the stability of a cylindrical current-carrying plasma in a shearless magnetic field are studied. It is shown that for magnetic fields satisfying Bθ/Bz ≫1, nonideal effects drastically change the shape of the unstable spectrum of the ideal case. In particular, there is a physical mode at k∥ =m−nq=0 which, for a given value of the product of the coefficient of perpendicular viscosity and thermal conductivity, triggers large-scale steady convection in the plasma. This state is shown to be the analog in a plasma of the well-known stationary convection in ordinary hydrodynamics.
Landau damping in the presence of an electrostatic field
Authors: Singh R. N. , Singh V.
Published: March 2003

A kinetic formulation of Landau damping in the presence of a small-scale electrostatic field is given. Magnetospheric plasma waves are often subjected to such electric field configurations. It is argued that the resulting dispersion of plasma waves and their damping or growth rate may help in developing a diagnostic tool.
Spectrum of low-frequency, nonaxisymmetric oscillations in a cold, current-carrying plasma column
Authors: Appert K. , Vaclavik J. , Villard L.
Published: March 2003

The spectrum of low-frequency, m= ±1 oscillations in a cold, collisionless, cylindrical plasma comprises the frequencies of the lowest radial eigenmodes and the surface modes of the fast magnetosonic wave, all the global eigenmodes of the Alfvén wave (ion-cyclotron wave), and the Alfvén continuum. The spectrum of a homogeneous, currentless plasma cylinder is obtained from an analytical dispersion relation. This spectrum is compared with numerically obtained spectra of nonuniform plasmas with and without axial current. The connection of the magnetohydrodynamic (MHD) kink and internal kink instabilities with the global eigenmodes of the Alfvén wave is evidenced. Among all the modes of the magnetosonic wave which have frequencies in the Alfvén continuum, the (m= −1, k<0) -surface mode is the best candidate for antenna–plasma coupling because its conversion layer is located well into the plasma interior.
Neoclassical transport due to large orbit effects in bumpy tori
Authors: Hastings D. E.
Published: March 2003

In an asymmetric device like the ELMO Bumpy Torus (EBT), the radial width of the collision orbits is not determined fully until the self-consistent ambipolar potential is known. There is a class of potentials for which the orbit width is not small compared with the equilibrium scale lengths, thus calling into question standard linear diffusion theory. A diffusion equation is derived in the constants of motion space for a hot minority nearly Maxwellian species undergoing large orbits in a cold dense species. This equation is solved to all orders in the step-size parameter, and the solution is examined in various limits.
Nonresonant ELMO Bumpy Torus transport coefficients in the small electric field regime
Authors: Hastings D. E. , Tolliver J. S. , Hedrick C. L.
Published: March 2003

A conservative BGK collision operator is used to obtain nonresonant neoclassical transport coefficients for a bumpy torus when the ∇B drift dominates over the E×B drift. Previous nonresonant coefficients have considered the opposite limit. For large collisionalities, the diffusion coefficients are only weakly dependent on the E×B drift, while for small collisionalities, plateau diffusion coefficients are obtained which have an exponential dependence on the E×B drift.
Suppression of tearing mode growth by externally imposed resonant magnetic islands
Authors: Chu M. S. , Ikezi H. , Jensen T. H.
Published: March 2003

Externally imposed resonant magnetic islands may be used to slow down the growth rate of the tearing mode via Rutherford’s mechanism. However, this is effective only if the harmonic number of the externally imposed islands is sufficiently high.
Microwave absorption and plasma heating due to microwave breakdown in the atmosphere
Authors: Woo Wee , DeGroot J. S.
Published: March 2003

The propagation and absorption of microwaves above the breakdown threshold in the atmosphere with the self-consistent breakdown plasma are investigated. Typical hydrodynamic calculations show that an ionization front is rapidly formed which moves toward the microwave source and consequently decouples the microwaves from the original ionization region. By focusing the microwaves or using a reflector, ionization can be confined to localized regions where the microwave strength is high enough to cause breakdown even though the incoming microwaves are below threshold. In a strongly collisional atmosphere, it is found that the cutoff plasma density, which roughly equals the collisionless cutoff density multiplied by the collisionality, is much higher than the calculated maximum density. This results in high absorption that increases with microwave power, decreases with atmospheric pressure, and is quite independent of other parameters. In a weakly collisional atmosphere, breakdown easily creates a plasma density higher than the cutoff density and causes reflection. It is found that the reflection decreases with the collisionality of the system and is quite independent of the microwave strength. In general, the microwave field strength in the ionization regions is attenuated to the breakdown value at steady state, and the resulting electron temperature is about 2 eV, independent of the incident microwave flux. Limitations of the calculations are due to the availability of experimental data for the rate coefficients, but comparison to results from recent focused microwave experiments shows excellent agreement.
Linear theory of cusptron microwave tubes
Authors: Uhm Han S. , Kim Chung M. , Namkung Won
Published: March 2003

Stability properties of the negative-mass instability in a rotating annular electron beam (E layer) are investigated, in connection with applications on the cusptron microwave tubes. The analysis is carried out for an infinitely long E layer propagating through a magnetron-type conducting wall and propagating parallel to an applied axial magnetic field. It is assumed that the E layer is thin and very tenuous. A closed algebraic dispersion relation of the negative-mass instability is obtained, including the important influence of conducting boundaries on the mode control in microwave generation and amplification. It is shown that for typical present experimental beam parameters, the gain of the cusptron normalized to the excitation frequency can be comparable to that of the gyrotron. Moreover, under the appropriate geometric configuration, perturbations with azimuthal mode number N are dominantly unstable. This optimizes the microwave power output of the radiation with frequency ω≂Nωc, where ωc is the electron cyclotron frequency and N is the resonator number in the conducting wall.
Plasma heating by a relativistic electron beam
Authors: Janssen G. C. A. M. , Bonnie J. H. M. , Granneman E. H. A. , Krementsov V. I. , Hopman H. J.
Published: March 2003

Reported are measurements on the interaction between a relativistic electron beam (REB) with the parameters 800 kV, 6 kA, 50–150 nsec, and a plasma with a density of ne=1.0×1019 m−3–1.0×1020 m−3. The electron temperature during and after the beam pulse is obtained by means of Thomson scattering. Also measured is the angular distribution of the beam electrons as a function of time and position. By varying the angular spread of the beam it is possible to pass from a kinetic to a quasihydrodynamic interaction. In both regimes measurements are compared with the appropriate theoretical model. Energy transfer is largest in the quasihydrodynamic regime and amounts to 2.5×103 J/m3 or 2.2×1016 eV/cm3. The electron temperature reaches values of 150 eV and appears limited by the electron heat conduction along the magnetic field.
Observation of high-frequency fields due to the interaction of a relativistic electron beam with a plasma
Authors: Janssen G. C. A. M. , Granneman E. H. A. , Hopman H. J.
Published: March 2003

Reported are measurements on the high-frequency fields generated by a relativistic electron beam plasma interaction in the BP II experiment. Measurements are done at fixed positions as well as integrated over the length of the experiment. At fixed positions time-resolved spectroscopy on hydrogen and helium lines is applied. The integrated measurements are performed with a magnetic energy analyzer, which measures the energy distribution of the relativistic electron beam at the end of the experiment. Spectroscopic measurements of the dynamic Stark effect and forbidden transitions lead to hf fields with amplitudes in the order of 1 MV/m. The interpretation of the beam energy spectra suggests localized fields with field strengths as great as 108 V/m.
Ion acoustic nonlinearities in stimulated Brillouin scattering
Authors: Heikkinen J. A. , Karttunen S. J. , Salomaa R. R. E.
Published: March 2003

Reduction of stimulated Brillouin scattering by the generation of higher ion wave harmonics is investigated. A theory model is derived which includes all harmonics. A simplified model based on the truncation to second harmonic ion waves reveals several interesting features. When damping and detuning are neglected the reflectivity is drastically reduced in a low-density plasma (ne<0.25ncr), but the solution turns phase unstable at low reflectivity levels. Finite damping of ion waves extends the range of validity of the model. Still nonlinear damping of the primary decay may considerably exceed the linear damping, rendering the generation of the second harmonic an important mechanism. A large detuning of the second harmonic is required before it can be considered decoupled. The strong interaction between the ion waves introduces a power shift which is able to dynamically reduce the Brillouin reflectivity. Undersone parameter conditions the reflectivity approaches an oscillatory steady state. The accuracy of the second harmonic model rapidly improves as damping and detuning of the ion waves increase, suggesting that harmonic generation is important in a medium-length (L/Lg<10), slightly damped plasma.
Calculation of the wall-pressure field in a turbulent channel flow
Authors: Handler R. A. , Hansen R. J. , Sakell L. , Orszag S. A. , Bullister E.
Published: March 2003

The wall-pressure field in a turbulent channel flow has been obtained from a numerical solution of the Navier–Stokes equations. The root mean square pressure normalized with respect to the wall shear stress, as well as the convection velocity of the field, are in good agreement with experiment. Certain limitations of the calculation, associated with its finite spatial resolution, are discussed.
Magnetic fluctuations in high-βp tokamak plasmas
Authors: Lee J. K. , Chu M. S. , Helton F. J. , Park W.
Published: March 2003

The systematics of magnetic fluctuations resulting from the nonlinear evolution of high-βp long parallel wavelength resistive magnetohydrodynamic modes are studied for circular tokamak plasmas with a free boundary. Large-amplitude fluctuations cause increased magnetic field stochasticity with increasing εβp. These fluctuations, when coupled to an anomalous energy loss mechanism, may lead to βp saturation with respect to total input power.
Subsonic turbulent flow over a rearward facing segmented step
Authors: Gai S. L. , Sharma S. D.
Published: March 2003

Flow visualization experiments are conducted to study the phenomena of separation and reattachment behind a segmented step in subsonic turbulent flow. The results show that the effect of castellations on the step face is to produce strong three-dimensional disturbances in the initial recirculation region. This is followed by a complex reattachment process and a transition region in which the three-dimensional disturbances are attenuated, the flow eventually resembling a two-dimensional flow in the relaxation far downstream.
Soft x-ray array results on the ZT-40M reversed-field pinch
Authors: Wurden G. A.
Published: March 2003

Spatially resolved soft x-ray measurements on the ZT-40M reversed-field pinch using a surface barrier diode array are reported. In plasma discharges with x-ray sawteeth present, the sawteeth have an m=0, n=0 character. In normal discharges, without large-amplitude sawteeth, a new ly observed internal m=1 oscillation is described. This quasicontinuous m=1 mode may be a manifestation of a saturated magnetohydrodynamic tearing mode.
Suppression of the m=2 rotational instability in the field-reversed configuration by means of quadrupole fields
Authors: Harned Douglas S.
Published: March 2003

Hybrid simulations, which include kinetic ions, demonstrate that the m=2 rotational instability in the field-reversed configuration can be suppressed by the application of an external quadrupole magnetic field without stopping the plasma rotation.
The evolution of a flat eddy near a wall in an inviscid shear flow
Authors: Russell J. M. , Landahl M. T.
Published: March 2003

A flat eddy is defined by the condition ε=δ/l≪1, where l=(l−21+l−23)−1/2, and l1, l3, and δ are the streamwise, spanwise, and vertical scales of the eddy, respectively. An approximate solution of the initial value problem for the evolution of a flat eddy in an inviscid flow is obtained using a Lagrangian formulation together with an expansion of the Euler equations in a Taylor series in time. The effects of horizontal pressure gradients on the eddy evolution are found to be small for small times such that εU0t/l≪1, where U0 is a measure of the initial horizontal velocity. Nonuniformity of the spanwise stretching rate, together with the shearing of the eddy by the mean flow, is found to lead to the formation of internal shear layers, also when such layers were absent initially. The separate contributions to the streamwise fluctuation velocity due to redistribution of background momentum and direct driving by the pressure gradient are compared; the latter is found to be significantly smaller than the former and plays very little role in the shear layer formation process.
Simulation of drift-cyclotron-loss-cone modes in tandem mirrors with sloshing ions
Authors: Cohen Bruce I. , Maron Neil , Nevins William McCay
Published: March 2003

Particle simulations of drift-cyclotron-loss-cone modes in a tandem-mirror end cell confirm that sloshing-ion distributions and the concomitant trapping of low-energy ions by a depression in the ambipolar potential lead to improved microstablity. The simulations study the influences of neutral-beam-injection angle, the magnetic-mirror ratio, the size of the depression in the ambipolar potential, and warm streaming plasma on both plasma confinement and drift-cone instability. A simple nonlinear theory of velocity-space transport predicts wave amplitudes and end-loss rates at steady state that are reasonably consistent with the simulation results. This study supports both the basic strategy for sloshing-ion operation of tandem mirrors and corroborates the first measurements of ion-cyclotron modes in the TMX-Upgrade experiment at the Lawrence Livermore National Laboratory.
Parametric instabilities during electron cyclotron heating of tandem mirrors
Authors: Nicholson D. R.
Published: March 2003

Electron cyclotron resonance heating is one of the most commonly used methods of heating electrons in the plugs and in the thermal barriers of tandem mirrors. The intense coherent electromagnetic waves used for such heating are susceptible to parametric decay into other modes. Significant growth rates are found for the decay of either ordinary or extraordinary waves into two magnetized electron plasma waves. This and related effects may result in electron heating mechanisms rather different than those assumed in linear ray-tracing calculations. These results may help explain the unusual effects observed during heating of the Phaedrus tandem mirror device. In the general case, these instabilities may be strongly inhibited by density gradients.
Internal tilting and classical transport for field-reversed configurations based on the Maschke–Hernegger solution
Authors: Clemente R. A. , Grillo C. E.
Published: March 2003

It is shown that elongated field-reversed configurations based on the Maschke–Hernegger solution of the Grad–Shafranov equation are unstable to internal tilting. The particle transport properties across the flux surfaces of such a model are also considered in the limit of large elongation of the separatrix. An estimation of the time of confinement of particles in terms of classical conductivity, which is lower than previous estimates, is given.
Total heat transfer to cylindrical collectors in collisionless plasma flow
Authors: Chang Jen-Shih , Godard R. , Scott R. C.
Published: March 2003

The theory of total heat transfer to cylindrical collectors in collisionless plasma flow with no magnetic field is presented. The treatment has been done for orbit-motion-limited conditions (Debye length sufficiently large), for the speed ratio S=U/(2kT/m)1/2 of the flow velocity to the most probable ion thermal velocity from 0 to 10, and for various values of the nondimensional collector potential χp=qVp/kT.
Plasma sheath transmission factors for tokamak edge plasmas
Authors: Stangeby P. C.
Published: March 2003

Simple analytic relations are derived for the floating potential of a solid object in contact with a magnetized plasma as a function of the electron/ion mass ratio, the electron/ion temperature ratio, and the secondary electron emission coefficient. Simple analytic relations are also obtained for the sheath transmission factors, i.e., the particle and energy fluxes to floating and nonfloating surfaces. These analytic formulations, derived from a fluid model treatment of ion flow, are found to give results in close agreement with results derived from more complex analysis based on kinetic theory and numerical models. Tokamak scrape-off plasmas are often characterized by ion distributions which have finite drift velocities at the point where the ions enter the scrape-off plasma from the main plasma (via cross-field diffusion). The Bohm criterion governing the drift velocity of ions as they enter the electrostatic sheath in front of a limiter, divertor plate, or probe is generalized for this case.
Reflection of ion acoustic waves by the plasma sheath
Authors: Ibrahim I. , Kuehl H. H.
Published: March 2003

The reflection coefficient R for linear monochromatic ion acoustic waves incident on the transonic layer and sheath from the plasma interior is calculated. The treatment differs from previous analyses in that (1) the exact zero-order ion density and velocity profiles for a planar, bounded plasma are used, and the zero-order charge separation is not neglected, and (2) the first-order quantities near the transonic layer are considered in detail, including first-order charge separation, whereby it is found that no coupling to the beam modes exists, and that the functional form of the first-order solution is completely determined. It is shown that the upper bound for ‖R‖ is (1)/(3) . The largest reflection occurs for frequencies which are small compared with the ionization frequency, and generally decreases with increasing frequency. By Fourier superposition, the reflection of a pulse is computed. For a narrow incident pulse, the reflected pulse is greatly distorted and is small compared with the incident pulse. For a broad pulse, the reflected pulse is similar in shape to the incident pulse, and has a magnitude which is approximately (1)/(3) of the incident pulse.
Linear relativistic gyrokinetic equation
Authors: Littlejohn Robert G.
Published: March 2003

The linear, relativistic gyrokinetic equation is derived, in a form appropriate for relativistic electrons in fusion research. The frequency of the perturbing wave satisfies ω≪Ωe. The use of generally covariant transformation rules, as well as first-order Hamiltonian perturbation theory, greatly simplifies the derivation. New results are presented for relativistic guiding center motion.
The azimuthal correlations of velocity and temperature fluctuations in an axisymmetric jet
Authors: Sreenivasan K. R.
Published: March 2003

The structure of turbulence in an axisymmetric jet at the nozzle exit Reynolds number of 4×104 was examined at several streamwise stations (1.0≤x/D≤40), by measuring, for a few fixed radial positions, two-point correlation functions of the streamwise velocity and temperature fluctuations with azimuthal separation. By an orthogonal Fourier decomposition of the azimuthal correlation functions, an interpretation of these correlations and of the large-scale evolution is provided.