bluuebunny/crossref_metadata_2025 huggingface dataset:
A test of equivalence of the variable-hard-sphere and inverse-power-law models in the direct-simulation Monte Carlo method
Authors: Koura Katsuhisa , Matsumoto Hiroaki , Shimada Toru
Published: July 2002

A test of the equivalence of the variable-hard-sphere (VHS) and inverse-power-law (IPL) models in rarefied gas flows is made for the molecular diffusion in a heat-bath gas and the two-dimensional rarefied gas flow around a circular cylinder using the null-collision Monte Carlo method. For the most severe test, the softest practical model corresponding to the Maxwell molecule is chosen. It is shown that the VHS diffusion is slower than the IPL one in the viscosity mean-free-path (mean-collision-time) scale but agrees with the IPL one in the diffusion scale. The VHS and IPL flow fields around a circular cylinder reveal a small but appreciable discrepancy in the usual viscosity scale. These facts should call attention to the equivalence of the VHS and IPL models especially when the molecular diffusion may play an important role in a (gas mixture) flow, where the diffusion scale may be preferable to the viscosity scale.
Period doubling in the wake of a three-dimensional cylinder
Authors: Rockwell D. , Nuzzi F. , Magness C.
Published: July 2002

Forced excitation of a long cylinder having a mild variation of diameter along its midspan generates period-doubled vortex formation. The onset of this localized, period-doubled response involves formation of diamond-shaped vortical structures during every other cycle of the cylinder oscillation.
Transonic flow of a fluid with positive and negative nonlinearity through a nozzle
Authors: Chandrasekar D. , Prasad Phoolan
Published: July 2002

The one-dimensional transonic flow of an inviscid fluid, which at large values of the specific heats exhibits both positive (Γ̄>0) and negative (Γ̄<0) nonlinearity regions {Γ̄=(1/ρ)[∂(ρa)/∂ρ]s} and which remains in a single phase, is studied. By assuming that Γ̄ changes its sign in the small neighborhood of the throat of the nozzle where transonic flow exists and introducing a new scaling of the independent variables, an approximate first-order partial differential equation (PDE) with a nonconvex flux function is derived. It governs both the steady transonic flows and the upstream moving waves near sonic point. The existence of continuous and discontinuous steady transonic flows when the throat area is either a maximum or a minimum is shown. The existence of standing sonic discontinuities and rarefaction shocks in the transonic flow are noted for the first time. Unlike in the classical gas flows, there are two sonic points and continuous transonic flows are possible only through one of them. The numerical evolution of those transonic waves that have both positive and negative nonlinearity in the same pulse is studied and some comments are made on the local stability of the particular steady flows.
Gap size effects on centrifugally and rotationally driven instabilities
Authors: Mutabazi Innocent , Normand Christiane , Wesfreid José Eduardo
Published: July 2002

The rotation effects on centrifugally driven instabilities in curved channel flow with a finite gap are investigated. An inviscid criterion of stability is formulated to explain the behavior of the flow when rotation and curvature effects compete to either stabilize or destabilize the flow. The stability of curved Poiseuille flow with finite gap size is studied, and it is shown that the asymmetry between the directions of rotation is enhanced when the gap size increases.
Boundary-layer receptivity due to a wall suction and control of Tollmien–Schlichting waves
Authors: Bodonyi R. J. , Duck P. W.
Published: July 2002

A numerical study of the generation of Tollmien–Schlichting (TS) waves due to the interaction between a small free-stream disturbance and a small localized suction strip on an otherwise flat surface has been carried out using finite-difference methods. The nonlinear steady flow is of the viscous–inviscid interactive (i.e., triple-deck) type while the unsteady disturbed flow is assumed to be governed by the triple-deck approximation of the unsteady Navier–Stokes equations linearized about this mean flow. Numerical solutions illustrate the growth or decay of the TS waves generated by the interaction between the free-stream disturbance and the suction strip, depending on the value of the scaled Strouhal number. An important result of this receptivity problem is the numerical determination of the amplitude of the Tollmien–Schlichting waves and the demonstration of the possible active control of the growth of Tollmien–Schlichting waves.
Hydrodynamic stability of high-viscosity cylindrical liquid bridges
Authors: Nicolás J. A.
Published: July 2002

The hydrodynamic stability of cylindrical liquid bridges is studied in the asymptotic limit when viscosity is paramount. The problem has been solved analytically in terms of a series of biorthogonal eigenfunctions (the Papkovich–Fadle functions), by using an orthogonalization method devised by Smith [Aust. J. Sci. Res. 5, 227 (1952)] for solving elasticity problems. It is found that the growth factor is real for all the values of the slenderness, leading to pure damping or growing motions for the stable or the unstable modes, respectively. This analysis is also valid when the viscous to capillary forces ratio is of order unity and the slenderness is close to the static stability limit. The results are compared with previous ones obtained from Rayleigh’s viscous model for jets or from Cosserat’s one-dimensional model, showing a very good agreement for long columns.
Experimental evidence supporting Kolmogorov’s refined similarity hypothesis
Authors: Thoroddsen S. T. , Van Atta C. W.
Published: July 2002

In a recent Letter, Hosokawa and Yamamoto [Phys. Fluids A 3, 457 (1992)] have provided evidence against Kolmogorov’s refined similarity hypothesis based on the results of direct numerical simulations of isotropic turbulence [J. Phys. Soc. Jpn. 60, 1852 (1991)] with Reλ close to 100. Here experimental results are presented from a wind-tunnel study of a cylinder wake, at Reλ of 550, which in spirit confirm the hypothesis, especially for the smaller scales in the inertial range. The present experimental velocity spectra contain one and one-half decades of inertial range. The apparent basic disagreement between the numerical simulations and the experiments may be due to the difference in Reλ, or may be symptomatic of a more subtle problem.
Decay of a laminar shear layer
Authors: Alston Thomas M. , Cohen Ira M.
Published: July 2002

This study considers the analysis of the developing flow due to the mixing of two uniform incompressible fluid streams with different velocities. Using the method of matched asymptotic expansions, solutions valid in three regions of the free shear layer are obtained. The expansion process uses a small parameter based on the difference between free-stream velocities and, unlike previous studies, is applied to the complete Navier–Stokes equations. The controversy involving the choice of a proper third boundary condition for the free shear layer between parallel streams is avoided in the present approach. This is achieved by matching the downstream solutions, valid inside and outside the mixing layer, to the solution describing the flow in the initial mixing region near the start of mixing. Results of the analysis indicate that the transverse velocity, in the developing free shear layer between two uniform streams, approaches similarity at a much slower rate than the longitudinal velocity. The analysis also showed that, in the early stages of the mixing process between two uniform streams, the faster stream is initially displaced away from the shear layer. This initial flow displacement is not predicted by the downstream mixing-layer similarity solution.
Double-plasma instability near the ion plasma frequency
Authors: Barrett P. J. , Greaves R. G.
Published: July 2002

If plasma leaks through a negatively biased grid from a positively biased source chamber into an outer region where its potential is free to oscillate, the potential can go into a large-amplitude spontaneous oscillation at a frequency set by the ion transit time through the sheath. This is accompanied by density fluctuations that free stream away from the grid, spatially damped. The frequency and temporal growth rate are investigated under various plasma conditions.
Theory of free-electron laser instability for a relativistic electron beam propagating in a dielectric-loaded waveguide
Authors: Choi Jeong-Sik , Kim Dong-Eon , Choi Duk-In , Yang Soon-Chul , Uhm Han S.
Published: July 2002

A free-electron laser (FEL) with a dielectric-loaded waveguide operating in an undulator (multiple mirror) field is analyzed. The stability properties are investigated self-consistently on the basis of the linearized Vlasov–Maxwell equations for an electron distribution function, in which all electrons have a Lorentzian distribution in the axial canonical momentum. Using appropriate boundary conditions, a dispersion relation is derived in the low density approximation; νb/γb≪1, and the growth rates of several types of mode couplings are computed. Even for a mildly relativistic electron beam (γb≤1.5), the typical maximum growth rate of instability is a few percent of c/Rw. As the axial momentum spread increases, the growth rate decreases substantially while the instability bandwidth increases. For the long-wiggler wavelength (LWW) mode, which only occurs in the dielectric-loaded waveguide, Cerenkov interaction plays an important role in the free-electron laser instability. In the case of the short-wiggler wavelength (SWW) mode, the frequency of the free-electron laser is greatly enhanced in mildly relativistic electron beams with appropriate choices of physical parameter values. Therefore, intense submillimeter microwaves might be produced by making use of a mildly relativistic electron beam with γ≊1.1. A wide band free-electron laser amplifier is also possible with a proper choice of external parameters, such as wiggler wavenumber k0, dielectric constant, and beam energy γb.
Equilibrium of a plasma in the fluid- and Vlasov–Maxwell systems
Authors: Mahajan Swadesh M. , Li Wann-Quan
Published: July 2002

It is shown that a recently constructed exact solution of the Vlasov equation describing a plasma with density and temperature gradients can be expressed in terms of the constants of motion. The distribution function is then used to illustrate the differences between a Vlasov and a one-fluid description. In fluid theory, only the pressure profile is determined (unless one postulates an equation of state), while the Vlasov description leads to a separate determination of density (g) and temperature (ψ2) profiles; the equation of state, g=ψ3−2/β, comes out naturally in the latter case.
q measurements during sawtooth oscillations in a low q tokamak
Authors: Moyer R. A. , Goetz J. A. , Dexter R. N. , Prager S. C.
Published: July 2002

The central safety factor during a sawtooth oscillation in material limiter tokamak discharges (scrapeoff plates inserted to the separatrix) in the TOKAPOLE II poloidal divertor tokamak [Phys. Rev. Lett. 49, 734 (1982)] is measured to be significantly less than 1 (approximately 0.7) during a sawtooth oscillation. This result is identical to that observed earlier in the same device in magnetic limiter discharges. Thus the presence of scrapeoff plasma beyond the divertor separatrix is not responsible for the absence of total reconnection.
Anomalous particle pinch for collisionless plasma
Authors: Terry P. W.
Published: July 2002

The particle transport arising from the convection of nonadiabatic electron density by ion temperature gradient driven turbulence is examined for trapped electron collisionality regimes. In the lower temperature end of this regime, trapped electrons are collisional and the particle flux is outward ( in the direction of the gradients). When the trapped electrons are collisionless, there is a temperature threshold above which the electron temperature gradient driven particle flux changes sign and becomes inward. The magnitude of the nonadiabatic electron contribution to the growth rate is found to be potentially of the same order as the ion contribution.
Nonviability of some nonlocal electron heat transport modeling
Authors: Prasad M. K. , Kershaw D. S.
Published: July 2002

Using exact analytical solutions, it is shown that two recent models of nonlocal electron heat transport are beset with mathematical anomalies leading to unphysical results. First, heat flow with a nonlocal heat flux does not smooth out steep temperature gradients in any finite time. Second, a computation of the thermoelectric field from a vanishing nonlocal current flux is an ill-posed problem leading to instabilities that violate the very assumption on which the model is based. It is verified that these anomalies can lead to a negative entropy production rate, which implies local thermodynamic instability. In particular, it is proved that a temperature distribution that is initially positive can later become negative for certain nonuniform electron density distributions. These results provide a basic understanding of the various difficulties encountered in numerical implementations of these models.
Optical diagnosis of electric fields in a beam-driven turbulent plasma
Authors: Dovrat Amikam , Benford Gregory
Published: July 2002

Optical diagnostics using laser fluorescence techniques are used to measure the rms electric field in a superstrongly turbulent, relativistic beam–plasma system. This yields the mapping of 〈E2〉 as a function of radial location r and time t. This 〈E2(r,t)〉 allows studies of growth and evolution of turbulent fields, their diffusion, and decay. Fluctuating electric fields occur when a 700 keV, 4 kA, 2 μsec electron beam propagates into a 20 cm diam, 1.5 m long drift tube filled with 10 mTorr of helium plasma. Stark effect shifts appear in suitable forbidden and allowed transitions, originating from the same upper energy level for the measurement: Hei 6632 Å and Hei 5015.7 Å. The spectral bandwidth includes the forbidden line and its satellites. Using the ratio of the intensity of the forbidden plus satellite lines, to the allowed line intensity, yields the rms field as the combined field of oscillation near the plasma frequency. Fields up to 28 kV/cm result. These results can be explained by an analytical model of production of strong electric fields by beam–plasma instability, including modulational transfer in k space, plasma heating, radiation, and wave convection. Comparison between experiment and the numerically integrated model shows good agreement.
Low-frequency current drive and helicity injection
Authors: Chan V. S. , Miller R. L. , Ohkawa T.
Published: July 2002

For ω≪Ωi, where Ωi is the ion cyclotron frequency, circularly polarized waves can drive current far exceeding the current resulting from linearly polarized waves. Further, the efficiency can be independent of plasma density. In some cases, this circular polarization may be interpreted in terms of helicity injection. For tokamak applications, where the wavenumber in the toroidal direction is a real quantity, wave helicity is injected only with finite Ez waves, where z is the direction of the static magnetic field. The Alfvén waves are possible current drive candidates but, in the cylindrical model considered, the compressional wave is weakly damped because Ez =0, while the shear Alfvén wave is totally absorbed at the surface because of finite Ez. A mixture of the two modes is shown to drive an oscillatory surface current even though the efficiency is high and independent of density. A more promising current drive candidate is a fast wave that propagates to the plasma interior and is damped by the minority cyclotron resonance. Near the minority mode conversion region, the fast wave is left-handed circularly polarized and it has a small but finite Ez component at high electron temperatures. The current drive efficiency, although not as high as that of the Alfvén wave, is still good and independent of density, making it attractive for fusion reactors.
Operating characteristics of a continuous-wave-relevant quasioptical gyrotron with variable mirror separation
Authors: Fliflet A. W. , Hargreaves T. A. , Manheimer W. M. , Fischer R. P. , Barsanti M. L. , Levush B. , Antonsen T.
Published: July 2002

Results from a quasioptical gyrotron experiment with a 20–28 cm mirror separation are presented, showing operation at powers up to 150 kW and efficiencies up to 12%. The output coupling would be varied from 0.4%–3% by changing the mirror separation and operating frequency. Operation was obtained over frequencies ranging from 95–130 GHz by changing the axial magnetic field, limited on the low end by waveguide cutoff in the diagnostics and at the high end by the maximum magnetic field achievable. The output power varied by approximately a factor of 2 over this range. Frequency variation of 4% was achieved by varying only the electron gun voltage; however, the output power also varied substantially due to the fact that the electron beam power was changing dramatically. Efficiency optimization by variation of output coupling and tapering of the magnetic field has been demonstrated. Regions of single-mode operation at powers up to 125 kW have been characterized and compared to recently developed theory.
A fluid treatment of the plasma presheath for collisionless and collisional plasmas
Authors: Scheuer J. T. , Emmert G. A.
Published: July 2002

The presheath region of an unmagnetized plasma is treated using ion fluid equations in the collisional and collisionless regimes. Effects of neutral gas–plasma interactions are included through source terms in the fluid equations. Ion–ion collisional effects are included through the closure conditions of the fluid equations. In the collisionless regime, the perpendicular ion temperature is assumed constant and independent of the parallel ion temperature. In the collisional limit, the parallel and perpendicular temperatures are assumed equal throughout the presheath. Profiles of ion density, flow velocity, parallel temperature, and energy flux obtained by integrating the fluid equations compare well to previously published kinetic results in the collisionless case. Ion–ion Coulomb collisions are shown to have a negligible effect on the ion particle and energy flux into the sheath.
Alternate transport
Authors: Boozer Allen H. , Baldwin David E. , Horton C. Wendell , Dominguez Réne R. , Glasser Alan H. , Krommes John A. , Neilson G. H. , Shaing Ker-Chung , Sadowski Walter L. , Weitzner Harold
Published: July 2002

The understanding of tokamak transport depends on the exploration of a wide range of theoretical models and of a variety of toroidal experiments. This report considers the contributions that nontokamak, but toroidal, experiments can make to our understanding of tokamak transport as well as theoretical alternatives to the standard drift wave model of tokamak transport.
Theory of resistivity-gradient-driven turbulence in a differentially rotating plasma
Authors: Kim Y. B. , Diamond P. H. , Biglari H. , Terry P. W.
Published: July 2002

The effects of a radially sheared poloidal flow on the structure of resistivity-gradient-driven turbulence in tokamak edge plasmas are self-consistently investigated. Sheared flow induces a coupling between turbulent radial diffusion and poloidal shearing, which results in enhanced decorrelation and a concomitant reduction in the size of the turbulent convection cells. These effects result in the suppression of resistivity-gradient-driven turbulence in the presence of strongly sheared poloidal flows. While the effects of sheared rotation are ultimately more pronounced at high kθ, the onset of enhanced decorrelation occurs first for low kθ modes. In addition to the trivial rotation-induced Doppler shift, sheared poloidal flows also induce a mode frequency that is comparable in size to the enhanced turbulent decorrelation rate, and whose sign varies with the sign of the flow shear. The mode frequency and flow shear can effectively render the turbulent diffusion nonresonant. The implications of these results for this and other models of edge turbulence are discussed.
Nonlinear theory and experiment of collective free electron lasers
Authors: Wurtele Jonathan S. , Chu Ronson , Fajans Joel
Published: July 2002

A theoretical and experimental study of the nonlinear performance of a free electron laser (FEL) amplifier operating in the collective (Raman) regime is reported. The FEL generates up to ∼100 kW of rf power at a frequency of 9.3 GHz and an efficiency of ∼10%. Power saturation, efficiency, and synchrotron oscillations are studied as a function of rf input power, electron beam energy, current, wiggler field amplitude, and axial distance within the helical wiggler. The influences of the nonlinear electron motion in the ponderomotive potential and space-charge waves are studied by measurements of the dependence of gain and efficiency on the initial radiation intensity. Good agreement with a nonlinear theory that takes cognizance of electron trapping in the combined ponderomotive and space-charge potential well is obtained.
Enhanced pinch effect due to the electrostatic potential
Authors: Shaing K. C. , Hazeltine R. D.
Published: July 2002

An inward particle pinch appears necessary to explain experimental results in tokamaks. Neither the neoclassical pinch effect, which is too small, nor the off-diagonal quasilinear term, which is usually outward in the trapped particle regime, can account for the observations. A mechanism for an enhanced inward pinch is proposed, based on results for an asymmetric magnetic field bump [Phys. Fluids 15, 2211 (1972)]. Because turbulent fluctuations also break toroidal symmetry, an enhanced inward pinch driven by the fluctuations and the Ohmic inductive field E is expected. To demonstrate this effect, an inward particle flux is calculated for a model tokamak configuration that has an electrostatic potential bump Φ0 at toroidal angle ζ=2π. For the parameter regime r/R<eΦ0/Te<1, the flux is found to be Γ=−4.47K(q)(r/R)L(Φ0)(vte/Rνe) 1/2cNE/B, where r(R) is minor (major) radius, B is the magnetic field strength, vte is the electron thermal speed, νe is the electron–ion collision frequency, q is the safety factor, and K(q) and L(Φ0) are functions of q and Φ0, respectively. The results are also applicable to an asymmetric potential bump created externally to enhance the inward pinch flux of high energy, collisionless particles.
Collisionless plasma presheath in a nonuniform magnetic field
Authors: Hussein Makarem A. , Emmert G. A.
Published: July 2002

The flow of a collisionless plasma in a nonuniform magnetic field is numerically simulated for a wide range of magnetic field strengths. An iterative scheme for solution of the Vlasov equation and quasineutrality is developed in which a Monte Carlo description for the ion dynamics is used. Ions are launched from distributed sources with finite temperature while electrons are assumed to obey the Boltzmann relation. The dependence of the electrostatic potential profile on both the spatial distribution of the particle source and the magnetic field strength profiles is explicitly investigated. The simulation results are compared with a recently published analytical solution [Phys. Fluids B 1, 725 (1989)].
Higher beta at higher elongation in the DIII-D tokamak
Authors: Lazarus E. A. , Chu M. S. , Ferron J. R. , Helton F. J. , Hogan J. T. , Kellman A. G. , Lao L. L. , Lister J. B. , Osborne T. H. , Snider R. , Strait E. J. , Taylor T. S. , Turnbull A. D.
Published: July 2002

A theoretical and experimental evaluation of axisymmetric stability and axisymmetric control has led to a modification of the vertical position control in the DIII-D tokamak, which now allows operation to within a few percent of the ideal magnetohydrodynamic (MHD) n=0 limit. It is found that the onset the departure from rigid shift behavior in D-shaped plasmas limits plasma elongation to 2.5 in DIII-D. The possibility of avoiding the vertical instability in future tokamaks with highly elongated plasmas is discussed. Recent experiments have focused on utilizing this capability for axisymmetric control to construct plasma shapes optimized to increase the achievable beta. Operation near the axisymmetric stability limit allows an increase in the achieved normalized current Ip/aBT, where Ip is the total plasma current, a is the minor radius, and BT is the toroidal field. Based on stability calculations, an equilibrium was developed to achieve marginal stability simultaneously to axisymmetric, kink, and ballooning instabilities. In the experiment, the shape was altered to higher elongation during the high-beta phase as the current profile broadened. A record high beta for DIII-D of 11% was achieved. The high-beta phase of the discharge lasted 40 msec, approximately one confinement time.
Tokamak error fields and locked modes
Authors: Reiman A. , Monticello D.
Published: July 2002

The effects of externally applied, resonant magnetic field perturbations on tokamak plasmas are discussed. An analytical model assuming tokamak ordering and narrow islands clarifies the conditions under which error fields are amplified or diminished by self-consistent effects. A strong amplification of error fields near marginal tearing stability is found to broaden the range of parameters over which deleterious tearing effects are seen. Three-dimensional numerical solutions indicate that saturated tearing mode island widths can be sensitive to the presence of small nonaxisymmetric perturbations. It is argued that the theoretical conclusions are consistent with observations in error compensation experiments, and are also consistent with observations of enhanced growth for locked modes.
Particle simulation model of transport in a bounded, Coulomb collisional plasma
Authors: Procassini Richard J. , Birdsall Charles K.
Published: July 2002

The transport of particles and energy in a fully ionized, collisional plasma is studied through the use of a kinetic transport model. A particle-in-cell (PIC) code has been coupled to a Monte Carlo, binary particle model of Coulomb collisions, to provide a fully kinetic, self-consistent description of transport and potential formation in a single spatial dimension and two velocity components (parallel and perpendicular to the spatial coordinate). The dependence of plasma transport on Coulomb collisionality is investigated by varying the normalized collision frequency within the range 10−2≤ν*≡νc0/νbe0≤5, where νc0 is the average electron/ion collision frequency and νbe0 is the frequency at which thermal electrons bounce between the collector sheath potential drops located adjacent to the absorbing plates at each end of the system. Collisions between charged-plasma and recycled-neutral particles are omitted in this study. For finite values of ν*, the heat conduction flux is found to be reduced from the value predicted by classical, hydrodynamic transport theory. The electron heat conduction flux is shown to lie between 12% and 21% of the free-streaming thermal flux qefs≡nev∥,tekTe, where ne, v∥,te, and kTe are the steady-state values of the electron density, parallel thermal velocity, and temperature, respectively. The variation of several transport quantities with collisionality is presented, and the results are compared against those from other collisional plasma transport models.
Experimental study of the relaxation cycle of a decaying spheromak in an external magnetic field
Authors: Ono Y. , Yamada M. , Janos A. C. , Levinton F. M.
Published: July 2002

For high electron temperature plasma discharges in the S-1 spheromak device [Plasma Physics and Controlled Nuclear Fusion Research, 1984 (IAEA, Vienna, 1985), Vol. 2, p. 535], ‘‘sawtooth’’- like oscillations appear on signals of magnetic field, flux, q value, and electron temperature. Based on the internal magnetic field profiles measured by magnetic probe scans, the mechanisms and causes of these oscillations are revealed. The cycle of one oscillation consists of a toroidal current peaking phase and a subsequent relaxation phase. In the peaking phase, resistive current decay at the edge causes the spheromak to deviate from the initial minimum-energy Taylor state. The deviation was revealed experimentally by the preferential decay of toroidal flux over poloidal flux. A simple calculation shows that a peaking of the electron temperature profile is the most probable cause for the preferential decay of toroidal flux over poloidal flux. During the peaking phase, q decreases so low as to make the configuration unstable to low-n magnetohydrodynamic (MHD) modes (mainly n=2 mode). In the relaxation phase, these modes invoke current redistribution (relaxation), restoring the Taylor state. A significant finding in the relaxation phase is that a reversed toroidal field similar to that of the reversed-field pinch (RFP) configuration is sometimes measured at the edge of the plasma for a brief period. The disappearance or resistive decay of the reversed toroidal flux is attributed to a flux conversion through the magnetic reconnections caused by the low-n modes.
Effects of a generalized presheath source in flowing magnetized plasmas
Authors: Chung K.-S. , Hutchinson I. H.
Published: July 2002

This paper extends the previous kinetic model [Phys. Rev. A 38, 4721 (1988)] of probing objects in flowing magnetized plasmas by generalizing cross-field transport and adding ionization to the source in the Boltzmann equation along the presheath. Ion sheath current density and ratio (R) of upstream to downstream current are obtained as a function of normalized plasma drift velocity (M), equivalent viscosity ratio (α), ion temperature (Ti∞), and ionization rate (σi). The form R=exp(M/Mc) fits the results very well and the calibration factor (Mc) is obtained. Here Mc decreases as α increases, Ti∞ increases, or σi increases. Comparisons with fluid and kinetic models are presented.
Drift wave vortices in inhomogeneous plasmas
Authors: Su X. N. , Horton W. , Morrison P. J.
Published: July 2002

The effects of density and temperature gradients on drift wave vortex dynamics are studied using a fully nonlinear model with the Boltzmann density distribution. The equation based on the full Boltzmann relation, in the short wavelength (∼ρs) region, possesses no localized monopole solution, while in the longer wavelength [∼(ρsrn)1/2] region the density profile governs the existence of monopolelike solutions. In the longer wavelength regime, however, the results of analysis show that due to the inhomogeneity of the plasma the monopoles cannot be localized sufficiently to avoid coupling to propagating drift waves. Thus, the monopole drift wave vortex is a long-lived coherent structure, but it is not precisely a stationary structure since the coupling results in a ‘‘flapping’’ tail. The flapping tail causes energy of the vortex to leak out, but the effect of the temperature gradient-induced nonlinearity is to reduce the leaking of this energy.
Asymptotic theory of the nonlinearly saturated m=1 mode in tokamaks with q<1
Authors: Thyagaraja A. , Haas F. A.
Published: July 2002

In this paper the necessary and sufficient conditions required for the existence of a nonlinearly saturated m=1 tearing mode in tokamaks with q0<1 are considered in cylindrical tokamak ordering using the asymptotic techniques developed by one of the authors in an earlier paper [Phys. Fluids 24, 1716 (1981)]. The outer equations for the helical perturbation amplitude ψ1(r) are solved exactly, in closed form for an arbitrary mean profile ψ0(r) in leading order. This is shown to result in a ‘‘no disturbance’’ theorem: the m=1 perturbation must be confined to within the radius ri such that q(ri)=1. The bifurcation relation for the nondimensional perturbation amplitude is then constructed by solving the nonlinear inner critical layer equations using an ordered iterative technique. For monotonically increasing q profiles, the equation has a solution if and only if the toroidal current density of the unperturbed equilibrium has a maximum within ri and the parameter d log q(r)/[d log η(r)] [where η(r) is the resistivity profile consistent with the q profile of the unperturbed equilibrium] is sufficiently small at ri. The considerations are extended to nonmonotonic profiles as well. When the conditions are met, a nonlinearly saturated m=1 tearing mode is shown to exist with a novel island structure, quite different from those obtained from the usual Δ′ analysis, which is shown to be inappropriate to the present problem. The relevance of the results of the present theory to sawtooth phenomena reported in JET [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. 1, p. 377] and other tokamaks is briefly discussed. The solution constitutes an analytically solved test case for numerical simulation codes to leading orders in a/R and the shear parameter d log q/d log η.
Gyrokinetic simulation of microinstabilities in high temperature tokamaks
Authors: Tsang K. T. , Cheng C. Z.
Published: July 2002

A new numerical approach to investigate the stability of low-frequency fluctuations in high temperature tokamaks is developed by solving the gyrokinetic equations for the ion and electron directly as an initial value problem. The advantage of this approach is the inclusion of many important kinetic features of the problem without approximations. The ion-temperature-gradient mode is investigated as an example of this new simulation technique. Both the adiabatic electron model and the full drift-kinetic electron model are studied. Numerical results show that the full drift-kinetic electron model is more unstable.
Experimental test of quasilinear theory
Authors: Tsunoda S. I. , Doveil F. , Malmberg J. H.
Published: July 2002

The use of the random phase approximation in quasilinear theory has been controversial for some time. For the bump-on-tail instability this approximation leads to the neglect of mode coupling effects mediated by the resonant particles. Recently, it has been argued theoretically and numerically that resonant particle mediated mode coupling effects actually play an important role, and that the statistically averaged effect of this mode coupling is a zeroth-order increase in the growth rate. The quasilinear theory of the interaction between a warm beam and a slow wave structure is formally identical to the quasilinear theory of the interaction between a warm beam and a plasma in the weak beam limit. Strong mode coupling effects have been experimentally observed when a weak warm beam interacts with waves on a slow wave structure. When a statistical average is done over the mode coupling, however, the predicted zeroth-order increase in the growth rate is not observed.
A theory of relativistic electron beam bunching by the plasma wake-field effects
Authors: Uhm Han S.
Published: July 2002

Modulated current profile of a relativistic electron beam is theoretically investigated in terms of the propagation distance in a chamber containing a diffused plasma, which exerts wake-field effects on the beam. Neglecting the beam-head erosion, it is found that the beam current profile at a specified propagation distance is expressed in terms of only the time t0, at which the beam segment enters the chamber. Energy modulation along the beam pulse is also calculated.
Enhancement of the velocity diffusion in longitudinal plasma turbulence
Authors: Cary John R. , Doxas Isidoros , Escande D. F. , Verga A. D.
Published: July 2002

Velocity diffusion has been observed to be significantly above that given by quasilinear theory in both test-particle and self-consistent simulations of the dynamics of turbulent electrostatic fields. Test-particle simulations and analytic calculations for motion in a set of randomly phased waves find the diffusion coefficient to be enhanced by a factor of 2.5 at intermediate values of the overlap parameter. Self-consistent simulations find that the turbulent spectrum can evolve naturally into a form such that the diffusion enhancement is present when the linear growth time of the modes is long compared with the effective discretization time, which is nearly an order of magnitude greater than the resonant broadening time.
Anomalous ion thermal diffusion from ηi modes
Authors: Beklemishev A. D. , Horton W.
Published: July 2002

Models of ion temperature gradient-driven turbulence are reexamined in terms of the structure of the turbulent spectrum for radially localized modes to explain the significant difference between the radial profiles of ion heat conductivity inferred from local turbulence models and those observed in experiments. The strong radial inhomogeneity of the effective density of the turbulence spectrum in k is shown to produce a significant increase of the fluctuation level and ion thermal diffusion toward the plasma edge as compared with the local models under certain conditions. The conditions required are stated and should be tested with numerical simulations.
Interferometry on the compact toroid formation experiments at Phillips Laboratory
Authors: Ruden E. L. , Mullins B. W. , Dearborn M. E. , Coffey S. K.
Published: July 2002

Interferometric measurements of the line-averaged free electron density of plasma compact toroids as part of preliminary toroid formation experiments are presented. These experiments are in preparation for compact toroid acceleration experiments to be performed at Phillips Laboratory’s High Energy Plasma Division. The primary instrument used was a Michelson (double pass) interferometer operating with a HeNe laser at a wavelength of 633 nm. Density measurements of hydrogen and argon plasmas are correlated with magnetic field and spectroscopic measurements to characterize the toroid mass, velocity, and impurity distribution.
Detailed 266 nm Thomson scattering measurements of a laser-heated plasma
Authors: Tracy M. D. , De Groot J. S. , Estabrook K. G. , Cameron S. M.
Published: July 2002

Collective Thomson scattering at 266 nm is used to obtain spatially resolved, two-dimensional electron density, temperature, and radial drift profiles of a collisional laser plasma (critical density, nc =1×1021 cm−3). An ultraviolet diagnostic wavelength minimizes the complicating effects of inverse bremsstrahlung and refractive turning in the coronal region of interest, where electron densities approach nc/10. Laser plasmas of this type are important because they model some of the aspects of the plasmas found in high-gain laser-fusion pellets irradiated by long pulse widths (tL≳10 nsec), where laser light is absorbed mostly in the corona. The experimental results and lasnex [Comments Plasma Phys. Controlled Fusion 2, 51 (1975)] simulations agree within a percent standard deviation of 40% for electron density and 50% for electron temperature and radial drift velocity. Thus it is shown that the hydrodynamics equations with classical coefficients and the numerical approximations in lasnex are valid models of laser-heated, highly collisional plasmas.
Formation and stability of polarization sheaths of a cross-field beam
Authors: Cai Dong Sheng , Buneman Oscar
Published: July 2002

A layer of plasma (such as that comoving with the space shuttle) can travel across a magnetic field if it is flanked by ‘‘polarization sheaths’’ that create the E field necessary for E×B drift. Both the spontaneous evolution and the stability of these positive and negative sheaths can be studied by their analogy with the charge configuration in magnetron-type electron devices. That is, conservation of canonical momentum results in ‘‘Brillouin flow,’’ i.e., shear flows in a pure ion sheath on one side and a pure electron sheath on the other side of the neutral plasma beam. The equilibrium parameters of this flow were obtained and the stability of the equilibria was analyzed. Both the ion and electron sheaths were found to be one-dimensionally stable and two-dimensionally unstable in a mode variously known as ‘‘magnetron,’’ ‘‘slipping stream,’’ or ‘‘diocotron.’’ This mode connects up with the Kelvin–Helmholtz instability in neutral matter. The analytical results were confirmed both qualitatively and quantitatively by numerical simulations. The saturation of the unstable modes, observed in the simulation, allows one to estimate how the polarization sheaths eventually diffuse.
Profile consistency in equilibria of field-reversed configurations
Authors: Steinhauer Loren C. , Ishida Akio
Published: July 2002

Experimental evidence is presented for a regulatory principle governing field-reversed configuration (FRC) equilibria. This leads to a form of ‘‘profile consistency’’ with which the current profile exhibits a remarkable correlation with xs (the ratio of the separatrix radius to the coil radius). The proposed explanation is that these equilibria are regulated by an instability which maintains the profile at a marginally stable condition.
Production and characterization of hot, long-scale-length laser plasmas
Authors: Seka W. , Craxton R. S. , Bahr R. E. , Brown D. L. , Bradley D. K. , Jaanimagi P. A. , Yaakobi B. , Epstein R.
Published: July 2002

Experimental data and hydrodynamic simulation results are presented for long-scale-length plasmas produced by the multibeam OMEGA fusion laser, with ≲1.5 kJ on target at wavelength 351 nm. The appropriately time-delayed beams from this system allow the production of millimeter-sized plasmas with ≳1 keV electron temperatures maintained over extended periods of time (≳1 nsec). The two-dimensional plasma expansion and the temporal evolution of the electron temperature and density are found to be in very good agreement with simulations. The plasma thus produced forms an excellent tool for investigating laser–plasma interaction processes under conditions relevant to future laser-fusion reactor targets.
Soft x-ray measurement of internal tearing mode structure in a reversed-field pinch
Authors: Chartas George , Hokin Samuel
Published: July 2002

The structure of internally resonant tearing modes has been studied in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] with a soft x-ray detector system consisting of an imaging array at one toroidal location and several detectors at different toroidal locations. The toroidal mode numbers of dominant m=1 structures are in the range n=5,6,7. The modes propagate with phase velocity v=1−6×106 cm/sec, larger than the diamagnetic drift velocity vd∼5×105 cm/sec. Phase locking between modes with different n is manifested as a beating of soft x-ray signals which is strongest near the resonant surfaces of the modes (r/a=0.3–0.5) and exhibits characteristics indicative of nonlinear mode coupling.
Two-dimensional direct simulation of deuterium-fiber-initiated Z pinches with detailed comparison to experiment
Authors: Sheehey Peter , Hammel Jay E. , Lindemuth Irvin R. , Scudder David W. , Shlachter Jack S. , Lovberg Ralph H. , Riley Ronald A.
Published: July 2002

Deuterium-fiber-initiated Z-pinch experiments have been simulated using a two-dimensional resistive magnetohydrodynamic model, which includes many important experimental details, such as ‘‘cold-start’’ initial conditions, thermal conduction, radiation, actual discharge current versus time, and grids of sufficient size and resolution to allow realistic development of the plasma. When the fiber becomes fully ionized (at a time depending on current ramp and fiber thickness), the simulations show rapidly developing m=0 instabilities, which originated in the corona surrounding the fiber, drive intense nonuniform heating and rapid expansion of the plasma column. Diagnostics generated from the simulation results, such as shadowgrams and interferograms, are in good agreement with experiment.
Self-consistent, nonlocal electron heat flux at arbitrary ion charge number
Authors: Sanmartín Juan R. , Ramírez J. , Fernández-Feria R. , Minotti F.
Published: July 2002

A single, nonlocal expression for the electron heat flux, which closely reproduces known results at high and low ion charge number Z, and ‘‘exact’’ results for the local limit at all Z, is derived by solving the kinetic equation in a narrow, tail-energy range. The solution involves asymptotic expansions of Bessel functions of large argument, and (Z-dependent) order above or below it, corresponding to the possible parabolic or hyperbolic character of the kinetic equation; velocity space diffusion in self-scattering is treated similarly to isotropic thermalization of tail energies in large Z analyses. The scale length H characterizing nonlocal effects varies with Z, suggesting an equal dependence of any ad hoc flux limiter. The model is valid for all H above the mean-free path for thermal electrons.
Energetic particle stabilization of the interchange mode in helically symmetric plasmas
Authors: Zheng L.-J. , Tsai S.-T.
Published: July 2002

The ELMO snaky torus concept [Furth and Boozer (private communication, 1983)] is investigated in a helically symmetric model with low beta and large aspect ratio (the curvature radius of the magnetic axis over the plasma radius). The equilibria are obtained by expansions in the inverse aspect ratio and plasma beta. Application of the energetic-particle- modified Mercier criterion [Zheng and Tsai, Phys. Fluids B 4, 1416 (1992)] to the analytical equilibria confirms that the interchange mode can be stabilized in helically symmetric systems in the low-beta regime by suitably introducing drift-nonreverse energetic particles. The results show that, for effective stabilization, it is better that the pressure profile of the energetic component depend on the radius quadratically, and peak at midplane in the poloidal angle, and the volume occupied by the energetic particles, the aspect ratio, and the ratio of the curvature to the torsion of the magnetic axis should be large when the energetic particles are drift nonreverse.
Ion flow in a strongly sheared electric field
Authors: Tao Y.-Q. , Conn R. W. , Schmitz L. , Tynan G.
Published: July 2002

Ion orbits and equilibrium ion flow in crossed electric and magnetic fields are examined for the case of a strongly nonuniform electric field such as found in edge plasmas of tokamak fusion experiments and in space plasmas. It is shown that the E×B drift approximation no longer applies, either to the motion of a single ion or to the collective response of the ion species when the absolute value of the shear parameter, ‖ζ‖, defined as the absolute value of the ratio of the gradient of E×B speed to ion gyrofrequency, is order one. It is also found that the ion velocity is strongly dependent on the electric field geometry. The results suggest that the existence of a strongly sheared electric field does not necessarily indicate the existence of strongly sheared plasma flow, and that the spatial shape of the electric field, when ‖ζ‖ is order one, may be a dominant factor in determining the resulting plasma flow speed.
The effect of shape in the three-dimensional ablative Rayleigh–Taylor instability. I: Single-mode perturbations
Authors: Dahlburg J. P. , Gardner J. H. , Doolen G. D. , Haan S. W.
Published: July 2002

The nonlinear saturation amplitudes attained by Rayleigh–Taylor perturbations growing on ablatively stabilized laser fusion targets are crucial in determining the survival time of those targets. For a given set of baseline simulation parameters, the peak amplitude is found to be a progressive function of cross-sectional perturbation shape as well as of wave number, with three-dimensional (3-D) square modes and two-dimensional (2-D) axisymmetric bubbles saturating later, and at higher amplitudes than two-dimensional planar modes. In late nonlinear times hydrodynamic evolution diverges; the 3-D square mode bubble continues to widen, while the 2-D axisymmetric bubble fills in.
Ion mobility and transport barriers in the tokamak plasmas
Authors: Xiao H. , Hazeltine R. D. , Zhang Y. Z. , Valanju P. M.
Published: July 2002

The character of charged particle motion in an axisymmetric toroidal system with a constant radial electric field is investigated both analytically and numerically. Ion radial mobility caused by the combined effects of the radial electric field and charge exchange is found. A simple moment argument in the banana regime matches the simulation results well. Relation of present work and high confinement (H-mode) experiment is also discussed.