bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of untrapped electrons on the sideband instability in a helical wiggler free electron laser
Authors: Davidson Ronald C. , Wurtele Jonathan S.
Published: July 2002

The detailed influence of an untrapped-electron population on the sideband instability in a helical wiggler free electron laser is investigated for small-amplitude perturbations about a constant-amplitude (â0s =const) primary electromagnetic wave with slowly varying equilibrium phase δ0s. A simple model is adopted in which all of the trapped electrons are deeply trapped, and the equilibrium motion of the untrapped electrons (assumed monoenergetic) is only weakly modulated by the ponderomotive potential. The theoretical model is based on the single-particle orbit equations together with Maxwell’s equations and appropriate statistical averages. Moreover, the stability analysis is carried out in the ponderomotive frame, which leads to a substantial simplification in deriving the dispersion relation. Detailed stability properties are investigated over a wide range of dimensionless pump strength ΩB /Γb ck0 and fraction of untrapped electrons fu =n̂u /n̂b. When both trapped and untrapped electrons) are present, there are generally two types of unstable modes, referred to as the sideband mode, and the untrapped-electron mode. For fu =0, only the sideband instability is present. As fu is increased, the growth rate of the sideband instability decreases, whereas the growth rate of the untrapped-electron mode increases until only the untrapped-electron mode is unstable for fu =1. It is found that the characteristic maximum growth rate of the most unstable mode varies by only a small amount over the entire range of fu from fu =0 (no untrapped electrons to fu =1 (no trapped electrons). The present analysis suggests that the linear and nonlinear evolution of the beam electrons and radiation field may be substantially modified by the present of an untrapped-electron component when fu ≳0.2.
Acoustic and entropy wave input–output relationships for a Faraday magnetohydrodynamic generator. I. Theory
Authors: Barton J. P.
Published: July 2002

A numerical procedure to determine the response of an ideal quasi-one-dimensional Faraday magnetohydrodynamic (MHD) generator to inputted linear acoustic and entropy (temperature) waves is developed from fundamental considerations. The results of the numerical procedure are expressed in terms of nine frequency-dependent reflection and transmission coefficients. These reflection and transmission coefficients can be used in MHD generator duct system design and can also be used to predict possible unstable (amplifying) MHD generator duct behavior.
Reduction of the hot electron energies with increasing pulse rise time in CO2-laser–target interaction
Authors: Decoste R. , Côté C. , Lavigne P. , Alaterre P. , Matte J.-P.
Published: July 2002

The effect of increasing the laser pulse rise time on the hot electron energy distribution is studied using one or three prepulses near or on the rise of a main CO2-laser pulse. For some slower rising pulses, the pressure from a hot and long gradient plasma balances the ponderomotive laser force in such a way that the hot electron energies are significantly reduced while the overall resonance absorption fraction remains almost the same. The experimental results are modeled with a 1-D hydrocode using multigroup fast electrons and a dependence of the absorption and the hot electron temperature on the density gradient scale length at the critical density. Qualitative agreement is obtained over a wide range of laser pulse conditions and rise times.
F-θ pumping with partial relaxation
Authors: Strauss H. R. , Harned Douglas S.
Published: July 2002

F-θ pumping is studied in a 1-D model including the hyperresistivity caused by tearing mode turbulence. When the hyperresistivity is sufficiently large compared to the resistivity, a positive, although time-varying, toroidal current can be maintained without a net toroidal voltage. For less hyperresistivity, the toroidal current periodically changes sign.
Electron beam propagation in the ion-focused regime
Authors: Buchanan H. Lee
Published: July 2002

In this paper the features of propagation of intense relativistic beams are discussed in the environment of low-density (≲1 Torr), partially ionized gas. Beam phenomena in this ion-focused regime (IFR) are found to be analogous to those at much higher density. Several instabilities and dissipative mechanisms have been identified and are compared to their high-density counterparts. Scaling laws are derived for quantitative estimation of these effects. In general, it is found that long propagation distances are possible in the laboratory suggesting that this regime may be useful for controlled beam transport and conditioning.
Effect of electric fields and fluctuations on confinement in a bumpy torus
Authors: Hiroe S. , Glowienka J. C. , Hillis D. L. , Wilgen J. B. , Chen G. L. , Cobble J. A. , El Nadi A. M. , Goyer J. R. , Solensten L. , Casson W. H. , Hankins O. E. , Quon B. H.
Published: July 2002

In order to understand the relationships between confinement and space potential (electric field) and between confinement and density fluctuations, plasma parameters in the ELMO Bumpy Torus Scale (EBT-S)[in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Tokyo, 1974), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] have been measured systematically for a wide range of operating conditions. Present EBT plasma parameters do not show a strong dependence on the potential profile, but rather exhibit a correlation with the fluctuations. The plasma pressure profile is found to be consistent with the profile anticipated on the basis of the flute stability criterion for a marginally stable plasma. For a heating power of 100 kW, the stored energy density is found to be restricted to the range between 4.5×1013 eV-cm−3 and 7×1013 eV-cm−3. The lower limit remains constant regardless of heating power and pertains to plasmas lacking an equilibrium and/or stability. The upper limit increases with heating power and is found to result from the onset of instabilities. In between the two limits is a plasma that is in an equilibrium state and is marginally stable. Operational trajectories exist that take the EBT plasma from one limit to the other.
Gyrotron simulations without particles
Authors: Antonsen Thomas M. , Levush Baruch
Published: July 2002

A new method for simulating gyromicrowave devices is suggested. The electron trajectories are represented in a Fourier series in initial gyrophases. Linear theory is reproduced by using three Fourier amplitudes. The nonlinear regime can be described by a relatively small number of amplitudes.
Wavenumber selection of convection rolls in a box
Authors: Arter Wayne , Bernoff A. , Newell A. C.
Published: July 2002

The dynamics of two-dimensional Rayleigh–Bénard convection rolls are studied in a finite layer with no-slip, fixed temperature upper and lower boundaries and no-slip insulating side walls. The dominant mechanism controlling the number of rolls seen in the layer is an instability concentrated near the side walls. This mechanism significantly narrows the band of stable wavenumbers although it can take a time comparable to the long (horizontal) diffusion time scale to operate.
Cusp width and power flow study at a high power magnetic multipole ion source
Authors: Horiike Hiroshi , Akiba Masato , Ohara Yoshihiro , Okumura Yoshikazu , Tanaka Shigeru
Published: July 2002

Power flow of the arc discharge on a high power magnetic multipole ion source is studied experimentally. Power flux distribution across line cusps is obtained using a probe made of a thermocouple. This confirms the fact that the full-width leakage is almost twice the size of the ion Larmor radius. It is shown that the arc discharge had strong spatial distribution in the arc chamber. The absolute value of the power flux is determined by comparing the integrated values of the distribution with heat loading measured by the coolant temperature rise. The result revealed that a very high power flux of more than 500 W/cm2 is concentrated on the anode wall at the line cusp position for a moderate arc discharge condition. It is also shown that a large portion of arc power is carried by the neutral particles. The composition of the source plasma near the grid is estimated from the power flux on the extraction grid.
Dissipative trapped particle modes in tandem mirrors
Authors: Berk H. L. , Chen C. Y.
Published: July 2002

The theory for the dissipative trapped particle modes is developed for a tandem mirror taking into account equilibrium rotational effects, electron temperature gradients, and an axial ambipolar potential, for a broad range of collisional parameters. The electrons are treated as a Maxwellian plasma that occupies the central cell and anchor cell regions. It is assumed that the eigenfunction is piecewise constant with abrupt transitions between the anchor and central cell regions. It is found that when ω/νp≳1, with ω the mode frequency and νp the Pastukhov loss rate, that the energy conservation structure of the collision operator produces important changes to previously developed theories. A solution to the problem is achieved by using the solution for the lifetime of an electron in an ambipolar trap, taking into account the global energy conservation. The energy conservation structure also allows a self-consistent description of dissipative instabilities when thermal gradient and electric fields are present. At very high collision frequency, a new dispersion relation is obtained, which exhibits an axially rotational shear drive coupled to radial temperature gradients producing instability. Numerical studies are presented for some parameters, with the deviation from previous theory highlighted.
Relativistic structure of stochastic wave–particle interaction
Authors: Akimoto K. , Karimabadi H.
Published: July 2002

Stochastic interactions of charged particles with electrostatic waves propagating at arbitrary angles to an external magnetic field are studied based on a relativistic canonical Hamiltonian formalism. The present theory, however, is valid also for electromagnetic waves after a slight modification. The stochasticity threshold is derived utilizing Chirikov’s criterion. It is found that relativistic effects are important for electrons interacting with relatively high phase velocity waves even at nonrelativistic initial energies. In particular, the relativistic generalization of a previous theory [Phys. Rev. Lett. 34, 1613 (1975); Phys. Fluids 21, 2230 (1978)] moves the degeneracy of primary resonances in nearly perpendicular directions to the angles where the parallel phase speed approximately equals the speed of light. It was also demonstrated for the first time that initially low energy electrons can gain relativistic energies (γ≫1) by means of the stochastic interaction with an electrostatic wave, where γ is the relativistic factor. Moreover, properties of second-order islands that form within primary islands have been studied. Finally, the nature of the stochastic particle energization by electrostatic waves is compared with that by electromagnetic waves, and the results are applied to the problem of electron acceleration during ionospheric modification experiments.
Macroscopic plasma oscillation bursts (fishbones) resulting from high-energy populations
Authors: Coppi B. , Migliuolo S. , Porcelli F.
Published: July 2002

Oscillation bursts (fishbones) of magnetically confined plasmas are associated with the excitation of an m0=1 mode when the ideal magnetohydrodynamic (MHD) threshold for the instability of this mode is reached. Near this threshold and in the absence of an effective ‘‘viscous’’ dissipative process, this mode remains marginally stable as a result of finite ion Larmor radius effects and has a real frequency of oscillation near the ion diamagnetic frequency. The release of the mode excitation energy related to the gradient of the plasma pressure, in the case of perpendicular neutral beam injection, is allowed by the resonant interaction of the mode with fast trapped ions that precess around the torus as a result of the curvature and the gradient of the confining magnetic field. This process plays the role of an effective viscosity. A consequence of the presented interpretation is that fishbone oscillations may also be excited in the case of parallel beam injection. In fact, for relatively low values of the beam transverse pressure, the basic mode frequency is related to the parameters of the target plasma and does not depend on the magnetic drift frequency of the beam particles that are injected nor on their velocity distribution.
Exact solutions of magnetohydrodynamic equations for fluids in a circular magnetic field
Authors: Namikawa Tomikazu , Hamabata Hiromitsu
Published: July 2002

Exact solutions of the nonlinear magnetohydrodynamic equations for a highly conducting fluid within an axisymmetric container are obtained. Attention is focused on fluids in which the unperturbed velocity and magnetic field are axially symmetric and purely zonal. It is shown that there are exact solutions with large amplitude but restricted form, indicating that an arbitrary disturbing force produces other motions as well as Alfvén waves propagating along an azimuthal magnetic field whose strength varies with radius.
Splitting instability of a high beta tokamak with noncircular cross section
Authors: Nakayama Yuichiro , Sato Tetsuya , Matsuoka Keisuke
Published: July 2002

The splitting of a tokamak with elliptical and bean-shaped cross sections is studied for finite beta plasmas. When the plasma beta exceeds a critical value, an elliptical tokamak is subject to an ideal pressure-driven instability, which deforms the ellipse in such a way that a thinned plasma current sheet is formed around the magnetic axis. As a result, magnetic reconnection is nonlinearly driven and the ellipse is split. The bean-shaped tokamak, however, is stable against a splitting perturbation for limited equilibria that could be numerically constructed. An interesting similarity to the energy relaxation process in a force-free plasma, namely, a two-step evolution (initial occurrence of an ideal magnetohydrodynamic instability and subsequent occurrence of driven reconnection), is discussed.
Radiation focusing in the cyclotron autoresonance maser
Authors: Kleva Robert G. , Levush Baruch , Sprangle P.
Published: July 2002

In the cyclotron autoresonance maser, an electromagnetic wave is amplified by interaction with an electron beam. Because of the copropagating electron beam, the refractive index seen by the wave is modified from the vacuum index and the dielectric properties of the electron beam can alter the propagation of the radiation beam. This phenomenon is studied through the use of a source-dependent modal expansion of the fully three-dimensional radiation field. In the exponential gain regime, the natural tendency of the radiation beam to spread diffractively is overcome and the beam is focused. The nature of the focusing is found to depend on the relative magnitude of the Doppler-shifted wave frequency and the gyrofrequency.
Synchrotron radiation from dynamically evolving runaway tails
Authors: Moghaddam-Taaheri Ebraahim , Vlahos Loukas
Published: July 2002

A quasilinear code (2-D in velocity and wave vector space) based on the Ritz–Galerkin method and finite elements is used to study the synchrotron emission from a dynamically evolving runaway tail. The evolution of runaway tails was studied recently using the same code [Phys. Fluids 28, 3356 (1985)]. The velocity distribution derived from the above code is used to estimate the synchrotron radiation in different time steps. The applicability of these results to the Princeton Large Torus [Phys. Rev. Lett. 49, 1255 (1982)] and solar microwave bursts is also commented on.
The streaming flow initiated by oscillating cascades of cylinders and their stability
Authors: Yan B. , Ingham D. B. , Morton B. R.
Published: July 2002

The steady fluid flow that is initiated by a cascade of cylinders, which oscillates harmonically in an unbounded, incompressible, viscous fluid that is otherwise at rest, is investigated, both numerically and experimentally. Finite-difference techniques are used to obtain the numerical solutions, and the streaming flow results show a reasonable agreement with the experimental data. It is found, both numerically and experimentally, that for large values of the streaming Reynolds number, Rs, the flow is not symmetrical about the axis of the oscillation, i.e., there exists a critical value of Rs, Rs0 say, such that the streaming flows for Rs≳Rs0 are unstable and those for Rs<Rs0 are stable. In order to understand this asymmetry, a stability analysis is performed, and the agreement between the theory and the experimental results is encouraging.
Transient rotational flow of an Oldroyd-B fluid over a disk
Authors: Tsamopoulos John , Borkar Abhay
Published: July 2002

The transient flow of an Oldroyd-B fluid over an infinite disk set in rotation impulsively is studied under the similarity assumption. The unsteady velocity and stress field is calculated exactly for short times by a power series expansion in time. The order of magnitude of the velocity and stress components is found to depend on the relative magnitude of the Deborah number (De) and the ratio of solvent to polymeric viscosities (μr). When either one becomes very small, a solution using singular perturbations and Laplace transforms is developed. It is found that the diffusive mechanism for momentum transfer, which exists for about μr≳0.1 (depending on De) dramatically changes and turns into a propagating wave for μr<0.1. Numerical calculations are used to determine the extent of validity of the present results.
Four vortices on doubly periodic paths
Authors: Rott Nicholas
Published: July 2002

Plane vortex configurations in ideal flow are considered for which the total ‘‘mass’’ of the vortex strengths, their ‘‘moments,’’ and their ‘‘polar moments of inertia’’ all vanish. These properties are conserved for all times. The simplest nontrivial realization of such a configuration requires four vortices. For this case, which belongs to the more extended family of four-vortex problems that are known to be integrable [Phys. Fluids 31, 2796 (1989); Phys. Fluids A 2, 1477 (1990)], some simple closed-form results are given. The analysis shows that the paths are periodic in a ‘‘configuration plane’’ moving with the vortices as well as in the absolute fluid plane. A ‘‘winding number’’ is determined from the analysis, which gives the ratio of the two periods. Patterns of the vortex paths are determined by a program based on the step-by-step integration of the equations of motion, which is—beyond a certain level of the analysis—still the more practical method of solution. Results showing the typical behavior of the motion paths for different winding numbers are presented.
Eddy viscosity transport model for turbulent flow
Authors: Durbin P. A. , Mansour N. N. , Yang Z.
Published: July 2002

An eddy viscosity transport model that is integrable to no-slip walls is described. The transport equation accommodates significant departure of the turbulence from equilibrium. The model uses an elliptic relaxation equation to avoid a need for damping functions; nonlocal wall effects are produced by solving the elliptic equation. Length scales are determined locally, without reference to distance to a surface. The model bifurcates from a laminar to a turbulent solution at a reasonable critical Reynolds number in plane channel flow. An assessment of the model is provided via boundary layer computations: these show that it behaves well in zero and adverse pressure gradients, although premature separation occurs in a strong adverse pressure gradient. Good results are obtained for the trailing edge separation bubble of an aerofoil at large angle of attack.
A simple theory of conditional mean velocity in turbulent scalar-mixing layer
Authors: Li J. D. , Bilger R. W.
Published: July 2002

This paper presents the experimental results on conditional mean transverse velocity across a turbulent scalar mixing layer. The velocity is conditioned on mixture fraction. It is found that the distribution of the conditional mean transverse velocity is closely related to the local mean-mixture fraction in physical space. Near the local mean-mixture fraction a linear relationship exists between the conditional mean transverse velocity and the mixture-fraction fluctuations. Departure from the linear relationship at large mixture-fraction fluctuations is mainly due to the nonlinear distribution of the mean mixture-fraction profile and it is not, as has been suggested by others, due to the limiting velocity of the large eddies in the flow.
Turbulence measurements in nonpenetrative thermal convection
Authors: Prasad Ajay K. , Gonuguntla P. V.
Published: July 2002

Nonpenetrative thermal convection at Ra=2×107 in a water layer of large-aspect ratio has been studied using Particle Image Velocimetry (PIV) to measure two-dimensional velocity vectors in a vertical plane. A sequence of 117 PIV photographs has been obtained with sufficient time separation between photographs to ensure statistical independence at least for the turbulent motions with time scales on the order of z*/w*. The grid spacing of the resulting velocity fields is adequate to resolve the smallest structures in the flow. Results include the development of motion with the onset of heating. The data set, obtained for fully developed turbulent flow, has been analyzed to obtain mean and rms velocities, vertical velocity skewness, two-point correlations, and stochastic estimates of velocity fields conditioned upon one and two-point events. A significant result is the absence of a discernible large scale circulation or mean flow with length scale on the order of the horizontal dimension of the fluid layer. Instead, the two-point correlations and stochastic estimates reveal the presence of roll cells whose horizontal length scale is equal to the layer depth. The measured values of vertical velocity skewness are in agreement with atmospheric measurements and large eddy simulations. The overall physical interpretation of the data suggests a seemingly chaotic and disordered motion, underlying which is a roll-cell mode with unit aspect ratio, punctuated by plumes emanating from the lower boundary.
Uniform shear flow in a binary mixture with general repulsive interactions
Authors: Marín C. , Garzó V.
Published: July 2002

A kinetic model for a binary mixture under uniform shear flow is exactly solved. The model incorporates a temperature dependence of the collision frequencies that allows the consideration of general repulsive interactions. The rheological properties of the mixture are obtained as functions of the shear rate, the parameters of the mixture (particle masses, concentrations, and force constants), and a parameter characterizing the interaction considered. In addition, the velocity distribution functions are explicitly obtained. While the transport coefficients are hardly sensitive to the interaction potential, the distribution functions are clearly influenced by the interaction parameter. In the tracer limit, a transition to an alternative state recently found in the context of Boltzmann equation is exactly identified in the case of Maxwell molecules. For non-Maxwell molecules, preliminary results suggest that this transition is also present although the phenomenon is less significant. A comparison with previous results derived from the a Boltzmann equation for Maxwell molecules is also carried out.
Calculation of velocity structure functions for vortex models of isotropic turbulence
Authors: Saffman P. G. , Pullin D. I.
Published: July 2002

Velocity structure functions (up′−up)m are calculated for vortex models of isotropic turbulence. An integral operator is introduced which defines an isotropic two-point field from a volume-orientation average for a specific solution of the Navier–Stokes equations. Applying this to positive integer powers of the longitudinal velocity difference then gives explicit formulas for (up′−up)m as a function of order m and of the scalar separation r. Special forms of the operator are then obtained for rectilinear stretched vortex models of the Townsend–Lundgren type. Numerical results are given for the Burgers vortex and also for a realization of the Lundgren-strained spiral vortex, and comparison with experimental measurement is made. In an Appendix, we calculate values of the velocity-derivative moments for the Townsend–Burgers model.
Dynamics of non-Brownian rodlike particles in a nonuniform elongational flow
Authors: Cloitre M. , Mongruel A.
Published: July 2002

This paper deals with the orientation of non-Brownian rodlike particles in a nonuniform elongational flow at low-Reynolds number. Experimentally, an elongational flow is created by introducing a small circular orifice into a cylindrical channel. The flow field is studied using a tracer anemometry technique. On the axis of symmetry, the deformation field is that of a pure elongation with a nonuniform elongation rate. It is well described by the analytical solution obtained for an infinite contraction ratio. This flow is used to study the dynamics of non-Brownian rodlike particles subject to nonuniform elongation. The orientation dynamics of a particle depends on the history of the deformation that it has experienced. The effects of small perturbations existing in the vicinity of the orifice are greatly amplified downstream. Consequently, particle motion does not exhibit the symmetry with respect to the orifice which is expected at low-Reynolds number.
Connection between corner vortices and shear layer instability in flow past an ellipse
Authors: Wang Z. Jane , Liu J. G. , Childress S.
Published: July 2002

We investigate, by numerical simulation, the shear layer instability associated with the outer layer of a spiral vortex formed behind an impulsively started thin ellipse. The unstable free shear layer undergoes a secondary instability. We connect this instability with the dynamics of corner vortices adjacent to the tip of the ellipse by observing that the typical turnover time of the corner vortex matches the period of the unstable mode in the shear layer. We suggest that the corner vortex acts as a signal generator, and produces periodic perturbation which triggers the instability.
The coupling of impurity ions with scrape-off layer plasma ions
Authors: Parbhakar K.
Published: July 2002

One-dimensional analysis of momentum transport equations of electrons, plasma ions, and one impurity charge state shows that both the nonambipolar current and the thermal forces can influence the coupling of impurity and plasma ions. The solution indicates that, in general, a steady-state drift can exist between the impurity and plasma ions. In cases where thermal forces are negligible, the impurity ions are strongly coupled to the plasma ions only if the transport is ambipolar. In this case the plasma ions and the impurities move as a single fluid. However, if nonambipolar currents are present, a steady-state drift is possible between the plasma ions and the impurity ions even if thermal forces are neglected. In this case, the imbalance due to the electron frictional force (∼Z2) acting on the impurity charge state and the plasma ions is balanced by the impurity and plasma ion friction.
Experimental beta limits of symmetric linear heliac configurations
Authors: Spanjers G. G. , Nelson B. A. , Ribe F. L. , Jarboe T. R.
Published: July 2002

Helically symmetric heliac equilibria [H. P. Furth, Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1966), Vol. 1, p. 103] are formed on the High Beta Q Machine (HBQM) [C. M. Greenfield, Phys. Fluids B 2, 133 (1990)] by using a fast-rising central conductor (hardcore) current in conjunction with a shock-heated l=1 stellarator configuration. The equilibria are found to possess a high global beta and the plasma pressure is approximately a flux-surface quantity. Under the effects of plasma, the magnetic well is found to deepen and the rotational transform is greatly increased and becomes highly sheared, owing to plasma currents induced by the fast-rising hardcore current. In the second phase of the experiment, the equilibrium fields of the symmetric heliac are lowered while maintaining the same shock heating in an attempt to raise the global beta. No substantial change in global beta is seen, indicating that the configuration forms at the beta limit in the shock-heated HBQM, and that the plasma beta seen in the first phase of the experiment is the symmetric heliac beta limit.
Reduced Braginskii equations
Authors: Yagi M. , Horton W.
Published: July 2002

A set of reduced Braginskii equations is derived without assuming flute ordering and the Boussinesq approximation. These model equations conserve the physical energy. It is crucial at finite β that the perpendicular component of Ohm’s law be solved to ensure ∇⋅j=0 for energy conservation.
Modification of tokamak edge turbulence using feedback*
Authors: Richards B. , Uckan T. , Wootton A. J. , Carreras B. A. , Bengtson Roger D. , Hurwitz P. , Li G. X. , Lin H. , Rowan W. L. , Tsui H. Y. W. , Sen A. K. , Uglum J.
Published: July 2002

Using active feedback, the turbulent fluctuation levels have been reduced by as much as a factor of 2 in the edge of the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Fusion Technol. 1, 479 (1981)]. A probe system was used to drive a suppressor wave in the TEXT limiter shadow. A decrease in the local turbulence-induced particle flux has been seen, but a global change in the particle transport at the present time has not been observed. By changing the phase shift and gain of the feedback network, the amplitude of the turbulence was increased by a factor of 10.
Screening modification on the trajectory of the projectile electron in the electron–ion excitation in dense plasmas
Authors: Jung Young-Dae
Published: July 2002

A behavior of the trajectory of the projectile electron in the electron-impact excitation of hydrogenic ions in dense plasmas is investigated using modified hyperbolic-orbit method in the semiclassical approximation. The interaction potential is obtained by the Debye–Hückel model of the screened Coulomb interaction for the interesting domain of the Debye length Λ/aZ≥10. The screening effects on the eccentricity of the projectile path produce a significant change in the excitation cross section near excitation threshold. The result shows that the trajectory of the projectile electron becomes a hyperbolic path rather than a parabolic path because of the plasma-screening effects. For higher incident energies, the plasma-screening effects on the projectile electron are decreased.
Explicit analytical solution of the nonlinear Vlasov–Poisson system
Authors: Škarka V. , Mahajan S. M. , Fijalkow E.
Published: July 2002

In order to describe the time evolution of an inhomogeneous collisionless plasma, the nonlinear Vlasov equation is solved perturbatively, using the subdynamics approach and the diagrammatic techniques. The solution is given in terms of a double perturbation series: one with respect to the nonlinearities and the other with respect to the interaction between particles. The infinite sum of interaction terms can be performed exactly due to the property of dynamical factorization. Following the methodology, the exact solution in each order with respect to nonlinearities is computed. For a choice of initial perturbation the first-order exact solution is numerically integrated in order to find the local density excess. The approximate analytical solution is found to be in excellent agreement with exact numerical integration as well as with ab initio numerical simulations. Analytical computation gives a better insight into the problem, and it has the advantage to be simpler, and also accessible, in some range of parameters where it is difficult to find numerical solutions.
Ion-acoustic compressive and rarefactive solitons in an electron-beam plasma system
Authors: Yadav L. L. , Tiwari R. S. , Sharma S. R.
Published: July 2002

Using the general formulation of reductive perturbation method, the Korteweg–de Vries (KdV) equation is derived for an electron-beam plasma with hot isothermal beam and plasma electrons and warm ions. The soliton solution of the KdV equation is discussed in detail. It is found that above a critical velocity of electron-beam two additional ion-acoustic soliton branches appear. It is found that corresponding to two linear modes, the system supports the existence of compressive as well as rarefactive solitons depending upon the plasma parameters, while corresponding to other two wave modes, the system supports only rarefactive solitons. The effect of different parameters on the characteristics of solitons have been investigated in detail.
The condition for cutoff layer vanishing in minority fundamental heating
Authors: Shen L. F. , Xiang N. , Yu G. Y.
Published: July 2002

The dispersion relation of a fast wave in a thermal (minority) hydrogen–deuterium plasma is investigated near the ion cyclotron frequency of hydrogen. It is found that the plasma shield for waves between the two-ion hybrid resonance and the minority fundamental resonance may be reduced by the thermal effects of plasma. By using zero Larmor radius approximations, the condition for cutoff layer vanishing due to the thermal effects of plasma is obtained analytically. The formula derived is compared with the numerical results from the full thermal-plasma dispersion relation, and excellent agreement is found. The results show that the cutoff layer may disappear in practical minority hydrogen–deuterium heating experiments.
Stability of straight tokamak equilibria without wall stabilization
Authors: Lortz D. , Zeiler A.
Published: July 2002

A specific straight tokamak equilibrium surrounded by vacuum and without a conducting wall is proved to be stable to all ideal magnetohydrodynamic (MHD) modes by analytically evaluating linear stability. The analysis starts with a circular-cylindrically symmetric equilibrium which has a piecewise constant current profile. The effect of corrugation, which is of great importance to the stability of axisymmetric modes, is taken into account by using perturbation methods. Stability of axisymmetric modes is proved by solving the eigenvalue problem up to second order in the corrugation amplitude. Whereas elliptical corrugation (N=2) leads to instability for arbitrary current density, an equilibrium with N≥3 may be stabilized to axisymmetric modes by current reversal. To treat nonaxisymmetric global modes, the potential energy is evaluated using tokamak scaling. A sufficient stability criterion is derived according to which the equilibrium is stable to nonaxisymmetric modes if the current density in the outer plasma area is reversed and in the center sufficiently peaked, and if the safety factor q at the magnetic axis is greater than unity, increasing monotonically toward the plasma edge.
Experimental study of growth rate scaling and quasicoherent states in plasma turbulence
Authors: Tham P. , Sen A. K.
Published: July 2002

Experimental results of the scaling of linear growth rate with the electrostatic fluctuation levels are presented. A quadratic scaling was obtained for the ion temperature gradient-driven instability (ITG), characteristic of weak turbulence. For the rotationally driven E×B mode, a linear scaling was obtained, characteristic of strong turbulence. Moreover, the linear growth rate scaling for the E×B mode exhibited two distinct behaviors, corresponding to a broadening and a narrowing of the spectral width as a function of increasing fluctuation level. The spectral broadening is associated with the usual turbulent broadening, while the spectral narrowing is attributed to the presence of coherent structures via self-organization. Radial harmonics were detected and shown to contribute to the spread in the spectral width. These structures were also studied via a bispectral analysis. The evolution to the quasicoherent state was also found to result in the presence of a fewer number of radial harmonics in the saturated state. It revealed the presence of three-wave coupling involving the radial and azimuthal harmonics, suggesting a possible mechanism for the nonlinear saturation of the mode and anomalous transport.
Spreading particle trajectories near a perfectly reflecting surface in a tilted magnetic field
Authors: Cohen Ronald H. , Ryutov Dmitri D.
Published: July 2002

A description is given for electron trajectories near a specularly reflecting surface in a magnetic field that intersects the surface at a shallow angle. A simple analytical theory of electron motion is presented. It is shown that an electron experiences a large number of reflections from the wall and makes a long path along the surface, before eventually returning to the plasma. The importance of this phenomenon for the sheath structure and electron cross-field transport is briefly discussed.
Wave-induced chaotic radial transport of energetic electrons in a laboratory terrella experiment
Authors: Warren H. P. , Mauel M. E.
Published: July 2002

This paper reports the observation of wave-induced chaotic radial transport of energetic electrons in a laboratory terrella, the Collisionless Terrella Experiment (CTX) [H. P. Warren and M. E. Mauel, Phys. Rev. Lett. 74, 1351 (1995)]. Electron cyclotron resonance heating is used to create a localized population of energetic electrons which excite the hot electron interchange instability. The electrostatic fluctuations driven by this instability have time-evolving spectra which resonate with the precessional drift motion of the hot electrons. We have established that the amplitude, frequency, and azimuthal mode number of the observed instabilities meet the conditions for the onset of chaotic particle motion. Electron transport is observed with a gridded particle detector. Increases in the flux of energetic particles to the detector are well correlated with the presence of fluctuations which meet the conditions for global chaos. Greatly diminished transport is observed when the fluctuations lead to thin, radially localized bands of chaos. The flux of energetic electrons to the detector is strongly modulated. By examining time-dependent Hamiltonian phase space flows, the modulation is shown to be the result of phase space correlations. A transport simulation based on the Hamiltonian motion of energetic electrons reproduces the frequency and modulation depth of the observed electron flux and allows for comparison between the Hamiltonian and quasilinear descriptions of transport.
On the stability of the internal kink mode in the banana regime
Authors: Fogaccia G. , Romanelli F.
Published: July 2002

The stability of the internal kink mode is investigated taking into account the kinetic response associated to the trapped thermal ions. Ion–ion collisions and diamagnetic effects in the layer are also considered. A significant stabilizing contribution is obtained, even at low-β values, on the mode, which might be stable, on present experiments, even though predicted unstable according to the Bussac criterion [Bussac et al., Phys. Rev. Lett. 35, 1638 (1975)]. In addition, a trapped-ion instability is found, characterized by mode frequency of the order of the trapped-ion bounce-averaged magnetic drift frequency.
Neoclassical and anomalous transport in axisymmetric toroidal plasmas with electrostatic turbulence
Authors: Sugama H. , Horton W.
Published: July 2002

Neoclassical and anomalous transport fluxes are determined for axisymmetric toroidal plasmas with weak electrostatic fluctuations. The neoclassical and anomalous fluxes are defined based on the ensemble-averaged kinetic equation with the statistically averaged nonlinear term. The anomalous forces derived from that quasilinear term induce the anomalous particle and heat fluxes. The neoclassical banana-plateau particle and heat fluxes and the bootstrap current are also affected by the fluctuations through the parallel anomalous forces and the modified parallel viscosities. The quasilinear term, the anomalous forces, and the anomalous particle and heat fluxes are evaluated from the fluctuating part of the drift kinetic equation. The averaged drift kinetic equation with the quasilinear term is solved for the plateau regime to derive the parallel viscosities modified by the fluctuations. The entropy production rate due to the anomalous transport processes is formulated and used to identify conjugate pairs of the anomalous fluxes and forces, which are connected by the matrix with the Onsager symmetry.
The effect of an external torque on low to high confinement transitions
Authors: Newman D. E. , Carreras B. A. , Diamond P. H.
Published: July 2002

A number of experiments have attempted, with varying degrees of success, to influence the low to high confinement mode (L–H) transition through the application of an external flow shear forcing mechanism. In order to theoretically describe this type of experiment, an external torque is included in the phase transition model of Diamond et al. [Phys. Rev. Lett. 72, 2965 (1994)] for the L–H bifurcation. These equations exhibit a bifurcation between an L-mode fixed point and an H-mode fixed point with variation of a critical parameter, corresponding to the edge gradient, across a threshold value. With the addition of the external torque [from biased limiter, probe, or radio frequency (rf) wave], the character of the transition changes in a manner analogous to the addition of an external magnetic field to a ferromagnetic material. In addition to the change in the transition dynamics, there is a marked decrease in the threshold level. In this simple model, the decrease in threshold level is proportional to the applied torque to the two-thirds power, in approximate functional agreement with that seen by some experiments.
Transport experiments in Alcator-C-Mod
Authors: Greenwald M. , Boivin R. L. , Bonoli P. , Christensen C. , Fiore C. , Garnier D. , Goetz J. , Golovato S. , Graf M. , Granetz R. , Horne S. , Hsu T. , Hubbard A. , Hutchinson I. , Irby J. , Kurz C. , LaBombard B. , Lipschultz B. , Luke T. , Marmar E. , McCracken G. , Niemczewski A. , O’Shea P. , Porkolab M. , Rice J. , Reardon J. , Schachter J. , Snipes J. , Stek P. , Takase Y. , Terry J. , Umansky M. , Watterson R. , Wolfe S. , Bombarda F. , May M. , Welch B.
Published: July 2002

A series of transport experiments has been carried out in Alcator-C-Mod. [Phys Plasmas 1, 1511 (1994)]. Data from both Ohmic and ICRF (ion cyclotron range of frequencies) heated plasmas can be fitted with an L-mode (low mode) scaling law. The Ohmic τE’s show no scaling with density in any regime and can reach values of 2–3 times neo-Alcator. Impurity confinement has been studied with the laser blow-off technique with τI showing nearly linear scaling with plasma current. Ohmic and ICRF H modes are obtained over a wide range of discharge parameters, extending the range in the international database for nB, by almost a factor of 10. The power threshold for ELM-free (edge localized mode) discharges is in rough agreement with the scaling P/S=0.044nB. Energy diffusivities of Ohmic and ICRF heated plasmas have been measured from local analysis of plasma profiles and power fluxes. The same analysis produces a value for plasma resistivity which lies between the Spitzer and neoclassical calculations. Analysis of plasma transients have yielded values for particle diffusivity and convection velocity.
Hall term enhancement of current drive in a bounded inhomogeneous plasma
Authors: Pandey B. P. , Avinash K. , Kaw P. K. , Sen A.
Published: July 2002

The problem of helicity injection and current drive due to ion-cyclotron waves in a bounded inhomogeneous plasma is reexamined. It is shown that Hall term contributions (which had been neglected earlier in the work of Mett and Taylor [Phys. Fluids B 4, 73 (1992)]), even in the limit of small corrections of the order of ω/Ωi (ω and Ωi are the wave and the ion-cyclotron frequencies, respectively), improve substantially the parallel electromotive force (EMF) for the resonance layer.
High-power electron-beam transport in long gas cells from 10−3 to 103 Torr nitrogen
Authors: Sanford T. W. L.
Published: July 2002

Measurements and analyses show that the 13 TW, pulsed Hermes-III [J. J. Ramirez, Proceedings of the 1989 Particle Accelerator Conference, Chicago, 1989 (IEEE, New York, 1989), p. 1446] electron beam, with current near the Alfvén limit, has two regimes of stable transport: a low-pressure window (between ∼1 and ∼100 mTorr) that is dominated by propagation in the semicollisionless IFR (ion-focused regime), and a high-pressure window (between ∼1 and ∼100 Torr) that is dominated by propagation in the resistive CDR (collision-dominated regime). Below ∼1 mTorr, there is insufficient ionization to confine the beam; between the windows, the two-stream and hollowing instabilities disrupt propagation; and above ∼100 Torr, the resistive hose instability degrades propagation.
Influence of trapped electrons on the dielectric properties of the Earth’s radiation belts
Authors: Grishanov N. I. , Elfimov A. G. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

The dielectric tensor of an axisymmetric magnetosphere is derived by solving the Vlasov equations for trapped particles for a two-dimensional plasma model with circular magnetic field lines. Because of the nonuniformity of the geomagnetic field any Fourier harmonic of the perturbed current density (after its Fourier expansion over the geomagnetic latitude) is proportional to the sum over all harmonics of the oscillating electric field; that leads to additional wave dissipation effects. The resonant conditions for wave–particle interactions are discussed taking into account the cyclotron and bounce resonances; these conditions in magnetospheric plasmas are entirely different from the Landau damping resonant factor in the uniform magnetic field. The numerical calculations of the longitudinal permeability in the Earth’s radiation belts (the equatorial distance of the magnetic field line is five Earth’s radii) are carried out for short-period oscillations with the frequency ω∼2 s−1 and longitudinal wave numbers n=1,...,15. The different energy levels of the trapped electrons, T∼40–4000 eV, are considered. It is shown that, in the low-frequency region, the imaginary part of the longitudinal permeability decreases as ∼T−2.5 whereas the electron temperature increases. This decrease is stronger than ∼T−1.5 for plasmas in a straight magnetic field. For a given wave frequency, the basic contribution of trapped electrons to the imaginary part of longitudinal permeability is associated with the low numbers of bounce resonances and the low longitudinal wave numbers.
Attraction of charged dust-grains in the presence of dust-lower-hybrid modes
Authors: Salimullah M. , Amin M. R.
Published: July 2002

The existence of the strong attractive force between the charged dust particulates in the presence of possible low-frequency dust-lower-hybrid modes in a magnetized dusty plasma has been examined. These modes arise on account of the inequalities of charge and number densities of electrons, ions, and dust particles, and finite Larmor radius effect of the massive dust grains in a hot magnetized dusty plasma.
Relativistic theory of cross-field transport and diffusion in a plasma
Authors: Mohanty J. N. , Baral K. C.
Published: July 2002

A modified Chapman–Enskog analysis is employed to solve the relativistic Boltzmann transport equation. This is in the context of binary Coulomb encounter in a weakly coupled and fully ionized nondegenerate electron–ion plasma across a magnetic field. Specific formulas concerning transport coefficients are derived analytically in the various limits of temperatures, viz., nonrelativistic, moderately relativistic, and ultrarelativistic temperatures. Early classical results are equally recovered. The dependence of coefficients on thermal regimes is quantitatively discussed and their markedly diminishing trends are analyzed.