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The results are presented for a basic heat transport experiment involving an off-axis heat source in which avalanche events occur. The configuration consists of a long, hollow, cylindrical region of elevated electron temperature embedded in a colder plasma and far from the device walls [Van Compernolle et al. Phys. Rev. E 91, 031102(R) (2015)]. The avalanche events are identified as sudden rearrangements of the pressure profile following the growth of fluctuations from ambient noise. The intermittent collapses of the plasma pressure profile are associated with unstable drift- Alfvén waves and exhibit both radial and poloidal dynamics. After each collapse, the plasma enters a quiescent phase in which the pressure profile slowly recovers and steepens until a threshold is exceeded and the process repeats. The use of reference probes as time markers allows for the visualization of the 2D spatio-temporal evolution of the avalanche events. Avalanches are observed only for a limited combination of heating powers and magnetic fields. At higher heating powers, the system transits from the avalanche regime into a regime dominated by sustained drift-Alfvén wave activity. This manuscript focuses on new results that illustrate the individual contributions to the avalanche process from density and temperature gradients in the presence of zero-order, sheared flows. [ABSTRACT FROM AUTHOR]

A calculation is presented of the radiation pattern of shear Alfvén waves generated by a burst of charged particles in a charge-neutral plasma with two-ions of differing charge-to-mass ratios. The wake pattern is obtained for the inertial and kinetic regimes of wave propagation. Due to the presence of two ion-species, the Alfvén waves propagate within two different frequency bands separated by a gap. One band is restricted to frequencies below the cyclotron frequency of the heavier species and the other to frequencies between the ion-ion hybrid frequency and the cyclotron frequency of the lighter species. The radiation pattern in the lower frequency band is found to exhibit essentially the same properties reported in a previous study [Van Compernolle et al., Phys. Plasmas 15, 082101 (2008)] of a single species plasma. However, the upper frequency band differs from the lower one in that it always allows for the Cherenkov radiation condition to be met. The methodology is extended to examine the Alfvénic wake of point-charges in the inertial and adiabatic regimes. The adiabatic regime is illustrated for conditions applicable to fusion-born alpha particles in ITER. [ABSTRACT FROM AUTHOR]

Pulses having a temporal Lorentzian shape arise naturally from topological changes in flow trajectories or phase-space orbits associated with deterministic chaos. The pulses can appear as random intermittent events in the time series of observable quantities, and they are the cause of exponential frequency power spectra previously observed in magnetically confined plasmas and various nonlinear systems. [ABSTRACT FROM AUTHOR]

The dynamics of transport at the edge of magnetized plasmas is deterministic chaos. The connection is made by a previous survey [M. A. Pedrosa et al., Phys. Rev. Lett. 82, 3621 (1999)] of measurements of fluctuations that is shown to exhibit power spectra with exponential frequency dependence over a broad range, which is the signature of deterministic chaos. The exponential character arises from Lorentzian pulses. The results suggest that the generalization to complex times used in studies of deterministic chaos is a representation of Lorentzian pulses emerging from the chaotic dynamics. [ABSTRACT FROM AUTHOR]

One of the fundamental waves in magnetized plasmas is the shear Alfvén wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfvén waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvénic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfvén waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfvén waves. [ABSTRACT FROM AUTHOR]

A simplified numerical study is made of the structures that are formed in a magnetized temperature filament due to oscillatory convection from large amplitude drift waves. This study is motivated by a recent experiment [D. C. Pace, M. Shi, J. E. Maggs et al., Phys. Plasmas 15, 122304 (2008)] in which Lorentzian-shaped temporal pulses are observed. These pulses produce a broadband, exponential frequency power spectrum. The model consists of an electron heat transport equation in which plasma convection arising from pressure-gradient driven drift-waves is included. It is found that above a critical wave amplitude, spatially complex structures are formed, which give rise to temporal pulses having positive and negative polarities at different radial positions. The temporal shape of the pulses can be fit by a Lorentzian function. The associated spatial structures exhibit temporally oscillatory heat plumes (positive polarity) and cold channels (negative polarity). The idealized effect of a static flow on these structures is explored. Depending on the flow direction (relative to the azimuthal propagation of the drift waves), the temporal Lorentzian pulses can be suppressed. [ABSTRACT FROM AUTHOR]

A calculation is presented of the radiation pattern of shear Alfvén waves excited by a burst of charged particles propagating along the confinement magnetic field. The characteristic wake patterns are obtained for the inertial and kinetic regimes of wave propagation. In the inertial regime, the waves are only excited by particles moving slower than the Alfvén speed. The radiated wake exhibits an inverted V-shape due to the backward-wave nature of the modes. In the kinetic regime, particles moving faster as well as slower than the Alfvén speed can radiate propagating modes. The super Alfvénic particles, however, excite modes with relatively short transverse scales. The motorboat type of wake of the kinetic modes is more typical of the Cherenkov process obtained in scalar dielectrics. The predictions are in agreement with experimental observations [B. Van Compernolle et al., Phys. Plasmas 13, 09112 (2006)] and computer simulations [F. S. Tsung et al., Phys. Plasmas 14, 042101 (2007)] in which a burst of fast electrons generated by resonant absorption in a magnetized plasma excites a pulse of large-amplitude Alfvén waves. [ABSTRACT FROM AUTHOR]

A broad experimental survey is made of the properties of spontaneously generated magnetic fluctuations in a large linear device in which the plasma density has different cross-field gradient scales. It includes steep gradients at the plasma-wall edge as well as at an interior plasma-plasma interface, thus phenomena of interest to magnetic fusion research as well as to space plasma physics is illustrated. Fluctuations in uniformly magnetized columns and plasmas with an axial gradient in the confining magnetic field are studied. Some of the highlights include the identification of a universal spectrum for drift-Alfvén turbulence and the role of partial reflections in shaping the spectra. [ABSTRACT FROM AUTHOR]

This analytic study investigates the nonlinear plasma response when a shear Alfvén wave of large transverse scale interacts with a field-aligned density perturbation whose transverse scale is comparable to the electron skin depth. The interaction between the large-scale wave and the density perturbation produces a small-scale shear mode [T. Drozdenko and G. J. Morales, Phys. Plasmas 7, 823 (2000)], which facilitates the transfer of energy to the plasma particles. The beat of the large-scale wave with the small-scale wave can lead to ponderomotive forces and flows in the plasma, or, if the field amplitudes of the small-scale wave are large enough to produce oscillatory velocities comparable to the electron thermal velocity, a streaming instability may develop. In this study, it is demonstrated that nonlinear effects arise in regions remote from the seed perturbation, and estimates of the amplitude of the large-scale wave necessary to produce significant ponderomotive density changes or trigger significant streaming instabilities are presented. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A systematic study is made of the spontaneous growth of fluctuations in temperature, density, and magnetic field in a narrow (on the order of the electron skin depth) field-aligned temperature filament embedded in a large magnetized plasma. Two broad classes of fluctuation ("low" and "high" frequency modes) have been identified and studied in detail. A high-frequency drift-Alfvén mode grows at frequencies about one tenth the ion gyrofrequency in the region of the filament where the temperature gradient is large. The measured radial profiles of the density and magnetic field fluctuations associated with this mode agree well with theoretical predictions. The high-frequency mode has been observed to exhibit several interesting nonlinear features, including steepening wave form, progression in azimuthal mode number, coupling to the low frequency mode with subsequent sideband generation, and eventually a transition to broad band turbulence. The nature of the low-frequency mode which has frequencies about one fiftieth of the ion gyrofrequency is less certain, but it has been identified as a spatially localized, azimuthally symmetric mode consisting primarily of temperature fluctuations. Both the high and low-frequency modes give rise to electron heat transport at rates in excess of the classical values. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A technique is presented for constructing a self-consistent Vlasov equilibrium for a bounded single species plasma confined by a strong magnetic field. The equilibrium incorporates particle losses to the surrounding wall due to large gyroradii, a feature not previously considered. Specific examples of the density and potential profiles including particle losses are given. [ABSTRACT FROM AUTHOR]

A simple model is developed that isolates the essential features of a magnetically confined plasma undergoing anomalous transport and whose properties are manipulated by externally injected radial currents. The model exhibits the following intrinsic properties (not built-in a priori): Ohmic confinement improves with density; L-mode scaling depends on heating power as P-0.6; a transition to H-mode behavior is induced by radial currents on the order of 10 A. The model is benchmarked against the experiments performed in the Continuous Current Tokamak (CCT) [R. J. Taylor et al., Phys. Rev. Lett. 63, 2365 (1989)]; excellent quantitative and qualitative agreement is obtained with most of the observations. Some discrepancies are identified and their significance is elucidated. The new effects explored with the model are H-modes induced by multiple electrodes, auxiliary heating in the presence of radial currents, and the concept of H mode as a ‘‘phase transition.’’ [ABSTRACT FROM AUTHOR]

The inverse linear mode conversion between an electrostatic ion Bernstein wave and an electromagnetic fast Alfvén wave in tokamak plasmas is investigated using a self-adjoint system of equations of fourth order that includes cyclotron and Landau damping processes. Over a broad range of cases investigated, the reciprocity of the mode conversion coefficients between the ion Bernstein and fast waves is observed to hold, within numerical accuracy, even in the presence of strong cyclotron absorption. It is also found that strong bulk ion heating occurs at twice the cyclotron frequency for direct launching of an ion Bernstein wave into a plasma that contains no ionic species that are resonant at the fundamental cyclotron frequency. [ABSTRACT FROM AUTHOR]

A numerical and analytical study is presented of the nonlinear phase-space trajectories of particles accelerated by resonant electric fields in a nonuniform plasma. The fields are represented by a driven Airy pattern characteristic of resonant excitation, and thus the results have relevance to several topics of current interest. The acceleration processes can be classified into three regimes depending upon the initial energy of the particles. The first type is a diffusive interaction which occurs for high-energy particles. The second type is slowing down due to temporary trapping inside potential wells having a spatially decreasing phase velocity. The third type is a phase-independent acceleration of slow particles. The comparison velocity used to classify these regimes is ωL/(kDL/[Square_Root]3)2/3, where ω is the resonant frequency, L the gradient scale length, and kD the Debye wave number. Simple analytic expressions are found that explain the basic features of numerical test particle orbits and illuminate the existence of the three categories of interaction. [ABSTRACT FROM AUTHOR]

A kinetic theory of electrostatic cyclotron waves in a single component plasma slab of density n[sub o](x) immersed in a uniform magnetic field B[sub o]&zcirc; is presented. The space charge electric field E[sub o](x)○ in such a plasma modifies the single particle gyrofrequency from Ω = qB[sub o]/mc to Ω[sub 1] (x) = [Ω² - ω²[sub p] (x) ][sup 1/2] in the limit r[sub c] /L « 1 (where r[sub c] is the orbit size and L [sup -1] = d In E[sub o]/ Ω Finite Larmor dx), causing the upper hybrid frequency to have the value (Ω²[sub 1] + ω²[sub p]) [sup 1/2] = radius effects introduce a velocity dependence into the single particle gyrofrequency Ω[sub 1], leading to energy transfer to the particles located at the resonant layers where ω - k[sub y] v[sub E] (x) = Ω[sub 1] (x) [v[sub E] (x) ŷ being the ExB drift velocity]. This energy transfer mechanism is operative even when k[sub z] = 0. Another nonzero Larmor radius effect is the appearance of thermal modes that are the analogs of Bernstein modes of neutral plasmas. When driven by an external capacitor plate antenna, these modes exhibit behavior similar to Tonks-Dattner resonances. [ABSTRACT FROM AUTHOR]

An analytic study is made of the effects produced by a localized modulation of the electrical conductivity of a magnetized plasma carrying a field-aligned current. The modulation is assumed to be caused by a heating source external to the plasma. The formulation used is applicable to modulation frequencies ω below the ion-cyclotron frequency ω[sub ci], and permits the calculation of the radiation pattern which contains shear as well as compressional Alfvén modes and near fields. For ω/ω[sub ci] « 1, the radiation appears primarily in the form of a highly collimated shear mode traveling along the field lines passing through the spot where the modulation occurs. [ABSTRACT FROM AUTHOR]

The high-frequency electromagnetic field generated by a density-modulated, helical electron beam propagating in a magnetized plasma is calculated. The magnetic fluctuations are found to exhibit spatially localized (evanescent) resonances at harmonics of the electron-cyclotron frequency, whose width is determined by the pitch angle of the beam, and whose existence is a consequence of the helical geometry. In addition, electrostatic modes are radiated near the hybrid frequencies, and electromagnetic modes are radiated above the upper-hybrid frequency. The predicted frequency spectrum and mode structure in configuration space are in good agreement with experimental observations of discrete emission lines at the electron-cyclotron harmonics [Phys. Fluids B 5, 3789 (1993)]. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The sheath formed between a magnetized plasma and a particle absorbing wall is examined for the case in which the magnetic field intercepts the wall at a small angle 0°<ε<=9°, where sin ε=B·n/|B|, and n is the unit normal to the wall. The model is time-independent and one-dimensional (1-D) with all functions varying only in the direction normal to the wall. The ions are modeled by a Maxwellian velocity distribution which is modified by the condition that ions, which would have hit the wall, are absent. For the electrons a fluid description is used, including the effects of electron–neutral collisions. The transport of particles due to turbulent electrostatic fluctuations is modeled by a constant electric field perpendicular to both B and n. It is found that in the range of angles under consideration, there are two distinct regimes of sheath formation. If ε<=ν=ν/Ωe (grazing incidence), where ν is the electron–neutral collision frequency and Ωe is the electron cyclotron frequency, then the properties of the sheath are determined by a parameter λ which is the ratio of the convective (E×B) and diffusive electron flows. If λ<=1, the wall potential is negative and the sheath scale length is on the order of an ion gyroradius. If λ>=1, the wall potential is positive and, for large λ, the sheath is characterized by two scales: a short length, which is a decreasing function of λ, adjacent to the wall, and the ion gyroradius farther from the wall. For ε>ν, (oblique incidence) the potential at the wall is negative with a magnitude close to that of the unmagnetized plasma and is only weakly dependent on ε. In addition, for this case, the sheath scale length is on the order of an ion gyroradius and is weakly dependent on ε, larger values of ε resulting in a slightly shorter scale length. [ABSTRACT FROM AUTHOR]

This study provides an analytic solution to the general problem of mode conversion in an unmagnetized plasma. Specifically, an electromagnetic wave of frequency ω propagating through a plasma with a parabolic density profile of scale length Lp is examined. The mode conversion points are located a distance Δ0 from the peak of the profile, where the electron plasma frequency ωp(z) matches the wave frequency ω. The corresponding reflection, transmission, and mode conversion coefficients are expressed analytically in terms of parabolic cylinder functions for all values of Δ0. The method of solution is based on a source approximation technique that is valid when the electromagnetic and electrostatic scale lengths are well separated. For large Δ0, i.e., (cLp/ω)1/2<Δ0

For mode conversion in an unmagnetized plasma with a parabolic density profile of scale length L, analytic expressions, in terms of parabolic cylinder functions, for the energy flux coefficients (reflection, transmission, and mode conversion) and the fields for both the "direct" problem (incident electromagnetic wave converting to a Langmuir wave) and the "inverse" problem (incident Langmuir wave converting to an electromagnetic wave) are derived for the case where the incident wave frequency ω matches the electron plasma frequency ω[sub p] at the peak of the density profile. The mode conversion coefficient for the direct problem is equal in magnitude to that of the inverse problem, and the corresponding reflection and transmission coefficients satisfy energy conservation. In contrast to the linear profile problem, the conversion efficiency depends explicitly on the value of the collision frequency (in the cold, collisional limit) or electron temperature (in the warm, collisionless limit), but a transformation of parameters relates the results for these two limits. [ABSTRACT FROM AUTHOR]

It is demonstrated that particle-simulation techniques can explore a broad range of dynamical behavior exhibited by non-neutral plasmas. New features isolated by this approach include collective relaxation through generation of core and halo populations; self-organization without radial transport as the plasma is cooled; spontaneous generation of solitonlike structures upon axial reflection from the external confining potential. [ABSTRACT FROM AUTHOR]

A simple model is presented that yields analytical expressions for the heating efficiency of beatwave excitation in a plasma with a linear density profile. The effect of self-consistent Landau damping by tail electrons is included without recourse to WKB approximations. [ABSTRACT FROM AUTHOR]

A theoretical and experimental investigation is made of a wave resonator based on the concept of wave reflection along the confinement magnetic field at a spatial location where the wave frequency matches the local value of the ion-ion hybrid frequency. Such a situation can be realized by shear Alfvén waves in a magnetized plasma with two ion species because this mode has zero parallel group velocity and experiences a cut-off at the ion-ion hybrid frequency. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is expected that a magnetic well configuration in a two-ion plasma can result in an Alfvén wave resonator. Such a concept has been proposed in various space plasma studies and could have relevance to mirror and tokamak fusion devices. This study demonstrates such a resonator in a controlled laboratory experiment using a H+-He+ mixture. The resonator response is investigated by launching monochromatic waves and impulses from a magnetic loop antenna. The observed frequency spectra are found to agree with predictions of a theoretical model of trapped eigenmodes. [ABSTRACT FROM AUTHOR]

A systematic study is made of the axial and transverse heat transport that arises when a microscopic temperature filament is generated, under controlled conditions, in a magnetized plasma of large dimensions. For early times and relatively small temperature gradients the study conclusively demonstrates the two-dimensional pattern characteristic of the classical theory based on Coulomb collisions. The full nonlinear dependence of the transverse and axial electron heat conductivities is sampled through temperature changes in the range δT[sub e]/T[sub e]∼1-10. The dependence on the confining magnetic field is explored over a factor of 3 (∼ factor of 10 in transverse conductivity). It is found that under quiescent conditions, the observed behavior agrees with classical theory within the experimental uncertainties. However, over long times and/or for steep temperature gradients, fluctuations develop spontaneously and cause a significant departure from the predictions of the classical theory. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]