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Molecular dynamics simulations are employed to study the field ion emission from ionic liquids. Here, an all-atom polarizable force field is selected to model the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4). This force field presents a superior ability to reproduce the transport properties of the liquid, and it is used for the first time in the analysis of field ion emission. Initially, the case of a suspended droplet of ionic liquid is employed to describe the characteristics of the liquid at equilibrium. Then, a simulation comprising a nano-droplet of ionic liquid attached to a wall and exposed to a uniform electric field is employed to study the emission of ions. The current emitted, the beam composition, the energy deficit, and the mean characteristics of the process are reported. An extensive analysis is then carried out based on the process characteristics, the theory of field evaporation, and previous numerical solutions. The results suggested that steady field evaporation is unlikely to take place in most of the cases simulated. Field evaporation appears to be limited to the use of low electric fields, which makes the rate of emission decrease significantly. Despite this limitation, insights are made regarding the role of droplet polarization in decreasing the vaporization energy of the ions. The emission process observed in the simulations seems to be related to the stability limit of the droplet and the tearing of its surface, resembling a periodic jet-type of emission. [ABSTRACT FROM AUTHOR]

The features of an unexpected, large event that arises spontaneously during a basic heat transport experiment are presented. It consists of the sudden collapse of the radial plasma pressure profile, akin to disruption events observed in toroidal magnetic confinement devices. The experiment is performed on the Large Plasma Device at the University of California, Los Angeles (UCLA). It uses a LaB6 thermionic emitter of annular shape to induce off-axis heating of a cold, afterglow plasma, in a linear magnetic geometry. The temporal evolution consists of three regimes. During an early, quiescent period, classical heat transport along and across the magnetic field arises from Coulomb collisions. After significant pressure gradients develop, drift-Alfvén waves become unstable. Upon reaching large amplitude, they trigger avalanche events that flatten the outer part of the heated region, which, in turn, quenches the instability. Due to the sustained heating, the pressure profile rebuilds and the process repeats, leading to a relatively long, second regime that displays multiple avalanches, but suddenly, the annular pressure profile is observed to collapse. After this collapse, the system enters a third regime with large fluctuations. Before the collapse, a rapid, runaway heating environment arises whose time evolution exhibits a self-similar dependence on the applied voltage. The time evolution, morphology, and scaling of the collapse event are presented, and an examination is made of the underlying mechanisms. [ABSTRACT FROM AUTHOR]

A numerical investigation is made of the response of a thermalized charge in contact with a chaotic thermostat to an externally applied, oscillating electric field, with and without a confining magnetic field. This study is based on the recent theoretical development of a chaotic thermostat [G. J. Morales, Phys. Rev. E 99, 062218 (2019)] for which the static properties of a thermalized charge (spatial diffusion and mobilities) were previously established. It is found here that in the unmagnetized case, to extract significant power from the oscillating electric field requires that the oscillatory velocity be larger than the thermal velocity. The spatial diffusion caused by the self-consistent, chaotic fluctuations is shown to be reduced, and even suppressed, by the coherent oscillations. The frequency-dependent mobilities, including cyclotron resonance, are determined, and the nonlinear modifications produced by large electric fields are explored. The value of the mobilities and the modifications to the diffusion coefficient exhibit non-monotonic dependencies on the control parameters. [ABSTRACT FROM AUTHOR]

An experiment is performed on a large plasma device operated by the Basic Plasma Science Facility at the University of California, Los Angeles, in which an electrically floating structure is placed near the end of the 18-m magnetized plasma column. The structure consists of a flat carbon plate that acts as a mask for a smaller, ring-shaped LaB6 emissive surface whose temperature can be externally controlled. This configuration has been previously used to study electron heat transport and pressure-driven avalanches [B. Van Compernolle and G. J. Morales, Phys. Plasmas 24, 112302 (2017)] by biasing the LaB6 ring-cathode with respect to a distant anode in a cold afterglow plasma. In contrast, the present study is performed during the active portion of the steady-state discharge in which the nominal plasma parameters are determined by the injection of an electron beam from a BaO cathode at the opposite end. It is found that, even without an applied bias on the LaB6 cathode, the self-consistent potential and current profiles are modified near the end plate as the LaB6 temperature is increased, resulting in density increases on the field lines in contact with the ring-cathode. In the absence of enhanced ionization, at the largest cathode temperatures, the ambient density can be doubled. A theoretical model is presented that provides a quantitative explanation for the experimental observations. [ABSTRACT FROM AUTHOR]

A recently developed model of nonlocal transport in two dimensions (2D), the "iterative method" [J. E. Maggs and G. J. Morales, Phys. Rev. E 99, 013307 (2019)], is applied to the published results of ECRH experiments [Takahashi et al. AIP Conf. Proc. 1580, 145 (2014)] performed in the Large Helical Device. It is found from the temperature profiles obtained from the 2D model that the sharply peaked electron temperature profiles observed in such experiments, and previously associated with local transport and "internal transport barriers" or Core Electron-Root Confinement, are also consistent with nonlocal transport. It is shown that the iterative method is suited to handling both scenarios. [ABSTRACT FROM AUTHOR]

A LaB6 thermionic emitter of annular shape is used in the Large Plasma Device at the University of California, Los Angeles to create off-axis heating conditions for various transport studies. Since the emitter is biased relative to a distant anode, which is many collision lengths away, the entire magnetized plasma develops a self-consistent, potential structure that simultaneously generates transverse and axial flows with shear. This study uses swept Langmuir probe techniques and Mach probes to map the flow patterns and their dependence on bias and plasma parameters. By implementing additional biasing configurations, it is possible to control the magnitude of the flows and their shear strength. The experimental measurements, including the self-consistent currents, are compared to predictions of a model that incorporates the boundary conditions associated with thermionic injection, combined with a Braginskii transport code for the electron temperature. [ABSTRACT FROM AUTHOR]

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 theoretical and numerical modeling study is made of a novel heating configuration recently implemented in the Large Plasma Device at the University of California, Los Angeles. The injection of an electron beam from a masked LaB6 cathode into a magnetized plasma results in a hollow, cylindrical filament of elevated temperature. The hot cylindrical ring has an axial extent that is about one-thousand times larger than its thickness, and the peak temperature can be ten times larger than that of the surrounding plasma. The simultaneous positive and negative radial pressure gradients provide an ideal platform for the investigation of transport phenomena of contemporary interest, including avalanches [Van Compernolle et al., Phys. Rev. E 91, 031102 (2015)] and nonlocal transport. The present study delineates both the parameter regimes achievable by classical transport and the linear stability of the self-consistent profiles, including temperature and density gradients. An avalanche model is developed based on the self-consistent evolution of drift-wave eigenfunctions in nonlinearly modified profiles of electron temperature and plasma density. [ABSTRACT FROM AUTHOR]

In 1991 a manuscript describing an instrument for studying magnetized plasmas was published in this journal. The Large Plasma Device (LAPD) was upgraded in 2001 and has become a national user facility for the study of basic plasma physics. The upgrade as well as diagnostics introduced since then has significantly changed the capabilities of the device. All references to the machine still quote the original RSI paper, which at this time is not appropriate. In this work, the properties of the updated LAPD are presented. The strategy of the machine construction, the available diagnostics, the parameters available for experiments, as well as illustrations of several experiments are presented here. [ABSTRACT FROM AUTHOR]

An electromagnetic, 3D gyrokinetic particle code is used to study the relaxation of a magnetized electron temperature filament embedded in a large, uniform plasma of lower temperature. The study provides insight into the role played by unstable drift-Alfvén waves observed in a basic electron heat transport experiment [D. C. Pace et al., Phys. Plasmas 15, 122304 (2008)] in which anomalous cross-field transport has been documented. The simulation exhibits the early growth of temperature-gradient-driven, drift-Alfvén fluctuations that closely match the eigenmodes predicted by linear theory. At the onset of saturation, the unstable fluctuations display a spiral spatial pattern, similar to that observed in the laboratory, which causes the rearrangement of the temperature profile. After saturation of the linear instability, the system exhibits a markedly different behavior depending on the inclusion in the computation of modes without variation along the magnetic field, i.e., kz=0. In their absence, the initial filament evolves into a broadened temperature profile, self-consistent with undamped, finite amplitude drift-Alfvén waves. But the inclusion of kz=0 modes causes the destruction of the filament and damping of the drift-Alfvén modes leading to a final state consisting of undamped convective cells and multiple, smaller-scale filaments. [ABSTRACT FROM AUTHOR]

An investigation is made of a shear Alfven wave resonator for burning plasma conditions expected in the ITER device. For small perpendicular scale-lengths the shear mode, which propagates predominantly along the magnetic field direction, experiences a parallel reflection where the wave frequency matches the local ion-ion hybrid frequency. In a tokamak device operating with a deuterium-tritium fuel, this effect can form a natural resonator because of the variation in local field strength along a field line. The relevant kinetic dispersion relation is examined to determine the relative importance of Landau and cyclotron damping over the possible resonator parameter space. A WKB model based on the kinetic dispersion relation is used to determine the eigenfrequencies and the quality factors of modes trapped in the resonator. The lowest frequency found has a value slightly larger than the ion-ion hybrid frequency at the outboard side of a given flux surface. The possibility that the resonator modes can be driven unstable by energetic alpha particles is considered. It is found that within a bandwidth of roughly 600 kHz above the ion-ion hybrid frequency on the outboard side of the flux surface, the shear modes can experience significant spatial amplification. An assessment is made of the form of an approximate global eigenmode that possesses the features of a resonator. It is identified that magnetic field shear combined with large ion temperature can cause coupling to an ion-Bernstein wave, which can limit the instability. [ABSTRACT FROM AUTHOR]

A radial fractional transport model [Kullberg et al., Phys. Rev. E 87, 052115 (2013)], that correctly incorporates the geometric effects of the domain near the origin and removes the singular behavior at the outer boundary, is compared to results of off-axis heating experiments performed in the Rijnhuizen Tokamak Project (RTP), ASDEX Upgrade, JET, and DIII-D tokamak devices. This comparative study provides an initial assessment of the presence of fractional transport phenomena in magnetic confinement experiments. It is found that the nonlocal radial model is robust in describing the steady-state temperature profiles from RTP, but for the propagation of heat waves in ASDEX Upgrade, JET, and DIII-D the model is not clearly superior to predictions based on Fick's law. However, this comparative study does indicate that the order of the fractional derivative, a, is likely a function of radial position in the devices surveyed. [ABSTRACT FROM AUTHOR]

Ray tracing calculations are performed for shear Alfvén waves in two-ion species plasmas in which the magnetic field varies with position. Three different magnetic topologies of contemporary interest are explored: a linear magnetic mirror, a pure toroidal field, and a tokamak field. The wave frequency is chosen to lie in the upper propagation band, so that reflection at the ion-ion hybrid frequency can occur for waves originally propagating along the magnetic field direction. Calculations are performed for a magnetic well configuration used in recent experiments [S. T. Vincena et al., Geophys. Res. Lett. 38, L11101 (2011) and S. T. Vincena et al., Phys. Plasmas 20, 012111 (2013)] in the Large Plasma Device (LAPD) related to the ion-ion hybrid resonator. It is found that radial spreading cannot explain the relatively low values of the resonator quality factor (Q) measured in those experiments, even when finite ion temperature is considered. This identifies that a damping mechanism is present that is at least an order of magnitude larger than dissipation due to radial energy loss. Calculations are also performed for a magnetic field with pure toroidal geometry, without a poloidal field, as in experiments being planned for the Enormous Toroidal Plasma Device. In this case, the effects of field-line curvature cause radial reflections. A poloidal field is included to explore a tokamak geometry with plasma parameters expected in ITER. When ion temperature is ignored, it is found that the ion-ion hybrid resonator can exist and trap waves for multiples bounces. The effects of finite ion temperature combine with field line curvature to cause the reflection point to move towards the tritium cyclotron frequency when electron temperature is negligible. However, for ITER parameters, it is shown that the electrons must be treated in the adiabatic limit to properly describe resonator phenomena. [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]

An experimental investigation of the low-frequency density fluctuations in the plasma edge region of the TJ-K stellarator [N. Krause et al., Rev. Sci. Inst. 73, 3474 (2002)] finds that the ensemble-averaged frequency spectra exhibit a near exponential frequency dependence whose origin can be traced to individual pulses having a Lorentzian temporal shape. Similar features have been previously observed [D. C. Pace et al., Phys. Plasmas 15, 122304 (2008)] in a linear magnetized device under conditions in which cross-field pressure gradients are present. The reported observation of such features within the turbulent environment of a toroidal confinement device provides support for the conjecture that the underlying processes are a general feature of pressure gradients. Also presented is the magnetic field strength dependence of the pulse widths and the waiting time distribution between pulses. [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]

The results of a theoretical modeling study and experimental investigation of the propagation properties of shear Alfvén waves of small transverse scale in a plasma with two ion species are reported. In the two ion plasma, depending on the mass of the heavier species, ion kinetic effects can become prominent, and significant parallel electric fields result in electron acceleration. The theory predicts the appearance of frequency propagation gaps at the ion-ion hybrid frequency and between harmonics of the lower cyclotron frequency. Within these frequency bands spatial structures arise that mix the cone-propagation characteristics of Alfvén waves with radially expanding ion Bernstein modes. The experiments, performed at the Basic Plasma Science Facility (BaPSF) at UCLA, consist of the spatial mapping of shear waves launched by a loop antenna. Although a variety of two ion-species combinations were explored, only results from a helium-neon mix are reported. A clear signature of a shear wave propagation gap, as well as propagation between multiple harmonics, is found for this gas combination. The evanescence of shear waves beyond the reflection point at the ion-ion hybrid frequency in the presence of an axial magnetic field gradient is also documented. [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]

Two different experiments involving pressure gradients across the confinement magnetic field in a large plasma column are found to exhibit a broadband turbulence that displays an exponential frequency spectrum for frequencies below the ion cyclotron frequency. The exponential feature has been traced to the presence of solitary pulses having a Lorentzian temporal signature. These pulses arise from nonlinear interactions of drift-Alfvén waves driven by the pressure gradients. In both experiments the width of the pulses is narrowly distributed resulting in exponential spectra with a single characteristic time scale. The temporal width of the pulses is measured to be a fraction of a period of the drift-Alfvén waves. The experiments are performed in the Large Plasma Device (LAPD-U) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] operated by the Basic Plasma Science Facility at the University of California, Los Angeles. One experiment involves a controlled, pure electron temperature gradient associated with a microscopic (6 mm gradient length) hot electron temperature filament created by the injection a small electron beam embedded in the center of a large, cold magnetized plasma. The other experiment is a macroscopic (3.5 cm gradient length) limiter-edge experiment in which a density gradient is established by inserting a metallic plate at the edge of the nominal plasma column of the LAPD-U. The temperature filament experiment permits a detailed study of the transition from coherent to turbulent behavior and the concomitant change from classical to anomalous transport. In the limiter experiment the turbulence sampled is always fully developed. The similarity of the results in the two experiments strongly suggests a universal feature of pressure-gradient driven turbulence in magnetized plasmas that results in nondiffusive cross-field transport. This may explain previous observations in helical confinement devices, research tokamaks, and arc plasmas. [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]

An exploratory investigation, both computational and experimental, is made of the diagnostic possibilities of a fast wave-based method for measuring the ion density and temperature profiles of tokamak plasmas. The concept consists of simultaneously launching several discrete frequencies and detecting their amplitude and phase at various toroidal locations. This study focuses on an array of frequencies that match the second harmonic of the ion gyrofrequency at various radial locations in the plasma profile. The method is explored with simple, wave-propagation codes for parameters corresponding to the Electric Tokamak (ET) [R. J. Taylor, et al., Nucl. Fusion 42, 46 (2002)] and ITER [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)]. An experimental investigation of the concept has been performed in a large tokamak (ET) in which six frequencies are launched and detected at four toroidal locations. Positive results obtained indicate that the concept and other variants deserve to be pursued in greater depth, using more sophisticated codes. [ABSTRACT FROM AUTHOR]

A simulation and modeling study is made of the nonlinear interaction of an electromagnetic pulse, in the O-mode polarization, with a magnetized plasma having a cross-field density gradient. For small amplitudes, the pulse propagates up to the cutoff layer where an Airy pattern develops. Beyond a certain power level, the ponderomotive force produced by the standing electromagnetic fields carves density cavities. The excess density piled up on the side of the cavities causes secondary, field-aligned plasma resonances to arise. Strong electron acceleration occurs due to the short scale of the secondary resonant fields. The fast electrons exiting the new resonant layers induce a return current system in the background plasma. This generates a packet of shear Alfvén waves of small transverse scale and increasing frequency. The results provide insight into microscopic processes associated with a recent laboratory investigation in which large-amplitude Alfvén waves have been generated upon application of high-power microwaves [B. Van Compernolle et al., Phys. Plasmas 13, 092112 (2006)]. [ABSTRACT FROM AUTHOR]

A mathematical model describing the operation of an Alfvén wave maser in a laboratory environment is constructed from a continuous differential equation. The model incorporates the essential features of maser operation, namely, a resonator with a semitransparent boundary, a unidirectional amplification region, and a nonuniform magnetic field that provides frequency filtering. The spectral features and the parameter scaling predicted by the model are in agreement with laboratory measurements of an Alfvén wave maser [J. E. Maggs and G. J. Morales, Phys. Rev Lett. 91, 035004 (2003)]. The model provides a useful tool to explore a variety of operational scenarios for users of this novel wave source, including studies of Alfvénic interactions of relevance to space and fusion plasmas. [ABSTRACT FROM AUTHOR]

A frequency selective Alfvén wave resonator results from the application of a locally nonuniform magnetic field to a plasma source region between the cathode and anode in a large laboratory device. When a threshold in the plasma discharge current is exceeded, selective amplification produces a highly coherent (δω/ω<5×10-3), large amplitude shear Alfvén wave that propagates out of the resonator, through a semitransparent mesh anode, into the adjacent plasma column where the magnetic field is uniform. This phenomenon is similar to that encountered in the operation of masers/lasers at microwave and optical frequencies. The current threshold for maser action is found to depend upon the confinement magnetic field strength B0. Its scaling is consistent with the condition for matching the drift speed of the bulk plasma electrons with the phase velocity of the mode in the resonator. The largest spontaneously amplified signals are obtained at low B0 and large plasma currents. The magnetic fluctuations δB associated with the Alfvén maser can be as large as δB/B0≈1.5% and are observed to affect the plasma current. Steady-state behavior leading to coherent signals lasting until the discharge is terminated can be achieved when the growth conditions are well-above threshold. The maser is observed to evolve in time from an initial m=0 mode to an m=1 mode structure in the transition to the late steady state. The laboratory phenomenon reported is analogous to the Alfvén wave maser proposed to exist in naturally occurring, near-earth plasmas. [ABSTRACT FROM AUTHOR]

A particle-in-cell (PIC) code has been developed that is capable of describing the propagation of compressional and shear Alfvén waves excited from a boundary. The code is used to elucidate the properties of Alfvén wave cones radiated from sources having transverse scale comparable to the electron skin depth. Good agreement between theoretical predictions and simulation results is found over a wide range of frequencies. An investigation has been undertaken of the effect of hot ions on the Alfvén wave cones. The PIC simulations demonstrate that as the ion temperature is increased there is a reversal in the cone angle. The reversal implies that there is a cross-field focusing of the shear Alfvén waves. This is a feature which is presently being considered in studies of field-line resonances in the earth’s magnetic field. The PIC results also illustrate the damping of shear modes due to the Doppler-shifted cyclotron resonance with hot ions. [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 examines the effect of cross-field flow on a microscopic current channel. To illustrate the subtle interplay between finite electron inertia and flows, a simple model of excitation of a microscopic current channel is considered. The model consists of a slab antenna exciter with a transverse width on the order of the electron skin-depth driven at a single frequency. The antenna is fixed in the laboratory frame and embedded within a plasma that has a uniform drift across the confining magnetic field. The combined effects of the plasma flow and the intrinsic, collisionless, cross-field expansion of the current channel lead to standing wave structures across the confining magnetic field. The resulting parallel electric fields generate an array of current filaments of alternating polarity which individually have transverse width smaller than the original channel. These results may help interpret laboratory and spacecraft measurements of Alfvénic turbulence and could lead to the development of a diagnostic tool to map plasma flows. © 2001 American Institute of Physics. [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]

A numerical study is made of the effects caused by incoherent fluctuations in the minority density concentration on the absorption of fast Alfvén waves at the ion–ion hybrid resonance. This work extends to the incoherent regime of a previous study by Romero and Morales [Phys. Fluids B 2, 2007 (1990)], which surveyed in detail the effects of single-mode (coherent) fluctuations. The system is represented by a Budden equation that properly describes the resonance and cutoff structure arising from spatial variations of the equilibrium magnetic field. For wave incidence such that the cutoff is encountered before the resonance layer, the absorption coefficient increases monotonically as the fluctuation amplitude is increased. A specific case is presented in which the absorption changes from ∼0% to ∼70% for minority density fluctuations δn/n0∼60% that correspond to electron density variations of less than 5%. For wave incidence such that the resonance is encountered before the cutoff, the reflection coefficient increases for minority density fluctuations below 5% of the ambient value. For larger density fluctuations, the reflected energy saturates at ∼20% of the incident wave energy. [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]

An analytic and numerical study is made of the effects caused by coherent density fluctuations, localized near the ion–ion hybrid resonance, on the propagation and absorption of fast Alfvén waves. The system is represented by a Budden equation that incorporates static fluctuations in the ion density profiles. The unperturbed system contains a resonance at x=0 and a cutoff at x=d; the Alfvén wave number at resonance is kA. Two cases are considered: fluctuations in the ion concentration ratio η and bulk density fluctuations. When the incident wave directly approaches the resonance, the fluctuation in the concentration ratio δη/η0 is less than 20%, and kAd<1, a ‘‘renormalized’’ Budden potential yields analytic expressions in good agreement with numerical results. In this regime, the reflection coefficient exhibits a new contribution arising from the modifications of the resonance layer. For direct approach, kAd>1, and concentration fluctuations 2%<δη/η0<5%, enhanced reflection leads to decreased absorption. For these parameters, the net absorption is reduced by more than 10% for noise wave numbers larger than 10 kA. However, the efficiency of the resonant absorption process remains unaffected. For kAd>2 and fluctuations δη/η0>5%, a regime of multiple resonances is encountered in which substantial absorption results. For indirect approach, a similar process takes place: The reflection coefficient is reduced from its unperturbed ∼1 value to ∼0. For bulk fluctuations, backscattering is the dominant physical process. [ABSTRACT FROM AUTHOR]

It is shown that second-order (ρ/L) corrections in the expression for the equilibrium distribution function are required to obtain a self-adjoint system of equations for ion cyclotron range of frequencies (ICRF) wave propagation in a nonuniform plasma (ρ denotes the ion-Larmor radius and L the gradient scale length). Self-adjointness results in energy conservation in the absence of absorption mechanisms, as expected. A Lagrangian density is obtained, which yields both the wave operator and its adjoint, and directly results in the conservation law of the truncated system. [ABSTRACT FROM AUTHOR]

An analytic study is made of the second-order modifications produced on the fast-tail electron distribution function of a nonuniform plasma subjected to resonant excitation by wave sources. The source models considered can represent excitation by external electromagnetic waves propagating obliquely to the plasma density gradient, mode conversion of electrostatic whistlers, beat of two transparent electromagnetic waves, and direct conversion from ripples in the density profile. The calculation treats the Landau damping provided by fast-tail electrons self-consistently and is applicable to plasmas having a long density scale length L, i.e., (kDL)1/3 >1, where kD is the Debye wavenumber of the warm background electrons. A threshold condition is found for the formation of a positive slope in the tail distribution by the various excitation mechanisms. [ABSTRACT FROM AUTHOR]

A nonlinear theory of wave resonances in a two-dimensional ion layer confined under the surface of liquid helium is presented. The ion layer is modeled as a two-dimensional cold plasma fluid. In addition to the usual nonlinearities present in the continuity equation and the equation of motion, the theory considers a nonlinear dependence of the mass of a plasma particle on its velocity, as suggested by indirect experimental evidence. Secular perturbation theory is used to find the plasma response when the damped, nonlinear system is driven externally. For typical experimental parameters, the mass nonlinearity is found to be the dominant nonlinear effect, giving rise to a backbending of the resonance curve. [ABSTRACT FROM AUTHOR]