Volume 6, Issue 12, December 1999
 LETTERS


Demonstration of a 10 kW average power 94 GHz gyroklystron amplifier
View Description Hide DescriptionThe experimental demonstration of a high average power Wband (75–110 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 118 AW peak output power and 29.5% electronic efficiency in the mode using a 66.7 kV, 6 A electron beam at 0.2% rf duty factor. At this operating point, the instantaneous full width at halfmaximum (FWHM) bandwidth is 600 MHz. At 11% rf duty factor, the gyroklystron has produced up to 10.1 kW average power at 33% electronic efficiency with a 66 kV, 4.15 A electron beam. This represents world record performance for an amplifier at this frequency. At the 10.1 kW average power operating point, the FWHM bandwidth is 420 MHz. At higher magnetic fields and lower beam voltages, larger bandwidths can be achieved at the expense of peak and average output power.

Generalized action invariants for drift waveszonal flow systems
View Description Hide DescriptionGeneralized action invariants are identified for various models of drift waveturbulence in the presence of the mean shear flow. It is shown that the wave kinetic equation describing the interaction of the small scale turbulence and large scale shear flow can be naturally writen in terms of these invariants. Unlike the wave energy, which is conserved as a sum of small and largescale components, the generalized action invariant is shown to correspond to a quantity which is conserved for the small scale component alone. This invariant can be used to construct canonical variables leading to a different definition of the wave action (as compared to the case without shear flow). It is suggested that these new canonical action variables form a natural basis for the description of the drift waveturbulence with a mean shear flow.

Dust–Coulomb waves in dense dusty plasmas
View Description Hide DescriptionDusty plasmas can be considered as tenuous, dilute or dense when the dust fugacity parameter satisfies ∼1, or where and R denote, respectively, the dust number density, the plasma Debye length and the dust grain size (radius), and is the dust plasma parameter. Dense dusty plasmas are shown to support a new kind of ultra lowfrequency electrostatic dust mode which may be called the “Dust–Coulomb Wave” (DCW). In contrast to the dust–acoustic wave(DAW) and the dust–lattice wave (DLW) which exist even for constant grain charge, DCWs are accompanied by dust charge as well as number density perturbations which are proportional to each other. For frequencies much smaller than the grain charging frequency, DCWs propagate as normal modes with the phase speed where is the charge (mass) of the dust grains. In the long wavelength limit, the DCW phase speed is much smaller than that of DAW and scales as Thus, for a given wave number, the frequency regime for the existence of DCW is much lower than the DAW regime. A comparison between the three types of dust–modes (DCWs, DAWs, and DLWs) has been carried out.
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 AMERICAN PHYSICAL SOCIETY CENTENNIAL PAPERS


Tests of causality: Experimental evidence that sheared flow alters turbulence and transport in tokamaks
View Description Hide DescriptionA prime goal in physics research is the development of theories which have the universality needed to explain a wide range of observations. Developed over the past decade, the model of turbulence decorrelation and stabilization by sheared flow has the universality needed to explain the turbulence reduction and confinement improvement seen in the edge and core of a wide range of magnetic confinement devices. Because the shear, turbulence, and transport are all intimately intertwined in multiple feedback loops, devising experiments to test whether shear causes a change in turbulence and transport has been a major challenge for experimentalists. Over the past five years, there have been at least four clear demonstrations of causality performed in tokamak plasmas, both at the plasma edge on Doublet IIID (DIIID) [Plasma Physics and Controlled Fusion Research 1985 (International Atomic Energy Agency, Vienna, 1986) Vol. I, p.159] and Tokamak Experiment for Technologically Oriented Research (TEXTOR) [Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991) Vol. I, p. 473] and further into the plasma core on DIIID and TokamakFusion Test Reactor [Phys. Plasmas 5, 1577 (1998)]. This paper discusses these tests in detail; the results agree with the expectations from the shear model. This paper also discusses similarities between flow shear effects in plasmas and in neutral fluids and provides examples of flow shear reduction of turbulence in neutral fluids under the proper conditions.

Role of computer modeling of plasmas in the 21st century
View Description Hide DescriptionFor much of their history plasmas have been characterized by complex unpredictable behavior. This stemmed from their general nonlinear turbulent behavior, from the difficulties of carrying out controlled experiments, and from the limitations of theory. Recently this situation has changed quite dramatically with the phenomenal growth in the capabilities of computer modeling. Contact between predictions and experiments have been made over a broad range of problems. In the 21st century the power of modeling will continue to grow; the techniques for using the tool will grow and our ability to understand the complex results will improve. In this endeavor the most critical factor is the human factor. Humans must create the models; they must make sense of the results; they must condense the results to a simplified form that is useful to others. Given the importance of plasmas to human activities and in the universe, these advances point to important developments.

Transport in accretion disks
View Description Hide DescriptionAstrophysical accretiondisks are powered by the release of gravitational potential energy as gas spirals down onto a compact star or black hole. The dynamics and evolution of accretiondisks depend upon how angular momentum is transported outward from one fluid element to another. The nature of this process was unclear for many years. Since the early 1990s, however, considerable progress has been made in understanding how turbulence arises and transportsangular momentum in astrophysical accretiondisks.Accretiondisks are generally highly conducting plasmas; the equations governing their evolution are those of ideal magnetohydrodynamics. Although a hydrodynamical disk would be locally stable, the combination of a weak subthermal magnetic field and outwardly decreasing differential rotation rapidly generates magnetohydrodynamical turbulence via a remarkably simple linear instability. Thus, turbulentaccretiondisks are fundamentally magnetohydrodynamical in nature.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Laboratory studies of magnetic vortices. I. Directional radiation of whistler waves based on helicity injection
View Description Hide DescriptionA novel principle for the directional excitation of whistler waves is demonstrated in a laboratory experiment. It is based on helicity conservation of electron magnetohydrodynamic fields in plasmas. Whistler wave packets propagating in opposite directions to a static magnetic field have opposite signs of helicity. Injection of helicity of one sign produces radiation in one direction. This is accomplished with an antenna consisting of a loop linked through a torus. Directionality of 20 dB is readily achieved. The direction of radiation is electronically reversible. Transmission between two antennas is unidirectional, hence nonreciprocal. Possible applications include secure communication, direction finding, and efficient power deposition in radio frequency(rf)heating.

Laboratory studies of magnetic vortices. II. Helicity reversal during reflection of a magnetic vortex at a conducting boundary
View Description Hide DescriptionThe reflection of a magnetic vortex from a conducting boundary is studied experimentally in a large laboratory plasma. The parameter regime is that of electron magnetohydrodynamics and the vortex consists of a spheromaklike magnetic field perturbation propagating in the whistler mode along a uniform background magnetic field. In this work we focus on the helicity properties of the vortexmagnetic field, electron velocity, and vorticity. The reflection conserves magnetic energy but reverses the sign of all helicities. The change in topology arises from a selfconsistent reversal of one linked vector field without involving helicity injection, reconnection, or dissipation processes. The breakdown of helicity conservation and the frozenin concept is explained by the presence of a vacuumlike sheath at the plasma–boundary interface.

Quantum drift waves
View Description Hide DescriptionThe results of theoreticalanalysis of the oscillation spectrum of a plasma placed in a very strong quantizing magnetic field are presented. It is shown that in ultraquantum limit, when all particles occupy the first Landau level, in an inhomogeneous quantized twocomponent plasma, volume quantum drift waves arise. These waves become unstable under some circumstances.

Uniformdensity, spherical electron focus
View Description Hide DescriptionAn equilibrium electron distribution is exhibited which forms a uniform electron density focus within a spherical system. Such a focus may be used to form a spherical, harmonic well for ion focusing as previously discussed. A selfconsistent density and spacecharge potential are calculated and the optimum focus radius is determined. Nonideal effects on electron and ion motion in the resulting electrostatic well are briefly discussed and strategies for their minimization are derived.

Flow driven resistive wall instability
View Description Hide DescriptionThe stability of a perfectly conducting fluid layer flowing along a magnetic field, parallel to a finitely conducting thin wall is examined. Finite layer width and compressibility of the fluid are shown to significantly lower the flow velocity required for instability to set in. The effect of axial flow on the stability of a cylindrical pinch surrounded by a resistive wall is examined. Flow is shown to have a destabilizing effect.

Covariant descriptions of the relativistic guidingcenter dynamics
View Description Hide DescriptionThe relativistic guiding center dynamics of charged particles is described in terms of noncanonical variables. The gyrokinetic transformation is obtained using the perturbative Lagrangian approach with a fully relativistic, fourdimensional covariant formulation. It is shown that the definition of the ignorable gyrophase (as well as those of the magnetic moment and the gyrocenter energy) is not unique, and allows for several free functions in the gyrokinetic transformation. This freedom can be interpreted as a choice of the reference frame. One of these frames, namely that moving with the relativistic version of the drift velocity, generates the simplest and intuitive description.

Autoresonant (nonstationary) excitation of a collective nonlinear mode
View Description Hide DescriptionThe autoresonant (nonlinear phase locking) manipulation of the diocotron mode in a nonneutral plasma is investigated. Autoresonance is a very general phenomenon in driven nonlinear oscillator and wave systems. By sweeping or chirping the drive frequency, autoresonance allows the amplitude of a nonlinear wave to be controlled without the use of feedback. The experimental results, including a novel scaling relation, are in excellent agreement with a simple theoretical model. These are the first controlled laboratory studies of autoresonance in a collective plasma system.

Collisional deltaf scheme with evolving background for transport time scale simulations
View Description Hide DescriptionThe approach is extended for simulating the transport timescale evolution of nearMaxwellian distributions in collisional plasmas. This involves simultaneously advancing weighted marker particles for representing the intrinsically kinetic component and fluid equations for the parameters of the shifted Maxwellian background The issue of increasing numerical noise in a collisional algorithm, due to marker particle weight spreading, is addressed in detail, and a solution to this problem is proposed. To obtain higher resolution in critical regions of phase space, a practical procedure for implementing sources and sinks of marker particles is developed. As a proof of principal, this set of methods is applied for computing electrical Spitzer conductivity as well as collisional absorption in a homogeneous plasma.

A selfconsistent analysis of a collisional presheath
View Description Hide DescriptionA onedimensional plasma discharge is analyzed under steady state conditions. Using simple models for source and collisions, a firstorder differential equation is obtained that simultaneously describes both the bulk and the presheath of the plasma. This equation is numerically solved in various regimes and physically interesting quantities such as the ratio of bulk to edge density and the size of the inertial terms in the bulk region are presented. Analytic expressions are obtained for profiles when collision frequency is assumed constant. Findings include for highly collisional systems, significant flow in the bulk plasma, and modified characteristics.

Dielectric tensor for inhomogeneous plasmas in inhomogeneous magnetic field
View Description Hide DescriptionThe derivation of explicit expressions for the effective dielectrictensor to be utilized in the dispersion relation for weakly inhomogeneous plasmas is discussed. The general expressions obtained are useful for situations with simultaneous existence of weak inhomogeneities in density and magnetic field. The particular case of a Maxwellian distribution in velocity space for the electron population is discussed, and relatively compact expressions for the dielectrictensor are obtained, which depend on the inhomogeneous plasma dispersion function introduced by [Gaelzer et al., Phys. Rev. E 55, 5859 (1997)] and ultimately on the wellknown Fried–Conte function and its derivatives.
 Nonlinear Phenomena, Turbulence, Transport

Bäcklund transformations, a simple transformation and exact solutions for dustacoustic solitary waves in dusty plasma consisting of cold dust particles and twotemperature isothermal ions
View Description Hide DescriptionDusty plasma with inertial dust fluid and twotemperature ions admits both compressive and rarefactive solitary waves. The Kortewegde Vries equations (KdVtype equations) with cubic nonlinearity at the critical density of lowtemperature isothermal ions are considered to discuss properties of dustacoustic solitary waves. In the vicinity of the critical density of lowtemperature ions, a KdVtype equation with mixed nonlinearity is discussed. The method of characteristics is used and the Bäcklund transformations (BTs) are employed to generate new solutions from the old ones. Another new solution of the KdV–mKdV equation is obtained using a simple transformation between the sineGordon equation and a linear equation combined with an extension of the tanh method of Malfliet.

Nonlinear relativistic gyrokinetic VlasovMaxwell equations
View Description Hide DescriptionA set of selfconsistent nonlinear gyrokineticequations is derived for relativistic charged particles in a general nonuniform magnetized plasma. Full electromagneticfield fluctuations are considered with spatial and temporal scales given by the lowfrequency gyrokinetic ordering. Selfconsistency is obtained by combining the nonlinear relativistic gyrokinetic Vlasov equation with the lowfrequency Maxwell equations in which charge densities and current densities are expressed in terms of moments of the gyrokinetic Vlasov distribution. For these selfconsistent gyrokineticequations, a lowfrequency energy conservation law is also derived.

Evidence for secondorder oscillations at the Best frequency in direct numerical simulations of the Vlasov equation
View Description Hide DescriptionQuantitative evidence for secondorder oscillations occurring at the frequency predicted by Best [Physica 74, 183 (1974)] and by Sedláček and Nocera [J. Plasma Phys. 48, 367 (1992)] is provided by direct, ab initio numerical simulations of the Vlasov equation. These oscillations are relevant both for their intrinsic connection to the more customary plasma echo (also retrieved), for their diagnostic applications to the study of turbulence, and for the high accuracy needed to reproduce them. This latter fact is used as a sensitive test for the numerical integration, which indeed reproduces both known and new features of the oscillations, including nonlinear Landau damping, particle trapping, and the oscillations’ frequencies, in excellent agreement with theory.

Threedimensional particle simulation of plasma instabilities and collisionless reconnection in a current sheet
View Description Hide DescriptionGeneration of anomalous resistivity and dynamical development of collisionless reconnection in the vicinity of a magnetically neutral sheet are investigated by means of a threedimensional particle simulation. For no external driving source, two different types of plasma instabilities are excited in the current layer. The lower hybrid drift instability (LHDI) is observed to grow in the periphery of current layer in an early period, while a drift kink instability (DKI) is triggered at the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LHDI. A reconnectionelectric field grows at the neutral sheet in accordance with the excitation of the DKI. When an external driving field exists, the convective electric field penetrates into the current layer through the particle kinetic effect and collisionless reconnection is triggered by the convective electric field earlier than the DKI is excited. It is also found that the anisotropic ion distribution is formed through the anomalous ion heating by the DKI.