Volume 18, Issue 3, March 2011

We study the lasermatter interaction via opticalfield ionization of a medium in a focused ultrashort laser pulse by means of the finitedifferencetimedomain modeling of Maxwell’s equations. General aspects of the ionizationinduced dynamics with TE and TMpolarized laser pulses are analyzed. It is shown that there are two qualitatively different regimes of the interaction depending on the angle of the laser beam focusing. At comparatively low angles plasma distributions are smooth; however, due to departure from the quasioptical behavior of light rays in selfgenerated plasma, lateral largescaled plasma structures can also be produced, leading to additional beam focusing and correspondingly to higher electron densities. At tight focusing, smallscaled plasma structures are generated that strongly influence field distribution, energy deposition, and scattering characteristics. The influence of electron collisions and the Kerr effect are also analyzed.
 REVIEW ARTICLE


Realizing steadystate tokamak operation for fusion energy
View Description Hide DescriptionContinuous operation of a tokamak for fusion energy has clear engineering advantages but requires conditions beyond those sufficient for a burning plasma. The fusion reactions and external sources must support both the pressure and the current equilibrium without inductive current drive, leading to demands on stability, confinement, current drive, and plasmawall interactions that exceed those for pulsed tokamaks. These conditions have been met individually, and significant progress has been made in the past decade to realize scenarios where the required conditions are obtained simultaneously. Tokamaks are operated routinely without disruptions near pressure limits, as needed for steadystate operation. Fully noninductive sustainment with more than half of the current from intrinsic currents has been obtained for a resistive time with normalized pressure and confinement approaching those needed for steadystate conditions. One remaining challenge is handling the heat and particle fluxes expected in a steadystate tokamak without compromising the core plasma performance.
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 LETTERS


Effects of hyperbolic rotation in Minkowski space on the modeling of plasma accelerators in a Lorentz boosted frame
View Description Hide DescriptionThe effects of hyperbolic rotation in Minkowski space resulting from the use of Lorentz boosted frames of calculation on laser propagation in plasmas are analyzed. Selection of a boost frame at the laser group velocity is shown to alter the laser spectrum, allowing the use of higher boost velocities. The technique is applied to simulations of laser driven plasma wakefield accelerators, which promise much smaller machines and whose development requires detailed simulations that challenge or exceed current capabilities. Speedups approaching the theoretical optima are demonstrated, producing the first direct simulations of stages up to 1 TeV. This is made possible by a million times speedup thanks to a frame boost with a relativistic factor as high as 1300, taking advantage of the rotation to mitigate an instability that limited previous work.

Heating of ions by lowfrequency Alfvén waves in partially ionized plasmas
View Description Hide DescriptionIn the solar atmosphere, the chromospheric and coronalplasmas are much hotter than the visible photosphere. The heating of the solar atmosphere, including the partially ionized chromosphere and corona, remains largely unknown. In this letter, we demonstrate that the ions can be substantially heated by Alfvén waves with very low frequencies in partially ionized lowbeta plasmas. This differs from other Alfvén wave related heating mechanisms such as ionneutral collisional damping of Alfvén waves and heating described by previous work on resonant Alfvén wave heating. We find that the nonresonant Alfvén wave heating is less efficient in partially ionized plasmas than when there are no ionneutral collisions, and the heating efficiency depends on the ratio of the ionneutral collision frequency to the ion gyrofrequency.

Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas
View Description Hide DescriptionThe effect of poloidal asymmetry of impurities on impurity transport driven by electrostaticturbulence in tokamakplasmas is analyzed. It is found that if the density of the impurity ions is poloidally asymmetric then the zeroflux impurity density gradient is significantly reduced and even a sign change in the impurity flux may occur if the asymmetry is sufficiently large. This effect is most effective in low shear plasmas with the impurity density peaking on the inboard side and may be a contributing factor to the observed outward convection of impurities in the presence of radio frequency heating.

Steady plasma channel formation and particle acceleration in an interaction of an ultraintense laser with nearcritical density plasma
View Description Hide DescriptionA welldefined nearsteady plasma channel is successfully formed in an interaction of an intense shortpulse laser with a nearcritical underdense plasma. The laser is well selffocused in the plasma channel. Our 2.5dimensional particleincell simulations also demonstrate a violent electron acceleration with a effective gradient of several tens of GeV/cm by a strong longitudinal chargeseparated field. At the same time, a strong ion acceleration appears at the rear plasma boundary. The results present a regime for the plasma channel formation and the particle acceleration by ultrashort laserplasma interaction.

Anomalous selfgenerated electrostatic fields in nanosecond laserplasma interaction
View Description Hide DescriptionElectrostatic (E) fields associated with the interaction of a wellcontrolled, highpower, nanosecond laser pulse with an underdense plasma are diagnosed by protonradiography. Using a current threedimensional wave propagation code equipped with nonlinear and nonlocal hydrodynamics, we can model the measured Efields that are driven by the laser ponderomotive force in the region where the laser undergoes filamentation. However, strong fields of up to 110 MV/m measured in the first millimeter of propagation cannot be reproduced in the simulations. This could point to the presence of unexpected strong thermal electron pressure gradients possibly linked to ion acoustic turbulence, thus emphasizing the need for the development of full kinetic collisional simulations in order to properly model laserplasma interaction in these strongly nonlinear conditions.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Stabilizing effect of a nonresonant radio frequency drive on the diocotron instability
View Description Hide DescriptionIt has been experimentally shown that the rotation radius of a nonneutral plasma column around the longitudinal axis of a Malmberg–Penning trap experiences a growth in amplitude (diocotron instability), leading to the loss of the plasma on the surface of the confining electrodes. A new stabilization mechanism has been investigated with the help of systematic experiments in the ELTRAP (ELectron TRAP) device where a highfrequency, lowamplitude drive has been applied on an azimuthally sectored electrode. An effective confining force is created, which reduces the offset of the column from the center. This interpretation and its theoretical analysis show a qualitative agreement with the experimental findings, where a net confinement effect is present for a wide range of drive amplitudes and frequencies.

Numerical study on rectangular microhollow cathode discharge
View Description Hide DescriptionRectangular microhollow cathodedischarge in argon is investigated by using twodimensional timedependent selfconsistent fluid model. The electric potential, electric field, particle density, and mean electron energy are calculated. The results show that hollow cathode effect can be onset in the present configuration, with strong electric field and high mean electron energy in the cathode fall while high density and quasineutral plasma in the negative glow. The potential well and electric filed reversal are formed in the negative glow region. It is suggested that the presence of large electron diffusion flux necessitates the field reversal and potential well.

Generalized matching criterion for electrostatic ion solitary propagations in quasineutral magnetized plasmas
View Description Hide DescriptionBased on the magnetohydrodynamics model, an exact arbitraryamplitude general solution is presented for oblique propagation of solitary excitations in two and threecomponent quasineutral magnetoplasmas, adopting the standard pseudopotential approach. It is revealed that the necessary matching criterion of existence of such oblique nonlinear propagations in two and threefluid magnetoplasmas possesses global features. These features are examined for the cases of electronion and electronpositronion magnetoplasmas with diverse equations of state. This study also reveals that for electronion magnetoplasmas with plasma frequencies larger than the cyclotron frequency a critical angle of exists at which propagation of solitary excitation is not possible. The Coriolis effect on allowed soliton matching condition in rotating magnetoplasmas is also considered as an extension to this work. Current investigation can have important implications for nonlinear wavedynamics in astrophysical as well as laboratory magnetoplasmas.

Transverse instability and magnetic structures associated with electron phase space holes
View Description Hide DescriptionElectron phase space holes (electron holes) are found to be unstable to the transverse instability. Twodimensional (2D) electromagnetic particleincell simulations are performed to investigate the structures of the fluctuating magnetic field associated with electron holes. The combined actions between the transverse instability and the stabilization by the background magnetic field lead a onedimensional electron hole into several 2D electron holes which are isolated in both the and directions. The electrons trapped in these 2D electron holes suffer the electric field drift due to the existence of the perpendicular electric field, which generates the current along the direction. Then, the unipolar and bipolar structures are formed for the parallel cut of the fluctuating magnetic field along the and directions, respectively. At the same time, these 2D electron holes move along the direction, and the unipolar structures are formed for the parallel cut of the fluctuating magnetic field along the direction.

Multipactor theory for multicarrier signals
View Description Hide DescriptionThis work presents a new theory of multipactor under multicarrier signals for parallelplate geometries, assuming a homogeneous electric field and onedimensional electron motion. It is the generalization of the nonstationary multipactor theory for singlecarrier signals [S. Anza et al.,Phys. Plasmas17, 062110 (2010)]. It is valid for multicarrier signals with an arbitrary number of carriers with different amplitude, arbitrary frequency, and phase conditions and for any material coating. This new theory is able to model the real dynamics of the electrons during the multipactor discharge for both single and double surface interactions. Among other parameters of the discharge, it calculates the evolution in time of the charge growth, electron absorption, and creation rates as well as the instantaneous secondary emission yield and order. An extensive set of numerical tests with particleincell software has been carried out in order to validate the theory under many different conditions. This theoretical development constitutes the first multipactor theory which completely characterizes the multipactor discharge for arbitrary multicarrier signals, setting the first step for further investigations in the field.

Anisotropic dissipative effects on the buoyancy instability with background heat flux
View Description Hide DescriptionThe linear buoyancy instability in magnetized plasmas is investigated in the presence of anisotropicresistivity and viscosity by taking into account the background heat flux. The magnetic field is assumed to be homogeneous and has both horizontal and vertical components. The heat is primarily transported along the magnetic force lines when the gyro radius is much less than the mean collision free path. The Hall term is examined first and shows a damping effect on the magnetothermal instability. The heatfluxdriven buoyancy instability (HBI) is then investigated by taking into account the parallel resistivity (PR), crossfield resistivity (CR), and the anisotropicviscosity. The general dispersion relation (DR) is derived and discussed in several special cases. We show that only the CR and viscosity exert effects on the DR in the first case. The critical condition for the occurrence of HBI is modified by the CR coupled with the viscosity and the value of the instability growth rate is diminished by them. The effects due to the PR (resp. viscosity) on the HBI are examined next. The PR (resp. viscosity) is shown to alter not only the growth rate but also the instability criterion. There exists an unstable mode when the temperature decreases in the direction of gravity while this case is proven to be magnetothermally stable in the ideal magnetohydrodynamic limit. A new unstable mode is solely induced by the presence of PR (resp. viscosity). When the PR and CR are both taken into account, the resistivity is shown to induce a damping mode rather than an instability. Finally, considering the PR and viscosity simultaneously, it is found that a new unstable mode is excited when the PR is not equal to the viscosity, or else, dissipation effects do not alter the instability criterion and just cut down the growth rate.

Ionizationinduced dynamics of lasermatter interaction in a focused laser pulse: A comparative analysis
View Description Hide DescriptionWe study the lasermatter interaction via opticalfield ionization of a medium in a focused ultrashort laser pulse by means of the finitedifferencetimedomain modeling of Maxwell’s equations. General aspects of the ionizationinduced dynamics with TE and TMpolarized laser pulses are analyzed. It is shown that there are two qualitatively different regimes of the interaction depending on the angle of the laser beam focusing. At comparatively low angles plasma distributions are smooth; however, due to departure from the quasioptical behavior of light rays in selfgenerated plasma, lateral largescaled plasma structures can also be produced, leading to additional beam focusing and correspondingly to higher electron densities. At tight focusing, smallscaled plasma structures are generated that strongly influence field distribution, energy deposition, and scattering characteristics. The influence of electron collisions and the Kerr effect are also analyzed.

Rotation of a magnetized plasma
View Description Hide DescriptionThe plasma rotation in the axial magnetic field of the linear machine Mistral [A. Escarguel, Eur. Phys. J. D56, 209 (2010)] is well described by the assumption that the electrons injected from the source exit radially from the central column and are subject to the Lorentz force. Electrons and ions rotate together by ambipolarity. The solution of the momentum equations foresees correctly the observed radial dependence of the ionic radial velocitymeasured by laser induced fluorescence. The resolution of these equations is also in good agreement with the measured dependence of the rotation frequency on the applied magnetic field and on the background pressure.

Methodology for turbulence code validation: Quantification of simulationexperiment agreement and application to the TORPEX experiment
View Description Hide DescriptionA methodology for plasma turbulence code validation is discussed, focusing on quantitative assessment of the agreement between experiments and simulations. The present work extends the analysis carried out in a previous paper [P. Ricci et al., Phys. Plasmas16, 055703 (2009)] where the validation observables were introduced. Here, it is discussed how to quantify the agreement between experiments and simulations with respect to each observable, how to define a metric to evaluate this agreement globally, and—finally—how to assess the quality of a validation procedure. The methodology is then applied to the simulation of the basic plasma physicsexperimentTORPEX [A. Fasoli et al., Phys. Plasmas13, 055902 (2006)], considering both twodimensional and threedimensional simulation models.

Numerical simulation of laminar plasma dynamos in a cylindrical von Kármán flow
View Description Hide DescriptionThe results of a numerical study of the magnetic dynamo effect in cylindrical von Kármán plasma flow are presented with parameters relevant to the Madison Plasma Couette Experiment. This experiment is designed to investigate a broad class of phenomena in flowing plasmas. In a plasma, the magnetic Prandtl number can be of order unity (i.e., the fluidReynolds number is comparable to the magnetic Reynolds number). This is in contrast to liquid metal experiments, where is small (so, ) and the flows are always turbulent. We explore dynamo action through simulations using the extended magnetohydrodynamic NIMROD code for an isothermal and compressible plasma model. We also study twofluid effects in simulations by including the Hall term in Ohm’s law. We find that the counterrotating von Kármán flow results in sustained dynamo action and the selfgeneration of magnetic field when the magnetic Reynolds number exceeds a critical value. For the plasma parameters of the experiment, this field saturates at an amplitude corresponding to a new stable equilibrium (a laminar dynamo). We show that compressibility in the plasma results in an increase of the critical magnetic Reynolds number, while inclusion of the Hall term in Ohm’s law changes the amplitude of the saturated dynamo field but not the critical value for the onset of dynamo action.

Nonlinear collisionless plasma wakes of small particles
View Description Hide DescriptionThe wake behind a spherical particle smaller than the Debye length in flowing plasma is calculated using a particleincell code. The results with different magnitudes of charge reveal substantial nonlinear effects down to values that for a floating particle would correspond to a particle radius . The peak potential in the oscillatory wake structure is strongly suppressed by nonlinearity, never exceeding times the unperturbed ion energy. By contrast, the density peak arising from ion focusing can be many times the ambient. Strong heating of the ions occurs in the nonlinear regime. Direct ion absorption by the particle is not important for the far wake unless the radius exceeds , and is therefore never significant (for the far wake) in the linear regime. Reasonable agreement with fullscale linear response calculations are obtained in the linear regime. The wake wavelength is confirmed and an explanation, in terms of the conical potential structure, is proposed for experimentallyobserved oblique alignment of differentsized grains.

Gyrokinetic analysis of tearing instabilities in a collisionless plasma
View Description Hide DescriptionUsing a gyrokinetic description, an analytic investigation of tearing instabilities is carried out for a collisionless tokamak plasma, with particular emphasis on delineating the effects associated with Landau and resonances. The linear characteristics of driven tearing modes are studied by including short wavelength variations across the confining magnetic field and long wavelength variations along the field. For the case when electrons are adiabatic and ions are fluidlike, the dispersion relation is solved analytically for mode widths lying between electron and ion excursion lengths. It is shown that electron Landau damping effect can significantly influence the tearing mode growth rate by making it proportional to in contrast to earlier kinetic results, which show a linear dependence on . The growth rate can further slow down when compressional mode coupling effects are taken into account. Likewise, analytic conditions for the growth of the gyrokinetic tearing mode in the presence of electron resonance effect are obtained for both the driven global mode as well as the large branch of this instability and expressions for the real frequency and growth rate of the modes are given. Our analytic results, besides providing physical insights into the influence of these ‘resonance’ effects, can also serve as useful benchmark signatures to look for in large scale numerical gyrokinetic simulations.

Generation of shear Alfvén waves by a rotating magnetic field source: Threedimensional simulations
View Description Hide DescriptionThe paper discusses the generation of polarized shear Alfvén waves radiated from a rotating magnetic field source created via a phased orthogonal twoloop antenna. A semianalytical threedimensional cold twofluid magnetohydrodynamics model was developed and compared with recent experiments in the University of California, Los Angeles large plasma device. Comparison of the simulation results with the experimental measurements and the linear shear Alfvén wave properties, namely, spatiotemporal wave structure, a dispersion relation with nonzero transverse wave number, the magnitude of the wave dependences on the wave frequency, show good agreement. From the simulations it was found that the energy of the Alfvén wave generated by the rotating magnetic field source is distributed between the kinetic energy of ions and electrons and the electromagnetic energy of the wave as: is the energy of the electromagnetic field, is the kinetic energy of the ion fluid, and is the kinetic energy of electron fluid for the experiment. The wavemagnetic field power calculated from the experimental data and using a fluid model differ by and is for the experimental parameters. In both the experiment and the threedimensional twofluid magnetohydrodynamics simulations the rotating magnetic field source was found to be very efficient for generating shear Alfvén waves.

The particle distributions of asymmetric kinetic electrostatic structures
View Description Hide DescriptionWe give the energy distributions of electrons and ions supporting a steady state electrostatic structure in a collisionless plasma. The electric potential of the structure is skew asymmetrically distributed in space. We show that the jump discontinuous, logarithmically singular electron and ion distributions may be reduced to elliptic integrals. We give the coefficients of the logarithmic terms and the jumps at the discontinuities and we show that they are reciprocally proportional. We calculate bounds for the potential skew asymmetry and show that these bounds are regulated by the boundary conditions of the particle distributions. Despite singularities, our treatment reproduces a smooth space distribution of the potential amplitude and electron and ion distributions that are smooth at one of the boundaries of the electrostatic structure.