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Recently, the general electromagnetic fluctuation theory for magnetized plasmas has been used to study the steady-state fluctuation spectra and the total intensity of low-frequency collective weakly damped modes for parallel wave vectors in Maxwellian plasmas. Now, we address the same question with respect to an arbitrary direction of the wave-vector. Here, we analyze this problem for equal mass plasmas. These plasmas are a very good tool to study various plasma phenomena, as they considerably facilitate the theoretical consideration and at the same time provide with their clear physical picture. Finally, we compare our results in the limiting case of parallel wave vectors with the previous study. [ABSTRACT FROM AUTHOR]

The spectrum of Lyapunov exponents is powerful tool for the analysis of the complex system dynamics. In the general framework of nonlinear dynamics, a number of the numerical techniques have been developed to obtain the spectrum of Lyapunov exponents for the complex temporal behavior of the systems with a few degree of freedom. Unfortunately, these methods cannot be applied directly to analysis of complex spatio-temporal dynamics of plasma devices which are characterized by the infinite phase space, since they are the spatially extended active media. In the present paper, we propose the method for the calculation of the spectrum of the spatial Lyapunov exponents (SLEs) for the spatially extended beam-plasma systems. The calculation technique is applied to the analysis of chaotic spatio-temporal oscillations in three different beam-plasma model: (1) simple plasma Pierce diode, (2) coupled Pierce diodes, and (3) electron-wave system with backward electromagnetic wave. We find an excellent agreement between the system dynamics and the behavior of the spectrum of the spatial Lyapunov exponents. Along with the proposed method, the possible problems of SLEs calculation are also discussed. It is shown that for the wide class of the spatially extended systems, the set of quantities included in the system state for SLEs calculation can be reduced using the appropriate feature of the plasma systems. [ABSTRACT FROM AUTHOR]

The results of an experiment on discharges in long atmospheric pressure air gaps at a pulsed voltage of amplitude up to 800 kV and risetime 150-200 ns have been analyzed. In the experiment, a radiation pulse of photon energy >5 keV and duration 10-20 ns was observed. In analyzing the experimental data it was supposed that a streamer is a plasma protrusion whose surface is equipotential to the cathode surface. It has been shown that the x-ray pulse results from the switch of electrons into the mode of "runaway" from the head of anode-directed streamers. For the electrons injected in the electrode gap from the streamer head, conditions for their switching into the mode of continuous acceleration are realized due to the enhanced electric field at the head. The predicted maximum of the spectrum of the bremsstrahlung generated by the runaway electron beam is around 15 keV. The presence of a maximum in the bremsstrahlung spectrum is due to that the photons emitted by electrons are absorbed by atoms of the gas in which the discharge operate. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is commonly perceived to drive flow and particle acceleration in flares of solar, stellar, and astrophysical disk coronae but the relative roles of different acceleration mechanisms in a given reconnection environment are not well understood. While outflow fast mode shocks have been predicted analytically, we show for the first time via direct numerical simulations that such shocks do indeed occur in the outflows of fast reconnection when an obstacle is present. These shocks are distinct from the slow mode Petschek inflow shocks. If Fermi acceleration of electrons operates in the weak fast shocks, the associated compression ratios will induce a Fermi acceleration particle spectrum that is significantly steeper than strong fast shocks commonly studied, but consistent with the demands of solar flares. While this is not the only acceleration mechanism operating in a reconnection environment, it is plausibly a ubiquitous one. [ABSTRACT FROM AUTHOR]

The system size dependence of electron acceleration during large-scale magnetic island coalescence is studied via a two-dimensional particle-in-cell simulation. Using a simulation box that is larger than those used in previous studies, injection by merging line acceleration and subsequent reacceleration inside a merged island are found to be the mechanisms for producing the most energetic electrons. This finding and knowledge of the reacceleration process enable us to predict that the high energy end of the electron energy spectrum continues to expand as the merged island size increases. Both the merging line acceleration and the reacceleration within a merged island require the island coalescence process to be so dynamic as to involve fast in-flow toward the center of a merged island. Once this condition is met in an early stage of the coalescence, it is likely to stay in the subsequent phase. In other words, if the thin elongated current sheet is initially able to host the dynamic magnetic island coalescence process, it will be a site where repeated upgrades in the maximum energy of electrons occur in a systematic manner. [ABSTRACT FROM AUTHOR]

The nonthermal effects on the propagation of the ion-acoustic soliton are investigated in generalized Lorentzian electron-ion plasmas. The soliton solution of the Korteweg-de Vries equation is obtained as a function of the spectral index and modified stretched coordinate in the generalized Lorentzian plasma. It is found that the nonthermal effect on the dispersive term is found to be stronger than that on the nonlinear term. It is shown that the nonthermal effect of the Lorentzian plasma strongly suppresses the stretched coordinate of the ion-acoustic soliton. It is also shown that the nonthermal effect increases the position of the ion-acoustic soliton. In addition, the nonthermal effects on the position of the ion-acoustic soliton are found to be more important in the forward direction. It is found that the nonthermal effect strongly suppresses the amplitude of the ion-acoustic soliton in Lorentzian electron-ion plasmas. It is also found that the nonthermal effect on the amplitude of the ion-acoustic soliton is more significant in the backward direction. [ABSTRACT FROM AUTHOR]

An accurate independent diagnosis of the temperature and the ionization state of the sample plasmas is important to get a set of well-characterized opacity data in experiment. An analysis of using x-ray scattering was presented for this purpose. Information of the ionization state of the sample plasmas could only be clearly detected when both the Compton and Rayleigh features of the scattering spectra are detectable in one laser shot. Only those plasmas with a few bounded electrons left can provide such kind of scattering spectra. The requirements for using the x-ray scattering as diagnosis are rigorous. Under a radiation field produced by laser-heated hohlraum, only limited number of sample elements can be heated to the ionization state required. Careful design is needed to achieve an x-ray scattering experiment. This work can be helpful to establish a diagnostic flattop for the opacity experiments. [ABSTRACT FROM AUTHOR]

A formalism for analyzing the effect of ion-neutral gas interactions on the flow of ions between nearly transparent electrodes in spherical geometry has been developed for atomic ions in a weakly ionized plasma, so that the important atomic effects are charge exchange and ion impact ionization. The formalism is applied to spherical, gridded, inertial-electrostatic confinement (IEC) devices. The formalism yields detailed information about the energy spectra of the ions and fast neutral atoms, and the resulting fusion rate for 3He ions in a background 3He gas. The results are illustrated with an example calculation for the Wisconsin IEC device operating on 3He. [ABSTRACT FROM AUTHOR]

A nonlinear gyrokinetic simulation code is used to test the quasilinear transport approximation (QLTA) with simulated nonlinear spectral (potential field) intensity. Two common forms of the QLTA are defined. The first uses the linear mode spectrum (mQLTA) and the second uses the complete frequency spectrum (fQLTA) for the nonlinear spectral intensity. The mQLTA is tested via two-step linear then nonlinear simulations convoluting a quasilinear weight with a nonlinear field intensity spectral weight to get the quasilinear transport in comparison with the actual nonlinear transport. The fQLTA is tested via one-step simulations that have ion and electron “plasma species” at full densities and “tracer species” at negligible densities (and making no contribution to the Poisson field solve equation). If the tracer and plasma gyrokinetic equations are identical, then so are their respective energy and particle diffusivities. Comparing tracer and plasma (actual) diffusivities, when the tracer equation nonlinearity is deleted, provides a quantifiable test of the fQLTA form. The mQLTA preserves ambipolarity but the two-step test includes only the leading linear normal modes at each wave number. The one-step test of the fQLTA subsumes all normal modes but precludes ambipolar particle flow. The mQLTA and fQLTA quasilinear weights (per normal mode) are shown to be identical for a commonly used (but unphysical) mode frequency line width model. In successful cases, quasilinear diffusivities are typically 1.4–1.8 (1.2–1.4) larger than actual diffusivities for mQLTA (fQLTA). The QLTA is expected to make best predictions in the ratios of energy and particle flows. Electron to ion energy flow ratios are well approximated but both forms of the QLTA appear to breakdown most evidently for ratios of particle to energy flows in cases with strongly pinched (and impractically large) particle flows. An example of the so-called passive tracer diffusivity, which includes only linear and nonlinear E×B motion, is given for comparison with actual diffusivities. [ABSTRACT FROM AUTHOR]

An atmospheric-pressure argon discharge plasma column is generated by making use of a single-electrode configuration with the power supply operating at a frequency of 45 kHz. It is observed that corona, glowlike plume, and filamentary discharges evolve individually with increasing applied voltage. It is in the filamentary state with average electron density of order 1012 cm-3 that plasma column grows up in the tube with increasing applied voltage. Its discharge characteristics are determined by measuring electrical parameters (voltage, conduction current, and average absorbed power) and optical emission spectroscopy. [ABSTRACT FROM AUTHOR]

Gyrokinetic turbulence simulations covering both electron and ion spatio-temporal scales self-consistently are presented. It is found that for experimentally realistic transport levels at long wavelengths, electron temperature gradient modes may yield substantial or even dominant high-wavenumber contributions to the electron heat flux. It is investigated in which way this behavior is reflected in several experimentally accessible quantities as, for instance, density or frequency spectra. [ABSTRACT FROM AUTHOR]

The GEM gyrokinetic δf simulation code [Y. Chen and S. Parker, J. Comput. Phys. 189, 463 (2003); and ibid.220, 839 (2007)] is shown to reproduce electron temperature gradient turbulence at the benchmark operating point established in previous work [W. M. Nevins, J. Candy, S. Cowley, T. Dannert, A. Dimits, W. Dorland, C. Estrada-Mila, G. W. Hammett, F. Jenko, M. J. Pueschel, and D. E. Shumaker, Phys. Plasmas 13, 122306 (2006)]. The electron thermal transport is within 10% of the expected value, while the turbulent fluctuation spectrum is shown to have the expected intensity and two-point correlation function. [ABSTRACT FROM AUTHOR]

The University of Manchester Linear System (ULS) [M. G. Rusbridge et al., Plasma Phys. Controlled Fusion 42, 588 (2000)] is an experiment in which a steady-state plasma stream is confined along a longitudinal magnetic field. The plasma passes through a tapered orifice into a separate gas target chamber (GTC) where it interacts with neutral gas at pressures of up to 15 mTorr. The upstream plasma beam is 6–14 mm in diameter with electron temperatures between 2 and 15 eV, and densities in the range of 1017–1019 m-3. The primary aim of this study is to investigate physical processes relevant to gas target divertors in tokamaks. Upstream parameters, in terms of electron temperature and density, can be varied in the ULS enabling the investigation of electron-ion recombination (EIR) or molecular-activated recombination (MAR) processes. Detached recombining plasmas have been studied in the GTC using an axially scanning spectroscopic system with a spatial resolution of less than 5 mm. We have investigated two distinct plasma regimes having the same electron temperature upstream, but upstream densities that differ by an order of magnitude. Vibrationally excited molecules, which are a necessary prerequisite for MAR, have been detected in both these cases. In the higher density case, which ultimately detaches via EIR processes, these excited molecules are present upstream of the EIR region. [ABSTRACT FROM AUTHOR]

One-dimensional Eulerian Vlasov-Maxwell simulations are presented that show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. A low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single-hot-spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a Gauss-Hermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping. [ABSTRACT FROM AUTHOR]

Electron-ion collisions in the presence of a strong laser field lead to a distribution of fast electrons with maximum energy Emax=(k0+2v0)2/2(a.u.), where k0 is the impact and v0 the quiver velocity of the electron. The energy spectrum is calculated by two approaches: (1) The time-dependent Schrödinger equation is numerically solved for wave packet scattering from a one-dimensional softcore Coulomb potential. Multiphoton energy spectra are obtained demonstrating a separation of the energy spectrum into an exponential distribution for transmission and a plateau distribution for reflection. (2) The energy spectrum is analytically calculated in the framework of classical instantaneous Coulomb collisions with random impact parameters and random phases of the laser field. An exact solution for the energy spectrum is obtained from which the fraction of fast electrons in the plateau region can be estimated. [ABSTRACT FROM AUTHOR]

Using high spatial and temporal resolution spectroscopy, direct measurements of the impurity dynamics during an edge-localized mode (ELM) cycle have revealed rich details of both the ELM event and the inter-ELM period. The increased transport associated with the ELM event is observed to affect all the particle species (electrons, ions, and impurities) in a similar manner over a wide range in plasma conditions. The density perturbation (and hence convective energy loss) is found to be insensitive to edge collisionality while the temperature perturbation (and hence conductive energy loss) decreases as the edge collisionality increases. Analysis of the response of the profiles to the ELM indicates that the initial response to the ELM is a rapid (<0.5 ms) decrease in the impurity density in the high gradient region in the edge coincident with a brief increase in the temperature and rotation velocity, which then drop on a slightly longer time scale (∼1 ms) than the initial density decrease. Transport is then observed to continually improve as the Er shear increases during the inter-ELM period. Analysis suggests that this correlation in conjunction with the loss of Er shear at the ELM event may be the underlying reason for the decrease in the conductive loss as the edge collisonality is increased. [ABSTRACT FROM AUTHOR]

Extended spectroscopic measurements of hydrogen emission into the vacuum ultraviolet region on the Maryland Centrifugal Experiment [R. F. Ellis, A. B. Hassam, S. Messer, and B. R. Osborn, Phys. Plasmas 8, 2057 (2001)] indicate, from opacity on the Lyman series, ∼6% atomic hydrogen coexisting with protons, electrons, and carbon-impurity ions in a plasma region where the temperature is ∼15 eV. An explanation is offered, based upon charge exchange of entering cold hydrogen atoms with plasma protons, followed by collisional excitation and radiative decay. [ABSTRACT FROM AUTHOR]

In a recent experiment at Max-Born Institut, Berlin, strong modulations have been observed in energy spectra of MeV ions that were accelerated by ultrashort intense laser pulses, τL=35 fs, I=1019 W/cm2, off water droplets. This experiment is studied analytically as well as by numerical particle-in-cell simulations in one and two dimensions and it is shown how classical isothermal fluid expansion models fail in the present case. The paper investigates alternative models which claim to describe the ion spectral modulations and a mechanism that generates proton spectra similar to the experimental ones due to a simultaneous acceleration of several ion components. Finally, the issue of directional anisotropy in the spectra of accelerated ions for the case of a spherical target is discussed. [ABSTRACT FROM AUTHOR]

Coherent terahertz and millimeter wave radiation from laser accelerated electron bunches has been measured. The bunches were produced by tightly focusing (spot diameter &ap;6 mm) a high peak power (up to 10 TW), ultra-short (&ges;50 fs) laser pulse from a high repetition rate (10 Hz) laser system (0.8 μm), onto a high density (>1019 cm³) pulsed gas jet of length &ap;1.5 mm. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation in the 0.3-19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured radiated energy for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 μJ/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be used as a powerful tool for measuring the properties of laser accelerated bunches at the exit of the plasma accelerator. Preliminary spectral measurements indicates that bunches as short as 30-50 fs have been produced in these laser driven accelerators. [ABSTRACT FROM AUTHOR]

A Lagrangian fluid model ~cold plasma, fixed ions is developed for analyzing the coupling between electron plasma waves. This model shows that a small wave number electron plasma wave ( v2 ,k2)will strongly affect a large wave number electron plasma wave ( v1 ,k1), transferring its energy into daughter waves or sidebands at ( v11n v2 ,k11nk2) in the lab frame. The accuracy of the model is checked via particle-in-cell simulations, which confirm that the energy in the mode at ( v1 ,k1) can be completely transferred to the sidebands at ( v11n v2 ,k11nk2) by the presence of the electron plasma mode at ( v2 ,k2). Conclusive experimental evidence for the generation of daughter waves via this coupling is then presented using time- and wave number-resolved spectra of the light from a probe laser coherently Thomson scattered by the electron plasma waves generated by the interaction of a two-frequency CO2 laser with a plasma.. [ABSTRACT FROM AUTHOR]

An experiment is carried out to ascertain the role of energetic electrons in the excitation of plasma turbulence, near the inhomogeneity of a magnetic filter field. This is done in two steps. First, a magnetic filter arrangement is used to eliminate energetic electrons from the plasma. Second, electrons of 0-20 eV energy are deliberately introduced between the main filter and an additional filter located in the target chamber. It is observed that plasma near the additional filter is quiescent in the absence of energetic electrons. When the energetic electrons are introduced, the plasma near the additional filter becomes turbulent. The frequency spectrum of the turbulent fluctuation is broad, with a peak around 100 kHz which lies in the lower hybrid frequency range. [ABSTRACT FROM AUTHOR]

It has been demonstrated that in the presence of weak collisions, described by the Lenard-Bernstein (LB) collision operator, the Landau-damped solutions become true eigenmodes of the system and constitute a complete set [C.-S. Ng et al., Phys. Rev. Lett. 83, 1974 (1999) and C. S. Ng et al., Phys. Rev. Lett. 96, 065002 (2004)]. We present numerical results from an Eulerian Vlasov code that incorporates the Lenard-Bernstein collision operator [A. Lenard and I. B. Bernstein, Phys. Rev. 112, 1456 (1958)]. The effect of collisions on the numerical recursion phenomenon seen in Vlasov codes is discussed. The code is benchmarked against exact linear eigenmode solutions in the presence of weak collisions, and a spectrum of Landau-damped solutions is determined within the limits of numerical resolution. Tests of the orthogonality and the completeness relation are presented. [ABSTRACT FROM AUTHOR]