Volume 16, Issue 5, May 2009
Index of content:

As multikeV xray radiators, hohlraums and halfraums with inner walls coated with metallic materials (called liner) have been tested for the first time with laser as the energy drive. For titanium, conversion efficiencies (CEs) are up to for emission into , integrating between 4.6 and 6.5 keV when a large diameter hohlraum is used. Germanium CE is into between 9 and 13 keV. The highest CEs have been obtained with a 1 ns squared pulse and phase plates giving laser absorption near 99%. These high CEs are due to longlasting, good plasma conditions for multikeV xray production maintained by plasma confinement inside the plastic cylinder and plasma collision leading to a burst of x rays at a time that depends on target size. As photon emitters at 4.7 keV, titaniumlined hohlraums are the most efficient solid targets and data are close to CEs for gas targets, which are considered as the upper limit for xray yields since their low density allows good laser absorption and low kinetics losses. As 10.3 keV xray emitters, exploded germanium foils give best results one order of magnitude more efficient than thick targets; doped aerogels and lined hohlraums give similar yields, about three times lower than those from exploded foils.
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Formation of a localized acceleration potential during magnetic reconnection with a guide field
View Description Hide DescriptionMagnetic reconnection near the surface of the sun and in the Earth’s magnetotail is associated with the production of highly energetic electrons. Direct acceleration in the reconnectionelectric field has been proposed as a possible mechanism for energizing these electrons. Here, however, we use kinetic simulations of guidefield reconnection to show that in twodimensional (2D) reconnection the parallel electric field, in the reconnection region is localized and its structure does not permit significant energization of the electrons. Rather, a large fraction of the electrons become trapped due to a sign reversal in , imposing strict constraints on their motions and energizations. Given these new results, simple 2D models, which invoke direct acceleration for energizing electrons during a single encounter with a reconnection region, need to be revised.

Nested multilayered X pinches for generators with megaampere current level
View Description Hide DescriptionA symmetric X pinch configuration that is conducive to using large numbers of wires on pulsed power generators has been tested at 1 MA. Using an initial configuration of wires before their twisting, similar to nested cylindrical wire arrays, enables a geometrically simple, compact, multilayerwire configuration at the X pinch crossing region. Multilayer X pinches with the same or different materials in the inner and outer wire layers were tested. Optimization resulted in X pinch radiation sources with peak power comparable to the most successful single layer X pinch, but with a compact, single bright X radiation source more reliably obtained using the nested configuration.

Static analysis of possible emittance growth of intense charged particle beams with thermal equilibrium distribution
View Description Hide DescriptionPossible emittance growths of intense, nonuniform beams during a transport in a focusing channel are derived as a function of nonlinear field energy and space charge tune depression factors. The nonlinear field energy of the beam with thermal equilibrium distribution is estimated by considering the particle distribution across the cross section of the beam. The results show that the possible emittance growth can be suppressed by keeping the beam particle in thermal equilibrium distribution during the beamtransport.

Electron scale structures in collisionless magnetic reconnection
View Description Hide DescriptionThe early timedependent phase of collisionless reconnection, which is dominated by electron dynamics, is investigated using electronmagnetohydrodynamic simulations. Simulations initialized with multiwavelength perturbations lead to reconnection at multiple sites and the interaction of electron flows generated at the neighboring sites leads to secondary instabilities. These electronscale processes limit the size of the current sheet and lead to an outofplane magnetic field with nested quadrupoles. These structures have important implications for multispacecraft missions exploring Earth’s magnetotail.

A physical parametrization of coupled transverse dynamics based on generalized Courant–Snyder theory and its applications
View Description Hide DescriptionA physical parametrization of coupled transverse dynamics is developed by generalizing the Courant–Snyder (CS) theory for one degree of freedom to the case of coupled transverse dynamics with two degrees of freedom. The four basic components of the original CS theory, i.e., the envelope equation, phase advance, transfer matrix, and CS invariant, all have their counterparts with remarkably similar expressions in the generalized theory. Applications of the new theory are given. It is discovered that the stability of coupled dynamics is completely determined by the generalized phase advance.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Effect of ion and ionbeam mass ratio on the formation of ionacoustic solitons in magnetized plasma in the presence of electron inertia
View Description Hide DescriptionThe propagation of ionacoustic solitary waves in magnetized plasma with cold ions and ionbeams together with electron inertia has been investigated theoretically through the Korteweg–de Vries equation. Subject to the drift velocity of the ion beam, the existence of compressive solitons is found to become extinct as ( ion mass/ionbeam mass) tends to 0.01 when ( is the beam velocity/phase velocity). Interestingly, a transitional direction of propagation of solitary waves has been unearthed for change over, from compressive solitons to rarefactive solitons based on and ( of the angle made by the wave propagation direction with the direction of the magnetic field) for fixed ( mass/ion mass). Further, the direction of propagation of ionacoustic waves is found to be the deterministic factor to admit compressive or rarefactive solitons subject to beam outsource.

Selfexcited surface plasmonpolaritons at the interface of counterstreaming plasmas
View Description Hide DescriptionSurfaceplasma modes are coupled electromagnetic/electrostatic (plasmonpolariton) excitations of free electrons near the vacuumplasma or plasmaplasma interface. The surface plasmonpolariton propagates along the surface plane and decays on both sides of the boundary. The effect of counterstreaming on the surface plasmonpolariton excitation is examined. It is shown that the twostream instability can excite selfconsistently the surface modes at the interface of two counterstreaming plasmas. The dispersion relation is derived and the exact numerical solutions are plotted for comparison to the excitations of a nonstreaming plasmaplasma interface. Such plasma models are of interest in electronic signal transmission, as well as in astrophysical applications and in beamplasma experiments.

Investigation of spatiotemporal behavior of the plasma density during the development of the thermocurrent instability
View Description Hide DescriptionConsidering a weakly ionized, collisional, quasineutral plasma placed in an electric field under the condition of nonresonant Cerenkov radiation and using the hydrodynamic model, a nonlinear diffusionlike equation with negative diffusion coefficient is obtained, which describes the dynamics of plasma density during the development of the thermocurrent instability. This equation is solved by the Adomian decomposition method. According to this solution, the spatiotemporal behavior of the plasma density during the growth of the thermocurrent instability is investigated. It is shown that the development of the thermocurrent instability causes the initial perturbations in the plasma density to grow. Also, it is shown that the increment in these perturbations continues until the growth of the thermocurrent instability ceases due to the breakdown of the quasineutrality condition. In this case, it is seen that the profile of the plasma density does not change any more and gets to a constant limit.

On the electron whistler dispersion law in a cold plasma with light ions and heavy charged particulates
View Description Hide DescriptionThe dispersion equation of electron whistler waves in a cold plasma with two light ions of comparable gyrofrequencies and heavy charged particulates is derived. It is valid in a very wide frequency range above the highest ion cutoff frequency when the wave frequency is essentially less than the electron plasma frequency. The derived electron whistlerdispersion law is expressed through the relative contents of the two light ions and the electrons, as well as the characteristic frequencies of the magnetized plasma, as the lower hybrid resonance frequency, the two highest ion cutoff frequencies, the gyrofrequencies of the light ions, and the electron gyro and plasma frequencies. The approximation of vanishingly small gyrofrequencies of the heavy ions permits to determine with a relevant accuracy the electron whistlerdispersion law using the features of electron whistler spectrograms only. Estimates of the relative charge density of the light ions are obtained and the dispersion laws of the adjacent branches, i.e., the electron whistler waves and the socalled ion cyclotron whistlers are calculated. For the electron whistler waves, the presence of negative ions can be the origin of a manyfold increase in the lower cutoff frequency; a merging effect of the cutoff frequencies of the adjacent branches can also appear.

Warm electromagnetic lower hybrid wave dispersion relation
View Description Hide DescriptionLower hybrid (LH) waves can interact resonantly with both electrons and ions transferring energy between the species. For this reason the properties of LH waves are of interest. Most treatments of LH waves include either electromagnetic (EM) or warm plasmaeffects but not both. Here a new analytic dispersion relation for LH waves, including both EM and warm plasmaeffects, is derived and shown to be more consistent than the previous analytic dispersion relations with numerical results. These comparisons show a very good agreement of the real part of the frequency and reasonable agreement of the imaginary part for a wide range of parameters. It is found that ion magnetization effects, which have been neglected in all previous analytic treatments of LH waves, are surprisingly important. When ion magnetization effects become important the continuous LH mode breaks up into a series of segments of ion Bernstein modes.

Hydromagnetic waves and instabilities in kappa distribution plasma
View Description Hide DescriptionStability properties of hydromagnetic waves (shear and compressional Alfven waves) in spatially homogeneous plasma are investigated when the equilibrium particle velocity distributions in both parallel and perpendicular directions (in reference to the ambient magnetic field) are modeled by kappa distributions. Analysis is presented for the limiting cases and for which solutions of the dispersion relations are analytically tractable. Here is the ratio of the wave phase speed and the electron (ion) thermal speed. Both low and high ( pressure/magnetic pressure)plasmas are considered. The distinguishing features of the hydromagnetic waves in kappa distribution plasma are (1) both Landau damping and transittime damping rates are larger than those in Maxwellianplasma because of the enhanced highenergy tail of the kappa distribution and (2) density and temperature perturbations in response to the electromagnetic perturbations are different from those in Maxwellianplasma when . Moreover, frequency of the oscillatory stable modes (e.g., kinetic shear Alfven wave) and excitation condition of the nonoscillatory (zero frequency) unstable modes (e.g., mirror instability) in kappa distribution plasma are also different from those in Maxwellianplasma. Quantitative estimates of the differences depend on the specific choice of the kappa distribution. For simplicity of notations, same spectral indices and have been assumed for both electron and ion population. However, the analysis can be easily generalized to allow for different values of the spectral indices for the two charged populations.

Magnetic reconnection with pressure tensor in electron magnetohydrodynamics
View Description Hide DescriptionThe dissipation mechanisms of reconnection and the pressure gradient effects on tearing mode with guide magnetic field are analyzed systematically by including the electron pressure tensor in electron magnetohydrodynamics. It is found that which dissipation mechanism dominates, either pressurebased dissipation or inertiabased dissipation, has a great relation with the relative scaling orders between the electron thermal Larmor radius and electron inertia skin depth. The effects of pressure gradient also depend on the relative magnitude between parallel and perpendicular equilibrium pressure gradients. When the pressurebased dissipation is dominant, the condition that pressure drives or suppresses tearing mode instability also depends on the relative magnitude between parallel and perpendicular equilibrium pressure gradients.

Filamentation of laser in a magnetized plasma under relativistic and ponderomotive nonlinearities
View Description Hide DescriptionFilamentation of a circularly polarized short pulse laser propagating along the direction of ambient magnetic field in plasma is studied. The nonlinearity arises through the combined effect of relativistic mass variation and ponderomotive force induced electron cavitation. The growth rate is maximum for an optimum filament size, . and increases with plasma density and ambient magnetic field.

Linear coupling of Alfven waves and acoustictype modes in dense quantum magnetoplasmas
View Description Hide DescriptionA coupled dispersion relation of low frequency shear Alfven waves and electrostatic waves in a dense quantum magnetoplasma is derived by using hydrodynamic model. The dispersive contribution of electron quantum effects is discussed for dynamic as well as static ions. The dominant role of electron Fermi pressure is highlighted and its comparison with the quantum pressure arising due to quantum Bohm potential is presented. For illustrative purpose, the results are analyzed numerically. The relevance of present work with the dense astrophysical and laboratory plasmas is pointed out with possible consequences.

Tests of collision operators using laboratory measurements of shear Alfvén wave dispersion and damping
View Description Hide DescriptionMeasurements of shear Alfvén waves are used to test the predictions of a variety of different electron collision operators, including several Krook collision operators as well as a Lorentz collision operator. New expressions for the collisional warmplasma dielectric tensor resulting from the use of the fully magnetized collisional Boltzmann equation are presented here. Theoretical predictions for the parallel phase velocity and damping as a function of perpendicular wave number are derived from the dielectric tensor. Laboratory measurements of the parallel phase velocity and damping of shear Alfvén waves were made to test these theoretical predictions in both the kinetic and inertial parameter regimes and at several wave frequencies . Results show that, in the inertial regime, the best match between measurements and theory occur when any of the Krook operators are used to describe electron collisions. In contrast, the best agreement in the kinetic regime is found when collisions are completely ignored.

Parker problem in Hall magnetohydrodynamics
View Description Hide DescriptionThe Parker problem in Hall magnetohydrodynamics(MHD) is considered. Poloidal shear superposed on the toroidal ion flow associated with the Hall effect is incorporated. This is found to lead to a triple deck structure for the Parker problem in Hall MHD, with the magnetic field falling off in the intermediate Hallresistive region more steeply (like ) than that (like ) in the outer ideal MHD region.

Nonlinear Phenomena, Turbulence, Transport

Implementation and application of two synthetic diagnostics for validating simulations of core tokamak turbulence
View Description Hide DescriptionThe deployment of multiple highresolution, spatially localized fluctuationdiagnostics on the DIIID tokamak [J. L. Luxon, Nucl. Fusion42, 614 (2002)] opens the door to a new level of core turbulencemodel validation. Toward this end, the implementation of synthetic diagnostics that model physical beam emission spectroscopy and correlation electron cyclotron emission diagnostics is presented. Initial results from their applications to local gyrokinetic simulations of two locations in a DIIID mode discharge performed with the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys.186, 545 (2003)] are also discussed. At normalized toroidal flux , we find very good agreement between experiment and simulation in both the energy flows and fluctuation levels measured by both diagnostics. However, at , GYRO underpredicts the observed energy flows by roughly a factor of 7, with rms fluctuation levels underpredicted by a factor of 3. Interestingly, at both locations we find good agreement in the shapes of the radial and vertical density correlation functions and in the shapes of the frequency power spectra. At both locations, the attenuation of the GYROpredicted fluctuations due to the spatial averaging imposed by the diagnostics’ spot sizes is significant, and its incorporation via the use of synthetic diagnostics is shown to be essential for quantitative comparisons such as these.

A novel mechanism for exciting intrinsic toroidal rotation
View Description Hide DescriptionBeginning from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers two physically distinct mechanisms by which microturbulence may drive intrinsic rotation. The first mechanism, which emanates from convection of parallel momentum, has already been analyzed [O. D. Gurcan et al., Phys. Plasmas14, 042306 (2007); R. R. Dominguez and G. M. Staebler, Phys. Fluids B5, 3876 (1993)] and was shown to follow from radial electric field shear induced symmetry breaking of the spectrally averaged parallel wave number. Thus, this mechanism is most likely active in regions with steep pressure gradients or strong poloidal flow shear. The second mechanism uncovered, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. This novel means of driving intrinsic rotation, while nominally higher order in an expansion of the mode frequency divided by the ion cyclotron frequency, does not depend on radial electric field shear. Thus, while the magnitude of the former mechanism is strongly reduced in regions of weak radial electric field shear, this mechanism remains unabated and is thus likely relevant in complementary regimes.

Finite orbit width effect in ion collisional transport in TJII
View Description Hide DescriptionThe validity of the traditional local diffusive approach and of the use of monoenergetic calculations has been studied for the stellarator TJII [Alejaldre et al., Fusion Technol.17, 131 (1990)]: it is shown to be doubtful, under some circumstances, even in a purely collisional description of transport. The diffusion in physical space starting from Diracdeltalike initial conditions has been studied using the code Integrator of Stochastic Differential Equations for Plasmas by Castejón et al. [Plasma Phys. Controlled Fusion49, 753 (2007)]. Particles may experience large radial excursions from their original magnetic surfaces in a single collisional time. The contribution of these particles to the flux may make it nondiffusive; nonGaussian density distributions, characterized by long tails, are observed. In the velocity space, there are important variations in the average particle kinetic energy after one collision time. We discuss the effect of this fact over the calculation of monoenergetic transport coefficients and their convolution. A simple analysis based on Hurst exponents has shown nevertheless that the description of transport by means of a pinch term and an effectivetransport coefficient is more correct than expected.

Probability distribution function for selforganization of shear flows
View Description Hide DescriptionThe first prediction of the probability distribution function (PDF) of selforganized shear flows is presented in a nonlinear diffusionmodel where shear flows are generated by a stochastic forcing while diffused by a nonlinear eddy diffusivity. A novel nonperturbative method based on a coherent structure is utilized for the prediction of the strongly intermittent exponential PDF tails of the gradient of shear flows. Numerical simulations using Gaussian forcing not only confirm these predictions but also reveal the significant contribution from the PDF tails with a large population of supercritical gradients. The validity of the nonlinear diffusionmodel is then examined using a threshold model where eddy diffusivity is given by discontinuous values, elucidating an important role of relative time scales of relaxation and disturbance in the determination of the PDFs.
