Distribution by Scientific Domains
Distribution within Physics and Astronomy

Selected Abstracts

Towards Fast Measurement of the Electron Temperature in the SOL of ASDEX Upgrade Using Swept Langmuir Probes

H.W. Müller
Abstract On ASDEX Upgrade first experiments were made using single probes with a voltage sweep frequency up to 100kHz. Possibilities and limitations using fast swept probes with a standard diagnostic and analysis tools are discussed. A good agreement between the data derived from fast swept single probe characteristics and floating as well as saturation current measurements was found. In a stationary (non ELMing) plasma the data of the fast swept probe are compared to standard slow swept probes (kHz range) showing an improvement of the measurement by faster sweeping. While ELM filaments already could be resolved the access of electron temperature fluctuations in small scale turbulence still has to be improved. Further comparisons are done in ELMy H-mode with combined ball-pen probe/floating potential measurements which can deliver electron temperatures with 25 , s time resolution at reduced spatial resolution compared to pin probes. During ELMs the electron temperatures derived from the ball-pen probe and fast swept single probes agreed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Ball-Pen Probe Measurements in L-Mode and H-Mode on ASDEX Upgrade

J. Adamek
Abstract Experimental investigations of the plasma potential, poloidal electric field and electron temperature during L-mode and ELMy H-mode were performed on ASDEX Upgrade by means of a probe head containing four ball-pen probes and four Langmuir probes. This allows to measure simultaneously the floating and plasma potential at the same time which are related by the electron temperature. Thus a combination of ball-pen probes and Langmuir probes offers the possibility to determine the electron temperature directly with high temporal resolution. This novel temperature measurement method is compared to standard techniques. The influence of the electron temperature on the usual calculation of the poloidal electric field from the gradient of the floating potential is determined by a comparison to the poloidal electric field derived from the plasma potential (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Physics Programme for Initial Operation of Wendelstein 7-X

H.-S. Bosch
Abstract It is the mission of the Wendelstein 7-X (W7-X) stellarator to demonstrate the reactor potential of the optimized stellarator line. Therefore W7-X has to demonstrate high-power, steady-state operation. After completion of its construction and commissioning of the device, a first operational phase is planned to investigate basic stellarator properties and to prepare later steady-state operation of Wendelstein 7-X. The physics programme for this initial operation period has been developed recently to comply with the physics goals of W7-X and will be discussed in this paper (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Extrapolation of the W7-X Magnet System to Reactor Size

F. Schauer
Abstract The fusion experiment Wendelstein 7-X (W7-X), presently under construction at the Greifswald branch institute of IPP, shall demonstrate the reactor potential of a HELIAS stellarator. HELIAS reactors with three, four and five periods have been studied at IPP since many years. With a plasma axis induction of 5 T, corresponding to about 10 T maximal induction at the coil, it was shown that such reactors are feasible. Now the possibility is being investigated to increase the conductor induction up to the 12 T , range, corresponding to > 5.5 T at the plasma axis. This improves the stellarator confinement properties but does not change the basic physics with respect to the previously analyzed machines. In particular the 5periodic HELIAS type, HSR5, is considered which evolves from W7-X by linear scaling of the main dimensions by a factor of four. Recent progress in superconductor technology and the extensive development work performed for ITER are taken into account. The latter is particularly relevant since by coincidence the circumferences of the HSR5 and the ITER toroidal field coils are practically the same. For the presented 12 T reactor version, the HSR50a, also the conductor and structural requirements are comparable to the corresponding ITER specifications. Therefore, advantage can be taken of these similarities for the stellarator reactor magnet design. The input was provided by the new code "MODUCO" which was developed for interactive coil layout. It is based on Bézier curve approximations and includes the computation of magnetic surfaces and forces (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Configuration Space Control for Wendelstein 7-X

T. Dodson
Abstract The Wendelstein 7-X stellarator (W7-X) is a superconducting fusion experiment presently under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. W7-X is a device with extreme geometrical complexity due to the close-packing of components inside the cryostat and their complex three-dimensional shapes. The task of the Configuration Space Control department is to ensure that these components do not collide with each other under the defined set of configurations such as during assembly, at cool down, or during operation at various coil currents, among others. To fulfill this task, sophisticated tools and procedures were developed and implemented within the realm of a newly founded division that focuses on design, configuration control, and configuration management. This paper will discuss the Configuration Space Control process, explore the advantages to the project resulting from the process, and demonstrate its application in the analysis of the cryogenic cooling pipes of Module 5 (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Generalized Spitzer Function with Finite Collisionality in Toroidal Plasmas

W. Kernbichler
Abstract The drift kinetic equation solver NEO-2 [1] which is based on the field line integration technique has been applied to compute the generalized Spitzer function in a tokamak with finite plasma collisionality. The resulting generalized Spitzer function has specific features which are pertinent to the finite plasma collisionality. They are absent in asymptotic (collisionless or highly collisional) regimes or in results drawn from interpolation between asymptotic limits. These features have the potential to improve the overall ECCD efficiency if one optimizes the microwave beam launch scenarii accordingly (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effect of Energetic-Ion-Driven MHD Instabilities on Energetic-Ion-Transport in Compact Helical System and Large Helical Device

M. Isobe
Abstract This paper describes 1) representative results on excitation of energetic-particle mode (EPM) and toroidicity-induced Alfvén eigenmode (TAE) and consequent beam-ion losses in CHS, and 2) recent results on beam-ion transport and/or losses while EPMs are destabilized in LHD. Bursting EPMs and TAEs are often excited by co-injected beam ions in the high-beam ion pressure environment and give a significant effect on co-going beam ions in both experiments. It seems that in CHS, resonant beam ions are lost within a relatively short-time scale once they are anomalously transported due to energetic-ion driven MHD modes, whereas unlike CHS, redistribution of beam ions due to energetic-ion driven MHD modes is seen in LHD, suggesting that not all anomalously transported beam ions escape from the plasma (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Optimization Study of ICRF Heating in the LHD and HSX Configurations

S. Murakami
Abstract Two global simulation codes, TASK/WM (a full wave solver) and GNET (a 5-D drift kinetic equation solver), are combined to simulate the ICRF heating in the 3D magnetic configuration. The combined code is applied to the ICRF minority heating in the LHD configuration. An optimization of the ICRF heating is considered in changing the magnetic configurations and the resonance surfaces in the LHD plasmas using GNET code. It is found that the heating efficiency is improved about 30% with the heating power of 10MW in the optimized heating scenario from that of the present standard off-axis heating scenario. Also the ICRF minority heating is studied in the HSX plasma and it is found that the ICRF heating of about 100kW is still effective to heat the plasma even , /a , 1/7.5 for tail ions (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Magnetic Resonances in ECR-Heated Plasmas of the TJ-II Heliac

D. López-Bruna
Abstract Low order rational values of the rotational transform ,magnetic resonances, for brevity, can be present inside ECR,heated plasmas of the TJ-II Heliac, in low magnetic shear conditions, without causing damage to confinement. Moreover, in agreement with previous experience in the TJ-II, the resonances seem to benefit confinement and are associated to changes in the radial electric field, at least in the density gradient region. These results encourage considering magnetic resonances as possible external actuators on confinement in stellarator/heliotron devices (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Influence of Low-Order Rational Surfaces on the Radial Electric Field of TJ-II ECH Plasmas

O. Bondarenko
Abstract Dynamic magnetic configuration scan experiments have been conducted in order to investigate the influence of the rotational transform on plasma rotation, radial electric field and turbulence. The main magnetic resonances (7/4 and 5/3 in this work) make a positive contribution to the local radial electric field, which depends on the plasma density and plasma radius. A local reduction in the level of density fluctuations due to the influence of low order rational surfaces is also observed. These results could explain local transport changes associated to low order rational surfaces observed in TJ-II ECH plasmas (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

ECCD Experiments Using the Upgraded Launching System in Heliotron J

K. Nagasaki
Abstract Electron cyclotron current drive (ECCD) experiments have been made in Heliotron J by using an upgraded EC launching system. A focused Gaussian beam is injected with the parallel refractive index, N||, ranging from -0.05 to 0.6. Ray tracing calculation shows that the EC power is more localized than that in the previous launching system. In the initial ECCD experiment, the EC injection angle has been successfully scanned as designed. Comparison between perpendicular and oblique launching experiments shows that for standard configuration, the EC current of 1 kA is driven at N|| , 0.3 in the Fisch-Boozer direction, and the bootstrap current flows up to 1 kA. A large increase in electron cyclotron emission (ECE) signals has been observed when the EC current was driven (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Configuration Studies on the Heliotron Fusion Energy Reactor with Split-Type Helical Coils

N. Yanagi
Abstract Configuration optimization is examined for the heliotron fusion energy reactor FFHR in order to find sufficient clearances between the ergodic region outside the nested magnetic surfaces and blankets at the inboard side of the torus. The standard configuration of FFHR, which is similar to that of LHD, has a relatively large major radius of the helical coils in order to satisfy this requirement. It has been found, as an alternative design, that equivalent clearances are obtained with a shorter major radius both by employing a lower helical pitch parameter and splitting the helical coils in the poloidal cross-section at the outboard side. Splitting the helical coils also provides another configuration that ensures magnetic well formation in the fairly large nested magnetic surfaces with outward shifted configurations. Optimization is being carried out for these configurations by adjusting the pitch modulation parameter to improve the particle confinement (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Simulation Study of the MHD Stability Beta Limit in LHD by TASK3D

M. Sato
Abstract The numerical method for analysis of the "MHD stability beta limit" based on a hierarchy integrated simulation code TASK3D has been developed. The numerical model for the effect of the MHD instabilities is introduced such that the pressure profile is flattened around the rational surface due to the MHD instabilities. The width of the flattening of the pressure gradient is determined from the width of the eigenmode structure of the MHD instabilities (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Verification of the 2D Tokamak Edge Modelling Codes for Conditions of Detached Divertor Plasma

V. Kotov
Abstract The paper discusses verification of the ITER edge modelling code SOLPS 4.3 (B2-EIRENE). Results of the benchmark against SOLPS 5.0 are shown for standard JET test cases. Special two-point formulas are employed in SOLPS 4.3 to analyze the results of numerical simulations. The applied relations are exact in frame of the equations solved by the B2 code. This enables simultaneous check of the parallel momentum and energy balances and boundary conditions. Transition to divertor detachment is analyzed quantitatively as it appears in the simulations in terms of the coupled momentum and energy balance (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Multi-Fluid Modeling of Low-Recycling Divertor Regimes

R. D. Smirnov
Abstract The low-recycling regimes of divertor operation in a single-null NSTX magnetic configuration are studied using computer simulations with the edge plasma transport code UEDGE. The edge plasma transport properties pertinent to the low-recycling regimes are demonstrated. These include the flux-limited character of the parallel heat transport and the high plasma temperatures with the flattened profiles in the scrape-off-layer. It is shown that to maintain the balance of particle fluxes at the core interface the deuterium gas puffing rate should increase as the divertor recycling coefficient decreases. The radial profiles of the heat load to the outer divertor plate, the upstream radial plasma profiles, and the effects of the cross-field plasma transport in the low-recycling regimes are discussed. It is also shown that recycling of lithium impurities evaporating from the divertor plate at high surface temperatures can reverse the low-recycling divertor operational regime to the high-recycling one and may cause thermal instability of the divertor plate (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Turbulent Dynamics of Beryllium Seeded Plasmas at the Edge of Tokamaks

R.V. Shurygin
Abstract Numerical simulation of turbulent MHD dynamics of beryllium seeded plasmas at the edge of tokamaks is performed. The model is based on the 4-fluid {,, n, pe, pi } reduced nonlinear Braginsky's MHD equations. Neutral hydrogen flow from the wall is described with a diffusion model. Beryllium line radiation is taken into consideration. The Be ion distribution over ionization states is calculated using the reduced model. Electron impact ionization, three body, photo- and dielectronic recombination and charge-exchange with neutral hydrogen are taken into account. Coronal equilibrium is not supposed. Simulations are performed for T-10 parameters. Radial distributions of averaged temperatures and their fluctuation levels, species flows, impurity radiation power, and impurity ions concentrations are obtained as functions of the Be concentration at the wall. The impurity radiation is shown to act on the turbulent oscillation level significantly if the total Be concentration at the wall exceeds 3 · 1011cm,3. The impurity turbulent transversal flow is directed inward and exceeds neoclassical flow significantly. The parallel conductivity and, as a consequence, turbulent transport are increased significantly by impurity radiation. The radiation loss dependence on the neutral Hydrogen concentration at the wall is also examined. The hydrogen concentration increasing the plasma density also rises. The relative beryllium concentration decreases. In total, these two effects are compensated, and the level of radiation losses is changed insignificantly (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Stabilization of Radiation-Condensation Instability by Light Impurity Injection

A. A. Pshenov
Abstract As it has been shown in [1,2], Radiation-Condensation Instability (RCI) may initiate Microfaceted Asymmetric Radiation from the Edge (MARFE) in tokamaks (see also review papers [3-5]). Nevertheless, experiments demonstrate the stable regimes with strongly radiated edge plasmas after Ne injection [6-8] or in siliconized discharges. Two effects destabilize radiative plasmas, the decrease of radiation losses Q with the electron temperature Te increase, and the increase of Q with electron and impurity densities rise. The finite relaxation time of impurity distribution over ionization states [6] as well as the thermal force acting on the growth rate doesn't shift the instability margin. Hence, one can examine the stability margin using the approximation of the coronal equilibrium. Radiation losses of intrinsic impurities like beryllium, carbon and nitrogen usually decrease with the temperature increase at the temperature range typical for the edge (see Fig. 1, curve 1). The situation may be significantly different for impurity mixtures. Radiation losses L , Q /(nenI)normalized by electron and impurity densities ne and nI for the mixture of carbon and neon are shown in Fig. 1, curves 2-5. One can see that ,Q/,T > 0 for practically any temperature at the edge if the concentration ratio nNe/nC , 5. Hence, one can expect the stabilization of RCI by injection of additional impurity and achievement of stable regime with the strongly radiated edge plasmas. The stability of plasmas with few impurity mixtures is examined in the present paper numerically (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Accumulation Process of High-Z Impurity in Toroidal Rotating Tokamak Plasma

K. Hoshino
Abstract The accumulation process of high-Z impurity in toroidal rotating tokamak plasma is investigated. A new inward pinch of high-Z impurity due to the ionization/recombination processes is derived using an analytic model. This inward pinch is driven by the large deviation of a drift orbit from a magnetic surface and the resultant variation of the charge state along the drift orbit. The pinch velocity increases with increasing toroidal rotation speed in both the co-direction and the ctr-direction. The inward pinch expected by the analytic model is really observed in the numerical simulation using the IMPGYRO code (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dependency of Tritium Retention in Graphite on Temperature Control of Molecular Dynamics

A. Ito
Abstract We have investigated the carbon plasma facing material and hydrogen atom interaction by the use of molecular dynamics simulation to clarify chemical erosion processes on divertor plate. The present paper is our first try at elucidation of temperature dependence by the molecular dynamics. Temperature was controlled by using Langevin thermostat method. As a result, the retention of hydrogen atom achieve steady state, and the CH4 was generated, which was not found MD simulations without a temperature control method. About 30 percent of injected hydrogen atoms are retained. CH4 yields has a peak at 600 K, which accords with experimental results. A dominant path of CH4 generation found by the present molecular dynamics simulation is as follows: a CH is detached from eroded surface and then it grows into CH4 adsorbing hydrogen atoms via CH2 and CH3. In addition, we propose the problem that the hydrogen atom retention and CH,, yields depend on the thermal relaxation time in MD simulation using temperature control methods (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Magnetohydrodynamic Simulation of Tungsten Wire in Wire-Array Z Pinch

D.-K. Kim
Abstract The magnetohydrodynamic behavior of tungsten wire ablating in wire-array Z pinch discharge on MAGPIE is simulated in a two-dimensional fine-grid domain using the GORGON code. A nonideal resistivity model has been implemented in the simulation to obtain plasma transport coefficients in the high density regime along with a screened hydrogenic model to calculate the radiative cooling. Starting from the initial state of warm dense plasma, the evolution of ablated wire is demonstrated to show its explosion and implosion dynamics as a function of discharge time and then the computed profile of electron density is compared with the contour lines reproduced from the measurement by a laser interferometer during the early stage of discharge. The comparison overall shows a fair agreement in terms of the magnitude and the profile shape while some discrepancies can be attributed to the simplified description of the internal wire core physics (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Chemical-Picture-Based Modeling of Thermodynamic Properties of Dense Multicharged-Ion Plasmas Using the Superconfiguration Approach

P.A. Loboda
Abstract Using the chemical-picture representation of plasmas as a mixture of various ions and free electrons, a consistent description of thermodynamics of dense multicharged-ion plasmas is being developed that involves the effects of Coulomb non-ideality and degeneracy of plasma electrons; contribution of the excited ion states (on the base of the superconfiguration approach) that may exist under an appropriate truncation of ion energy spectra due to plasma effects; hard-sphere-model representation of the finite-volume effects of plasma ions with the model parameters (effective ion sizes) corresponding to superconfigurations yielding the greatest contribution to partition functions. We present the calculated data for average ionization, Grüneisen coefficient, and specific heat of aluminum and iron plasmas at temperatures of 0.03,3 keV and densities 10,3 , 10,5 of their normal material densities. Calculated thermodynamic functions and shock Hugoniots are compared with other theoretical and experimental data (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Composition of Partially Ionized Systems Using the PlanckLarkin Partition Function of Mid-Z ions

A. Sengebusch
Abstract We report on the composition of warm, dense plasmas of M-shell ions. Assuming local thermal equilibrium, the abundance of different ionization stages is given by a set of coupled Saha-equations. In order to avoid discontinuities at the Mott density, the partition functions have to account for pressure ionization due to continuum lowering consistently. The Planck-Larkin renormalization of bound and scattering states is well elaborated for hydrogenic systems. This paper shows the consistent extension to moderately ionized Mid-Z elements. We present results for solid-density polymere and titanium plasmas (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Big Consequences of Small Changes (Non-locality and non-linearity of Hartree-Fock equations)

M.Ya. Amusia
Abstract It is demonstrated that non-locality and non-linearity of Hartree-Fock equations dramatically affect the properties of their solutions that essentially differ from solutions of Schrödinger equation with a local potential. Namely, it acquires extra zeroes, has different coordinate asymptotic, violates so-called gauge-invariance, has different scattering phases at zero energy, has in some cases several solutions with the same set of quantum numbers, usually equivalent expressions of current and Green's functions became non-equivalent. These features result in a number of consequences for probabilities of some physical processes, leading e. g. to extra width of atomic Giant resonances and enhance considerably the ionization probability of inner atomic electrons by a strong field (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Equation of State of Strongly Coupled Quark,Gluon Plasma , Path Integral Monte Carlo Results

V.S. Filinov
Abstract A strongly coupled plasma of quark and gluon quasiparticles at temperatures from 1.1Tc to 3Tc is studied by path integral Monte Carlo simulations. This method extends previous classical nonrelativistic simulations based on a color Coulomb interaction to the quantum regime. We present the equation of state and find good agreement with lattice results. Further, pair distribution functions and color correlation functions are computed indicating strong correlations and liquid-like behavior (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Collisionless Plasma Expansion in the Presence of a Dipole Magnetic Field

H. B. Nersisyan
Abstract The collisionless interaction of an expanding high,energy plasma cloud with a magnetized background plasma in the presence of a dipole magnetic field is examined in the framework of a 2D3V hybrid (kinetic ions and massless fluid electrons) model. The retardation of the plasma cloud and the dynamics of the perturbed electromagnetic fields and the background plasma are studied for high Alfvén,Mach numbers using the particle,in,cellmethod. It is shown that the plasma cloud expands excluding the ambient magnetic field and the background plasma to form a diamagnetic cavity which is accompanied by the generation of a collisionless shock wave. The energy exchange between the plasma cloud and the background plasma is also studied and qualitative agreement with the analytical model suggested previously is obtained (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dynamics of Dislocations in a 2D Plasma Crystal

V. Nosenko
Abstract Recent experimental results on the dislocation dynamics in a two-dimensional plasma crystal are reviewed. A single layer of micron-size microspheres was suspended in the sheath of a capacitively coupled rf discharge. The particles self-organized in a triangular lattice. Edge dislocations were created in pairs in this plasma crystal when the internal shear stress built up above a threshold, due to the crystal's slow differential rotation. Basic stages of dislocation nucleation were identified and studied, from gradual pile-up of shear strain in the crystal to eventual escape of free dislocations. After nucleation, dislocations moved supersonically with respect to the shear waves and generated shear-wave Mach cones (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Characterization of a 2.8 kJ Small Plasma Focus Using a Five Phase Radiative Model

Sh. Al-Hawat
Abstract A radiative five-phase plasma focus model (axial phase, inward radial phase, reflected shock phase, radiative phase and expanded phase) was applied to a 2.8 kJ plasma focus device to find the structure of the plasma focus formation and to calculate the plasma parameters and emitted radiation from the plasma pinch. To verify the model, the radiation probability of such device in neon plasma has been studied; a linear approximation method was applied by a FORTRAN program which has been written for this purpose. The theoretical and experimental results of the temporal development of current and voltage at 0.9 mbar of neon and spike voltages at different filling gas pressures were obtained and compared. In addition to that our plasma focus (PF) device was compared with different PF devices in relation to the plasma energy density and the drive parameter (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Investigation of the Start Transient in a Hall Thruster

H. Liu
Abstract A two dimensional axisymmetric fully kinetic Particle-in-Cell/Monte Carlo Collision (PIC-MCC) model is used to describe the ignition process in a Hall thruster. A current peak and latter the periodic oscillation of current and electric potential are found. The corresponding evolutions of plasma density, electric potential and atom density during the ignition process are introduced in the paper. In addition, influences of mass flow rate and discharge potential on current peak are modeled and analyzed. The simulated results are consistent with former experimental results. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effect of Oscillating Sheath on Near-Wall Electron Current Profile in Hall Thrusters

D. R. Yu
Abstract The radial profile of the axial electron current in Hall thrusters was measured; however, the significant decay of the current density peak in the near-wall region can't be explained effectively by the steady sheath theory. As the sheath exhibits an oscillating character when the thruster is in operation, its effect on the near-wall current profile has been studied in this paper. To obtain a comprehensive knowledge, we have considered a wide sheath oscillation frequency span which includes two asymptotic frequency cases at high and low ends. Based on the case studied, either an analytical treatment or a numerical simulation is applied. The results show that the current density peak has a fastest damping speed away from the wall at the asymptotic low frequency. With the increase of the frequency, both the decay and the spatial "wavelength" of the current profile decrease. The decay finally disappears at the asymptotic high frequency with a constant spatial "wavelength". Moreover, the sheath oscillation amplitude can enhance the decay and enlarge the spatial "wavelength". Taking into account of the realistic situation in Hall thrusters, the significant impact of the oscillating sheath on the near-wall electron current profiles can be anticipant. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Using the Steepened Plasma Profile and Wave Breaking Threshold in Laser-Plasma Interaction

P. Zobdeh
Abstract In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self,focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new profile. We show the amplitude and period of electrical field oscillation are increased by reducing the steepened scale length. Also our numerical results identify that by reducing the steepened scale length, the electrical field is increased to wave breaking threshold limit. This high gradient electrical field causes the effective beam loading during the wave breaking phenomenon. The wave breaking can be the initial point for other acceleration regime as cavity or channel guiding regime. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]