Volume 11, Issue 10, October 2004
 LETTERS


Anomalous inhibition of electron transport in laser–matter interaction at subrelativistic intensities
View Description Hide DescriptionElectron transport in femtosecond laserirradiated solid targets is investigated by means of onedimensional particleincell simulations that include a model of collisional ionization, binary collisions and field ionization, while treating ions as individual particles. In particular, heat and particle fluxes in conductor and insulator targets are compared at the onset of relativistic laser intensities, i.e., at . Simulations show that fast electrons generate a longitudinal electric field of the order in the bulk material that acts to inhibit heat flux in insulators, while the electric fields observed in metals are weaker and electrons penetrate deeper into the target. The bulk heat transport is found to be similar in both materials and mainly Spitzerlike, with a noticeable contribution by fast electrons.

Refraction effects on the cavity formation and interaction of an intense ultrashort laser pulse with a gas jet
View Description Hide DescriptionFormation of a plasma cavity with a shock wave in gas jetsirradiated by tightly focused femtosecond laser pulses causes the wave break of the laser wake field at the front of the shock wave and, as a result, the injection of electrons into the acceleration phase of the wakefield wave. A strong crescentlike deformation of the cavity and a change in electron signal are observed with gas density growth. It is attributed to a mutual effect of the cavity on the laser pulse propagation and break of the plasma wake field due to refraction of the laser pulse.

Interface velocity of laser shocked targets
View Description Hide DescriptionThe interface velocity of iron partially releasing into a lithium fluoride (LiF) window has been measured using VISAR technique. Corrections to the fringepervelocity relationship are presented. A good agreement with hydrodynamic simulations is found at laser intensities . For higher intensities, velocities appear to be lower than the numerical predictions. A strong modification of compressed LiF index of refraction behavior might explain such a discrepancy.

Comparison of a copper foil to a copper wirearray Z pinch at
View Description Hide DescriptionResults from the first solid foil implosion on the Z accelerator are reported. The foil implosion is compared to a 300wirearray implosion with the same material and the same diameter, height, and total mass. Though both the foil and the array produced comparable xray yields, the array’s radiation burst was twice as powerful and half as long as the foil’s. These data along with xray backlighting images and inductance measurements suggest that the foil implosion was more unstable than the wirearray implosion.
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 ARTICLES

 Lasers, Particle Beams, Accelerators, Radiation Generation

Comparative analysis of gyrotron backwardwave oscillators operating at different cyclotron harmonics
View Description Hide DescriptionA comparative analysis between the fundamental and second cyclotron harmonics of gyrotron backwardwave oscillators (gyroBWOs) is presented. The simulation results reveal that nonlinear field contraction is a common feature for both harmonic interactions. Besides, the electron transit angle, used to characterize the axial modes of the fundamental harmonic mode at the startoscillation conditions, is found to be applicable even for the second harmonic mode. Each axial mode of either the fundamental harmonic or the second harmonic modes is maintained at a constant value of the electron transit angle while changing the operating parameters, such as magnetic field and beam voltage. Extensive numerical calculations are conducted for the startoscillation currents and tuning properties. Moreover, singlemode operating regimes are suggested where the second harmonic gyroBWO could generate a considerable output power, comparing with the fundamental harmonic gyroBWO.
 Magnetically Confined Plasmas, Heating, Confinement

Transport reduction by shear flows in dynamical models
View Description Hide DescriptionThe reduction in the transport of particles and heat by a strong mean shear flow is studied, in the context of interchange and iontemperature gradientturbulencemodels. Compared to passive scalar transport, a stronger reduction in the transport (scaling with the shearing rate as ) results from a severe reduction in the amplitude of turbulent velocity in both models. However, the cross phase is only modestly reduced, as in the scalar field case. These results are in qualitative agreement with the results from both gyrokinetic and gyrofluid simulations of toroidal iontemperature gradientturbulence [Lin et al., Phys. Rev. Lett. 83, 3645 (1999); Falchetto and Ottaviani, Phys. Rev. Lett. 92, 025002 (2004)], but contradict recent claims in some literature, highlighting the importance of the detailed properties of the flow in determining the overall transport level.
 Lasers, Particle Beams, Accelerators, Radiation Generation

Space charge limited flow of a thin electron beam confined by a strong magnetic field
View Description Hide DescriptionAn approximate analytic theory is developed and implemented numerically for calculating the space charge limited current and electric field of a thin cylindrical beam or currentsheet between two wide parallel electrodes. The flow is confined by a sufficiently strong magnetic field. Assuming that the potential and current density are almost homogeneous in the direction transversal to the flow, the beam current and its profile are computed by a variational method. The average current density scales as the reciprocal of the beam width when the latter becomes very small. The total cylindrical beam current thus decreases proportionly to its diameter while the total current of a sheet becomes almost independent of the width in this regime.
 Magnetically Confined Plasmas, Heating, Confinement

Statistical investigation of transport barrier effects produced by biasing in a nonfusion magnetoplasma
View Description Hide DescriptionThe effects of a biasing potential on the statistical properties of the fluctuating flux, particle density and radial velocitytime series in the nonfusion magnetoplasma Thorello [C. Riccardi, D. Xuantong, M. Salierno, L. Gamberale, and M. Fontanesi, Phys. Plasmas 4, 3749 (1997)] are studied. The shape evolution of the probability distribution function (PDF) of the temporal coarsegrained fluctuating flux is investigated as a function of the radial position in the plasma equatorial plane, for different values of the bias potential applied to a limiter. A reduction of cross correlations between density and radial velocity, concomitant with an increase in the shearing rate of the radial electric field, is found near the center of the plane as a function of the bias potential. The partial densityvelocity cross correlations, evaluated at different time scales, reach their asymptotic values for times of the order of those describing the change in shape of the fluctuating flux PDF. The magnitude of the shearing rate of the radial electric field, calculated at different radial positions in the plane, is consistent with the expected decorrelation mechanism of turbulent transport.
 Inertially Confined Plasmas, Dense Plasmas, Equations of State

Effect of current rate on energy deposition into exploding metal wires in vacuum
View Description Hide DescriptionThis paper presents direct experimental proof of a significant increase of energy deposition into a metal core before voltage breakdown with the current rate for nanosecond exploding wires in a vacuum. This effect is demonstrated for nine different refractory and nonrefractory metals. The strongest influence of current rate was demonstrated for tungstenwires. Increasing the current rate from 20 to changes the wire core from a solid to a clusterlike state. For nonrefractory metals such as , , , and , fast explosion allows deposition inside a metal core 1.5–2.9 times the atomization enthalpy before voltage breakdown. The slow explosion, with , gives 2–3 times less energy deposition before voltage breakdown than the fastexplosion mode. The currentrate effect is important for optimization of wire ablation, reduction of the mass left behind in the wirearray load, and final xray yield in modern multiMA wirearray Zpinch facilities.
 Magnetically Confined Plasmas, Heating, Confinement

Electron Bernstein wave generation in a linear plasma system
View Description Hide DescriptionElectron Bernstein wave(EBW)generation via mode conversion is observed in a short cylindrical electron cyclotron resonance (ECR) plasma system. ECR plasma with input microwave power varying from is produced at an operating pressure of with hydrogen gas with a maximum plasma density of at the center and plasma temperature of . The axial magnetic field required for ECR in the system is produced by two identical magnetic coils arranged in the Helmholtz configuration. The fundamental ECR surface is located at the geometrical center of the plasma system. Microwaves of from a magnetron in mode are launched radially to have extraordinary wave in plasma. The EBW is observed to be generated at the upper hybrid resonance layer at a distance of from the center towards the microwave source. The EBW is assumed to be generated through mode conversion which is inferred by the observation of the parametric decay of the incident wave. The experimental results pertaining to the EBW detection and characterization are discussed in this paper.
 Basic Plasma Phenomena, Waves, Instabilities

Magnetosonic modes with a beam of dust or secondary ions
View Description Hide DescriptionChargeddust particles on nearKeplerian orbits in planetary rings drift relative to the corotating background plasma, and this relative streaming may drive dustmodified magnetosonic waves unstable. Using a magnetofluid model, this situation is revisited, and it is shown that a correct description can be obtained at frequencies and Doppler shifts that are small compared to the electron gyrofrequency. An excellent agreement is reached between the analytical approximation and the numerical solution of the full or reduced dispersion law. A very lowfrequency, small wave number beam instability may occur at supermagnetosonic beam velocities of the dust component, but, based on the existing data, is unlikely to play a role in planetary rings. Instability is also found for submagnetosonic speeds, which might be relevant to Saturn’s E ring.

Spatially limited ion acoustic wave activity in lowpressure helicon discharges
View Description Hide DescriptionIon acoustic wave phenomena are studied and compared in two lowpressure argon discharges created by helicon sources. The wave amplitudes are spatially localized near the edge of a plasma column as the amplitudes of the “mirror waves” that are separated from the helicon source frequency by the ion wave frequency. Dependencies of the ion wave on radial position, pressure, input power, and magnetic field are investigated. Measurements of the wavelength show that the wave is traveling azimuthally at approximately the ion sound speed in the direction of electron gyration. Although the wave spectra are indicative of a parametric decay phenomenon, it seems more likely that the radial plasma pressure gradient drives an ion acoustic instability which then modulates the helicon source.
 Magnetically Confined Plasmas, Heating, Confinement

Rotational stability of plasma blobs
View Description Hide DescriptionThe stability of plasma blobs which have both density and temperature higher than the surrounding plasma, and can transport heat as well as particles, is considered. It is shown that the internal blob temperature profile can drive azimuthal rotation or spin about the blob axis, which produces a robust rotational instability in the interchange limit . The theory includes the effects of the centrifugal and Coriolis forces, the sheared velocity , and the axial sheathboundary condition. Estimates show that finiteLarmorradius stabilization is ineffective, but the sheath conductivity can be strongly stabilizing. The blob rotational instability has only a small direct impact on the particle and energy transport, but it serves as a useful diagnostic for the underlying blob spin, which is an important variable in determining the blob’s radial velocity. A separate branch of temperaturegradientdrivensheathinstabilities, predicted in the eikonal limit, is not observed for low mode numbers.

Improved twopoint model for limiter scrapeoff layer
View Description Hide DescriptionAn analytical model for a limiter scrapeoff layer (SOL) is proposed, which takes selfconsistently into account both conductive and convective contributions to the heat transport in SOL. The particle flows in the SOL main part are determined by considering the recycling of neutrals. The model allows us to interpret the results of numerical simulation by the code EMC3EIRENE [Y. Feng, F. Sardei, P. Grigull, K. McCormick, J. Kisslinger, D. Reiter, and Y. Igitkhanov, Plasma Phys. Controlled Fusion 44, 611 (2002)] for the edge region of Tokamak Experiment for Technology Oriented Research (TEXTOR) [Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (Institute for Electrical and Electronics Engineers, Piscataway, NJ, 1995), p. 470].

Damping of Alfvén eigenmodes on localized electrons in stellarators
View Description Hide DescriptionThe electron collisional damping of various kinds of Alfvén eigenmodes (AE) in stellarators is studied. Simple expressions for the damping rates of various welllocalized AEs are obtained and applied to evaluating the damping rates in the Helias reactor [H. Wobig et al., Fusion Energy 1998, 17th Conference Proceedings, Yokohama, Japan (IAEA, Vienna, 1999), Vol. 4, p. 1235] and quasipoloidal stellarator [J. F. Lyon et al., Fusion Energy 2002, 19th Conference Proceedings, Lyon, France (IAEA, Vienna, 2002), Paper IC/P05]. The estimates made indicate that the collisional damping prevents the destabilization of AEs localized at the plasma periphery of the Helias reactor. A new mechanism that can considerably affect the collisional damping of Alfvén waves in stellarators is predicted: it is found that under certain conditions the collisionless orbit transformations between the locally trapped states and locally passing states significantly change the damping rate, so that the latter becomes weakly (logarithmically) dependent on the collision frequency.
 Lowtemperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Fluctuation threshold and profile resilience in weakly ionized plasma in a curved, unsheared magnetic field
View Description Hide DescriptionA study of electrostaticfluctuations in a weakly ionized plasma confined by a purely toroidalmagnetic field is presented. At low field the plasma is quiescent and transport is provided by stationary flows along open equipotential surfaces. For above a certain threshold, strong electrostaticfluctuations are excited, and the electron pressure profile on the lowfield side is resilient with scale length much shorter than the threshold scale length for flute instability. At threshold the fluctuations exhibit the signatures of a monochromatic drift mode, and above threshold they are dominated by flute modes growing on the lowfield side. Threshold behavior and profile resilience are explained as a twostage process, where drift waves act as a seed for the unstable flute modes, and where a negative feedback mechanism involving anomalous transport regulates the electron pressure profile to maintain the imposed particle flux.
 Basic Plasma Phenomena, Waves, Instabilities

Microwavegenerated lowfrequency plasma wave excited in the periphery of the evanescent layer
View Description Hide DescriptionThe observation of microwavecreated lowfrequency electrostatic waves from the evanescent layer in a pulsed plasma discharge and effects of a weak magnetic field on its propagation characteristics are reported. The creation of the electrostatic wave depends on the density variations along the direction of microwave propagation. It is observable when a part of the microwave is reflected and another part of it eventually gets absorbed in the resonanceabsorption layers on two sides of the evanescent layer. The velocity of the wave is one order of magnitude higher than that of the ionacoustic wave. Under the application of a very weak magnetic field (affecting only the electrons) perpendicular to the direction of propagation of both the excited plasma waves and the microwaves, the waves are still observable only with altered velocity and characteristics. At higher magnetic fields the electrostatic waves are completely subdued under a continuous instability independent of presence or absence of the microwaves.
 Radiation: Emission, Absorption, Transport

Gold emissivities for hydrocode applications
View Description Hide DescriptionThe Radiom model [M. Busquet, Phys Fluids B 5, 4191 (1993)] is designed to provide a radiativehydrodynamic code with nonlocal thermodynamic equilibrium (nonLTE) data efficiently by using LTE tables. Comparison with benchmark data [M. Klapisch and A. BarShalom, J. Quant. Spectrosc. Radiat. Transf. 58, 687 (1997)] has shown Radiom to be inaccurate far from LTE and for heavy ions. In particular, the emissivity was found to be strongly underestimated. A recent algorithm, Gondor [C. Bowen and P. Kaiser, J. Quant. Spectrosc. Radiat. Transf. 81, 85 (2003)], was introduced to improve the gold nonLTE ionization and corresponding opacity. It relies on fitting the collisional ionization rate to reproduce benchmark data given by the Averroès superconfiguration code [O. Peyrusse, J. Phys. B 33, 4303 (2000)]. Gondor is extended here to goldemissivity calculations, with two simple modifications of the twolevel atom line source function used by Radiom: (a) a larger collisional excitation rate and (b) the addition of a Planckian source term, fitted to spectrally integrated Averroès emissivity data. This approach improves the agreement between experiments and hydrodynamic simulations.
 Nonlinear Phenomena, Turbulence, Transport

Wave damping as a critical phenomenon
View Description Hide DescriptionThe evolution of a monochromatic Langmuir wave in a collisionless plasma is studied using Vlasov simulations for a wide range of initial amplitudes. Three types of initial electron distributions are considered: Maxwellian, Lorentzian, and dilute warm Maxwellian plus dense cold component. It is shown that there exists a critical initial amplitude that separates the damping and nondamping asymptotic regimes. Depending on the initial amplitude there are three main types of asymptotic evolution: (i) monotonic linear Landau damping below the threshold, ; (ii) the critical case , when the field damps algebraically as ; (iii) at initial damping followed by a period of subsequent exponential growth and then irregular oscillations about a nonzero amplitude. This threshold is well described as a critical phenomenon, showing powerlaw dependencies on the distance from the threshold not only for field quantities, which are expected of secondorder phase transitions in thermodynamics, but also for temporal ones. The critical exponent for both the initial damping and growth phases differ from those expected if the threshold is caused by O’Neil’s particle trapping in the wave potential. However, trapping affects the critical exponents well above the threshold and explains the characteristic frequency of oscillations above the threshold. It is found that for a Maxwellian plasma the threshold amplitude corresponds to the condition that the trapping (bounce) frequency equals the modulus of the theoretical Landau damping rate ; at the threshold, . For Lorentzian and Maxwellianpluscold component plasmas this ratio is and , respectively. The temporal and the field scalings are thus interrelated, suggesting that the inclusion of the temporal dimension is vital for critical phenomena in collisionless plasmas, in contrast to thermodynamic systems where the very small characteristic time to achieve equilibrium removes time from the scaling.
 Ionospheric, Solarsystem, and Astrophysical Plasmas

Exact solutions for steady reconnective annihilation revisited
View Description Hide DescriptionThis work complements the previous studies on steady reconnective magnetic annihilation in three different geometries: the twodimensional Cartesian and polar ones and the threedimensional (3D) cylindrical one. A special class of diffusive solutions is found analytically in explicit form for all of the three geometries. In the 3D case it is extended to a much wider class of exact solutions describing reconnective magnetic annihilation at the separatrix spine line of a magnetic null point. One of the obtained solutions provides an explicit expression for the CraigFabling solution. It is also identified which of the steady flow regimes found are dynamically accessible.