Volume 21, Issue 7, July 2014
Index of content:

The cosmic magnetic fields in regions of low plasma pressure and large currents, such as in interstellar space and gaseous nebulae, are forcefree in the sense that the Lorentz force vanishes. The threedimensional ArnoldBeltramiChildress (ABC) field is an example of a forcefree, helical magnetic field. In fluid dynamics, ABC flows are steady state solutions of the Euler equation. The ABC magnetic field lines exhibit a complex and varied structure that is a mix of regular and chaotic trajectories in phase space. The characteristic features of field line trajectories are illustrated through the phase space distribution of finitedistance and asymptoticdistance Lyapunov exponents. In regions of chaotic trajectories, an ensembleaveraged variance of the distance between field lines reveals anomalous diffusion—in fact, superdiffusion—of the field lines. The motion of charged particles in the forcefree ABC magnetic fields is different from the flow of passive scalars in ABC flows. The particles do not necessarily follow the field lines and display a variety of dynamical behavior depending on their energy, and their initial pitchangle. There is an overlap, in space, of the regions in which the field lines and the particle orbits are chaotic. The time evolution of an ensemble of particles, in such regions, can be divided into three categories. For short times, the motion of the particles is essentially ballistic; the ensembleaveraged, mean square displacement is approximately proportional to t ^{2}, where t is the time of evolution. The intermediate time region is defined by a decay of the velocity autocorrelation function—this being a measure of the time after which the collective dynamics is independent of the initial conditions. For longer times, the particles undergo superdiffusion—the mean square displacement is proportional to t^{α} , where α > 1, and is weakly dependent on the energy of the particles. These superdiffusive characteristics, both of magnetic field lines and of particles moving in these fields, strongly suggest that theories of transport in threedimensional chaotic magnetic fields need a shift from the usual paradigm of quasilinear diffusion.
 LETTERS


Possible energy gain for a plasmalinerdriven magnetoinertial fusion concept
View Description Hide DescriptionA onedimensional parameter study of a MagnetoInertial Fusion (MIF) concept indicates that significant gain may be achievable. This concept uses a dynamically formed plasma shell with inwardly directed momentum to drive a magnetized fuel to ignition, which in turn partially burns an intermediate layer of unmagnetized fuel. The concept is referred to as Plasma Jet MIF or PJMIF. The results of an adaptive mesh refinement Eulerian code (Crestone) are compared to those of a Lagrangian code (LASNEX). These are the first published results using the Crestone and LASNEX codes on the PJMIF concept.

LH transition dynamics in fluid turbulence simulations with neoclassical force balance
View Description Hide DescriptionSpontaneous transport barrier generation at the edge of a magnetically confined plasma is reproduced in fluxdriven threedimensional fluid simulations of electrostatic turbulence. Here, the role on the radial electric field of collisional friction between trapped and passing particles is shown to be the key ingredient. Especially, accounting for the selfconsistent and precise dependence of the friction term on the actual plasma temperature allows for the triggering of a transport barrier, provided that the input power exceeds some threshold. In addition, the barrier is found to experience quasiperiodic relaxation events, reminiscent of edge localised modes. These results put forward a possible key player, namely, neoclassical physics via radial force balance, for the low to highconfinement regime transition observed in most of controlled fusion devices.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Electron acoustic wave driven vortices with nonMaxwellian hot electrons in magnetoplasmas
View Description Hide DescriptionLinear dispersion characteristics of the Electron Acoustic Wave (EAW) and the corresponding vortex structures are investigated in a magnetoplasma in the presence of nonMaxwellian hot electrons. In this regard, kappa and Cairns distributed hot electrons are considered. It is noticed that the nonthermal distributions affect the phase velocity of the EAW. Further, it is found that the phase velocity of EAW increases for Cairns and decreases for kappa distributed hot electrons. Nonlinear solutions in the form of dipolar vortices are also obtained for both stationary and nonstationary ions in the presence of kappa distributed hot electrons and dynamic cold electrons. It is found that the amplitude of the nonlinear vortex structures also reduces with kappa factor like the electron acoustic solitons.

Triple plasmon resonance of bimetal nanoshell
View Description Hide DescriptionIn this paper, light absorption spectra properties of a bimetal multilayer nanoshell based on quasistatic approach are investigated. Comparing with silverdielectricsilver and silverdielectricgold nanoshells, golddielectricsilver nanoshells have three intense and separated plasmon peaks which are more suitable for multiplex biosensing. Calculations show that relatively small thickness of outer silver shell and large dielectric constant of middle dielectric layer of golddielectricsilver nanoshell are suitable to obtain the triple plasmon resonance.

Vlasov multidimensional model dispersion relation
View Description Hide DescriptionA hybrid model of the Vlasov equation in multiple spatial dimension D > 1 [H. A. Rose and W. Daughton, Phys. Plasmas 18, 122109 (2011)], the Vlasov multi dimensional model (VMD), consists of standard Vlasov dynamics along a preferred direction, the z direction, and N flows. At each z, these flows are in the plane perpendicular to the z axis. They satisfy Euleriantype hydrodynamics with coupling by selfconsistent electric and magnetic fields. Every solution of the VMD is an exact solution of the original Vlasov equation. We show approximate convergence of the VMD Langmuir wave dispersion relation in thermal plasma to that of VlasovLandau as N increases. Departure from strict rotational invariance about the z axis for small perpendicular wavenumber Langmuir fluctuations in 3D goes to zero like θ ^{N} , where θ is the polar angle and flows are arranged uniformly over the azimuthal angle.

Jeans self gravitational instability of strongly coupled quantum plasma
View Description Hide DescriptionThe Jeans selfgravitational instability is studied for quantum plasma composed of weakly coupled degenerate electron fluid and nondegenerate strongly coupled ion fluid. The formulation for such system is done on the basis of two fluid theory. The dynamics of weakly coupled degenerate electron fluid is governed by inertialess momentum equation. The quantum forces associated with the quantum diffraction effects and the quantum statistical effects act on the degenerate electron fluid. The strong correlation effects of ion are embedded in generalized viscoelastic momentum equation including the viscoelasticity and shear viscosities of ion fluid. The general dispersion relation is obtained using the normal mode analysis technique for the two regimes of propagation, i.e., hydrodynamic and kinetic regimes. The Jeans condition of selfgravitational instability is also obtained for both regimes, in the hydrodynamic regime it is observed to be affected by the ion plasma oscillations and quantum parameter while in the kinetic regime in addition to ion plasma oscillations and quantum parameter, it is also affected by the ion velocity which is modified by the viscosity generated compressional effects. The Jeans critical wave number and corresponding critical mass are also obtained for strongly coupled quantum plasma for both regimes.

Linear growth rates of resistive tearing modes with subAlfvénic streaming flow
View Description Hide DescriptionThe tearing instability with subAlfvénic streaming flow along the external magnetic field is investigated using resistive MHD simulation. It is found that the growth rate of the tearing mode instability is larger than that without the streaming flow. With the streaming flow, there exist two Alfvén resonance layers near the central current sheet. The larger perturbation of the magnetic field in two closer Alfvén resonance layers could lead to formation of the observed cone structure and can largely enhance the development of the tearing mode for a narrower streaming flow. For a broader streaming flow, a larger separation of Alfvén resonance layers reduces the magnetic reconnection. The linear growth rate decreases with increase of the streaming flow thickness. The growth rate of the tearing instability also depends on the plasma beta (β). When the streaming flow is embedded in the current sheet, the growth rate increases with β if β < βs, but decreases if β > βs. The existence of the specific value βs can be attributed to competition between the suppressing effect of β and the enhancing effect of the streaming flow on the magnetic reconnection. The critical value βs increases with increase of the streaming flow strength

The effect of plasma background on the instability of two nonparallel quantum plasma shells in whole K space
View Description Hide DescriptionIn this paper, quantum fluid equations together with Maxwell's equations are used to study the stability problem of nonparallel and nonrelativistic plasma shells colliding over a “background plasma” at arbitrary angle, as a first step towards a microscopic understanding of the collision shocks. The calculations have been performed for all magnitude and directions of wave vectors. The colliding plasma shells in the vacuum region have been investigated in the previous works as a counterstreaming model. While, in the presence of background plasma (more realistic system), the colliding shells are mainly nonparalleled. The obtained results show that the presence of background plasma often suppresses the maximum growth rate of instabilities (in particular case, this behavior is contrary). It is also found that the largest maximum growth rate occurs for the twostream instability of the configuration consisting of counterstreaming currents in a very dilute plasma background. The results derived in this study can be used to analyze the systems of three colliding plasma slabs, provided that the used coordinate system is stationary relative to the one of the particle slabs. The present analytical investigations can be applied to describe the quantum violent astrophysical phenomena such as white dwarf stars collision with other dense astrophysical bodies or supernova remnants. Moreover, at the limit of , the obtained results described the classical (sufficiently dilute) events of colliding plasma shells such as gammaray bursts and flares in the solar winds.

Feedbackassisted extension of the tokamak operating space to low safety factor^{a)}
View Description Hide DescriptionRecent DIIID and RFXmod experiments have demonstrated stable tokamak operation at very low values of the edge safety factor q(a) near and below 2. The onset of n = 1 resistive wall mode (RWM) kink instabilities leads to a disruptive stability limit, encountered at q(a) = 2 (limiter plasmas) and q 95 = 2 (divertor plasmas). However, passively stable operation can be attained for q(a) and q 95 values as low as 2.2. RWM damping in the q(a) = 2 regime was measured using active MHD spectroscopy. Although consistent with theoretical predictions, the amplitude of the damped response does not increase significantly as the q(a) = 2 limit is approached, in contrast with damping measurements made approaching the pressuredriven RWM limit. Applying proportional gain magnetic feedback control of the n = 1 modes has resulted in stabilized operation with q 95 values reaching as low as 1.9 in DIIID and q(a) reaching 1.55 in RFXmod. In addition to being consistent with the q(a) = 2 external kink mode stability limit, the unstable modes have growth rates on the order of the characteristic wall eddycurrent decay timescale in both devices, and a dominant m = 2 poloidal structure that is consistent with ideal MHD predictions. The experiments contribute to validating MHD stability theory and demonstrate that a key tokamak stability limit can be overcome with feedback.

Xray conversion of ultrashort laser pulses on a solid sample: Role of electron waves excited in the preplasma
View Description Hide DescriptionFlat silicon samples were irradiated with 40 fs, 800 nm laser pulses at an intensity at the best focus of 2·10^{18} Wcm^{−2}, in the presence of a preplasma on the sample surface. Xray emission in the spectral range from 2 to 30 keV was detected inside and outside the plane of incidence, while varying preplasma scale length, laser intensity, and polarization. The simultaneous detection of 2ω and 3ω/2 emission allowed the contributions to the Xray yield to be identified as originating from laser interaction with either the nearcritical density (nc) region or with the nc/4 region. In the presence of a moderate preplasma, our measurements reveal that, provided the preplasma reaches a scalelength of a few laser wavelengths, Xray emission is dominated by the contribution from the interaction with the under dense plasma, where electron plasma waves can grow, via laser stimulated instabilities, and, in turn, accelerate free electrons to high energies. This mechanism leads also to a clear anisotropy in the angular distribution of the Xray emission. Our findings can lead to an enhancement of the conversion efficiency of ultra short laser pulses into Xrays.

On the Debye–Hückel effect of electric screening
View Description Hide DescriptionThe paper considers nonlinear selfconsistent electric potential equation (Sec. I ), due to a cloud made of a single species of electric charges, satisfying a Boltzmann distribution law (Sec. II ). Exact solutions are obtained in a simple logarithmic form, in three cases: (Sec. III ) spherical radial symmetry; (Sec. IV ) plane parallel symmetry; (Sec. V ) a special case of azimuthalcylindrical symmetry. All these solutions, and their transformations (Sec. VI ), involve the DebyeHückel radius; the latter was originally defined from a solution of the linearized selfconsistent potential equation. Using an exact solution of the selfconsistent potential equation, the distance at which the potential vanishes differs from the DebyeHückel radius by a factor of . The preceding (Secs. II–VI ) simple logarithmic exact solutions of the selfconsistent potential equations involve no arbitrary constants, and thus are special or singular integrals not the general integral. The general solution of the selfconsistent potential equation is obtained in the plane parallel case (Sec. VII ), and it involves two arbitrary constants that can be reduced to one via a translation (Sec. VIII ). The plots of dimensionless potential (Figure 1), electric field (Figure 2), charge density (Figure 3), and total charge between ζ and infinity (Figure 4), versus distance normalized to DebyeHückel radius ζ ≡ z/a, show that (Sec. IX ) there is a continuum of solutions, ranging from a charge distribution concentrated inside the DebyeHückel radius to one spreadout beyond it. The latter case leads to the limiting case of logarithmic potential, and stronger electric field; the former case, of very concentrated charge distribution, leads to a fratricide effect and weaker electric field.

On transition from Alfvén resonance to forced magnetic reconnection
View Description Hide DescriptionWe revisit the transition from Alfvén resonance to forced magnetic reconnection with a focus on the property of their singularities. As the driven frequency tends to zero, the logarithmic singularity of Alfvén resonance shifts to the powerlaw singularity of forced reconnection, due to merging of the two resonance layers. The transition criterion depends on either kinetic effects or dissipations that resolve the singularity. As an example, a small but finite resistivity is introduced to investigate the transition process. The transition threshold is then obtained as the driven frequency reaches a level of .

Strong selffocusing of a coshGaussian laser beam in collisionless magnetoplasma under plasma density ramp
View Description Hide DescriptionThe effect of plasma density ramp on selffocusing of coshGaussian laser beam considering ponderomotive nonlinearity is analyzed using WKB and paraxial approximation. It is noticed that coshGaussian laser beam focused earlier than Gaussian beam. The focusing and defocusing nature of the coshGaussian laser beam with decentered parameter, intensity parameter, magnetic field, and relative density parameter has been studied and strong selffocusing is reported. It is investigated that decentered parameter “b” plays a significant role for the selffocusing of the laser beam as for , strong selffocusing is seen. Further, it is observed that extraordinary mode is more prominent toward selffocusing rather than ordinary mode of propagation. For , with the increase in the value of magnetic field selffocusing effect, in case of extraordinary mode, becomes very strong under plasma density ramp. Present study may be very useful in the applications like the generation of inertial fusion energy driven by lasers, laser driven accelerators, and xray lasers. Moreover, plasma density ramp plays a vital role to enhance the selffocusing effect.

Conditions for reflection and transmission of an ion acoustic soliton in a dusty plasma with variable charge dust
View Description Hide DescriptionModified Kortewegde Vries (mKdV) equations are derived for the incident, reflected, and transmitted waves in order to examine the soliton reflection and its transmission through an inhomogeneous plasma comprising ions, dust grains with fluctuating charge and two types of electrons, namely nonisothermal electrons and isothermal electrons. All the mKdV equations are coupled at the point of reflection and solved for the reflected soliton. Unlike others, a relation is established between the velocity shifts of the incident, reflected and transmitted solitons, and based on a critical value of the shift of incident soliton the strengths of the soliton reflection and transmission are talked about. Conditions are obtained for the soliton reflection and its transmission, and a comparative study is made for the two cases of fixed and fluctuating charges on the dust grains.

Selffield effects on instability of wave modes in a twostream freeelectron laser with an axial magnetic field
View Description Hide DescriptionFree electron lasers (FEL) play major roles in the Raman Regime, due to the charge and current densities of the beam selffield. The method of perturbation has been applied to study the influence of selfelectric and selfmagnetic fields. A dispersion relation for twostream free electron lasers with a helical wiggler and an axial magnetic field has been found. This dispersion relation is solved numerically to investigate the influence of selffields on the FEL coupling and the twostream instability. It was found that selffields can produce very large effects on the FEL coupling, but they have almost negligible effects on twostream instability.

Surface electromagnetic wave equations in a warm magnetized quantum plasma
View Description Hide DescriptionBased on the singlefluid plasma model, a theoretical investigation of surface electromagnetic waves in a warm quantum magnetized inhomogeneous plasma is presented. The surface electromagnetic waves are assumed to propagate on the plane between a vacuum and a warm quantum magnetized plasma. The quantum magnetohydrodynamic model includes quantum diffraction effect (Bohm potential), and quantum statistical pressure is used to derive the new dispersion relation of surface electromagnetic waves. And the general dispersion relation is analyzed in some special cases of interest. It is shown that surface plasma oscillations can be propagated due to quantum effects, and the propagation velocity is enhanced. Furthermore, the external magnetic field has a significant effect on surface wave's dispersion equation. Our work should be of a useful tool for investigating the physical characteristic of surface waves and physical properties of the bounded quantum plasmas.

Reconnection properties in collisionless plasma with open boundary conditions
View Description Hide DescriptionCollisionless magnetic reconnection in a Harris current sheet with different initial thicknesses is investigated using a D Darwin particleincell simulation with the magnetosonic open boundary condition. It is found that the thicknesses of the ion dissipation region and the reconnection current sheet, when the reconnection rate reaches its first peak, are independent of the initial thickness of the current sheet; while the peak reconnection rate depends on it. The peak reconnection rate increases with decrease of the current sheet thickness as , where is the initial current sheet halfthickness.

An axially propagating twostream instability in the Hall thruster plasma
View Description Hide DescriptionCollective Thomson scattering experiments reveal the presence of highfrequency, axial electron density fluctuations at millimetric wavelengths in the Hall thruster plasma. The properties of these fluctuations are investigated experimentally and via linear kinetic theory. The relative drift of electrons and ions in the axial direction is found to be insufficient to cause excitation of the observed mode. Instead, the mode is determined to be a twostream instability arising due to the velocity difference between singly and doubly charged ion populations in the plume.

Iontemperaturegradient sensitivity of the hydrodynamic instability caused by shear in the magneticfieldaligned plasma flow
View Description Hide DescriptionThe crossmagneticfield (i.e., perpendicular) profile of ion temperature and the perpendicular profile of the magneticfieldaligned (parallel) plasma flow are sometimes inhomogeneous for space and laboratory plasma. Instability caused either by a gradient in the iontemperature profile or by shear in the parallel flow has been discussed extensively in the literature. In this paper, (1) hydrodynamic plasma stability is investigated, (2) real and imaginary frequency are quantified over a range of the shear parameter, the normalized wavenumber, and the ratio of densitygradient and iontemperaturegradient scale lengths, and (3) the role of inverse Landau damping is illustrated for the case of combined iontemperature gradient and parallelflow shear. We find that increasing the iontemperature gradient reduces the instability threshold for the hydrodynamic parallelflow shear instability, also known as the parallel KelvinHelmholtz instability or the D'Angelo instability. We also find that a kinetic instability arises from the coupled, reinforcing action of both freeenergy sources. For the case of comparable electron and ion temperature, we illustrate analytically the transition of the D'Angelo instability to the kinetic instability as (a) the shear parameter, (b) the normalized wavenumber, and (c) the ratio of densitygradient and iontemperaturegradient scale lengths are varied and we attribute the changes in stability to changes in the amount of inverse ion Landau damping. We show that near a normalized wavenumber k ⊥ ρi of order unity (i) the real and imaginary values of frequency become comparable and (ii) the imaginary frequency, i.e., the growth rate, peaks.

Improvement of charged particles transport across a transverse magnetic filter field by electrostatic trapping of magnetized electrons
View Description Hide DescriptionA study on the transport of charged particles across a magnetic filter field has been carried out in a double plasma device (DPD) and presented in this manuscript. The DPD is virtually divided into two parts viz. source and target regions by a transverse magnetic field (TMF) which is constructed by inserting strontium ferrite magnets into two stainless steel rectangular tubes. Plasma electrons are magnetized but ions are unmagnetized inside the TMF region. Negative voltages are applied to the TMF tubes in order to reduce the loss of electrons towards them. Plasma is produced in the source region by filament discharge method and allowed to flow towards the target region through this negatively biased TMF. It is observed that in the target region, plasma density can be increased and electron temperature decreased with the help of negatively biased TMF. This observation is beneficial for negative ion source development. Plasma diffusion across the negatively biased TMF follows Bohm or anomalous diffusion process when negative bias voltage is very less. At higher negative bias, diffusion coefficient starts deviating from the Bohm diffusion value, associated with enhanced plasma flow in the target region.
