Volume 23, Issue 2, February 2016

The initial state recurrence in numerical simulations of the VlasovPoisson system is a wellknown phenomenon. Here, we study the effect on recurrence of artificial collisions modeled through the LenardBernstein operator [A. Lenard and I. B. Bernstein, Phys. Rev. 112, 1456–1459 (1958)]. By decomposing the linear VlasovPoisson system in the FourierHermite space, the recurrence problem is investigated in the linear regime of the damping of a Langmuir wave and of the onset of the bumpontail instability. The analysis is then confirmed and extended to the nonlinear regime through an Eulerian collisional VlasovPoisson code. It is found that, despite being routinely used, an artificial collisionality is not a viable way of preventing recurrence in numerical simulations without compromising the kinetic nature of the solution. Moreover, it is shown how numerical effects associated to the generation of fine velocity scales can modify the physical features of the system evolution even in nonlinear regime. This means that filamentationlike phenomena, usually associated with low amplitude fluctuations contexts, can play a role even in nonlinear regime.
 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Effects of electromagnetic wiggler and ion channel guiding on equilibrium orbits and waves propagation in a free electron laser
View Description Hide DescriptionIn this paper, an analysis of equilibrium orbits for electrons by a simultaneous solution of the equation of motion and the dispersion relation for electromagnetic wavewiggler in a freeelectron laser(FEL) with ionchannel guiding has been presented. A fluid model has been used to investigate interactions among all possible waves. The dispersion relation has been derived for electrostatic and electromagnetic waves with all relativistic effects included. This dispersion relation has been solved numerically. For group I and II orbits, when the transverse velocity is small, only the FEL instability is found. In group I and II orbits with relatively large transverse velocity, new couplings between other modes are found.

On the rogue wave propagation in ion pair superthermal plasma
View Description Hide DescriptionEffects of superthermal electron on the features of nonlinear acoustic waves in unmagnetized collisionless ion pair plasma with superthermal electrons have been examined. The system equations are reduced in the form of the nonlinear Schrodinger equation. The rogue wave characteristics dependences on the ionic density ratio (ν = n–0/n+0), ionic mass ratio (Q = m+/m−), and superthermality index (κ) are investigated. It is worth mentioning that the results present in this work could be applicable in the Earth's ionosphereplasmas.

Collisional effects on the numerical recurrence in VlasovPoisson simulations
View Description Hide DescriptionThe initial state recurrence in numerical simulations of the VlasovPoisson system is a wellknown phenomenon. Here, we study the effect on recurrence of artificial collisions modeled through the LenardBernstein operator [A. Lenard and I. B. Bernstein, Phys. Rev. 112, 1456–1459 (1958)]. By decomposing the linear VlasovPoisson system in the FourierHermite space, the recurrence problem is investigated in the linear regime of the damping of a Langmuir wave and of the onset of the bumpontail instability. The analysis is then confirmed and extended to the nonlinear regime through an Eulerian collisional VlasovPoisson code. It is found that, despite being routinely used, an artificial collisionality is not a viable way of preventing recurrence in numerical simulations without compromising the kinetic nature of the solution. Moreover, it is shown how numerical effects associated to the generation of fine velocity scales can modify the physical features of the system evolution even in nonlinear regime. This means that filamentationlike phenomena, usually associated with low amplitude fluctuations contexts, can play a role even in nonlinear regime.

Quantum mechanical gradB drift velocity operator in a weakly nonuniform magnetic field
View Description Hide DescriptionThis paper presents the analytical solution for quantum mechanical gradB drift velocity operator by solving the Heisenberg equation of motion. Using the time dependent operators, it is shown the analytical solution of the position operators in and of the particle in the presence of a weakly nonuniform magnetic field. It is also shown numerically that the gradB drift velocity operator agrees with the classical counterpart.
 Nonlinear Phenomena, Turbulence, Transport

Wakes and precursor soliton excitations by a moving charged object in a plasma
View Description Hide DescriptionWe study the evolution of nonlinearion acoustic wave excitations due to a moving charged source in a plasma. Our numerical investigations of the full set of cold fluid equations go beyond the usual weak nonlinearity approximation and show the existence of a rich variety of solutions including wakes, precursor solitons, and “pinned” solitons that travel with the source velocity. These solutions represent a large amplitude generalization of solutions obtained in the past for the forced Korteweg deVries equation and can find useful applications in a variety of situations in the laboratory and in space, wherever there is a large relative velocity between the plasma and a charged object.

Nonlinear vortex structures with perpendicular shear flow, hot ions, and nonthermal distribution of electrons
View Description Hide DescriptionCoupling of drift vortex and the ion acoustic modes in the linear and nonlinear regimes are investigated with sheared ion flow perpendicular to the ambient magnetic field in a plasma comprising of hot ions and nonthermal population of electrons. In this regard, generation of nonlinearvortex structures in the presence of kappa, Cairns, and qnonextensive electron distributions are investigated in detail, and comparison with the Maxwellian distribution is also made. The appositeness of the present investigation in the matter of auroral Fregion is also pointed out.

Transport equation for plasmas in a stationaryhomogeneous turbulence
View Description Hide DescriptionFor a plasma in a stationary homogeneous turbulence, the FokkerPlanck equation is derived from the nonlinear Vlasov equation by introducing the entropy principle. The ensemble average in evaluating the kinetic diffusiontensor, whose symmetry has been proved, can be computed in a straightforward way when the fluctuating particle trajectories are provided. As an application, it has been shown that a mean parallel electric filed can drive a particle flux through the StokesEinstein relation, independent of the details of the fluctuations.
 Magnetically Confined Plasmas, Heating, Confinement

Vapor shielding models and the energy absorbed by divertor targets during transient events
View Description Hide DescriptionThe erosion of divertor targets caused by high heat fluxes during transients is a serious threat to ITER operation, as it is going to be the main factor determining the divertor lifetime. Under the influence of extreme heat fluxes, the surfacetemperature of plasma facing components can reach some certain threshold, leading to an onset of intense materialevaporation. The latter results in formation of cold dense vapor and secondary plasma cloud. This layer effectively absorbs the energy of the incident plasma flow, turning it into its own kinetic and internal energy and radiating it. This so called vapor shielding is a phenomenon that may help mitigating the erosion during transient events. In particular, the vapor shielding results in saturation of energy (per unit surface area) accumulated by the target during single pulse of heat load at some level Emax. Matching this value is one of the possible tests to verify complicated numerical codes, developed to calculate the erosion rate during abnormal events in tokamaks. The paper presents three very different models of vapor shielding, demonstrating that Emax depends strongly on the heat pulse duration, thermodynamic properties, and evaporation energy of the irradiated target material. While its dependence on the other shielding details such as radiation capabilities of material and dynamics of the vapor cloud is logarithmically weak. The reason for this is a strong (exponential) dependence of the target materialevaporation rate, and therefore the “strength” of vapor shield on the target surfacetemperature. As a result, the influence of the vapor shielding phenomena details, such as radiation transport in the vapor cloud and evaporated materialdynamics, on the Emax is virtually completely masked by the strong dependence of the evaporation rate on the target surfacetemperature. However, the very same details define the amount of evaporated particles, needed to provide an effective shielding to the target, and, therefore, strongly influence resulting erosion rate. Thus, Emax cannot be used for validation of shielding models and codes, aimed at the target materialerosion calculations.

Verification of gyrokinetic particle simulation of currentdriven instability in fusion plasmas. III. Collisionless tearing mode
View Description Hide DescriptionA finitemass electron fluid model for low frequency electromagnetic fluctuations, particularly the collisionless tearing mode, has been implemented in the gyrokinetic toroidal code. Using this fluid model, linear properties of the collisionless tearing mode have been verified. Simulations verify that the linear growth rate of the single collisionless tearing mode is proportional to De^{2}, where De is the electron skin depth. On the other hand, the growth rate of a double tearing mode is proportional to De in the parameter regime of fusion plasmas.

Diverse waveparticle interactions for energetic ions that traverse Alfvén eigenmodes on their first full orbit
View Description Hide DescriptionNeutralbeam ions that are deflected onto loss orbits by Alfvén eigenmodes(AE) on their first bounce orbit and are detected by a fastion loss detector (FILD) satisfy the “local resonance” condition proposed by Zhang et al. [Nucl. Fusion 55, 22002 (2015)]. This theory qualitatively explains FILD observations for a wide variety of AEparticle interactions. When coherent losses are measured for multiple AE, oscillations at the sum and difference frequencies of the independent modes are often observed in the loss signal. The amplitudes of the sum and difference peaks correlate weakly with the amplitudes of the fundamental losssignal amplitudes but do not correlate with the measured mode amplitudes. In contrast to a simple uniformplasma theory of the interaction [Chen et al., Nucl. Fusion 54, 083005 (2014)], the losssignal amplitude at the sum frequency is often larger than the losssignal amplitude at the difference frequency, indicating a more detailed computation of the orbital trajectories through the mode eigenfunctions is needed.
 Inertially Confined Plasmas, High Energy Density Plasma Science, Warm Dense Matter

Threedimensional singlemode nonlinear ablative RayleighTaylor instability
View Description Hide DescriptionThe nonlinear evolution of the singlemode ablative RayleighTaylor instability is studied in three dimensions. As the mode wavelength approaches the cutoff of the linear spectrum (shortwavelength modes), it is found that the threedimensional (3D) terminal bubble velocity greatly exceeds both the twodimensional (2D) value and the classical 3D bubble velocity. Unlike in 2D, the 3D shortwavelength bubble velocity does not saturate. The growing 3D bubbleacceleration is driven by the unbounded accumulation of vorticity inside the bubble. The vorticity is transferred by mass ablation from the RayleighTaylor spikes to the ablatedplasma filling the bubble volume.

Scaling magnetized liner inertial fusion on Z and future pulsedpower accelerators
View Description Hide DescriptionThe MagLIF (Magnetized Liner Inertial Fusion) concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] has demonstrated fusion–relevant plasma conditions [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] on the Z accelerator with a peak drive current of about 18 MA. We present 2D numerical simulations of the scaling of MagLIF on Z as a function of drive current, preheat energy, and applied magnetic field. The results indicate that deuteriumtritium (DT) fusion yields greater than 100 kJ could be possible on Z when all of these parameters are at the optimum values: i.e., peak current = 25 MA, deposited preheat energy = 5 kJ, and Bz = 30 T. Much higher yields have been predicted [S. A. Slutz and R. A. Vesey, Phys. Rev. Lett. 108, 025003 (2012)] for MagLIF driven with larger peak currents. Two high performance pulsedpower accelerators (Z300 and Z800) based on lineartransformerdriver technology have been designed [W. A. Stygar et al., Phys. Rev. ST Accel. Beams 18, 110401 (2015)]. The Z300 design would provide 48 MA to a MagLIF load, while Z800 would provide 65 MA. Parameterized Theveninequivalent circuits were used to drive a series of 1D and 2D numerical MagLIF simulations with currents ranging from what Z can deliver now to what could be achieved by these conceptual future pulsedpower accelerators. 2D simulations of simple MagLIF targets containing just gaseous DT have yields of 18 MJ for Z300 and 440 MJ for Z800. The 2D simulated yield for Z800 is increased to 7 GJ by adding a layer of frozen DT ice to the inside of the liner.
 Lasers, Particle Beams, Accelerators, Radiation Generation

Laser beat wave resonant terahertz generation in a magnetized plasma channel
View Description Hide DescriptionResonant excitation of terahertz (THz) radiation by nonlinear mixing of two lasers in a ripplefree self created plasma channel is investigated. The channel has a transverse static magnetic field and supports a THz Xmode with phase velocity close to the speed of light in vacuum when the frequency of the mode is close to plasma frequency on the channel axis and its value decreases with the intensity of lasers. The THz is resonantly driven by the laser beat waveponderomotive force. The THz amplitude scales almost three half power of the intensity of lasers as the width of the THz eigen mode shrinks with laser intensity.

Angular distribution of plasma luminescence emission during filamentation in air
View Description Hide DescriptionWe experimentally measure the angular distribution of plasmaluminescence emission during the filamentation of linearly polarized femtosecond laser pulses. It is found that the luminescence from N2 shares the same intensity in all directions, while that from is more intense in the direction parallel to that of the laser polarization. The isotropic emission behavior of luminescence from N2 illustrates that the formation of excited does not result from the dissociative recombination; on the other hand, the linear increase of the strength of 337 nm signal with pressure indicates that the collision plays a negligible role, excluding the collision assisted intersystem crossing scheme. However, the intersystem crossing does not rely on the collision, making it a possible scheme. This study will be helpful to the understanding of mechanism of plasmaluminescence emission during femtosecondfilamentation.

Hollow structure formation of intense ion beams with sharp edge in background plasmas
View Description Hide DescriptionThe transport of intense ion beams with sharp radial beam edge in plasmas has been studied with twodimensional electromagnetic particle simulations. The initial solid beam evolves into a hollow beam due to the nonlinear sharp transverse force peak in the regions of beam edge. The magnitude and nonlinearity of this peak are enhanced as the ion beam travels further into the plasma, due to the selfconsistent interactions between the beam ions and the plasmaelectrons. This structure formation is shown to be independent on the beam radius.

Overmoded subterahertz surface wave oscillator with pure TM01 mode output
View Description Hide DescriptionOvermoded Otype Cerenkov generators using annular electron beams are facing the problem of multimodes output due to the inevitable structural discontinuities. A simple but effective method to achieve the pure TM01 mode output is applied on the 0.14 THz overmoded surface waveoscillator (SWO) in this paper. In spite of still using an overmoded slow wave structure to ensure the easy fabrication, the followed smooth circular waveguide is shrinkingly tapered to the output waveguide with appropriate radius that it cuts off other higher modes except TM01 mode. Moreover, the modified device here has the same power capacity as the previous one according to the numerical analysis. By optimized lengths of the transition waveguide and tapered waveguide, particleincell simulation results indicate that the subterahertz wave with output power increased 14.2% at the same frequency is obtained from the proposed SWO under the previous input conditions, and importantly, the output power is all carried by TM01 mode as expected. Further simulation results in the pulse regime confirm the feasibility of the optimized structure in the actual experiments. This simple and viable design is also applicable to overmoded devices in the lower frequency band of subterahertz wave.
 LowTemperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Selfpulsing in a lowcurrent hollow cathode discharge: From Townsend to glow discharge
View Description Hide DescriptionWe investigate the selfpulsing phenomenon of a low current cavity discharge in a cylindrical hollow cathode in pure argon. The waveforms of pulsed current and voltage are measured, and the timeaveraged and timeresolvedimages of hollow cathodedischarge are recorded by using highspeed intensified charge coupled device camera. The results show that the selfpulsing is a mode transition between lowcurrent stage of Townsend discharge and highcurrent stage of glow discharge. During the selfpulsing, the current rising time relates to the dissipation of space charges, and the decay time relates to the reconstruction of the virtual anode by the accumulation of positive ions. Whether or not space charges can form and keep the virtual anode is responsible for the discharge mode and hence plays an important role in the selfpulsing phenomenon in low current hollow cathodedischarge.

Up and downstream sheaths in an ionbeamplasma system
View Description Hide DescriptionIon sheaths formed in the up and downstream sides of a negatively biased metal plate/mesh in an ionbeambackgroundplasma system were experimentally investigated in a double plasma device.Measured potential profiles near the plate exhibit asymmetric structure, showing thicker sheath in the downstream side. The presence of the ion beam causes the shrink of the sheaths on both sides. The sheath thickness decreases with the increase of beam energy and density. Furthermore, the sheaths near the mesh are substantially thinner than that near the plate because of the partial transmission of the mesh to the ions. In addition, the increase of neutral gas pressure leads to the reduction of the beam energy and density, resulting in the increase of the sheath thickness.
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 BRIEF COMMUNICATIONS


Control of plasma properties in a short directcurrent glow discharge with active boundaries
View Description Hide DescriptionTo demonstrate controlling electron/metastable density ratio and electron temperature by applying negative voltages to the active (conducting) discharge wall in a lowpressure plasma with nonlocal electron energy distribution function, modeling has been performed in a short (lacking the positivecolumn region) directcurrent glow discharge with a cold cathode. The applied negative voltage can modify the trapping of the lowenergy part of the energetic electrons that are emitted from the cathodesheath and that arise from the atomic and molecular processes in the plasma within the device volume. These electrons are responsible for heating the slow, thermal electrons, while production of slow electrons (ions) and metastable atoms is mostly due to the energetic electrons with higher energies. Increasing electron temperature results in increasing decay rate of slow, thermal electrons (ions), while decay rate of metastable atoms and production rates of slow electrons (ions) and metastable atoms practically are unchanged. The result is in the variation of electron/metastable density ratio and electron temperature with the variation of the wall negative voltage.

Hyperresistive forced magnetic reconnection
View Description Hide DescriptionWe study Taylor's model of forced magnetic reconnection mediated by plasma hyperresistivity. This includes both linear and nonlinear regimes of the process. It is shown how the onset of plasmoid instability occurs in the strongly nonlinear regime of forced reconnection.
