Volume 23, Issue 12, December 2016
 LETTERS


Critical role of electron heat flux on Bohm criterion
View Description Hide DescriptionBohm criterion, originally derived for an isothermalelectron and coldion plasma, is often used as a rule of thumb for more general plasmas. Here, we establish a more precise determination of the Bohm criterion that are quantitatively useful for understanding and modeling collisional plasmas that still have collisional meanfreepath much greater than plasma Debye length. Specifically, it is shown that electron heat flux, rather than the isothermal electron assumption, is what sets the Bohm speed to be with the electron and ion parallel temperature at the sheath entrance and mi the ion mass.

Effects of preheat and mix on the fuel adiabat of an imploding capsule
View Description Hide DescriptionWe demonstrate the effect of preheat, hydrodynamic mix and vorticity on the adiabat of the deuteriumtritium (DT) fuel in fusion capsule experiments. We show that the adiabat of the DT fuel increases resulting from hydrodynamic mixing due to the phenomenon of entropy of mixture. An upper limit of mix, Mclean/MDT ≥ 0.98, is found necessary to keep the DT fuel on a low adiabat. We demonstrate in this study that the use of a high adiabat for the DT fuel in theoretical analysis and with the aid of 1D code simulations could explain some aspects of 3D effects and mix in capsule implosion. Furthermore, we can infer from our physics model and the observed neutron images the adiabat of the DT fuel in the capsule and the amount of mix produced on the hot spot.

The physics mechanisms of the weakly coherent mode in the Alcator CMod Tokamak
View Description Hide DescriptionThe weakly coherent mode (WCM) in Imode has been studied by a sixfield twofluid model based on the Braginskii equations under the BOUT++ framework for the first time. The calculations indicate that a tokamak pedestal exhibiting a WCM is linearly unstable to drift Alfven wave (DAW) instabilities and the resistive ballooning mode. The nonlinear simulation shows promising agreement with the experimental measurements of the WCM. The shape of the density spectral and location of the spectral peak of the dominant toroidal number mode n = 20 agrees with the experimental data from reflectometry. The simulated mode propagates in electron diamagnetic direction is consistent with the results from the magnetic probes in the laboratory frame, a large ratio of particle to heat diffusivity is consistent with the distinctive experimental feature of Imode, and the value of the simulated χe at the edge is in the range of experimental errors of χeff from the experiment. The prediction of the WCM shows that free energy is mainly provided by the electron pressure gradient, which gives guidance for pursuing future Imode studies.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Ponderomotive processes as proxies for breaking of ion acoustic solitary waves
View Description Hide DescriptionWave breaking is a ubiquitous nonlinear phenomenon in plasma that is followed by sudden drop of wave amplitude after a wave steepening. We perform fluid simulation of the ion acoustic solitary waves (IASWs) to investigate the start time of the wave steepening and breaking process. This simulation demonstrates that a long wavelength perturbation in the electron and ion equilibrium densities evolves into two long wavelength IASWs. These IASWs steepens and breaks into short wavelength solitary structures, which become stable ion acoustic solitons at later time. From the detailed analysis of simulation output, we accomplish the criteria for steepening and breaking of the IASWs based on the (a) acceleration of IASWs (b) balance between maximum potential energy and the maximum electron kinetic energy. Furthermore, we examined the ponderomotive potential and the ponderomotive frequency of the electrons and ions during the process of the generation, steepening and breaking of these IASWs. It is observed that the maximum ponderomotive potential of both electrons and ions enhances during the steepening and attains the maximum close to the breaking of the IASWs. The simulation shows that the electron (ion) average ponderomotive frequency is considerably higher than the electron plasma frequency in the initial phase of generation of IASWs, which rapidly oscillates and approaches to frequencies much smaller than electron (ion) plasma frequency. These ponderomotive frequencies remain unchanged until the start of steepening of the IASWs; however, both frequencies are found to increase during the steepening and breaking of these IASWs. Based on this information, we propose that the ponderomotive potential and ponderomotive frequencies of electrons and ions can be used as proxies to determine the steepening and breaking time of the IASWs. We find that the onset time of the wave breaking varies inversely with the thermal velocity of the electrons and the amplitude of the initial density perturbation (IDP), while it is directly proportional to the width of the IDP. It is also noted that the number of solitons formed in the system and their characteristics depends on the electron temperature, width, and amplitude of the IDP.

Hybrid simulations of a parallel collisionless shock in the large plasma device
View Description Hide DescriptionWe present twodimensional hybrid kinetic/magnetohydrodynamic simulations of planned laserablation experiments in the Large Plasma Device. Our results, based on parameters that have been validated in previous experiments, show that a parallel collisionless shock can begin forming within the available space. Carbondebris ions that stream along the magneticfield direction with a blowoff speed of four times the Alfvén velocity excite strong magnetic fluctuations, eventually transferring part of their kinetic energy to the surrounding hydrogen ions. This acceleration and compression of the background plasma creates a shock front, which satisfies the Rankine–Hugoniot conditions and can therefore propagate on its own. Furthermore, we analyze the upstream turbulence and show that it is dominated by the righthand resonant instability.

Modelling nonlinear electrostatic oscillations in plasmas
View Description Hide DescriptionThe nonlinear 1D plasma electrostatic oscillation is formulated in an analytic framework that allows closedform analytic solutions along the characteristics, and solved numerically in configuration space. Additionally, a novel iterative analytical form for the finiteamplitude oscillation solution is derived, which compares favourably with the other two techniques. A fresh insight into the evolution of the oscillation is gained, including defining the least achievable density in the nonlinear oscillation as half of the equilibrium value, and relating the associated maximum density achievable in terms of that minimum.

Spatially resolved behavior of laserproduced copper plasma along expansion direction in the presence of static uniform magnetic field
View Description Hide DescriptionWe report on the spatially resolved optical emission spectroscopic study of laserproduced copper plasma in the presence of static uniform magnetic field in air ambient at atmospheric pressure. The response of copper atomic/ionic lines to magnetic field along the axial direction of plasma is different. It is attributed to the difference in populating process (electron impact excitation and recombination) of each transition. In the present work, we introduced air pressure to calculate the stopping radius and found it to be around the distance at which the intensity is pronounced. The electron density varied as n e = 9.2z ^{−0.33} without magnetic field and in the presence of 0.3 T magnetic field, it varied as n e = 7.9z ^{−0.27}. The electron temperature variation with distance from the target in the absence and presence of magnetic field is found to be T e = 1.1z ^{−0.23} and T e = 0.9z ^{−0.18}. The electron density and temperature decay slowly along the plasma expansion direction in the presence of magnetic field. It is due to magnetic confinement of plasma. We demonstrated that the thermal conductivity of plasma is enhanced in the presence of magnetic field. From the spatial evolution of the electron density and temperature, we estimated the approximate dimension of the core and tail region of the plasma and found an increase in the core dimension in the presence of magnetic field. The increase in core dimension is in agreement with the intensity variation of ionic line. It is attributed to an increase in heat transfer due to an increase in thermal conductivity in the presence of magnetic field. The present work may help optimize the distance from target to enhance spectral line intensity in optical emission spectroscopy in the presence of magnetic field.

A technique to control crossfield diffusion of plasma across a transverse magnetic field
View Description Hide DescriptionA study to control charged particle transport across a transverse magnetic field (TMF), popularly known as the magnetic filter in a negative ion source, has been carried out in a double plasma device. In the experimental setup, the TMF placed between the two magnetic cages divides the whole plasma chamber into two distinct regions, viz., the source and the target on the basis of the plasma production and the corresponding electron temperature. The plasma produced in the source region by the filament discharge method diffuses into the target region through the TMF. Data are acquired by the Langmuir probe and are compared in different source configurations, in terms of external biasing applied to metallic plates inserted in the TMF plane but in the orthogonal direction. The effect of the direction of current between the two plates in either polarity of bias in the presence of TMF on the plasma parameters and the crossfield transport of charge particles are discussed.

Investigating the electron density of multiMeV Xrayinduced air plasmas at low pressures based on electromagnetic resonant cavity analysis
View Description Hide DescriptionWe investigate air plasmas generated by multiMeV pulsed Xrays at pressures ranging from 10^{−5} to 10^{−1} mbar. The experimental approach used for these studies is based on measurements of resonant frequencies damping and shift for different electromagnetic modes within a cylindrical cavity. Timeintegrated electron densities in Xrayinduced air plasmas are inferred from the damping rate of the measured magnetic fields and their corresponding frequency shifts. In the present study, electron densities ranging from 10^{8} to 10^{9 }cm^{−3} at pressures ranging from 10^{−3} to 10^{−1} mbar have been measured. Experimental results were confronted to 3D MaxwellVlasov ParticleInCell simulations incorporating a radiationinduced electric conductivity model. The method used in this work enables determining microscopic and macroscopic physical quantities within low pressure air plasmas generated by pulsed Xray.

MHD instabilities developing in a conductor exploding in the skin effect mode
View Description Hide DescriptionThe results of experiments with exploding copper conductors, performed on the MIG facility (providing currents of amplitude of about 2.5 MA and rise time of 100 ns), are analyzed. With an frame optical camera, largescale instabilities of wavelength 0.2–0.5 mm were detected on the conductor surface. The instabilities show up as plasma “tongues” expanding with a sound velocity in the opposite direction to the magnetic field gradient. Analysis performed using a twodimensional MHD code has shown that the structures observed in the experiments were formed most probably due to flute instabilities. The growth of flute instabilities is predetermined by the development of thermal instabilities near the conductor surface. The thermal instabilities arise behind the front of the nonlinear magnetic diffusion wave propagating through the conductor. The wavefront on its own is not subject to thermal instabilities.

Suppression of diamagnetism by neutrals pressure in partially ionized, highbeta plasma
View Description Hide DescriptionSuppression of diamagnetism in a partially ionized plasma with high beta was experimentally investigated by the use of Langmuir and Hall sensor probes, focusing on a neutrals pressure effect. The plasma beta, which is the ratio of plasma to vacuum magnetic pressures, varied from ∼1% to >100% while the magnetic field varied from ∼120 G to ∼1 G. Here, a uniform magnetized argon plasma was operated mostly in an inductive mode, using a helicon plasma source of the Large Helicon Plasma Device [S. Shinohara et al., Phys. Plasmas 16, 057104 (2009)] with a diameter of 738 mm and an axial length of 4860 mm. Electron density varied from 5 × 10^{15} m^{−3} to <3 × 10^{18} m^{−3}, while an argon fill pressure was varied from ∼0.02 Pa to 0.75 Pa as well as the magnetic field mentioned above, with the fixed radio frequency (rf) and power of 7 MHz and ∼3.5 kW, respectively. The observed magnetic field reduction rate, a decrease of the magnetic field divided by the vacuum one, was up to 18%. However, in a certain parameter regime, where the product of ion and electron Hall terms is a key parameter, the measured diamagnetic effect was smaller than that expected by the plasma beta. This suppressed diamagnetism is explained by the neutrals pressure replacing magnetic pressure in balancing plasma pressure. Diamagnetism is weakened if neutrals pressure is comparable to the plasma pressure and if the coupling of plasma and neutrals pressures by ionneutral collisions is strong enough.

Formation of a collisionless shock wave in a multicomponent plasma
View Description Hide DescriptionWe discuss the theory of the formation of a quasitransverse collisionless shock wave in a multicomponent plasma. We show that in a plasma with a significant admixture of cold heavy ions, a specific MHD mode can be excited. This mode plays the same role for the collisionless shock formation as a quasitransverse fast magnetosonic wave in a plasma with one sort of ions. As a result of this mode excitation, the solar wind velocity threshold for the formation of a collisionless shock becomes significantly less than in the case of a plasma with only light ions. We derive a nonlinear differential equation which describes a shock wave when perturbations become strong enough. Based on our theoretical results, we argue that upstream of the magnetic pileup region of Mars or Venus, an additional shock wave may be formed.
 Nonlinear Phenomena, Turbulence, Transport

Interaction of solitons for obliquely propagating magnetoacoustic waves in stellar atmosphere
View Description Hide DescriptionWe study here the nonlinear oblique propagation of magnetoacoustic waves in dense plasmas with degenerate electrons by deriving KadomtsevPetviashvili (KP) equation for small but finite amplitude perturbations. The two soliton interaction has been studied by finding the solution of the KP equation using the Hirota bilinear formalism. For illustrative purposes, we have used the plasma parameters typically found in white dwarf stars for both the fast and slow modes of magnetoacoustic waves. It has been observed that the soliton interaction in the fast and slow modes is strongly influenced by the predominant and weak dispersive coefficients of the KP equation. The single soliton behavior has also been explained for the fast and slow magnetoacoustic modes.

Amplitude and size scaling for interchange motions of plasma filaments
View Description Hide DescriptionThe interchange dynamics and velocity scaling of bloblike plasma filaments are investigated using a twofield reduced fluid model. For incompressible flows due to buoyancy, the maximum velocity is proportional to the square root of the relative amplitude and the square root of its crossfield size. For compressible flows in a nonuniform magnetic field, this square root scaling only holds for ratios of amplitudes to crossfield sizes above a certain threshold value. For small amplitudes and large sizes, the maximum velocity is proportional to the filament amplitude. The acceleration is proportional to the amplitude and independent of the crossfield size in all regimes. This is demonstrated by means of numerical simulations and explained by the energy integrals satisfied by the model.

Acoustic solitons in a magnetized quantum electronpositronion plasma with relativistic degenerate electrons and positrons pressure
View Description Hide DescriptionThe obliquely nonlinear acoustic solitary propagation in a relativistically quantum magnetized electronpositron (ep) plasma in the presence of the external magnetic field as well as the stationary ions for neutralizing the plasma background was studied. By considering the dynamic of the fluid ep quantum and by using the quantum hydrodynamics model and the standard reductive perturbation technique, the ZakharovKuznetsov (ZK) equation is derived for small but finite amplitude waves and the solitary wave solution for the parameters relevant to dense astrophysical objects such as white dwarf stars is obtained. The numerical results show that the relativistic effects lead to propagate the electrostatic bell shape structures in quantum ep plasmas like those in classical pairion or pair species for relativistic plasmas. It is also observed that by increasing the relativistic effects, the amplitude and width of the ep acoustic solitary wave will decrease. In addition, the wave amplitude increases as positron density decreases in magnetized ep plasmas. It is indicated that by increasing the strength of the magnetic field, the width of the soliton reduces and it becomes sharper. At the end, we have analytically and numerically shown that the pulse soliton solution of the ZK equation is unstable and have traced the dependence of the instability growth rate on electron density. It is found that by considering the relativistic pressure, the instability of the soliton pulse can be reduced. The results can be useful to study the obliquely nonlinear propagation of small amplitude localized structures in magnetized quantum ep plasmas and be applicable to understand the particle and energy transport mechanism in compact stars such as white dwarfs, where the effects of relativistic electron degeneracy become important.
 Magnetically Confined Plasmas, Heating, Confinement

On the measurement of the threshold electric field for runaway electron generation in the Frascati Tokamak Upgrade
View Description Hide DescriptionExperiments have been carried out to evaluate the threshold electric field for runaway generation during the flattop phase of ohmic discharges in the Frascati Tokamak Upgrade tokamak. An investigation of the conditions for runaway electron generation and suppression has been performed for a wide range of plasma parameter values. The measured threshold electric field is found to be significantly larger ( times) than predicted by the relativistic collissional theory of runaway generation, , and can be explained to a great extent by an increase of the critical electric field due to the effect of the electron synchrotron radiation losses. These findings are consistent with the results of an ITPA joint experiment to study the onset, growth, and decay of relativistic runaway electrons [Granetz et al., Phys. Plasmas 21, 072506 (2014)]. Confirmation of these results for disruptions with high electric field might imply significantly lower requirements on electron densities for suppression and prevention of runaway formation in ITER.

Helical temperature perturbations associated with radially asymmetric magnetic island chains in tokamak plasmas
View Description Hide DescriptionThe simple analysis of Rutherford [Phys. Fluids 16, 1903 (1973)] is generalized in order to incorporate radial magnetic island asymmetry into the nonlinear theory of tearing mode stability in a lowβ, large aspectratio, quasicylindrical, tokamak plasma. The calculation is restricted to cases in which the radial shifts of the island X and Opoints are (almost) equal and opposite. For the sake of simplicity, the calculation concentrates on a particular (but fairly general) class of radially asymmetric island magnetic fluxsurfaces that can all be mapped to the same symmetric fluxsurfaces by means of a suitable coordinate transform. The combination of island asymmetry (in which the radial shifts of the X and Opoints are almost equal and opposite) and temperatureinduced changes in the inductive current profile in the immediate vicinity of the island is found to have no effect on tearing mode stability.

Stationary MHD equilibria describing azimuthal rotations in symmetric plasmas
View Description Hide DescriptionWe consider the stationary magnetohydrodynamical (MHD) equilibrium equation for an axisymmetric plasma undergoing azimuthal rotations. The case of cylindrical symmetry is treated, and we present two semianalytical solutions for the stationary MHD equilibrium equations, from which a number of physical properties of the magnetically confined plasma are derived.

Current drive with combined electron cyclotron wave and high harmonic fast wave in tokamak plasmas
View Description Hide DescriptionThe current driven by combined electron cyclotron wave (ECW) and high harmonic fast wave is investigated using the GENRAY/CQL3D package. It is shown that no significant synergetic current is found in a range of cases with a combined ECW and fast wave (FW). This result is consistent with a previous study [Harvey et al., in Proceedings of IAEA TCM on Fast Wave Current Drive in Reactor Scale Tokamaks (Synergy and Complimentarily with LHCD and ECRH), Arles, France, IAEA, Vienna, 1991]. However, a positive synergy effect does appear with the FW in the lower hybrid range of frequencies. This positive synergy effect can be explained using a picture of the electron distribution function induced by the ECW and a very high harmonic fast wave (helicon). The dependence of the synergy effect on the radial position of the power deposition, the wave power, the wave frequency, and the parallel refractive index is also analyzed, both numerically and physically.

Improvements to an ion orbit loss calculation in the tokamak edge
View Description Hide DescriptionAn existing model of collisionless particle, momentum, and energy ion orbit loss from the edge region of a diverted tokamak plasma has been extended. The extended ion orbit loss calculation now treats losses of both thermal ions and fast neutral beam injection ions and includes realistic flux surface and magnetic field representations, particles returning to the plasma from the scrape off layer, and treatment of xtransport and xloss. More realistic flux surface geometry allows the intrinsic rotation calculation to predict a peaking in the profile closer to the separatrix, which is consistent with experiment; and particle tracking calculations reveal a new mechanism of “xtransport pumping,” which predicts larger ion losses when coupling conventional ion orbit loss and xloss mechanisms, though still dominated by conventional ion orbit loss. Sensitivity to these ion orbit loss model enhancements is illustrated by fluid predictions of neoclassical rotation velocities and radial electric field profiles, with and without the enhancements.