Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session JO7: Waves, Turbulence and Transport |
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Chair: Tiffany Desjardins, Los Alamos National Laboratory Room: 212 AB |
Tuesday, November 1, 2016 2:00PM - 2:12PM |
JO7.00001: Plasma sheath model in the presence of field-induced electron emission Jiba Dahal, Venkattraman Ayyaswamy Microplasmas have become an active area of research during the last two decades with several applications including nanomaterial synthesis, electronics, lighting, biomedicine, and metamaterials for controlling electromagnetic waves. The advances in micro/nanofabrication and the further miniaturization of plasma devices have contributed to the increasing role of new physical mechanisms that were previously neglected. Electric field-induced emission of electrons is one such mechanism that is gaining significance particularly with the discovery of novel electrodes that demonstrate excellent field emission properties. These field emitted electrons and their interaction with microdischarges has shown to affect both pre-breakdown and post-breakdown regimes of operation. The current work focuses on the development of self-consistent sheath model that includes the effects of field-induced electron emission. Sheath models presented earlier accounts for other emission mechanisms such as thermionic and secondary electron emission, the strong influence of electric field on electron emission is shown to lead to unique interplay. The results obtained from the sheath model for various parameters including current-voltage characteristics, and ion/electron number density are validated with PIC-MCC results. [Preview Abstract] |
Tuesday, November 1, 2016 2:12PM - 2:24PM |
JO7.00002: Ion transport and entrapment in electrostatic virtual cathodes Chi-Shung Yip, Noah Hershkowitz, Greg Severn Axial ion motions in virtual cathodes are studied with Laser Induced Fluorescence. Virtual cathodes are formed using small (A$_{\mathrm{electrode}}$/A$_{\mathrm{loss}}$ \textless (m$_{\mathrm{e}}$/m$_{\mathrm{i}})^{\mathrm{1/2}})$ electrodes biased at near to or higher than the plasma potential far from the electrode in multi-dipole confined filament discharges of argon gas. The virtual cathodes are electrostatic, with no magnetic fields present near the electrode to confine ions. An emissive probe is employed to measure the full potential profile from the bulk up to the surface of the electrode. A planar Langmuir probe is employed to measure the electron temperature T$_{\mathrm{e}}$, the plasma density n$_{\mathrm{e\thinspace }}$and the Debye length. Reflected ions are not observed in significant amounts unless the electrode is biased at least 1 T$_{\mathrm{e}}$/e above the bulk plasma potential. When the electrode is biased at at least 1 T$_{\mathrm{e}}$/e above the plasma potential, significant retardation of ion velocities in much of the potential profile is observed along with reflected ions. Ion temperature rises significantly when retardation occurs. However, such increases in temperature do not account for the full energy that ions gain from the potential drop. The role of pumping by the saddle point electric fields is under investigation. [Preview Abstract] |
Tuesday, November 1, 2016 2:24PM - 2:36PM |
JO7.00003: Ion loss in weakly collisional three ion species plasmas Noah Hershkowitz, Chi-Shung Yip, Greg Severn Ion velocity distribution functions (ivdfs) at sheath-presheath boundaries are studied with laser-induced fluorescence in weakly collisional Xe-Kr-Ar and Xe-Ar-Ne plasmas using tunable diode lasers for Ar$+$ and Xe$+$ ions. The argon and xenon ivdfs are measured at the sheath-presheath boundary near a negatively biased boundary plate. The plasma potential profiles are measured by an emissive probe throughout the sheath and presheath. Relative ion concentrations resulting from the neutral gas mixtures are calculated using G-H. Kim's loss-production balance method. Ne$+$ and Kr$+$ ions are added to plasmas with approximately equal amounts of Ar$+$ and Xr$+$ ions, where Ar$+$ and Xe$+$ ions are previously observed to reach the sheath edge at nearly the system sound speed. In two ion species plasmas, instability enhanced collisional friction was demonstrated to dictate the escape velocities of the ions. When three ion species are present, it is demonstrated that as the concentration of the third ion species increases, Xe$+$ and Ar$+$ ion velocities at the sheath edge deviates from the common sound speed and approach their individual Bohm velocities. These observations are consistent with the predictions of the instability enhanced collisional friction theory [Preview Abstract] |
Tuesday, November 1, 2016 2:36PM - 2:48PM |
JO7.00004: Gyrokinetic GDC turbulence simulations: confirming a new instability regime in LAPD plasmas M.J. Pueschel, G. Rossi, D. Told, P.W. Terry, F. Jenko, T.A. Carter Recent high-beta experiments at the LArge Plasma Device have found significant parallel magnetic fluctuations in the region of large pressure gradients. Linear gyrokinetic simulations show the dominant instability at these radii to be the gradient-driven drift coupling (GDC) mode, a non-textbook mode driven by pressure gradients and destabilized by the coupling of ExB and grad-$B_\parallel$ drifts. Unlike in previous studies, the large parallel extent of the device allows for finite-$k_z$ versions of this instability in addition to $k_z=0$. The locations of maximum linear growth match very well with experimentally observed peaks of $B_\parallel$ fluctuations. Local nonlinear simulations reproduce many features of the observations fairly well, with the exception of Bperp fluctuations, for which experimental profiles suggest a source unrelated to pressure gradients. In toto, the results presented here show that turbulence and transport in these experiments are driven by the GDC instability, that important characteristics of the linear instability carry over to nonlinear simulations, and -- in the context of validation -- that the gyrokinetic framework performs surprisingly well far outside its typical area of application, increasing confidence in its predictive abilities. [Preview Abstract] |
Tuesday, November 1, 2016 2:48PM - 3:00PM |
JO7.00005: Electromagnetic turbulence and transport in increased $\beta$ LAPD Plasmas Giovanni Rossi, Troy Carter, MJ Pueschel, Frank Jenko, Paul Terry, Daniel Told The new LaB6 plasma source in LAPD has enabled the production of magnetized, increased β plasmas (up to ~15\%). We report on the modifications of pressure-gradient-driven turbulence and transport with increased plasma β. Density fluctuations decrease with increasing β while magnetic fluctuations increase. B⊥ fluctuations saturate while parallel (compressional) magnetic fluctuations increase continuously with $\beta$. At the highest $\beta$ values δB||/δB⊥ ~ 2 and δB/B ~ 1\%. The measurements are consistent with the excitation of the Gradient-driven Drift Coupling (GDC). This instability prefers k|| = 0 and grows in finite $\beta$ plasmas due to density and temperature gradients through the production of parallel magnetic field fluctuations and resulting ∇B|| drifts. Comparisons between experimental measurements and theoretical predictions for the GDC will be shown. Direct measurements of electrostatic particle flux have been performed and show a strong reduction with increasing $\beta$. No evidence is found (e.g. density profile shape) of enhanced confinement, suggesting that other transport mechanisms are active. Preliminary measurements indicate that electromagnetic transport due to parallel magnetic field fluctuations at first increases with $\beta$ but is subsequently suppressed at higher $\beta$ values. [Preview Abstract] |
Tuesday, November 1, 2016 3:00PM - 3:12PM |
JO7.00006: Investigation of MHD Instabilities in Jets and Bubbles Using a Compact Coaxial Plasma Gun in a Background Magnetized Plasma Y. Zhang, D.M. Fisher, B. Wallace, M. Gilmore, S.C. Hsu A compact coaxial plasma gun is employed for experimental investigation of launching plasma into a lower density background magnetized plasma. Experiments are being conducted in the linear device HelCat at UNM. Four distinct operational regimes with qualitatively different dynamics are identified by fast CCD camera images. For regime I plasma jet formation, a global helical magnetic configuration is determined by a B-dot probe array data. Also the m$=$1 kink instability is observed and verified. Furthermore, when the jet is propagating into background magnetic field, a longer length and lifetime jet is formed. Axial shear flow caused by the background magnetic tension force contributes to the increased stability of the jet body. In regime II, a spheromak-like plasma bubble formation is identified when the gun plasma is injected into vacuum. In contrast, when the bubble propagates into a background magnetic field, the closed magnetic field configuration does not hold anymore and a lateral side, Reilgh-Taylor instability develops. Detailed experimental data and analysis will be presented for these cases. [Preview Abstract] |
Tuesday, November 1, 2016 3:12PM - 3:24PM |
JO7.00007: Intermittency and turbulent dissipation at the tail of the MHD cascade Frank Jenko, Alejandro Banon Navarro, Bogdan Teaca, Daniel Told, Daniel Groselj, Paul Crandall We present the first systematic and detailed study of intermittency and turbulent dissipation in weakly collisional Alfvenic turbulence, using high resolution gyrokinetic and fully kinetic simulations spanning the range of scales between the ion and the electron gyroradii. The electron dynamics are found to be strongly intermittent and dominated by linear phase mixing, while nonlinear phase mixing dominates the weakly intermittent ions. These effects are quantified via phase space measures, generalizing techniques known from fluid turbulence. Moreover, real space structures that have a higher than average heating rates are shown not to be confined to current sheets. This novel result is at odds with previous studies, which use the electromagnetic work in the local electron fluid frame as a proxy for turbulent dissipation to argue that heating follows the intermittent spatial structure of the electric current. Furthermore, we show that electrons are dominated by parallel heating while the ions prefer the perpendicular heating route. We comment on the far reaching implications of the results presented here for the interpretation of computer simulations, laboratory experiments, and space observations. [Preview Abstract] |
Tuesday, November 1, 2016 3:24PM - 3:36PM |
JO7.00008: Fluid nonlinear frequency shift of nonlinear ion acoustic waves in multi-ion species plasmas in small wave number region Qingsong Feng, Chengzhuo Xiao, Qing Wang, Chunyang Zheng, Zhanjun Liu, Lihua Cao, Xiantu He The properties of the nonlinear frequency shift (NFS) especially the fluid NFS from the harmonic generation of the ion-acoustic wave (IAW) in multi-ion species plasmas has been researched by Vlasov simulation. The pictures of the nonlinear frequency shift from harmonic generation and particles trapping are shown to explain the mechanism of NFS qualitatively. The theoretical model of the fluid NFS from harmonic generation in multi-ion species plasmas is given and the results of Vlasov simulation are consistent to theoretical result of multi-ion species plasmas. When the wave number $k\lambda_{De}$ is small, such as $k\lambda_{De}=0.1$, the fluid NFS dominates in the total NFS and will reach as large as nearly $15\%$ when the wave amplitude $|e\phi/T_e|\sim0.1$, which indicates that in the condition of small $k\lambda_{De}$, the fluid NFS dominates in the saturation of stimulated Brillouin scattering especially when the nonlinear IAW amplitude is large. [Preview Abstract] |
Tuesday, November 1, 2016 3:36PM - 3:48PM |
JO7.00009: ABSTRACT MOVED TO jp10.171 |
Tuesday, November 1, 2016 3:48PM - 4:00PM |
JO7.00010: Electron plasma wave filamentation in the kinetic regime Pavel Lushnikov, Harvey Rose, Denis Silantyev We consider nonlinear electron plasma wave (EPW) dynamics
in the kinetic wavenumber regime, $0.25 |
Tuesday, November 1, 2016 4:00PM - 4:12PM |
JO7.00011: ABSTRACT WITHDRAWN |
Tuesday, November 1, 2016 4:12PM - 4:24PM |
JO7.00012: Impact of Ion Acoustic Wave Instabilities in the Flow Field of a Hypersonic Vehicle on EM Signals Saba Mudaliar, Vladimir Sotnikov Flow associated with a high speed air vehicle (HSAV) can get partially ionized. In the absence of external magnetic field the flow field turbulence is due to ion acoustic wave (IAW) instabilities. Our interest is in studying the impact of this turbulence on the radiation characteristics of EM signals from the HSAV. We decompose the radiated signal into coherent and diffuse parts. We find that the coherent part has the same spectrum as that of the source signal, but it is distorted because of dispersive coherent attenuation. The diffuse part is expressed as a convolution (in wavenumber and frequency) of the source signal with the spectrum of electron density fluctuations. This is a constrained convolution in the sense that the spectrum has to satisfy the IAW dispersion relation. A quantity that characterizes the flow is the mean free path (MFP). When the MFP is large compared to the thickness of the flow the coherent part is significant. If the MFP is larger than the thickness of the flow the diffuse part is the dominant part of the received signal. In the special case when the source signal frequency is close the electron plasma frequency, there can exist in the flow region Langmuir modes in addition to the EM modes. The radiation characteristics of EM source signals from the HSAV in this case are quite different. [Preview Abstract] |
Tuesday, November 1, 2016 4:24PM - 4:36PM |
JO7.00013: Effect of RF Waves on Ion Temperature Gradient Modes S Sen, J Martinell, K Imadera, Y Kishimoto The ion-temperature-driven modes are studied in the presence of radio frequency waves by the use of the Gyro-Kinetic simulation Code and ASTRA Code. It is shown that the radio frequency waves through the ponderomotive force can stabilize the ion-temperature-gradient instabilities and contrary to the usual belief no radio frequency wave-induced flow generation hypothesis is required. This might be a major way to create a transport barrier in the fusion energy generation. [Preview Abstract] |
Tuesday, November 1, 2016 4:36PM - 4:48PM |
JO7.00014: Effects of temperature dependence of electrical and thermal conductivities on the heating of a one dimensional conductor Foivos Antoulinakis, Peng Zhang, Y. Y. Lau, David Chernin Dependence of electrical conductivity on temperature gives rise to electrotheramal instability, an important instability for Z-pinches [1]. In other areas, ohmic heating limits the operation of nanoscale circuits such as graphene electronics, carbon nanofiber based field emitters, and nanolasers [2]. For many applications, it is important to consider the temperature dependence of the thermal and electrical conductivities when calculating the effects of ohmic heating. We examine the effects of linear temperature dependence of the electrical and thermal conductivities on the heating of a one-dimensional conductor by solving the coupled non-linear steady state electrical and thermal conduction equations. We find that there are conditions under which no steady state solution exists. In the special case in which the temperature dependence of the electrical conductivity may be neglected, we have obtained explicit expressions for these conditions. The maximum temperature and its location within the conductor are examined for various boundary conditions. We note that the absence of a steady state solution may indicate the possibility of thermal runaway. [1] K. J. Peterson, et al., Phys. Plasmas, 20, 056305 (2012). [2] P. Zhang, et al., IEEE J. Quantum Electronics, 52, 2000207 (2016). [Preview Abstract] |
Tuesday, November 1, 2016 4:48PM - 5:00PM |
JO7.00015: Evolution of Multiple Double Layer in Glow discharge and its inherent Properties Prince Alex, Saravanan A, Suraj Sinha Formation and evolution of multiple anodic double layers (MADLs) were experimentally studied in glow discharge plasma. The boundary condition for the existence of MADL was identified in terms of threshold bias and ambient working pressure. The MADL formation is accompanied by an explosive growth in anode current and consequent current-voltage characteristics follows a hysteresis loop. The analysis yield that stable MADLs is only observed when the control voltage V$_{\mathrm{2}}$ is between a certain critical values ( V$_{\mathrm{q}}$ \textless V$_{\mathrm{2}}$ \textless V$_{\mathrm{r}}$ and V$_{\mathrm{t}}$ \textless V$_{\mathrm{2}}$ \textless V$_{\mathrm{u}}$ ). At V$_{\mathrm{q}}$ , electron drift velocity exceed the electron thermal velocity ($\nu_{\mathrm{d}}$ \textasciitilde 4$\nu_{\mathrm{te}})$ results in Buneman instability and leads the formation of MADL. Above V$_{\mathrm{r}}$ MADL begins to decay and at large V$_{\mathrm{2}}$ where $\nu_{\mathrm{d}}$ \textgreater \textgreater $\nu_{\mathrm{te}}$ MADL completely transforms to an intense high current carrying unstable anode glow. The floating potential analysis carried out using three axially positioned electrostatic probes shows a bipolar signature of DL with \begin{figure}[htbp] \centerline{\includegraphics[width=1.34in,height=0.17in]{130720161.eps}} \label{fig1} \end{figure} as the control parameter is varied. The floating potential analysis also shows that hysteresis arises due to the difference in magnitude of electric field required to align the space charges in the DL sheet at the control voltage changes forward and backward. The effect of pressure on MADL indicates that the MADL structure advances towards anode surface as the pressure is increases. The power dumped (W) in the MADL is estimated to decrease with increase in pressure while the same increase in the anode glow. [Preview Abstract] |
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