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 UP10: Poster Session VIII (Waves, Turbulence and Shocks)Poster
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Room: Exhibit Hall 1 |
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UP10.00001: WAVES, TURBULENCE AND SHOCKS |
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UP10.00002: Observations of Macroscopic Shocks in the Laboratory Douglass Endrizzi, Lauren Laufman-Wollitzer, Mike Clark, Joseph Olson, Rachel Myers, Cary Forest, Hiroshi Gota A magnetized coaxial plasma gun (MCPG) built by Tri Alpha Energy has been installed on the Wisconsin Plasma Astrophysics Lab (WiPAL) vacuum vessel. The MCPG fires a dense ($10^{18} m^{-3}$) and warm (10-30 eV) compact toroid (CT) at speeds of order 100 km/s. The CT is characterized using $\dot{B}$ magnetic diagnostics, multi-tip temperature probes, Ion saturation density probes, and a fast Phantom camera. The CT is injected into vacuum field, neutral gas, and plasmas of various beta. Results and evidence for propagating shocks will be presented. [Preview Abstract] |
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UP10.00003: Collision Experiment of an Arched Plasma-Filled Flux Rope and a Target Cloud of Initially Neutral Gas Pakorn Wongwaitayakornkul, Paul Bellan, Hui Li, Shengtai Li Shocks occur in the co-rotating interaction regions just beyond the solar corona, in the corona during CME events, and when the solar wind impacts Earth's magnetosphere. The Caltech solar loop experiment investigates shock physics by creating an arched plasma-filled flux rope that expands to collide with a pre-injected, initially-neutral gas. We focus the investigation on the situation of a heavy-gas plasma (Argon) impacting a much lighter neutral gas cloud (Hydrogen). The neutral gas target cloud ionizes immediately upon being impacted and plasma-induced shock waves propagate in the target cloud away from the impact region. Analysis of data from magnetic probes, Langmuir probes, a fast camera, and spectroscopic measurements will be presented. The measurements suggest that a thin, compressed, ionized layer of hydrogen is formed just downstream of the Argon plasma loop and that thin, supersonic shocks form further downstream and propagate obliquely away from the plasma loop. Numerical simulation of an ideal MHD plasma is underway to enable comparison of the measurements with the predictions of MHD theory. [Preview Abstract] |
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UP10.00004: Laboratory studies of stagnating plasma flows with applications to inner solar system and stellar bow shocks T.E. Weber, R.J. Smith, S.C. Hsu Supercritical magnetized collisionless shocks are thought to play a dominant role in the overall partition of energy throughout the universe by converting flow kinetic energy to other forms such as thermal and supra-thermal populations, magnetic field enhancement, turbulence, and energetic particles. The Magnetized Shock Experiment (MSX) at LANL creates conditions similar to those of inner solar system and stellar bow shocks by accelerating hot (100s of eV during translation) dense (10$^{\mathrm{22}}$ -- 10$^{\mathrm{23}}$ m$^{\mathrm{-3}})$ Field Reversed Configuration (FRC) plasmoids to 100s of km/s; resulting in $\beta \approx $ 1, collisionless plasma flows with M$_{\mathrm{sonic}}$ and M$_{\mathrm{Alfv\mbox{\'{e}}n}}$ $\approx $ 10. The drifting FRC can be made to impinge upon a variety of static obstacles including: a strong mirror or cusp magnetic field (mimicking magnetically excited shocks such as the Earth's bow shock), plasma pileup from a solid obstacle (similar to the bow shocks of Mercury and the Moon), and a neural gas puff (bow shocks of Venus or the comets). Characteristic shock length and time scales that are both large enough to observe yet small enough to fit within the experiment, enabling study of the complex interplay of kinetic and fluid processes that mediate cosmic shocks and can generate non-thermal distributions, produce density and magnetic field enhancements much greater than predicted by fluid theory, and accelerate particles. An overview of the experimental program will be presented, including recent results. [Preview Abstract] |
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UP10.00005: Experiments and Diagnostics for Investigation of Shock Formation in Colliding Magnetized Plasma Flows Andrew Hamilton, James Caplinger, Vladimir Sotnikov, Chris Plechaty The problem of producing collisional and collisionless shocks in the laboratory is of great interest for numerous space plasma applications$^{\mathrm{1}}$. One approach is based on the idea of combining the strong magnetic fields created during the vaporization and ionization of two parallel wires resulting in a high velocity plasma flow $^{\mathrm{2}}$. In support of laboratory experiment we propose to use pulse power generator with the following parameters: 2.5kA, 80kV, rise time of 5ns. Magnetic fields of 10$^{\mathrm{4\thinspace }}$Gauss, very near the wires, are created due to the currents produced by the pulse power generator. This will allow us to investigate interaction of two colliding plasma flows with frozen magnetic fields in opposite directions. Parameters of the flow will correspond to that produced in the process of wire implosion experiments in the Antenna and Electromagnetic Technology Branch's Plasma Physics Sensors Laboratory (PPSL). Currently with the existing power in the pulse power generator collisional radiative shocks can be created in the vicinity of a reconnection region in such a system. \begin{enumerate} \item B.T. Draine and C.F. McKee, ``Theory of Interstellar Shocks,'' Annual Rev. Astron. Astrophysics, 1993, pp. 373--- 432. \item M.A. Mal'kov and V.I. Sotnikov, ``Lower Hybrid Drift Instability and Reconnection of Magnetic Field Lines of Force,'' Soviet Journal of Plasma Physics, Sept 1985, pp.626---631 \end{enumerate} [Preview Abstract] |
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UP10.00006: ABSTRACT WITHDRAWN |
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UP10.00007: Shocks in 2D Yukawa systems : thermodynamics and kinetic properties. Michael S. Murillo, Mathieu Marciante The study of shock propagation has become a common way to obtain statistical information on a medium, as one can relate properties of the undisturbed medium to the shock dynamics through the Rankine-Hugoniot relations. However, theoretical investigations of shock dynamics are often done through idealized fluid models, which neglect kinetic properties. In this poster we study the propagation of shock waves in plasmas at the particle level, using molecular dynamics simulations to model the propagation of stationary shock waves in a two-dimensional Yukawa plasma. Stationary shocks are generated by a piston moving at constant speed, and macroscopic thermodynamic quantities such as temperature and pressure are computed from statistical averages. This theoretical investigation comprises two parts. First, we present the thermodynamic equilibrium properties of Yukawa plasmas under shock dynamics. Next, we focus on the influence of the kinetic aspects of the plasma, showing how transport coefficients of fluid models are related to the microscopic dynamics in phase-space and apply it to the Yukawa plasma case. [Preview Abstract] |
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UP10.00008: Confinement and Transport in a Laboratory Magnetosphere Ethan Peterson, Michael Clark, Christopher Cooper, Douglass Endrizzi, John Wallace, David Weisberg, Cary Forest Measurements of density, temperature, diamagnetic currents, and ion flows throughout a dipole magnetosphere immersed in a homogeneous plasma are presented. A 1-D ambipolar diffusion transport model developed for multi-cusp confinement systems is adapted for a dipole magnetosphere geometry and compared to measurements. In addition, differential azimuthal flow is imposed on the magnetosphere through electrically driven flow at the boundary of the machine. Modifications to the transport and confinement due to differential rotation are presented as well. [Preview Abstract] |
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UP10.00009: On-Off intermittency detected at the onset of turbulence in a magnetized plasma column Thiery Pierre The transition to turbulence is investigated in a rotating linear magnetized plasma column (MISTRAL device) and the role of the noise is emphasized. The destabilization is induced by injection of electrons on the axis of the device biasing the anode of the source plasma. Starting from a rotating plasma, that can be compared to a laminar regime in fluid dynamics, the slight increase of the potential of the source plasma leads to the onset of intermittent bursts in the edge corresponding to the expulsion of plasma blobs and to the transient destruction of the stable rotating plasma column. The statistical analysis of the time series of the density at the onset of the intermittency is performed. The distribution of the recurrence time of the turbulent bursts and the distribution of the duration of the laminar phases are analyzed. At the threshold, a power law is found for the distribution of the laminar duration with critical exponent -3/2. This dynamical behavior is similar to On-off intermittency (Platt, Spiegel, Tresser, PRL 70, 279,1993) induced by Gaussian noise superimposed on the control parameter. When the control parameter is increased, the distribution evolves towards an exponential decay law. [Preview Abstract] |
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UP10.00010: Stirring a slightly magnetized column of plasma Victor D\'esangles, Guillaume Bousselin, Alexandre Poy\'e, Marc Moulin, Ludovic de Poucques, Nicolas Plihon The von-K\`arm\`an plasma experiment (VKP) is a cylindrical, low pressure, high density plasma experiment which confines the plasma thanks to an axial magnetic field. Currents are radially driven between a hot emissive cathode and an anode which apply a Lorentz force on the plasma together with the magnetic field. We demonstrate that current driven radially sets the plasma into rotation. LIF technique at 668.43 nm as well as Mach probes measurements have been developed and used in different regimes in order to measure the velocity of plasma and relate it to the current driven between the electrodes. The LIF signal shows an important widening which corresponds to doppler shift effect due to the velocity of the ions. This widening can be related to the Mach probes signals. In the long term views, each end of the plasma column will be rotating in an opposite direction, such as to create a large shear-layer, resulting in a von-K\`arm\`an-type flow. [Preview Abstract] |
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UP10.00011: Transition from avalanche dominated transport to drift-wave dominated transport in a basic laboratory experiment Bart Van Compernolle, George Morales, James Maggs, Richard Sydora Results of a basic heat transport experiment involving an off-axis heat source are presented. Experiments are performed in the Large Plasma Device (LAPD) at UCLA. A ring-shaped electron beam source injects low energy electrons (below ionization energy) along a strong magnetic field into a preexisting, large and cold plasma. The injected electrons are thermalized by Coulomb collisions within a short distance and provide an off-axis heat source that results in a long, hollow, cylindrical region of elevated plasma pressure embedded in a colder plasma, and far from the machine walls. The off-axis source is active for a period long compared to the density decay time, i.e. as time progresses the power per particle increases. Two distinct regimes are observed to take place, an initial regime dominated by avalanches, identified as sudden intermittent rearrangements of the pressure profile, and a second regime dominated by sustained drift-Alfv\'en wave activity. The transition between the two regimes is sudden, affects the full radial profile and is preceded by the growth of drift Alfv\'en waves. Langmuir probe data will be shown on the evolution of the density, temperature and flow profiles during the transition. The character of the sustained drift wave activity will also be presented. [Preview Abstract] |
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UP10.00012: Nonlinear Convective Heat Transport in Multiple Interacting Magnetized Electron Temperature Filaments Scott Karbashewski, Richard Sydora, Bart Van Compernolle, George Morales, James Maggs Results are presented from basic heat transport experiments and gyrokinetic simulations of multiple magnetized electron temperature filaments in close proximity. This arrangement samples cross-field transport from nonlinear drift-Alfven waves and large scale convective cells. Experiments are performed in the Large Plasma Device (LAPD) at UCLA. A biased LaB6 cathode injects low energy electrons (below ionization energy) along a strong magnetic field into a pre-existing large and cold plasma forming an electron temperature filament embedded in a colder plasma, and far from the machine walls. A carbon masking plate with several holes (each 1cm diameter, 1.5cm apart) is used to create 3 electron temperature filaments. By covering two holes in the mask drift-Alfven and thermal waves from a single filament have been characterized and compared to previous studies with a different electron beam source (Pace et al., Phys. Plasmas, 15, 122304 (2008)). The observed eigenmode structures also compares favorably with recent 3D gyrokinetic simulations (Sydora et al., Phys. Plasmas, 22, 102303 (2015)). The 3-filament case exhibits a complex wave pattern and enhanced cross-field transport. Detailed mode analysis and comparison with nonlinear simulations is reported. [Preview Abstract] |
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UP10.00013: Recent Experimental and Numerical Results on Turbulence, Flows and Global Stability Under Biasing in a Magnetized Linear Plasma M. Gilmore, T.R. Desjardins, D.M. Fisher Ongoing experiments and numerical modeling on the effects of flow shear on electrostatic turbulence in the presence of electrode biasing are being conducted in helicon plasmas in the linear HelCat (Helicon-Cathode) device. It is found that changes in flow shear, affected by electrode biasing through Er x Bz rotation, can strongly affect fluctuation dynamics, including fully suppressing the fluctuations or inducing chaos. The fundamental underlying instability, at least in the case of low magnetic field, is identified as a hybrid resistive drift-Kelvin-Helmholtz mode. At higher magnetic fields, multiple modes (resistive drift, rotation-driven interchange and/or Kelvin-Helmholtz) are present, and interact nonlinearly. At high positive electrode bias (V \textgreater 10Te), a large amplitude, global instability, identified as the potential relaxation instability is observed. Numerical modeling is also being conducted, using a 3 fluid global Braginskii solver for no or moderate bias cases, and a 1D PIC code for high bias cases. Recent experimental and numerical results will be presented. [Preview Abstract] |
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UP10.00014: Turbulence and Transport in Multi-Ion Species Plasmas in the Large Plasma Device Jeffrey Robertson Understanding of turbulence and transport in multi-ion-species plasmas is important for establishing predictive capability for burning tokamak plasmas with comparable densities of D and T. Fundamental modifications to drift-wave instabilities and resulting turbulence are expected from theoretical studies, including new instabilities driven by dissimilar ion density gradients [1]. Even in pure ion species plasmas, transport mysteries remain regarding dependence on ion mass such as the isotope scaling of turbulent transport [2]. Recently, experiments have been performed on the Large Plasma Device at UCLA in which mixed Hydrogen-Helium plasmas were created and the relative concentration was varied systematically. The properties of edge turbulence and transport rates were documented and initial results will be presented. Experimental results are will also be compared to linear drift-wave instability theory in plasmas with multiple ion species. [1] New Paradigm for the Isotope Scaling of Plasma Transport Paradox Sokolov, V. and Sen, A. K., Phys. Rev. Lett. 92, 165002 (2004). [2] Isotope scaling and $\eta $i mode with impurities in tokamak plasmas Dong, J. Q. and Horton, W. and Dorland, W., Physics of Plasmas, 1, 3635-3640 (1994). [Preview Abstract] |
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UP10.00015: Role of edge turbulence and shear flows in density limit on HL-2A tokamak Rongjie Hong, George Tynan, Min Xu, Lin Nie, Dong Guo, Rui Ke, Ting Long, Yifang Wu, Boda Yuan The tokamak density limit has long been suspected as a consequence of the enhanced turbulent transport in edge plasmas. In this study, evolutions of the turbulence and shear flows were investigated at different normalized density $\bar{n}_e / n_{G}$ in the plasma boundary region of HL-2A tokamak using Langmuir probes. As the density limit was approached, the equilibrium profile of density was flattened in the Scrape-Off Layer (SOL) and steepened inside the separatrix, while the edge cooling was observed from the electron temperature profile. The turbulent cross-field transport also increased substantially with the $\bar{n}_e / n_{G}$ and the collisionality. In addition, the amplitude of the poloidal phase velocity decreased at higher densities. This destruction of the shear layer was associated with the collapse of the Reynolds stress and thus the reduction in the nonlinear energy transfer from high-frequency fluctuations to low-frequency shear flows. These observations indicate an important role of the edge turbulence and the turbulence-driven shear flow in the underlying physics of tokamak density limit. [Preview Abstract] |
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UP10.00016: Cross-diagnostic comparison of fluctuation measurements in a cylindrical argon plasma Adam Light, Saikat Chakraborty Thakur, George Tynan The advent of fast imaging diagnostics, which provide two-dimensional measurements on relevant plasma time scales, has proven invaluable for interpreting plasma dynamics in laboratory devices. Despite its success, imaging remains a qualitative aid for many studies, because intensity is difficult to map onto a single physical variable for use in a theoretical model. This work continues our exploration of the relationship between visible-light imaging and other diagnostics in the Controlled Shear Decorrelation Experiment (CSDX). CSDX is a well-characterized linear machine producing dense plasmas relevant to the tokamak edge ($T_e \sim 5$ eV, $n_e \sim 10^{13}$/cc). Visible light from ArI and ArII line emission is collected at high frame rates using a fast digital camera, floating potential and ion-saturation current are measured by an array of electrostatic probe tips, and average profiles of ion temperature and velocity are obtained using laser-induced fluorescence (LIF). We present a detailed comparison between these measurements, including temporal, spatial, and spectral properties in various operational regimes. [Preview Abstract] |
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UP10.00017: Spontaneous transition from drift wave turbulence to multi-instability turbulence: bifurcation, hysteresis and plasma detachment. Saikat Chakraborty Thakur, Rongjie Hong, Kyle Adriany, George Tynan Recent studies in the Controlled Shear Decorrelation eXperiment show a spontaneous self-organized global transition in the plasma dynamics via a transport bifurcation during the transition to broadband turbulence [1, 2] with increasing magnetic field (B). For B \textless B$_{\mathrm{crit}}$, the plasma is dominated by density gradient driven resistive drift waves. For B \textgreater B$_{\mathrm{crit}}$ the plasma is characterized by steepened density and ion temperature gradients and both azimuthal and parallel velocity shear layers, along with multiple plasma instabilities. In this new equilibrium, we find high azimuthal mode number fluctuations rotating in the ion diamagnetic drift direction at the core, resistive drift waves near the density gradient and turbulence driven sheared flows near the edge. The plasma also seems to simultaneously detach from the end of the device and the length of the plasma column shortens. We use spectroscopy to study the detachment, which also follows the hysteresis curves associated with the transport bifurcation that led to steepened profiles. We find that the value of B$_{\mathrm{crit}}$, depends on operating pressure, gas flow rate and RF input power. [1] L. Cui \textit{et. al.}, \textit{PoP }\textbf{23} 055704 (2016) [2] S. C. Thakur \textit{et. al.}, \textit{PSST} \textbf{23} 044006 (2014) [Preview Abstract] |
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UP10.00018: Understanding the Impact of Parallel Boundary Conditions on Turbulence in CSDX through Nonlocal Simulations Payam Vaezi, Christopher Holland, Saikat Takhur, George Tynan The Controlled Shear Decorrelation Experiment (CSDX) linear plasma device provides a unique platform for investigating the underlying physics of self-regulating drift-wave turbulence/zonal flow dynamics. A minimal model of 3D equations of drift-reduced nonlocal cold ion fluid, which evolves density, vorticity, and electron temperature fluctuations, with proper sheath boundary conditions is used to simulate dynamics of the turbulence in CSDX and its response to changes in parallel boundary conditions. These simulations carried out using BOUndary Turbulence (BOUT$++)$ framework, and use equilibrium electron density and temperature profiles taken from experimental measurements. The simulation results show that the choice of axial insulating or conducting boundary conditions influences the turbulence structure and zonal flow formation, resulting in less broadband (more quasi-coherent) turbulence in conducting boundary condition. These initial results are qualitatively consistent with experimental observations, and progress in more quantitative validation analysis of the model using synthetic diagnostics will be presented. This work is supported by US DoE under DE-FG02-06ER54871. [Preview Abstract] |
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UP10.00019: Experimental Investigation of Turbulent-driven Sheared Parallel Flows in the CSDX Plasma Device George Tynan, Rongjie Hong, Jiacong Li, Saikat Thakur, Patrick Diamond Parallel velocity and its radial shear is a key element for both accessing improved confinement regimes and controlling the impurity transport in tokamak devices. In this study, the development of radially sheared parallel plasma flows in plasmas without magnetic shear is investigated using laser induced fluorescence, multi-tip Langmuir and Mach probes in the CSDX helicon linear plasma device. Results show that a mean parallel velocity shear grows as the radial gradient of plasma density increased. The sheared flow onset corresponds to the onset of a finite parallel Reynolds stress that acts to reinforce the flow. As a result, the mean parallel flow gains energy from the turbulence that, in turn, is driven by the density gradient. This results in a flow away from the plasma source in the central region of the plasma and a reverse flow in far-peripheral region of the plasma column. The results motivate a model of negative viscosity induced by the turbulent stress which may help explain the origin of intrinsic parallel flow in systems without magnetic shear. [Preview Abstract] |
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UP10.00020: Intrinsic Axial Flows in CSDX and Dynamical Symmetry Breaking in ITG Turbulence Jiacong Li, P.H. Diamond, R. Hong, S.C. Thakur, X.Q. Xu, G.R. Tynan Toroidal plasma rotation can enhance confinement when combined with weak magnetic shear [Mantica, PRL, 2011]. Also, external rotation drive in future fusion devices (e.g. ITER) will be weak. Together, these two considerations drive us to study intrinsic rotations with weak magnetic shear. In particular, a global transition is triggered in CSDX when magnetic field B exceeds a critical strength threshold [Cui, PoP, 2016]. At the transition an ion feature emerges in the core turbulence. Recent studies show that a dynamical symmetry breaking mechanism in drift wave turbulence [Li, PoP, 2016] can drive intrinsic axial flows in CSDX, as well as enhance intrinsic rotations in tokamaks. Here, we focus on what happens when ion features emerge in CSDX, and how ion temperature gradient (ITG) driven turbulence drives intrinsic rotations with weak magnetic shear. The effect of dynamical symmetry breaking in ITG turbulence depends on the stability regime. In a marginally stable regime, dynamical symmetry breaking results in an augmented turbulence viscosity (chi-phi). However, when ITG is far from the stability boundary, a negative increment in turbulent viscosity is induced. [Preview Abstract] |
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UP10.00021: Modeling studies of transport bifurcation phenomena in a collisional drift wave turbulence. Rima Hajjar, Patrick Diamond, Georges Tynan, Arash Ashourvan Self-organization of drift wave turbulence via particle transport and Reynolds stresses is a mechanism for turbulence suppression and reduction of cross field transport. This energy transfer mechanism between microscale drift waves and mesoscale zonal flows can create a transport bifurcation and trigger the formation of an internal transport barrier. We report here on studies investigating transport bifurcation dynamics in the CSDX linear device using a 1D reduced turbulence and mean field evolution model. This two-mixing scale Hasegawa-Wakatani based model evolves spatio-temporal variations of three plasma fields: the mean density $n$, the mean vorticity $u$ and the turbulent potential enstrophy $e$. The model adopts inhomogeneous potential vorticity mixing on a mixing length the expression of which is related to the Rhines' scale and to the mode scale (i.e. is $\nabla n$ and $\nabla u$ dependent). The model is based on expressions for turbulent fluxes of $n$, $u$ and $e$ derived from mixing length concepts. Turbulent particle and enstrophy transport are written as diffusive, but a residual stress part is included in the expression for the vorticity flux. Mixed boundary conditions are used at both ends of the domain and an external boundary fueling source is added. Simulation results show a steepening in the particle density profiles with \textbf{B} along with the formation of a net flow shear layer resulting from the vorticity mixing. These results suggest that the system dynamic is capable of sustaining the plasma core by means of a purely diffusive particle flux, without any explicit inward particle pinch. [Preview Abstract] |
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UP10.00022: Characterization of magnetic dynamos driven by non-Gaussian, correlated velocity fluctuations R. Sanchez, D. E. Newman, J.M. Reynolds-Barredo The generation of magnetic dynamos by turbulent fluctuations with non-Gaussian, non-Markovian features has been studied by means of meshless, Lagrangian numerical schemes inspired in those of Smoothed-Particle Hydrodynamics. The power of this numerical approach has allowed us to fully characterize the dynamo generation over a wide range of values of the exponents that characterize the statistics and correlation of the turbulent fluctuations. In particular, the tail exponent of the probability density function of the velocity fluctuations, $\alpha$, and the Hurst exponent $H$ of the turbulent velocity along its Lagrangian trajectories. Our numerical results indicate that, over some significant exponent ranges, magnetic dynamo generation is observed in spite of the absence of any mean flow helicity, which is in contrast to what is expected in the more traditional Gaussian, Markovian case. [Preview Abstract] |
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UP10.00023: Model for avalanches in a hollow pressure filament M. J. Poulos, G. J. Morales A theoretical and modeling study is made of a novel heating configuration recently implemented in the LAPD device at UCLA [1]. The geometry essentially consists of a hollow pressure filament embedded in a cold, magnetized plasma. An eigenmode-based stability analysis is made of drift-waves excited by simultaneous gradients in density and temperature. A Braginskii transport code that includes the ExB flows of the unstable modes is used to self-consistently evolve the profile modifications in the presence of external heating. A shooting code is used to calculate the evolving mode structures as the profiles are modified. It is found that intermittent avalanches are triggered and their properties are in good agreement with the experimental observations. During the recovery phase after an avalanche the large difference between the relaxation times of density and temperature results in azimuthal filamentation of the profiles. New insights are obtained that suggest future laboratory and theoretical studies. [1] B. Van Compernolle, G. J. Morales, J. E. Maggs , and R. D. Sydora, Phys. Rev. E 91, 031102 (2015). [Preview Abstract] |
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UP10.00024: Revisiting the Physics of Long - Short Interaction P.H. Diamond, N.T. Katt, G.T. Katt An important example of disparate scale interaction in confined plasmas is that of the interplay of larger scale drift wave turbulence with smaller scale ETG turbulence. Here, we examine electron heat avalanches in the presence of both drift wave and ETG activity. We explore: i.) the rapid release of ETG to the passage of a larger scale avalanche. The ETG has the effect of limiting the sharpness of the crest of the avalanche by switching on rapid parasitic transport. This suggests that should the profiles be close to ETG marginality, the small scale modes will sharply inhibit the penetration of drift wave avalanches into that zone. Transport will occur primarily via the ETG, on the tilted crests of the large scale drift wave avalanches. ii.) the interplay of two response times (drift wave and ETG) relating the instantaneous and mean heat fluxes. In particular, the instantaneous flux can relax to mean flux on either of the (faster) ETG time scale or the (slower) drift wave time scale, depending on which is excited. These combine to yield a multi-time-scale telegraph equation for the heat avalanche. [Preview Abstract] |
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UP10.00025: Ion pressure gradient effects on Kelvin-Helmholtz and interchange instabilities David Russell, James Myra In the flow-free state, radial force-balance implies that the poloidal components of the ExB and ion diamagnetic drifts, \textasciitilde grad(Pi) / n, are mirrored : vE $+$ vdi $=$ 0. Analysis [1] of the linearized fluid equations shows that the mirrored state is stable in the absence of the interchange drive, \textasciitilde grad(Pe$+$Pi) / n, i.e., the K-H instability is absent. With the interchange drive present, the mirrored-state growth rate passes through a global \textit{minimum} value with \textit{increasing} ion pressure gradient, due to sheared ExB flow and diamagnetic suppression, admitting a stability interval in a neighborhood of the minimum if other damping mechanisms are present. The K-H instability is recovered, absent the interchange drive, if force-balance is generalized to include neoclassical poloidal flows (vE $+$ vdi $+$ vnc $=$ 0, vnc \textasciitilde grad(Ti)) [2], so that mirroring is lost. Implications for achieving a quiescent H-mode are discussed, and SOLT simulations, which include nonlinear ion pressure effects, are compared with the linear picture. [1] J.R. Myra et al., J. Plasma Phys. \textbf{82}, 905820210 (2016). [2] L. Ch\^{o}n\'{e} et al., Phys. Plasmas \textbf{21}, 070702 (2014). [Preview Abstract] |
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UP10.00026: Transition Dynamics and Hysteresis of Plasma Edge Transport in a Flux-driven System Xueyun Wang, Chuankui Sun, Ao Zhou, Dianjing Liu, Bo Li, Xiaogang Wang Transition of low to high confinement regimes due to competition between interchange turbulence and mean shear flow in edge plasmas are explored. We find that two regimes with different transport levels exist during the nonlinear evolution of the interchange mode. In the first (L-) regime, the large-scale turbulent eddies dominate with the high level of transport. By increasing the input heat flux above a certain threshold, the transition to the second (H-) regime occurs, in which strong mean \textbf{E}$\times $\textbf{B} shear flows are generated. The large-amplitude oscillations which form both clockwise and counter-clockwise cycles in the phase space of turbulence intensity and mean flow energy during a transition are demonstrated by the nonlinear energy transfer. We show that the open field line region is critical to the formation of Reynolds stress gradients, which produces the mean shear flows localized in the edge through the turbulent momentum transport. The back transition to the L-regime is studied by reducing the input heat flux to a level much lower than the threshold for the forward transition, indicating a significant hysteresis. [Preview Abstract] |
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UP10.00027: Momentum Transport and Stable Modes in Kelvin-Helmholtz Turbulence Adrian Fraser, Paul Terry, Ellen Zweibel, MJ Pueschel Ubiquitous in astrophysical and fusion systems, where turbulent momentum transport is of interest, the Kelvin-Helmholtz (KH) instability features unstable and stable modes at the same scales. We show that KH turbulence, in keeping with recent findings for other turbulence types, can have stable modes affect transport and move systems away from the usual energy cascade through an inertial range. Using a threshold parameter, we evaluate energy transfer to stable modes and its associated impact on turbulent amplitudes and transport, demonstrating the possibility of stable-mode-regulated KH systems. A quasilinear momentum transport calculation is performed to quantify the reduction in momentum transport due to stable modes. Finally, comparisons are made to gyrokinetic simulations driven by shear flows; linearly stable companion modes are identified, and their impact on turbulence is quantified. [Preview Abstract] |
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UP10.00028: Directional change of particles in dissipative drift-wave turbulence Benjamin Kadoch, Wouter J.T. Bos, Kai Schneider We analyze the statistical properties of Lagrangian particle transport in dissipative drift-wave turbulence modeled by the Hasegawa-Wakatani system. The angle between subsequent particle displacement increments is evaluated as a function of the timelag and thus multi-scale geometric statistics can be performed [Bos et a., PRL, 114, 214502, 2015]. The evolution of the mean angle with the time lag is studied and the probability density function of the directional change are analyzed for the different flow regimes. By varying the adiabaticity parameter the flow regime can be modified from the hydrodynamic limit to a geostrophic limit, including the quasi adiabatic regime which has some relevance for edge turbulence of fusion plasmas in tokamaks. [Preview Abstract] |
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UP10.00029: Full-f gyrokinetic study of Electron Temperature Gradient Mode Jugal Chowdhury, Seung-Hoe Ku, Robert Hager, Randy Michael Churchill, Choong-Seock Chang, Yang Chen, Scott E. Parker There has been renewed interest on the possibility of having a substantial electron heat flux by the electron temperature gradient (ETG) modes, although they are small scale instabilities. Inside a steep transport barrier the low-k instabilities are usually suppressed to a significant degree, but some residual low-k turbulence modes can still exist. In the edge pedestal, two interesting questions remain to be answered. How will the strong background plasma dynamics interact with the ETG modes in producing transport, and how will the residual low-k turbulence affect the ETG modes. The strong background plasma dynamics include the self-organized plasma profile and rotation profile, and the ExB shearing profile. There have been gyrokinetic studies for the ETG mode using reduced delta-f methods in which the driving forces from background plasma gradients are fixed and, at the same time, simplified without the magnetic drift driver. The questions raised here requires a full-f simulation. In this presentation, we study the first question under the simplification that all other turbulence modes are completely suppressed in the edge pedestal. We use the full-f 5D gyrokinetic code XGC1 with diverted magnetic geometry. We will compare the results with the simplified delta-f simulations. [Preview Abstract] |
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UP10.00030: A Full Eulerian Vlasov-Maxwell Study of Turbulent Dynamics and Dissipation Jason TenBarge, James Juno, Ammar Hakim The development of a detailed understanding of turbulence in magnetized plasmas has been a long standing goal of the broader scientific community, both as a fundamental physics process and because of its applicability to a wide variety of phenomena. Turbulence in a magnetized plasma is the primary mechanism responsible for transforming energy at large injection scales into small-scale motions, which are ultimately dissipated as heat in systems such as the solar corona and wind. At large scales, the turbulence is well described by fluid models of the plasma; however, understanding the processes responsible for heating a weakly collisional plasma such as the solar wind requires a kinetic description. We present the first fully kinetic Eulerian Vlasov-Maxwell study of turbulence using the Gkeyll simulation code. We focus on the pristine distribution function dynamics that are possible with the Eulerian approach. We also present the signatures and form of dissipation as diagnosed via field-particle correlation functions. [Preview Abstract] |
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UP10.00031: Studying the interaction mechanisms of the tearing mode and drift-wave turbulence S.D. James, D.P. Brennan, C. Holland, O. Izacard Understanding the mechanisms through which turbulence and MHD instabilities interact is vital to the success of magnetically confined fusion. Simulating the self-consistent evolution of turbulence and MHD instabilities is a challenging numerical problem due to the disparate scales involved. We use a newly developed code, TURBO, to perform nonlinear simulations of a three-field model which couples the evolution of drift-wave turbulence to Ohm's Law. TURBO evolves the density, vorticity, and magnetic flux in a slab geometry using an equilibrium with prescribed stability properties and turbulent drives. By imposing a propagating boundary condition on the magnetic flux we examine the dependence of an asymmetry in the density flux on the propagation velocity of the boundary condition. We present results showing the influence of the turbulence on the stability of the tearing mode and the energy transport between them via a turbulent resistivity. For the case of a static island in a poloidal flow, results indicate that the energy transport and density flux display a spatial asymmetry in the poloidal direction and are peaked away from the X-point. A recent study of ITG turbulence in the presence of a magnetic island found analogous effects and we discuss the relation to our work. [Preview Abstract] |
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UP10.00032: Transport of particles in chaotic magnetic fields -- transition between superdiffusion and normal diffusion F. Holguin, A. K. Ram Magnetic fields in regions of low plasma pressure and large currents, such as in interstellar space and gaseous nebulae, are force-free as the Lorentz force vanishes. The Arnold-Beltrami-Childress (ABC) field and the Archontis field are examples of three-dimensional, force-free, helical, chaotic magnetic fields. They correspond to single and double Beltrami flows, respectively. The spatial transport of particles is superdiffusive in the ABC field [1] and normally diffusive in the Archontis field. If these spatially fluctuating fields are added onto a larger amplitude uniform magnetic field, the particle transport across the uniform field depends on the energy of the particles – a mix of normal and super diffusion for low energies, and superdiffusion for high energies. In the presence of fluctuating fields with sinusoidal time variation, the particles not only undergo cross-field diffusion but also gain energy. We present results on the cross-field diffusion of particles and on their energization. The transition between normal diffusion and superdiffusion is discussed within the realm of spatial transport. [1] A.K. Ram \textit{et al.}, \textit{Phys. Plasmas} \textbf{21}, 072309 (2014). [Preview Abstract] |
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UP10.00033: Oscillation-center theory for waves D. E. Ruiz, I. Y. Dodin Linear waves, both quantum and classical, can experience ponderomotive effects when propagating in modulated media. This phenomenon is analogous to the ponderomotive effect encountered by charged particles in high-frequency electromagnetic fields. Using the Weyl calculus and generalized Lie transformations for waves, we obtain a first-principle variational theory that describes the slowly-varying, oscillation-center dynamics of waves. In this approach, the characteristic wavelength of the modulations are allowed to be comparable to that corresponding to the wave. Quantum-like effects, such as the photon recoil effect, are retained. Examples and numerical results are presented. The theory is applied to several physical systems, such as: a Schr\"{o}dinger particle interacting with an oscillating electrostatic field, an electromagnetic (EM) wave propagating in a density-modulated plasma, and a Klein-Gordon particle interacting with arbitrary background and oscillatory EM fields. This work can serve as basis for future studies on the modulational instability. [Preview Abstract] |
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UP10.00034: Variational principles for dissipative waves I. Y. Dodin, D. E. Ruiz Variational methods are a powerful tool in plasma theory. However, their applications are typically restricted to conservative systems or require doubling of variables, which often contradicts the purpose of the variational approach altogether. We show that these restrictions can be relaxed for some classes of dynamical systems that are of practical interest in plasma physics, particularly including dissipative plasma waves. Applications will be discussed to calculating dispersion relations and modulational dynamics of individual plasma waves and wave ensembles. [Preview Abstract] |
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UP10.00035: Chirped nonlinear resonance dynamics in phase space Lazar Friedland, Tsafrir Armon Passage through and capture into resonance in systems with slowly varying parameters is one of the outstanding problems of nonlinear dynamics. Examples include resonant capture in planetary dynamics , resonant excitation of nonlinear waves, adiabatic resonant transitions in atomic and molecular systems and more. In the most common setting the problem involves a nonlinear oscillator driven by an oscillating perturbation with a slowly varying frequency, which passes through the resonance with the unperturbed oscillator. The process of resonant capture in this case involves crossing of separatrix and, therefore, the adiabatic theorem cannot be used in studying this problem no matter how slow is the variation of the driving frequency. It will be shown that if instead of analyzing complicated single orbit dynamics in passage through resonance, one considers the evolution of a distribution of initial conditions in phase space, simple adiabaticity and phase space incompressibility arguments yield a solution to the resonant capture probability problem. The approach will be illustrated in the case of a beam of charged particles driven by a chirped frequency wave passing through the Cherenkov resonance with the velocity distribution of the particles. [Preview Abstract] |
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UP10.00036: Analytical and numerical treatment of drift-tearing modes in plasma slab V.V. Mirnov, C.C. Hegna, C.R. Sovinec, E.C. Howell Two-fluid corrections to linear tearing modes includes 1) diamagnetic drifts that reduce the growth rate and 2) electron and ion decoupling on short scales that can lead to fast reconnection. We have recently developed an analytical model that includes effects 1) and 2) and important contribution from finite electron parallel thermal conduction. Both the tendencies 1) and 2) are confirmed by an approximate analytic dispersion relation that is derived using a perturbative approach of small ion-sound gyroradius $\rho_{s}$. This approach is only valid at the beginning of the transition from the collisional to semi-collisional regimes. Further analytical and numerical work is performed to cover the full interval of $\rho_{s}$ connecting these two limiting cases. Growth rates are computed from analytic theory with a shooting method. They match the resistive MHD regime with the dispersion relations known at asymptotically large ion-sound gyroradius. A comparison between this analytical treatment and linear numerical simulations using the NIMROD code with cold ions and hot electrons in plasma slab is reported. [Preview Abstract] |
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UP10.00037: A Model of Energetic Ion Effects on Pressure Driven Tearing Modes in Tokamaks M. R. Halfmoon, D. P. Brennan Previous analysis of toroidal confinement experiments has shown that energetic ions interact with and affect MHD mode stability, which has been modeled and simulated for ideal MHD instabilities and resistive wall modes [1]. In addition, the 2/1 tearing mode was found to be damped or stabilized by energetic ions, with significant effects on the slow growing resistive mode. This study focuses on the mode-particle interactions between energetic ions and pressure-driven, slow growing tearing modes which have been shown to be driven unstable in experiments as pressure increases [2]. Using a reduced analytic description of a high aspect ratio tokamak equilibrium with a fixed total current and variable magnetic shear; we add in the effects of high energy particles as a modification to the perturbed pressure [1]. We find the importance of global magnetic shear (s$=$1/q dq/dr) on the particle precession drift frequency, which determines whether or not the energetic particle population resonates with the mode. We find that for s\textgreater s$_{\mathrm{crit}}$ at the radial position of the pressure jump, particles have a stabilizing effect on the 2/1 tearing mode. However, we find that for s\textless s$_{\mathrm{crit}}$, particles drive mode development. 1. B. Hu, and R. Betti, "Resistive wall mode in collisionless quasistationary plasmas." Physical Review Letters \quad 93.10 (2004): 105002. 2. D.P. Brennan, et al "Energetic particle effects on~n~$=$ 1 resistive MHD instabilities in a DIII-D hybrid discharge," Nucl. Fusion 52, 033004 (2012). [Preview Abstract] |
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UP10.00038: Evolution of relativistic electron current beam in a cold plasma with fixed background of ions Roopendra Singh Rajawat, Sudip Sengupta, Predhiman K Kaw A numerical study of evolution of relativistic electron current beam in a cold homogeneous plasma with immobile ions has been carried out using one dimensional electrostatic relativistic particle-in-cell code. It is found that the beam current when longitudinally perturbed by imposing a relativistically intense wave, diminishes with time due to phase mixing effects, arising because of spatial variation of relativistic mass. Studies have been conducted for various flow velocities($v_{0}/c$) and relativistic intensities ($\frac{eE_{0}}{m\omega_{pe} c}$) of the perturbed wave. Rate of decay of current decreases with increasing flow velocity for a fixed ($\frac{eE_{0}}{m\omega_{pe} c}$); and for a given initial current the final magnitude of current decreases with increasing relativistic intensity of the perturbed wave. [Preview Abstract] |
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UP10.00039: Current filamentation model for the Weibel/Filamentation instabilities Chang-Mo Ryu, Cong Tuan Huynh, Chul Min Kim A current filamentaion model for a nonrelativistic plasma with e$+$/e- beam has been presented together with PIC simulations, which can explain the mangetic field enhancement during the Weibel/ Filamentation instabilities. This filament model assumes the Hammer -Rostoker equilibrium. In addition, this model predicts preferential acceleration/deceleration for electron-ion plasmas depending on the injected beam to be e$+$/e-. [Preview Abstract] |
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UP10.00040: 2D Plasma Photonic Crystals in resonantly pumped Cesium Vapor Fabio Righetti, Mark Cappelli Plasma photonic crystals (PCs) afford the opportunity for dynamic reconfigurability. In this presentation we describe the conditions required for constructing an all-plasma PC that can interact with sub mm-wavelength radiation. Conditions required for this interaction are high plasma densities (\textgreater 10$^{\mathrm{14}}$ cm$^{\mathrm{-3}})$ and small lattice constant (\textless 1 mm). We describe the construction of a two-dimensional photonic crystal composed of several sub-millimeter plasma filaments in a 1 Torr heated cesium vapor cell. The cesium is ionized by 1 W continuous-wave laser excitation with the wavelength centered around the 852 nm resonance line. The filaments are produced by focusing the laser through a microlens array with a 500 \textmu m pitch. Small departures from line center are found to produce a strong variation in the plasma filament structure and density. Stark broadening measurements of the cesium 9F-5D transition at 647.4 nm yield plasma density. We present preliminary terahertz transmission spectrum of the two-dimensional plasma photonic crystal structure. Experimental results are compared to numerical simulations which predict the presence of bandgaps in regions of both negative and positive plasma dielectric constant. [Preview Abstract] |
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UP10.00041: Nonlinear interaction between BAE and BAAE YAQI LIU, Huasen Zhang, Zhihong Lin The beta-induced Alfven-acoustic eigenmode (BAAE) in toroidal plasmas is verified by GTC simulations. The BAAE can be excited by realistic energetic particle density gradient, even though the stable BAAE (in the absence of energetic particles) is heavily damped by the thermal ions. In the simulations with reversed magnetic shear, BAAE frequency sweeping is observed and poloidal mode structure has a triangle shape with a poloidal direction similar to that observed in tokamak experiments. When we decrease the tokamak size ITER to present-day tokamak, the most unstable modes change from BAAE to BAE (beta-induced Alfven eigenmode). For a certain tokamak size, BAE and BAAE coexist with similar linear growth rates. At nonlinear stage, BAE modes saturate first, while BAAE modes continue to grow until nonlinear modes with beating wave (sum of BAE and BAAE frequency) and positive frequencies are excited. In the long time simulation, amplitudes of BAE, BAAE, and beat waves oscillate, indicating mode energy nonlinearly transfers between them. Zonal fields suppress the mode coupling and energy transfer between BAE to BAAE, and reduce frequency chirping and saturation amplitudes. The growth rate of the zonal fields is about twice of the linear growth rate of BAE/BAAE. [Preview Abstract] |
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UP10.00042: Effect of diffusivity on the propagation and the Landau damping of the space-charge wave in a turbulent plasma column Myoung-Jae Lee, Young-Dae Jung, Kyu-Sun Chung When collisions of electrons with ions are involved, the random fluctuating electric field would affect the projectile electrons in a turbulent plasma since the response of the random field fluctuations is important in the binary encounters as well as in the interaction potentials. The dispersion relation for the space-charge wave propagating in a cylindrically-bounded turbulent plasma is derived in terms of the cylinder radius and the roots of the Bessel function of the first kind which appears as the boundary condition by employing the longitudinal dielectric permittivity that contains the diffusivity based on the Dupree theory of turbulent plasma. We find that the wave frequency for a lower-order root of the Bessel function is higher than that of a higher-order root. We also find that the Landau damping of the wave is greatest for the lowest order root, but it is suppressed significantly as the order of the root increases. The wave frequency and the Landau damping are enhanced as the cylinder size increases. We find that the diffusivity of turbulent plasma would enhance the damping of space-charge waves for a small wave number. As the wave number increases, the diffusivity is less effective to the damping of the wave. [Preview Abstract] |
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UP10.00043: Role of Inhomogeneous Flow on Plasma Turbulence Gabriela Vasquez, S Sen In this paper the effect of a radially varying parallel equilibrium flow on the stability of the Rayleigh–Taylor (RT) mode is studied in a plasma medium. It is shown that the parallel flow curvature can completely stabilize the RT mode. The flow curvature also has a robust effect on the radial structure of the mode. Possible implications of these findings are also discussed. [Preview Abstract] |
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UP10.00044: Effect of viscosity on propagation of MHD waves in astrophysical plasma Alemayehu Cherkos We determine the general dispersion relation for the propagation of magnetohydrodynamic (MHD) waves in an astrophysical plasma by considering the effect of viscosity with an anisotropic pressure tensor. Basic MHD equations have been derived and linearized by the method of perturbation to develop the general form of the dispersion relation equation. Our result indicates that an astrophysical plasma with an anisotropic pressure tensor is stable in the presence of viscosity and a strong magnetic field at considerable wavelength. [Preview Abstract] |
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UP10.00045: Nonlinear dispersive evolution of coherent trapped particle structures in collisionless plasmas Debraj Mandal, Devendra Sharma The nonlinear limit of the collective perturbations in plasma is characterized by the onset of amplitude dependence in the wave dispersion. In a special class of nonlinear effects having origin in plasma kinetic theory, this amplitude dependence is removed only by collisions such that perturbations have no linear counterpart in collisionless limit and must follow a nonlinear dispersion relation (NDR). Exploring whether these fundamentally nonlinear perturbations can be driven unstable without entropy production might transform the character of the linear threshold based operating mechanism of the plasma turbulence that relies on well defined discrete spectrum prescribed by the linear plasma dispersion. In our multiscale, exact mass ratio, kinetic simulations the evolution of fundamentally nonlinear trapped particle structures is explored on both fast and slow ion and electron acoustic branches of the associated Nonlinear dispersion relation, respectively. The propagating structures that mutually interact exhibit a near continuum of the phase velocities and show microscopic evolution of the separatrix between streaming and trapped particle regions in the phase space, describing the subtle continuity between discrete and continuum bases of the plasma turbulence. [Preview Abstract] |
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UP10.00046: Computational stutuies of Langmuir solitons and oscillating two stream instabilities S.H. Chang, Y. Nishimura By a Particle-in-Cell simulation, generation of Langmuir solitons \footnote{E. J. Valeo and W. L. Kruer, Phys. Rev. Lett. 33, 750 (1974).} is investigated. Onset of oscillating two stream instability (OTSI) \footnote{K. Nishikawa, J. Phys. Soc. Jpn., 24, 916 (1968).} is observed subsequent to the collapse of the solitons. The saturation mechanism of OTSI is discussed by by estimating the separatrix of resonating islands in the phase space. [Preview Abstract] |
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UP10.00047: Vlasov Simulations of Ladder Climbing and Autoresonant Acceleration of Langmuir Waves Kentaro Hara, Ido Barth, Erez Kaminski, Ilya Dodin, Nathaniel Fisch The energy of plasma waves can be moved up and down the spectrum using chirped modulations of plasma parameters, which can be driven by external fields. Depending on the discreteness of the wave spectrum, this phenomenon is called ladder climbing (LC) or autroresonant acceleration (AR) of plasmons, and was first proposed by Barth \textit{et al. }[Barth \textit{et al.} PRL \textbf{115} 075001 (2015)] based on a linear fluid model. Here, we report a demonstration of LC/AR from first principles using fully nonlinear Vlasov simulations of collisionless bounded plasma [Hara \textit{et al.} PoP \textbf{22} 022104 (2015)]. We show that, in agreement to the basic theory, plasmons survive substantial transformations of the spectrum and are destroyed only when their wave numbers become large enough to trigger Landau damping. [Preview Abstract] |
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UP10.00048: A Vlasov equation to describe the Alfven wave CVK spectrum Fred Skiff We explore a Vlasov description of the Alfven wave to enable a kinetic analysis of experimental data of wave-particle interactions (suprathermal electron parallel-velocity distribution functions). By neglecting the displacement current, the Alfven wave is described by a Case-Van Kampen (CVK) continuum analogous to the electron plasma wave and the ion acoustic wave. Use of the appropriate diagonalizing transform (projection onto CVK modes) provides a way of analyzing the interaction of a plasma antenna structure with the plasma. This Vlasov description reduces to the usual two-fluid description of kinetic and inertial Alfven waves in the appropriate limits. [Preview Abstract] |
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UP10.00049: Effects of compressional magnetic perturbation on kinetic Alfven waves Ge Dong, Amitava Bhattacharjee, Zhihong Lin Kinetic Alfven waves play a very important role in the dynamics of fusion as well as space and astrophysical plasmas. The compressional magnetic perturbation $\delta $B\textbar \textbar can play important role in kinetic Alfven waves (KAW) and various instabilities at large plasma $\beta $. It could affect the nonlinear behavior of these modes significantly even at small $\beta $. In this study, we have implemented $\delta $B\textbar \textbar in gyrokinetic toroidal code (GTC). The perpendicular Ampere's law is solved as a force balance equation. Double gyroaveraging is incorporated in the code to treat the finite Larmor radius effects related to $\delta $B\textbar \textbar terms. KAW is studied in slab geometry as a benchmark case. A scan in $\beta $ for the KAW dispersion relation shows that as $\beta $ approaches 1 (\textgreater 0.3), the effects of $\delta $B\textbar \textbar becomes important. Connections are made with other existing studies of KAWs in the fusion and space plasma literature. This new capability of including $\delta $B\textbar \textbar in GTC could be applied to nonlinear simulations of modes such as kinetic ballooning and tearing modes. [Preview Abstract] |
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UP10.00050: A stringent limit on the amplitude of Alfv\'enic perturbations in high-beta low-collisionality plasmas Jonathan Squire, Eliot Quataert, Alexander Schekochihin It is shown that low-collisionality plasmas cannot support linearly polarized shear-Alfv\'en fluctuations above a critical amplitude $\delta B_{\perp}/B_{0} \sim \beta^{\,-1/2}$, where $\beta$ is the ratio of thermal to magnetic pressure. Above this cutoff, a developing fluctuation will generate a pressure anisotropy that is sufficient to destabilize itself through the parallel firehose instability. This causes the wave frequency to approach zero, interrupting the fluctuation before any oscillation. The magnetic field lines rapidly relax into a sequence of angular zig-zag structures. Such a restrictive bound on shear-Alfv\'en-wave amplitudes has far-reaching implications for the physics of magnetized turbulence in the high-$\beta$ conditions prevalent in many astrophysical plasmas, as well as for the solar wind at $\sim\! 1 \mathrm{AU}$ where $\beta > 1$. [Preview Abstract] |
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UP10.00051: Spontaneous excitation of waves by an intense ion beam on the Large Plasma Device Shreekrishna Tripathi, Bart Van Compernolle, Walter Gekelman, Patrick Pribyl, William Heidbrink A hydrogen ion beam (15 keV, 10 A) has been injected into a large magnetized plasma ($n \approx 10^{10}$--$10^{13}$ cm$^{-3}$, $T_e = 5.0$--$15.0$ eV, $B = 0.6$--$1.8$ kG, He$^+$ and H$^+$ ions, 19 m long, 0.6 m diameter) for performing fast-ion studies on the Large Plasma Device (LAPD). The beam forms a helical orbit (pitch-angle $\approx 7^\circ$--$55^\circ$), propagates with an Alfv\'{e}nic speed (beam-speed/Alfv\'{e}n-speed = $0.2$--$3.0$), and significantly enhances the electron temperature and density when injected during the plasma afterglow. We report results on spontaneous generation of Alfv\'{e}n waves and electrostatic waves in the lower-hybrid range of frequencies by the beam. Roles of normal and anomalous Doppler-shifted ion-cyclotron resonances in destabilizing the Alfv\'{e}n waves were examined by measuring the phase-speed of waves and relevant parameters of the plasma using a variety of diagnostic tools (retarding-field energy analyzer, three-axis magnetic-loop, Dipole, and Langmuir probes). Conditions for the maximum growth of these waves were determined by varying the parameters of the beam and ambient plasma and examining the mode-structures in the fluctuation-spectra. Reference: Tripathi et al., Phys. Rev. E 91, 013109 (2015) [Preview Abstract] |
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UP10.00052: First Laboratory Observation of a Shear Alfv\'{e}n Wave Parametric Instability S Dorfman, T Carter, S Vincena, P Pribyl, G Rossi, Y Lin, R Sydora Alfv\'{e}n waves, a fundamental mode of magnetized plasmas, are ubiquitous in lab and space. The non-linear behavior of these modes is thought to play a key role in important problems such as the heating of the solar corona, solar wind turbulence, and Alfv\'{e}n eigenmodes in tokamaks. In particular, theoretical predictions show that these Alfv\'{e}n waves may be unstable to various parametric instabilities. Recent results from the Large Plasma Device at UCLA have recorded the first observation of a sheer Alfv\'{e}n wave parametric instability in the laboratory [Dorfman and Carter, PRL 2016]. When a single finite $\omega/\Omega_i$, finite $k_\perp$ Alfv{\'e}n wave is launched above a threshold amplitude, three daughter waves are observed: two sideband Alfv{\'e}n waves co-propagating with the pump and a low frequency nonresonant mode. Frequency and parallel wave number matching relations are satisfied. Although these features are consistent with the $k_\perp=0$ modulational instability theory, the theoretical growth rate is too small to explain observations. Efforts are underway to determine the nature of the perpendicular (to the background magnetic field) nonlinear drive, conduct comparative simulation studies, and identify parametric instabilities in spacecraft data. [Preview Abstract] |
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UP10.00053: Experimental Evaluation of Energy Transfer between Fast Ions and Alfven Eigenmodes Kenichi Nagaoka, Masaki Osakabe, Mitsutaka Isobe, Kunihiro Ogawa, Yasuhiro Suzuki, Shinji Kobayashi, Satoshi Yamamoto, Yoshizumi Miyoshi, Yuto Katoh, Josep Maria Fontdecaba, Enrique Ascasibar Recently, a new wave-particle analyzer was proposed to identify interaction between fast ions and Alfven eigenmodes [K. Nagaoka, 67$^{\mathrm{th}}$ annual meeting of APS-DPP, savanna, 2015]. A data acquisition system for the wave-particle interaction analysis was developed for particle counting mode operation of neutral particle detectors. We recently applied the system to the Si-FNA detector signals in LHD and Heliotron J, and NPA signals in TJ-II. The first experimental results obtained in three devices are presented and the importance of the optimization of line of sight will be discussed. [Preview Abstract] |
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UP10.00054: Ion-Acoustic Wave Scattering description using Case-Van Kampen modes Jorge Berumen, Feng Chu, Ryan Hood, Sean Mattingly, Fred Skiff We present an experimental characterization of the ion acoustic wave scattering using Case-Van Kampen modes. The experiment is performed in a cylindrical, magnetized, singly-ionized Argon inductively-coupled gas discharge plasma that is weakly collisional with typical conditions: n\textasciitilde 10$^{\mathrm{9}}$cm$^{\mathrm{-3}}$ T$_{\mathrm{e}}$\textasciitilde 7 eV and B\textasciitilde 1 kG. A 5 ring antenna with diameter similar to the plasma diameter is used for launching the waves. A survey of the ion velocity distribution function's zeroth and first order as well as density fluctuations at different frequencies is done using Laser-Induced Fluorescence (LIF) as the main diagnostics method. Analysis of the scattering of the waves and its dependence on wave frequency is done utilizing Case-Van Kampen modes and the use of Morrison's G-transform [1]. Bibliography: [1] F. Skiff, H. Gunell, C.S. Ng A. Bhattacharjee, and W.A. Noonan. Electrostatic degrees of freedom in non-maxwellian plasma. Physics of Plasmas, 9(5):1931, 2002. [Preview Abstract] |
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UP10.00055: Multi-wave coupling and non-linear interactions in DC planar magnetron microdischarges. Nicolas Gascon, Christopher Young, Tsuyohito Ito, Mark Cappelli We study azimuthal wave structures in two planar DC magnetron microdicharges (\textasciitilde 1-10W) operated with argon. Segmented anode/electrodes and high frame rate camera imaging of plasma emission are used to characterize azimuthal modes and transitions as evidenced in the spatial and temporal variation in the discharge current. The dominant stable mode structure varies with discharge voltage and electrode distance, and is observed rotating in the negative $E$ x $B$ direction. This negative drift direction is attributed to a local field reversal arising from strong density gradients that drive excess ions towards the anode. Observed mode transitions are shown to be consistent with models of gradient drift-wave dispersion in such a field reversal when the fluid representation includes ambipolar diffusion parallel to the magnetic field direction. Azimuthal wave dispersion ($f$-$k)$ spectra obtained from two-point signal analysis ($f=$100 kHz-100 MHz and $k=$0-200 m$^{\mathrm{-1}})$, reveal rich and complex waves and transient structures, and in some operating conditions, multi-wave coupling and nonlinear interactions. Preliminary analysis of these structures point to energy transfer mechanisms consistent with classic turbulence models, such as described by the Hasegawa-Mima equations. [Preview Abstract] |
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UP10.00056: Radiation efficiency for exciting whistler modes of electric and magnetic antennas: a comparison J M Urrutia, R L Stenzel Low frequency whistler modes ($\omega<\omega_c/2$) are excited in a large uniform laboratory plasma with electric dipoles and magnetic loop antennas oriented perpendicular to the ambient magnetic field. The antennas are driven under identical plasma conditions with short pulses from the same rf source so as to avoid nonlinear effects. The wave propagation and rf field topology are measured with rf probes. As expected, a magnetic loop antenna excites much stronger whistler modes than an electric dipole antenna. This is because the dipole electric field is shielded by sheaths and its current is a small displacement current compared to the conduction current of a closed loop antenna. A power ratio of $P_{\mathrm{loop}} / P_{\mathrm{dipole}} \simeq$ 8000 has been observed. The radiation resistances have also been obtained from first principles ($R_{\mathrm{rad}}=P_{\mathrm{rad}}/I^2_{\mathrm{rms}}$), but cannot be compared since the currents are vastly different. It is interesting to note that the electric dipole excites a wave whose topology resembles that of an $m=1$ helicon mode. The loop has an elongated shape of the same length as the electric dipole (15~cm) and excites an $m=0$ mode. These results are relevant to whistler wave injections experiments into space plasmas. [Preview Abstract] |
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UP10.00057: Helicon and Trivelpiece-Gould modes in uniform unbounded plasmas R. L. Stenzel, J. M. Urrutia Helicon modes are whistler modes with angular orbital momentum caused by phase rotation in addition to the axial phase propagation. Although these modes have been associated with whistler eigenmodes in bounded plasma columns, they do exist in unbounded plasmas. Experiments in a large laboratory plasma show the wave excitation with phased antenna arrays, the wave field topology and the propagation of helicons. Low frequency whistlers can have two modes with different wavelengths at a given frequency, called helicons and Trivelpiece-Gould modes. The latter are whistler modes near the oblique cyclotron resonance. The oblique propagation is due to short radial wavelengths near the boundary. In unbounded plasmas, the oblique propagation arises from short azimuthal wavelengths. This has been observed in high-mode number helicons (e.g., $m=8$). It creates wave absorption in the center of the helicon mode. The strong absorption of the wave can heat electrons and create perpendicular wave-particle interactions. These results may be of interest in space plasmas for scattering of energetic electrons and in helicon plasma sources for plasma processing and thruster applications. [Preview Abstract] |
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UP10.00058: Experimental Investigation of Parametric Excitation of Whistler Waves Nathan Zechar, Vladimir Sotnikov, James Caplinger, Mark Hopkins Previous theoretical work has shown that a parametric interaction between electrostatic lower oblique resonance (LOR) and ion acoustic waves (IAW) can produce electromagnetic whistler waves in a cold magnetized plasma. It was also demonstrated theoretically that this interaction can more efficiently generate electromagnetic whistler waves than by direct excitation using a conventional loop antenna. For the purpose of experimentally validating the above result, an experiment was designed and built utilizing a helicon array plasma source capable of high density and spatial uniformity. We first demonstrate the ability to directly excite whistler waves along with the familiar resonant surfaces which comprise the LOR. Next we will attempt to generate and observe ion acoustic waves as well as test their agreement with the linear dispersion relation. Finally, we will investigate the existence of any nonlinear interaction which indicates the desired parametric excitation and attempt to analyze the efficiency of this method of excitation. [Preview Abstract] |
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UP10.00059: Investigation of Parametric Excitation of Whistler Waves Using 3D Particle-In-Cell Simulations James Caplinger, Vladimir Sotnikov, Daniel Main, David Rose, Ioana Paraschiv Previous theoretical work has shown that a parametric interaction between quasi-electrostatic lower oblique resonance (LOR) and lower frequency ($\omega $ \textless $\omega $LH) ion acoustic or extremely low frequency (ELF) waves can produce electromagnetic whistler waves in a cold magnetized plasma. It was also demonstrated theoretically that this interaction can more efficiently generate electromagnetic whistler waves than by direct excitation by a conventional loop antenna, operating at a single frequency. For the purpose of numerically validating the above result, a series of particle-in-cell simulations were carried out. We first demonstrate the ability to accurately model whistler wave excitation producing the familiar resonant surfaces which comprise the LOR using a modeled loop antenna. Next we demonstrate the ability to generate ion acoustic waves as well as ELF waves, both of which are shown to agree with the expected linear dispersion relations. Finally, we investigate the existence of any nonlinear interaction which indicates the desired parametric excitation and attempt to analyze the efficiency of this method of excitation and radiated power going into the whistler part of the VLF wave spectrum. [Preview Abstract] |
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UP10.00060: Nonlinear Coherent Structures of Alfv\'{e}n Wave in a Collisional Plasma. Sayanee Jana, Samiran Ghosh, Nikhil Chakrabarti The Alfv\'{e}n wave dynamics is investigated in the framework of Lagrangian two-fluid model in a cold magnetized collisional plasma in presence of finite electron inertia. In the quasi-linear limit, the dynamics of the nonlinear Alfv\'{e}n wave is shown to be governed by a modified Korteweg-de Vries Burgers (mKdVB) equation. In this mKdVB equation, the electron inertia is found to act as a source of dispersion and the electro-ion collision serves as a dissipation. In the long wavelength limit, we have also investigated wave modulation characteristics of the nonlinear Alfv\'{e}n wave. The dynamics of this modulated wave is shown to be governed by a damped nonlinear Schr\"{o}dinger equation (NLSE). These nonlinear equations are analysed by means of analytical and numerical simulation to elucidate the various aspects of the phase-space dynamics of the nonlinear wave. Results reveal that nonlinear Alfv\'{e}n wave exhibits shock, envelope and breather like structures. Numerical simulations also predict the formation of Alfv\'{e}nic rogue waves, rogue wave holes and giant breathers. These results could be useful for understanding the salient features of the Alfv\'{e}nic magnetic field structures from observational data in very low-$\beta $magnetized collisional plasmas in space and laboratory. [Preview Abstract] |
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UP10.00061: Observation of Kinetic Alfvén Eigenmodes with RMP Penetration in J-TEXT Tokamak Linzi Liu, Qiming Hu, Ge Zhuang kinetic Alfvén eigenmodes excited in J-TEXT Ohmic plasmas are observed when the penetration of external applied resonant magnetic perturbations (RMPs) occurs. These modes are identified localized in gaps (20-80kHz) triggered by kinetic thermal ion effect [Chen L and Zonca F 2016 Reviews of Modern Physics] in the Alfvén continuum. One of these modes, which have the highest frequency, is modulated by the RMP intensity, and its frequency converge to the nonlinearly modified beta-induced Alfvén eigenmode (BAE) continuum accumulation point [Biancalani A 2011 PPCF]. The experimental observation of this type of mode agrees with BAE whose frequency is proportional to the magnetic island half width. Another type of mode with lower frequency(~35kHz) well settled down in the thermal ion transit frequency range is found that it has no relation with RMP strength and magnetic island width, as indicated in previous work [Linzi L 2015 PPCF]. The investigation of the third type of modes shows that the mode frequency is in the range of diamagnetic drift frequency (~20kHz) and its damping mechanism should involve the fast particle effect. All of the modes above are observed only in low density (~1×10^{19}) along with slide-away discharge. [Preview Abstract] |
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UP10.00062: Excitation of Electron Acoustic Waves in Plasmas of the SINP-MaPLE Device. Satyajit Chowdhury, Subir Biswas, Nikhil Chakrabrati, Rabindranath Pal Electron acoustic wave (EAW) is the low frequency branch of the undamped electrostatic plasma wave and has low phase velocity [Physics of Fluids, 28, 2439-2441 (1985)]. In order to overcome Landau damping the EAW needs a non-Maxwellian electron velocity distribution with a flat region near the phase velocity, or equivalently, a plasma with two temperature electron species with a relative velocity between them. The ECR produced plasmas of the MaPLE device [Review of scientific instruments,~81(7), 073507, (2010)] at Saha Institute of Nuclear Physics provide such characteristics as observed by retarded field energy analyzer and single Langmuir probe. Experiments are carried out to exploit this feature by putting a negatively biased mesh launcher inside the plasma and energizing it with sinusoidal voltages from a function generator with frequencies varying near the ion plasma frequency. Circular mesh probes along the axis of the device serve as detectors for wave propagation. Experimental results show EAWs are indeed launched and propagate along the magnetic field direction. The dispersion curve experimentally obtained shows the phase velocity matching satisfactorily with the estimated theoretical values. Changing the bias on the launcher the electron distribution function is varied, which, in turn, controls the wave amplitude. Detailed experimental results will be presented. [Preview Abstract] |
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UP10.00063: Synergetic effects between chaotic and self-consistent effects observed for wave particle interaction in a TWT upgrade Fabrice Doveil, Didier Guyomarc'h, Meirielen Caetano da Sousa, Yves Elskens Beside industrial uses, Traveling Wave Tubes (TWT) are useful to mimic and study plasma-like wave-particle interaction. We upgraded a TWT, whose slow wave structure is a 4 m long helix (diameter 3.4 cm, pitch 1 mm) of Be-Cu wire in a vacuum glass tube. At one end, a cathode injects electrons, radially confined by a constant axial magnetic field. Movable probes, capacitively coupled to the helix, launch and monitor waves with an arbitrary waveform at a few tens of MHz. At the other end of the helix, a trochoidal analyzer allows to reconstruct the beam electron distribution function after its self-consistent interaction with the waves. The new device's observed dispersion relation agrees very well with a sheath model. The measured probe-helix coupling coefficients are used to reconstruct the spatial evolution of a launched wave as it interacts with the beam. For low beam intensity, chaotic effects are observed on the beam. For larger beam intensity, growth and saturation of a launched wave is observed. [1] G. Dimonte, J.H. Malmberg, Phys. Fluids 21, 1188 (1978). [2] S.I. Tsunoda, F. Doveil, J.H. Malmberg, Phys. Rev. Lett. 58, 1112 (1987). [3] F. Doveil, A. Macor, A. Aissi, Cel. Mech. Dyn. Astr. 102, 255 (2008). [Preview Abstract] |
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UP10.00064: Co-existence of Kelvin Helmholtz and Drift wave Instabilities in IMPED. Sayak Bose, P.K. Chattopadhyay, J. Ghosh, Y.C. Saxena The Kelvin-Helmholtz (KH) and drift wave (DW) instabilities are excited simultaneously in the \textbf{I}nverse \textbf{M}irror \textbf{P}lasma \textbf{E}xperimental \textbf{D}evice (IMPED). The unique control features of IMPED[1] enables one to modify the radial profiles of density, potential and electron temperature as per experimental requirement. The instabilities are identified by measuring the wavelength, amplitude of density and potential fluctuations and radial profiles of density and plasma potential. These instabilities are found to interact nonlinearly with each other leading to the formation of side bands. Bispectral analysis has been used to experimentally confirm the nonlinear coupling. The side bands are observed to be asymmetrical under most experimental conditions. But, symmetric side bands are observed at atypical experimental conditions. The method of excitation and control of these instabilities and the probable mechanism of power distribution in side bands is presented. \begin{enumerate} \item Bose et al. Rev. Sci. Instrum. 86, 063501 (2015). \end{enumerate} [Preview Abstract] |
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