Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session PP8: Poster Session VI: Mixing Mini-Conference, Sources, Edge and Divertor, MHD, Waves, Instabilities & Reconnection |
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Room: Plaza ABC |
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PP8.00001: Mix mitigation with external magnetic fields and hot-spot ion viscosity in Rayleigh-Taylor unstable inertial confinement fusion plasmas Bhuvana Srinivasan, Xian-Zhu Tang Rayleigh-Taylor instabilities (RTI), an interchage instability commonly observed in inertial confinement fusion capsules, causes mixing of hot and cold fuels leading to energy loss from the hot-spot. Large externally applied magnetic fields can slow the growth of the RT while damping short-wavelength RT modes. Further mix mitigation is possible through hot-spot ion viscosity if ignition quality temperatures are achieved since viscosity has a strong dependence on ion temperature, $\sim T^{5/2}$. During deceleration when the gas-ice interface is RT unstable, the Reynold's number, $Re$, is laminar in the hot-spot due to high viscosity and it is turbulent in the ice. Even with a disparate $Re$ profile across the gas-ice interface, the ion viscosity can be strong enough to damp short-wavelength RTI and short-wavelenth Kelvin-Helmholtz modes. In the absence of physical dissipation, the peak vorticity and the peak magnetic field suffer from ultraviolet divergence. Viscosity saturates the peak vorticity and resistivity saturates the peak magnetic field. A study of vorticity and magnetic field saturation will be presented using a visco-resistive Hall-MHD model in addition to results showing mix mitigation using magnetic fields and ion viscosity. [Preview Abstract] |
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PP8.00002: Mixing the directions of thermodynamic time Alexander Klimenko, Ulrich Maas Conventional thermodynamics is formulated and tested in the world populated almost solely by matter; it can be consistently extended to antimatter in two mutually excluding ways: CP-invariant or CPT-invariant. While CP-invariant thermodynamics is more or less conventional; its CPT-invariant counterpart results in different directions of the thermodynamic time for matter and antimatter and in their thermodynamic antagonism -- antimatter seems to us very hot due to having negative apparent temperatures. In spite of the similarity of thermodynamic properties of matter and antimatter, CPT-invariant thermodynamics favours conversion of antimatter into matter (in our time). While thermodynamic properties of systems and antisystems under conditions of CPT invariance are more or less clear, properties of mixtures of particles and antiparticles pose a more difficult problem. The major difficulty is in evaluating implications of mixing of two different time primers with the opposite direction of induced thermodynamic time. In the present work we analyse the effect of mixing of directions of time resulting from mixing of matter and antimatter under conditions of CPT-invariant thermodynamics. [Preview Abstract] |
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PP8.00003: Diffusivity of Mixtures in Warm Dense Matter Regime Tomorr Haxhimali, Robert Rudd, Julie Jackson, A. Bruce Langdon, James Glosli, Frank Graziani Modeling of ionic diffusion in warm dense plasma mixtures has been of longstanding interest in astrophysics and in Inertial Confinement Fusion. In this work we employ classical Molecular Dynamics (MD) to calculate diffusion coefficients in mixed plasmas. In the MD study we make use of the Yukawa potential as an effective ion-ion interaction potential that accounts for the screening effects of the electrons. We focus in binary asymmetric mixtures between Deuterium and Argon at Temperatures from 10-100eV and ion densities from 10$^{23}$-10$^{25}$ ion/cc. In uniform mixed systems we use Green-Kubo techniques to calculate self-diffusivities and Maxwell-Stefan diffusivities over a range of conditions. The new results from this study show that a simple linear relations between Maxwell-Stefan diffusivity and self-diffusivities is not always valid. The interdiffusivity that enters in Fickian equation can be related to the Maxwell-Stefan diffusivities through the thermodynamic factor. The latter requires knowledge of the equation of state of the mixture. We compare these results with classical kinetic theories that assume binary collisions. To test these Green-Kubo approaches and to estimate the activity contribution we have also employed large-scale non-equilibrium, non-uniform mixed, MD. [Preview Abstract] |
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PP8.00004: Equation of State Models for Low-Z Materials at High Energy Densities Konstantin V. Khishchenko Models of thermodynamic properties of materials over a wide range of parameters are necessary for numerical simulations of processes at high energy densities including mixing in fusion plasmas. Accuracy of calculation results is determined mainly by adequacy of equation of state (EOS) of a medium. In the present work, different wide-range EOS models for low-Z elements and compounds are considered, such as Thomas--Fermi or Hartree--Fock--Slater plasma models. A semiempirical model of thermodynamic potential free energy with taking into account polymorphic phase transformations, melting, evaporation and ionization is presented. EOS calculations are carried out for hydrogen, deuterium, lithium, beryllium, carbon and hydrocarbon compounds in a broad region of the phase diagram. Obtained results are compared with available data of experiments at high pressures and temperatures in shock and release waves. [Preview Abstract] |
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PP8.00005: Characterization of Hard X-ray Radiation Produced From Gas Targets During Laser-Plasma Interactions With an Ultra-intense, Terawatt Class Laser System Zhen Zhao, William Schumaker, Keegan Behm, Michael Vargas, Vladimir Chvykov, Victor Yanovsky, Anatoly Maksimchuk, Alexander Thomas, Karl Krushelnick X-rays produced via the interaction of an ultra-short, ultra-intense laser pulse with gas targets exhibit many desirable qualities such as compact sizes, short pulse durations, and short-pulse probe capabilities. These features make the use of such x-rays promising for applications ranging from medicine to homeland security. Using HERCULES, a 300 TW, 800 nm Ti:Sapphire laser system, the properties of hard x-rays produced via the interaction of a $\sim30$ fs laser pulse with various gas targets are characterized. The different x-ray generation mechanisms studied include nonlinear Thomson scattering and betatron x-rays from laser wakefield acceleration. Gas targets include a gas jet nozzle and staged gas cells. [Preview Abstract] |
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PP8.00006: Two-stream instability at soft X-ray wavelengths for increasing brightness of Compton sources Nikolai Yampolsky, Chengkun Huang, Gian Luca Delzanno, Dmitry Shchegolkov We propose a novel scheme which may result in the next generation Compton source for soft X-rays. The scheme is based on creating the distribution of relativistic electron beam which consisting of several energy bands and allowing for the two-stream instability to develop resulting in a short scale density modulation. The multi-stream beam distribution is created within a single bunch through a series of manipulations with the electron beam phase space. As a result, several well separated energy bands with close energies can be formed. The wavelength of the microbunching caused by the two-stream instability strongly depends on the beam parameters and can be reduced down to soft X-ray wavelengths that are not achievable with other mechanisms. The microbunching can be used for significant improvement of Compton sources brightness which is estimated to be 5-6 orders of magnitude. [Preview Abstract] |
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PP8.00007: Modeling Nonlinear Thomson/Compton scattering of LWFA GeV electron bunches Joana Luis Martins, Marija Vranic, Jorge Vieira, Thomas Grismayer, Ricardo Fonseca, Luis Silva Laser-wakefield accelerators have been shown to produce bunches on the GeV energy level in few cm of plasma. There is growing interest on the possibility of using them in all-optical schemes for X-ray/Gamma-ray radiation sources, where the laser pulses Thomson/Compton scatter with these bunches. These scenarios can also provide a means to detect signatures of radiation damping. With laser pulses already available, with focused intensities on the order of 10$^{21}$ W/cm$^2$, a scheme where a GeV energy electron bunch scatters the laser and looses approximately half of its energy is possible. This and similar scenarios will be explored numerically with a combination of PIC simulations performed with the OSIRIS 2.0 framework (with radiation damping) and the post-processing of the particle trajectories to obtain the radiation spectrum with quantum corrections. The role of this corrections and the damping on the spectrum shape and emitted energy will be explored. The results will also be compared with spectra obtained from a modified version of OSIRIS 2.0 where QED processes were implemented to model the radiation emission and the cooling of the electrons through a Monte-Carlo module. [Preview Abstract] |
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PP8.00008: Interaction of Ultra relativistic Fireball beam (e$^{-}$e$^{+})$ with Plasma Nitin Shukla, Jorge Vieira, Patric Muggli, Luis Silva Plasma instabilities are strong candidates for seed magnetic field generation to explain radiation processes in Gamma Ray Bursters. Mimicking these extreme conditions in laboratory is still an open challenge. Currently available electron and positron bunches at Stanford Linear Accelerator (SLAC) may provide ideal conditions to investigate magnetic field generation and amplification in the laboratory through the Weibel or Current Filamentation Instability (WI/CFI) [1]. In order to address this possibility we resort to PIC simulations modelling the interaction of fireball bunches with plasmas considering the SLAC bunch parameters. We find that by keeping the number of bunch particles constant that WI/CFI grows faster than Oblique Instabilities (OI) for shorter bunches (with lengths smaller than the plasma wavelength) which provide sufficiently large peak fireball bunch densities. On the contrary, for longer bunches, with lower peak densities, OI grow faster than WI/CFI. We find that both OI and WI/CFI saturate by increasing the beam temperature. Analytical results are in agreement with simulations. \\[4pt] [1] P. Muggli, S.F. Martins, J. Vieira, and L.O. Silva arXiv:1306.4380 (2013). [Preview Abstract] |
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PP8.00009: Particle-In-Cell simulations of electron beam microbunching instability in three dimensions Chengkun Huang, Y. Zeng, M.D. Meyers, S. Yi, B.J. Albright, T.J.T. Kwan Microbunching instability due to Coherent Synchrotron Radiation (CSR) in a magnetic chicane is one of the major effects that can degrade the electron beam quality in an X-ray Free Electron Laser. Self-consistent simulation using the Particle-In-Cell (PIC) method for the CSR fields of the beam and their effects on beam dynamics have been elusive due to the excessive dispersion error on the grid. We have implemented a high-order finite-volume PIC scheme that models the propagation of the CSR fields accurately. This new scheme is characterized and optimized through a detailed dispersion analysis. The CSR fields from our improved PIC calculation are compared to the extended CSR numerical model [1] based on the Lienard-Wiechert formula in 2D/3D. We also conduct beam dynamics simulation of the microbunching instability using our new PIC capability. Detailed self-consistent PIC simulations of the CSR fields and beam dynamics will be presented and discussed. \\[4pt] [1] C.-K. Huang et. al, Phys. Rev. STAB 16, 010701 (2013). [Preview Abstract] |
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PP8.00010: Strong terahertz generation from relativistic laser-solid interactions Yutong Li Terahertz radiation has been attracted much interest due to increasingly wide applications. Though THz radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasmas, with an advantage of no damage limit, are promising medium to generate strong THz radiation. We have symmetrically studied strong THz radiation from solid targets driven by relativistic laser pulses. The experiments were carried out using the Xtreme Light II laser system at the Institute of Physics, Chinese Academy of Sciences, and the COMET sub-picosecond laser system at the Lawrence Livermore National Laboratory, respectively. THz radiation with a pulse energy of tens micorJ/sr (driven by femtosecond laser), even $\sim$ mJ/sr (driven by sub-picosecond laser) is observed. In this talk, the THz polarization, temporal waveform, angular distribution and energy dependence on the laser energy will be presented. We find that the radiation is dependent on the preplasma density scale length. [Preview Abstract] |
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PP8.00011: Experimental observation of frequency up-conversion by flash ionization phenomena Noboru Yugami, Takamitsu Otsuka, Yasuhiko Sentoku, Akinori Nishida, Ryosuke Kodama When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization predicted by S. C. Wilks et.al. The theory requires not only the plasma creation in time much shorter than an oscillation period of the electromagnetic wave but also plasma length much longer than a wavelength of it. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency up-conversion from 0.35 THz to 3.3 THz by the irradiance of the Ti:sapphire laser in ZnSe crystal. The increment of the terahertz wave frequency is good agreement of the theory. [Preview Abstract] |
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PP8.00012: Experimental and simulation study of electric field screening of carbon fiber field emitters Wilkin Tang, Don Shiffler, Matthew LaCour, Ken Golby, Tim Knowles Field emitter arrays have the potential to provide high current density, low voltage operation, and high pulse repetition for radar and communication. It is well known that packing density of the field emitter arrays significantly affect the emission current\footnote{L. Nilsson, O. Groning, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J. M. Bonard, and K. Kern,''Scanning Field Emission From Patterned Carbon Nanotube Films'', Appl. Phys. Lett. 76, 2071 (2000).} because individual emitters screen each other from the imposed electric field. Previous experiments were conducted with 1000s of field emitters which makes the analysis of electric field screening difficult. Here we describe experiments with small numbers of emitters--a dual-cathode and four-cathode configuration.\footnote{W. Tang, D. Shiffler, K. Golby, M. LaCour and T. Knowles, ``Experimental Study of Electric Field Screening by the Proximity of Two carbon Fiber Cathodes'', J. Vac. Sci. Technol. B 30, 061803 (2012).} The experiments utilize carbon fiber field emitters (two and four) with variable spacing to investigate the effect of electric field screening on current emission. Analytic model and Particle-in-cell simulations are performed to compare with the experiments. [Preview Abstract] |
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PP8.00013: Parallel Simulation of Underdense Plasma Photocathode Experiments David Bruhwiler, Bernhard Hidding, Yunfeng Xi, Gerard Andonian, James Rosenzweig, Estelle Cormier-Michel The underdense plasma photocathode concept (aka Trojan horse) [1,2] is a promising approach to achieving fs-scale electron bunches with pC-scale charge and transverse normalized emittance below 0.01 mm-mrad, yielding peak currents of order 100 A and beam brightness as high as $10^{19} A/m^2/rad^2$, for a wide range of achievable beam energies up to 10 GeV. A proof-of-principle experiment will be conducted at the FACET user facility in early 2014. We present 2D and 3D simulations with physical parameters relevant to the planned experiment.\\[4pt] [1] Hidding et al., PRL 108:035001 (2012).\\[0pt] [2] Xi et al., PRST-AB 16:031303 (2013). [Preview Abstract] |
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PP8.00014: Brillouin Flow in Recirculating Planar Magnetron D.H. Simon, Y.Y. Lau, M. Franzi, G. Greening, R.M. Gilgenbach We examine the Brillouin flow in the conventional magnetron, inverted magnetron, and planar magnetron, with respect to the equilibrium, stability, and operating conditions. This renewed interest was prompted by our recent invention of the recirculating planar magnetron (RPM) [1], where rapid start up utilizes the negative mass instability in the inverted magnetron configuration [2]. Given that Brillouin flow is the most likely state in a crossed-field gap, and that various embodiments of the RPM consist of the conventional, inverted, and planar magnetron, it is necessary to study equilibrium, stability, and operating conditions at the same footing. To study startup, we solve the eigenvalue problem that governs the stability of Brillouin flow, including the effects of the resonant cavities that form the slow wave structures.\\[4pt] [1] R. M. Gilgenbach, et al., IEEE Trans. Plasma Sci. 39, 980 (2011).\\[0pt] [2] D. M. French, et al., Appl. Phys. Lett. 97, 111501 (2010). [Preview Abstract] |
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PP8.00015: Characterizing Electrical and Thermal Breakdown of Metamaterial Structures for HPM Applications T. Wynkoop, M. Gilmore, A.G. Lynn, S. Prasad, E. Schamiloglu The use of metamaterials (MTMs) has been proposed to increase the performance and efficiency of high power microwave (HPM) sources. However, by nature, MTMs are composed of subwavelength structures and are prone to electrical breakdown. In order to investigate the survivability of potential MTM structures in an HPM environment, two test stands are being constructed to characterize MTM electrical and thermal response. First, the SINUS-6 electron beam accelerator with maximum deliverable power of 4.2 GW(700 kV, 6 kA) , and pulse duration of 12 ns will be utilized. MTM's will be placed in close proximity to the beam, and breakdown will be characterized via fast imaging, and survey and high resolution spectroscopy. Secondly, a low current electron gun with V$_{\mathrm{beam}} \le $ 50 kV, that can operate from ns pulsed to steady state, will investigate thermal loading and charging. Ultimately, results of this characterization will be used to develop robust MTM resonant/slow wave structures for HPM applications. [Preview Abstract] |
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PP8.00016: The current understanding of transitions from large to small ELMs and to edge turbulence with no ELMs X.Q. Xu A suite of two-fluid models has been implemented in BOUT$++$ for all ELM regimes and fluid turbulence. A suite of gyro-fluid models is under development for pedestal turbulence and transport. A suite of 3D neutral and impurity models is also under development for SMBI, recycling, gas puffing, and for sputtering from RF antennas and divertor plates. Progress in several key areas of research will be presented: upshift of PB instability thresholds due to background turbulence; ELM power deposition on the divertor plates; identification of top pedestal micro-turbulence zone for ELM spreading and pedestal peak gradient MHD zone for ELM crashing For a ballooning-dominated equilibrium, we find that both pressure gradient and pedestal density can control the transition from large ELMs to small ELMs. Small elms can be either resistive or ideal P-B modes; the density dependence of the Elm size is due to ion diamagnetic stabilization, not due to collisionality. The flux limited expressions of parallel thermal diffusivities show weak or no collisionality dependence, even in the SOL. A decrease of the ELM size with density is a natural consequence for ballooning modes. For a peeling-dominated equilibrium and for typical experimental scenarios with natural transition between peeling dominated and ballooning dominated equilibria during the pedestal buildup, the scaling characteristics of the ELMs size will also be presented. [Preview Abstract] |
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PP8.00017: BOUT++ simulations of edge turbulence in Alcator C-Mod's EDA H-mode E.M. Davis, M. Porkolab, J.W. Hughes, B. LaBombard, P.B. Snyder, X.Q. Xu Energy confinement in tokamaks is believed to be strongly controlled by plasma transport in the pedestal. The pedestal of Alcator C-Mod's Enhanced $D_{\alpha}$ (EDA) H-mode ($\nu^* > 1$) is regulated by a quasi-coherent mode (QCM), an edge fluctuation believed to reduce particle confinement and allow steady-state H-mode operation. \texttt{ELITE} calculations indicate that EDA H-modes sit well below the ideal peeling-ballooning instability threshold, in contrast with ELMy H-modes. Here, we use a 3-field reduced MHD model in \texttt{BOUT++} to study the effects of nonideal and nonlinear physics on EDA H-modes. In particular, incorporation of realistic pedestal resistivity is found to drive resistive ballooning modes (RBMs) and increase linear growth rates above the corresponding ideal rates. These RBMs may ultimately be responsible for constraining the EDA pedestal gradient. However, recent high-fidelity mirror Langmuir probe measurements indicate that the QCM is an electron drift-Alfv\'{e}n wave - not a RBM. Inclusion of the parallel pressure gradient term in the 3-field reduced MHD Ohm's law and various higher field fluid models are implemented in an effort to capture this drift wave-like response. [Preview Abstract] |
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PP8.00018: Simulations of Edge Current Driven Kink Modes with BOUT$++$ code G.Q. Li, X.Q. Xu, P.B. Snyder, A.D. Turnbull, T.Y. Xia, C.H. Ma, P.W. Xi Edge kink modes (or peeling modes) play a key role in the ELMs. The edge kink modes are driven by peak edge current, which comes from the bootstrap current. We calculated sequences of equilibria with different edge current using CORSICA by keeping total current and pressure profile fixed. Based on these equilibria, with the 3-field BOUT$++$ code, we calculated the MHD instabilities driven by edge current. For linear low-n ideal MHD modes, BOUT$++$ results agree with GATO results. With the edge current increasing, the dominant modes are changed from high-n ballooning modes to low-n kink modes. The edge current provides also stabilizing effects on high-n ballooning modes. Furthermore, for edge current scan without keeping total current fixed, the increasing edge current can stabilize the high-n ballooning modes and cannot drive kink modes. The diamagnetic effect can stabilize the high-n ballooning modes, but has no effect on the low-n kink modes. Also, the nonlinear behavior of kink modes is analyzed. [Preview Abstract] |
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PP8.00019: Six-field two-fluid simulations of ELM power depositions on divertor target in real tokamak geometry using BOUT$++$ code T.Y. Xia, X.Q. Xu, M.E. Fenstermacher The six-field two-fluid model based on the Braginskii equations in BOUT$++$ simulation framework is used to study the edge localized modes (ELMs) in realistic tokamak discharges of DIII-D and EAST with the experimentally measured profiles of density, radial electric field, electron and ion temperatures as the initial conditions. The simulations with two different resolutions on the lower single-null geometry are done to describe the evolutions of pedestal energy loss, density profile and heat flux on divertor through the ELM event. The simulation for high resolution shows much faster energy loss than the low resolution one, and leads to the twice of the amplitude for ion heat flux. Our high simulations show that the total energy loss for the small ELM with high frequency is well consistent with the measurement. The amplitudes of heat flux on divertor target are comparable with the early time evolutions of the IR heat flux measurement. Plasma sheath boundary conditions (SBC) are implemented at the divertor plate and they can effectively broaden the heat flux distribution at the outer plate compared to the Dirichlet boundary conditions. The poloidal structures of the heat flux on divertor target will be reported in this paper. [Preview Abstract] |
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PP8.00020: Analysis of different responses of ion and electron in six-field two-fluid ELM simulations Chenhao Ma, Xueqiao Xu We report simulation results of a Landau-Fluid (GLF) extension of the BOUT++ six-field two-fluid Braginskii model which contributes to increasing the physics understanding of ELMs. Landau-Fluid closure can fill the gap for parallel dynamics between hot, collisionless pedestal region and cold, collisional SOL region in H-mode plasmas. Our goal is extending the classical parallel heat flux with Landau-Fluid closures and making comparisons with other closure models. Our simulations show that for weakly collisional pedestal plasmas, the calculated growth rate with Landau-Fluid closure introduces more effective damping on the peeling-ballooning modes than that with the classical thermal diffusivity. Further nonlinear simulation shows that ELM size with Landau-Fluid Closure is smaller than that with classical thermal diffusivity. We find an ELM crash has two phases: fast initial crash of ion temperature perturbation on the Alfven time scale and slow turbulence spreading. Turbulence transport phase is a slow encroachment of electron temperature perturbation due to the ELM event into pedestal region which is due to a positive phase shift around $\pi/2$ between electron temperature and potential on pedestal region while ion temperature is in-phase with potential. [Preview Abstract] |
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PP8.00021: Modeling of Tokamak Divertor Plasma for Weakly Collisional Parallel Electron Transport M.V. Umansky, A.M. Dimits, I. Joseph, T.D. Rognlien Tokamak edge transport codes, such as UEDGE, rely on a collisional fluid plasma model valid only when $\lambda_{e,i} \ll L$ where $\lambda$ is collisional mean free path, $L$ the spatial scale of the problem. This condition is at best marginally satisfied in present-day tokamak edge plasmas, and likely to be violated in next-generation devices. In particular, departures from the Spitzer electron parallel heat conduction have a strong influence on poloidal energy flux in tokamak edge, and poses a serious limitation on the utility of existing transport models. This raises demands for developing accurate and efficient representation of parallel transport within edge-plasma modeling codes for the non-collisional and weakly collisional regimes. Such a representation has been developed in the context of Landau-fluid (LF) models [1]. Practical implementation of LF parallel transport models in the framework of an edge modeling code has become possible due to the recent invention of an efficient non-spectral method for non-local closure operators [2]. Here we describe the implementation of an LF-based model for the parallel plasma transport in the framework of the UEDGE code. \\[4pt] [1] Beer and Hammett, Phys. Plasmas 3, 4046 (1996).\\[0pt] [2] Dimits et al., Bull. Am. Phys. Soc. v. 57, n. 12 (2012) [Preview Abstract] |
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PP8.00022: Tokamak turbulence simulations using BOUT$++$ framework in core region S.S. Kim, X.Q. Xu, H. Jhang, Tongnyeol Rhee, P.W. Xi, P.H. Diamond, A. Dimits, M. Umansky, G.Y. Park Development of a self-consistent, core-edge integrated simulation capability is a long standing problem in fusion simulation program. Such capability would yield insight into questions related to global profile dynamics originating from L to H and internal transport barrier (ITB) transitions. Starting from a tokamak edge plasma simulation code, BOUT$++$ has evolved into a versatile framework that can be used to simulate a wide range of fluid models in complicated magnetic geometry. For the realization of the self-consistent core-edge coupled simulation, we developed a core gyro-Landau-fluid code using BOUT$++$ framework. The primary physics goal of this development is to realize ITB formation in the presence of non-resonant modes and to study effects of flat q-profile and rotation shear on core profile de-stiffening. Initial efforts focused on the self-consistent simulation of core ITG turbulence and code verification. Verification of the code was realized by comparing linear growth rates calculated from BOUT$++$ with those from gyrokinetic codes. Global nonlinear simulations using 3$+$1 fields model were performed for ITG turbulence. Details of the code development and preliminary physics results will be presented. [Preview Abstract] |
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PP8.00023: BOUT++ flux-driven simulation of edge transport barrier formation with sheared equilibrium flows G.Y. Park, S.S. Kim, T. Rhee, H.G. Jhang, P.H. Diamond, X.Q. Xu The BOUT++ three-dimensional electromagnetic turbulence simulation code [1] is used to study edge transport barrier (ETB) formation and its underlying dynamics. A set of reduced MHD equations is solved including the effects of both equilibrium shear and turbulence driven zonal flows. The form of equilibrium flow profiles can be either proportional to the equilibrium pressure gradient or analytically given. We have applied flux-driven boundary condition near the inner simulation boundary to inject a finite amount of heat flux into the simulation domain and reach the steady flux-driven states. It has been found that externally imposed equilibrium shear flow can trigger ETB formation. Large turbulence is observed to be generated near and propagate into the pedestal region and strongly suppressed there by the local equilibrium flow shear. It has also been found that actual ETB formation is significantly influenced by various effects, i.e., turbulence driven zonal flow and its damping rate, outgoing heat flux level, etc. Detailed dynamics of edge transport barrier formation and its parametric dependence on varying parameters (zonal flow damping rate, heat source and sink rates, etc) will be discussed. \\[4pt] [1] B.D.Dudson, et al., Comput. Phys. Commun. 180, 1467 (2009) [Preview Abstract] |
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PP8.00024: Validations of BOUT$++$ transport simulations with HL-2A experiments using SMBI Z.H. Wang, X.Q. Xu, D.L. Yu, A.P. Sun, J.Q. Dong, L.H. Yao In BOUT$++$ code framework, a new trans-neut module has been developed to deal with neutrals and plasmas transport during fueling of super-sonic molecule beam injection (SMBI) or gas puffing (GP) [1]. It modifies BOUT$++$ code of boundary plasma turbulence to study dynamics of neutrals transport and interactions with plasma during fueling. The model couples plasma density, heat and momentum transport equations with neutrals density and momentum transport equations for atoms and molecules. Particle interactions of dissociation, ionization, recombination and charge-exchange have been included. Particle recycling is also considered at both wall and divertor plates. A local molecule flux boundary condition is applied to model SMBI. It is found that neutrals can penetrate deeply across the separatrix. Simulations are done in a realistic HL-2A tokamak geometry. The initial profiles are specified same as the experiment and they are kept stable via radial dependent diffusion coefficients. The simulations of penetration depth and mean profiles during SMBI will be validated with HL-2A experiments. This work supported by NSFC, Grant No. 11205053 and US DOE under DE-AC52-07NA27344. \\[4pt] [1] Z H Wang, X Q Xu, et al, submitted to Nuclear Fusion, 2013. [Preview Abstract] |
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PP8.00025: Development of PIC-Fluid hybrid scheme for impurity generation and transport in BOUT++ framework Xiaotao Xiao, Xueqiao Xu Impurity generation and transport are an important topic of research in burning plasmas in order to avoid a significant degradation of the fusion capabilities of a reactor device. It is a critical issue for RF experiments due to the phenomenon of rf-enhanced impurity generation. In tokamaks, the impurity transport is usually complicated by the combination of turbulent-driven transport and neoclassical transport, So developing the PIC module in BOUT++ framework, which simulates tokamak edge plasmas using fluid models, will enhance the capability to efficiently simulate both turbulence and neoclassical physics in realistic geometry. The research will be carried out mainly in two steps: a test particle module, in which the orbits is advanced in given background plasma with turbulent electromagnetic field from BOUT++ edge turbulence simulations to yield the spatial distribution of impurities in edge plasmas from given sources at the divertor plates and at the protection limiters near RF antennas; and then a PIC-fluid hybrid module, in which background plasma and the turbulent electromagnetic fields will change with the impurity particle sources. The main issues such as particle weighting and sorting scheme, the communication between the fluid and the PIC parts, are discussed. [Preview Abstract] |
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PP8.00026: Simulations of plasma response to RMP with BOUT$++$ code Bin Gui, Xueqiao Xu BOUT$++$ code is a framework which developed to simulate 3-dimenisonal fluid equations in curvilinear coordinates (Dudson, et al., Computer Physics Communications, 2009). Here we developed an ideal two-field model (vorticity and Ampere's law) to simulate the influence of resonant magnetic perturbation (RMP) on the pedestal plasmas. The vacuum RMP field is self-consistently calculated and included in the two-field model. The current sheets at resonant surface are found, and the radial magnetic field distribution of vacuum RMP and total magnetic field strength are compared. The influence of resistivity on the current sheets is also studied in this model. Based on this work, the radial magnetic field perturbation includes the vacuum RMP component and plasma response in the pedestal region is obtained. By applying the magnetic field perturbation in the peeling-ballooning mode simulation, the influence of RMP on the pedestal plasma could be studied in L-mode, H-mode and ELM discharges. [Preview Abstract] |
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PP8.00027: Peeling-Ballooning mode simulation in ``Snowflake'' divertor configuration using BOUT++ Jingfei Ma, Xueqiao Xu, Dmitri Ryutov, Maxim Umansky ``Snowflake'' divertor, one of the two innovative divertor concepts [1,2], was introduced to solve the issue of large heat loads on plasma facing components and the resulting material erosion, especially during ELMing H-mode, by spreading particle flux to two additional divertor plates. In our work, two-fluid code BOUT++ is used to conduct linear peeling-ballooning (P-B) mode simulations in both standard (SD) and snowflake (SF) divertor geometry generated from DIII-D ELMing H-mode equilibrium. The purpose of this work is to explore how the changes of edge magnetic topology due to implementation of SF geometry will affect P-B mode instabilities. The results are: (1) Linear P-B mode behaviors are greatly affected by magnetic shear at outer middle plane. The growth rate in SF geometry is larger due to the smaller local magnetic shear. (2)Due to the smaller local shear, global mode structures are more strongly ballooning (more radially extended yet less poloidally extended) in SF geometry for moderate toroidal mode numbers. (3) Diamagnetic drift provides stabilizing effects on P-B mode in SF geometry, but not in SD geometry.\\[4pt] [1] M.Kotschenreuther et al., 2004 IAEAFEC. IC/P6-43\\[0pt] [2] D.D. Ryutov, PHYSICS OF PLASMAS 14, 064502 (2007) [Preview Abstract] |
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PP8.00028: Model of a convective, fully detached snowflake divertor D.D. Ryutov, S.I. Krasheninnikov, T.D. Rognlien A general description of a fully-detached divertor based on a snowflake configuration is presented. We exploit a characteristic feature of this divertor: the presence of a significant zone of a very low poloidal magnetic field around the second-order (or near-second-order) poloidal field null. The virtual absence of the poloidal field should give rise to intense curvature-driven plasma convection in this zone and a resulting effective sharing of the plasma outflow between the four divertor legs; the broadening of the flow in each of the legs can also be expected [1-3]. Taken together, these features bring the divertor plasma to the state where the full detachment can be anticipated. A possible shape of the divertor is presented, which involves five ``domes,'' similar the single dome envisaged for the ITER divertor. The resulting estimated heat loads do not exceed 1.5 MW/m2. This configuration also reduces the peak ELM heat flux owing to both convective spreading of the heat flux and increased connection length.\\[4pt] [1] D.D. Ryutov et al, PPCF, 54, 124050 (2012).\\[0pt] [2] V.A. Soukhanovskii et al, 19, 082504 (2012).\\[0pt] [3] H. Reimerdes et al, PPCF, 55, Dec. 2013. [Preview Abstract] |
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PP8.00029: Extension of Gyro-Landau Fluid Equations to Higher Order Ilon Joseph, Andris Dimits Gyro-Landau fluid theory can be used to accurately model turbulent fluctuations over the wide range of collisionality present in tokamak edge plasmas. Here, the theory is extended to more accurately treat the effects of finite perturbation amplitude and finite collisionality. At 2$^{nd}$ order in amplitude, the gyro-averaged Hamiltonian is modified by quadratic correlations in the gyro-phase dependent part of the effective potential. The quadratic terms can be expressed through a bilinear 4D spatial integral operator that approximately splits into the product of $\Gamma _{0}^{1/2}$ operators. In order for the system to conserve energy, the Poisson equation must retain quadratic terms in density and potential and has a similar approximation. Landau closures based on fitting linear dispersion relations [1] for the core plasma typically neglect finite collisionality and nonlinearity in the closure itself. A generalization of the technique developed in [2] to treat the Chapman-Enskog fluid expansion yields a nonlinear extension of the linear closures. \\[4pt] [1] G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. \textbf{64}, 3019 (1990).\\[0pt] [2] Z. Y. Chang and J. D. Callen, Phys. Fluids B \textbf{4}, 1167 (1992). [Preview Abstract] |
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PP8.00030: Axisymmetric curvature-driven instability in a model toroidal geometry W.A. Farmer, D.D. Ryutov A model problem is presented which qualitatively describes a pressure-driven instability which can occur in the divertor region of a tokamak where the poloidal field becomes small. The model problem is described by a horizontal slot with a vertical magnetic field which plays the role of the poloidal field. Line-tying boundary conditions are applied at the planes defining the slot. A toroidal field lying parallel to the planes is assumed to be very strong, thereby constraining the possible structure of the perturbations. Axisymmetric perturbations which leave the toroidal field unperturbed are analyzed. Ideal magnetohydrodynamics is used, and the instability threshold is determined by the energy principle. Because of the boundary conditions, the Euler equation is, in general, non-separable except at marginal stability. This problem may be useful in understanding the source of heat transport into the private flux region in a snowflake divertor which possesses a large region of small poloidal field, and for code benchmarking as it yields simple analytic results in an interesting geometry. [Preview Abstract] |
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PP8.00031: Simulations of Turbulence in Tokamak Edge and Effects of Self-Consistent Zonal Flows* Bruce Cohen, Maxim Umansky Progress is reported on simulations of electromagnetic drift-resistive ballooning turbulence in the tokamak edge. This extends previous work [1] to include self-consistent zonal flows and their effects. The previous work [1] addressed simulation of L-mode tokamak edge turbulence using the turbulence code BOUT that solves Braginskii-based plasma fluid equations in tokamak edge domain. The calculations use realistic single-null geometry and plasma parameters of the DIII-D tokamak and produce fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes that compare favorably to experimental data. In [1] the effect of sheared ExB poloidal rotation is included with an imposed static radial electric field fitted to experimental data. In the new work here we include the radial electric field self-consistently driven by the microturbulence, which contributes to the sheared ExB poloidal rotation (zonal flow generation). We present simulations with/without zonal flows for both cylindrical geometry, as in the UCLA Large Plasma Device, and for the DIII-D tokamak L-mode cases in [1] to quantify the influence of self-consistent zonal flows on the microturbulence and the concomitant transport. *This work was performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344 at the Lawrence Livermore National Laboratory. \\[4pt] [1] B. I. Cohen et al., Phys. Plasmas \textbf{20}, 055906 (2013) [Preview Abstract] |
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PP8.00032: Statistical physics of inter-ELM time interval sequences Anthony Webster, Richard Dendy, Sandra Chapman We report recent studies of the statistical properties of the sequence of time intervals between successive edge localised modes (ELMs). We have compared theoretically derived and empirical probability density functions (pdfs) for the waiting time intervals between ELMs from 85 long steady H-mode plasmas from the Joint European Torus (JET). The Weibull distribution provides a good fit to both type I and type III ELMs, with different parameters. We infer (A J Webster and R O Dendy, Phys Rev Lett 110, 155004 (2013)) that the type III ELMs were generated by a memoryless process, whereas the type I ELMs were consistent with build-up and release. Delay time analysis (F A Calderon, R O Dendy, S C Chapman, A J Webster et al, Phys. Plasmas 20, 042306 (2013)) of six similar JET H-mode plasmas with different levels of gas puffing strongly suggests that the underlying ELMing process is low dimensional. A current study of a dataset of 15,000 ELMs from two weeks of equivalent JET plasmas yields a combined pdf for inter-ELM time intervals which, surprisingly, displays a series of sharp maxima. All three studies show that rigorous statistical analysis of inter-ELM time intervals can contribute to quantitative classification of ELM types and to physical insight into the ELMing processes. [Preview Abstract] |
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PP8.00033: Simulations of ELMs in realistic tokamak geometry with the nonlinear MHD code JOREK Isabel Krebs, Matthias Hoelzl, Stephen Jardin, Karl Lackner, Sibylle Guenter Edge-localized modes (ELMs) are relaxation-oscillation instabilities which occur at the edge of high confinement (H-mode) plasmas, ejecting particles and energy. The suitability of H-mode as operational regime for future fusion devices depends crucially on the occurrence and detailed dynamics of ELMs. We simulate ELMs in realistic ASDEX Upgrade geometry including the scrape-off layer using the nonlinear MHD code JOREK. Emphasis is put on including many toroidal Fourier harmonics in the simulations in order to study nonlinear interaction between these. Several experimental observations, such as a toroidal and poloidal localization of the perturbation and a drive of Fourier components with low toroidal mode numbers, are reproduced by the simulations. A simple model describing the three-wave interaction by quadratic terms in the equations is used to explain and interpret the nonlinear evolution of the toroidal Fourier spectrum in the simulations. It is investigated how sheared toroidal plasma rotation influences the nonlinear coupling between the toroidal Fourier harmonics. A benchmark of the two-fluid versions of JOREK and M3D-C1 is in progress. [Preview Abstract] |
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PP8.00034: Computational analysis of two-fluid edge plasma stability in tokamak geometries Tom Neiser, Derek Baver, Troy Carter, Jim Myra, Phil Snyder, Maxim Umansky In H-mode, the edge pressure gradient is disrupted quasi-periodically by Edge Localized Modes (ELMs), which leads to confinement loss and places large heat loads on the divertor. This poster gives an overview of the peeling-ballooning model [1] for ELM formation and presents recent results of 2DX, a fast eigenvalue code capable of solving equations of any fluid model [2]. We use 2DX to solve reduced ideal MHD equations of two-fluid plasma in the R-Z plane, with toroidal mode number resolving the third dimension. Previously, 2DX has been successfully benchmarked against ELITE and BOUT$++$ for ballooning dominated cases in simple shifted circle geometries [3-4]. We present follow-up work in simple geometry as well as similar benchmarks for full X-point geometry of DIII-D. We demonstrate 2DX's capability as computational tool that supports nonlinear codes with linear verification and as experimental tool to identify density limits, map the spatial distribution of eigenmodes and investigate marginal stability of the edge region.\\[4pt] [1] P.B. Snyder et al., Phys. Plasmas~9, 2037~(2002)\\[0pt] [2] D.A. Baver et al., Comp. Phys. Comm. 182, 1610 (2011)\\[0pt] [3] B.D. Dudson et al., Comp. Phys. Comm. 180, 1467 (2009)\\[0pt] [4] www.lodestar.com/research/vnv/ [Preview Abstract] |
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PP8.00035: Simulation of a tokamak edge plasma with the kinetic code COGENT M. Dorf, R. Cohen, M. Dorr, J. Hittinger, T. Rognlien, P. Colella, D. Martin, P. McCorquodale Progress on the development of the continuum gyrokinetic code COGENT for edge plasma simulations is reported. The COGENT code models an axisymmetric gyrokinetic equation coupled to the long-wavelength limit of the gyro-Poisson equation. COGENT is distinguished by application of fourth-order conservative discretization, and mapped multiblock grid technology to handle the geometric complexity of the tokamak edge. The code has also a number of model collision operator options, which have been successfully verified in neoclassical simulations. Our recent development work has focused on incorporation of the full (nonlinear) Fokker-Planck collision model. The implementation of the Fokker-Plank operator is discussed in detail, and the results of the initial verification studies are presented. In addition, we report on progress and status of the newly available divertor version of the COGENT code that includes both closed and open magnetic field line regions and a model for recycled neutral gas. [Preview Abstract] |
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PP8.00036: Global $\delta f$ neoclassical calculations in a pedestal Matt Landreman, F. Parra, P.J. Catto, D.R. Ernst, I. Pusztai Conventional calculations of neoclassical flows, current, and fluxes may not be valid in the pedestal since the strong gradients violate the assumed ordering, yet accurate calculation of these quantities is important for understanding edge stability and confinement. We have therefore developed a new radially global continuum neoclassical code PERFECT [1] which allows some radial scale lengths to be as small as the poloidal ion gyroradius. A strong radial electric field with strong shear is also included. In contrast to conventional neoclassical calculations, sources of particles and energy must be determined self-consistently to find the correction to the Maxwellian. The full linearized Fokker-Planck collision operator is implemented, arbitrary collisionality is allowed, and an arbitrary number of species are permitted. Efficiency is aided by a new spectral discretization for velocity space [2] and a preconditioned Krylov-space solver. At large aspect ratio, precise agreement is obtained between the code and recent analytic theory that accounts for finite orbit width effects. At realistic aspect ratio, strong poloidal asymmetries can arise in the flow, breaking the usual form for flows on a flux surface.\\[4pt] [1] PPCF 54, 115006 (2012).\\[0pt] [2] J Comp Phys 243, 130 (2013). [Preview Abstract] |
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PP8.00037: Applications of the ArbiTER edge plasma eigenvalue code D.A. Baver, J.R. Myra, M.V. Umansky ArbiTER is a flexible eigenvalue code designed for plasma physics applications. This code uses an equation and topology parser to determine how a particular set of linearized model equations is spatially discretized. The resulting matrix form is then solved using the SLEPc [1] eigensolver package. The equation and topology parsers permit a wide variety of capabilities, including variable numbers of dimensions, both finite difference and finite element methods, and irregular boundary conditions. Recent upgrades also permit parallel operation and the solution of source-driven problems. Two applications of this code will be presented, both as demonstrations of capability and as benchmark cases. One of these is the calculation of resistive ballooning modes with fully kinetic electrons. This will demonstrate the capacity for solving kinetic problems. The other is the use of extended spatial domains for ballooning stability analysis. This will demonstrate the utility of extra dimensions in calculations with fluid models. Work supported by the U.S. DOE grant DE-SC0006562.\\[4pt] [1] http://www.grycap.upv.es/slepc/ [Preview Abstract] |
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PP8.00038: Ballooning Stability of Separatrix Spanning Modes J.R. Myra, D.A. Baver, D.A. D'Ippolito, M.V. Umansky, L.L. LoDestro, R.J. Goldston, J.H. Nichols The ideal ballooning stability of the near-separatrix tokamak plasma and its possible relation to the Greenwald density limit, as discussed in [1], motivates the present work. We consider a sequence of CORSICA-generated equilibrium shapes with varying elongation and examine the marginal stability of infinite-n and finite-n separatrix-spanning modes using the 2DX [2] and ArbiTER [3] eigenvalue codes. The elongation scaling of the result provides a test of the proposed density-limit theory. A new computationally efficient technique for dealing with the phase variation of moderate-n modes across the branch cut in field-line following coordinates will also be discussed.\\[4pt] [1] R. J. Goldston and T. Eich, 24th IAEA Fusion Energy Conference, San Diego, USA, October 8 - 13, 2012, paper IAEA-CN-197/TH/P4-19 \newline [2] D. A. Baver, J. R. Myra and M.V. Umansky, Comp. Phys. Comm. \textbf{182}, 1610 (2011). \newline [3] D. A. Baver et al., this conference [Preview Abstract] |
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PP8.00039: KInetic Effect on Dynamics of Plasma Coherent Structures Seiji Ishiguro, Hiroki Hasegawa Kinetic effects on plasma blob dynamics have been studied by means of a three dimensional electrostatic plasma particle simulation code with particle absorbing boundaries. In the particle simulation, an external magnetic field $B$ is pointing into the $z$ direction (corresponding to the toroidal direction). The strength of magnetic field increases in the positive x direction (corresponding to the counter radial direction), i.e., $\partial B / \partial x > 0$. A coherent structure is initially set as a column along the external magnetic field and propagates in the $-x$ direction. In this study, we have investigated the dependence of blob propagation on the ion-to-electron temperature ratio and the magnetic field strength. When the magnetic field strength is decreased (or the ion-to-electron temperature ratio is increased), we have found that the symmetry breaking in a blob profile occurs. This fact is thought to indicate that the effect of gyro motion of plasma particles induces the symmetry breaking. [Preview Abstract] |
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PP8.00040: Configuration of Spontaneous Flows in a Plasma Coherent Structure Hiroki Hasegawa, Seiji Ishiguro Spontaneous particle flows in a plasma coherent structure (blob) have been studied with a three dimensional electrostatic plasma particle simulation code. In the particle simulation, the particle absorbing boundaries are placed on the both ends in the $z$ direction (corresponding to the toroidal direction) and one end in the $x$ direction (corresponding to the counter radial direction). The former boundaries and the latter one corresponds to end plates (divertor plates) and the first wall. A coherent structure is initially set as a column along the external magnetic field and propagates to the first wall because the magnetic field is set as $\partial B / \partial x \neq 0$. In this study, we have investigated the configuration of spontaneous particle flows in a plasma coherent structure. We have found that a spiral current system in a plasma blob and characteristic features in the velocity distribution of plasma particles in a blob. [Preview Abstract] |
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PP8.00041: Parallel flow acceleration at the presence of magnetic trapping in an open field line plasma Zehua Guo, Xianzhu Tang, Chris McDevitt Tokamak scrape-off layer (SOL) plasma spans a range of collisionality that poses a challenge for both kinetic and fluid modeling. In high-performance shots, the SOL plasma can be sufficiently hot that although the electrons remain a nearly isotropic Maxwellian distribution, the ions can become anisotropic due to magnetic trapping of the $1/R$ toroidal field around the mid-plane. Similarly the electrons and ions tend to have different temperatures. Capturing the physics hence requires at least three temperatures, $T_e, T_{\perp_i}, T_{\perp_e}.$ The ion parallel flow acceleration, which is closely tied to ambipolar potential, becomes particularly interesting in comparison with the standard SOL fluid model predictions. Here we illustrate the underlying physics in a simplified linear geometry where the combined effect of magnetic trapping and plasma sheath is resolved by kinetic simulations. Specifically, we find that parallel flow is accelerated across the transonic point far away from the sheath entrance. Remarkably, a modified CGL model with collisional effect is found to be useful for understanding the new physics results. Work supported by OFES. [Preview Abstract] |
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PP8.00042: Global electromagnetic simulations of tokamak scrape-off layer turbulence Federico Halpern, Paolo Ricci, Sebastien Jolliet, Joaquim Loizu, Annamaria Mosetto We discuss recent studies addressing the properties of tokamak SOL turbulence using a global, electromagnetic, fluid drift-reduced Braginskii model. Non-linear simulations are carried out using the Global Braginskii Solver (GBS) code [1], which is capable of carrying out self-consistent, global three-dimensional simulations of the plasma dynamics in the tokamak SOL. The simulations involve plasma profile formation in the SOL as a power balance between plasma flux from the core, the turbulent radial transport, and the losses at the plasma sheath where the magnetic field lines intersect with the vessel. A gradual approach in increasing complexity has made possible (a) to determine the dominant instabilities driving the SOL turbulence, (b) to identify the mechanisms that saturate the growth of the linear modes and therefore regulate the level of radial transport, and (c) to study the role of electromagnetic effects in enhanced transport regimes. The non-linear dynamics revealed by the simulations agree with the analytical estimates that have been carried out. A scaling for the SOL width in circular limited plasmas has been derived and compared against experimental data from several tokamaks.\\[4pt] [1] P. Ricci et al., Plasma Physics and Controlled Fusion, 2012, 54, 124047. [Preview Abstract] |
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PP8.00043: Divertor heat-load width from gyrokinetic neoclassical and turbulence simulation in XGC1 Seung-Hoe Ku, Robert Hager, C.S. Chang, Jianying Lang, D.P. Stotler Divertor heat-load width is one of the critical issues for magnetic fusion and ITER. For a more complete first-principles study of the problem, XGC1 gyrokinetic simulation is performed in diverted geometry including neutral particles, gyrokinetic ions, and drift-kinetic electrons. Neoclassical, X-transport (orbit loss), spontaneous radial and poloidal electric field generation, and turbulence bob physics are solved together. A stable logical sheath algorithm is used for the determination of wall sheath potential, without the actual resolution of the Debye sheath profile. The contributions of various physics elements to the divertor heat-load width are investigated: baseline neoclassical physics, blobs, neutral particles, and 3D RMP. DIII-D, NSTX, C-Mod, and JET geometry will be compared. Comparison with the drift-based model for heat-load width [1] and the purely neoclassical XGC0 result [2] and comparison with experimental observations will also be presented. [Preview Abstract] |
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PP8.00044: Theory of Advanced Magnetic Divertors Michael Kotschenreuther, Prashant Valanju, Swadesh Mahajan, Brent Covele The magnetic field structure in the SOL is the most important determinant of divertor physics. A comprehensive analytical and numerical methodology is developed to investigate SOL magnetic fields in the backdrop of two advanced divertor geometries- the X-divertor (XD) proposed and discussed in 2004, and the snowflake divertor (SFD) of 2007-2010. The analysis shows that XD and SFD represent very distinct and readily distinguishable magnetic geometries, epitomized through a differentiating metric, the Divertor Index (DI). In terms of this simple metric, the XD (DI \textgreater\ 1) and the SFD (DI \textless\ 1) fall on opposite sides of the standard divertor SD (DI$=$1). Amongst other things, DI signifies the rate of convergence (divergence) of the flux surfaces near the divertor plate; the flux surfaces of SFD are more convergent contracting) than the SD while the XD flux surfaces are less convergent, in fact, divergent (flaring). These different SOL magnetics imply different physics, particularly with respect to detachment dynamics. It is also shown that some experiments on NSTX and DIII-D match both the prescription and the predictions of the 2004 XD paper. [Preview Abstract] |
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PP8.00045: Advanced Divertor Design and Application under Modern Superconducting Tokamak Constraints Brent Covele, Mike Kotschenreuther, Swadesh Mahajan, Prashant Valanju With current ITER projections already predicting divertor exhaust heat loads in the 5-10 MW/m$^{2}$ range, i.e. at the maximum tolerance, it is clear that the divertor heat load problem will only be exacerbated for future superconducting tokamaks, as well as perhaps some modern tokamaks today. Thus, an advanced divertor, such as the X-Divertor (XD), Super-X Divertor (SXD), or Snowflake (SF) will become a virtual necessity to reduce incident heat flux at the target plates. Using the 2D magnetic equilibrium code CORSICA, we explore the possibilities of creating an advanced divertor for a next-generation superconducting tokamak (I$_{\mathrm{p}} =$ 15 MA, B$_{\mathrm{T}} =$ 5.3 T, R $=$ 6.2 m) under nominal engineering constraints. Advanced divertors were achieved with no in-vessel PF coils, PF current densities below 30 MA/m$^{2}$, and vertical maintenance access, all of which are favorable conditions for tokamaks today. Both the XD and SF divertors are readily achievable while maintaining core plasma performance, and the advantages and disadvantages of each are discussed in turn. Some thought is given as to how the divertor cassette will need to be modified to accommodate advanced divertors. [Preview Abstract] |
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PP8.00046: Comparison Between Experiments and EMC3-Eirene Simulations of the Snowflake Divertor in TCV G.P. Canal, T. Lunt, Y. Feng, H. Reimerdes, B.P. Duval, B. Labit, W.A.J. Vijvers, S. Coda, T.W. Morgan, F. Nespoli, B. Tal, G. De Temmerman In reactor-size machines like DEMO, conventional divertor solutions are not expected to be sufficient to keep the heat load within the operating limits of the plasma-facing components. The ``snowflake'' (SF) divertor has emerged as a potential way to reduce the heat loads. EMC3-Eirene simulations of the plasma- and neutral particle-transport in the scrape-off layer of SF plasmas were performed for various distances between primary and secondary X-points. Anomalously large cross-field transport coefficients had to be chosen to match the experimental particle and heat flux profiles at the primary strike points. Although these profiles are well matched, the heat fluxes at the strike points in the private flux region are underestimated compared to those obtained experimentally, suggesting an additional cross-field transport mechanism not included in the simulation. The model also predicts the formation of a high density plasma blob at the primary X-point for small distances between X-points, which has not yet been seen experimentally, further supporting the hypothesis of an additional cross-field transport mechanism. The influence of particle drifts on the particle and heat flux profiles will be discussed. [Preview Abstract] |
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PP8.00047: Power exhaust in all geometric variations of the snowflake divertor on TCV Wouter Vijvers, Gustavo Canal, Basil Duval, Benoit Labit, Holger Reimerdes, Stefano Coda, Tilmann Lunt, Thomas Morgan, Greg de Temmerman The snowflake (SF) divertor is recognized as a potential exhaust solution for large-scale, high-performance tokamaks. TCV has advanced to a detailed study of the transport through the SF's scrape-off layer (SOL), null region and divertor legs to determine the optimal geometry and quantify parallel and cross-field transport. Experimental SF plasmas have two closely spaced x-points, leading to two additional strike points (SPs) and a larger region of low poloidal field than in a conventional divertor. The relative x-point positions determine the divertor geometry and hence the exhaust properties. The results show that if parallel transport is dominant, either the HFS or LFS SOL power can be distributed to two SPs, with the power ratio depending on the SOL width, inter-x-point distance (D) and geometrical divertor asymmetry. Cross-field transport allows power to reach SPs in the private flux region. Experiments show significant power reaching such SPs already at large D, particularly during ELMs, enabling a 2-3x reduction in flux to the main SPs. As EMC3-Eirene simulations predict much smaller SP powers, additional transport mechanisms beyond perpendicular diffusion are considered. The SF's beneficial magnetic properties are shown to be enhanced in reactor-size devices. [Preview Abstract] |
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PP8.00048: Simulation for heat flux mitigation by gas puffing in KSTAR Seung Bo Shim, Vladislav Kotov, Suk-Ho Hong, Reiter Detlev, Jin Yong Kim, Yong Su Na, Hae June Lee Control of heat flux is very important to achieve high performance long pulse operation in tokamaks. There are so many efforts to reduce the heat flux like change of divertor structure, snowflake divertor, and RMP, etc. Detachment by gas puffing is used for long time to reduce the heat flux. In this paper edge plasma scenarios of KSTAR are analyzed numerically by well-known B2-Eirene code package(SOLPS4.3). High performance discharges with heating power $\approx $ 8 MW and core flux $\approx $ 10$^{21}$ s$^{-1}$ is used. Gas puffed on the outer mid-plane(OMP), both divertors is likely to stay attached. So, gas puffed on the outer target, one is near the private flux region(PFR) and the other is near the scrape-off-layer(SOL). When gas puffed near the SOL is still attached, and it is worse than gas puff from OMP because it is too close to cryo-pump. The case near the PFR shows high recycling region easily compared with OMP case. When one forth gas puffed on the PFR, results are similar with OMP case. But it is still not good for detachment operation. Detachment operation window is too small for the gas puffing on the PFR. [Preview Abstract] |
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PP8.00049: Effects of toroidal field direction and heating power on divertor asymmetry and scrape-off layer flow in EAST Shaocheng Liu, Houyang Guo, Liang Wang, Huiqian Wang, Kaifu Gan, Guosheng Xu, Liang Chen, Ning Yan, Wei Zhang, Ran Chen, Linming Shao, Hao Xiong, Siye Ding, Guanghai Hu, Yelu Liu, Nan Zhao, YongLiang Li, Xiang Gao, Xianzu Gong Divertor asymmetry and scrape-off layer (SOL) flow have been systematically investigated in the EAST, with respect to toroidal field direction, divertor configuration, power injection methods and heating power. Divertor plasma exhibits an outboard-enhanced in-out asymmetry in heat flux in LSN configuration for both normal and reversed field directions. USN exhibits an inboard-favored asymmetry for normal field, while exhibits a balanced or even outboard-favored asymmetry for reversed field. DN has the strongest in-out asymmetry in heat flux, favoring the outer divertor. The in-out asymmetry ratio of q$_{t,out}$/q$_{t,total}$ increases with the power across the separatrix P$_{loss}$. The characteristics of the measured SOL parallel flow under various discharge conditions are consistent with the Pfirsch-Schl\"uter flow with the parallel Mach number M$_{\vert \vert }$ decreasing with the line averaged density but increasing with P$_{loss}$. [Preview Abstract] |
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PP8.00050: Implication of a shock front in the X-point region of ASDEX Upgrade on the evolution of divertor detachment Marco Wischmeier, Steffen Potzel, Daniel Carralero, Stefan H. Muller Existing numerical fluid transport code packages containing an as complete as possible model of our current understanding of the volumetric processes in the divertor are not able to qualitatively reproduce the experimental observations in the high field side, HFS, divertor of ASDEX Upgrade during detachment. In contrast to numerical models it is observed experimentally that as the ion flux density to the inner divertor drops a region of high density $is formed in the X-point region of the far Scrape-Off Layer, SOL, of the HFS divertor extending away from the separatrix. In the X-point region supersonic flows are theoretically predicted and measured experimentally. In a Gedankenexperiment we compare the critical conditions for the onset and offset of a shock front and its implications for the spatial change of the recycling along the divertor target. If the poloidal temperature gradient was of the order of the neutrals mean free path then a shock front could form close to the X-point. We compare the implications to the experimental dependencies under which the high density in the far SOL of the HFS X-point region appears. [Preview Abstract] |
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PP8.00051: Evaluating Stellarator Divertor Designs with EMC3 Aaron Bader, D.T. Anderson, Y. Feng, C.C. Hegna, J.N. Talmadge In this paper various improvements of stellarator divertor design are explored. Next step stellarator devices require innovative divertor solutions to handle heat flux loads and impurity control. One avenue is to enhance magnetic flux expansion near strike points, somewhat akin to the X-Divertor concept in Tokamaks. The effect of judiciously placed external coils on flux deposition is calculated for configurations based on the HSX stellarator. In addition, we attempt to optimize divertor plate location to facilitate the external coil placement. Alternate areas of focus involve altering edge island size to elucidate the driving physics in the edge. The 3-D nature of stellarators complicates design and necessitates analysis of new divertor structures with appropriate simulation tools. We evaluate the various configurations with the coupled codes EMC3-EIRENE, allowing us to benchmark configurations based on target heat flux, impurity behavior, radiated power, and transitions to high recycling and detached regimes. [Preview Abstract] |
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PP8.00052: Secondary electron emission and the bifurcation of the heat flux to the targets in fusion plasmas Wonjae Lee, Sergei Krasheninnikov The presence of secondary electron emission (SEE) from plasma facing components in fusion devices can result in a strong localization of the heat flux from plasma to the wall and subsequent wall erosion. Usually, the impact of the SEE is considered assuming the Maxwellian distribution of the electrons coming to the surface. As a result, the SEE coefficient only depends on electron temperature and not on the wall potential. Therefore, the solution for the wall potential found from the ambipolarity of plasma flow to the wall (including SEE) is unique. However, the tail of electron distribution function in the SOL of fusion devices can be far from Maxwellian due to preferential loss of fast electrons. Consequently, the SEE coefficient will depend on the wall potential and multiple solutions of the ambipolarity equation can be possible corresponding to different regimes of plasma flow to the wall: with and without SEE effects. [Preview Abstract] |
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PP8.00053: Transport of Tungsten and Beryllium Droplets in ITER R.D. Smirnov, B.T. Brown, S.I. Krasheninnikov, T.D. Rognlien Recent experiments on ASDEX-Upgrade [1] and JET [2] tokamaks demonstrate that melting of tungsten and beryllium plasma-facing components and ejection of molten material in form of droplets can occur in transient plasma events relevant to ITER. As the experiments and previous simulations [3] show, the ejected material can significantly affect impurity content and discharge stability in present tokamaks. Thus, it is important to evaluate transport and possible impact of the molten ejectile on ITER operation. In this work we present results of modeling with DUSTT/UEDGE code of tungsten and beryllium droplet transport in ITER. The ejection of various quantities of the droplets with different sizes and velocities from top, mid-plane, and divertor locations is simulated. The increase of core impurity influx and critical amounts of the ejected material, which can cause discharge termination, are evaluated. The produced by droplet ejection long range impurity transport and re-deposition are also discussed.\\[4pt] [1] K. Krieger et al., Phys. Scr. T145 (2011) 014067.\\[0pt] [2] G. Sergienko et al., PFMC-14 (2013) A104.\\[0pt] [3] R.D. Smirnov et al., J. Nulc. Mater. 415 (2011) S1067. [Preview Abstract] |
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PP8.00054: Time-dependent recycling modeling with edge plasma transport codes A. Pigarov, S. Krasheninnikov, T. Rognlien, S. Taverniers, E. Hollmann First,we discuss extensions to Macroblob approach which allow to simulate more accurately dynamics of ELMs, pedestal and edge transport with UEDGE code. Second,we present UEDGE modeling results for H mode discharge with infrequent ELMs and large pedestal losses on DIII-D. In modeled sequence of ELMs this discharge attains a dynamic equilibrium. Temporal evolution of pedestal plasma profiles, spectral line emission, and surface temperature matching experimental data over ELM cycle is discussed. Analysis of dynamic gas balance highlights important role of material surfaces. We quantified the wall outgassing between ELMs as 3X the NBI fueling and the recycling coefficient as 0.8 for wall pumping via macroblob-wall interactions. Third,we also present results from multiphysics version of UEDGE with built-in, reduced, 1-D wall models and analyze the role of various PMI processes. Progress in framework-coupled UEDGE/WALLPSI code is discussed. Finally, implicit coupling schemes are important feature of multiphysics codes and we report on the results of parametric analysis of convergence and performance for Picard and Newton iterations in a system of coupled deterministic-stochastic ODE and proposed modifications enhancing convergence. [Preview Abstract] |
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PP8.00055: 3D Equilibrium Reconstruction with Magnetic Islands for DIII-D ELM Suppression Experiments Allan Reiman, Donald Monticello We discuss 3D equilibrium calculations performed with the PIES2012 code for nonaxisymmetric DIII-D shots. The shots were part of a series of experiments studying the suppression of edge localized modes (ELMs) using externally imposed nonaxisymmetric magnetic fields. Our calculations are benchmarked against those of other codes, and are compared with experimental observations. [Preview Abstract] |
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PP8.00056: The PIES2012 Code for Calculating 3D Equilibria with Islands and Stochastic Regions Donald Monticello, Allan Reiman, Daniel Raburn We have made major modifications to the PIES 3D equilibrium code to produce a new version, PIES2012. The new version uses an adaptive radial grid for calculating equilibrium currents. A subset of the flux surfaces conform closely to island separatrices, providing an accurate treatment of the effects driving the neoclassical tearing mode. There is now a set of grid surfaces that conform to the flux surfaces in the interiors of the islands, allowing the proper treatment of the current profiles in the islands, which play an important role in tearing phenomena. We have verified that we can introduce appropriate current profiles in the islands to suppress their growth, allowing us to simulate situations where islands are allowed to grow at some rational surfaces but not others. Placement of grid surfaces between islands is guided by the locations of high order fixed points, allowing us to avoid spectral polution and providing a more robust, and smoother convergence of the code. The code now has an option for turning on a vertical magnetic field to fix the position of the magnetic axis, which models the horizontal feedback positioning of a tokamak plasma. The code has a new option for using a Jacobian-Free Newton Krylov scheme for convergence. The code now also contains a model that properly handles stochastic regions with nonzero pressure gradients. [Preview Abstract] |
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PP8.00057: Modified edge-localized mode (ELM) structures and dynamics under static n=1 resonant MP in the KSTAR tokamak J. Lee, G.S. Yun, M. Kim, M.J. Choi, W. Lee, H.K. Park, C.W. Domier, N.C. Luhmann, Jr., Y.M. Jeon, S.G. Lee Magnetic perturbations (MPs) are one of the methods to suppress or mitigate the edge-localized modes (ELMs) by enhancing particle transport and keeping the edge pressure gradient below a threshold. In the 2012 KSTAR campaign, static $n=1$ resonant MPs [1] altered both the spatial structure and temporal dynamics of ELMs. In particular, the electron cyclotron emission (ECE) images [2] of the plasma edge region showed that the ELM filaments still remained without burst during the entire ELM-crash-suppressed period. The observation suggests that the MP did not prevent the growth of ELM perturbation, but instead kept the growth below a critical value. During this period, the apparent poloidal motion of the ELM filaments was random, suggesting that the MP caused the plasma edge to become stationary in the laboratory frame. The poloidal spacing between the filaments reduced from $\sim 20$ cm before MP to $\sim 13$ cm after MP, which suggests a transition to a higher toroidal mode number.\\[4pt] [1] Y.M. Jeon et al. Phys. Rev. Lett. 109 (2012)\\[0pt] [2] G.S. Yun et al., Phys. Rev. Lett. 81 (2011) [Preview Abstract] |
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PP8.00058: The effects of weakly 3-D equilibrium on MHD stabiliyt of tokamak pedestals C.C. Hegna The stability of MHD modes is evaluated in the presence of an equilibrium perturbed by a topology-preserving 3-D distortion. The theory employs a perturbation approach assuming that the 3-D amplitude is small. In general, the 3-D distortion is destabilizing as it lowers the critical conditions for instability for the least stable mode. The theory is specialized to the MHD stability of pedestal modes in the presence of shielded RMP fields. Previous work has demonstrated that local MHD stability properties (and hence microinstabilities) can be significantly altered by the presence of applied 3-D fields [1]. In this work, we expand these calculations in an effort to address whether RMP fields can affect `global' peeling-ballooning modes. For this application, the dominant 3-D modification is due to the localized resonant current responses at rational surfaces. These localized currents couple harmonics with different toroidal numbers and produce an MHD eigenmode with multiple toroidal harmonics. The physics of how the localized current structures affect the MHD stability of tokamak pedestals will be discussed.\\[4pt] [1] T. M. Bird and C. C. Hegna, \emph{Nucl. Fusion} {\bf 53}, 013004 (2013) [Preview Abstract] |
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PP8.00059: 3D Boundary Behavior in RFX-mod: Role of Advected Quantities P. Scarin, M. Agostini, L. Carraro, G. Ciaccio, G. Spizzo, N. Vianello The boundary of the RFX-mod Reversed Field Pinch device is characterized by weak magnetic chaos affecting ion and electron diffusion. Particle transport is determined by magnetic islands and by the electrostatic potential which arises from the interaction of the ambipolar constraint on electron-ion motion, with plasma-wall interaction (PWI). We will present an extensive summary of experimental observations, indicating that plasma pressure, floating potential, particle influx, plasma flow (or equivalently, radial electric field E$^{\mathrm{r}})$ and turbulence structures are modulated according to the the MHD modes (m/n$=$0/1 and 1/7) that determine the geometry of the edge. The distortions (``ripple'') of magnetic flux surfaces, along the toroidal and poloidal directions, are due to the presence of magnetic islands, whose X-points in the RFP are characterized by a negative charge excess (electron accumulation). Apparently, RMPs in Tokamaks are characterized by electron accumulation near the O-points of those islands, due to a different level of magnetic chaos. The presence of an E$^{\mathrm{r}}$ modulation along the angles, makes the edge plasma sensitive to advected quantities through particle energy, energy-exchanging collisions and particle recycling at the wall. We will also explore the feedback of the change of turbulence properties on MHD, by analyzing the dependence of the 0/1 mode amplitude on the Prandtl number. [Preview Abstract] |
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PP8.00060: NIMROD Computation of Plasma Response to Resonant Magnetic Perturbations in DIII-D P. Zhu, C.R. Sovinec Plasma response is believed to be a key element in the suppression and mitigation of edge localized modes by resonant magnetic perturbations (RMPs) in tokamak experiments. We compute the plasma response to RMP in DIII-D discharges \#126006 and \#142603 using the extended MHD models implemented in the NIMROD code. The I-coil vacuum field is imposed as the boundary condition at the tokamak wall location. Plasma responses to RMP are obtained by following the linear and nonlinear evolution of the configuration into steady state subject to the RMP boundary condition. Such a steady state can be only reached if the unstable toroidal component edge localized modes in these discharges are suppressed by physical mechanisms in the MHD model or excluded numerically from the computation. The rotational and two-fluid effects on the toroidal and poloidal spectra of the perturbed magnetic field on each magnetic flux surface in response to RMPs are evaluated in these simulations. [Preview Abstract] |
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PP8.00061: Particle simulation in stochastic magnetic fields at tokamak edge C.C. Chang, Y. Nishimura, C.Z. Cheng An orbit following simulation code is developed incorporating magnetic perturbation. While magnetic field lines can exhibit stochastic behavior in the presence of incommensurate magnetic perturbations,\footnote{In the simulation model, the magnetic perturbations are externally imposed; T. E. Evans, R. A. Moyer, et al., Nature Physics {\bf 2}, 419 (2006).} the particle motions are also influenced by the mirror force and the perturbed electric fields. Remnants of lowest order magnetic islands can also play an important role in regulating the particle and heat transport. Effective perpendicular transport can be enhanced in the presence of trapped particles; how the mirror force influences the transport in stochastic magnetic fields is examined. This work is supported by National Science Council of Taiwan, NSC 100-2112-M-006-021-MY3 and NCKU Top University Project. [Preview Abstract] |
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PP8.00062: Applications of 3D Equilibrium Reconstruction Samuel Lazerson, Joachim Geiger, Yuri Gribov, Stuart Hudson, David Gates The STELLOPT 3D equilibrium reconstruction code has utilized to model the W7-X diagnostic response to bootstrap current profile variation, the ITER diagnostic response to +/- 2 cm boundary variations, and full 3D equilibrium reconstructions for the DIII-D plasma with applied RMP fields. Field line tracing codes indicate a +/- 5 cm ability to shift divertor strike points providing limited capabilities to control divertor heat loads in W7-X. Parameter space maps of bootstrap current variation suggest a diagnostic set capable of predicting bootstrap current evolution and providing feedback control of the plasma. Modeling of the ITER 15MA scenario suggests such coils will perturb the plasma edge up to +/- 2 cm, with significant effects on magnetic diagnostic response. Full 3D reconstructions of the DIII-D plasma with applied RMP's have been carried out with STELLOPT. Forward modeling of the equilibrium using the Stepped Pressure Equilibrium Code (SPEC) suggests the presence of islands and corroborates the notion of mode penetration in this shot. [Preview Abstract] |
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PP8.00063: Gyrokinetic particle simulation of internal kink modes in DIII-D Joseph McClenaghan, Zhihong Lin Magnetohydrodynamic (MHD) instabilities excited by equilibrium current in toroidal fusion devices play important roles in plasma stability and confinement. Kinetic effects are important in the excitation and saturation of the MHD modes, as well as resulting transport. In this work, we have applied Gyrokinetic Toroidal Code (GTC) to study kinetic effects in current-driven MHD modes. As the first step, we have performed GTC simulation of the n=m=1 internal kink mode, which has been studied extensively in tokamak experiments, theory and MHD simulations. We compare the dispersion relation and mode structure from the simulation to the ideal MHD theory in a low beta limit to verify the gyrokinetic simulation of current-driven MHD modes. As a next step, we simulate the internal kink mode in a realistic DIII-D geometry. [Preview Abstract] |
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PP8.00064: A Mechanism for Soft Beta Limits in NSTX Stephen Jardin, Nate Ferraro, Jin Chen, Josh Breslau, Stefan Gerhardt It is well known that exceeding the beta limits in a tokamak or ST in some regions of parameter space can lead to a disruption, whereas in other regions one encounters a ``soft limit'' where transport is increased locally but the discharge continues. To better understand the mechanism which leads to a soft beta limit, we have modeled part of the evolution of a NSTX discharge that is near or at the beta limit for interchange modes near the magnetic axis. Using the implicit 3D MHD code M3D-$C^{1}$ we find that as the central safety factor, q$_{0}$, slowly decreases toward unity due to resistive diffusion, a single n$=$3 interchange-like mode is the first to become unstable. As this mode grows in amplitude, it nonlinearly drives n$=$6, n$=$9, and other modes. In the early growth stages, the n$=$3 mode mostly distorts the magnetic surfaces. However, once the other modes reach comparable amplitude, Poincare plots of the magnetic field show that parallel thermal conductivity, which is much larger than perpendicular thermal conductivity ($\kappa_{\vert \vert} \gg \kappa_{\bot}$), can cause rapid transport of the electron energy from the center to an annular region surrounding the center, reducing the central pressure that was driving the instability. In many cases, the magnetic surfaces will reform and the configuration, now with lower central pressure, will become axisymmetric again. This work was supported by the US DOE contract no. DEAC02-09CH11466 and the SciDAC Center for Extended Magnetohydrodynamic Modeling. [Preview Abstract] |
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PP8.00065: New picture of the 1/1 internal kink and sawtooth in compressible toroidal plasmas Linda Sugiyama The $m=1$, $n=1$ internal kink mode and the sawtooth crash have been analyzed extensively in magnetically confined toroidal plasmas. Nevertheless, many questions remain. A new analysis, with the aid of numerical simulation, shows that small parameter expansions such as large aspect ratio break down in general for the MHD compressible toroidal 1/1 instability with realistically small growth rates. The perpendicular momentum rate of change $\rho\partial\mathbf{v}_\perp/\partial t$ must be very small compared to the individual terms in $-\rho(\mathbf{v}\cdot\nabla)\mathbf{v}|_\perp+\mathbf{J}\times\mathbf{B}|_\perp-\nabla_\perp p$. The lowest order mode still has the standard 1/1 internal kink form, but the $\mathbf{v}_\perp$ magnitude and growth rate are determined by the higher order terms. Terms containing $\tilde{B}_\phi$, nominally associated with the compressional Alfv\'en wave are important. One corollary is that reduced MHD (RMHD) fails completely and Sweet-Parker-type reconnection never develops. At a critical nonlinear amplitude, associated with the growth of the higher toroidal harmonics, a fast, explosive crash begins with rapidly accelerating velocity growth that matches observations. Other transverse MHD instabilities experience analogous effects. [Preview Abstract] |
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PP8.00066: Generalized resistive wall boundary conditions for cylindrical and toroidal geometry in NIMROD Andrea Montgomery, C.C. Hegna, C.R. Sovinec, S.E. Kruger, S.A. Sabbagh A generalized resistive wall boundary condition is implemented in NIMROD, making it possible to study both cylindrical and toroidal geometries with arbitrary axisymmetric shaping. The magnetic fields inside the computational domain are matched at the wall with external fields found using a vacuum-field solver. With this extended capability, NIMROD is used to study the stability of resistive wall modes for an advanced scenario ITER constructed equilibrium with normalized beta of 2.9 (above the n$=$1 no-wall stability limit) and toroidal rotation. The non-linear, self-consistent interactions between a rotating plasma, unstable modes and a resistive wall are considered. Methods to extend the generalized boundary condition physics to include external resonant magnetic perturbations will be discussed. [Preview Abstract] |
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PP8.00067: Resistive Wall Mode Simulations With JOREK-STARWALL Rachel McAdams, Ian Chapman, Howard Wilson, Matthias Hoelzl, Guido Huysmans, Yueqiang Liu, Peter Merkel Resistive Wall Modes (RWMs) are one of the main limitations to operation at high plasma pressure and weak magnetic shear, as is required in so-called Advanced Tokamak scenarios. These scenarios may be a route to a viable fusion power station. The implementation of a coupled JOREK-STARWALL code, wherein STARWALL calculates the magnetic field in the vacuum region surrounded by a resistive wall, is used. The wall incorporates realistic geometry, with three dimensional effects. A typical cylindrical equilibrium, unstable to RWMs, is used to compare simulated growth rates from JOREK-STARWALL to linear, analytic growth rate predictions in order to illustrate and benchmark the influence of the resistive wall. Furthermore, the stability of ITER advanced scenarios to RWMs is explored with ITER wall geometry. [Preview Abstract] |
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PP8.00068: Dynamics of tokamak plasma surface current in 3D ideal MHD model Sergei A. Galkin, V.A. Svidzinski, L.E. Zakharov Interest in the surface current which can arise on perturbed sharp plasma vacuum interface in tokamaks was recently generated by a few papers (see [1-4] and references therein). In dangerous disruption events with plasma-touching-wall scenarios, the surface current can be shared with the wall leading to the strong, damaging forces acting on the wall [2] A relatively simple analytic definition of $\delta $-function surface current proportional to a jump of tangential component of magnetic field nevertheless leads to a complex computational problem on the moving plasma-vacuum interface, requiring the incorporation of non-linear 3D plasma dynamics even in one-fluid ideal MHD. The Disruption Simulation Code (DSC), which had recently been developed in a fully 3D toroidal geometry with adaptation to the moving plasma boundary, is an appropriate tool for accurate self-consistent $\delta $function surface current calculation. Progress on the DSC-3D development will be presented. Self-consistent surface current calculation under non-linear dynamics of low m kink mode and VDE will be discussed.\\[4pt] [1] L.E. Zakharov, Phys. Plasmas, \textbf{15}, 062507 (2008)\\[0pt] [2] A.J. Webster, Phys. Plasmas \textbf{17}, 110708 (2010)\\[0pt] [3] A.J. Webster, Phys. Plasmas \textbf{18}, 112507 (2011)\\[0pt] [4] L.E. Zakharov, S.A. Galkin, S.N. Gerasimov, Phys. Plasmas \textbf{19}, 055703 (2012) [Preview Abstract] |
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PP8.00069: Toroidal rotation and halo current produced by disruptions Henry Strauss, Linda Sugiyama, Roberto Paccagnella, Joshua Breslau, Stephen Jardin In several experiments including JET, it was observed that disruptions were accompanied by toroidal rotation. There is a concern that there may be a resonance between rotating toroidal perturbations and the resonant frequencies of the ITER vacuum vessel, causing enhanced damage. MHD simulations with M3D demonstrate that disruptions produce toroidal rotation. The toroidal velocity can produce several rotations of the sideways force during a disruption. Edge localized modes (ELMs) also produce poloidal and toroidal rotation. A theory of rotation produced by MHD activity will be presented. In the case of ELMs, the theory gives toroidal rotation Alfven Mach number, $M_\phi \approx 10^{-2} \beta_N. $ This is consistent with a scaling for intrinsic toroidal rotation in H mode tokamaks. It was also discovered on JET that disruptions were accompanied by toroidal variation of the plasma current $I_\phi$. From $\nabla \cdot {\bf j} = 0, $ the toroidal current variation $\Delta I_\phi$ is proportional to the 3D halo current, $ \oint J_n R d l,$ where $J_n$ is the normal current density at the wall. The 3D halo current is calculated analytically and computationally. A bound on $\Delta I_\phi/ I_{\phi}$ is found, proportional to the halo current fraction and toroidal peaking factor. [Preview Abstract] |
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PP8.00070: Guiding-center simulation of runaway electron confinement during tokamak disruptions Akinobu Matsuyama, Masatoshi Yagi, Yasuhiro Kagei During disruptions of tokamak discharges, runaway electrons are often generated and may cause a substantial damage to the plasma facing component. For this, runaway generation and transport mechanisms have been paid much attention in recent years. This paper reports a modeling of runaway confinement using a relativistic guiding-center following code ETC-Rel. The generation process is here included as a marker source term by Monte-Carlo sampling, and three-dimensional trajectories of MeV-order electrons are traced in realistic tokamak geometry with collisions and radiation. Simulation results are illustrated for evaluating (1) the energy distribution function with inductive electric field for ITER- and JT-60U-grade disruptions, and (2) the loss rate with low-order magnetic perturbations yielding the island overlapping. Effects of cross-field drift on the threshold of globally chaotic runaway trajectories are discussed, showing the possibility for high-energy part of runaways to be lost even below the stochastic threshold of the magnetic field-lines. The guiding-center model allows us to investigate such a runaway loss process in presence of magnetic perturbation without any diffusive-type approximation. [Preview Abstract] |
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PP8.00071: Hall-MHD resistive tearing modes in finite-aspect-ratio cylindrical geometry J.J. Ramos Analytic dispersion relations for the Hall-MHD resistive tearing instability in slab geometry [1-3] have been useful to carry out verification tests of extended-MHD numerical codes [4]. In order to provide additional benchmarks, the corresponding theory is worked out for the geometry of a periodic plasma cylinder of finite aspect ratio. While this problem is conceptually identical to the slab geometry one, the algebraic complication brought about by the Hall terms in the new geometry is non-trivial. Modifications of the dispersion relations that provide new benchmarking material are associated with the finite values of the ratio $k_z r/m$ between the axial and azimuthal wave numbers. The results are parametrized in the physically relevant phase diagram spanned by the plasma beta and the Hall parameter $k d_i$. \\[4pt] [1] V.V. Mirnov, C.C. Hegna and S.C. Prager, Phys. Plasmas 11, 4468 (2004). \\[0pt] [2] E. Ahedo and J.J. Ramos, Plasma Phys. Control. Fusion 51, 055018 (2009). \\[0pt] [3] E. Ahedo and J.J. Ramos, Phys. Plasmas 19, 072519 (2012). \\[0pt] [4] C.R. Sovinec, J.R. King and the NIMROD Team, J. Comp. Phys. 229, 5803 (2010). [Preview Abstract] |
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PP8.00072: Computational Investigation of Extended-MHD Effects on Tokamak Plasmas Jacob R. King, Scott E. Kruger We present studies with the extended-MHD NIMROD code of the tearing instability and edge-localized modes (ELMs). In our first study we use analytics and computations to examine tearing in a large-guide field with a nonzero pressure gradient where previous results show drift effects are stabilizing [Coppi, PoF (1964)]. Our work finds three new results: (1) At moderately large ion gyroradius the mode rotates at the electron drift velocity and there is no stabilization. (2) With collision-less drift reconnection, computations must also include electron gyroviscosity and advection. And (3) we derive a dispersion relation that exhibits diamagnetic stabilization and describes the transition between the electron-fluid-mediated regime of (1) and the semi-collisional regime [Drake and Lee, PoF (1977)]. Our second study investigates the transition from an ideal- to an extended-MHD model in an ELM unstable tokamak configuration. With the inclusion of a full generalized Ohm's law the growth rate is enhanced at intermediate wave-numbers and cut-off at large wave-numbers by diamagnetic effects consistent with analytics [Hastie et al., PoP (2003)]. Adding ion gyroviscosity to the model is stabilizing at large wave-numbers consistent with recent results [Xu et al., PoP (2013)]. [Preview Abstract] |
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PP8.00073: Stabilization of NTMs using real-time equilibrium reconstruction on TCV Doohyun Kim, Timothy P. Goodman, Olivier Sauter, Hoang Bao Le, Jean-Marc Moret In tokamak plasmas, Neoclassical Tearing Modes (NTMs) can limit $\beta $ values to below the ideal MHD limit and degrade plasma confinement [1]. Therefore, to reach the high performance tokamak regime, control and stabilization of NTMs is essential; it can be achieved using localized electron cyclotron heating and current drive (ECH/ECCD) [2]. In previous TCV experiments, NTM stabilization was obtained as ECH/ECCD deposition was swept in one direction until the mode disappeared [3]. We now enhance our control of NTMs using a real-time (RT) version of the equilibrium code LIUQE [4]. RT-LIUQE gives the safety factor (q) profile, and from pre-calculated ray-tracing and an assumption of the mode rational surface, a target q is converted to a requested EC launcher angle. When an NTM appears, one, or several, EC beams are directed at the NTM and the mode is successfully stabilized. \textit{This work was supported in part by the Swiss National Science Foundation.} \\[4pt] [1] Sauter O. \textit{et al} 1997 \textit{Phys. Plasmas} \textbf{4} 1654\\[0pt] [2] La Haye R.J. 2006 \textit{Phys. Plasmas} \textbf{13 }055501\\[0pt] [3] Felici F. \textit{et al }2012 \textit{Nucl. Fusion} \textbf{52} 074001\\[0pt] [4] Hofmann F. \textit{et al }1988 \textit{Nucl. Fusion} \textbf{28} 1871 [Preview Abstract] |
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PP8.00074: Observation of the bifurcation of tearing modes due to supersonic gas injected into the J-TEXT plasmas Xiande Feng, Ge Zhuang, Zhoujun Yang, Jinshui Xiao, Jie Chen, Xiwei Hu The influence on the magnetohydrodynamic (MHD) behaviors due to the gas injected by Super-sonic Molecular Beam Injection (SMBI) has been observed on J-TEXT tokamak. The change of local electron temperature and density due to the injected supersonic gas trigger a relaxation instability, and then the relaxation could bifurcate into two different types of tearing modes' behaviors. One is the tearing mode with single m/n$=$2/1 helicity (m and n are poloidal and toroidal mode numbers, respectively), another is the tearing modes with different helicities (m/n$=$2/1, 3/2). The mechanism responsible for the bifurcation is closely related to changes of the pressure, and lately the plasma current density profile, due to the gas injection. [Preview Abstract] |
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PP8.00075: Theory and simulation of the magnetic island-induced TAE mode Carson Cook, Chris Hegna, Don Spong, Steve Hirshman In this work, we develop the theory of the shear Alfven continuum in the presence of a magnetic island in a toroidal equilibrium. The shear Alfven spectrum of a magnetically confined plasma influences the stability properties of the system.~ Discrete Alfven eigenmodes are of particular interest to fusion plasmas.~ The frequencies of these modes lie in the gaps of the Alfven continuum, and thus the modes do not experience continuum damping and can be driven unstable by energetic particles. The effects of magnetic islands on the ellipticity-induced Alfven eigenmode and the beta-induced Alfven eigenmode have been studied in some detail (see e.g. Biancalani et al, \textit{Plasma Phys. Control. Fusion }53, 025009 (2011)). However, the effects of islands on the toroidicity-induced Alfven eigenmode (TAE) has not been investigated. The magnetic island-induced TAE (MiTAE) gap will be discussed along with the discrete MiTAE mode. Numerical simulation results using the SIESTA equilibrium code will be presented and compared to theory. Using the Hessian matrix from a SIESTA equilibrium, the Alfven eigenmodes and frequencies can be computed. A simple toroidal equilibrium with an island will be studied and the computed MiTAE structure and frequency will be compared to the analytical prediction. [Preview Abstract] |
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PP8.00076: Dynamic evolutions of toroidal Alfven eigenmodes driven by energetic particles Zhiwei Ma, Jia Zhu, GuoYong Fu A kinetic simulation code based on a reduced model is developed to study dynamic evolutions of a single/multiple toroidicity-induced shear Alfven eigenmode (TAE) driven by energetic particles. In single TAE simulations, it is found for zero background damping that the wave amplitude in nonlinear phase can either saturate for weak particle drives or slowly increase for strong drives. This slow nonlinear growth in strong drive cases is found to be associated with broadening and overlapping of resonances between the wave and trapped particles. For the near-marginal-stability case with a large background damping, the mode nonlinear evolution exhibits strong upward and downward frequency chirping in multiple branches. An hole/clump formation is observed clearly in the corresponding evolution of particle distribution. In multiple TAE simulations, passing particles are found to cause a longer duration of the linear growth phase if resonant regions of two modes are quite perfectly overlapped. But deeply trapped particles play a dominant role on the nonlinear growth no matter whether the resonant regions of two modes are overlapped or not. From anisotropic and isotropic simulations, it is suggested that passing particles have a negative effect on the mode nonlinear development. For the near marginal stability case, the upward and downward frequency chirping in multiple branches are affected due to multiple mode interaction effect. [Preview Abstract] |
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PP8.00077: Energetic ion excited long-lasting ``sword'' modes in tokamak plasmas with low magnetic shear Xiaogang Wang, Ruibin Zhang, Wei Deng, Yi Liu An $m$/$n=$1 mode driven by trapped fast ions with a sword-shape envelope of long-lasting (for hundreds of milliseconds) magnetic perturbation signals, other than conventional fishbones, is studied in this paper. The mode is usually observed in low shear plasmas. Frequency and growth rate of the mode and its harmonics are calculated and in good agreements with observations. The radial mode structure is also obtained and compared with that of fishbones. It is found that due to fast ion driven the mode differs from magnetohydrodynamic long lived modes (LLMs) observed in MAST and NSTX. On the other hand, due to the feature of weak magnetic shear, the mode is also significantly different from fishbones. The nonlinear evolution of the mode and its comparison with fishbones are further investigated to analyze the effect of the mode on energetic particle transport and confinement. [Preview Abstract] |
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PP8.00078: Nonlinear Alfv\'en eigenmode studies through optimized PIC simulations Wenjun Deng, Guo-Yong Fu A marker optimization technique for $\delta f$ PIC simulations has been developed and implemented in the kinetic/MHD hybrid code M3D-K. The technique removes markers to achieve importance sampling for $\delta f$, so as to save markers as well as computing time. The technique saves about 95\% of markers in a nonlinear simulation of toroidal Alfv\'en eigenmode (TAE) driven by energetic particles. The technique is then applied to studies of nonlinear dynamics of energetic particles, TAE and reversed shear Alfv\'en eigenmode (RSAE). In our single mode study, a marginally unstable $n = 2$ RSAE is excited by energetic particles. The mode exhibits frequency chirping up and down in the nonlinear stage. The down chirping component has higher amplitude than the up chirping component. Such frequency chirping is due to the nonlinearities of the energetic particles, since the background MHD is linear in this simulation. The frequency chirping is probably related to hold-clump formation in energetic particle distribution function as predicted by analytic theory. In our multiple mode study, two TAEs with similar growth rates are excited simultaneously by energetic particles. The nonlinear mode-mode coupling effect is being investigated. [Preview Abstract] |
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PP8.00079: ESC-EEC-EPC code system for plasma core and edge equilibrium and particle orbits Xujing Li A new Edge Equilibrium Code (EEC), which is a new solver of the Grad-Shafranov equation complementing the existing ESC code (based on Fourier representation) is presented. EEC, being developed specifically for the near edge region with an arbitrary shape of the plasma boundary, uses adaptive flux coordinates with Hermite finite element representation. A special routine for fast solving the sparse matrix equations was created for EEC. The edge solution of EEC is matched with the core solution from ESC through a virtual boundary and the two codes communicate as two parallel processes. This approach addresses the future needs in enhancing functionality of EEC without conflicting with the interface of both codes. The code was complemented by Edge Particle Code (EPC) for massive calculation of collisional particle orbits using GPU. The resulting ESC-EEC-EPC code system acquired unmatched ability (a) in fast free and fixed boundary equilibrium calculations for arbitrary plasma shapes, (b) in using both $r-z$ and different flux coordinates, (c) in choosing different combinations of input profiles, (d) in performing equilibrium reconstruction together with variances analysis, and (e) in assessing the diagnostics used for equilibrium reconstruction. [Preview Abstract] |
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PP8.00080: Ertel's vorticity theorem and new flux surfaces in multi-fluid plasmas Elie Hameiri Based on an extension to plasmas of Ertel's classical vorticity theorem in fluid dynamics, it is shown that for each species in a multi-fluid plasma there exists a set of nested surfaces that have this species' fluid particles confined within them. Variational formulations for the plasma evolution and its equilibrium states are developed, based on the new surfaces and all of the dynamical conservation laws associated with them. It is shown that in the general equilibrium case, the energy principle lacks a minimum and cannot be used as a stability criterion. A special limit of the variational principle yields single-fluid magnetohydrodynamic plasma equilibria and can be used to approximate the equilibrium state of a two-fluid plasma in a perturbative way. [Preview Abstract] |
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PP8.00081: Anisotropic pressure effects on magnetospheric MHD equilibrium M. Furukawa We have studied effects of anisotropic pressure on magnetospheric MHD equilibrium analytically and numerically. The plasma is confined by only poloidal magnetic field generated by an internal ring current. The anisotropic pressure is assumed to be the CGL tensor form [1]. The diamagnetic current has two components; (i) the one remains at isotropic pressure and (ii) the other arises due to pressure anisotropy. Note that both components flow in the toroidal direction in this configuration. If the perpendicular pressure ($p_\perp$) is larger than the parallel pressure ($p_\parallel$), (i) and (ii) flow in the opposite direction in wide region of the confinement. Thus the change of magnetic field from the vacuum field is reduced even at high beta[2]. We have also examined beta limit. It can exceed unity locally. Especially when $p_\parallel > p_\perp$, the beta limit can be explained by using the analytic expression of diamagnetic current. \\[4pt] [1] G. F. Chew et al, Proc. R. Soc. London Ser. A 236, 112 (1956).\\[0pt] [2] M. Furukawa, The 12th Asia Pacific Physics Conference of AAPPS (Makuhari Messe, Chiba, Japan, July 2013), D1-PWe-01. [Preview Abstract] |
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PP8.00082: A direct comparison between single-fliud and Hall-MHD turbulence Hideaki Miura, Keisuke Araki Effects of the Hall term on energy transfer in MHD turbulence and on intermittency are studied numerically through a direct comparison of the single-fluid MHD turbulence and Hall MHD turbulence, aiming at clarifying and modeling the small-scales so that we can carry out macroscopic simulations of torus plasma and solar wind plasmas with the appropriate contributions by the small scales. Direct numerical simulations of freely decaying incompressible turbulence are carried out. In the single-fluid MHD turbulence, both the enstrophy density and the current density show a typical sheet-like structure. On the other hand, the enstrophy density in Hall MHD turbulence shows a tubular structure which is similar to that in hydrodynamic turbulence. Such a transition can cause a qualitative change of the micro-transport by the fluid flow. We can show that the JxB force is superior to the advection term in the momentum equation. It is conjectured that the magnetic pressure, a part of the JxB force, can play a significant role to form the tubular structure of the enstrophy density once the Hall term is introduced. We will examine the conjecture through detailed analysis of the local field structures as well as through the energy transfer function analysis. [Preview Abstract] |
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PP8.00083: Extended MHD Simulation of Kelvin-Helmholtz Instability in a 2D Slab Tomoharu Hatori, Hideaki Miura, Atsushi Ito, Masahiko Sato, Ryosuke Goto Shear flow of plasma in magnetic confinement fusion devices can play important roles to achieve high-performance plasma.On one hand, it can improve plasma confinement.On the other hand, it can cause magnetohydrodynamic (MHD) instabilities such as Kelvin-Helmholtz (KH) instability.Although KH instability has been researched intensively in a (single-fluid) MHD theory, the effects of the ion inertia length (two-fluid effect) or finite Larmor radius (FLR effect) to KH modes have not yet been well investigated, especially for parameters suitable for magnetically confined plasmas.These small scale effects are important when the shear is strong, e.g. in the edge region of H-mode tokamaks.In this study, numerical simulations of the KH instability in a 2D slab are carried out by our nonlinear extended MHD code.Evolution of KH modes due to sheared-flow perpendicular to an equilibrium magnetic field is concerned.Two-fluid terms show stabilizing effect, while FLR terms destabilizing. Wave numbers that growth rates are affected by those effects vary by beta, which correspond to the ratio of the Larmor radius to the ion inertia length. Discussion about nonlinear evolution and saturation will be presented. [Preview Abstract] |
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PP8.00084: Hall and gyro-viscous effects to the Rayleigh-Taylor instability in a 2D slab Ryosuke Goto, Hideaki Miura, Atsushi Ito, Masahiko Sato, Tomotoharu Hatori Small scale effects such as the Finite Larmor Radius (FLR) effect and the Hall term which are ignored in the single-fluid MHD model can be important for the growth of the high wave number unstable modes such as the ballooning instability. Here we consider a simple Rayleigh-Taylor (R-T) instability in a 2D slab, and study the effect of the Hall term and the FLR effect to the R-T instability. The FLR effect is modeled as the gyro-viscous tensor [1, 2]. It is shown that the linear growth rate of the high wave number modes are reduced by the FLR effect and increased by the Hall term. However, when the Hall term and the FLR effect are added simultaneously, high wave number modes are strongly reduced. We will compare results of linear stability analysis to those of nonlinear simulations, and study some aspects of nonlinear growth under the effect of the FLR and the Hall terms by the use of an appropriate index such as the mixing width. In the Hall case, mixing width is slightly increased compared with MHD case. However growth rate reduces when the Hall term and the gyro-viscosity are added simultaneously, mixing width reaches comparable level with MHD case.\\[4pt] [1] S. I. Braginskii, Rev. Plasma Phys., 1, 205 (1965).\\[0pt] [2] P. Zhu et al., Phys. Rev. Lett., 101, 085005 (2008). [Preview Abstract] |
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PP8.00085: A pressure-driven model for the quasi periodical oscillations of the Single Helical States in Reversed Field Pinch plasmas Roberto Paccagnella In this work a model that could explain the experimentally observed quasi periodical oscillations in electron temperature and perturbed magnetic field in a Reversed Field Pinch is discussed. An ohmically heated plasma in which an interplay between thermal conduction and heat transport, on one side, and the magneto-hydro-dynamical stability, on the other side, is studied. It is shown that, by making some simple and physically reasonable assumptions, a set of equations can be obtained showing a variety of periodical or quasi periodical oscillations for the relevant dynamical variables. [Preview Abstract] |
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PP8.00086: Nonlinear Excitation of Acoustic Modes by Large Amplitude Alfv\'{e}n waves in the Large Plasma Device (LAPD) S. Dorfman, T. Carter, P. Pribyl, S.K.P. Tripathi, B. Van Compernolle, S. Vincena, R. Sydora Alfv\'{e}n waves, a fundamental mode of magnetized plasmas, are ubiquitous in lab and space. While the linear behavior of these waves has been extensively studied [1], non-linear effects are important in many real systems, including the solar wind and solar corona. In particular, a parametric decay process in which a large amplitude Alfv\'{e}n wave decays into an ion acoustic wave and backward propagating Alfv\'{e}n wave may play an important role in coronal heating and/or in establishing the spectrum of solar wind turbulence. Recent counter-propagating Alfv\'{e}n wave experiments have recorded the first laboratory observation of the Alfv\'{e}n-acoustic mode coupling at the heart of this parametric decay instability [2]. The resonance in the observed beat process has several features consistent with ponderomotive coupling to an ion acoustic mode, including the measured dispersion relation and spatial profile. Strong damping observed after the pump Alfv\'{e}n waves are turned off is under investigation. New experiments and simulations also aim to identify decay instabilities from a single large-amplitude Alfv\'{e}n wave. \\[4pt] [1] W. Gekelman, et. al., Phys. Plasmas 18, 055501 (2011).\\[0pt] [2] S. Dorfman and T. Carter, Phys. Rev. Lett. 110, 195001 (2013). [Preview Abstract] |
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PP8.00087: Prospects for studying temperature-anisotropy-driven instabilities in a high-beta laboratory plasma T.A. Carter, S. Dorfman, L. Bardoczi, A. Geraldini, J. Robertson, G. Rossi, S. Tang, S. Tripathi, S. Vincena, W. Gekelman The mirror and firehose instabilities are driven unstable in magnetized, high-beta plasmas with anisotropic ion distribution functions. Evidence for the action of these instabilities has been found in space plasmas, in particular solar wind observations [1], and they are thought to be important in a number of astrophysical plasmas (e.g. accretion disks). Studying these important instabilities in the lab requires a high-beta, magnetized plasma and the creation of sufficient temperature anisotropy. We will discuss prospects for laboratory experiments making use of the Enormous Toroidal Plasma Device (ETPD) at UCLA. Firehose-unstable ion distributions might be driven in plasmas flowing into an expanding magnetic field (similar to the solar wind). Enhanced anisotropy could be provided by the formation of a double layer in the expanding plasma, which leads to the production of ion beams in expanding laboratory plasmas [2]. We will report on: initial experiments in LAPD studying expanding plasmas, measurements of plasma parameters in ETPD and on theoretical projections for acheivable anisotropy and instability thresholds in ETPD. \\[4pt] [1] S.D. Bale, et al., PRL 103, 211101 (2009).\\[0pt] [2] C. Charles, et al., PoP 11, 1706 (2004). [Preview Abstract] |
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PP8.00088: Excitation of Alfven waves by a spiraling ion beam in the Large Plasma Device Shreekrishna Tripathi, Bart Van Compernolle, Walter Gekelman, Patrick Pribyl, William Heidbrink, Troy Carter A hydrogen ion beam (15 kV, 10 A) has been obliquely injected from the end of the Large Plasma Device (LAPD) into a large magnetoplasma ($n \approx 10^{12}$ cm$^{-3}$, T$_e \approx 4$ eV, B = 1.0 - 1.8 kG, 19 m long, 0.6 m diam) for performing fusion-relevant fast-ion studies. The beam was produced using a recently upgraded ion source that utilizes a hot-cathode LaB$_6$ plasma source and a multi-aperture three-grid beam-extractor. Measurements of the beam profiles at multiple axial locations (up to 18 m distance from the source) have evinced a spiraling ion-beam (current-density $\approx$ 60 mA/cm$^2$, pitch angle in the plasma $\approx$ 53$^{\circ}$) that propagates with an Alfvenic speed (beam speed/Alfven speed = 0.5 - 1.2). Although the beam generates other waves, we will focus on the spontaneous generation of shear Alfven waves by the beam. To investigate the role of the resonant wave-particle interaction, an Alfven wave in the direction of the beam propagation was launched from an antenna. The ratio of beam-speed to wave phase-speed was varied. Initial results demonstrate spatial growth of the launched wave under suitable conditions for the resonant wave particle interaction. [Preview Abstract] |
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PP8.00089: Ion beam generated modes in the lower hybrid frequency range in a laboratory magnetoplasma Bart Van Compernolle, Shreekrishna Tripathi, Walter Gekelman, Patrick Pribyl The interaction of a fast ion beam with a low $\beta$ plasma has been studied in the laboratory. Experiments were performed at the LArge Plasma Device (LAPD) at UCLA. The experiments were done in a Helium plasma ($n \simeq 10^{12} \ \mbox{cm}^{-3}$, $B_0$ = 1000 G - 1800 G, $f_{pe}/f_{ce} \simeq 1 - 5$, $T_e \simeq 4 \ \mbox{eV}$, $v_{te} \ll v_A$). The ion beam is either a Helium beam or Hydrogen beam with energies ranging from 5 keV to 18 keV. The fast ion velocity is on the order of the Alfv\'en velocity. The beam is injected from the end of the machine, and spirals down the linear device. Waves were observed below $f_{ci}$ in the shear Alfv\'en wave regime, and in a broad spectrum above $f_{ci}$ in the lower hybrid frequency range. The wave generation was studied for various plasma parameters, as well as for different beam energies and pitch angles. The waves were measured with 3-axis electric and magnetic probes. Detailed measurements of the 2D perpendicular mode structure will be shown. Progress on a theoretical framework of the wave generation by the ion beam will be presented along with comparisons to the measured wave properties. [Preview Abstract] |
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PP8.00090: Geometric theory of wave kinetics and the ray-tracing controversy I.Y. Dodin, N.J. Fisch An invariant, geometric formulation of linear wave kinetics is proposed that allows casting \textit{any} wave equation (WE) in a quantumlike form. The wave amplitude is described by the Schr\"odinger equation, which, absent dissipation, has a Lagrangian form. Any approximations made to the Lagrangian preserve the conservative form of WE, automatically preventing standard errors (e.g., at guessing a WE from a dispersion relation or at approximating the dielectric tensor with its local value). The wave action is naturally introduced as a Hermitian operator (density matrix). The associated kinetic equation (KE) accounts for both diffraction and mode coupling and conserves wave quanta. Contrary to a popular misconception, taking the geometrical-optics limit does \textit{not} necessarily lead to what is known as the ``wave kinetic equation.'' This undermines the applicability of ray tracing for common practical applications; e.g., the correct KE manifestly prohibits stochasticity at $t \to \infty$. [Preview Abstract] |
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PP8.00091: Alfv\'{e}n Wave Behavior in Partially Ionized Plasmas and a Strong Density Gradient in the Hot hELicon eXperiment (HELIX) Stephanie Sears, Jerry Carr Jr., Robert VanDervort, Greg Lusk, Richard Magee, Earl Scime Damping of Alfv\'{e}n waves is one of the most likely mechanisms for ion heating in the solar corona. Ion-neutral collisions have significant but poorly-understood effects on energy transfer and Alfv\'{e}n wave propagation in partially ionized plasmas, such as those found in the solar chromosphere. The neutral density in HELIX varies strongly with radius, giving access to a wide range of Alfv\'{e}n dynamics across the plasma column. The ratio of ion-cyclotron collision frequency in the solar atmosphere varies from 10-6 to 10, while in HELIX the ratio varies from about 0.02 to 0.5. With the use of a new internal wave-launching antenna close to the high-density core and a small-scale magnetic sense coil probe, the behavior of radially confined Alfv\'{e}n waves is measured and characterized in helium. These propagation measurements, along with LIF observations of the temperature and drift of a minor argon ion component in the majority helium plasma, are compared to observations in the corona. [Preview Abstract] |
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PP8.00092: Simulation of Ion Diffusion in Kinetic Alfven Waves Yu Lin, Xueyi Wang, Jay Johnson Kinetic Alfven waves (KAWs) have been suggested to play an important role in the plasma transport. Previously, we carried out a three-dimensional (3-D) hybrid simulation [Lin, Johnson, and Wang, PRL, 2012] for mode conversion from a fast wave to KAWs at the magnetopause, in which the magnetic field is in the $\hat{z}$ direction perpendicular to the density gradient ($\hat{x}$). KAWs with $k_x \rho_i\sim 1$ generated by linear mode conversion were found to nonlinearly decay to modes with $k_y \rho_i \sim 1$. The transfer of energy to large perpendicular and azimuthal $k_y$ modes has been suggested to provide large transport across plasma boundaries. In order to understand the ion diffusion in these KAWs, we now show a further 3-D hybrid simulation combined with test particle calculations for ion cross-field line diffusion in KAWs. A system of uniform plasma is driven by a steady KAW with $k_\perp = k_x$. A turbulence spectrum is obtained. Dependence of the ion diffusion coefficient $D_\perp$ on the driver amplitude, frequency, wave vector, ion beta, $T_e/T_i$, and the particle energy is obtained. The results are also compared with those based on the quasilinear theory. The 3-D results are compared with the 2-D runs ($k_y=0$) to show the importance of 3-D physics. [Preview Abstract] |
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PP8.00093: Impact of plasma core profiles on MHD stability at tokamak edge pedestal Nobuyuki Aiba, Hajime Urano Suppression and/or mitigation of large amplitude edge localized modes (ELMs) is one of the critical issues for ITER. In JET quasi-double-null plasmas, both high-$\beta_{\mathrm{p}}$ and high-$l_{\mathrm{i}}$ are necessary to obtain small amplitude ``grassy'' ELMs [1], and the origin of grassy ELM is thought as the ballooning mode [2], where $l_{\mathrm{i}}$ is plasma internal inductance. We pay attention to these conditions with the assumption that pedestal transport phenomenon in grassy ELM regime is governed by the kinetic ballooning mode stability as EPED1 model [3], and discuss numerically impact of plasma core profiles on MHD stability at tokamak edge pedestal. For example, by making core pressure profile peaked with fixed pedestal width predicted with EPED1 model, current density increases near magnetic axis; namely, $l_{\mathrm{i}}$ becomes larger. This widens the difference between poloidal flux on axis and that at plasma surface, and reduces the pressure gradient and current density near pedestal. Such a change of pedestal condition due to varying plasma core profiles has impact on edge MHD stability.\\[4pt] [1] G. Saibene et al., Nucl. Fusion 45 (2005) 297.\\[0pt] [2] N. Oyama et al., Nucl. Fusion 50 (2010) 064014.\\[0pt] [3] P. B. Snyder et al, Nucl. Fusion 49 (2009) 085035. [Preview Abstract] |
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PP8.00094: A spherical Couette experiment to observe linear MHD instabilities at medium Reynolds numbers Elliot Kaplan, Benjamin Gohl, Thomas Gundrum, Martin Seilmayer, Frank Stefani Turbulent spherical Couette flows in a strong axial magnetic field (Re $\in (10^4, 10^6)$, Ha $\in (0,3000)$) have given rise to a nonaxisymmetric instability that resembled the long sought-after, nonlinear, Magnetorotational Instability [Sisan (2004)]. Subsequent theoretical and numerical investigations have likened the observed instability to linear instabilities in either the Shercliff layer or the return flow [Hollerbach (2009), Gissinger (2011)]. These investigations also turned up a stability region in Re/Ha space between these (MHD) instabilities and the (HD) jet instability. Presented here are designs and plans for a spherical Couette experiment---presently under construction in Rossendorf---that aims to measure these instablities in the space betwixt the simulations and the turbulent experiments (Re $\in (10^3,10^4)$, Ha $\in (0,160)$). [Preview Abstract] |
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PP8.00095: GeFi Simulation of Electron-Ion Hybrid Instability Lei Qi, Yu Lin, Xueyi Wang, Ami Dubois, Edward Thomas Shear flow instabilities play an important role in laboratory as well as space plasmas. Excitation of the electron-ion hybrid (EIH) instability in a magnetized plasma with a transverse electric field and thus a localized electron cross-field flow is investigated using our gyrokinetic electron and fully-kinetic ion (GeFi) particle simulation model. Regime with $\rho _{\mathrm{e}}$\textless L$_{\mathrm{E}}$ \textless $\rho_{\mathrm{i}}$ is considered, where $\rho_{\mathrm{i}}$ and $\rho_{\mathrm{e}}$ are the electron and ion Larmor radii, respectively, and L$_{\mathrm{E}}$ represents the scale length of the shear flow profile. Both linear and nonlinear physics are studied. First, for the shear flow profile in a slab geometry, the simulation model is benchmarked by comparison of the eigen mode structure obtained from the linear GeFi results in a uniform plasma density with that from the linear theory [e.g., Ganguli, Lee, and Palmadesso, 1987], and good agreement is obtained. Vortex-like structures are also observed in the electrostatic potential. Second, in the nonlinear GeFi simulation, the EIH mode instability saturates in a time scale t$=$0.2/$\Omega_{\mathrm{i}}$ ($\Omega_{\mathrm{i}}$ is the ion gyrofrequency), and it nonlinearly evolves to a lower hybrid mode. Third, linear and nonlinear EIH instabilities in a plasma with a nonuniform density is also simulated. Finally, the GeFi simulation is carried out in a cylindrical geometry for conditions of the Auburn Linear Experiment for Instability Studies (ALEXIS) experiment. The results are compared with the ALEXIS measurements of the EIH instability. [Preview Abstract] |
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PP8.00096: Laboratory Investigation of the Dynamics of Shear Flows in a Plasma Boundary Layer Ami DuBois, Edward Thomas, William Amatucci, Gurudas Ganguli For a wide variety of laboratory and space plasma environments, theoretical predictions state that plasmas are unstable to transverse and parallel inhomogeneous flows over a very broad frequency range. Specifically, for a velocity shear oriented perpendicular to a uniform background magnetic field, the shear scale length ($L_{E})$ compared to the ion gyro-radius ($\rho_{i})$ determines the character of the shear driven instability that may prevail. An interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column. For the first time, the continuous variation of $\rho_{i}$/$L_{E}$, and the associated transition of the instability regimes driven by the shear flow mechanism, is demonstrated in a single laboratory experiment under identical plasma conditions. This work characterizes the compression/relaxation of boundary layers often generated in a variety of space plasma processes. [Preview Abstract] |
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PP8.00097: ABSTRACT WITHDRAWN |
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PP8.00098: Basic Experiments on the Production and Identification of Toroidal ETG Modes Abed Balbaky, Vladimir Sokolov, Amiya Sen First parametric studies of the transition from the slab mode to toroidal (curvature) branch of ETG mode in CLM are reported along with comparison with theory. CLM was operated in the mirror configuration with cell length (50cm-100cm) and mirror ratio (1-2.3). This allows for $R_c \sim 1.2m$ and provides sufficient toriodicity to excite the toroidal branch of ETG, $ {\varepsilon}{_n} = {L}{_T{_e}}/{R{_c}} \sim .003 > {\varepsilon}{_{crit}} = {k_\|} / 2 {k_\perp} \sim .0005$ [1]. A simple fluid model has been developed to predict changes in ETG mode frequency and growth rate as a function of toriodicity. When operating near our maximum of ${\varepsilon}{_n} \sim .003$ our model predicts a shift in the mode frequency on the order of ~30kHz, and a slight increase in growth rate. Experimentally we see a shift of ~100kHz, and a modest increase in mode amplitude, which roughly agrees with the theoretical estimates. This level of agreement is very similar over our full range of possible curvatures. \\[4pt] [1] J.Y. Kim and W.Horton, Phys. Fluids B 3, 1167 (1991) [Preview Abstract] |
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PP8.00099: Study of two-stream instability in low-pressure discharge Guiqiu Wang, Igor Kaganovich, Alexander Khrabrov, Dmytro Sydorenko, Hongyue Wang Electron emission from discharge chamber wall is important in low-pressure discharges, such as capacitively coupled plasma (CCP), divertor plasmas, direct current cathode discharges, direct current magnetrons, multipactors, electrostatic, Hall thruster and so on. It is well known that the electrons emitted from the wall are accelerated into plasma by the electric field in the sheath adjacent to the wall and form an electron beam. Such beams on the one hand play an important role in the maintenance of discharge and affect plasma and sheath characteristics, on the other hand, they may excite the two-stream instability in the plasma. As a result, the beam electrons are slowed down and the plasma electrons are heated. In this work, a one-dimensional Particle-in-Cell (PIC) simulation is carried out to study these effects in low-pressure discharge. The relationship between Electron velocity distribution function (EVDF) of plasma-beam system and the two-stream instability whether happens is discussed and the dispersion relation is studied in detail when the two-stream instability occurs. [Preview Abstract] |
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PP8.00100: Pitch angle scattering of an anisotropic electron beam: linear theory and PIC simulations Xiangrong Fu, S. Peter Gary, Misa Cowee, Dan Winske, Kaijun Liu An anisotropic electron beam ($T_{\perp b}/T_{\| b}>1$) drifting along the background magnetic field through a cold background plasma is investigated by both linear theory and 2D PIC simulations. There are two possible instabilities in this scenario: the electron beam instability and the electromagnetic whistler instability. Linear analysis shows that the beam instability grows much faster than the whistler instability in our parameter regime. As a result of the beam instability, the beam is slowed down and heated in the parallel direction. In the 2D case, the electric field of obliquely propagating modes heats electrons in the perpendicular direction, and thus scatters some electrons outside the loss cone of an inhomogeneous magnetic field configuration. The linear theory also predicts that as the temperature anisotropy of the beam increases, the wave propagation aligns more with the background magnetic field, implying a reduction in the perpendicular heating. 2D PIC simulations confirm the prediction of the linear theory and show that fewer electrons are scattered outside a loss cone for for larger $T_{\perp b}/T_{\|b}$. We also investigate the parameter regime in which the whistler instability is comparable to the beam instability. LA-UR-13-25045. [Preview Abstract] |
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PP8.00101: Surface Waves in a Semi-Bounded Collisional Plasma Modjtaba Moaied, Yuriy Tyshetskiy, Sergey Vladimirov Surface waves (SWs) in a plasma half-space are studied taking into account electron-neutral collisions. The spectrum and damping of SWs are obtained. It is shown that the SWs are strongly damped for wavelengths less than a minimum wavelength which is significantly depends on plasma parameters. The relative importance of collisional and Cherenkov damping of SWs is investigated and is graphically shown for a range of plasma parameters and SW wavelengths. The SWs are electromagnetic waves which propagate along the interface between two different media with different signs of the dielectric permittivities. SWs have had applications in many different fields and are the subject of many theoretical, numerical and experimental investigations. In most of previous works,\footnote{R. L. Guernsey (1969).}$^,$\footnote{S. V. Vladimirov and M. Y. Yu (1994).}$^,$\footnote{A. A. Rukhazde and B. Shokri (1998).} the entire analyses are carried without inclusion of collisions between particles. Yet collisions can be significant in low-temperature plasmas. The collisions in a quantum plasma are shown to significantly affect the SW properties.\footnote{Yu. Tyshetskiy et al. (2012).} In this work, we investigate the effects of collision on the SW properties for classical plasmas. The collisions in a collisional plasma are shown to play an important role in damping of SWs, changing the wavelength range in which SWs are weakly damped, compared to those in a collisionless plasma. [Preview Abstract] |
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PP8.00102: Current driven instability in finite beta plasmas Istvan Pusztai, Peter J. Catto, Felix I. Parra, Michael Barnes The induced electric field in a tokamak drives a parallel electron current flow. In an inhomogeneous, finite beta plasma, when this electron flow is non-negligible compared to the ion thermal speed, the Alfv\'{e}n mode wave solutions of the electromagnetic gyrokinetic equation can become an almost purely growing kink mode. Using the new ``low-flow'' version of the gyrokinetic code GS2 developed for momentum transport studies [Barnes et al 2013 to appear in Phys. Rev. Lett., arXiv: 1304.3633], we are able to model the effect of the induced parallel electric field on the electron distribution to study the impact of a current on stability. We identify high mode number kink modes in GS2 simulations and make comparisons to analytical theory in a sheared magnetic geometry. We find a reassuring agreement with analytical results both in terms of parametric dependences of mode frequencies and growth rates, and regarding the radial mode structure. [Preview Abstract] |
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PP8.00103: A direct Vlasov simulation of nonlinear plasma waves Kentaro Hara, Iain Boyd, Igor Kaganovich A direct Vlasov simulation, which solves the collisionless Vlasov equation directly on a discretized phase space, achieves good resolution of velocity distribution functions in comparison to particle methods. In this presentation, nonlinear electron plasma waves (EPWs) and ion acoustic waves (IAWs) are investigated with a fully-kinetic one-dimensional Vlasov simulation. A parallelized Vlasov simulation is employed since grid resolution of the discretized phase space is required to be fine enough in order to capture the nonlinear waves with higher harmonic modes. The primary goal is benchmarking our simulation with results obtained from another Vlasov code and verification with the nonlinear theories [R. L. Berger \textit{et al.}, Phys. Plasmas, \textbf{20}, 032107 (2013)]. The frequency shift of nonlinear plasma waves is investigated by applying an initial density perturbation or an external driver potential. It has been observed that the plasma frequency decreases for EPWs and increases for IAWs for $T_{e}/T_{i}=10$, which agrees with Berger's simulation and theories. A further investigation varying the generation of the nonlinear wave such as driver amplitude and duration time will be performed and discussed. [Preview Abstract] |
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PP8.00104: Scattering of High Frequency Electromagnetic Waves in the Presence of Low Frequency Density Irregularities James Lundberg, Tony Kim, Vladimir Sotnikov, Evgeny Mishin, David Ross, Thomas Mehlhorn Presence of plasma can strongly influence propagation properties of electromagnetic signals used for surveillance and communication. In particular, we are interested in mechanisms of generation of low frequency plasma turbulence in the ionosphere and inside a plasma sheath of reentry and hypersonic vehicles and in similar applications. We will discuss generation of low frequency density irregularities due to the presence of plasma flows with velocity shear and interchange instability. Next, influence of excited wave turbulence on scattering of high frequency electromagnetic waves used for communication purposes will be presented. Finally, scattering cross-sections due to interaction of high frequency EM waves with density irregularities produced by different types of low frequency plasma turbulence will be discussed. [Preview Abstract] |
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PP8.00105: Interchange and Flow Velocity Shear Instabilitiesin the Presence of Finite Larmor Radius Effects Tony Kim, James Lundberg, Vladimir Sotnikov, Evgeny Mishin, Thomas Genoni, David Rose, Thomas Mehlhorn Ionospheric irregularities cause scintillations of electromagnetic signals that can severely affect navigation and transionospheric communication, in particular during Equatorial Plasma Bubbles (EPBs) events. However, the existing ionospheric models do not describe density irregularities with typical scales of several ion Larmor radii that affect UHF and L bands. These irregularities can be produced in the process of nonlinear evolution of interchange or flow velocity shear instabilities. The model of nonlinear development of these instabilities based on two-fluid hydrodynamic description with inclusion of finite Larmor radius effects will be presented. The derived nonlinear equations will be solved numerically. The high-resolution simulations will be driven by the ambient conditions corresponding to the AFRL C/NOFS satellite low-resolution data during EPBs. [Preview Abstract] |
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PP8.00106: Simultaneous Multi-angle Radar Observations of Langmuir Turbulence Excited by RF Ionospheric Interactions at HAARP J.P. Sheerin, N. Rayyan, N. Watanabe, B.J. Watkins, W.A. Bristow, P.A. Bernhardt The high power HAARP HF transmitter is employed to generate and study strong Langmuir turbulence (SLT) in the interaction region of overdense ionospheric plasma. Diagnostics included the Modular UHF Ionospheric Radar (MUIR) sited at HAARP, the SuperDARN-Kodiak HF radar, and HF receivers to record stimulated electromagnetic emissions (SEE). Dependence of diagnostic signals on HAARP HF parameters, including pulselength, duty-cycle, aspect angle, and frequency were recorded. Short pulse, low duty cycle experiments demonstrate control of artificial field-aligned irregularities (AFAI) and isolation of ponderomotive effects. Among the effects observed and studied are: SLT spectra including cascade, collapse, and co-existence spectra and an outshifted plasma line under certain ionospheric conditions. High time resolution studies of the temporal evolution of the plasma line reveal the appearance of an overshoot effect on ponderomotive timescales. Bursty turbulence is observed in the collapse and cascade lines. For the first time, simultaneous multi-angle radar measurements of plasma line spectra are recorded demonstrating marked dependence on aspect angle with the strongest interaction region observed displaced southward of the HF zenith pointing angle. Numerous measurements of the outshifted plasma line are observed. Experimental results are compared to previous high latitude experiments and predictions from recent modeling efforts. [Preview Abstract] |
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PP8.00107: Study of Linear and Nonlinear Wave Excitation Feng Chu, Jorge Berumen, Ryan Hood, Sean Mattingly, Frederick Skiff We report an experimental study of externally excited low-frequency waves in a cylindrical, magnetized, singly-ionized Argon inductively-coupled gas discharge plasma that is weakly collisional. Wave excitation in the drift wave frequency range is accomplished by low-percentage amplitude modulation of the RF plasma source. Laser-induced fluorescence is adopted to study ion-density fluctuations in phase space. The laser is chopped to separate LIF from collisional fluorescence. A single negatively-biased Langmuir probe is used to detect ion-density fluctuations in the plasma. A ring array of Langmuir probes is also used to analyze the spatial and spectral structure of the excited waves. We apply coherent detection with respect to the wave frequency to obtain the ion distribution function associated with externally generated waves. Higher-order spectra are computed to evaluate the nonlinear coupling between fluctuations at various frequencies produced by the externally generated waves. Parametric decay of the waves is observed. [Preview Abstract] |
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PP8.00108: Stability of Inhomogeneous Equilibria of Hamiltonian Continuous Media Field Theories George Hagstrom There are a wide variety of $1+1$ Hamiltonian continuous media field theories that exhibit phase space pattern formation. In plasma physics, the most famous of these is the Vlasov-Poisson equation, but other examples include the incompressible Euler equation in two-dimensions and the Hamiltonian Mean Field (or XY) model. One of the characteristic phenomenon that occurs in systems described by these equations is the formation of cat's eye patterns in phase space as a result of the nonlinear saturation of instabilities. Corresponding to each of these cat's eyes is a spatially inhomogeneous equilibrium solution of the underlying model, in plasma physics these are called BGK modes, but analogous solutions exist in all of the above systems. Here we analyze the stability of inhomogeneous equilibria in the Hamiltonian Mean Field model and in the Single Wave model, which is an equation that was derived to provide a model of the formation of electron holes in plasmas. We use action angle variables and the properties of elliptic functions to analyze the resulting dispersion relation construct linearly stable inhomogeneous equilibria for in the limit of small numbers of particles and study the behavior of solutions near these equilibria. [Preview Abstract] |
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PP8.00109: Study of Auroral Electron Acceleration in the Laboratory J.W.R. Schroeder, F. Skiff, G.G. Howes, C.A. Kletzing, T.A. Carter, S. Dorfman Particle interactions with Alfv\'en waves have been proposed as a possible means for accelerating electrons and generating aurorae. Auroral theory states that electron acceleration by inertial Alfv\'en waves varies with the perpendicular wavenumber and Alfv\'en wave amplitude. Traditional diagnostics are not sensitive to the predicted small fluctuations of populations in the tail of the distribution function. A novel approach accurately measures this region of the distribution function using the absorption of a small-amplitude high frequency whistler wave. Inertial Alfv\'en waves ($v_{\mbox{th}}/v_A \sim 0.2$) with $\delta B/B \sim 10^{-5}$ are launched in an overdense plasma at the Large Plasma Device (LaPD) with $B=1800$ G. Under these conditions, only the whistler mode propagates parallel to the background field at frequencies just below the electron cyclotron frequency. Initial results show the dielectric perturbation of the distribution function by an Alfv\'en wave. We present further analysis of measurements of the electron distribution function under plasma conditions relevant to the auroral magnetosphere using a range of Alfv\'en wave parameters and compare results to theoretical predictions. [Preview Abstract] |
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PP8.00110: Essential Tokamak Geometric Effects for Global Ballooning Mode Studies Huasheng Xie, Yong Xiao Although often used to study the turbulent transport in finite beta Tokamak plasma, the analytical equilibrium usually implemented in the gyrokinetic simulation would be inadequate in either describing the essential physics or quantitatively comparing with analytical theory. By revisiting the Grad-Shafranov equation and local s-alpha model, we find the usually used concentric or shifted-circle equilibrium is not enough for studying the global behavior of the finite beta fusion plasmas, for example, it may miss the second unstable region or give incorrect growth rate. The essential global shifted-circle geometry for studying finite-beta modes is discussed and verified. This semi-analytical equilibrium is also implemented in gyrokinetic particle code (GTC) and benchmarked for ideal ballooning mode. In addition, preliminary results on kinetic ballooning mode will be presented. [Preview Abstract] |
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PP8.00111: RECONNECTION |
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PP8.00112: Chaos in Magnetic Flux Ropes Walter Gekelman, Tim DeHaas, Bart Van Compernolle Magnetic Flux Ropes Immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Full three dimensional magnetic field and flows are measured at thousands of time steps. Each collision results in magnetic field line generation and the generation of a quasi-seperatrix layer and induced electric fields. Three dimensional magnetic field lines are computed by conditionally averaging the data. The permutation entropy can be calculated from the time series of the magnetic field data or flows is used to calculate the positions of the data on a Jensen Shannon complexity map. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. Other types of chaotic dynamical models (Gissinger , Lorentz and Henon) also fall on the map and can give a clue to the nature of the turbulence. The ropes fall in the region of the C-H plane where chaotic systems lie. The entropy and complexity change in space and time, which reflects the change and possibly type of chaos associated with the ropes.\\[4pt] [1] C. Bandt, B. Pompe, Phys. Rev. Lett., 88,174102 (2007)\\[0pt] [2] O. Russo et al., Phys. Rev. Lett., 99, 154102 (2007), J. Maggs, G.Morales, 55, 085015 (2013) [Preview Abstract] |
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PP8.00113: Ion Flows Associated with Two Flux Ropes in a Background Plasma Timothy DeHaas, Walter Gekelman, Bart Van Compernolle Magnetic flux ropes are ubiquitous as they are located on and near the sun, presumably other stars, and near the earth and other planets. They consist of helical field lines which vary in pitch due to the electric current flowing along a background magnetic field. Multiple braided flux ropes have been observed in the solar corona, and their unraveling is theorized to be the signature of magnetic reconnection. Two flux ropes (L$=$10 m, A$=$7 cm$^{2}$, J$=$10 amp/cm$^{2})$ were created in the Large Plasma Device (LAPD) at UCLA (Bo$=$330 G, $n_{o} =$ 10$^{12}$ cm$^{-3}$, T$^{e}=$4eV, ~Ar). These kink unstable ropes violently twist and oscillate about a central axis. A quasi-separatrix layer (QSL) forms as the ropes collide and the magnetic field lines reconnect. Through the use of a six-faced Mach probe, volumetric data was taken to determine the three-dimensional plasma flow. Volumetric magnetic fields were obtained through use of a three-axis magnetic probe. The three-dimensional data is conditionally averaged to construct the average flux rope dynamic. In this experiment, the ropes are shown to twist, interact, then merge; while the plasma flows are shown to spiral around the two flux ropes in a singular O-point. As they collide and a QSL is formed and an induced electric field is generated, slowing parallel ion flows. [Preview Abstract] |
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PP8.00114: Quasi-separatrix layer analysis of line-tied tearing modes in kinetic simulation Zachary Billey, Ellen Zweibel, John Finn, William Daughton It has been shown under linear resistive MHD theory that line-tied field geometry, such as occurs in stellar and accretion disk coronae, can affect the behavior of tearing modes, especially the resistive scaling. The effect is determined by the relation between geometric width, determined by the system length along the guide field, and the tearing layer width. We extend these results into the nonlinear and kinetic regimes by analyzing simulations of a Harris sheet in line-tied slab geometry using the particle in cell code VPIC. In periodic systems, one can trace flux surfaces and magnetic islands by following field lines through many periods of the system. This is not the case with line-tied boundaries. We will instead use field-line integrated diagnostics based on the quasi-separatrix layer theory. A QSL is a thin region where the magnetic field behaves as if reconnecting along a hyperbolic closed field line although no true closed hyperbolic line exists. We will identify reconnecting regions by comparing the potential difference along field lines due to ideal effects with the potential difference due to non-ideal effects. We also calculate the squashing factor for comparison. [Preview Abstract] |
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PP8.00115: Coexistence of drift-like and tearing instabilities in non-uniform plasma V.V. Mirnov, C.C. Hegna, J.P. Sauppe, C.R. Sovinec The simultaneous presence of drift-like and tearing instabilities is investigated with the use of two-fluid extended MHD code NIMROD and analytical methods. The model includes electron compressibility and the physics of electron-ion decoupling on short scales as well as the effect of diamagnetic flows caused by non-uniform density profile. Linear numerical simulations are performed for plasma slab with cold ions and hot electrons in a doubly periodic box bounded by two perfectly conducting walls. Magnetic shear configuration utilizes a sinusoidal profile for the reconnecting magnetic field which is unstable with respect to current-driven drift-tearing instability. This instability is characterized by a clear magnetic perturbation. Additionally, there is an unstable pressure-gradient driven mode suggestive of a resistive-drift type with largely electrostatic perturbations. Both modes are observed in NIMROD simulations. Simplified two-fluid linear analytical model confirms coexistence of drift-tearing and resistive-drift unstable modes. We investigate the nature (physical or numerical) of the newly observed drift-like mode and its role in the dynamics of the system. [Preview Abstract] |
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PP8.00116: Generation of superthermal electrons in single X-line reconnection Jan Egedal, William Daughton During magnetic reconnection, stress in the magnetic field is reduced and the process is often accompanied by an explosive release of magnetic energy. In the Earth's magnetotail, reconnection energizes electrons up to hundreds of keV, and in solar flare events a large fraction the released energy is channeled into the electrons, resulting in superthermal populations in the MeV range. In recent numerical and theoretical models, geometries with multiple reconnection sites have been studied in order to enhance the energy transfer to the electrons. Meanwhile, using a kinetic simulation, here we show that in low beta plasmas, electron energization occurs at large scales and with high efficiency in the exhaust of single X-line reconnection. Furthermore, the numerical electron heating spectra are consistent with those observed during solar flare events. [Preview Abstract] |
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PP8.00117: Quantifying the Reconnection Rate in Magnetopause Boundary Layers William Daughton, Takuma Nakamura, Vadim Roytershteyn, Homa Karimabadi, Burlen Loring Solar wind entry into the magnetosphere occurs in thin boundary layers, which typically have large magnetic and/or velocity shear across them. Magnetic reconnection is the dominant entry mechanism, but in regions with strong nearly perpendicular velocity shear, the Kelvin-Helmholtz instability dominates and can lead to vortex induced reconnection. In both of these limits, recent fully kinetic simulations feature the development of 3D turbulence, characterized by electron-scale current sheets and interacting flux ropes. In the presence of this complex 3D dynamics, computing the reconnection rate by integrating the parallel electric field is problematic due to the chaotic magnetic field lines and large electric field fluctuations. Here we explore a new idea that exploits the connection between the magnetic topology and the mixing of electrons that originate from separate sides of the current layer. The 3D reconnection rate and local signatures of reconnection are contrasted with 2D in the limits of both weak and strong velocity shear. [Preview Abstract] |
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PP8.00118: Mechanisms of Energization of Plasma Particles in the Magnetic Reconnection Layer of a Laboratory Plasma M. Yamada, J. Yoo, J. Jara Almonte, H. Ji, C. Myers, C. Swanson Mechanisms of the energization of plasma particles in the magnetic reconnection layer has been studied by monitoring the behavior of electrons and ions in MRX [1, 2]. The measured profiles of plasma parameters are quantitatively analyzed in the context of the two-fluid reconnection physics [1] and compared with the recent numerical simulation results. The electron heating is observed to extend beyond the electron diffusion region and considered to be due to energization by magnetic instabilities of incoming electrons trapped in the magnetic mirror. This energization often occurs impulsively. Ions are accelerated by an electrostatic field across the separatrices to the plasma exhaust region of the reconnection layer and become thermalized through re-magnetization by the exiting magnetic fields. In this paper, the acceleration and heating of ions and electrons which extents much wider than the length scale of the ion skin depth, is addressed quantitatively for the first time in a laboratory reconnection layer. The results will bring a new insight into the conversion mechanism of magnetic energy to that of plasma particles during magnetic reconnection.\\[4pt] [1] M. Yamada, R. Kulsrud, H. Ji, Rev. Mod. Phys. v.82, 602 (2010)\\[0pt] [2] J. Yoo et al, Phys. Rev. Letts. 110, 215007 (2013) [Preview Abstract] |
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PP8.00119: Dependence of ion dynamics on upstream density asymmetry and guide field Jongsoo Yoo, Masaaki Yamada, Hantao Ji, Clayton Myers, Jonathan Jara-Almonte, Charles Swanson, Peter Bolgert In the Magnetic Reconnection Experiment (MRX), the upstream density ratio and guide field strength are systematically varied to study their effect on the observed ion dynamics. The guide field significantly modifies both the Hall current profile and the in-plane electrostatic potential, thereby changing the ion flow profile. The ion outflow speed decreases as the guide field increases, which is consistent with a previous study [1]. The upstream density asymmetry also breaks the symmetry of the ion flow pattern and quadrupole field. The stagnation point of the ion flow is shifted to the lower density side, and the magnitude of the quadrupole field is larger on the higher density side. These observations are consistent with recent numerical simulation results. The radial pressure balance and reconnection rate with different upstream density ratios are also discussed. \\[4pt] [1] T. Tharp \textit{et al.}, \textit{Phys. Rev. Lett.}, \textbf{109}, 165002, 2012 [Preview Abstract] |
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PP8.00120: Electrostatic Microinstabilities within the Electron Diffusion Layer J. Jara-Almonte, W. Daughton, H. Ji, M. Yamada Both numerical simulations and laboratory experiments have extensively studied the electron skin-depth scale structure of the electron diffusion layer, but neither have fully resolved both the scale-seperation and physics on scales between the Debye length and the skin-depth. Here, the first fully kinetic 2D simulations of anti-parallel magnetic reconnection at realistic electron temperatures ($c/v_{the} = 64$ and $\omega_{pe}/\Omega_e = 32$) are presented. Macroscopic features such as the reconnection rate or layer width are found to be insensitive to the electron temperature. When the electron temperature becomes sufficiently low, the electron diffusion layer becomes unstable to electrostatic instabilities involving multiple streaming populations near the X-line. This leads to multiple electron holes within the electron diffusion layer which interact non-linearly to generate turbulence which may be important in understanding the electron heating within the diffusion layer observed in experiments. [Preview Abstract] |
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PP8.00121: Experimental measurement of ion distribution function during magnetic reconnection C. Swanson, M. Yamada, H. Ji, J. Yoo, C.E. Myers, J. Jara-Almonte, P. Bolgert The Magnetic Reconnection Experiment (MRX) investigates the process of magnetic reconnection in a laboratory setting. Efficient heating of electrons and ions has been detected and documented during fast reconnection. In order to explore acceleration of charged particles beyond their thermal speeds, seen in space satellite measurements and numerical kinetic simulations, a five-electrode retarding-field energy analyzer was developed to measure the electron and ion distributions in MRX. All electrodes are referenced to the plasma-facing aperture by capacitors to accommodate the dynamically varying plasma potential. An in-line isolation amplifier was placed into the probe shaft to avoid possible picking up electric noise. All electrodes but the collector are electroformed grids of nickel, of which the aperture and rejecter grids have sub-Debye-length spacing. A first version of this probe is designed to measure ions entering the probe aperture perpendicular to local magnetic field. Initial data will be presented and compared with the numerical predictions by kinetic simulations. [Preview Abstract] |
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PP8.00122: A new Schwarzschild optical system for two-dimensional EUV imaging of MRX plasmas P. Bolgert, M. Bitter, P. Efthimion, K.W. Hill, H. Ji, C.E. Myers, M. Yamada, J. Yoo, S. Zweben This poster describes the design and construction of a new Schwarzschild optical system for two-dimensional EUV imaging of plasmas. This optical system consists of two concentric spherical mirrors with radii R$_{1}$ and R$_{2}$, and is designed to operate with certain angles of incidence $\theta_{1}$ and $\theta _{2}$. The special feature of this system resides in the fact that all the rays passing through the system are tangential to a third concentric circle; it assures that the condition for Bragg reflection is simultaneously fulfilled at each point on the two reflecting surfaces if the spherical mirrors are replaced by spherical multi-layer structures. A prototype of this imaging system will be implemented in the Magnetic Reconnection Experiment (MRX) at PPPL to obtain two-dimensional EUV images of the plasma in the energy range from 18 to 62 eV; the relative intensity of the emitted radiation in this energy range was determined from survey measurements with a photodiode. It is thought that the radiation at these energies is due to Bremsstrahlung and line emission caused by suprathermal electrons. [Preview Abstract] |
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PP8.00123: Equilibrium force balance and eruptive instabilities in solar-relevant laboratory magnetic flux ropes C.E. Myers, M. Yamada, E.V. Belova, H. Ji, J. Yoo, J. Jara-Almonte Quasi-statically driven line-tied magnetic flux ropes are studied in the context of storage-and-release eruptions in the solar corona. The Magnetic Reconnection Experiment (MRX) facility is utilized to produce these arched low-$\beta$ flux ropes. Detailed \emph{in situ} magnetic measurements and supporting MHD simulations permit quantitative analysis of the plasma behavior. We find that the orientation of the applied potential magnetic field arcade with respect to the flux rope footpoints (i.e., the electrodes) is key. With an arcade that is aligned \emph{parallel} to the footpoints, force free currents induced in the expanding flux rope modify the pressure and tension in the arcade to produce a confined, quiescent discharge and a saturated kink instability. In an \emph{obliquely} aligned arcade, on the other hand, a sigmoidal equilibrium forms that can dynamically erupt. Both the kink instability and the torus instability are studied as candidate eruptive mechanisms---the latter by varying the vertical gradient of the potential field arcade. New 2D magnetic measurements of these equilibrium and eruptive features facilitate comparisons to solar observations and modeling. [Preview Abstract] |
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PP8.00124: Development of Electromagnetic Particle Simulation Code in an Open System for Investigation of Magnetic Reconnection H. Ohtani, R. Horiuchi, S. Usami In order to investigate magnetic reconnection from the microscopic viewpoint, we have developed a three-dimensional electromagnetic particle simulation code in an open system (PASMO) [1]. For performing the code on a distributed memory and multi-processor computer system with a distributed parallel algorithm, we distributed only information of particles and did not decompose the domain in the previous PASMO code. However, in the case that the memory size on one node of computer is limited, the previous code could not be performed for large-scale simulation because all field data were duplicated on each parallel process. In order to overcome this problem, we decompose the domain, in which the field variable defined by three coordinates is distributed. The processor performs the field solver in the mapped domain, and carries out the particle pusher for the particles which exist in the domain. In this paper, we develop the open boundary condition with the domain decomposition algorithm and perform more large-scale particle simulations. We will discuss the performance of the new PASMO and the simulation results on the magnetic reconnection.\\[4pt] [1] H.Ohtani and R.Horiuchi: Plasma and Fusion Research, Vol.4, 024 (2009). [Preview Abstract] |
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PP8.00125: Collisionless Reconnection Comparison Between Gyrokinetic and Particle-in-Cell Simulations Jason TenBarge, Homa Karimabadi, William Daughton, Gregory Howes Magnetic reconnection is thought to be responsible for a large portion of the energy conversion between magnetic shear and particle energy in systems as wide ranging as stellar coronae, the solar wind, and terrestrial fusion devices. The gyrokinetic system of equations has long been employed to study turbulence in toroidally confined fusion plasmas and has more recently been applied to study Alfv\'{e}nic turbulence relevant to the solar wind. In both of these systems, magnetic reconnection is assumed to play some, perhaps significant, role in converting magnetic to particle energy. However, a rigorous comparison between numerical simulations of the kinetic particle-in-cell (PIC) and asymptotically derived gyrokinetic systems has never been performed. We present the results of such a comparison between the PIC code VPIC and the gyrokinetic code AstroGK/GS2 performed at low plasma beta, $\beta = 0.01$, relevant to fusion devices and the solar corona and moderate beta, $\beta = 1$, relevant to the solar wind. The comparison seeks to determine under what parameters, e.g., $\delta B \ll B_g$, the PIC system converges to the gyrokinetic result and how well the converged results compare in terms of energy transport. [Preview Abstract] |
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PP8.00126: The Emergence, Motion, and Disappearance of Magnetic Null Points Nicholas Murphy, Clare Parnell, Andrew Haynes, David Pontin Magnetic reconnection frequently occurs at and around magnetic null points. We derive exact expressions for the motion of a magnetic null point in a smoothly varying magnetic field. We define $\textbf{x}_n$ as the position of a null, $\textbf{U}=d\textbf{x}_n/dt$ as the null's velocity, and $\textbf{M}$ as the Jacobian matrix of the magnetic field at the null. By evaluating the derivative of the magnetic field following the motion of the null, we find the null velocity to be $\textbf{U}=-\textbf{M}^{-1}\partial \textbf{B}/\partial t$ with all quantities evaluated at the null point. For resistive MHD, this reduces to $\textbf{U}=\textbf{V}(\textbf{x}_n)-\eta\textbf{M}^{-1}\nabla^2\textbf{B}$. This expression indicates that any difference between the plasma flow velocity at the null and the velocity of the null itself is due to resistive diffusion of the magnetic field. Null points must diffuse in and out of existence. Null-null pairs first appear (or disappear) as a single degenerate null with singular $\textbf{M}$, and then instantaneously move apart (together) infinitely fast. An expression describing the motion of separators cannot depend solely on local parameters and must include information on connectivity changes due to reconnection along the entire field line. [Preview Abstract] |
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PP8.00127: Properties of Magnetic Reconnection as a function of magnetic shear Yi-Hsin Liu, William Daughton, Homa Karimabadi, Hui Li, Peter Gary, Fan Guo Observations of reconnection events at the Earth's magnetopause and in the solar wind show that reconnection occurs for a large range in magnetic shear extending to the very low shear limit. Here we report on our study of the effect of the magnetic shear on details of reconnection such as its structure and rate, using 2D and 3D kimetic simulations and analytical theory. Contrary to all previous theories, we find that the electron diffusion region bifurcates into two or more distinct layers in regimes with weak magnetic shear.\footnote{Yi-Hsin Liu et al. Phys. Rev. Lett. {\bf 110}, 265004, 2013} This new morphology is explained by oblique tearing modes which produce flux ropes while simultaneously driving enhanced current at multiple resonance surfaces. This physics persists into the nonlinear regime leading to multiple electron layers embedded within a larger Alfv\'enic inflow and outflow, a feature that could be observed by NASA's up-coming Magnetospheric Multiscale mission. We have extended the study to lower shear cases and found a new regime where the rate becomes much smaller and the properties of the reconnection changes significantly. We will discuss this new regime and offer a new analytical model that predicts key aspects of this regime. [Preview Abstract] |
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PP8.00128: Magnetic Reconnection in Crossed, Plasma-filled Flux Tubes Zachary Tobin, Paul Bellan Magnetic flux tubes are a fundamental feature of solar coronal loops and astrophysical jets, as well as fusion devices, such as tokamaks and spheromaks. The dynamics of arched, plasma-filled flux tubes are being studied in experiments at Caltech. These flux tubes are subject to magnetohydrodynamic forces, expanding, undergoing kink instabilities, and magnetically reconnecting. An upgraded experiment arranges for two of these flux tubes to cross over each other, so that the flux tubes undergo magnetic reconnection at the crossover point, forming one long flux tube and one short flux tube. According to theoretical predictions, this reconnection should also result in a half-twist in the flux tubes at the crossover point, with the twist propagating along each tube as Alfv\'en waves. Initial observations indicate these flux tubes magnetically reconnect with each other as predicted: merging of flux tubes occurs if the currents and magnetic fields of both tubes are all parallel, but if one of the magnetic fields is directed antiparallel, no merging occurs. If the flux tubes are formed adjacent, rather than crossing over each other, they do not merge. Flux tubes of different species generate protrusions not seen in single-species pairs. [Preview Abstract] |
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PP8.00129: Nonlinear dynamics and saturation of magnetic islands A. Poye, A. Smolyakov, O. Agullo, S. Benkadda, X. Garbet Saturation of magnetic islands is investigated numerically and analytically . The asymptotic matching theory is extended to include higher order and asymmetry effects. Role of m=0 nonlinear harmonic is investigated. In regimes of large values of the tearing mode stability parameter $\Delta ^{\prime }$ island dynamics is investigated numerically. In these regimes, the island dynamics exhibit a number of transient features such as coalescence instability, X-point collapse and plasmoid generation. It is shown that conditions (and characteristics) for these transient instabilities depend on the viscosity and resistivity however the final width of the saturated island is independent of the viscosity and resistivity values. [Preview Abstract] |
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PP8.00130: Plasmoid Chain Dynamics in Three-Dimensional Kinetic Simulations S. Markidis, P. Henri, G. Lapenta, A. Divin, M. Goldman, D. Newman, E. Laure We study the dynamics of a plasmoid chain with three dimensional Particle-in-Cell simulations. The evolution of the system with and without a uniform guide field, whose strength is 1/3 the asymptotic magnetic field, is investigated. The plasmoid chain forms by spontaneous magnetic reconnection: the tearing instability rapidly disrupts the initial current sheet generating several small-scale plasmoids, that rapidly grow in size coalescing and kinking. The plasmoid kink is mainly driven by the coalescence process. The presence of guide field strongly influences the evolution of the plasmoid chain. Without a guide field, a main reconnection site dominates and smaller reconnection regions are included in larger ones, leading to an hierarchical structure of the plasmoid-dominated current sheet. On the contrary in presence of a guide field, plasmoids have approximately the same size and the hierarchical structure does not emerge, a strong core magnetic field develops in the center of the plasmoid in the direction of the existing guide field, and bump-on-tail instability, leading to the formation of electron holes, is detected in proximity of the plasmoids. [Preview Abstract] |
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PP8.00131: Large Magnetic Reconnection Simulations with Anisotropic Fluid Closure Obioma Ohia, Jan Egedal, Vyacheslav S. Lukin, William S. Daughton, Ari Le Collisionless magnetic reconnection, a process linked to coronal mass ejections, solar flares, and magnetic substorms, has been widely studied through fluid models and fully kinetic simulations. Though fluid models often reproduce the fast reconnection rate of fully kinetic simulations, significant differences are observed in the structure of the reconnection regions. However, guide-field fluid simulations implementing new equations of state that accurately account for the anisotropic electron pressure [1] reproduce the detailed reconnection region observed in kinetic simulations [2]. Implementing this two-fluid simulation using the HiFi framework [3], we study the large-scale dynamics of the electrons layers as a function of various plasma parameters including the guide magnetic field.\\[4pt] [1] Le A et al., Phys. Rev. Lett. 102, 085001 (2009). \newline [2] Ohia O, et al., Phys. Rev. Lett. In Press (2012). \newline [3] Lukin VS, Linton MG, Nonlinear Proc. Geoph. 18, 871 (2011) [Preview Abstract] |
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PP8.00132: A Kinetic Approach to Shear Driven Magnetic Reconnection for Multi-Scale Modeling of CME Initiation Carrie Black, Spiro Antiochos, Rick DeVore, Kai Germaschewski, Judy Karpen In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consisting of an upward force due to the magnetic pressure of the sheared field balanced by a downward tension due to overlying, un-sheared field is widely believed to be disrupted by magnetic reconnection. Therefore, understanding initiation of solar explosive phenomena requires a true multi-scale model of reconnection onset driven by the buildup of magnetic shear. While, the application of a magnetic field shear is a trivial matter in MHD simulations, it is significantly challenging to do so in a PIC code. The driver must be implemented in a self-consistent manner and with boundary conditions that avoid the generation of waves that destroy the applied shear. In this work, we describe such a driver for 2.5D, aperiodic, PIC system and discuss the implementation of driver consistent boundary conditions that allow a net electric current to flow through the walls. Preliminary tests of these boundaries with a MHD equilibrium are shown. [Preview Abstract] |
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PP8.00133: Forced Magnetic Reconnection at an X-point: Particle-In-Cell and Ten-Moment Extended MHD Simulations Liang Wang, Naoki Bessho, Amitava Bhattacharjee, Kai Germaschewski, Ammar Hakim We will present comparative numerical studies of current sheet formation and forced magnetic reconnection at an X-point, beginning from a potential field. The problem will be simulated by the fully kinetic Particle Simulation Code (PSC) [1] and an extended ten-moment MHD code Gkeyll [2] that retains important kinetic physics, particularly, electron inertia and full electron/ion pressure tensors. Our goals are to investigate the similarities and differences between the two models, and to seek suitable parameterization of kinetic effects in the fluid models. The simulation domain is restrained in 2-D and is closed by conducting wall boundaries. The reconnection is forced by in-plane flows imposed on two opposite boundaries, where the forcing flows converge at the two boundary centers, and are slow compared to the characteristic Alfv\'en speed. We will compare results on the time-dependence of the reconnecting electric field (suitably normalized), as well as the structure of current sheets from PSC, Gkeyll, and an MHD code, varying ion-to-electron mass ratio and domain size. This study is carried out under the auspices of a Focus Topic in the NASA Living With a Star Targeted Research and Technology Program. \\[4pt] [1] Fox, eta, Phys Plasmas, 2012\\[0pt] [2] Hakim, J Fusion Energy, 2008 [Preview Abstract] |
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PP8.00134: Diamagnetic Stabilization of Cylindrical $m=2$ Double-Tearing Modes Stephen Abbott, Kai Germaschewski Double Tearing Modes (DTMs) have been explored as possible sources of large scale instability in non-monotonic $q$ profiles, as well as generators of strong sheared flows in the late nonlinear phase. The reversed-shear profiles which give rise to DTMs may also be accompanied by an Internal Transport Barrier (ITB). The ITB introduces pressure gradients that both couple the DTM to an ideal instability and (with appropriate physics) induce diamagnetic flows, similar to the ideally unstable $m=1$ kink-tearing mode. Using the Hall-MHD code MRC-3D we show that the diamagnetic drifts along the current sheets caused by the equilibrium pressure gradient have a stabilizing effect on the reconnecting linear DTM but may not be sufficient to prevent onset of the `explosive growth phase' if the mode is allowed to evolve into the nonlinear phase. MRC-3D is one of a suite of codes using the `libmrc' computational framework. It supports nonuniform grids in curvilinear coordinates with implicit and massively parallel explicit time integration. Its extended MHD model includes the Hall and electron pressure tensor terms in Ohm's law. [Preview Abstract] |
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PP8.00135: Magnetic Reconnection Driven by the Nernst Effect Chang Liu, W. Fox, A. Bhattacharjee, A. Joglekar, A. Thomas Magnetic reconnection in high-energy-density plasmas has been the subject of recent observations and PIC simulations. In laser-plasma experiments, laser-driven hot spots on a target can give rise to strong magnetic fields due to the Biermann battery effect. The hot spots can also produce strong heat flux perpendicular to the magnetic field, bringing into play the Nernst effect. Recently, using the Vlasov-Fokker-Planck code IMPACTA, which relies on a perturbative expansion of the electron distribution function holding ions fixed, Joglekar and Thomas (JT) have shown that the Nernst effect can play a significant role in magnetic reconnection. Since the domain of applicability of the expansion constrains the realm of validity of JT's results, we have undertaken a 2D PIC study of the Nernst effect, including complete kinetic dynamics of electrons as well as ions. We analyze the results using a broad range of dimensionless parameters, including plasma beta, the mass ratio of electrons and ions, and the Lundquist and Nernst numbers. We have found that the Nernst term contributes dominantly to support the the out-of-plane electric field upstream of the reconnection layer, consistent with JT's results. Variations on these results as a function of plasma parameters will be discussed. [Preview Abstract] |
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PP8.00136: Experimental studies of magnetic reconnection in 3D geometries Arturs Vrublevskis, Jan Egedal Magnetic reconnection has been predominantly investigated in two dimensions. However, depending on the topology and geometry of the magnetic field, a rich collection of magnetic reconnection scenarios is possible in 3D including configurations with magnetic nulls. In the experiments at the Versatile Toroidal Facility (VTF) we form a flux rope along the background toroidal magnetic field and then pulse a separate coil locally producing an opposing time varying dipole field. This drives asymmetric reconnection as well as produces a pair of 3D null points along the flux rope. We can explore configurations where a field line (spine) directly connects the nulls as well as the more complex configurations where the nulls are no longer directly linked. We diagnose the plasma with arrays of Rogowski coils, magnetic coils, and Langmuir probes and observe the effects of different topologies on the nature of reconnection. [Preview Abstract] |
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PP8.00137: Transition in Electron Physics of Magnetic Reconnection in Weakly Collisional Plasma Ari Le, Vadim Roytershteyn, Homa Karimabadi, William Daughton, Jan Egedal, Cary Forest Using self-consistent fully kinetic simulations with a Monte Carlo treatment of the Coulomb collision operator, we explore the transition between collisional and kinetic regimes of magnetic reconnection in high-Lundquist-number current sheets. Recent research in collisionless reconnection has shown that electron kinetic physics plays a key role in the evolution. Large-scale electron current sheets may form, leading to secondary island formation and turbulent flux rope interactions in 3D. The new collisional simulations demonstrate how increasing collisionality modifies or eliminates these electron structures in the kinetic regimes. Additional basic questions that are addressed include how the reconnection rate and the release of magnetic energy into electrons and ions vary with collisionality. The numerical study provides insight into reconnection in dense regions of the solar corona, the solar wind, and upcoming laboratory experiments at MRX (Princeton) and MPDX (UW-Madison). [Preview Abstract] |
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PP8.00138: Triggered Reconnection at 1 MA on COBRA John Greenly, Kate Blesener, Charles Seyler, Xuan Zhao We present new results in the study of magnetic reconnection of flux generated by two parallel currents in exploding Al wires driven to 1 MA by the Cornell COBRA pulser. Magnetic and thermal energy are stored in the system as the current rises in 200 ns. The stored energy is then dissipated in reconnection and outflows triggered at the time of voltage reversal and the decline of external magnetic pressure. Data are presented from a new optical spectroscopy diagnostic with high spatial and spectral resolution. The flows are supersonic. Strongly radiating shocks are associated with the current sheet and outflow boundaries. PERSEUS MHD and XMHD simulations are presented to compare with experiment and characterize the reconnection regime. [Preview Abstract] |
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PP8.00139: The Coalescence of Magnetic Islands in a Large Aspect Ratio Current Sheet Jianhua Cheng The formation of secondary islands and their coalescence play an important role in dissipating energy during magnetic reconnection. Recently, we have studied magnetic reconnection initiated by the tearing instability using a hybrid simulation with Lorentz force ions and fluid electrons. For current sheets with small aspect ratios, only a single island is present. For current sheets with large aspect ratios, multiple islands form and eventually coalesce into one large elongated island. We have observed ion pressure anisotropy near the $X$ point, which may prevent the full contraction of the islands and hence keep them from breaking into smaller islands. In addition, we find that a larger fraction of the dissipated magnetic energy is converted into the ion kinetic energy as the aspect ratio increases. Asymptotically, the ratio reaches slightly over $50\%$. Ion heating is identified by the ion energy spectra inside the island region which exhibits a slightly larger tail than the Maxwell distribution function. We believe that the bipolar in-plane electric field associated with the Hall term is related to the ion acceleration. Diagnostics from tracer particles are presented to illustrate how and where the ions are accelerated. [Preview Abstract] |
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PP8.00140: Quasi-linear and nonlinear stabilization of the m=1 kink-tearing mode in cylindrical geometry Kai Germaschewski, Stephen Abbott, Amitava Bhattacharjee The $m=1$ kink-tearing mode is believed to play an important role in sawtooth crashes in tokamaks. Experimental data show that the sawtooth crash happens on time scales faster than what can be explained by resistive MHD reconnection models and a nonlinear impulsive growth phase is observed. Building on our prior computational study of the nonlinear evolution of the cylindrical $m=1$ mode, we investigate in detail the nonlinear evolution of the mode in two regimes in an extended MHD model: (1) A nonlinear explosive growth phase mitigated by a shortening of the current sheet length for approximately force-free equilbrium configurations and (2) diamagnetic stabilization through a steepening pressure gradient that causes a poloidal separation of stagnation point of the flow and the magnetic X line as well as asymmetric flow in equilibria with initial pressure gradients. This work employs the Magnetic Reconnection Code (MRC), an extended MHD code that integrates Hall term and electron pressure gradient in a Generalized Ohm's Law. The code uses implicit time integration through the PETSc library and automatic code generation to create functions that evaluate the r.h.s. and sparse Jacobian. [Preview Abstract] |
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PP8.00141: Floating Potential Measurement of Magnetic Reconnection Region with High Guide Field in the UTST Merging Experiment Kotaro Yamasaki Electrostatic probe measurements of electron temperature [1] revealed a clear quadra-pole structure of floating potential profile around the X-point in the UTST merging/ reconnection experiment with high toroidal (guide) magnetic field. In the initial slow phase of reconnection, the 2D profile of floating potential has a long center electron layer with long outer ion layer but it is transformed into the quadra-pole structure when the sheet compression triggers fast reconnection. This result agrees well with the PIC simulation results by Horiuchi etc [2]. We confirmed that the polarity of the quadrupole structure completely reversed under reversed polarity of the guide field. This fact indicates that the quadrapole floating potential structure is caused by the guide field. Also, the IDS probe measurement documented a high speed reconnection outflow around 27km/s, almost equal to poloidal Alfv\'en speed. We will discuss the relationship between the potential structure, the outflow and ion heating around the X-point. [Preview Abstract] |
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