## Click here for the Research & Review Series information and talks

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## Reguar Weekly Theory Seminars:Thursdays at 10:45am in T169, refreshments @ 10:30am

**Please contact Fatima Ebrahimi if you would like to give a seminar, suggest a speaker or would like to be notified of seminars by email.**

**Please note: All visitor must have their PPPL employee host notify the Site Protection Division of any visitors to PPPL. Please contact the seminar coordinator Fatima Ebrahimi or the department administrator Jennifer Jones for entrance to the laboratory. **

#### Future seminars are subject to changes due to speakers availability. Local, flexible speakers may be asked to reschedule their seminars to give guests an opportunity to deliver talks.

#### SEE BELOW FOR DATES & TIMES OF UPCOMING AND DETAILS ON PAST SEMINAR

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## Thursday, August 14***NOTE SPECIAL TIME***

### 1:00 PM, in T169 Theory Seminar Room

#### Speaker: James Cho, Queen Mary, University of London and Harvard University

#### 'Dynamics of Close-In Extrasolar Planet Atmospheres'

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### Thursday, July 17

### 10:45am, in T169 Theory Seminar Room

Turbulent Magnetic Field Amplification in Young Galaxies

Jennifer Schober -- Institute of Theoretical Astrophysics, Heidelberg University

(Germany)

Magnetic fields play an important role in present-day galaxies, in particular by influencing the star formation process. In models of young galaxies magnetic fields are usually not considered as they are assumed not to be dynamical important at high redshifts. In the presence of turbulence, however, the small-scale or turbulent dynamo can amplify weak magnetic seed fields by randomly stretching, twisting and folding the field lines. The details of this process depend on the nature of turbulence, i.e. on the hydrodynamic and magnetic Reynolds numbers, and on the compressibility of the gas. With a model of a typical young galaxy, where turbulence is generated by accretion and supernova explosions, we determine the growth rate of the small-scale dynamo. We follow the exponential growth of the magnetic field on the viscous scale and also the subsequent transport of the magnetic energy to larger scales in the non-linear dynamo phase. Depending on the parameters of our model we find that equipartition of magnetic and kinetic energy, i.e. a field strength of roughly 10^(-5) G, is reached within 4 to 270 Myr. Thus, we expect that the turbulent dynamo can generate strong unordered fields already in very young galaxies.

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## Tuesday July 8th* Special Day*

##### 10:45am, in T169 Theory Seminar Room

Relaxation to neoclassical flow equilibrium in gyrofluid simulations

Bruce Scott, Max-Planck-IPP

EURATOM Association

The theorem for toroidal angular momentum conservation within gyrokinetic field theory is used as a starting point for consideration of slow transport of flows under quasistatic force balance. The content of the momentum by itself yields a relation between the electric field and the parallel flow, if conserved/transported quantities are taken as given. The relation of poloidal to parallel flow then yields the radial electric field in terms of the poloidal flow as in the standard case. If the toroidal Mach number is not small this gives an iterable solution for the electric field provided neoclassical theory has given the poloidal flow of each ion species. In a gyrokinetic computation, provided the collision operator is sufficient (is conservative) this is enough to recover neoclassical results in the appropriate limit but is also applicable to more general

situations. This treatment serves merely to underpin the ability of global gyrokinetic computations to treat neoclassical physics even when conventional ordering is relaxed. Finally, the pathway of relaxation to slowly varying conditions from an arbitrary initial state is detailed.

The time scale hierarchy is separated to have Alfven and then geodesic oscillations damp away, and then on the ion collisional time scale the electric field is established, and then on the much slower confinement time the conserved quantities are transported. All of the important

effects go through first order drifts, with the quadratic term in the Hamiltonian accounting for polarization.

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## Thursday, May, 29

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Professor Julia Mikhailova -- Department of Mechanical and Aerospace Engineering -- Princeton University

##### TITLE: "Laser-driven synchrotron-type emission from solid surfaces"

High-order harmonic emission from the interaction of relativistic-intensity laser pulses with solids offers the potential for intense ultrashort XUV/x-ray pulse generation. This highly nonlinear phenomenon was predicted by numerical simulations and later observed in many experiments. It is commonly explained in terms of the so-called “relativistic oscillating mirror” model (Doppler frequency upshift of light reflected by a moving surface). Here, a clear physical picture is presented for the generation of attosecond XUV/x-ray pulses from the interaction of relativistic-intensity laser pulses with overdense plasma slabs. The sub-cycle, field-controlled release and subsequent nanometer-scale acceleration of relativistic electron bunches under the combined action of the laser and ionic potentials give rise to synchrotron-like emission.This insight into the fundamental properties of the relativistic high-harmonic emission process allows for an analytical treatment of the effect. The high-frequency cutoff in the emission spectrum is explained in terms of the basic laws of synchrotron radiation. The emerging synchrotron-like radiation is confined to time intervals much shorter than the half-cycle of the driver field. This intuitive approach will be instrumental in analyzing and optimizing laser-driven relativistic sources of intense ultrashort XUV/X-ray pulses.

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## Thursday, April 24

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. Eun-Hwa Kim -- PPPL

##### TITLE: Role of the ion-ion hybrid resonance in the planetary magnetospheres

Ion cyclotron frequency range waves (or electromagnetic ion cyclotron wave, EMIC) have been often observed at Earth and Mercury’s magnetospheres. Because the presence of different ion species has an influence on the plasma’s dispersion characteristics near the ion gyrofrequencies, new multi-ion resonances, such as Buchsbaum and ion-ion hybrid (IIH) resonances, are added with each additional ion species. When the frequency of incoming fast compressional waves matches the ion-ion hybrid resonance condition in an increasing (or decreasing) heavy ion concentration or inhomogeneous magnetic field strength, wave energy from incoming compressional waves concentrates and mode converts to electromagnetic ion cyclotron (EMIC) waves. Mode conversion at this resonance has been simulated using a multi-fluid code showing that the resulting EMIC waves are strongly guided by the ambient magnetic field (B_{0}) and have linear polarization, therefore, the IIH resonance has been suggested to be the field-line resonance at Mercury and linearly polarized EMIC waves at Earth. In addition, because the IIH resonance frequency depends on B_{0} and the ratio of the ion densities, the ratio of the ion densities can be estimated using the IIH resonance frequency of the observed linearly polarized EMIC waves. Using 1D and 2D full wave codes, time-dependent multi-ion wave model, we discuss how such IIH resonance occurs in the Earth and Mercury’s magnetosphere and how to infer heavy ion density using the IIH resonances.

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## Friday, April, 18 * Special Day & Time*

##### 11am in T169 Theory Seminar Room

##### Speaker: Dr. Andrea Cole, Columbia University

##### TITLE:Variational Principles with Padé Approximants for Tearing Mode Analysis

Abstract:

Tearing modes occur in several distinct physical regimes, and it is often important to compute the inner layer response for these modes with various effects. There is a need for an approximate and efficient method of solving the inner layer equations in all these regimes. In this talk I introduce a method of solving the inner layer equations based on using a variational principle with Pade approximants. For all the regimes considered, the main layer equations to be solved are inhomogeneous, and Pade approximants give a convenient and efficient method of atisfying the correct asymptotic behavior at the edge of the layer. Results using this variational principle - Pade approximant method in three of these regimes are presented. These regimes are the constant-psi resistive-inertial (RI) regime, the constant-psi viscoresistive (VR) regime, and the non-constant-psi inviscid tearing regime. The last regime includes the constant-psi RI regime and the inertial regime. The results show that reasonable accuracy can be obtained very efficiently with Pade approximants having a small number of parameters.

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## Thursday, April, 17

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. Guoyong Fu -- PPPL

##### TITLE: "M3D-K simulations of energetic particle transport due to sawteeth, fishbone and TAE"

Recent results of M3D-K nonlinear simulations of energetic particle transport are presented. We investigate energetic particle redistribution due to sawteeth in tokamaks and due to fishbone and TAE in NSTX. The main results are

(1) Sawteeth: Test particle simulations are carried out to study the energetic particle transport due to a sawtooth crash. The results show that energetic particles are redistributed radially in plasma core depending on pitch angle and energy. For trapped particles, the redistribution occurs for particle energy below a critical value in agreement with previous theory. For co-passing particles, the redistribution is strong with little dependence on particle energy. In contrast, the redistribution level of counter-passing particles decreases as particle energy becomes large.

(2) Fishbone: Nonlinear simulations of beam-driven fishbone instability in NSTX have been carried out for weakly reversed q profiles with minimum of q just above unity. Result show nonlinear saturation with strong frequency chirping and beam ion profile flattening.

(3) TAE: Nonlinear simulations of multiple beam-driven TAEs in NSTX have been carried out. Preliminary results show mode saturation, frequency chirping and beam ion distribution flattening.

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## Thursday, March 13

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. G. Prof. Shvets

##### TITLE: "Multi-Dimensional Collective Instabilities of Laser and Particle Beams Relevant to High Energy Density Laboratory Plasma Science".

I will discuss how two of the oldest-known plasma instabilities, the Weibel Instability and the Raleigh-Taylor Instability, manifest themselves in the context of high energy density relativistic plasmas. The RTI will be described in the context of laser acceleration of thin targets by ultra-intense laser pulses. This acceleration regime, known as Radiative Pressure Acceleration (RPA) is very promising for developing compact sources of high energy monoenergetic ions that could potentially find numerous applications ranging from fast ignition to cancer treatment. I will discuss the first analytic model of a uniformly laser-accelerated target and discuss the stability of such targets to RTI. PIC simulations reveal that considerable deviations of the growth rate of the RTI from the conventional scaling can emerge for ultra-high laser intensities. Paths to suppressing RTI using multi-ion species targets will be discussed.

The Weibel Instability, which occurs when a beam-like plasma propagates through stationary plasma, has been proposed as a candidate for generation of strong magnetic fields and for sustaining collisionless shocks in astronomical environments. I will demonstrate that there are severe constraints limiting the amount of directed beam energy that can be converted into magnetic field. Analytic results for relativistic beams undergoing filamentation and collisionless thermalization in dense plasma will be presented. A new type of a self-focused beam equilibrium similar to Bennett Pinch will be discussed. Because transverse temperature of the beam can suppress WI entirely, I will discuss how quasi-electrostatic waves generated by the beam can recover the WI despite high temperature of the beam. The results of modeling WI usingf reduced-description codes will be presented.

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## Thursday, March 6

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Kimin Kim, Princeton Plasma Physics Laboratory

Title: Rotational resonances in 3D perturbed tokamaks, and their impact on neoclassical toroidal viscosity and kinetic stability

Resonance of perturbed orbits with toroidal ExB rotation, called the bounce-harmonic resonance, changes trapped bounce orbits to closed circuits. It dominantly contributes to perturbed pressures and neoclassical toroidal viscosity (NTV) torque in the rotating plasmas with nonaxisymmetric magnetic perturbations, while random phase-mixing reduces the nonambipolar particle transport for off-resonance particles. Such driving mechanism of NTV also plays an important role in the kinetic stabilization of slowly growing MHD modes such as the resistive wall mode (RWM), but acts differently due to shifts of resonance frequency condition. This talk will discuss the mechanism of bounce-harmonic resonance in perturbed tokamaks and its impacts on NTV transport and kinetic potential energy, using a guiding-center particle code, POCA. Theoretical and experimental analyses on the magnetic braking and kinetic stability modification of RWM in NSTX will be presented.

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## Thursday, February, 27

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. Haihong Che -- NASA/GSFC

##### TITLE: Electron Fluid Description of Wave-Particle Interactions in Strong Buneman Turbulence

To understand the nature of anomalous resistivity in magnetic reconnection, we investigate turbulence-induced momentum transport and energy dissipation during Buneman instability in force-free current sheets. Using 3D particle-in-cell simulations, we find that the macroscopic effects generated by wave-particle interactions in Buneman instability can be approximately described by a set of electron fluid equations. These equations show that the energy dissipation and momentum transports along current sheets are locally quasi-static but globally non-static and irreversible. Turbulence drag dissipates both the streaming energy of current sheets and the associated magnetic energy. The decrease of magnetic field maintains an inductive electric field that re-accelerates electrons. The net loss of streaming energy is converted into the heat of electrons moving along the magnetic field and increases the electron Boltzmann entropy. The growth of self-sustained Buneman waves satisfies a Bernoulli-like equation that relates turbulence-induced convective momentum transport and thermal momentum transport. Electron trapping and de-trapping drive local momentum transports, while phase mixing converts convective momentum into thermal momentum. The drag acts like a micro-macro link in the anomalous heating process. The dissipated magnetic energy is converted into the electron heat moving perpendicularly to the magnetic field and this heating process is decoupled from the heating of Buneman instability in the current sheets.

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## Thursday, February, 20

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. **Alexei Pankin**

##### Title: TBA

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Thursday, February, 6

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. Frank Chang

**TITLE: Physical Picture of 2-1/2D Driven Collisionless Magnetic Reconnection**

The physical picture of how electrons and ions flow, how the electric and magnetic fields change, and how particles gain energy will be presented for the 2-1/2D collisionless driven magnetic reconnection. The 2-1/2 dimensional collisionless reconnection studies are performed using the particle simulation PASMO code1 and theoretical analysis. In particular, we will provide the physical mechanism of how the poloidal current (including the Hall current in the downstream region) is generated and how the electrostatic potential is produced in the poloidal plane. The physical picture of how the quadrupole magnetic field and electrostatic potential are generated in the 2-dimensional (poloidal) plane is different from the one presented by Uzdensky and Kulsrud.2

C. Z. Cheng1,2, S. Inoue3, Y. Ono2,3, R. Horiuchi4

1 Institute of Space and Plasma Sciences, National Cheng Kung University, Taiwan

2 Department of Advanced Energy, University of Tokyo, Japan

3 Graduate School of Engineering, University of Tokyo, Japan

4 National Institute for Fusion Science, Japan

1 H. Ohtani and R. Horiuchi, Plasma Fusion Res., 4, 024 (2009)

2 D. A. Uzdensky and R. M. Kulsrud, Phys. Plasma, 13, 062305 (2006)

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## Thursday, January 30, 2014

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Hamid Saleem, National Centre for Physics Quaid-iAzam University Campus, Pakistan

##### TITLE: TBA

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## Thursday, January 16, 2014

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Lei Qi, Yu Lin and Xueyi Wang, Physics Department, Auburn

##### TITLE: GeFi Particle Simulation of Lower Hybrid Waves and Electron-ion Hybrid Instability

Lower hybrid wave (LHW) is a potential source to heat both electrons and ions to thermonuclear temperature and generate electric currents in fusion plasmas under Landau damping. Although linear Landau damping and nonlinear parametric instability of LHWs were well studied in theories, few particle simulations have been performed. In this talk, physics of LHWs is investigated with a gyro-kinetic electron and fully kinetic ion (GeFi) particle simulation model in the electrostatic limit. GeFi model is particularly suitable for plasma dynamics with wave frequencies lower than the electron gyrofrequency, and for problems in which the wave modes ranging from Alfvén waves to lower-hybrid/whistler waves that need to be handled on an equal footing with realistic electron-to-ion mass ratio. Firstly, the linear physics of lower hybrid waves and their nonlinear interactions with both electrons and ions through Landau damping are studied. Unlike most other wave modes, LHWs can resonantly interact with both electrons and ions, with the former being highly magnetized and the latter nearly unmagnetized around lower hybrid frequency. While the resonant electrons are trapped in the wave field in the nonlinear electron Landau damping, resonant ions are untrapped throughout the wave-particle interaction. Then, electron-ion hybrid instability driven by transversely sheared E×B flow, which plays an important role in laboratory and space plasmas, is studied by the GeFi model in the linear and nonlinear regimes. Electron-ion hybrid instability is in the lower hybrid frequency regime with ρe<LE <ρi, where ρi and ρe are the electron and ion Larmor radii, respectively, and LE represents the scale length of the shear flow profile. Realistic experimental parameters for Auburn Linear Experiment for Instability Studies (ALEXIS) device are adopted in GeFi particle simulations, and the results are compared with ALEXIS measurements.

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## Thursday, December 19, 2013

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Prof. Eliezer Hameiri

TITLE: "Multi-fluid and MHD plasmas with flow, a variational approach"

ABSTRACT:

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 can be constructed 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. A limit of the variational integral yields the two-fluid Hall-Magnetohydrodynamic (HMHD) model. A further special limit yields MHD equilibria and can be used to approximate the equilibrium state of a Hall-MHD plasma in a perturbative way.

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## Thursday, December 12, 2013

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. Luca Guazzotto

##### Title: "Perturbed equilibria in tokamaks"

Magnetic perturbations are an intrinsic part of tokamak physics. The plasma boundary is perturbed even during stationary stages of plasma discharges. Fast thermal quench (a sudden drop of the core plasma temperature) in the initial phase of disruptions suggests the existence of magnetic perturbations in the entire plasma volume. As a result, high voltage during this phase generates energetic runaway electrons, potentially very damaging for the vacuum vessel of tokamaks. The development of the theory and numerical models of perturbed equilibria is a ritical step for the understanding of the phenomena mentioned above.

At resonant surfaces, islands will form, altering the simple idealized picture of nested flux surfaces. In the present work we propose a general approach for describing perturbed plasma equilibria. The fundamental ingredients are: 1) an energy principle based description of the perturbed equilibrium with magnetic islands; 2) a quasi-linear model for finite-sized islands. Our special form of the energy principle, recently developed based on vector potential perturbations, is suitable for the problem and for the scale separation in MHD in general. It also covers ideal MHD stability and perturbations. In the vicinity of the islands we use a smooth current density model, which gives the matching conditions for the energy principle. This model removes the singularity at the resonant surfaces and significantly simplifies the otherwise ideal MHD calculations. A considerable amount of work has already been carried out on this topic. The STB package of codes, which includes the Reduced MHD version of the perturbed equilibrium model, was supplied to the ITER Organization in 2012. The next step is to extend the initial reduced model to arbitrary tokamak configurations, including NSTX.

Besides 2-D tokamak applications (plasma boundary, WTKM, RMP, thermal quench, runaway electrons), the perturbed equilibrium is envisioned as a key ingredient in 3-D MHD simulations of stellarators and tokamak disruptions. The recently introduced Reference Magnetic Coordinates reduce the 3-D magnetic fields near the resonant surfaces to the 2-D approximation, thus making a generic link to perturbed equilibria in tokamaks.

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##### Thursday, November 21, 2013

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. J .Perez

##### Title: TBA

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##### Thursday, November 7, 2013

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Dr. Scott Boardsen (NASA/GSFC)

##### Title: Recent progress on ultra low frequency (ULF) waves at Mercury

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## Thursday, Oct 17, 2013

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: R.M. Churchill, MIT

##### Title: Flux Surface Variation of Impurity Density and Flows in the Pedestal Region of Alcator C-Mod

Measured impurity density and flows in the pedestal region of Alcator C-Mod can deviate significantly on a flux surface from current model predictions. Comparing localized measurements at the low-field side (LFS) midplane and the high-field side (HFS) midplane, boron (B5+) impurity density asymmetries larger than 10 are observed in H-mode plasmas, with larger densities at the HFS. The LFS density pedestal varies in position and width with varying plasma conditions, while the HFS impurity density profile remains rather fixed. Impurity density asymmetries are not observed in plasmas with small gradients, i.e L-mode, suggesting the drive for the asymmetry may be the strong gradients in the H-mode pedestal region. However, impurity density asymmetries are also absent in I-mode plasmas, despite the presence of a strong radial gradient in temperature (with no main ion density pedestal). This indicates an interplay between the gradient scale lengths of the main ion density and temperature in the drive of the impurity density asymmetry. Possible and probable causes of these density and flow asymmetries will be explored, including localized sources, poloidally asymmetric radial transport, and ion-impurity friction.

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## Thursday, Oct 10, 2013

##### 10:45am, in T169 Theory Seminar Room

##### Speaker: Fatima Ebrihami, Dept. of Astrophysical Sciences and PPPL, Princeton University

##### Title: Magnetic reconnection process in transient coaxial helicity injection

Non-inductive current formation and its sustainment is one of the major physics objectives in NSTX as an advanced Spherical Torus (ST). A promising candidate for start-up current formation is Coaxial Helicity Injection (CHI). We numerically examine the physics of transient CHI for start-up in NSTX. Through resistive MHD simulations, we first obtain the minimum conditions required for generating closed flux and then explain the fundamental mechanism for magnetic reconnection and closed flux generation in transient CHI discharges. We find that at sufficiently low magnetic diffusivity (high Lundquist number), and with a sufficiently narrow injector flux footprint width, the oppositely directed field lines have sufficient time to reconnect (before dissipating), leading to the formation of closed flux surfaces. Simulations show that an X point is formed in the injector region, followed by formation of closed flux surfaces within 0.5 ms after the driven injector voltage and injector current begin to rapidly decrease. As the injector voltage is turned off, the fields lines tend to untwist in the toroidal direction and magnetic field compression exerts a radial J × B force and generates a bi-directional radial Etoroidal × Bpoloidal pinch flow to bring oppositely directed field lines closer together to reconnect. The reconnection process is shown to have transient Sweet-Parker characteristics ( http://link.aip.org/link/?PHP/ 20/090702&aemail=author). There are similarities between the transient Sweet-Parker reconnection found here and that reported in forced-reconnection laboratory plasmas of MRX

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## Thursday, Oct 3, 2013

##### 10:45pm, in T169 Theory Seminar Room

##### Speaker: J.P. Boeuf – LAPLACE, CNRS, University of Toulouse

#### Title: Rotating instabilities in low temperature magnetized plasmas

After a short introduction on the activities on low temperature magnetized plasmas at the LAPLACE laboratory in Toulouse (Hall thrusters, negative ion source for the ITER neutral beam injector), we will present and discuss recent results from PIC MCC (Particle-In-Cell Monte Carlo Collisions) simulations of low magnetized plasma columns, 1) under conditions of electron beam sustained plasma columns such as the MISTRAL device at the PIIM laboratory in Marseille, and 2) under conditions of cylindrical magnetron discharges. Typical plasma parameters in theses devices are : plasma density in the range [1014-1016 m-3], electron temperature [0.1-5 eV], magnetic field [10-50 mT], gas pressure [10-2 -1 Pa].

In the case of the MISTRAL device, the simulations show a low frequency ExB rotation of the plasma associated with an azimuthal non-uniformity of the plasma potential that reproduces well the exprimental results and provides a physical interpretation of the LIF (Laser Induced Fluorescence) measurements of the ion velocity distribution recently performed at the PIIM laboratory1. In the case of cylindrical magnetrons, where a larger current is drawn across the magnetic field by the applied voltage, the simulations predict the formation of a rotating instability associated with an ionization front (« rotating spoke ») whose properties are very similar to those observed in the experiments developed in the 1960s-1980s to study the concept of critical ionization velocity2 (CIV). The CIV concept had been introduced by Alfven in his theory of planet formation.

[1] C. Rebont, N. Claire, Th. Pierre, and F. Doveil, Phys. Rev Lett. 106 225006 (2011)

[2] A. Piel, E. Möbius and G. Himmel, Astrophysics and Space Science 72 211 (1980); N. Brenning, Space Science Review 59 209 (1992)

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## Thursday, Sep 19, 2013

##### 10:45pm, in T169 Theory Seminar Room

##### Speaker: Mike Campanell

#### Title: Effects of Electron Emission on Plasma-Surface Interaction

##### Abstract:

Secondary, thermionic and photon-induced electron emission from surfaces are important in many plasma applications. Most theoretical models of PSI with emission invoke assumptions that the sheath is time-independent, the wall potential is negative, ions enter the sheath at the Bohm velocity, the presheath is negligibly affected, and one wall is present [1]. It can be shown with basic theory and PIC simulations that these assumptions break down in a variety of situations. When emission is strong, the sheath potential can become positive, repelling ions from the wall and eliminating ion sputtering [2,3]. Emitted electrons entering the plasma can drastically affect the presheath structure too. If the electron mean free path is large, emitted electrons can transit the plasma and impact other walls; hence in low collisionality systems wall charging becomes a complex global flux balance problem [4]. Secondary emission can trigger sheath instabilities preventing plasma-wall systems from reaching steady state [5,6]. Implications of these effects are discussed for tokamak divertors, Hall thrusters, dusty plasmas, hot cathodes, RF discharges and spacecraft.

[1] G.D. Hobbs and J.A. Wesson, Plasma Phys. 9, 85 (1967).

[2] M.D. Campanell, A.V. Khrabrov and I. D. Kaganovich, Phys. Rev. Lett. 108, 255001 (2012).

[3] M.D. Campanell, submitted to Phys. Rev. E (2013).

[4] M.D. Campanell and H. Wang, submitted to Appl. Phys. Lett. (2013).

[5] M.D. Campanell, A.V. Khrabrov and I. D. Kaganovich, Phys. Rev. Lett. 108, 235001 (2012).

[6] M.D. Campanell et al., Phys. Plasmas 19, 123513 (2012).

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## Thursday, Sep12, 2013

##### 10:45pm, in T169 Theory Seminar Room

##### Speaker: Paul Schmit, Sandia National Laboratories

#### Title: Magnetic fields and tail-ion depletion in inertial confinement fusion

"The impact of embedded magnetic fields on Knudsen layer formation and tail-ion depletion [1] near steep density and temperature gradients in inertial confinement fusion is investigated for the first time. Magnetic fields change the energy scaling of the ion diffusivity in a way that eliminates the preferential losses of fast ions compared to thermal ions. Simple threshold criteria give conditions such that the restoration of the ion tail distribution is sufficient to recover much of the lost fusion reactivity. The tail-ion kinetic equations are solved for hot fuel bounded by a cold, nonreacting wall using a numerical stochastic differential equation solver, and the modified fusion reactivities are calculated. We find that modest magnetic fields too weak to magnetize thermal ions are still sufficient to restore much of the lost reactivity, consistent with the threshold conditions. We also find that the Maxwell-averaged fusion reactivities are recovered more fully in uniformly magnetized cylindrical targets compared to uniformly magnetized spherical targets. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] K. Molvig et al., PRL 109, 095001 (2012)."

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## Thursday, Sep 5, 2013

##### 10:45pm, in T169 Theory Seminar Room

##### Speaker: I.D. Kaganovich, D. Sydorenko, A.V. Khrabrov, E. Tokluoglu, E.A. Startsev, R.C. Davidson

#### Title: Collective Beam-Plasma Interactions for Fusion and Plasma Processing Applications

The beam-plasma interaction is one of the most important phenomena in plasma physics and its applications. A recent resurgence of interest in beam-plasma interactions is due tho the ability of advanced particle-in-cell codes to simulate many aspects of the complex processes observed in experiments on beam-plasma interactions relevant to plasma processing of semiconductors. I will give examples of beam self-organization phenomena that occur because of nonlinear saturation of instabilities and their subsequent evolution. The first example is an electron beam undergoing the Weibel filamentation instability. We demonstrate that the beam splits into many filaments and then the filaments undergo collective interactions and nonlinear mergers. The final state of the process is a single pinched beam surrounded by a wide halo [1]. The second example is an ion beam propagating in background plasma. An ion beam can effectively excite plasma waves and whistler waves when propagating in plasma [2]. The two-stream instability between the beam ions and plasma electrons cause further growth of plasma waves, which in turn leads to a significant enhancement in the plasma return current and a defocusing of the beam [3]. The final example is relevant to plasma processing applications. The two-stream instability of an intense electron beam in finite-length plasma with nonuniform density is investigated numerically. A new regime of the instability is observed where intense plasma oscillations occur near the plasma edges. Here, bulk electrons can be accelerated to substantial energies in the direction opposite to the beam propagation direction. The new regime appears stochastically and only for strong beam currents.

[1] V. Khudik, I. Kaganovich, and G. Shvets, PoP 19, 103106 (2012).

[2] M. Dorf, I. Kaganovich, E. Startsev, and R. C. Davidson, PoP 17, 023103 (2010).

[3] E. A. Startsev, I. D. Kaganovich, R. C. Davidson, “Effects of beam-plasma instabilities on neutralized propagation of intense ion beams in background plasma”, NIMA A, in press (2013).

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## Thursday, Aug 29, 2013

#### 10:45pm, in T169 Theory Seminar Room

#### Speaker: Nick Murphy (Harvard-Smithsonian Center for Astrophysics)

#### Title: Asymmetric Magnetic Reconnection in the Solar Atmosphere

Models of solar flares and coronal mass ejections typically predict the development of an elongated current sheet in the wake behind the rising flux rope. In reality, reconnection in these current sheets will be asymmetric along the inflow, outflow, and out-of-plane directions. We perform large-scale simulations to investigate the consequences of asymmetry during solar reconnection. We predict several observational signatures, including flare loops with a skewed candle flame shape, slow drifting of the current sheet, asymmetric hard X-ray emission and motion of flare loop footpoints, and rolling motions within the erupting flux rope. We perform simulations of the plasmoid instability with asymmetric upstream magnetic fields and show that islands grow preferentially into the weak field upstream region. The islands develop net vorticity because the outflow jets impact them obliquely rather than directly. All of these simulations show non-ideal plasma flow across X-points. To understand this, we derive exact expressions for the rate of motion of a magnetic null point. Finally, we discuss how comparisons to observations will necessary to fully understand the roles of global and 3D effects.

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## Thursday, Aug 22, 2013

#### 10:45pm, in T169 Theory Seminar Room

#### Speaker: Alain J. Brizard, Saint Michael's College

#### Title: Guiding-center effects in gyrokinetic momentum conservation laws for axisymmetric tokamak plasmas

Momentum conservation laws for the truncated gyrokinetic Vlasov-Poisson equations in axisymmetric tokamak geometry are derived by the Noether method. The talk will emphasize the role played by the guiding-center polarization as well as other higher-order guiding-center effects. A discussion on the relation between Littlejohn’s 1983 derivation of the guiding-center transformation and the guiding-center transformation will be included.

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## Friday, Aug 2, 2013 *** Specail Day***

#### 10:45pm, in T169 Theory Seminar Room

#### Speaker: Ido Barth, Racah Institute of Physics, Hebrew University of Jerusalem

#### Title: "Autoresonance - A Kinetic Perspective"

Autoresonance (AR) is a salient property of many driven nonlinear systems to stay in resonance when the system's parameters vary in space and/or time. The idea was used in relativistic particle accelerators since 1946, but only 45 years later recognized as extremely important in many other fields of physics. Presently, applications of AR exist in atomic and molecular physics, plasmas, fluids, nonlinear optics, planetary dynamics, and Josephson junctions. The salient feature of the AR is the existence of a sharp threshold on the amplitude of the oscillating, chirped frequency driving perturbation for transition to AR. In the talk, I will address the AR phase-locking transition of a thermally distributed ensemble and the bunching effect due to self fields. It will be shown that at temperature T, the capture probability is a smoothed step function of the driving amplitude with the step location and width scaling as 3/4 ( being the chirp rate) and 1/2 T , respectively [1]. However, at sufficiently low temperatures this width saturates to a finite value associated with zero point quantum fluctuations [2,3]. When the particle density increases, strong repulsive self-fields reduce the width of the threshold considerably, as the ensemble forms a localized autoresonant macro-particle [1]. This result played an essential role in the mixing scheme in anti-hydrogen formation experiment at CERN. Finally, I will address the quantum counterpart of the classical AR phenomenon, i.e. the ladder climbing, and the continuous transition between these two regimes [3,4].

[1] I. Barth, L. Friedland, E. Sarid, and A. G. Shagalov, PRL 103, 155001 (2009).

[2] K. W. Murch, R. Vijay, I. Barth, O. Naaman1, J. Aumentado, L. Friedland, and I. Siddiqi1, Nature Physics 7,

105 (2011).

[3] I. Barth, L. Friedland, O. Gat, and A.G. Shagalov, PRA 84, 013837 (2011).

[4] Y. Shalibo, Y. Rofe, I. Barth, L. Friedland, R. Bialczack, J. M. Martinis, and N. Katz, PRL 108, 037701 (2012).

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## Thursday, Aug 1, 2013

#### 10:45pm, in T169 Theory Seminar Room

#### Speaker: Michael Kraus, Max Planck Institute for Plasma Physics, Garching, Germany

#### Title: "Variational Integrators for Plasma Physics"

Variational integrators provide a systematic way to derive geometric numerical methods that preserve a discrete multisymplectic form (and therefore have good long time energy behaviour) as well as momenta associated to symmetries of the system by Noether’s theorem. After a short outline of the theory, the application of variational integrators to several prototypical theories from plasma physics is presented. These are the Vlasov-Poisson system in 1D as well as ideal and reduced magnetohydrodynamics in 2D. The Vlasov-Poisson integrators preserve the total particle number, momentum and energy as well as norms of the distribution function exactly (up to machine precision). The ideal MHD integrator preserves energy as well as cross helicity, and the reduced MHD integrator energy and several Casimirs of the system.These properties will be demonstrated by typical test cases from plasma and astro physics.

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## Thursday, July 25, 2013

#### 10:45pm, in T169 Theory Seminar Room

#### Speaker: Dr. Luca Guazzotto, University of Rochester

#### Title: Two-fluid equilibrium: theory, numerical solution, and applications

Flows, either driven or intrinsic, are commonly observed in tokamak experiments. Single-fluid MHD equilibria with flow are routinely considered, and various codes exist that include arbitrary or at least toroidal velocity in their implementation. A class of equilibria of particular interest is the "transonic equilibrium". In transonic equilibria tangential density and velocity discontinuities form between the edge, where the poloidal velocity is faster than the poloidal sound speed, and the core where it is slower. Our interest in obtaining a better model for transonic flows led us to consider two-fluid equilibria, in which ion inertia is retained, and ions and electrons flow on distinct surfaces. The axisymmetric equilibrium problem was reformulated, and a new two-fluid version of the MHD equilibrium code FLOW, named "FLOW2" was implemented. Even though our work on two-fluid transonic equilibrium is still in progress, useful results can already be obtained with FLOW2. In particular, it is possible to run equilibria for arbitrary axisymmetric systems, like for instance DIII-D and NSTX, and for arbitrary profiles of the input free functions. Details of the model and implementation are presented, together with sample applications.

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## Thursday, May 23, 2013 *** SPECIAL TIME***

#### 2:30pm, in T169 Theory Seminar Room

#### Speaker: Predrag S. Krstić, University of Tennessee, Knoxville TN 37996-6173

#### Title: ATOMISTIC SCIENCE FOR THE PLASMA-MATERIAL INTERFACE

Plasma-Material Interface (PMI) mixes materials of the two worlds, creating a dynamical surface which is one of the most challenging areas of multidisciplinary science, with many fundamental processes and synergies. The traditional trial-and-error approach to developing first-wall material and component solutions for current and future fusion devices is becoming prohibitively costly because of the increasing device size, curved toroidal geometry, access restrictions, and complex programmatic priorities. The experimentally validated atomistic theory/computation for studying the dynamics of the creation and evolution of the PMI under irradiation by heavy particles (atoms, molecules) at carbon, lithiated carbon and tungsten, as well as the emerging elastic and inelastic processes, in particular retention and sputtering chemistry will be presented.

Recent work with lithium coatings deposited on a variety of metallic and graphitic surfaces, in a number of tokamak fusion machines around the world, has provided evidence of the sensitive dependence plasma behavior has on these ultra-thin deposited films. Our computer simulations, done in collaboration with Japanese and French scientists, and validated by in-situ experiments at Purdue University and at NSTX have contributed to unraveling the mystery of this high fidelity control [1]. We will present quantum-classical atomistic calculations which elucidate roles of lithium and oxygen in amorphous-carbon on retention of hydrogen. We show that the presence of oxygen in the surface plays the key role in the uptake chemistry, while lithium’s main role is to bring the oxygen to the surface. D atoms preferentially bind with O and C-O when there is a comparable amount of oxygen to Li at surface. This finding well matches a number of experimental results, obtained within the last decade.

The plasma-facing walls of the next-generation fusion reactors will be exposed to high fluxes of neutrons and plasma-particles and will operate at high temperatures for thermodynamic efficiency. To this end we have been studying the evolution dynamics of vacancies and interstitials to high doses of tungsten surfaces bombarded by self-atoms, in preparation for simulations of plasma-surface interactions (erosion, hydrogen and helium uptake and fuzz formation) and recycling occurring in tungsten walls damaged by energetic neutrons**.**

[1] P. S. Krstic et al., Physical Review Letters 110, 105001 (2013)

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## Theory/CPPG Seminar

#### Thursday, May 23, 2013

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: John Jenkins, Computer Science Department, North Carolina State University

#### Title: Layout Optimization Techniques for Extreme-scale Analytics

Abstract:

Recent trends in I/O in an HPC context present significant, multi-dimensional challenges: coping with huge increases in the amount and complexity of data produced, effectively using increasingly complex I/O subsystems and hardware configurations, and allowing for swift data analysis under varying access patterns, to name a few. To address these challenges, advanced data reorganization and analytics techniques must be explored, placing particular focus on data reduction as a first-order constraint. To this end, I will present two technologies made to accelerate different data access workloads. First, I will introduce a precision-based technique for multiresolution analysis, extracting favorable performance and accuracy characteristics by exploiting the floating-point data format. I additionally explore the implications of performing varying analyses on reduced-precision GTS electrostatic potential data, highlighting the efficacy of providing a configurable, multi-resolution data decomposition. Second, I will discuss a parallel system for query-driven analysis that drives down storage and query processing costs by operating directly in a compressed data space. To conclude, I will present ongoing works that aim to tame the data complexity problem inherent in data layout optimizations: how transparent can these tools be made to the end-user, and can I/O libraries support them effectively?

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### ** SPECIAL DAY** Wednesday, May 15, 2013**

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: Wenjun Deng, PPPL

#### Title: Marker particle optimization for deltaf PIC simulation

In f particle-in-cell (PIC) plasma simulations, the markers are for re- solving the pertrubed distribution function f in phase space. Because f is unknown beforehand, typically a huge number of markers are loaded everywhere in phase space. Meanwhile, for some waves/instabilities, .g., Alfven eigenmodes excited by energetic particles, the f mode structure is localized in a small portion of the phase space, which is usually the resonant regions, and f is nearly zero elsewhere. Maintaining a large number of markers in the f 0 regions is a waste of computing time. Reducing markers in these regions can save a lot of computing time. In this work, a marker removal method is developed to achieve this time saving eect. In this presentation, this method is rst demonstrated in a simple bump-on-tail simulation in 2D phase space, then it is generalized for simulations in 5D gyro-center phase space. It can also be straightforwardly generalized for simulations in phase space of other dimensions. This method has been implemented in the kinetic/MHD hybrid code M3D-K and can save markers by about a factor of 20 in an n = 1 toroidal Alfven eigenmode (TAE) simulation. This method will accelerate nonlinear studies of energetic-particle-driven Alfven eigenmodes.

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#### Thursday, May 9, 2013

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: Mike Mauel, Columbia University

#### Title: Turbulent Pinch, Laboratory Magnetospheres, and the Economic Viability of Fusion

The turbulent pinch is among the most remarkable phenomena of magnetized plasma. First seen in planetary magnetospheres, the turbulent pinch causes self-organization, diffusion counter to gradients, and centrally-peaked plasma profiles from a outer source. While we don't yet have a self-consistent understanding of the turbulent pinch in tokamaks, in the magnetosphere, the pinch sets the profiles of the radiation belts, energizes the ring current, and is likely the process for the third electron radiation belt newly discovered by the Van Allen Probes. In this talk, I'll review observations of the turbulent pinch in laboratory magnetospheres, including adiabatic drift-resonant transport, low-frequency MHD turbulence, and steady-state, high-beta, "invariant profiles" observed in the superconducting levitated dipole experiment. In the second half of my talk, I want to discuss fusion research that looks beyond ITER. The next frontier in fusion research is economic viability. This means integrating technology advances with advancements in fusion plasma physics to engineer and test fusion confinement concepts that significantly simplify, reduce capital costs, and improve maintainability.

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#### Thursday, May 2, 2013

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: Peter Damiano (PPPL)

#### Title: Electron acceleration in Alfvenic Aurora

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#### Thursday, April 25, 2013

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: G.R. Dennis

Title:Research School of Physics, The Australian National University, Canberra, Australia

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**Friday, April 19, 2013 *** SPECIAL DAY*****

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: Joaquim Loizu (Switzerland)

#### Title: The role of the sheath in the magnetized plasma fluid turbulence

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#### Thursday, March 14, 2013

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: D. del-Castillo-Negrete, Oak Ridge National Laboratory

#### Title: "Nonlocal models of non-diffusive transport"

There is experimental, numerical, and theoretical evidence of the limited applicability of the standard diffusive transport paradigm. In this seminar we discuss a framework for the construction of effective non-local models of non-diffusive transport, and apply the resulting models to problems of interest to magnetically confined fusion plasmas. We begin by discussing signatures of non-diffusive behavior in numerical and experimental data. Following this, we present the statistical foundations of the models in the context of non-Gaussian (Levy type) and non-Markovian stochastic processes, and discuss numerical methods for the solution of non-local transport equations in one and two dimensions. We then discuss several applications of the models including: (i) non-diffusive Lagrangian transport in plasma turbulence; (ii) anomalous confinement time scaling, profile peaking, and up-hill transport; (iii) non-local transport in the presence of internal transport barriers; and (iv) modeling of non-local perturbative experiments in JET and LHD.

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#### Thursday, January 10th

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: Dr. Will Fox, University of New Hampshire

#### Title:Magnetic reconnection and laboratory astrophysics experiments with laser-produced plasmas

Magnetic reconnection has recently been observed and studied in high-energy-density, laser-produced plasmas. These experiments are interesting both for obtaining new data on reconnection, and may also be relevant for inertial fusion, as this "magnetic reconnection" geometry, with multiple, colliding, magnetized plasma bubbles, occurs naturally inside ICF hohlraums. I will present results from our simulation effort modeling previous magnetic reconnection experiments, and present results from set of new experiments that we have initiated on the OMEGA EP facility at the University of Rochester Lab for Laser Energetics (LLE). In these new experiments the seed magnetic field is generated by pulsing current through a pair of external foils using the MIFEDS current generator (Magneto-Inertial Fusion Electrical Discharge System) developed at LLE, allowing experiments with either a "reconnection" geometry with anti-parallel fields or "null" geometry with a collision between nominally unmagnetized plasma.

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#### Thursday, January 3, 2013

#### 10:45 AM, in T169 Theory Seminar Room

#### Speaker: Dr. Robert Leamon

#### Title: On the modulation of the solar activity cycles

We address the origin of the 11-year (quasi-)periodicity of the sunspot cycle by tying it to the significant temporal overlap of activity bands belonging to the 22-year magnetic activity cycle. Using a systematic analysis of ubiquitous coronal brightpoints, and the prevalent magnetic scale on which they form, we are able to observationally demonstrate the entirety of the 22-year magnetic activity cycle. The phases of the sunspot cycle occur as landmarks in the interaction and evolution of the overlapping activity bands in each hemisphere. The work presented establishes significant observational constraints for models of the origins of solar magnetic activity and will, as a result, improve our understanding of the structure of the heliosphere and the modulation of our star¹s radiative and particulate output.