Upcoming Theory Seminar Schedule

12/11 (THU) Dr. Roman Kolesnikov (PPPL)

12/18 (THU) Dr. John Wright (MIT)

Date/Time: December 5, 2008 (Friday) PM 1:30

Location: Theory Seminar Room (T169)

Title: Gyrokinetic particle simulation of CTEM turbulence

Speaker: Dr. Yong Xiao,UC Irvine

Abstract:

The nonlinear physics and transport properties of collisionless trapped electron mode (CTEM) turbulence are studied using the Gyrokinetic Toroidal Code (GTC). Detailed analysis of spatial and time scales in CTEM turbulence found that zonal flow shearing is the dominant mechanism for saturation and transport processes. The electron transport scaling in CTEM has a gradual transition from Bohm to GyroBohm scaling when the machine size is increased while all other dimensionless parameters are kept constant. The electron heat transport demonstrates some non-diffusive features even though its radial profile follows the global profile of turbulence intensity, whereas ion heat transport in CTEM is diffusive and follows the local intensity profile. Results of GTC simulations of turbulent transport of toroidal momentum and energetic particles will also be summarized.

Date/Time: December 3, 2008 (Wednesday) AM 10:45

Location: Theory Seminar Room (T169)

Title: The effect of plasma triangularity on turbulent transport: modelling TCV experiments by linear and non linear gyrokinetic simulations

Speaker: Mr. Alessando Marinoni, Centre de Recherches en Physique des Plasmas, Switzerland

Abstract:

The effect of plasma shape on confinement has been experimentally explored in the TCV tokamak revealing in particular that the core electron heat transport is significantly reduced by a negative triangularity configuration, which could indicate a (partial) stabilization of the microinstabilities at play in a conventional positive triangularity configuration. The present work is a theoretical investigation of the effect exerted by triangularity on plasma turbulence. In particular, it compares the TCV experimental results to non-linear local gyrokinetic simulations performed on the basis of actual MHD equilibrium reconstructions. In both the linear and non-linear phases, negative triangularity is found to have a stabilizing influence on ion-scale instabilities, specifically on the so-called Trapped Electron Mode (TEM) which is the dominant instability in the conditions of the TCV experiments considered; more specifically, the variation of the heat flux with triangularity calculated by the non-linear simulations is in fair agreement with the experimental results. The resulting stabilization is a result of a rather complex modification of the toroidal precessional drift of trapped particles exerted by negative triangularity.

Date/Time: SPECIAL DAY, November 26, 2008(Wednesday) AM 11:00

Location: Theory Seminar Room (T169)

Title: Warm simulations of low-frequency wave propagation in three-dimensional plasmas

Speaker: Nicolas Mellet, Centre de Recherches en Physique des Plasmas, Switzerland

Abstract:

The low-frequency waves are of great interest in plasmas (e.g. ICRF heating, destabilisation of global Alfvén modes by fast ions, etc …). In order to model kinetic effects affecting their propagation, a warm dielectric tensor has been introduced in the full wave code LEMan. This allows modeling Landau damping or the Kinetic Alfvén Wave for example. One of the main difficulties is to compute the parallel wave vector which is in fact related to the distribution function. This formulation requires an inversion of a polynomial matrix. Good results have been obtained in the Alfvén domain in 2D (tokamaks) and 3D configurations like LHD. Nevertheless this method is not adequate for ICRF. An iterative model has then been introduced whereby new parallel wave vector is computed at each step. Good agreement has been found between both methods in the Alfvén range of frequencies. Furthermore fast ion contributions have been added through a new term in the dielectric tensor derived from a bi-Maxwellian distribution function. LEMan has been then linked to the particle code VENUS in a self-consistent way.

Date/Time : SPECIAL DAY, November 25, 2008 (Tuesday) AM 10:45

Location : Theory Seminar Room (T169)

Title: Kinetic theory of geodesic acoustic and related modes.

Speaker: Dr. Andrei Smolyakov, University of Saskatchewan, Saskatoon, Canada

Abstract:
Kinetic theory of geodesic acoustic modes is developed with emphasis on the electromagnetic effects due to electron parallel motion. The higher dispersion effects on GAM are investigated. Several dispersion mechanisms are identified depending on the regime and mode localization. Two new electromagnetic (Alfven type) modes induced by averaged geodesic curvature are identified. It is shown that extended MHD (Grad hydrodynamics) exactly recovers the kinetic dispersion relation for GAMs. The mode coupling and mode polarization are investigated within the extended MHD model. The role of drift effects on GAMs will be discussed.

Date/Time: November 06, 2008 AM 10:45

Location: Theory Seminar Room (T169)

Title: Anisotropic Pressure and Magnetic Perturbations in Tokamaks

Speaker: Dr. Harry Mynick,PPPL

Abstract:
We compute the effect on a tokamak of applying a nonaxisymmetric magnetic perturbation. An equilibrium with scalar pressure yields zero net radial current, and therefore zero torque. Thus, the usual approach, which assumes scalar pressure, is not self-consistent, and masks the close connection which exists between that radial current and the in-surface currents, which provide shielding or amplification of the perturbation. Here, the pressure anisotropy and from this, both the radial and in-surface currents, are analytically computed. The surface-average of the radial current recovers earlier expressions for ripple transport, while the in-surface currents provide an expression for the amount of self-consistent shielding the plasma provides.

Date/Time: October 30, 2008 AM 10:45

Location: Theory Seminar Room (T169)

Title: Basic Physics of Collisionless Magnetic Reconnection

Speaker: Dr. Dmitri Uzdensky *

Department of Astrophysical Sciences, Princeton University

Abstract:
In this talk I will outline the overall qualitative picture of collisionless magnetic reconnection with an emphasis on developing basic physical understanding of this important plasma phenomenon. I will describe the fundamental processes and the main structural elements of the reconnection layer and explain what role each of them plays and how they interact with each other.

* in collaboration with the MRX group and Russell Kulsrud

Date/Time: September 4th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Dr. Yang Ren, University of Wisconsin - Madison

Title: Experimental Study of High Frequency Magnetic Fluctuations in MST

Abstract:

Reversed field pinch plasmas exhibit a broad spectrum of magnetic fluctuations, dominated by low frequency tearing modes (about 10-30 KHz) which are important for magnetic self-organization and transport. However, the origin of higher frequency fluctuations remains unclear. Here we propose that magnetic energy nonlinearly cascades from the
tearing mode fluctuations to the shorter wavelength, high frequency
fluctuations. This is suggested by observations that the high frequency
power always increases or decreases in concert with the tearing mode
amplitudes. The portion of the power spectrum adjacent to the low
frequency tearing modes exhibits a power law similar to expectations
from relevant nonlinear inertial cascade models. However, the power
falls more rapidly at higher frequency, suggesting dissipation is
important in a large portion of the spectrum. Interestingly, these
spectral features resemble magnetic turbulence measurements in space
plasmas. Radial profile measurements using a magnetic probe show that
the high frequency fluctuations are locally resonant modes which have
a much larger perpendicular k than the parallel k. The k-spectra also
exhibit power law structure similar to the frequency spectra.

Date/Time: July 24th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Alexander Pletzer, Tech-X.

Title: FACETS: Bringing whole device modeling capability to the fusion community.

Abstract:

The Framework Architecture for Core-Edge Transport Simulation (FACETS) is a multi-institutional SciDAC effort started in 2007 to deliver whole device tokamak transport modeling capability. Driven by the need to predict the temperature pedestal height, one of fusions most important performance parameter, FACETS aims at providing infrastructure for the self-consistent coupling of core, edge, and wall models. To do so, FACETS integrates the best available components from within the fusion and applied mathematics communities. These are presently GLF23/MMM95 for core turbulent flux calculations, UEDGE for edge transport, PETSc for access to robust and fast nonlinear solvers, with the integration of the neutral beam code NUBEAM being work in progress.

As a by product of FACETS's effort, a new plasma-wall interaction code WALLPSI has been written. Moreover, building on FACETS's infrastructure, we have also developed a core solver, perhaps the first, to scale to tens of processors. The FACETS core solver uses a manager-worker model to farm out the numerically expansive turbulent flux calculations across processes.

FACETS differs from other efforts (e.g. SWIM) by emphasizing implicit coupling between components, in particular between core-edge. Implicit coupling requires a tight coupling approach where each component is concurrently run (on its own communicator) within a single executable. Early core-edge coupling results are presented as well as validation of our core solver against ASTRA.

Date and Time: Friday, July 11 (10:30 AM)

Location: Display Wall Room

Speaker: Dr. John M. Finn, Lawrence Livermore National laboratory

Title: An optimal robust equidistribution method for two-dimensional grid generation based on Monge-Kantorovich optimization

Abstract:

I will present a new cell-area equidistribution method for grid adaptation, based on Monge-Kantorovich optimization (or Monge-Kantorovich optimal transport). The method is based on a rigorous variational principle, in which the L_2 norm of the grid displacement is minimized, constrained locally to produce a prescribed positive-definite cell volume distribution. The procedure involves solving the Monge-Ampere equation,a single, nonlinear, elliptic scalar equation with no free parameters, and with proved solution existence and uniqueness theorems. We show that, for sufficiently small grid displacement, this method also minimizes the mean grid-cell distortion, measured by the trace of the covariant metric tensor. We solve the Monge-Ampere equation numerically with a Jacobian-Free Newton-Krylov method. The ellipticity property of the Monge-Ampere equation allows multigrid preconditioning techniques to be used effectively, delivering a scalable algorithm under grid refinement. Several challenging test cases demonstrate that this method produces optimal grids in which the constraint is satisfied numerically to truncation error. We also compare this method to the well known deformation method [G. Liao and D. Anderson, Appl. Anal., v.44, p.285 (1992)]. We show that the new method achieves the desired equidistributed grid using comparable computational time, but with considerably better grid quality than the deformation method. I will present recent work with more general boundaries in 2D and in a cube in 3D.

Date/Time: July 7th (Thursday) 10:45 am

Location: Theory Seminar Room (T169)

Speaker: Dr. Jian-Zhou Zhu, Center for Nonlinear Studies Lost Alamos National Laboratory

Title: Partial (Incomplete) Thermalization in Turbulence

Abstract:

It is shown that the use of a high power alpha of the Laplacian in the dissipative term of hydrodynamical equations leads asymptotically to truncated inviscid *conservative* dynamics with a finite range of spatial Fourier modes. Those at large wavenumbers thermalize, whereas modes at small wavenumbers obey ordinary viscous dynamics [C. Cichowlas et al. Phys. Rev. Lett. 95, 264502 (2005)]. The energy bottleneck observed for finite alpha may be interpreted as incomplete thermalization. Slaving techniques used to track the stiffness in
solving the model equations will also be discussed.

This talk will include material from a recent paper with Uriel Frisch
et al., http://arxiv.org/abs/0803.4269. If you want to talk with Dr.
Zhu while he visits PPPL on July 7 or 8, please send me an email.

Date/Time: June 5th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Seung-ho Choi, University of Maryland

Title: Observations of magnetic fluctuations in the Maryland Centrifugal Experiment.

Abstract:

Initial results from magnetic probes on the Maryland Centrifugal eXperiment(MCX) [R. F. Ellis et al. Phys. Plasmas 8, 2057 (2001)] provide details of the propagation and azimuthal mode structure of magnetic fluctuations in the edge region. Magnetic coils placed azimuthally along the edge measure changes in the axial magnetic field during the time history of the plasma discharge. The eight evenly spaced coils can resolve azimuthal modes up to m=3. The plasma rotates azimuthally in MCX due to an applied radial electric field. Using a variety of different analysis of the data, it is inferred that the magnetic fluctuations are dominantly convected by the plasma rotation for several rotation periods before significant de-correlation. These findings help to identify the modes at the edge and indicate that there are a few low mode numbers that are dominant during the discharge. Also, the speed of rotation of the modes is found to change dramatically from the High Rotation (HR) state to a low rotation ordinary (O) state, consistent with a corresponding change in the plasma load voltage. The fluctuation spectrum becomes dominated by a single mode after the transition.

Date/Time: May 29th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Jianying Lang, Center for Integrated Plasma Studies, Univ. of Colorado, Boulder, Colorado

Title: Gyrokinetic delta-f particle simulation of Trapped Electron Mode turbulence and Toroidal Alfven Eigenmode.

Abstract:

Mode coupling theory and gyrokinetic turbulence simulation are used to study the nonlinear saturation mechanisms of collisionless trapped electron mode (CTEM) turbulence [1, 2]. Turbulence simulations show that the importance of zonal flow is parameter sensitive, but is well characterized by the ExB shearing rate formula. The importance of zonal flow is found to be sensitive to temperature ratio, magnetic shear and electron temperature gradient. For parameter regimes where zonal flow is unimportant, zonal density (a purely radial density perturbation) is generated and is found to be the dominant saturation mechanism. In fact, CTEM turbulence saturates at physically reasonable levels with or without zonal flow. This is in stark contrast to ion temperature gradient driven turbulence where the zonal flow has an order of magnitude effect on the saturation level. A toroidal mode coupling theory is developed that agrees well with simulation in the initial nonlinear saturation phase (before fully developed turbulence ensues). The theory predicts nonlinear generation of the zonal density and then the feedback and nonlinear saturation of the unstable mode. The spectrum changes from the linear stage to the nonlinear stage is also observed in CTEM turbulence and reported here. Finally, we have utilized GEM to investigate the Toroidal Alfven Eigenmode (TAE) in tokamak plasmas. In simulations, we have observed the TAE frequency falling in the gap, which agrees with the theory prediction [3]. The TAE frequency and mode structure in GEM also agree well with the eigenfrequency and eigenfunction from an eigenmode calculation.

References

[1] J. Lang, Y. Chen and S. E. Parker, Phys. Plasmas 14, 082315 (2007).

[2] J. Lang, S. E. Parker and Y. Chen, Phys. Plasmas 15, 055907 (2008).

[3] G. Y. Fu and J. W. Van Dam, Phys. Fluids B 1, 1949 (1989).

Date/Time: May 22th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Bruce Scott, Max-Planck-IPP, EURATOM Association

Title: Energetics and other Issues in Total-F
Gyrokinetic Computations of Tokamak Turbulence.

Abstract:

A total-f gyrokinetic formulation is given, based closely on the efforts by Hahm, Lee, and Brizard in the 1980s. The Lie transform theory is briefly reviewed to motivate the approximations used in the model. energetic consistency is discussed, with various approximate forms of the generalised potential following corresponding forms in the polarisation equations. Zonal flow energetics is addressed. The computational method, based on Arakawa's Hamiltonian-preserving
discretisation scheme, is described. First results from the axisymmetric versions (one electromagnetic, one electrostatic with adiabatic electrons) are shown, most especially the global Alfv\'en oscillation. Total-f electromagnetic computations must face the concurrent computation of the time dependent MHD background, which is not in exact equilibrium as the turbulence continually disturbs the
Pfirsch-Schlueter current balance. As an example, results from the fluid and gyrofluid models on the self consistent turbulence/MHD interaction are reviewed. Finally, a discussion about the role of flows in the gyrokinetic ordering is provoked.

Date/Time: May 20th (Tuesday) 10:45am

Location: Display Wall Room

Speaker: Dr. Denis Eremin, IPP, Germany

Title: Unstable drift-kinetic Alfven modes with helicity close to the
rotational transform.

Abstract:

An important feature of some modern facilities (such as optimized stellarators) with low shear is that the value of their rotational transform is close to a rational number. If a mode helicity is close to the rotational transform of the background magnetic field, the Alfv\'en continuum frequency can lie in the range of the diamagnetic frequencies of the background electrons. An analytical analysis shows that if the local electron diamagnetic frequency curve crosses the Alfv\'en continuum from above as one proceeds from the axis to the plasma edge at a radial location relatively distant from the axis, a family of unstable drift-kinetic Aflv\'en modes (DKAEs) arises due to the coupling between the drift and Alfv\'en waves having same poloidal numbers. The coupling is mediated by the parallel electric field and the mode is destabilized due to parallel resonances. The mode growth rate can be relatively large and comparable to the real part of the mode frequency. The growth rate peaks at a small radial number, so that the most unstable mode can appear global and be confused with the magnetohydrodynamic (MHD) modes which can occur in this part of the spectrum. The DKAE modes in question are also studied numerically, with an eigenvalue and a PIC gyrokinetic codes. An ongoing generalisation of the gyrokinetic PIC code to a straight helically symmetric stellarator geometry to study the influence of helically trapped particles is discussed.

Date/Time: May 15th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: P. F. Chen, Nanjing University, China

Title: Transition-Region Explosive Events: Reconnection Modulated By p-Mode waves.

Abstract:

Transition-region explosive events (TREEs) have long been proposed as a
consequence of magnetic reconnection. However, several critical issues have not been well addressed, such as the location of the reconnection site, their unusually short lifetime (about one minute), and the recently discovered repetitive behaviour with a period of three to five minutes. In this paper, we perform MHD numerical simulations of magnetic reconnection, where the effect of five-minute solar p-mode oscillations is examined. UV emission lines are synthesised on the basis of numerical results in order to compare with observations directly. It is found that several typical and puzzling features of the TREEs with impulsive bursty behaviour can only be explained if there exist p-mode oscillations and the reconnection site is located in the upper chromosphere at a height range of around 1900 km <h< 2150 km above the solar surface. Furthermore, the lack of proper motions of the high-velocity ejection may be due to a rapid change of temperature along the reconnection ejecta.

Date/Time: May 9th (Friday) 2:00pm

Location: Theory Seminar Room (T169)

Speaker: Zhe Gao, Department of Engineering Physics, Tsinghua University,
Beijing, China

Title: Nonlinear nonresonant forces by radio-frequency waves and current/flow drive.

Abstract:

Low-frequency waves were considered as an attractive mechanism of driving plasma current. However, electron trapping may dramatically reduce the current drive efficiency in the subthermal resonant regime. The possibility of increasing the drive efficiency by helicity injection is proposed and developed. This scheme was also refers as “helicity injection” or “dynamo effect” or “ponderomotive forces” drive. This mechanism was considered to be typically nonresonant because the ponderomotive forces act on the bulk plasma rather than on resonant particles.

However, we examine the nonlinear nonresonant force by applied rf waves. Along the dc magnetic field, for a single particle, the ponderomotive force is only in the direction of asymmetry of the wave field quadric. For cold plasma, the Reynolds stress acting on the fluid element fully counteracts the nonresonant force offered by the quasi-linear EM force. For warm plasma, the collisionless nonresonant force is also cancelled by the nonlinear kinetic pressure force. Therefore, in collisionless plasmas, all the forces are depending on the Landau resonant absorbing and none of nonresonant forces by low frequency waves can drive parallel current. The analysis is also extended to all directions in a general inhomogeneous background magnetic field. Even the local force exists across the field, the nonresonant force in the flux surface will be annihilated after a flux-surface average in toroidal plasmas.

Date/Time: May 8th (Thursday) 2:00pm

Location: Theory Seminar Room (T169)

Speaker: Double header: Igor Kaganovich and Mikhail Dorf, Princeton Plasma Physics Laboratory

Title: PHYSICS OF NEUTRALIZED DRIFT COMPRESSION FOR FOCUSING OF INTENSE BEAM PULSES IN BACKGROUND PLASMA

Abstract:

Neutralized drift compression offers effective way of particle beam focusing and current amplification. The beam intensity can be increased more than 100 times in both radial and longitudinal directions, totaling more than 10,000 times increase in the beam density during this process. Experimental results and challenges will be discussed. Optimal configuration of focusing elements to mitigate time-dependant focal plane will be discussed. Self-electric and magnetic fields can prevent tight ballistic focusing and have to be reduced by supplying neutralizing electrons. We present a survey of the present numerical modeling techniques and theoretical understanding of plasma neutralization of intense particle beams. Theory predicts that there is a sizable enhancement of the self-electric and self-magnetic fields due to the dynamo effect in presence of solenoidal magnetic field. The dynamo effect occurs due to the induced electron rotation, which twists the applied magnetic field and generates a self-magnetic field that is much larger than in the limit with no applied magnetic field. Excitation of helicon or whistler waves by the beam results in very complicated patterns of self-magnetic and electric field.

Date/Time: May 6th (Tuesday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Bruce Scott, Max-Planck-IPP EURATOM Association

Title: Ion Temperature Effects in Electromagnetic, Transcollisional
Gyrokinetic Computations of Edge Turbulence

Abstract:
A delta-f gyrokinetic formulation of edge turbulence is given which
allows weak or strong collisionality and inductivity in the parallel
dynamics. Despite the fluxtube ordering, no assumption of ballooning is
involved. Substantial energy in edge turbulence is resident in a stable
component representing dissipative shear Alfven waves, and this feature
of previous gyrofluid efforts is well captured. Scalings versus beta,
collisionality, magnetic shear, and ion temperature are given.
Differences to the gyrofluid results follow from the fact that most of
the particles are trapped, but for an ion/electron temperature ratio
(tau_i) of unity the results are qualitatively similar.
However, as tau_i is increased towards two, holding the ion temperature
gradient fixed, a sharp reduction of transport of well over an order of
magnitude is observed. Control tests show that this result is kinetic
(the gyrofluid model does not show it), it is nonlinear (linear growth
rate scaling does not show it) and that it does not follow from either
particle trapping or zonal flow phenomena. Gyroaveraging by the finite
ion gyroradius is the sole cause (necessary and sufficient ingredient).
Evidently the details involve nonlinear phase mixing of the responses
such that the spectral range k_perp rho_i > 0.5 is eliminated from the
dynamics. Nonlinear self sustainment in this range is a part of the
overall dynamics, and its elimination results in the reduction of
transport. Although this transport drop with increasing tau_i is
relevant to edge transport barrier formation, a full solution to that
problem will await completion of the ongoing development of total-f
gyrokinetic models which will address the transport and turbulence
combination with full self consistency.

JOINT THEORY/PST SEMINAR

Date/Time: April 24th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Dr. Yu. Petrov, Prairie View A&M, Prairie View, TX

Title: Status of Rotamak program at Prairie View.

Abstract:

Two Rotamak devices, where the steady plasma current is driven by applied Rotating Magnetic Field (RMF), are being operated at Prairie View A&M University: one with a spherical and another with a cylindrical chamber. We discuss latest experiments on plasma reshaping in cylindrical Rotamak. During 40-ms plasma discharge, a pulse current is briefly fed to a magnetic coil located at the midplane (middle coil). The internal magnetic field is scanned with pick-up coils in almost all cross-section of plasma. The use of the middle coil allows switching from D-shape to doublet shape plasma and back. We also compare main results of experiments in spherical and cylindrical chambers, in particular, the structure of self-generated toroidal magnetic field. The evidence is presented that in Rotamak-ST case (with external toroidal magnetic field) the mechanism of RMF penetration is different from that in Rotamak-FRC (no external tor. field); it is more efficient due to whistler wave excitation.

Date/Time: April 17th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Allen Boozer, Columbia University

Title: Perturbed Equilibria.

Abstract:

The response of plasmas to small external magnetic perturbations is an
important issue for both the tokamak and the stellarator programs.
Tokamak applications include (1) Control of magnetic field errors
(errors ten thousand times smaller than the main field can cause
disruptions). (2) Control of edge localized modes (ELMs) and resistive
wall modes (RWMs). (3) Determination of the level and the effects of
the toroidal torques produced by toroidal asymmetries. Stellarator
applications include: (1) Information useful for plasma and coil
design. (2) Specification of allowable construction tolerances. (3)
Assessment of intrinsic islands and magnetic surface quality. Ideal
MHD stability codes, such as DCON and CAS3D, give the fundamental
information, but appropriate postprocessors for these codes are
required for the applications. Jong-Kyu Park has written and
implemented IPEC, which is the required postprocessor for the DCON
code, and Carolin Nuehrenberg has appropriately modified the CAS3D
code. The methods and the results will be discussed with an emphasis
on the areas in which the fundamental understanding has been changed by
these new computational capabilities.

Date/Time: April 11th (Friday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Elena Belova, Princeton Plasma Physics Laboratory

Title: Numerical simulations of NBI driven GAE modes in NSTX.

Abstract:

Hybrid 3D code HYM is used to investigate beam ion effects on MHD modes in a NSTX, aiming at simulations of NSTX shots where chirping frequency GAE/CAE modes have been observed. The thermal plasma is modeled using the MHD equations, and full-orbit delta-f kinetic description is used for the beam ions. The simulations show that for large neutral beam injection velocities and strong anisotropy in the pitch-angle distribution, many Alfven modes are excited. Unstable GAEs modes for 2<n<7 and weakly unstable CAE for n>7 are observed. Scaling of the growth rate of GAE mode with beam ion density is stronger than linear due to significant modification of plasma equilibrium profiles. Profile modification is due to self-consistent beam ion effects, and it has indirect effect on the stability. It was demonstrated that phase velocity of the unstable GAE mode has opposite sign compared to the beam injection velocity, and the resonant particles satisfy Doppler-shifted cyclotron resonant conditions. Dependence of the growth rate on dissipation parameters is studied. Nonlinear simulations show that the GAE instability saturates at low amplitude.

Date/Time: April 10th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: E. A. Startsev, Princeton Plasma Physics Laboratory

Title: Dynamics of electromagnetic two-stream interaction processes during longitudinal and transverse compression of an intense ion beam pulse propagating through background plasma.

Abstract:

To achieve maximum energy density charged particle beam must be compressed radially and longitudinally while its space-charge is neutralized by background plasma. The beam propagating in plasma is subject to electrostatic two-stream instability and electromagnetic Weibel instability. The electrostatic two-stream instability may lead to longitudinal bunching of the beam pulse and eventual longitudinal beam heating. Consequently, this could degrade longitudinal compression of the beam. Similarly, the electromagnetic Weibel instability may cause transverse filamentation of the beam, which may degrade transverse compression. To achieve stronger transverse focusing, it has been proposed to pass the beam through a strong solenoidal magnetic field. The solenoidal magnetic field can extend long distance away from the solenoid into the neutralizing plasma where the beam is compressed longitudinally. In this paper, we review how transverse and longitudinal compression changes the dynamics of two-stream and Weibel instabilities. We also discuss how these instabilities are modified by the solenoidal magnetic field.

Date/Time: April 4th (Friday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Christine Nguyen, C.E.A. Cadarache, France

Title: Low frequency modes triggered by fast particles in Tore-Supra.

Abstract:

The recent observation of MHD modes destabilized by fast supra-thermal particles in Tore-Supra has triggered a theoretical and experimental program designed to model those modes and understand their interaction with fast particle populations. For the success of burning plasmas, this study is of major importance to understand and control the confinement of alpha particles, whose resonant interaction with MHD modes may be detrimental for plasma yields. Two particular types of instabilities are currently investigated in Tore-Supra, following the experimental observations of low-frequency modes: the Beta Alfvén Eigenmode (BAE) and the electron fishbone.
Using a variational formalism coupled to a Fourier mode decomposition, and a gyrokinetic-MHD model for the plasma, the BAE dispersion has been derived and found consistent with F. Zonca et al. earlier derivation in the ballooning representation. In particular, this derivation allows to identify Tore-Supra observed modes as BAEs despite of the ambiguity linked to the BAE/GAM degeneracy, and to calculate a threshold for BAE destabilization. This calculated threshold has been compared with experimental observations, using in particular the Monte-Carlo code PION to access the fast ion distribution function. This comparison validates the consideration of ion Landau damping of the mode sidebands as a main damping process.
Experiments designed to study electron fishbone modes have been conducted, and display a puzzling behavior that provides some insight in the interaction of those modes with the supra-thermal electron distribution. Some modes, observed with the ECE diagnostic are consistent with the traditional characteristics of precessional fishbone modes, and appear to be characterized by frequency jumps associated with a fast electron phase space redistribution. This redistribution of fast electrons is likely to lead to a modification of the q profile, which may have an impact on transport.

Speaker: Prof. Adam Burrows, Princeton University Astrophysics Dept.

Time: March 20, 10:45am

Place: Theory Conference Room

Title:

Abstract:

To address the theoretical supernova explosion problem
with physical fidelity requires the development and use of
sophisticated numerical radiation/hydrodynamic codes that
simulate the multi-dimensional flow in a variety of Mach-number
regimes. Though the latest simulations incorporate rotation,
multi-group radiative transfer, and magnetic fields, they
are not yet general-relativistic, do not solve the Boltzmann
equation in its full multi-D context, and are not fully 3D in space.
One must eventually do the calculations in six-dimensional phase
space (plus time), and such seven-dimensional calculations are
currently beyond reach. Nevertheless, there has been much recent
progress and this progress has been informed by numerical
experiments that will only get better in the next 3-5 years.
In this talk, I will discuss the latest physical ideas in the
theory of the mechanism of core-collapse supernovae and the
variety of results that have emerged from recent massive
computations. Moreover, I will motivate what more may need to be
done to solve in credible fashion the enigma of stellar death and
supernova explosion.

Date/Time: March 13th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Gennady Shvets, University of Texas at Austin

Title: Filamentation of high-current beams in plasmas: physics and applications to Fast Ignition.

Abstract:

The filamentation (a.k.a Weibel) instability of high-current beams propagating in background plasmas is one of the most basic and long-studied collective plasma processes. The dynamics and energetics of its nonlinear saturation is important for both laboratory and astrophysical plasmas. The WI is likely to play an important role in the Fast Ignitor scenario because it may result in the collective energy loss of a relativistic electron beam in both coronal and core plasma regions. Collisionless WI has been suggested as an important mechanism for relativistic collisionless shock formation in gamma ray bursts. In this talk I will describe the theoretical framework predicting the long-term evolution, structure, and coalescence energetics of beam/return current filaments during the Weibel instability of an electron beam in a collisionless plasma is developed. I will emphasize the strongly nonlinear stage of the instability, during which the beam density of filaments is compressed to the background plasma density, and the ambient plasma is fully evacuated. Our analytic and numerical results demonstrate that the beam filaments can carry super-Alfvenic currents by assuming current and density profiles similar to the Hammer-Rostoker equilibrium. This has profound

implications for the long-term evolution of the magnetic field and beam current and explains the long-standing puzzle: why magnetic field energy initially increases, but eventually decreases during the collisionless Weibel instability.

In collaboration with O. Polomarov, A. Sefkow, and I. Kaganovich. Supported by the US DOE grant DE-FG02-05ER54840.

Speaker: Dr. Jeronimo Garcia

Time: March 6 (2:00 PM)

Place: PPPL Display Wall Room

Title: ITER steady-state analysis with the CRONOS suite of codes

Abstract:

Integrated simulations with the CRONOS suite of codes, developed at CEA-Cadarache, are used to study the physics involved in the Internal Transport Barrier (ITB) sustainment and to identify the main obstacles for the establishment of a steady-state scenario in ITER. It is shown that any current drive inside the ITB leads to a progressive shrinking and disappearence of the barrier (known as the current misalignment effect) which means that Neutral Beam Current Drive, which is naturally localized in the central part of the plasma, proves to be of little use in these scenarios. In contrast, a pure Radio Frequency scenario is proposed showing that it provides a solution of principle to the current alignment problem. The main feature of this scenario is that there is a strong minimum negative magnetic shear to steadily sustain the ITB for 3000s, below which low performance inductive scenarios are recovered. The actual design of the ECRH/ECCD system in ITER can provide such a negative magnetic shear, leading to a clear dependence of the temperature gradient (with a well defined threshold) on the Pech/ parameter. The threshold obtained can be characterized as a second order phase transition as it has been done previously for the ITB formation of other completely different fusion devices as, e.g. the Large Helical Device (LHD) The extension of these scenarios to the future fusion demonstration commercial reactor (usually called DEMO) will be considered.

Speaker: Dr. Kathy Yelick, NERSC Director

Title: The Future of NERSC

Date: February 25 (Monday) at 3:00 PM

Place: PPPL Display Wall Room

Abstract:

NERSC provides production computing to the DOE Office of Science community
with over 3000 users working on 300 distinct projects. The NERSC systems
include the Cray XT4 system, know as Franklin, which has over 100 Teraflops
of peak performance and a sustained application performance of 19 Teraflops.
In addition, NERSC provides archival and online storage, cluster computing,
and application consulting services. I will highlight some of the science activities performed at NERSC and describe two new strategic directions for the next 10 years. The first significantly expands the support for analysis of large scientific data sets through a combination of hardware, software, and service activities. The second is to re-think high end computing system design from a power-efficiency perspective, which I believe is necessary to reach the target of Exascale computing. I will talk about initial efforts in these two areas, both using an application-driven approach to finding a general solution to these problems.

Date/Time: February 14th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: S. C. Jardin, Princeton Plasma Physics Laboratory

Title: The M3D-C1 Approach to Calculating Two-Fluid Equilibrium, Stability, and Magnetic Reconnection in Magnetized Plasmas

Abstract:

The M3D code [1] has proven itself to be an invaluable tool for the simulation and understanding of global nonlinear phenomena in magnetic fusion confinement devices. However, the structure of M3D is not optimal for computing in regimes where two-fluid (2F) effects dominate, or for times that are very long compared to the Alfven transit time. We have built upon many of the favorable features of the M3D approach to construct the M3D-C1 code [2], which is based on high-order, compact conformal finite elements with C1 continuity on an unstructured adaptive grid. The efficient split-implicit time advance is shown to be closely related to the ideal MHD energy principle, and allows time steps several orders of magnitude in excess of the Courant condition based on the Alfven or whistler waves. The full model consists of 8 3D scalar variables. Nontrivial, energy conserving, subsets of the full equations exist including 2-variable 3D reduced MHD which is a toroidal generalization of [3] and a 4-variable 3D reduced model which is a toroidal generalization of [4]. The structure of the code makes linear calculations exceptionally efficient. Illustrative results in 2F toroidal equilibrium, 3D linear stability and 2F magnetic reconnection are given. Future capabilities including a surrounding resistive wall and a scalable full 3D nonlinear time evolution are discussed.

Speaker: Dr. Scott Klasky

Date and Time: February, 12 (11:00AM)

Place: Display Wall Room (PPPL)

Title: End-to-end computing for petascale simulations.

Abstract: ORNL has embraced leadership-class computing, and has quickly
become one of the leading institutions for the DOE and NSF for large
scale computations. One of the most challenging problems associated with
running on the large computers is dealing with the huge amounts of data
that is generated. Researchers are quickly becoming overwhelmed with the
daunting task, not only of running their simulations on 100K processors,
but also of efficiently extracting and transporting the many TB's of
data generated by the simulations, and analyzing this data, and share it
with their colleagues in a timely manner. The impact of these challenges
and the overall time to solution is only growing as computers are
getting faster. In order to help address these challenges we have been
developing a suite of software solutions, which are gaining acceptance
by the largest codes that are part of the DOE open science.

Our suite of software solutions includes new API's (ADIOS) that allow
for both MPI-IO and asynchronous I/O through Remote Direct Memory Access
(RDMA), workflow automation using the Kepler workflow package, fast wide
area data transfers, and dashboards that combine data management,
provenance management, and data analysis for monitoring complex
simulations. In this talk I introduce these solutions and will show how
this technology is being used in several fusion SciDAC projects.

Date/Time: February 7th (Thursday) 10:45am

John A. Krommes
Princeton University, Plasma Physics Laboratory

Location: Theory Seminar Room (T169)

The remarkable similarity between the
scaling of kurtosis with squared skewness
for TORPEX density fluctuations
and sea-surface temperature fluctuations:
Baby steps toward a theory

The striking similarity between certain higher-order statistics of drift-interchange plasma turbulence in the TORPEX device [B. Labit et al., Phys. Rev. Lett. 98, 255002 (2007)] and sea-surface temperature fluctuations [P. Sura and P. D. Sardeshmukh, J. Phys. Oceanogr. (2007), in press] (SS) is described. A successful nonlinear Langevin theory due to SS is reviewed, then generalized to include linear wave propagation; it is shown to make a reasonable prediction for the shape of the kurtosis versus skewness curve for TORPEX. The relevance of these somewhat naive calculations will be discussed, and future research directions will be indicated.

Frank Jenko (from Garching) will give a seminar on

"Nonlinear Gyrokinetic Simulations for the W7-X and NCSX Stellarators"

this Thursday, Jan. 31, at 10 a.m. (earlier than the usual theory
seminar time), in the Theory Seminar Room.

Speaker: Prof. Jinchao Xu
Pennsylvania State University

Date and time: Feb. 1, 2008

Place: Display Wall Room

Title: Nonconforming Finite Elements for High Order
PDE Systems and Relevant Algebraic Solvers

Abstract:

In this talk, I will first discuss a few issues on finite element
methods for high order partial differential equations including
application of conforming elements (such as $C^1$ elements for fourth
order problems), possible danger in reducing a high order PDE into a
system of lower order PDEs, and the design of minimal order
nonconforming elements for 2m-th order PDEs in $R^n$ (for any $n \ge m
$). I will also talk about a few recent results on
optimal and practical solvers for Maxwell equations and Navier-Stokes
equations.

Speaker: Dr. Mark Adams
Columbia University/PPPL

Place: Display Wall Room

Time: January, 25 10:30 AM

Title: Toward optimal multigrid algebraic solvers in magnetohydrodynamics
simulations of fusion plasmas

Abstract:

Magnetohydrodynamic simulations of tokamak fusion plasmas exhibit a
large separation of temporal scales. To overcome the temporal
stiffness associated with the fast compressive and Alfven waves in
single-fluid resistive MHD, we consider the development of optimal
implicit algorithms. We strive to achieve "textbook" multigrid
efficiency in which the set of nonlinear equations is solved to
discretization accuracy at each time step, with a cost equivalent
to a few (less than 10) residual calculations (or work units).
We present results from a few canonical MHD problems: magnetic
reconnection in 2D and in the presence of a strong guide field.

Date/Time: January 24th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Hong Qin, Princeton Plasma Physics Laboratory

Title: Variational Symplectic Integrator for the Guiding Center Motion of Charged Particles for Long Time Simulations in General Magnetic Fields

Abstract:

A variational symplectic integrator for the guiding center motion of charged particles in general magnetic fields is developed for long time simulation studies of magnetized plasmas. Instead of discretizing the differential equations of the guiding center motion, the action of the guiding center motion is discretized and minimized to obtain the iteration rules for advancing the dynamics. The variational symplectic integrator conserves exactly a discrete Lagrangian symplectic structure, and has better numerical properties over long integration time, compared with standard integrators, such as the standard and variable time-step 4th order Runge-Kutta methods. Standard integrators only guarantee the error to be small in each time-step. The errors at different time-steps often accumulate coherently, and result in a large error over a large number of time-steps. The symplectic integrator conserves the symplectic structure exactly, and guarantees that the energy error is bounded by a small number for all the time-steps. Numerical examples with more than 25 million time-steps are given to demonstrate the superiority of the variational symplectic integrator. This significant improvement in long term simulation capability of gyrokinetics is a direct, otherwise-impossible result of the geometric formulation of the gyrokinetic theory using the modern language of differential geometry [H. Qin, et al, Physics of Plasmas 14, 056110 (2007)].

Date/Time: January 10th (Thursday) 10:45am

Location: Theory Seminar Room (T169)

Speaker: Leonid E. Zakharov, Princeton Plasma Physics Laboratory

Title: The kink mode during the disruptions in tokamaks

Abstract:

The talk explains the locked $m/n=1/1$ kink mode during the vertical
disruption event when the plasma has an electrical contact with the
plasma facing conducting surfaces. It is shown that the kink
perturbation can be in equilibrium state even with a stable safety
factor $q > 1$, if the halo currents, excited by the kink mode, can
flow through the conducting structure. This suggests a new explanation
of the toroidal asymmetry in magnetic measurements and so-called
sideway forces on the in-vessel components
during the disruption event.