Resistive wall mode stabilization by slow plasma rotation in DIII-D tokamak discharges with balances neurtral beam injection

E. J. Strait, A. M. Garofalo, G. L. Jackson, M. Okabayashi, H. Reimerdes, M. S. Chu, R. Fitzpatrick, R. J. Groebner, Y. In, R. J. LaHaye, M. J. Lanctot, Y. Q. Liu, G. A. Navratil, W. M. Solomon, H. Takahashi


Recent experiments in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] show that the resistive wall mode (RWM) can be stabilized by smaller values of plasma rotation than previously reported. Stable discharges have been observed with beta up to 1.4 times the no-wall kink stability limit and ion rotation velocity (measured from CVI emission) less than 0.3% of the Alfvén speed at all integer rational surfaces, in contrast with previous DIII-D experiments that indicated critical values of 0.7%–2.5% of the local Alfvén speed. Preliminary stability calculations for these discharges, using ideal magnetohydrodynamics with a drift-kinetic dissipation model, are consistent with the new experimental results. A key feature of these experiments is that slow plasma rotation was achieved by reducing the neutral beam torque. Earlier experiments with strong neutral beam torque used “magnetic braking” by applied magnetic perturbations to slow the rotation, and resonant effects of these perturbations may have led to a larger effective rotation threshold. In addition, the edge rotation profile may have a critical role in determining the RWM stability of these low-torque plasmas. © 2007 American Institute of Physics


An attempt to detect coronal mass ejections in Lyman-alpha using SOHO SWAN

M.L. Mays, O.C. St. Cyr, E. Quemerais, J.-L. Bertaux, S. Yashiro, R. Howard


In this study, the possibility that coronal mass ejections (CMEs) may be observed in neutral Lyman-α emission was investigated. An observing campaign was initiated for SWAN (Solar Wind ANisotropies), a Lyman-α scanning photometer on board the Solar and Heliospheric Observatory (SOHO) dedicated to monitoring the latitude distribution of the solar wind from its imprints on the interstellar sky background. This was part of SOHO Joint Observing Program (JOP) 159 and was an exploratory investigation as it was not known how, or even if, CMEs interact with the solar wind and interstellar neutral hydrogen at this distance (≈60 and 120 RS). The study addresses the lack of methods for tracking CMEs beyond the field-of-view of current coronagraphs (30 RS). In our first method we used LASCO, white-light coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope also on SOHO, to identify CME candidates which, subjeOI:ct to certain criteria, should have been observable in SWAN. The criteria included SWAN observation time and location, CME position angle, and extrapolated speed. None of the CME candidates that we discuss were identified in the SWAN data. For our second method we analyzed all of the SWAN data for 184 runs of the observing campaign, and this has yielded one candidate CME detection. The candidate CME appears as a dimming of the background Lyman-α intensity representing ≈10% of the original intensity, moving radially away from the Sun. Multiple candidate CMEs observed by LASCO and EIT were found which may have caused this dimming. Here we discuss the campaign, data analysis technique and statistics, and the results. © Springer 2007
DOI: 10.1007/s11207-007-0196-8


Ineraction of scrape-off layer currents with magnetohydrodynamical instabilities in tokamak plasmas

R. Fitzpatrick


A simple theoretical model is developed which describes how current eddies are excited in the scrape-off layer (SOL) of a large aspect-ratio, low- , circular cross- section tokamak by time-varying magnetohydrodynamical instabilities originating from within the plasma. This model is used to study the interaction of SOL currents with tearing modes and resistive wall modes in a typical  tokamak plasma. SOL currents are found to be fairly effective at braking the rotation of tearing modes, and to have a significant destabilizing effect on resistive wall modes.


Nonlinear tearing mode in inhomogeneous plasma: I. Unmagnetized islands

F.L. Waelbroeck


A theory of the nonlinear growth and propagation of magnetic islands in the semi-collisional regime is presented. The theory includes the effects of finite electron temperature gradients and uses a fluid model with cold ions in slab geometry to describe islands that are unmagnetized in the sense that their width is less than ρs, the ion Larmor radius calculated with the electron temperature. The polarization integral and the natural phase velocity are both calculated. It is found that increasing the electron temperature gradient reduces the natural phase velocity below the electron diamagnetic frequency and thus causes the polarization current to become stabilizing. © 2007 Plasma Phys. Controlled Fusion
DOI: 10.1088/0741-3335/49/6/014


Ion acceleration by hot electrons in microclusters

B.N. Breizman, A. Arefiev


A self-consistent analytical description is presented for collisionless expansion of a fully ionized cluster with a two-component electron distribution. The problem is solved for an initial ``water-bag'' distribution of hot electrons with no angular momentum, which reflects the mechanism of electron heating. This distribution evolves in time due to adiabatic cooling of hot electrons. The solution involves a cold core of the cluster, a thin double layer at the cluster edge, and a quasineutral flow with a rarefaction wave. The presented analysis predicts a substantial number of accelerated ions with energies greater than the cutoff energy of the initial distribution of the hot electrons. © 2007 Physics of Plasmas
DOI: 10.1063/1.2747633


Guiding, focusing, and sensing on the subwavelength scale using metallic wire arrays

G. Shvets, S. Trendafilov, J.B. Pendry, A. Sarychev


We show that tapered arrays of thin metallic wires can manipulate electromagnetic fields on the subwavelength spatial scale. Two types of nanoscale imaging applications using terahertz and midinfrared waves are enabled: image magnification and radiation focusing. First, the tapered wire array acts as a multipixel TEM endoscope by capturing an electromagnetic field profile created by deeply subwavelength objects at the endoscope’s tip and magnifying it for observation. Second, the image of a large mask at the endoscope’s base is projected onto a much smaller image at the tip. © Physical Review Letters 2007
DOI: 10.1103/PhysRevLett.99.053903


Effect of interplanetary shocks on the AL and DST indices

M. L. Mays, W. Horton, J. Kozyra, T.H. Zurbuchen, C. Haung, E. Spencer


The question of how much interplanetary shock (IP) events contribute to the geoeffectiveness of solar wind drivers is assessed through numerical experiments using the WINDMI model, a physics-based model of the solar wind-driven magnetosphere-ionosphere system. Analytic fits to solar wind input parameters (BIMF, usw, nsw) allowed shocks and associated shock-sheath plasma to be removed while leaving other features of the solar wind driver undisturbed. Percent changes in WINDMI-derived AL and Dst indices between runs with and without the observed shock and sheath signatures were taken as a measure of its relative contribution to the geoeffectiveness. The major magnetic storms during 15–24 April 2002 and 3–6 October 2000 were selected for this experiment. In both cases, the IP shock and sheath features contributed significantly to the geoeffectiveness of the solar wind driver. The magnetic field compressional jump is important to producing the changes in the AL during these two storm intervals. © 2007 American Geophysical Union
DOI: 10.1029/2007GL029844


Analysis of the 3-7 October 2000 and 15-24 April 2002 geomagnetic storms with an optimized nonlinearn dynamical model

E. Spencer, W. Horton, M.L. Mays, I. Doxas, J. Kozyra


A computationally optimized low-dimensional nonlinear dynamical model of the magnetosphere-ionosphere system called WINDMI is used to analyze two large geomagnetic storm events, 3–7 October 2000 and 15–24 April 2002. These two important storms share common features such as the passage of magnetic clouds, shock events from coronal mass ejections, triggered substorms, and intervals of sawtooth oscillations. The sawtooth oscillations resemble periodic substorms but occur in association with strong or building ring current populations and have injection regions that are unusually close to the Earth and unusually wide in magnetic local times (Henderson et al., 2006; Borovsky et al., 2007). The April 2002 event includes one of the best examples of sawtooth events ever observed. On 18 April 2002, sawtooth oscillations were clearly visible when solar wind conditions (IMF B z , density, pressure) were relatively steady with a slowly varying Dst. In this study, WINDMI is used to model the 3–7 October 2000 and 15–24 April 2002 geomagnetic activity. WINDMI results are evaluated focusing on the sawtooth intervals and the overall prediction of the westward auroral electrojet (AL) index and Dst index. The input to the model is the dynamo driving voltage derived from the fluctuating solar wind plasma and the interplanetary magnetic field measured by the ACE satellite. The output of the model is a field-aligned current proportional to the AL index and the energy stored in the ring current which is proportional to the Dst index. The model parameters are optimized using a genetic algorithm (GA) to obtain solutions that simultaneously have least mean square fit to the AL and Dst indices and also exhibit substorms of period 2–4 hours. The GA optimization results show that the model is able to predict the Dst index reliably and captures the timing and periodicity of the sawtooth signatures in the AL index reasonably well for both storm events. © 2007 American Geophysical Union
DOI: 10.1029/2006JA012019


Erratum; Collisionless magnetic reconnection with arbitrary guide-field

R. Fitzpatrick, F. Porcelli

© 2007 American Institute of Physics


Energy confinement scaling predictions for the stabilized tandem mirror

J. Pratt, W. Horton, H.L. Berk


The absence of toroidal curvature and the relatively weak internal parallel currents in a tandem mirror gives the system favorable stability and transport properties. GAMMA-10 experiments demonstrate that sheared plasma rotation suppresses turbulent radial losses through control of the radial potential profiles. Recent achievements of the GAMMA-10 include 3 keV ion confinement potentials and T e ≥ 800 eV. Total energy confinement times for the GAMMA-10 experiment exceed by an order of magnitude the corresponding empirical confinement times in toroidal devices. At the temperatures achieved in the GAMMA-10, the end loss rate τp ? 100 ms so that radial losses determine τE, as intended in tandem mirror reactor designs. Drift-wave results on radial confinement times developed using Bohm, gyro-Bohm, and electron temperature gradient (ETG) scalings imply that the tandem mirror has a qualitatively different form of drift-wave radial transport from that in toroidal devices. Drift-wave eigenmodes for the GAMMA-10 are analyzed for the fluctuating electrostatic potential and magnetic perturbations. © Springer  2007


Overview of recent progress in the Gamma 10 tandem mirror

T Cho, H Higaki, M Hirata, H Hojo, M Ichimura, K Ishii, K Islam, A Itakura, I Katanuma, J Kohagura, R Minami, Y Nakashima, T Numakura, T Saito, Y Tatematsu, M Yoshikawa, O Watanabe, Y Kubota, T Kobayashi, Y Yamaguchi, H Saimaru, Y Higashizono, Y Miyata, S Kiminami, K Shimizu, M Itou, T Ikuno, A Mase, Y Yasaka, K Sakamoto, M Yoshida, A Kojima, K Ogura, N Nishino, W Horton, T Kariya, T Imai, VP Pastukhov, S Miyoshi


(1) Four-time progress in ion-confining potentials Φc to 3.0 kV in comparison to Φc attained 1992-2002 is achieved in the hot-ion mode (Ti=several keV). A scaling of Φc, which favorably increases with plug electron-cyclotron heating (ECH) powers (PPECH), is obtained. (2) The advance in Φc leads to a finding of remarkable effects of radially sheared electric fields (dEr/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in Φc with increasing nc to ~1019 m-3 with the recovery of Φc with increasing PPECH is obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential Φc, generates a strong Er shear and peaked vorticity with the direction reversal of Err × B sheared flow near the Φc peak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in Te and Ti rises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW, 20 ms) in a standard tandem-mirror operation raises Te0 from 70 to 300 eV together with Ti[perpendicular]0 from 4.5 to 6.1 keV, and Ti//0 from 0.5 to 1.2 keV with p0=95 ms for Φc (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of p0/E0 is observed to be 1.7 for Φc trapped ions (consistent with Pastukhov's theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (ICH~0.3; nlc=4.5×1017 m-2). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (Te=600 eV and Ti=6.6 keV) with azimuthal Er×B sheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation. © 2007 Fusion Science and Technology


Effect of an error field on the stability of the resistive wall mode

R. Fitzpatrick


A simple model of the resistive wall mode (RWM) in a rotating tokamak plasma subject to a static error field is constructed, and then used to investigate RWM stability in a DIII-D-like [J.L. Luxon, Nucl. Fusion 42, 614 (2002)] plasma. An error field as small as 10 G (i.e. about 5X10-4 of the toroidal field) is found to significantly increase the critical plasma rotation frequency needed to stabilize the RWM.  Such an error field also profoundly changes the nature of the RWM onset. At small error-field amplitudes, the RWM switches on gradually as the plasma rotation is gradually reduced.  On the other hand, at large error-field amplitudes, there is a sudden collapse of the plasma rotation as the rotation frequency falls below some critical value.  This collapse is associated with a very rapid switch-on of the RWM. ©2007 American Institute of Physics
DOI: 10.1063/1.2446041


Effect of sheared flow on magnetic islands

F.L. Waelbroeck, R. Fitzpatrick, Daniela Grasso


The effect of sheared flow on a magnetic island is examined. In contrast to the density and temperature gradients which are flattened for sufficiently wide islands, it is found that the velocity gradient persists inside the separatrix whenever the constant-ψ approximation is satisfied. It follows that velocity shear has a negligible effect on island amplitude in that approximation. The effect of the violation of the constant-ψ approximation is explored by using the Kelvin-Stuart family of islands, and it is found that flattening is modest even when the separatrix encloses virtually all the current. ©2007 American Institute of Physics
DOI: 10.1063/1.2434251


Isothermal tokamak

P.J. Catto, R.D. Hazeltine


An isothermal tokamak, Isomak, is investigated to demonstrate that nearly exact, rigidly toroidally rotating Maxwellian solutions exist for both the ions and the electrons. For the ions this Maxwellian solution is valid in the limit in which unlike collisions of the ions with the electrons are weak and the induced electric field unimportant, while for Maxwellian electrons unlike collisions can be retained as long as the friction with the ions is small (electron-ion collision frequency smaller than the electron gyrofrequency). In such cases magnetically confined, exponentially decaying density profiles are allowed, minimizing contact with the wall or limiter. Indeed, the near Maxwellian behavior assures that radial particle and heat fluxes are small. In fact, for specially tailored ion and electron current drives it is possible to maintain the Maxwellians as exact steady state solutions of the full ion and electron kinetic equations. Three reasons to consider an Isomak are its usefulness as an ideal tokamak reference, its possible relevance to lithium-walled tokamaks, and its value in checking codes in the isothermal limit. © 2006 American Institute of Physics
DOI: 10.1063/1.2403090


The influence of the ion polarization current on magnetic island stability in a tokamak plasma

R. Fitzpatrick, F.L. Waelbroeck, F. Militello


The influence of the ion polarization current on the stability of a constant-ψ magnetic island in a tokamak plasma is investigated numerically using a reduced two-fluid model in two-dimensional slab geometry. The polarization current is found to be negligibly small when the island is either too narrow or too wide. However, under certain circumstances, there exists an intermediate regime in which the polarization current is appreciable, and has a stabilizing influence on the island. This effect may account for the metastable nature of neoclassical tearing modes in tokamak plasmas. © 2006 American Institute of Physics
DOI: 10.1063/1.2402914


Synoptic-scale nonlinear stationary magnetized Rossby waves in the ionospheric E-layer

T.D. Kaladze, W. Horton


Magnetized Rossby waves are produced by a dynamo electric field and represent the ionospheric generalization of tropospheric Rossby waves in a rotating atmosphere with a spatially inhomogeneous geomagnetic field. They are described by the modified Charney-Obukhov equation with a Poisson-bracket convective nonlinearity. This type of equation has solutions in the form of synoptic-scale nonlinear solitary dipole vortex structures of 1000–3000 km in diameter. With the use of equivalence conditions, various stationary nonlinear solutions are obtained and investigated analytically. The basic characteristics of stationary vortex structures for magnetized Rossby waves are investigated. © 2006 Springer
DOI: 10.1134/S1063780X06120038


Perturbative analysis of the tearing mode saturation

F. Militello, R.J. Hastie, F. Porcelli


The saturation of the tearing mode in a plasma column is investigated in the framework of the resistive magnetohydrodynamics approximation. In particular, a perturbative procedure is adopted to evaluate the structure of the magnetic island in three relevant physical conditions, depending on the model for the evolution of the resistivity, which may be affected by the growth of the mode. In cylindrical geometry, which is well suited to describe a large-aspect-ratio, low-beta tokamak plasma, the magnetic island is asymmetric with respect to the magnetic surface where reconnection occurs. New relations for the saturated island width ws as a function of the relevant features of the equilibrium current density profile, i.e., its gradient and curvature at the reconnecting surface, are obtained. Finally, equivalent relations are also derived in the slab limit. © 2006 American Institute of Physics
DOI: 10.1063/1.2375036


Exact nonlinear solution for the Hall-Alfvén wave in partially ionized plasmas

V. Krishan, M. Furukawa, S.M. Mahajan


It is shown that the exact nonlinear solution for the Hall–Alfvén waves can be obtained in a uniformly rotating weakly ionized plasma such as those which exist in various types of accretion disks. In addition this piece of work demonstrates
a method of eliminating the inaccuracies embedded in the literature on this subject. © 2006 Cambridge University Press


Equilibrium through self-organization and a stability condition derived from generalized self-organization theory

K. Yoshiomi, T. Toshiki, J.W. Van Dam


The equilibria and stability of the least-dissipated relaxed state are derived from a generalized theory of self-organization for open or closed general nonlinear and dissipative dynamical systems. An application of the general theory is presented for the two-fluid model of plasmas. The concept of selective decay, together with helicity invariance, is analytically shown to  have no connection with relaxed states derived from helicity invariance. © 2006 Cambridge University Press


Doubly negative metamaterials in the near infrared and visible regimes based on thin film nanocomposites

V. Lomakin, Y. Fainman, Y.Urzhumov, G. Shvets

Optics Express, Vol. 14, Issue 23, pp. 11164-11177


An optical metamaterial characterized simultaneously by negative permittivity and permeability, viz. doubly negative metamaterial (DNM), that comprises deeply subwavelength unit cells is introduced. The DNM can operate in the near infrared and visible spectra and can be manufactured using standard nanofabrication methods with compatible materials. The DNM’s unit cell comprise a continuous optically thin metal film sandwiched between two identical optically thin metal strips separated by a small distance form the film. The incorporation of the middle thin metal film avoids limitations of metamaterials comprised of arrays of paired wires/strips/patches to operate for large wavelength / unit cell ratios. A cavity model, which is a modification of the conventional patch antenna cavity model, is developed to elucidate the structure’s electromagnetic properties. A novel procedure for extracting the effective permittivity and permeability is developed for an arbitrary incident angle and those parameters were shown to be nearly angle-independent. Extensions of the presented two dimensional structure to three dimensions by using square patches are straightforward and will enable more isotropic DNMs. © 2006 Optical Society of America


Explanation of the JET n=0 Chirping Mode

H.L Berk, C.J. Boswell, D. Borba, A.C.A. Figueiredo, T. Johnson, M.F.F. Nave, S.D. Pinches, S.E. Sharapov, JET EFDA contributors


Persistent rapid up and down frequency chirping modes with a toroidal mode number of zero (n = 0) are observed in the JET tokamak when energetic ions, in the range of several hundred keV, are created by high field side ion cyclotron resonance frequency heating. Fokker–Planck calculations demonstrate that the heating method enables the formation of an energetically inverted ion distribution which supplies the free energy for the ions to excite a mode related to the geodesic acoustic mode. The large frequency shifts of this mode are attributed to the formation of phase space structures whose frequencies, which are locked to an ion orbit bounce resonance frequency, are forced to continually shift so that energetic particle energy can be released to counterbalance the energy dissipation present in the background plasma. © 2006Nucl. Fusion (www.iop.org/)


Whispering gallery mode microresonators as polarization converters

P. Bianucci, C. R. Fietz, J. W. Robertson, G. Shvets, C-K Shih

Optics Letters, Vol. 32, Issue 15, pp. 2224-2226


A ring resonator coupled to a waveguide can be used as a highly efficient polarization converter when the input is properly polarized. We model this phenomenon and verify the predictions with a demonstration of very efficient polarization conversion (>90%) on a silica microsphere coupled to a tapered optical fiber. © 2007 Optical Society of America


Revisiting linear gyrokinetics to recover ideal magnetohydrodynamics and missing finite Larmor radius effects

L. J. Zheng, M. T. Kotschenreuther, J. W. Van Dam


The linear gyrokinetics theory in the axisymmetric configuration is revisited. It is found that the conventional gyrokinetic theory needs to be repaired in order to recover the linear magnetohydrodynamics from the gyrokinetics and to obtain the finite Larmor radius effect on the magnetohydrodynamic modes in an ordering-consistent manner. Two key inclusions are: (1) the solution of the equilibrium gyrokinetic distribution function is carried out to a sufficiently high order; (2) the gyrophase-dependent part of the perturbed distribution function is kept. The new gyrokinetic theory developed in this paper can be used to extend directly the magnetohydrodynamic stability analysis to the gyrokinetic one without invoking the hybrid kinetic-fluid hypothesis. © 2007 American Institute of Physics
DOI: 10.1063/1.2746811


Nonlinear polarization conversion using microring resonators

C. Fietz, G. Shvets


We present a design of a polarization converter between linear, circular, and elliptic accomplished with an on-chip high-Q dielectric microring resonator. Nonlinear polarization switching can be accomplished at modest input intensities because of the high-intensity compression in the ring. We predict an optical bistability effect making the polarization of the transmitted light dependent on its spectral or intensity history. © 2007 Optical Society of America


Relativistic dynamical bistability and adiabatic excitation of strong plasma waves

O. Polomarov, G. Shvets


Adiabatic evolution of the nonlinear resonantly driven dynamical system generic to a variety of plasma physics problems, including generation of large-amplitude plasma waves in a plasma beat-wave accelerator, is studied. The properties of the resonant Hamiltonian and the dynamics of its phase space for adiabatically varying parameters are considered. It is shown that the system can exhibit bistability and the Hamiltonian of a bistable system always follows the same trajectory for the adiabatically varying driver regardless of whether the system is excited or left quiescent. Descriptions of the bistability, autoresonance, and their possible combination based on the properties of the resonant Hamiltonian are given. ©2007 American Institute of Physics
DOI: 10.1063/1.2710783


Polarization conversion in a silica microsphere

P. Bianucci, C.R. Fietz, J.W. Robertson, G. Shvets, C-K Shih


We experimentally demonstrate controlled polarization-selective phenomena in a whispering gallery mode resonator. We observed efficient (≈ 75%) polarization conversion of light in a silica microsphere coupled to a tapered optical fiber with proper optimization of the polarization of the propagating light. A simple model treating the microsphere as a ring resonator provides a good fit to the observed behavior.
© 2007 Optical Society of America


Computationally efficient description of relativistic electron beam transport in collisionless plasma

O. Polomarov, A.B. Sefkow, I. Kaganovich, G. Shvets


A reduced approach to modeling the electromagnetic Weibel instability and relativistic electron beam transport in collisionless background plasma is developed. Beam electrons are modeled by macroparticles and the background plasma is represented by electron fluid. Conservation of generalized vorticity and quasineutrality of the plasma-beam system are used to simplify the governing equations. The method is suitable for modeling the nonlinear stages of collisionless beam-plasma interaction. A computationally efficient code based on this reduced description is developed and benchmarked against a standard particle-in-cell code. The full-scale two-dimensional numerical simulation of the Weibel instability saturation of a low-current electron beam is presented. Using the present approach, linear growth rates of the Weibel instability are derived for the cold and finite-temperature beams. ©2007 American Institute of Physics
DOI: 10.1063/1.2710812


Shear Flows at the Tokamak Edge and Their Role in Core Rotation and the L-H Transition

A.Y. Aydemir


Pfirsch-Schlüter fluxes in tokamaks are shown to drive strong poloidal and toroidal shear flows that are localized to the edge and scrape-off layer in the presence of temperature gradients and finite bootstrap current in the pedestal. Within a magnetohydrodynamic model, the effect of these flows on core rotation and their role in the magnetic configuration dependence of the power threshold for the low- (L-) to high- (H-)mode transition are discussed. Theoretical predictions based on symmetries of the underlying equations, coupled with computational results, are found to be in general agreement with observations in the Alcator C-Mod tokamak [Phys. Plasmas 12, 056111 (2005)].  © 2007 The American Physical Society


Shear flows at the tokamak edge and their interaction with edge-localized modes

A.Y. Aydemir


Shear flows in the scrape-off layer (SOL) and the edge pedestal region of tokamaks are shown to arise naturally out of transport processes in a magnetohydrodynamic model. In quasi-steady-state conditions, collisional resistivity coupled with a simple bootstrap current model necessarily leads to poloidal and toroidal flows, mainly localized to the edge and SOL. The role of these flows in the grad-B drift direction dependence of the power threshold for the L (low) to H (high) transition, and their effect on core rotation, are discussed. Theoretical predictions based on symmetries of the underlying equations, coupled with computational results, are found to be in agreement with observations in Alcator C-Mod [Phys. Plasmas 12, 056111 (2005)]. The effects of these self-consistent flows on linear peeling/ballooning modes and their nonlinear consequences are also examined. © 2007 American Institute of Physics
DOI: 10.1063/1.2727330


Turbulent impulsive magnetic energy release from electron scale reconnection

W. Horton, J.-H. Kim, F. Militello, M. Ottaviani


Magnetic reconnection may occur as bursts of nonlinear plasma dynamics on the electron collisionless skin length scale de=c/ωpe, during which a large fraction of the magnetic energy is converted to plasma thermal energy and plasma flow energy. An example of such a bursty energy release event is given with a simple set of electron Hall equations. The energization mechanism is the cross-field compression of the electron gas between interacting magnetic islands. The electron energization appears to be consistent with the rapid electron energy flux changes measured by the Cluster spacecraft crossing thin current sheets at −17 RE in the geotail. The analysis is relevant to planned spacecraft missions for measuring electron scale magnetic reconnection events in the geomagnetic tail. © 2007 American Institute of Physics


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