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Mar 2013

Volume 20, Issue 3, Articles (03xxxx)

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Phys. Plasmas 20, 032106 (2013); http://dx.doi.org/10.1063/1.4794320 (10 pages)

M. Raghunathan and R. Ganesh
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back to top Magnetically Confined Plasmas, Heating, Confinement

Collisional damping of the geodesic acoustic mode

Zhe Gao

Phys. Plasmas 20, 032501 (2013); http://dx.doi.org/10.1063/1.4794339 (4 pages) | Cited 1 time

Online Publication Date: 1 March 2013

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The frequency and damping rate of the geodesic acoustic mode (GAM) is revisited by using a gyrokinetic model with a number-conserving Krook collision operator. It is found that the damping rate of the GAM is non-monotonic as the collision rate increases. At low ion collision rate, the damping rate increases linearly with the collision rate; while as the ion collision rate is higher than vti/R, where vti and R are the ion thermal velocity and major radius, the damping rate decays with an increasing collision rate. At the same time, as the collision rate increases, the GAM frequency decreases from the (7/4+τ)vti/R to (1+τ)vti/R, where τ is the ratio of electron temperature to ion temperature.
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52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Fast wave stabilization/destabilization of drift waves in a plasma

Pawan Kumar and V. K. Tripathi

Phys. Plasmas 20, 032502 (2013); http://dx.doi.org/10.1063/1.4794341 (5 pages)

Online Publication Date: 1 March 2013

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Four wave-nonlinear coupling of a large amplitude whistler with low frequency drift wave and whistler wave sidebands is examined. The pump and whistler sidebands exert a low frequency ponderomotive force on electrons introducing a frequency shift in the drift wave. For whistler pump propagating along the ambient magnetic field Bsmath with wave number math0, drift waves of wave number math = math⊥+k||math see an upward frequency shift when k⊥2/k02>4k||/k0 and are stabilized once the whistler power exceeds a threshold value. The drift waves of low transverse wavelength tend to be destabilized by the nonlinear coupling. Oblique propagating whistler pump with transverse wave vector parallel to math⊥ is also effective but with reduced effectiveness.
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52.35.Kt Drift waves
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)

The effect of emissive biased limiter on the magnetohydrodynamic modes in the IR-T1 tokamak

M. Ghasemloo, M. Ghoranneviss, M. K. Salem, R. Arvin, S. Mohammadi, and A. Nik Mohammadi

Phys. Plasmas 20, 032503 (2013); http://dx.doi.org/10.1063/1.4791658 (7 pages)

Online Publication Date: 1 March 2013

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A moveable emissive biased limiter (EBL) for the investigation of spatial and temporal structure of MHD modes in IR-T1 tokamak, based on mirnov oscillations, was designed and constructed. The biasing has been considered to improve the global confinement by setting up an electric field at the plasma edge. Radial electric field (Er) modifies edge plasma turbulence, plasma rotation, and transport. Mirnov oscillations using singular value decomposition (SVD) and wavelet techniques were analyzed. SVD algorithm has been employed to analyze the frequency and wavenumber harmonics of the MHD fluctuations. The time-resolved frequency component analysis has been performed using wavelets. The EBL was applied to plasma at 10 ms with negative polarity. The results show that after applying EBL, the m = 2 mode is grown, m = 3 mode is suppressed, and Hα radiation is decreased. Furthermore, results of the wavelet analysis of mirnov coil in the time range of 8–12 ms indicate that 1.5 ms after applying EBL, the MHD frequency is reduced from 45 kHz to 25 kHz.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Ra Plasma turbulence
52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties

On the formation of m = 1, n = 1 density snakes

Linda E. Sugiyama

Phys. Plasmas 20, 032504 (2013); http://dx.doi.org/10.1063/1.4793450 (11 pages) | Cited 1 time

Online Publication Date: 4 March 2013

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The m/n = 1/1 helical ion density “snake” located near the q = 1 magnetic surface in a toroidal, magnetically confined plasma arises naturally in resistive MHD, when the plasma density evolves separately from pressure. Nonlinear numerical simulations show that a helical density perturbation applied around q = 1 can form a quasi-steady state over q≳1 with math of opposite average sign to math. Two principal outcomes depend on the magnitude of math/n and the underlying stability of the 1/1 internal kink mode. For a small q<1 central region, a moderate helical density drives a new, slowly growing type of nonlinear 1/1 internal kink inside q<1, with small math and ∇math ≃ ∇(nmath). The hot kink core moves away from, or perpendicular to, the high density region near q ≃ 1, preserving the snake density during a sawtooth crash. The mode resembles the early stage of heavy-impurity-ion snakes in ohmic discharges, including recent observations in Alcator C-Mod. For a larger, more unstable q<1 region, the helical density perturbation drives a conventional 1/1 kink where math aligns with math, leading to a rapid sawtooth crash. The crash redistributes the density to a localized helical concentration inside q≲1, similar to experimentally observed snakes that are initiated by a sawtooth crash.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Fa Tokamaks, spherical tokamaks
52.65.Kj Magnetohydrodynamic and fluid equation
52.25.Vy Impurities in plasmas

Thermal ion effects on kinetic beta-induced Alfvén eigenmodes excited by energetic ions

Longyu Qi, J. Q. Dong, A. Bierwage, Gaimin Lu, and Z. M. Sheng

Phys. Plasmas 20, 032505 (2013); http://dx.doi.org/10.1063/1.4794287 (12 pages)

Online Publication Date: 7 March 2013

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Kinetic beta-induced Alfvén eigenmodes (KBAEs) driven by energetic ions are numerically investigated using revised AWECS code. The thermal ion density and temperature gradients are taken into account. It is found that the growth rate of the KBAEs increases with the thermal ion pressure gradient, and the contributions from the density gradient and temperature gradient of the thermal ions to the enhancement of the instability are comparable. The damping effect of thermal ion dynamics on the modes is also observed.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Fa Tokamaks, spherical tokamaks
52.65.Kj Magnetohydrodynamic and fluid equation
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties

Physics basis of Multi-Mode anomalous transport module

T. Rafiq, A. H. Kritz, J. Weiland, A. Y. Pankin, and L. Luo

Phys. Plasmas 20, 032506 (2013); http://dx.doi.org/10.1063/1.4794288 (13 pages) | Cited 1 time

Online Publication Date: 7 March 2013

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The derivation of Multi-Mode anomalous transport module version 8.1 (MMM8.1) is presented. The MMM8.1 module is advanced, relative to MMM7.1, by the inclusion of peeling modes, dependence of turbulence correlation length on flow shear, electromagnetic effects in the toroidal momentum diffusivity, and the option to compute poloidal momentum diffusivity. The MMM8.1 model includes a model for ion temperature gradient, trapped electron, kinetic ballooning, peeling, collisionless and collision dominated magnetohydrodynamics modes as well as model for electron temperature gradient modes, and a model for drift resistive inertial ballooning modes. In the derivation of the MMM8.1 module, effects of collisions, fast ion and impurity dilution, non-circular flux surfaces, finite beta, and Shafranov shift are included. The MMM8.1 is used to compute thermal, particle, toroidal, and poloidal angular momentum transports. The fluid approach which underlies the derivation of MMM8.1 is expected to reliably predict, on an energy transport time scale, the evolution of temperature, density, and momentum profiles in plasma discharges for a wide range of plasma conditions.
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52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Ra Plasma turbulence
52.55.Fa Tokamaks, spherical tokamaks

Nonideal fishbone instability excited by trapped energetic electrons

Y. Liu (刘宇), Z. T. Wang (王中天), Y. X. Long (龙永兴), J. Q. Dong (董家 齐), and C. J. Tang (唐昌建)

Phys. Plasmas 20, 032507 (2013); http://dx.doi.org/10.1063/1.4794738 (5 pages)

Online Publication Date: 7 March 2013

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It is shown that trapped energetic electrons can resonate with the collisionless m = 1 nonideal kink mode, therefore exciting the nonideal e-fishbone, which would often lead to a drop in soft x-ray emissivity and frequency chirping. The theory predictions agree well with the experimental observations of e-fishbone on HL-2A. It is also found that the effects of MHD energy of background plasma might be the reason for the observed phenomena: frequency chirping up and down, and V-font-style sweeping.
Show PACS
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Fa Tokamaks, spherical tokamaks
52.55.Tn Ideal and resistive MHD modes; kinetic modes

The excitation of geodesic acoustic mode flows by a resonant magnetic field and by resonant heating

Robert G. Kleva and A. B. Hassam

Phys. Plasmas 20, 032508 (2013); http://dx.doi.org/10.1063/1.4794837 (6 pages)

Online Publication Date: 12 March 2013

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Toroidal magnetohydrodynamic (MHD) simulations demonstrate that sheared poloidal flows in tokamaks can be generated by the resonant excitation of the geodesic acoustic mode (GAM). Poloidal flows are generated by two resonant excitation methods: oscillating currents in an external coil and an oscillating heat source. The coil current and the heat source oscillate in time at the local GAM frequency. The sheared poloidal flow generated by the excitation of the GAM may be useful for the suppression of plasma instabilities.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.50.Dg Plasma sources

The infinite interface limit of multiple-region relaxed magnetohydrodynamics

G. R. Dennis, S. R. Hudson, R. L. Dewar, and M. J. Hole

Phys. Plasmas 20, 032509 (2013); http://dx.doi.org/10.1063/1.4795739 (6 pages)

Online Publication Date: 15 March 2013

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We show the stepped-pressure equilibria that are obtained from a generalization of Taylor relaxation known as multi-region, relaxed magnetohydrodynamics (MRXMHD) are also generalizations of ideal magnetohydrodynamics (ideal MHD). We show this by proving that as the number of plasma regions becomes infinite, MRXMHD reduces to ideal MHD. Numerical convergence studies illustrating this limit are presented.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
02.60.-x Numerical approximation and analysis

Investigation of the transport shortfall in Alcator C-Mod L-mode plasmas

N. T. Howard, A. E. White, M. Greenwald, M. L. Reinke, J. Walk, C. Holland, J. Candy, and T. Görler

Phys. Plasmas 20, 032510 (2013); http://dx.doi.org/10.1063/1.4795301 (5 pages) | Cited 1 time

Online Publication Date: 18 March 2013

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A so-called “transport shortfall,” where ion and electron heat fluxes and turbulence are underpredicted by gyrokinetic codes, has been robustly identified in DIII-D L-mode plasmas for ρ>0.55 [T. L. Rhodes et al., Nucl. Fusion 51(6), 063022 (2011); and C. Holland et al., Phys. Plasmas 16(5), 052301 (2009)]. To probe the existence of a transport shortfall across different tokamaks, a dedicated scan of auxiliary heated L-mode discharges in Alcator C-Mod are studied in detail with nonlinear gyrokinetic simulations for the first time. Two discharges, only differing by the amount of auxiliary heating are investigated using both linear and nonlinear simulation of the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)]. Nonlinear gyrokinetic simulation of the low and high input power discharges reveals a discrepancy between simulation and experiment in only the electron heat flux channel of the low input power discharge. However, both discharges demonstrate excellent agreement in the ion heat flux channel, and the high input power discharge demonstrates simultaneous agreement with experiment in both the electron and ion heat flux channels. A summary of linear and nonlinear gyrokinetic results and a discussion of possible explanations for the agreement/disagreement in each heat flux channel is presented.
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52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
52.55.Fa Tokamaks, spherical tokamaks
52.65.Tt Gyrofluid and gyrokinetic simulations
52.80.-s Electric discharges

Effect of magnetic fluctuations on the confinement and dynamics of runaway electrons in the HT-7 tokamak

R. J. Zhou, L. Q. Hu, E. Z. Li, M. Xu, G. Q. Zhong, L. Q. Xu, and S. Y. Lin

Phys. Plasmas 20, 032511 (2013); http://dx.doi.org/10.1063/1.4795740 (8 pages)

Online Publication Date: 18 March 2013

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The nature of runaway electrons is such that the confinement and dynamics of the electrons can be strongly affected by magnetic fluctuations in plasma. Experimental results in the HT-7 tokamak indicated significant losses of runaway electrons due to magnetic fluctuations, but the loss processes did not only rely on the fluctuation amplitude. Efficient radial runaway transport required that there were no more than small regions of the plasma volume in which there was very low transport of runaways. A radial runaway diffusion coefficient of Dr ≈ 10 m2s-1 was derived for the loss processes, and diffusion coefficient near the resonant magnetic surfaces and shielding factor ϒ = 0.8 were deduced. Test particle equations were used to analyze the effect of magnetic fluctuations on runaway dynamics. It was found that the maximum energy that runaways can gain is very sensitive to the value of αs (i.e., the fraction of plasma volume with reduced transport). αs = (0.28−0.33) was found for the loss processes in the experiment, and maximum runaway energy could be controlled in the range of E = (4 MeV-6 MeV) in this case. Additionally, to control the maximum runaway energy below 5 MeV, the normalized electric field needed to be under a critical value Dα = 6.8, and the amplitude normalized magnetic fluctuations math needed to be at least of the order of math ≈ 3×10−5.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena

Effects of magnetic field on anisotropic temperature relaxation

Chao Dong, Haijun Ren, Huishan Cai, and Ding Li

Phys. Plasmas 20, 032512 (2013); http://dx.doi.org/10.1063/1.4795728 (11 pages)

Online Publication Date: 19 March 2013

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In a strongly magnetized plasma, where the particles' thermal gyro-radii are smaller than the Debye length, the magnetic field greatly affects the plasma's relaxation processes. The expressions for the time rates of change of the electron and ion parallel and perpendicular temperatures are obtained and calculated analytically for small anisotropies through considering binary collisions between charged particles in the presence of a uniform magnetic field by using perturbation theory. Based on these expressions, the effects of the magnetic field on the relaxation of anisotropic electron and ion temperatures due to electron-electron collisions, ion-ion collisions, and electron-ion collisions are investigated. Consequently, the relaxation times of anisotropic electron and ion temperatures to isotropy are calculated. It is shown that electron-ion collisions can affect the relaxation of an anisotropic ion distribution in the strong magnetic field.
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52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Fi Transport properties
52.25.Xz Magnetized plasmas

Traveling wave current drive theory for an arbitrary m-polar configuration

V. N. Duarte, R. A. Clemente, and R. Farengo

Phys. Plasmas 20, 032513 (2013); http://dx.doi.org/10.1063/1.4796089 (10 pages)

Online Publication Date: 21 March 2013

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An extension of the formalism employed to describe current drive in magnetized plasmas by means of traveling magnetic fields (or double-helix configuration) is presented. In all previous theoretical studies, only driving fields with dipolar topology have been employed and the figure of merit of the current drive mechanism has never been analyzed in terms of the dissipation in the power feeding circuit. In this paper, we show how to express the model equations in terms of the current amplitude in the coils, for an arbitrary number of equally spaced coils wound around the plasma column. We present a brief review of the existing theory and a theoretical formulation, valid for an arbitrary m-polar helical symmetry, which removes the above mentioned complications and limitations. In the limit of straight coils, our magnetic field expression agrees exactly with well-established results of the literature for rotating magnetic field current drive. Finally, we present initial numerical results from a recently developed code which consistently compares the steady driven nonlinear Hall currents and steady fields, corresponding to different configurations in terms of the Ohmic dissipation in the helical coils and discuss future perspectives.
Show PACS
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Xz Magnetized plasmas
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Fi Transport properties
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