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Jun 1999

Volume 6, Issue 6, pp. 2319-2643

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Frequency and power response of high-power plasma-filled backward-wave oscillators

D. M. Goebel, E. S. Ponti, R. L. Eisenhart, and R. W. Lemke

Phys. Plasmas 6, 2319 (1999); http://dx.doi.org/10.1063/1.873537 (4 pages) | Cited 4 times

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Backward-wave oscillators (BWO) have long been used as voltage-tunable signal sources, and more recently as high-power microwave (HPM) sources. The frequency at which a BWO oscillates is commonly viewed as being determined by the circuit geometry and the electron beam parameters. However, several techniques used to improve the efficiency and reduce the size of HPM BWOs, such as the Pasotron, cause the frequency to vary in discrete steps with changes in the beam parameters, and a plasma in these device can cause the frequency to chirp (increase significantly) during the pulse. End reflections in low-loss circuit designs and excessive plasma generation in these new source geometries produce nonclassical frequency and power behavior. © 1999 American Institute of Physics.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
52.75.-d Plasma devices
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back to top Basic Plasma Phenomena, Waves, Instabilities

Kinetic modes in a hot magnetized and weakly collisional plasma

S. De Souza-Machado, M. Sarfaty, and F. Skiff

Phys. Plasmas 6, 2323 (1999); http://dx.doi.org/10.1063/1.873504 (9 pages) | Cited 6 times

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Velocity space perturbations associated with low-frequency waves launched in a weakly collisional plasma are shown to consist of a discrete spectrum of modes. Collisions are modeled using an energy and momentum conserving one-dimensional Fokker–Planck operator. The linearized Vlasov–Poisson–Fokker–Planck system of equations is solved by expanding the perturbed ion-distribution function in terms of Hermite polynomials, from which an eigenvalue problem is set up. The eigenvalues and eigenvectors yield the ion acoustic mode that is weakly damped [J. Dougherty, Physics of Fluids 7, 1788 (1964)], as well as a discrete spectrum of kinetic modes. © 1999 American Institute of Physics.
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52.25.Dg Plasma kinetic equations
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Kn Thermodynamics of plasmas
52.20.-j Elementary processes in plasmas

Two-dimensional electron-magnetohydrodynamic instabilities

F. Califano, R. Prandi, F. Pegoraro, and S. V. Bulanov

Phys. Plasmas 6, 2332 (1999); http://dx.doi.org/10.1063/1.873538 (8 pages) | Cited 16 times

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The linear dispersion relation and the spatial structure of high frequency instabilities, with a mixed tearing-bending character is studied. These instabilities are driven by the electron velocity gradient in a collisionless electron plasma moving against a background of immobile neutralizing ions in an inhomogeneous magnetic field. As the angle between the perturbations and the magnetic field lines is varied, perturbations change from the tearing type (parallel propagation) to the bending type (perpendicular propagation). © 1999 American Institute of Physics.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.30.-q Plasma dynamics and flow

Propagation of fast surface waves in an ideal Hall-magnetohydrodynamic plasma slab

Ivan Zhelyazkov and Gottfried Mann

Phys. Plasmas 6, 2340 (1999); http://dx.doi.org/10.1063/1.873505 (9 pages) | Cited 6 times

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The oblique and parallel propagation of fast sausage and kink magnetohydrodynamic (MHD) surface waves in an ideal magnetized plasma slab is studied taking into account the Hall term in the generalized Ohm’s law. It is found that, in the case of incompressible plasmas, the combining action of the Hall effect and the oblique wave propagation makes possible the existence of multivalued solutions to the dispersion relations of both MHD surface modes—some of them, corresponding to positive values of the transverse wave number (ky), undergo a “propagation stop” at specific (numerically found) full wave numbers. It is curious that such multivalued dispersion curves appear even at purely parallel propagation in low-β plasmas (β→0). Another peculiarity of these surface modes (in both cases) is that with the growing of the wave number they can change their nature—from bulk to pseudosurface (or pure surface) and leaky waves. © 1999 American Institute of Physics.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)

Electrostatic modes and instabilities in nonideal dusty plasmas with sheared flows and grain charge fluctuations

N. N. Rao

Phys. Plasmas 6, 2349 (1999); http://dx.doi.org/10.1063/1.873506 (10 pages) | Cited 16 times

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The combined effects of nonideal contributions and grain charge fluctuations on the propagation of low-frequency electrostatic modes, such as the dust-acoustic waves and drift waves, as well as on the excitation of (parallel) Kelvin–Helmholtz instabilities in a dusty inhomogeneous magnetoplasma are investigated. In the low-frequency regime, dust-acoustic waves and drift waves are recovered with modifications due to the nonideal, as well as charge fluctuation contributions. Magnetized dusty plasmas support a dust temperature gradient-driven drift wave, which exists even in the absence of density inhomogeneities. In the dust gyro-frequency regime, the electrostatic dust cyclotron mode is modified by the transverse shear in the field-aligned flow. The other branch of the cyclotron mode becomes unstable when the shear flow exceeds a threshold value, which is determined by the ratio of the parallel to perpendicular component of the wave number. In general, the charge fluctuation leads to an overall decrease in the growth rate of the excited instability. For the Kelvin–Helmholtz configuration, the existence of a temperature gradient-driven instability that occurs when the relative flow speed between adjacent layers exceeds a critical value is shown. The latter is found to be much smaller than the dust-acoustic phase speed which is applicable for the density gradient-driven Kelvin–Helmholtz instability. On the other hand, the effects of the nonideal contributions in the two cases show quantitative behavior which are complementary to each other. For frequencies larger than the charging frequency, there is a net reduction in the growth rate of the instabilities due to the grain charge fluctuations. © 1999 American Institute of Physics.
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52.27.Lw Dusty or complex plasmas; plasma crystals
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Kt Drift waves
52.25.Gj Fluctuation and chaos phenomena
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.30.-q Plasma dynamics and flow
back to top Nonlinear Phenomena, Turbulence, Transport

Statistical theory of turbulent transport (non-Markovian effects)

A. Zagorodny and J. Weiland

Phys. Plasmas 6, 2359 (1999); http://dx.doi.org/10.1063/1.873507 (14 pages) | Cited 15 times

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A transition probability approach is used to study transport caused by linear instabilities in the real (coordinate) and velocity spaces. The general non-Markovian Fokker–Planck treatment is used to derive the diffusion coefficients. The Dupree–Weinstock renormalized diffusion is reproduced in the Markovian limit. For transport in real space, the non-Markovian diffusion coefficient which is part of one of the transport models used in predicting the performance of the International Thermonuclear Experimental Reactor, [D.R. Mikkelsen, G. Bateman, D. Boucher et al., Proceedings of the Sixteenth IAEA Fusion Energy Conference, Yokohama 1998 (IAEA, Vienna, 1999) Paper IAEA-CN-69/ITER P1/08], is reproduced in a more systematic way. © 1999 American Institute of Physics.
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52.35.Ra Plasma turbulence
52.25.Fi Transport properties
52.55.Fa Tokamaks, spherical tokamaks
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Influence of particle trapping on the propagation of ion cyclotron waves

H. Abbasi, N. L. Tsintsadze, and D. D. Tskhakaya

Phys. Plasmas 6, 2373 (1999); http://dx.doi.org/10.1063/1.873508 (7 pages) | Cited 13 times

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The influence of particle trapping on the nonlinear dynamics of ion cyclotron waves is investigated. It is shown that in the case when the frequency of externally applied electromagnetic waves is close to that of the ion cyclotron waves, the nonlinear coupling between the ion cyclotron waves and the resulting longitudinal perturbations is mainly caused by the pondermotive potential of ions. Due to the trapping of particles, the characteristics of the propagation of the stationary ion cyclotron waves, as well as the condition for their existence, change. Under some circumstances there is no possibility for their propagation. Depending on the amplitude, oscillatory and localized shapes are possible. Trapping of particles makes the characteristic length of modulation larger than the case when it is neglected. © 1999 American Institute of Physics.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)

A nonlinear dynamic model of relaxation oscillations in tokamaks

A. Thyagaraja, F. A. Haas, and D. J. Harvey

Phys. Plasmas 6, 2380 (1999); http://dx.doi.org/10.1063/1.873509 (13 pages) | Cited 8 times

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Tokamaks exhibit several types of relaxation oscillations such as sawteeth, fishbones and Edge Localized Modes (ELMs) under appropriate conditions. Several authors have introduced model nonlinear dynamic systems with a small number of degrees of freedom which can illustrate the generic characteristics of such oscillations. In these models, one focuses on physically “relevant” degrees of freedom, without attempting to simulate all the myriad details of the fundamentally nonlinear tokamak phenomena. Such degrees of freedom often involve the plasma macroscopic quantities such as pressure or density and also some measure of the plasma turbulence, which is thought to control transport. In addition, “coherent” modes may be involved in the dynamics of relaxation, as well as radial electric fields, sheared flows, etc. In the present work, an extension of an earlier sawtooth model (which involved only two degrees of freedom) due to the authors is presented. The dynamical consequences of a pressure-driven “coherent” mode, which interacts with the turbulence in a specific manner, are investigated. Varying only the two parameters related to the coherent mode, the bifurcation properties of the system have been studied. These turn out to be remarkably rich and varied and qualitatively similar to the behavior found experimentally in actual tokamaks. The dynamic model presented involves only continuous nonlinearities and is the simplest known to the authors that can yield features such as sawteeth, “compound sawteeth” with partial crashes, “monster” sawteeth, metastability, intermittency, chaos, periodic and “grassy” ELMing in appropriate regions of parameter space. The results suggest that linear stability analysis of systems, while useful in elucidating instability drives, can be misleading in understanding the dynamics of nonlinear systems over time scales much longer than linear growth times and states far from stable equilibria. © 1999 American Institute of Physics.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
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

Transient reconnection caused by the impact and switch-off of a transverse shear flow

T. Chen, Z. X. Liu, and X. X. Zhang

Phys. Plasmas 6, 2393 (1999); http://dx.doi.org/10.1063/1.873510 (8 pages) | Cited 1 time

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It is supposed that local and transient reconnection in the plasma boundary layer can be caused by the impact and switch-off of a single directional transverse shear flow. MHD (magnetohydrodynamic) simulation is used to investigate the reconnection processes in the two cases. It is found that if the inflow is homogeneous, it does not cause reconnection; if the inflow is shearing flow, no matter how great the shear of the flow is, it may cause reconnection either during impacting period or after stop impacting. It is pointed out that the sudden stop of external force may be an important triggering mechanism of energy transformation and reconnection in the plasma boundary layer region. © 1999 American Institute of Physics.
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52.40.Hf Plasma-material interactions; boundary layer effects
52.30.-q Plasma dynamics and flow
52.65.Kj Magnetohydrodynamic and fluid equation

Resistive drift wave turbulence in a three-dimensional geometry

Søren B. Korsholm, Poul K. Michelsen, and Volker Naulin

Phys. Plasmas 6, 2401 (1999); http://dx.doi.org/10.1063/1.873511 (8 pages) | Cited 10 times

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The Hasegawa–Wakatani model describing resistive drift waves is investigated analytically and numerically in a three-dimensional periodic geometry. After an initial growth of the energy the drift waves couple nonlinearly to convective cells, which eventually dominate the system completely. An approach to include more physical boundary conditions to the system is presented. This changes the results of the simulations significantly. © 1999 American Institute of Physics.
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52.35.Ra Plasma turbulence
52.35.Kt Drift waves
52.25.Fi Transport properties
52.65.-y Plasma simulation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.38.Bv Rayleigh scattering; stimulated Brillouin and Raman scattering

Kinetic analysis of the plasma boundary layer in an oblique magnetic field

Th. Daube and K.-U. Riemann

Phys. Plasmas 6, 2409 (1999); http://dx.doi.org/10.1063/1.873512 (9 pages) | Cited 12 times

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A kinetic analysis is presented to study the influence of a strong oblique magnetic field on the plasma boundary layer in front of an absorbing wall. The ion transport is provided by (a) magnetic field lines intersecting the wall and (b) by charge exchange collisions with neutrals. The Debye length λD is small compared with the ion gyroradius ρi and the electrons are Boltzmann distributed. For the limiting case λD→0 and cold neutrals, an efficient method is developed to calculate the self-consistent stationary solution of the ion Boltzmann equation. The resulting ion distribution functions exhibit an involved structure. The kinetic Bohm-criterion is found to be fulfilled marginally. To investigate the stability of the solution, the analysis is supplemented by a particle simulation. It is shown that the involved structure of the stationary ion velocity distribution is smeared out (a) by an ion cyclotron instability (cold gas) and (b) by a finite gas temperature. The potential profiles and ion distributions resulting from the self-consistent kinetic analysis are compared with expectations based on a previous hydrodynamic model. © 1999 American Institute of Physics.
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52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations
52.40.Hf Plasma-material interactions; boundary layer effects
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Numerical computation of collisionless drift wave turbulence

Frank Jenko and Bruce D. Scott

Phys. Plasmas 6, 2418 (1999); http://dx.doi.org/10.1063/1.873513 (7 pages) | Cited 12 times

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Collisionless drift waves are studied by means of nonlinear numerical simulations in a three-dimensional sheared slab geometry. The electron dynamics is described by a drift-kinetic equation, and the ions are treated as a cold fluid. The energy spectra of the turbulent fluctuations and the dependence of the resulting anomalous transport on various dimensionless plasma parameters are investigated. It is shown that this model resolves fundamental contradictions between experimental results and linear drift wave theory, especially the dependence of turbulent transport on radial position but also the scaling with ion mass. © 1999 American Institute of Physics.
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52.35.Ra Plasma turbulence
52.35.Kt Drift waves
52.65.-y Plasma simulation
52.25.Dg Plasma kinetic equations
52.25.Gj Fluctuation and chaos phenomena
52.25.Fi Transport properties
back to top Magnetically Confined Plasmas, Heating, Confinement

Influence of continuous spectrum on ballooning instabilities in plasmas with shear-flow

J. B. Taylor

Phys. Plasmas 6, 2425 (1999); http://dx.doi.org/10.1063/1.873514 (5 pages) | Cited 14 times

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The influence of shear-flow on stability of plasma ballooning modes is important for Tokamak experiments. In a static plasma, the growth rate of ballooning modes is readily determined using the “ballooning transformation,” but this is ineffective for plasmas with flow. One then has only the quasi-static approximation. This gives the growth rate in the limit that shear velocity Ω′→0, but no other information on the effect of shear-flow. Furthermore, it is invalid in typical cases because of the intervention of the stable magnetohydrodynamic continuum. In this paper, a simple model is used to investigate the influence of shear-flow on ballooning modes. This shows that the intervention of the continuum leads to a reduction in the growth rate proportional to ∣Ω′∣ for small Ω′. This is in accord with some numerical simulations—but contrary to the (Ω′)2 variation expected from a perturbation expansion. In fact, since the effect is nonanalytic in Ω′, it cannot be obtained from a perturbation expansion in Ω′ and an alternative formalism is first developed for dealing with this problem. © 1999 American Institute of Physics.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.30.-q Plasma dynamics and flow
52.55.Fa Tokamaks, spherical tokamaks

Effective temperatures, sawtooth mixing, and stochastic diffusion ripple loss of fast H+ minority ions driven by ion cyclotron heating in the Tokamak Fusion Test Reactor

M. P. Petrov, R. Bell, R. V. Budny, N. N. Gorelenkov, S. S. Medley, R. B. White, and S. J. Zweben

Phys. Plasmas 6, 2430 (1999); http://dx.doi.org/10.1063/1.873539 (7 pages) | Cited 11 times

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This paper presents studies of the H+ minority ions driven by Ion Cyclotron Radio Frequency (ICRF) heating in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Phys. Plasmas 5, 1577 (1998)] deuterium plasmas using primarily passive H° flux detection in the energy range of 0.2–1.0 MeV. The measured passive H+ energy spectra are compared with active (Li pellet charge exchange) results. It is shown that in the passive mode the main donors for the neutralization of H+ ions in this energy range are C5+ ions. The measured effective H+ tail temperatures range from 0.15 MeV at an ICRF power of 2 MW to 0.35 MeV at 6 MW. Radial redistribution of ICRF-driven H+ ions was detected when giant sawtooth crashes occurred during the ICRF heating. The redistribution affected ions with energy below 0.7–0.8 MeV. The sawtooth crashes displaces H+ ions outward along the plasma major radius into the stochastic ripple diffusion domain where those ions are lost in ∼10 msec. These observations are consistent with the model of the redistribution of energetic particles developed previously to explain the results of deuterium-tritium (DT) alpha-particle redistribution due to sawtooth oscillations observed in TFTR. The experimental data are also consistent with numerical simulations of H+ stochastic ripple diffusion losses.© 1999 American Institute of Physics.
Show PACS
52.55.Fa Tokamaks, spherical tokamaks
52.25.Kn Thermodynamics of plasmas
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
52.25.Fi Transport properties
52.50.Gj Plasma heating by particle beams

Calculations of wave excitation and dissipation in Tokamak Chauffage Alfvén wave heating experiment in Brazil

G. Amarante-Segundo, A. G. Elfimov, D. W. Ross, R. M. O. Galvão, and I. C. Nascimento

Phys. Plasmas 6, 2437 (1999); http://dx.doi.org/10.1063/1.873515 (6 pages) | Cited 8 times

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The results of numerical calculations of Alfvén wave absorption are discussed for the experimental conditions foreseen for the Tokamak Chauffage Alfvén wave heating experiment in Brazil (TCABR) [Nucl. Fusion 30, 503 (1996)]. In particular, the effect of toroidal mode coupling on the power deposition of Global Alfvén Wave (GAW) eigenmodes is carefully investigated. Resonant absorption of the toroidally coupled sidebands causes a broad power deposition close to the plasma boundary which can surpass the power deposition of the main GAW at the plasma center. However, the wave absorption can be somewhat optimized by a proper choice of mode spectrum. The excitation of a pure mode spectrum centered at the toroidal mode numbers n = −4 and −6 leads to better plasma coupling than a spectrum centered at n = −2. Finally, it is shown that a small population of light impurities in a hydrogen plasma can strongly modify the dispersion of the GAW and the toroidal Alfvén continuum.© 1999 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.50.Gj Plasma heating by particle beams
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Test particle transport in perturbed magnetic fields in tokamaks

M. de Rover, A. M. R. Schilham, A. Montvai, and N. J. Lopes Cardozo

Phys. Plasmas 6, 2443 (1999); http://dx.doi.org/10.1063/1.873516 (9 pages) | Cited 15 times

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Numerical calculations of magnetic field line trajectories in a tokamak are used to investigate the common hypotheses that (i) field lines in a chaotic field make a Gaussian random walk and (ii) that the poloidal component of the magnetic field is uniform in regions with a chaotic magnetic field. Both hypotheses are found invalid in typical tokamak conditions. A test particle transport model in the so-called “collisionless diffusion” limit is presented, based on the field line excursions in numerical simulations. Decorrelation mechanisms that effectively enhance the transport in a stochastic field are investigated. © 1999 American Institute of Physics.
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52.25.Fi Transport properties
52.55.Fa Tokamaks, spherical tokamaks
52.65.-y Plasma simulation
52.35.Ra Plasma turbulence
05.45.-a Nonlinear dynamics and chaos
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion

Thermal instabilities in the edge transport barrier

Weston M. Stacey

Phys. Plasmas 6, 2452 (1999); http://dx.doi.org/10.1063/1.873517 (10 pages) | Cited 18 times

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A linear analysis of the edge transport barrier identifies localized, two-dimensional (radial-perpendicular) thermal instabilities driven by both impurity radiation and atomic (ionization/charge-exchange/elastic scattering) cooling and suppressed by conductive and convective heat transport. These instabilities are stabilized by sufficiently large values of the edge temperature gradient. Numerical calculations indicate that such instabilities would be expected for edge conditions (electron, impurity and neutral atom densities, temperatures, gradient scale lengths) typical of current tokamak experiments. © 1999 American Institute of Physics.
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52.40.Hf Plasma-material interactions; boundary layer effects
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Kn Thermodynamics of plasmas

Global Alfvén eigenmodes in a stellarator with trapped energetic particles

Zh. N. Andrushchenko, O. K. Cheremnykh, and J. W. Edenstrasser

Phys. Plasmas 6, 2462 (1999); http://dx.doi.org/10.1063/1.873518 (10 pages) | Cited 1 time

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The presence of a super-thermal ion component in tokamak as well as in stellarator plasmas, can give rise to Alfvén-type instabilities. In the present paper an analytical investigation of global Alfvén eigenmodes (GAEs) in a stellarator with planar magnetic axis is performed and analytical expressions for the dominant mode numbers are derived. The effect of trapped highly energetic particles on the GAEs is investigated. The frequencies and corresponding growth rates of the discrete GAE spectrum are found for different distributions of the energetic particles. It is shown that for given background conditions, the slowing-down distributions yield the largest values for the growth rates. The inclusion of the highly energetic particles leads to a change of the conditions for the existence of GAEs and in the dominant mode numbers. A shear dependent threshold for the pressure of the trapped energetic particles has been found, above which GAEs cannot be excited. © 1999 American Institute of Physics.
Show PACS
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Jd Magnetic mirrors, gas dynamic traps
52.55.Fa Tokamaks, spherical tokamaks
52.30.-q Plasma dynamics and flow

Correlation among geodesic curvature of the magnetic field lines, plasma rotation and improved confinement regimes in present tokamak experiments

F. Alladio, F. Crisanti, A. Mancuso, P. Micozzi, O. Tudisco, K. H. Burrell, M. S. Chu, L. L. Lao, T. H. Osborne, R. Giannella, P. Lomas, D. O’Brien, V. Riccardo, and B. Schunke

Phys. Plasmas 6, 2472 (1999); http://dx.doi.org/10.1063/1.873519 (14 pages) | Cited 1 time

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In toroidal magnetic confinement configurations all the charged particles align their magnetic drifts with the flux surfaces if, and only if, the lines of force have zero geodesic curvature (omnigeneous configuration). In this condition, the neoclassical transport is zero; unfortunately, standard tokamak devices are far from meeting such a condition. Nevertheless, a minimization of the geodesic curvature of the lines of force in tokamaks minimizes the neoclassical transport, and could as well reduce any other transport terms depending on the noncoincidence between magnetic and drift surfaces. In particular, the geodesic curvature of the lines of force generates both the neoclassical Pfirsch–Schlüter current density j∥PS, as well as Hirshman factor q [Nucl. Fusion 18, 917 (1978)], which increases the moment of inertia of the magnetic configuration. The improved confinement observed in the VH-mode (very high confinement) and in reversed magnetic shear discharges of the DIII-D tokamak [Plasma Physics Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] and the high plasma performances of the Joint European Torus (JET) [Plasma Physics Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 27] can be correlated with the reduction of geodesic curvature. This reduction influences also the absolute value and the profile of the v∧B part of the radial electric field Er, which are invoked by many authors as main ingredients in reducing the anomalous transport in tokamak plasmas. © 1999 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.30.-q Plasma dynamics and flow
52.25.Fi Transport properties

Dynamic response of detached recombining plasmas to plasma heat pulse in a divertor simulator

N. Ohno, M. Tanaka, N. Ezumi, D. Nishijima, S. Takamura, S. I. Krasheninnikov, A. Yu. Pigarov, and J. Park

Phys. Plasmas 6, 2486 (1999); http://dx.doi.org/10.1063/1.873520 (9 pages) | Cited 10 times

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The experiments on the plasma heat pulse to the detached recombining helium plasma associated with the volumetric radiative and three-body recombination (EIR) have been performed in a linear divertor plasma simulator. Detailed observations of the time evolution of plasma parameters and helium Balmer series spectra show that the dynamic response of the detached recombining plasma to the heat pulse depends strongly on the heat transport through energetic electrons generated by the heat pulse. For the detached recombining plasma with a relatively low neutral pressure, it was found that the EIR is not sufficient to suppress an increase of ion flux to the target plate during the pulse. Several key characteristic time scales involved in this system are also analyzed. © 1999 American Institute of Physics.
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52.25.Fi Transport properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

A tearing mode model for cascades of high-frequency modes

K. Hallatschek

Phys. Plasmas 6, 2495 (1999); http://dx.doi.org/10.1063/1.873521 (8 pages) | Cited 2 times

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A model for cascades of high-frequency modes found in the ASDEX Upgrade (Axially Symmetric Divertor Experiment) tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1993 (International Atomic Energy Agency, Vienna, 1994), Vol. 1, p. 127] is described. In a cascade, modes of the form (m,n), m = n+1 are sequentially excited and stabilized. Mode numbers up to 23 can be reached by a domino effect, where each mode excites a mode with m,n increased by one and is extinguished subsequently. Despite the high mode numbers these modes can be interpreted as tearing modes destabilized by a region of very low magnetic shear and high current gradient. Their instability can be shown by a cylindrical estimate, the cascade process has been investigated in a quasilinear model. The onset of the cascade phenomenon is shown to be caused by an inverse resistivity profile during the activity of a (1,1) mode. © 1999 American Institute of Physics.
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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
52.25.Fi Transport properties
52.30.-q Plasma dynamics and flow

The radial electric field in a tokamak with reversed magnetic shear

P. Zhu, W. Horton, and H. Sugama

Phys. Plasmas 6, 2503 (1999); http://dx.doi.org/10.1063/1.873522 (10 pages) | Cited 19 times

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Neoclassical theory with the impurity rotational velocity is used to evaluate the radial electric field, Er, in a tokamak. The result of using the complete matrix method for the deuterium–carbon plasma is compared with a reduced analytic formula for determining Er [Ernst et al., Phys. Plasmas 5, 665 (1998)]. The analytic formula is shown to overestimate the Er magnitude and its gradient. Two transport measures of the effect of the Er shear are compared for the reversed shear and enhanced reversed shear discharges in Tokamak Fusion Test Reactor [Mazzucato et al., Phys. Rev. Lett. 77, 3145 (1996)]. It is shown that the combined Er and magnetic shear measure, Υs, from linear stability theory gives a higher correlation with the observed transition between the two discharges than the vorticity measure ωs from Er shear alone. © 1999 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Vy Impurities in plasmas

External kink modes in a Large Helical Device (LHD) equilibrium with self-consistent bootstrap current

J. L. Johnson, Katsuji Ichiguchi, Yuji Nakamura, Masao Okamoto, Masahiro Wakatani, and Noriyoshi Nakajima

Phys. Plasmas 6, 2513 (1999); http://dx.doi.org/10.1063/1.873523 (10 pages) | Cited 9 times

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Magnetohydrodynamic (MHD) stability studies of low-mode-number free-boundary kink modes in a finite-β Large Helical Device (LHD) [A. Ilyoshi, et al. Fusion Technology 17, 169 (1990)] equilibrium series with self-consistent bootstrap current find some serious free-boundary modes. They indicate that care must be taken in the design of high-β experiments. Since the LHD configuration is flexible, with the possibility of introducing or modifying dipole and quadrapole fields, unbalancing the currents in the helical coils, operating with a high-temperature divertor, and changing the collisionality regime by working with different values of temperature and density, paths to high-β operation should exist. Comparison of the experimental results with these theoretical predictions for the studied equilibrium sequence will provide understanding of the MHD stability properties of LHD. © 1999 American Institute of Physics.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Jd Magnetic mirrors, gas dynamic traps
52.25.-b Plasma properties

Avalanche runaway growth rate from a momentum-space orbit analysis

P. B. Parks, M. N. Rosenbluth, and S. V. Putvinski

Phys. Plasmas 6, 2523 (1999); http://dx.doi.org/10.1063/1.873524 (6 pages) | Cited 21 times

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The growth of avalanche runaway electrons, a potentially serious issue for disruptions in high-current tokamaks, was recently studied by Monte Carlo and numerical solutions of the relativistic Fokker–Planck equation. Here a new analytical formulation based on the analysis of the particle trajectories of the knocked out secondary electrons offers insight and yields an analytical expression for the growth rate over the entire parameter range of interest. The growth rates are compared with the numerical simulations and analytical solutions of the Fokker–Planck equation in various limits [M. N. Rosenbluth and S. V. Putvinski, Nucl. Fusion 37, 1355 (1997)], and are found to be in good agreement. © 1999 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.65.Pp Monte Carlo methods
52.27.Ny Relativistic plasmas
52.65.Ff Fokker-Planck and Vlasov equation

One-dimensional full-wave description of plasma emission and absorption in the ion cyclotron range of frequency in tokamaks

David Fraboulet, Alain Bécoulet, and Frédéric Nguyen

Phys. Plasmas 6, 2529 (1999); http://dx.doi.org/10.1063/1.873525 (15 pages)

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To maintain the ignition state in a tokamak fusion reactor, a control must be performed on the population of alpha-products, and this implies the ability to diagnose those α-particles. It is studied here whether the detection of emission radiated in the ion cyclotron range of frequency by a reactor plasma can provide useful information concerning fusion products, especially concerning their density profile. It is shown that the detection of the radiation emitted by the fast alpha particules along their cyclotron motion can give access to moments of their distribution function. This requires one to compute the phase of the emitted field, using a full-wave approach. Such a technique allows one to set in a convenient way the inverse problem of the determination of the emitting α-particles distribution through the radiation detection. A brief analysis of the expected situation in a reactor-relevant plasma is given. In parallel, the one-dimensional full-wave code developed in this frame is also useful for studying the physics of fast wave plasma heating. It enables one to take into account the mode conversion of the fast wave into the ion Bernstein wave that appears near each ion cyclotron resonance. Results show that higher order terms may significantly alter the energy partitioning, in hot plasma cases involving mode conversion heating and/or ion cyclotron high harmonics heating. © 1999 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.70.Gw Radio-frequency and microwave measurements
52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.50.Gj Plasma heating by particle beams
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
28.52.Cx Fueling, heating and ignition
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