bluuebunny/crossref_metadata_2025 huggingface dataset:
Pressure and conducting wall effects on kink modes in large aspect ratio tokamaks with large current gradients
Authors: Eriksson G.
Published: July 2002

The recently developed analytical theory of external kink modes in large aspect ratio tokamaks with large current gradients is extended to include effects due to locally large pressure gradients and a conducting wall. The results are presented in terms of simple integrals over the equilibrium profiles.
Beam orbit analysis in a wiggler by using a noncanonical perturbation method
Authors: Kishimoto Y. , Tokuda S. , Sakamoto K.
Published: July 2002

In a free-electron laser (FEL) with a long wiggler, an additional focusing field is required to keep beam particles from diverging. An additional long scale transverse motion induced by the focusing field modulates the longitudinal motion and sometimes destroys the axial phase relation between the beam particles and radiation field, even if the beam is successfully transmitted. In order to investigate such a delicate phase relation under a complex wiggler configuration, a systematic perturbation method that employs noncanonical variables and a Lie transformation developed by Cary and Littlejohn [Ann. Phys. 151, 1 (1983)] is presented. The method is applied to a focusing wiggler, which has a parabolically curved pole face. Radial space charge force is taken into account in the analysis and the maximum current that can propagate the wiggler is estimated for an electron beam whose energy is around 1 MeV. The effect of the beam current on the axial phase relation is also investigated and found to be small. The present noncanonical approach is found to be transparent and comprehensive for this kind of problem, compared with the standard canonical perturbation method and applicable to arbitrarily complex wigglers.
A field-reversal mechanism in a reversed-field pinch
Authors: Nagata A. , Sakamoto H. , Sato K. I. , Ashida H. , Amano T.
Published: July 2002

A field-reversal mechanism in a reversed-field pinch (RFP) is studied through a three-dimensional resistive compressible magnetohydrodynamic (MHD) simulation of the single- and multiple-helicity modes. As the magnetic Reynolds number increases, the m=1 fluctuating magnetic field grows exponentially and extends radially, and then the flow begins to form the vortex structure around the core of the plasma. This radial flow acts such as to push out the toroidal magnetic field. As the dynamo electric field induced by this interaction increases near the core of the plasma, the toroidal magnetic field at the wall decreases toward the negative value and the toroidal magnetic field reverses. As a result, it is found that the field reversal is achieved by the single-helicity evolution of the m=1 mode alone, without the (m=0; n≠0) modes, and the interaction of the radial flow and the toroidal magnetic field is the most dominant source for the dynamo action on the field-reversal process.
Kinetic self-organization: Creation of super ion acoustic double layer
Authors: Sato Tetsuya , Takamaru Hisanori ,   the Complexity Simulation Groupa)
Published: July 2002

A ‘‘super’’ ion acoustic double layer is realized through a particle simulation. This double layer is an illustration of self-organization in an open system, where a fresh, constant flux of electrons is continuously supplied from the upstream boundary and disordered (‘‘dirty’’) particles are discharged from the downstream boundary. The entropy production rate is found to be maximized in accordance with creation of normal (weak) and super (strong) double layers and to vanish as they disappear. The amplitude of the potential jump in such super double layers reaches an unexpectedly large magnitude, which is far above the electron thermal energy, and a superthermal electron beam is generated on the downstream side.
Effect of cylindrical curvature on nonlocal electron transport in a plasma
Authors: Bychenkov V. Yu. , Matte J. P. , Johnston T. W.
Published: July 2002

Following earlier analyses for the planar and spherical cases, nonlocal electron radial transport is treated for cylindrical high-Z plasmas. Comparison of the result with earlier ones shows that, for the same parameters, the magnitude of the nonlocal heat flux for the cylindrical case is intermediate between the planar and spherical cases, and somewhat closer to the spherical case.
Analysis of the self-oscillations instability due to the plasma coupling with an emissive hot cathode sheath
Authors: Arnas Capeau C. , Prasad G. , Bachet G. , Doveil F.
Published: July 2002

Experimental results are reported showing that plasma self-oscillations appear when the ion flux arriving at the hot emissive cathode double sheath boundary from a continuous plasma discharge is not high enough to sustain the discharge. Using the stability criterion of a double sheath and the particle balance equations, different conditions for the plasma stability are stated, in good qualitative agreement with experimental results. A nonlinear analysis giving the noninteger correlation dimension in a chaotic self-oscillations regime is also reported, showing that a complete understanding of this instability may be reached by a finite set of nonlinear equations.
Electrostatic fluctuations and related transport in the edge and core plasma of the Keda Tokamak-5C
Authors: Xu Yuhong , Wang Guiding , Zhai Kan , Wang Cheng , Liu Wandong , Yu Chang-Xuan
Published: July 2002

Using the swept double probe and triple probe technique, temperature, density and potential fluctuations have been measured in both the plasma edge and core of the Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190]. The fluctuation-driven particle and energy fluxes were also estimated from the correlations of 〈ñeẼθ〉 and 〈T̃eẼθ〉. Evidence has been found that there exist substantial electron temperature fluctuations of T̃e/Te≂0.4ñe/ne in both regions. At the edge of KT-5C, many of the fluctuation features, such as the velocity shear stabilization of turbulence, the non-Boltzmann relation and the role of fluctuations in the edge confinement, are very similar to those obtained from other tokamaks. In the plasma interior, the fluctuations do obey the simple Boltzmann relationship and the profiles of the normalized fluctuating levels ñe/ne and φ̃p/Te vary roughly as ñe/ne∝ne−3/2 and φ̃p/Te∝ne−1/2Te−1, respectively. In addition, a spiky, radially intermittent structure in the particle and energy fluxes with large levels at the low-q resonances and lower levels in between was found. The experimental results provide some new evidence, particularly in the plasma core, for a better understanding of the basic mechanism of turbulence.
General formulation and exact solution for two-dimensional field-aligned magnetohydrodynamic equilibrium flows
Authors: Hau L.-N.
Published: July 2002

A general formulation is presented for steady field-aligned magnetohydrodynamic (MHD) equilibrium flows with isotropic or gyrotropic pressures. Closure to the anisotropic MHD model is provided by a pair of double-polytropic energy equations, for which double-adiabatic and double-isothermal conditions are special limits of the model. For the latter case, a MHD counterpart of Bernoulli’s equation is derived. The study is then focused on the two-dimensional (∂/∂y=0 but By≠0) problems, for which a generalized Grad–Shafranov equation is developed for field-aligned MHD flow equilibria with isotropic or gyrotropic pressures. The formulation is put in a form that allows self-consistent solutions to be constructed numerically in a way similar to the static case; examples of such MHD equilibria are shown. An asymptotic formulation is also developed for stretched gyrotropic plasma configurations, which, however, is not applicable to two-dimensional planar configurations with regions of weak magnetic field strength, such as the geomagnetic tail.
Compressibility and rotation effects on transport suppression in magnetohydrodynamic turbulence
Authors: Yoshizawa Akira
Published: July 2002

Compressibility and rotation effects on turbulent transports in magnetohydrodynamic (MHD) flows under arbitrary mean field are investigated using a Markovianized two-scale statistical approach. Some new aspects of MHD turbulence are pointed out in close relation to plasma compressibility. Special attention is paid to the turbulent electromotive force, which plays a central role in the generation of magnetic and velocity fluctuations. In addition to plasma rotation, the interaction between compressibility and magnetic fields is shown to bring a few factors suppressing MHD fluctuations and, eventually, density and temperature transports, even in the presence of steep mean density and temperature gradients. This finding is discussed in the context of the turbulence-suppression mechanism in the tokamak’s high-confinement modes.
Anisotropy effects on toroidal ηi stability from collisionless two-fluid theory
Authors: Mondt J. P.
Published: July 2002

Ion temperature and temperature gradient anisotropy effects on the toroidal branch of the local ηi instability are analyzed using collisionless two-fluid theory. For isotropic gradients there is good agreement with the Braginskii model for the thresholds even for anisotropic ion temperature, if one averages over the degrees of freedom. But during rapid parallel or perpendicular heating anisotropy in both Ti and ηi is expected to occur. For parallel heating, upper ηi∥ thresholds several times the lower threshold occur at moderate Ti∥/Ti⊥ for moderate to large εn, with low growth rates in the intermediate regime. For perpendicular heating lower thresholds are found but are readily finite-Larmor-radius stabilized. Comparison with kinetic theory shows a sizable ηi range for which the instability is non-reactive with low growth rate. Growth rates and thresholds tend to agree better with kinetic theory for k√Ti⊥/(miωci2)<0.5 than the Braginskii model. The perturbo-theoretic limitations of both two-fluid theories are delineated.
Flux limiting due to electron impact excitation energy loss
Authors: Catto Peter J. , Krasheninnikov Sergei , Hazeltine R. D.
Published: July 2002

Large radiation losses in the electron energy balance equation due to electron impact excitation can cause the parallel scale length to become small enough that a short mean-free path expansion becomes questionable for the high-speed tail electrons. It then becomes necessary to limit the parallel electron heat flux so that it remains below its free-streaming value nT3/2/m, where n, T, and m are the electron density, temperature, and mass. A Boltzmann inelastic scattering col- lision operator is adopted to investigate whether electron impact excitation can lead to a self-consistent flux limit by depleting the electron tail.
The resistive interchange mode in the presence of equilibrium flow
Authors: Hou L. , Paris R. B. , Wood A. D.
Published: July 2002

The influence of a sub-Alfvénic equilibrium flow along the magnetic field and a weak fluid viscosity on the resistive gravitational interchange mode is investigated in plane slab geometry. With a uniform resistivity, the current layer is subject to both tearing and interchange so that a degree of mode-mixing is inevitable. In contrast to the tearing mode which is destabilized, sub-Alfvénic flow is shown to have a stabilizing effect on the linear resistive interchange mode.
On the cooling of the plasma fireball produced by a laser spark in front of liquids and solids
Authors: Kaganovich D. , Meerson B. , Zigler A. , Cohen C. , Levin J.
Published: July 2002

The results of experimental and theoretical investigation of the late, conductive–advective cooling of the fireball, produced by a laser spark in gas near a surface of condensed matter, are presented. The laser spark was initiated by the focusing of a 0.5 J 8 ns Nd:YAG laser. The fireball cooling rate was studied by using a fast frame camera and monochromatic imaging. A theoretical model is developed to describe the conductive–advective cooling stage. The model is reduced to a single nonlinear temperature diffusion equation. In the spherically symmetric case, a similarity solution found earlier is verified and employed for interpreting the experimental results.
Streaming instability in the grain charge fluctuations
Authors: Das A. C. , Misra A. K. , Goswami K. S.
Published: July 2002

Highly charged massive dust grains present in a dusty plasma may exhibit charge fluctuations in response to certain types of oscillations incorporated in the plasma. In this situation the grain charge becomes a time-dependent and self-consistent dynamic variable. The consequent modifications in the collective properties of a dusty plasma in response to longitudinal space-charge waves are investigated. It is shown this new effect gives rise to purely damped modes, in addition to causing a collisionless damping of the normal modes.
Demonstration of high-performance negative central magnetic shear discharges in the DIII-D tokamak
Authors: Rice B. W. , Burrell K. H. , Lao L. L. , Navratil G. , Stallard B. W. , Strait E. J. , Taylor T. S. , Austin M. E. , Casper T. A. , Chu M. S. , Forest C. B. , Gohil P. , Groebner R. J. , Heidbrink W. W. , Hyatt A. W. , Ikezi H. , La Haye R. J. , Lazarus E. A. , Lin-Liu Y. R. , Mauel M. E. , Meyer W. H. , Rettig C. L. , Schissel D. P. , St. John H. E. , Taylor P. L. , Turnbull A. D. ,   the DIII-D Team
Published: July 2002

Reliable operation of discharges with negative central magnetic shear has led to significant increases in plasma performance and reactivity in both low confinement, L-mode, and high confinement, H-mode, regimes in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. Using neutral beam injection early in the initial current ramp, a large range of negative shear discharges have been produced with durations lasting up to 3.2 s. The total noninductive current (beam plus bootstrap) ranges from 50% to 80% in these discharges. In the region of shear reversal, significant peaking of the toroidal rotation [fφ(0)∼30–60 kHz] and ion temperature [Ti(0)∼15–22 keV] profiles are observed. In high-power discharges with an L-mode edge, peaked density profiles are also observed. Confinement enhancement factors up to H≡τE/τITER-89P∼2.5 with an L-mode edge, and H∼3.3 in an edge localized mode (ELM)-free H mode, are obtained. Transport analysis shows both ion thermal diffusivity and particle diffusivity to be near or below standard neoclassical values in the core. Large pressure peaking in the L mode leads to high disruptivity with βN≡βT/(I/aB)≤2.3, while broader pressure profiles in the H mode gives low disruptivity with βN≤4.2.
Ion heat pulse after a sawtooth crash in the JAERI Fusion Torus-2M tokamak
Authors: Miura Y. , Okano F. , Suzuki N. , Mori M. , Hoshino K. , Takizuka T. ,   JFT-2M Group, Itoh K. , Itoh S.-I.
Published: July 2002

The ion heat pulse after a sawtooth crash is studied by a time-of-flight (TOF) neutral measurement. A rapid change of the bulk ion energy distribution near the edge is observed after a sawtooth crash. The delay time is measured, and the effective measuring position is estimated by a neutral transport code. Then a transient ion thermal conductivity, χiHP, of about (7–18) m2/s is evaluated for the low confinement mode (L-mode) plasma. The simple diffusive model with constant χiHP, however, does not explain the amplitude of the pulse in the ion energy distribution.
Nonlinear theory of the interaction of sound waves with an inhomogeneous magnetized plasma in the resonant slow wave layer
Authors: Ruderman M. S. , Goossens M. , Hollweg J. V.
Published: July 2002

The nonlinear theory of driven magnetohydrodynamic (MHD) waves in the resonant slow wave dissipative layer developed by Ruderman, Hollweg, and Goossens [Phys. Plasmas 4, 75 (1997)] is used to study the interaction of sound waves with a static one-dimensional planar magnetic plasma configuration. This configuration is a nonhomogeneous magnetic slab (region II) sandwiched by a homogeneous magnetic field free plasma (regions I) and a homogeneous magnetic plasma (region III). The equilibrium magnetic field is unidirectional. An incoming sound wave is launched from region I, and the equilibrium quantities and the parameters of the incoming sound wave are chosen in such a way that the wave is evanescent in region III and resonates with a slow local MHD wave in region II. The analysis is restricted to incoming sound waves with wave vectors in the plane determined by the equilibrium magnetic field and the direction of inhomogeneity, so that there is no resonance with a local Alfvén wave. Partial reflection of the incoming sound wave from region II generates an outgoing sound wave in region I. The nonlinearity parameter introduced by Ruderman et al. is assumed to be small, and a regular perturbation analysis is used to determine the wave solution. The present analysis shows that nonlinearity in the resonant slow wave dissipative layer causes the following new effects in comparison with results obtained on the basis of linear theory: (i) higher harmonic contributions are generated in the wave solution in the dissipative layer and also in the outgoing sound wave in addition to the fundamental harmonic. Outside the dissipative layer the amplitude of each harmonic contribution is of a higher order with respect to the small nonlinearity parameter than inside the dissipative layer. This property reflects the fact that nonlinearity is only taken into account in the dissipative layer, so that the higher harmonic contributions to the outgoing wave are only due to the interaction of the external plasma motions with the plasma motions in the dissipative layer; (ii) the coefficient of wave energy absorption and the absolute value of the jump in the normal velocity are decreased.
Rayleigh–Taylor instability evolution in ablatively driven cylindrical implosions
Authors: Hsing W. W. , Barnes Cris W. , Beck J. B. , Hoffman N. M. , Galmiche D. , Richard A. , Edwards J. , Graham P. , Rothman S. , Thomas B.
Published: July 2002

The Rayleigh–Taylor instability is an important limitation in inertial confinement fusion capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects of material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes (e.g., Bell–Plesset growth). Amplitude thresholds for the nonlinear regime are reduced, since the wavelength λ of a mode m decreases with convergence λ∼R/m, where R is the radius. Convergent effects have been investigated using an imploding cylinder driven by x-ray ablation on the NOVA laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)]. By doping sections of the cylinder with opaque materials, in conjunction with x-ray backlighting, the growth and feedthrough of the perturbations from the ablation front to the inner surface of the cylinder for various initial modes and amplitudes from early time through stagnation was measured. Mode coupling of illumination asymmetries with material perturbations is observed, as well as phase reversal of the perturbations from near the ablation front to the inner surface of the cylinder. Perturbation growth is observed due to convergence and compressibility alone, without the effects of acceleration, and scales as ∼1/ρR, where ρ is the mass density. Imaging is performed with an x-ray pinhole camera coupled to a gated microchannel plate detector.
Alpha-driven magnetohydrodynamics (MHD) and MHD-induced alpha loss in the Tokamak Fusion Test Reactor
Authors: Chang Z. , Nazikian R. , Fu G.-Y. , White R. B. , Zweben S. J. , Fredrickson E. D. , Batha S. H. , Bell M. G. , Bell R. E. , Budny R. V. , Bush C. E. , Chen L. , Cheng C. Z. , Darrow D. , LeBlanc B. , Levinton F. M. , Majeski R. P. , Mansfield, D. K. , McGuire K. M. , Park H. K. , Rewoldt G. , Synakowski E. J. , Tang W. M. , Taylor G. , von Goeler S. , Wong K. L. , Zakharov L. ,   TFTR Group
Published: July 2002

Alpha-driven toroidal Alfvén eigenmodes (TAEs) are observed as predicted by theory in the post-neutral beam phase in high central q (safety factor) deuterium–tritium (D–T) plasmas in the Tokamak Fusion Test Reactor (TFTR) [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)]. The mode location, poloidal structure, and the importance of q profile for TAE instability are discussed. So far no alpha particle loss due to these modes was detected due to the small mode amplitude. However, alpha loss induced by kinetic ballooning modes (KBMs) was observed in high-confinement D–T discharges. Particle orbit simulation demonstrates that the wave–particle resonant interaction can explain the observed correlation between the increase in alpha loss and appearance of multiple high-n (n⩾6, n is the toroidal mode number) modes.
International Thermonuclear Experimental Reactor: Physics issues, capabilities and physics program plans
Authors: Wesley John C. ,   ITER Joint Central Team
Published: July 2002

Present status and understanding of the principal plasma-performance determining physics issues that affect the physics design and operational capabilities of the International Thermonuclear Experimental Reactor (ITER) [ITER EDA Agreement and Protocol 2 (International Atomic Energy Agency, Vienna, 1994)] are presented. Emphasis is placed on the five major physics-basis issues—energy confinement, beta limit, density limit, impurity dilution and radiation loss, and the feasibility of obtaining partial-detached divertor operation—that directly affect projections of ITER fusion power and burn duration performance. A summary of these projections is presented and the effect of uncertainties in the physics-basis issues is examined. ITER capabilities for experimental flexibility and plasma-performance optimization are also described, and how these capabilities may enter into the ITER physics program plan is discussed.
Squire’s theorem for the magnetohydrodynamic sheet pinch
Authors: Seehafer N. , Schumacher J.
Published: July 2002

The stability of the quiescent ground state of an incompressible viscous fluid sheet bounded by two parallel planes, with an electrical conductivity varying across the sheet, and driven by an external electric field tangential to the boundaries is considered. It is demonstrated that irrespective of the conductivity profile, as magnetic and kinetic Reynolds numbers (based on the Alfvén velocity) are raised from small values, two-dimensional perturbations become unstable first.
Stimulated scattering of a whistler wave off ion-cyclotron and ion-acoustic modes in a dusty plasma
Authors: Annou R. , Tripathi V. K.
Published: July 2002

A whistler wave propagating through a magnetized dusty plasma undergoes stimulated Brillioun scattering off ion-cyclotron and ion-acoustic modes. The dust has little effect on nonlinear coupling. However, it reduces the growth rate by introducing linear damping on the low-frequency modes.
Time-resolved measurements of secondary Langmuir waves produced by the Langmuir decay instability in a laser-produced plasma
Authors: Labaune C. , Baldis H. A. , Bauer B. S. , Tikhonchuk V. T. , Laval G.
Published: July 2002

Direct observations of secondary Langmuir waves produced by the parametric decay instability of primary Langmuir waves are presented. The measurements have been obtained using Thomson scattering of a short-wavelength probe laser beam and are resolved in time, space, frequency, and wave number. The primary Langmuir waves were driven by stimulated Raman scattering (SRS) of a smoothed laser beam in a preformed plasma. Measurements of the amplitude of the density fluctuations associated with primary and secondary Langmuir waves show that the threshold of the Langmuir decay instability (LDI) is close to the threshold of the Raman instability. This is in agreement with theoretical predictions. However, the ratio of amplitudes of the density fluctuations associated with both secondary and primary Langmuir waves does not agree with existing theories of SRS saturation due to LDI cascading and/or strong Langmuir turbulence in homogeneous plasmas. An explanation based on the interaction beam intensity distribution produced by the random phase plate in the plasma is discussed.
Detuning effect of negative ions on ion-acoustic-decay instabilities of electromagnetic waves
Authors: Cho Guan Sik
Published: July 2002

It is proved analytically in the fluid-Maxwell theory of plasma that negative ions can stabilize the ion-acoustic-decay instability of electromagnetic waves by changing and making off-resonant the electron plasma wave frequency. The number density ratio of negative to positive ions that is needed to suppress the instability scales with the square root of the pump intensity difference from the minimum threshold.
Design of a distributed radiator target for inertial fusion driven from two sides with heavy ion beams
Authors: Tabak Max , Callahan-Miller Debra
Published: July 2002

The status of a distributed radiator heavy-ion target design is described. In integrated calculations this target ignited and produced 390–430 MJ of yield when driven with 5.8–6.5 MJ of 3–4 GeV Pb ions. The target has cylindrical symmetry with disk endplates. The ions uniformly illuminate these endplates in a 5 mm radius spot. Considerations which led to this design are discussed together with some previously unused design features: low-density hohlraum walls in approximate pressure balance with internal low-Z fill materials, radiation symmetry determined by the position of the radiator materials and particle ranges, and early time pressure symmetry possibly influenced by radiation shims. How this target scales to lower input energy or to lower beam power is discussed. Variant designs with more realistic beam focusing strategies are also discussed. Tradeoffs required for targets which accept higher particle energies are described.
Equatorial plasma instability in time-dependent equilibrium
Authors: Basu Bamandas
Published: July 2002

A theoretical model for plasma instability in the equatorial F region of the nighttime ionosphere that takes into account the time-dependent aspect of the ambient plasma, where the density gradient increases with time, and that allows variation of mode amplitudes both along the geomagnetic field line and along the vertical direction is presented. The instability is excited due to the combined effects of the gravity and an eastward electric field in the presence of a density gradient, and the time-dependent density gradient arises because of the E0×B0 upward drift of the plasma and the decreasing (with altitude) recombination rate. The model is used to study the linear evolution of the instability and some results of the analysis are reported. The spatially localized plasma modes, predicted by the model, are consistent with the experimental observations. Due to the time-dependent density gradient, growth of the plasma modes is enhanced over what is obtained when the time dependence is ignored, and this enhancement increases with time.
Nonlinear drift tearing modes in the presence of a resistive wall and field errors
Authors: Finn John M.
Published: July 2002

Nonlinear tearing modes with electron diamagnetism and resistive wall are studied, with parallel compression and particle diffusion. The resistive wall mode is linearly unstable, and may be diamagnetically stabilized. Whenever this mode is unstable, it locks nonlinearly to the wall, saturating as without diamagnetic effects. Two important parameters are the drag coefficient ν [Phys. Plasmas 5, 461 (1998)] and sound speed cs. For cs small, flow cancelling the diamagnetic propagation is spontaneously generated. For ν or cs large, this flow is decreased, and there is slow propagation. For cs large, the pressure becomes nearly constant along the flux surfaces, leading to slower propagation and smaller flow. If the resistive wall mode is stabilized by diamagnetism, there is a critical field error for locking and the hysteretic locking-unlocking bifurcation diagram is asymmetric, i.e., diamagnetic plasma rotation is more effective in unlocking or preventing locking. Also, there are bifurcations to slowly propagating locked states.
Saturation of alpha particle driven instability in Tokamak Fusion Test Reactor
Authors: Gorelenkov N. N. , Chen Y. , White R. B. , Berk H. L.
Published: July 2002

A nonlinear theory of kinetic instabilities near threshold [Berk et al., Plasma Phys. Rep. 23, 842 (1997)] is applied to calculate the saturation level of toroidicity-induced Alfvén eigenmodes (TAE), and to be compared with the predictions of δf method calculations (Y. Chen, Ph.D. thesis, Princeton University, 1998). Good agreement is observed between the predictions of both methods and the predicted saturation levels are comparable to experimentally measured amplitudes of the TAE oscillations in Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)].
Alfvén and fast wave forces, affecting ions in magnetic traps with closed magnetic surfaces
Authors: Tsypin V. S. , Nascimento I. C. , Galvão R. M. O. , Elfimov A. G. , Tendler M. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

General expressions for time- and surface-averaged radio frequency forces, affecting ions in closed toroidal devices, are obtained in this paper. Toroidal effects are included in these forces. These effects can, for example, be important to calculate Alfvén or fast wave forces in stellarators, spheromaks, or for toroidicity induced Alfvén wave eigenmodes (TAE) in axially symmetric tokamaks. The further simplification of obtained expressions should be fulfilled for the proper kind of rf waves and toroidal devices. It is hoped that these rf force expressions can, for example, be useful for the computer simulations of the transport barrier formation by Alfvén and fast waves in toroidal devices.
Stabilization of the external kink and control of the resistive wall mode in tokamaks
Authors: Garofalo A. M. , Turnbull A. D. , Strait E. J. , Austin M. E. , Bialek J. , Chu M. S. , Fredrickson E. , La Haye R. J. , Navratil G. A. , Lao L. L. , Lazarus E. A. , Okabayashi M. , Rice B. W. , Sabbagh S. A. , Scoville J. T. , Taylor T. S. , Walker M. L. ,   The DII-D Team
Published: July 2002

One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-n ideal magnetohydrodynamic instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes (RWMs). Previous experiments have demonstrated that plasmas with a nearby conducting wall can remain stable to the n=1 ideal external kink above the beta limit predicted with the wall at infinity. Recently, extension of the wall stabilized lifetime τL to more than 30 times the resistive wall time constant τw and detailed, reproducible observation of the n=1 RWM have been possible in DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159] plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the q=3 surface decreases below a critical frequency of 1–7 kHz (∼1% of the toroidal Alfvén frequency). The measured mode growth times of 2–8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constant τw. From its onset, the RWM has little or no toroidal rotation (ωmode⩽τw−1≪ωplasma), and rapidly reduces the plasma rotation to zero. These slowly growing RWMs can in principle be destabilized using external coils controlled by a feedback loop. In this paper, the encouraging results from the first open loop experimental tests of active control of the RWM, conducted in DIII-D, are reported.
Reduction of early-time perturbation growth in ablatively driven laser targets using tailored density profiles
Authors: Metzler Nathan , Velikovich Alexander L. , Gardner John H.
Published: July 2002

The effects of tailoring the density profile in a laser target in order to decrease imprinting of mass perturbations due to the long-wavelength modes are investigated analytically and numerically. Inverting the acceleration of the ablation front during the shock transit time could reduce the early-time mass perturbation amplitudes developed in the target after the shock transit. This principle was first suggested for mitigating the Rayleigh–Taylor (RT) instability of imploding Z-pinches [Velikovich et al., Phys. Rev. Lett. 77, 853 (1996); Phys. Plasmas 5, 3377 (1998)]. As the shock wave slows down propagating into higher density layers, the effective gravity near the ablation front has the same direction as the density gradient. This makes the mass perturbations near it oscillate at a higher frequency and at a lower amplitude than they normally would due to the “rocket effect” caused by mass ablation [Sanz, Phys. Rev. Lett. 73, 2700 (1994); Piriz et al., Phys. Plasmas 4, 1117 (1997)]. So, tailoring density profiles instead of using flat densities is demonstrated to reduce the “seed” mass perturbation amplitude at the onset of the exponential RT growth.
Ion sheath expansion for a target voltage with a finite risetime
Authors: Uhm Han S. , Kim Gon H. , Kim Young W. , Kim Gun W. , Han Seung H. , Hong Mun P.
Published: July 2002

Properties of the ion sheath expansion in plasma are investigated for a target voltage with a finite rise time. A theoretical model for ion sheath expansion is developed when the negative target voltage increases linearly. The theory predicts that the sheath expansion is proportional to the square root of time at the beginning and is proportional to the 56 power of time later on. An experimental measurement has been carried out and the measured data are compared with theoretical results. It is shown that the sheath front propagates very fast at the beginning and slows down later, even for continuously rising negative voltage on target.
Poloidal flows and enhanced reverse shear bifurcation in tokamaks
Authors: Srinivasan R. , Avinash K.
Published: July 2002

The effect of poloidal flow shear on transition to enhanced reverse shear (ERS) mode in tokamaks is studied. A model is examined where the flow-fluctuation equilibrium evolves in time as a function of pressure gradients. Under some conditions the transition to the ERS phase occurs via as s-fold catastrophe and is accompanied by a spike in poloidal flow generation where poloidal flow shear is reversed. The nature of the transition is found to depend critically on the ratio of factors controlling the damping and generation of poloidal flow shear. The turbulence level shows a periodic bursting behavior which is suppressed in the ERS phase. The relevance of these results to recent observations from the Tokamak Fusion Test Reactor [R. E. Bell et al., Phys. Rev. Lett. 81, 1429 (1998)] is briefly discussed.
Computer simulations of laser hot spots and implosion symmetry kiniform phase plate experiments on Nova
Authors: Peterson R. R. , Lindman E. L. , Delamater N. D. , Magelssen G. R.
Published: July 2002

LASNEX computer code simulations have been performed for radiation symmetry experiments on the Nova laser with vacuum and gas-filled hohlraum targets [R. L. Kauffman et al., Phys. Plasmas 5, 1927 (1998)]. In previous experiments with unsmoothed laser beams, the symmetry was substantially shifted by deflection of the laser beams. In these experiments, laser beams have been smoothed with Kiniform Phase Plates in an attempt to remove deflection of the beams. The experiments have shown that this smoothing significantly improves the agreement with LASNEX calculations of implosion symmetry. The images of laser produced hot spots on the inside of the hohlraum case have been found to differ from LASNEX calculations, suggesting that some beam deflection or self-focusing may still be present or that emission from interpenetrating plasmas is an important component of the images. The measured neutron yields are in good agreement with simulations for vacuum hohlraums but are far different for gas-filled hohlraums.
Sideband suppression in tapered wiggler free electron lasers including thermal spreads
Authors: Riyopoulos Spilios
Published: July 2002

Sideband growth in tapered wiggler free electron lasers (FELs) is studied in general, and the growth rate dependence on the tapering gradient is given analytically. While earlier work considered the zero energy spread limit with particles at the bottom of the bucket, here full energy spread across the bucket is considered, resulting in different growth scaling. Particles at a given bucket location, parameterized by the action J, go in an out of resonance with a given sideband frequency 2γz2Ω(J;z)=ω±−ωo, as the bounce frequency Ω(J;z) varies along the interaction length z. Therefore, the gain per sideband frequency is reduced relative to a fixed wiggler. Instead of being determined by the fraction particles that stay in resonance, proportional to the inverse shear (dΩ/dJ)−1, here the sideband gain is proportional to the fraction of the interaction length ∝(dΩ/dz)−1 that most particles spent in resonance. Other thing being equal, the sideband gain per frequency interval is reduced as G(ω)≃Go(ω)Lt/Lw, where Lt,Lw are the tapering and wiggler lengths and G0(ω) the gain for untapered wiggler. The maximum growth spreads over a wide frequency band yielding a continuum sideband spectrum.
Control and diagnostic uses of feedback
Authors: Sen A. K.
Published: July 2002

Recent results on multimode feedback control of magnetohydrodynamic (MHD) modes and a variety of diagnostic uses of feedback are summarized. First, is the report on reduction and scaling of transport under feedback. By controlling the fluctuation amplitudes and consequently the transport via feedback, it is found that the scaling of the diffusion coefficient is linear with root-mean-square rms fluctuation level. The scaling appears not to agree with any generic theory. A variety of other diagnostic uses of feedback have been developed. The primary goal is an experimental methodology for the determination of dynamic models of plasma turbulence, both for better transport understanding and more credible feedback controller designs. A specific motivation is to search for a low-order dynamic model, suitable for the convenient study of both transport and feedback. First, the time series analysis method is used for the determination of chaotic attractor dimension of plasma fluctuations. For E×B rotational flute modes it is found to be close to three, indicating that a low-order dynamic model may be adequate for transport prediction and feedback controller design. Second, a new method for direct experimental determination of nonlinear dynamical models of plasma turbulence using feedback has been developed. Specifically, the process begins with a standard three-wave coupling model and introduces a variable feedback gain. The power spectrum, delayed power spectrum, and bispectrum of fluctuations are then experimentally obtained. By varying the feedback gain continuously, an arbitrary number of numerical equations for a fixed number of unknowns can be generated. Their numerical solution yields the linear dispersion, as well as nonlinear coupling coefficients. This method has been successfully applied for E×B rotationally driven flute modes.
Measurement of cross-magnetic-field heat transport due to long-range collisions
Authors: Hollmann E. M. , Anderegg F. , Driscoll C. F.
Published: July 2002

Cross-magnetic-field heat transport in a quiescent pure ion plasma is found to be diffusive and to be dominated by long-range “guiding center” collisions. In these long-range collisions, which occur in plasmas with Debye length λD greater than cyclotron radius rc, particles with impact parameters rc<ρ⩽λD exchange parallel kinetic energy only. The resulting thermal diffusivity χL is independent of magnetic field B and plasma density n. The measured thermal diffusivity χ agrees within a factor of 2 with the long-range prediction χL=0.49nv̄b2λD2 over a range of 1000 in temperature, 50 in density, and 4 in magnetic field. This thermal diffusivity is observed to be up to 100 times larger than classical diffusivity from short-range velocity-scattering collisions. These long-range collisions are typically dominant in unneutralized plasmas, and may also contribute to electron heat transport in neutral plasmas.
Latest progress in steady state plasma research on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade
Authors: Ide S. ,   JT-60 Team
Published: July 2002

The JT-60U [Y. Kusama and the JT-60 team, Phys. Plasmas 6, 1935 (1999)] high performance plasmas, such as high poloidal-beta high-confinement mode (high βp H-mode) and reversed magnetic shear (RS) plasmas, have been optimized towards a regime of steady state operation concerning high confinement, high β and high bootstrap current fraction which are requirements of a steady state fusion plasma. What is essential to keep improved confinement in these plasmas is to maintain the internal and/or the edge transport barriers (ITB and/or ETB). A key issue to achieve non-inductive current drive relevant to a steady state fusion reactor is to increase the fraction of the bootstrap current and match the spatial profile to the optimum. In JT-60U, RS plasmas have been optimized not only for the high performance but also for reactor relevant steady state research. In 1999, as the result of the optimization, the equivalent deuterium–tritium (D–T) fusion gain (QDTeq) of 0.5 was sustained for 0.8 s, which is roughly equal to the energy confinement time, by utilizing feedback control of the stored energy. Furthermore, in a RS plasma with H-mode edge, a confinement enhancement factor of 3.6 was maintained for 2.7 s with a large bootstrap current fraction. In addition to the existing radio frequency and the negative-ion based neutral beam heating systems, the newly installed electron cyclotron range of frequency system extended the improved confinement study to a regime where the electron temperature is higher than the ion temperature. The required power to obtain the internal transport barrier in a RS plasma was found not to be sensitive to the toroidal magnetic field. The regime of improved confinement was extended to higher electron density using argon puffing.
Low collisionality steady state tokamaks
Authors: Weitzner Harold
Published: July 2002

The low collisionality scaling and expansion procedure is used in two cases to characterize a steady state tokamak maintained by a loop voltage. In the first problem, appropriate to the outer regions of a divertor tokamak, the safety factor, Q, is assumed large and the ion flow Mach number is not small. The steady state problem is reduced to an integral equation in the independent variable, the poloidal angle, and the equations which characterize the plasma state functions n, Ti, Te, and Φ are given. The second problem is appropriate in a tokamak core, and it is assumed that Q is not large and the ion flow Mach number is small. Explicit equations are given which characterize the plasma state functions; the bootstrap current and the plasma current are also given. With the results of both problems one can describe a divertor tokamak with large flows near the edge.
Luminescence line shape broadening mechanisms in GaInAs/AlInAs quantum wells
Authors: Welch D. F. , Wicks G. W. , Eastman L. F.
Published: July 2002

GaInAs/AlInAs single quantum wells have been grown lattice matched to InP substrates by molecular beam epitaxy. The quantum well thicknesses ranged from 15 to more than 100 Å. The low-temperature photoluminescence exhibited a monotonically increasing spectral linewidth with decreasing well thickness. Of the several possible broadening mechanisms of the quantum well photoluminescence, two mechanisms were found to dominate: one mechanism for thin wells and one for thick wells. Quantum wells thicker than 50 Å were found to have their low-temperature photoluminescence spectra broadened primarily by a transfer of electrons from the AlInAs cladding layers into the GaInAs quantum well. Wells thinner than 50 Å had their photoluminescence broadened mainly by interface roughness.
Direct method for the investigation of nonradiative recombination in semiconductors
Authors: Dersch Helmut , Amer Nabil M.
Published: July 2002

The dynamics of nonradiative recombination processes are investigated by measuring the thermal expansion and subsequent displacement of a sample surface caused by the absorption of a modulated laser beam. The displacement is detected with a Michelson interferometer for modulation frequencies of 1 Hz–100 kHz. We find that the frequency dependence of the signal is only weakly influenced by thermal or acoustic properties, and therefore solely reflects the dynamics of the nonradiative recombination processes. The feasibility of the method is demonstrated and results from crystalline and amorphous Si are presented.
Influence of oxide thickness on the transport properties of silicon metal-oxide-semiconductor systems
Authors: Gold A.
Published: July 2002

We calculate the effects of a finite oxide thickness on the transport properties of a two-dimensional interacting electron gas in a metal-oxide-semiconductor system at temperature zero. Due to the image potential the electron-impurity interaction and the electron-electron interaction are reduced in comparison to an infinite oxide thickness. For impurity scattering the mobility increases with decreasing oxide thickness d. For surface roughness scattering the opposite effect is found. The thickness effects become relevant for 2kFd≤1 and kF is the Fermi wave number. The influence on the metal insulator phase diagram is also evaluated.
Photocurrent spectroscopy in GaAs/AlGaAs multiple quantum wells under a high electric field perpendicular to the heterointerface
Authors: Yamanaka K. , Fukunaga T. , Tsukada N. , Kobayashi K. L. I. , Ishii M.
Published: July 2002

Photocurrent (PC) spectroscopy under high electric fields perpendicular to the heterointerface has been utilized to characterize subband structures in GaAs/AlGaAs multiple quantum wells (MQW’s). The PC spectra from MQW’s under high electric fields show clear exciton peaks corresponding to forbidden transitions between electron and hole subbands. From PC spectra, we have precisely determined the splitting energies of the two lowest electron and four highest hole subbands in 55-, 82-, and 105-Å-thick wells. The experimental results indicate that the conduction-band offset is 60±3% of the band-gap discontinuity. Our data are also in excellent agreement with finite square well calculations using a heavy-hole effective mass of 0.34 and a light-hole effective mass of 0.12.
Dependence of grain size on the substrate temperature of Si and Ge films prepared by evaporation under ultrahigh vacuum
Authors: Tsu R. , Gonzalez-Hernandez J. , Chao S. S. , Martin D.
Published: July 2002

Silicon and germanium films were evaporated in ultrahigh vacuum at substrate temperatures ranging from 0 to 600 °C, and this was followed by high-temperature annealing. The grain size of the annealed microcrystalline samples was measured by transmission electron microscopy. The maximum grain size occurs when the substrate temperature is in the range in which the deposited film changes from the amorphous to the crystalline state. Results have been analyzed on the basis of the competition between two nucleation mechanisms, one with and the other without foreign species present.