bluuebunny/crossref_metadata_2025 huggingface dataset:
Chandra X-ray Observatory high-resolution x-ray spectroscopy of stars: Modeling and interpretation
Authors: Huenemoerder David P.
Published: April 2004

The Chandra X-ray Observatory grating spectrometers allow study of stellar spectra at resolutions on the order of 1000. Prior x-ray observatories’ low resolution data have shown that nearly all classes of stars emit x rays. Chandra reveals details of line and continuum contributions to the spectra which can be interpreted through application of plasma models based on atomic databases. For cool stars with hot coronas interpreted in the solar paradigm, assumption of collisional ionization equilibrium allows derivation of temperature distributions and elemental abundances. Densities can be derived from He-like ion’s metastable transition lines. Abundance trends are unlike the Sun, as are the very hot temperature distributions. For young stars, there is evidence of accretion driven x-ray emission, rather than magnetically confined plasma emission. For some hot stars, the expected emission mechanism of shocked winds has been challenged; there is now evidence for magnetically confined thermal plasmas. The helium-like line emission in hot stars is susceptible to photoexcitation, which can also be exploited to derive wind structure.
Plasma dynamics in pulsed strong magnetic fields
Authors: Doron R. , Arad R. , Tsigutkin K. , Osin D. , Weingarten A. , Starobinets A. , Bernshtam V. A. , Stambulchik E. , Ralchenko Yu. V. , Maron Y. , Fruchtman A. , Fisher A. , Huba J. D. , Roth M.
Published: April 2004

Recent investigations of the interaction of fast-rising magnetic fields with multi-species plasmas at densities of 1013–1015 cm−3 are described. The configurations studied are planar or coaxial interelectrode gaps pre-filled with plasmas, known as plasma opening switches. The diagnostics are based on time-dependent, spatially resolved spectroscopic observations. Three-dimensional spatial resolution is obtained by plasma-doping techniques. The measurements include the propagating magnetic field structure, ion velocity distributions, electric field strengths, and non-Maxwellian electron energy distribution across the magnetic field front. It is found that the magnetic field propagation velocity is faster than expected from diffusion. The magnetic field evolution cannot be explained by the available theoretical treatments based on the Hall field (that could, in principle, explain the fast field propagation). Moreover, detailed observations reveal that magnetic field penetration and plasma reflection occur simultaneously, leading to ion-species separation, which is also not predicted by the available theories. A possible mechanism that is based on the formation of small-scale density fluctuations, previously formulated for astrophysical plasmas, may explain these results.
Electron heating and acceleration in the shock transition region: Background plasma parameter dependence
Authors: Shimada Nobue , Hoshino Masahiro
Published: April 2004

In addition to the well-known plasma parameters of Mach number (MA) and plasma β, the other plasma parameter of the frequency ratio of the electron plasma to gyration, ωpe/Ωce (∝n/B), is emphasized as another important parameter for the plasma heating/acceleration process in the shock transition region. The process of energy conversion from the ion bulk flow into energetic electrons through field activity due to plasma instabilities cannot be described only by such macro averaged parameters as MA and β. The evolution of plasma instabilities largely depends on the background plasma properties, i.e., magnetic field B and density n, which is represented by ωpe/Ωce. Recent studies have shown that electron phase space hole generation due to the nonlinear evolution of Buneman instability between reflected ions and inflow electrons plays an important role in particle energization in the shock transition region. However, even under the same Mach number and plasma β, the formation of electron phase space holes depends on the frequency ratio ωpe/Ωce. Moreover, depending on ωpe/Ωce, different heating and acceleration properties can be obtained. The effect of the frequency ratio ωpe/Ωce on the electron energization is investigated in the shock transition region by using periodic simulations with realistic mass ratio. When ωpe/Ωce⩽1 no electron phase space hole is generated and mild heating of the electrons with a nonthermal energy spectrum is observed. In the region of ωpe/Ωce ⩾ 10 a clear series of electron phase space holes is generated and, after that, ion phase space holes are generated in the resultant hot electrons. These ion phase space holes play an important role in the subsequent strong electron energization.
Control of internal transport barriers on Alcator C-Mod
Authors: Fiore C. L. , Bonoli P. T. , Ernst D. R. , Hubbard A. E. , Greenwald M. J. , Lynn A. , Marmar E. S. , Phillips P. , Redi M. H. , Rice J. E. , Wolfe S. M. , Wukitch S. J. , Zhurovich K.
Published: April 2004

Recent studies of internal transport and double transport barrier regimes in the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] have explored the limits for forming, maintaining, and controlling these plasmas. The C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions and the plasma has no internal particle or momentum sources. The double-barrier mode comprised of an edge barrier with an internal transport barrier (ITB) can be induced at will using off-axis ion cyclotron range of frequency (ICRF) injection on either the low or high field side of the plasma with either of the available ICRF frequencies (70 or 80 MHz). When an enhanced Dα high confinement mode (EDA H-mode) is accessed in Ohmic plasmas, the double barrier ITB forms spontaneously if the H-mode is sustained for ∼2 energy confinement times. The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar regardless of the trigger method. They are characterized by strong central peaking of the electron density, and a reduction of the core particle and energy transport. The control of impurity influx and heating of the core plasma in the presence of the ITB have been achieved with the addition of central ICRF power in both the Ohmic H-mode and ICRF induced ITBs. The radial location of the particle transport barrier is dependent on the toroidal magnetic field but not on the location of the ICRF resonance. A narrow region of decreased electron thermal transport, as determined by sawtooth heat pulse analysis, is found in these plasmas as well. Transport analysis indicates that a reduction of the particle diffusivity in the barrier region allows the neoclassical pinch to drive the density and impurity accumulation in the plasma center. An examination of the gyrokinetic stability at the trigger time for the ITB suggests that the density and temperature profiles are inherently stable to ion temperature gradient and trapped electron modes in the core inside of the ITB location.
Modeling of feedback and rotation stabilization of the resistive wall mode in tokamaks
Authors: Chu M. S. , Bondeson A. , Chance M. S. , Liu Y. Q. , Garofalo A. M. , Glasser A. H. , Jackson G. L. , La Haye R. J. , Lao L. L. , Navratil G. A. , Okabayashi M. , Remierdes H. , Scoville J. T. , Strait E. J.
Published: April 2004

Steady-state operation of the advanced tokamak reactor relies on maintaining plasma stability with respect to the resistive wall mode (RWM). Active magnetic feedback and plasma rotation are the two methods proposed and demonstrated for this purpose. A comprehensive modeling effort including both magnetic feedback and plasma rotation is needed for understanding the physical mechanisms of the stabilization and to project to future devices. For plasma with low rotation, a complete solution for the feedback issue is obtained by assuming the plasma obeys ideal magnetohydrodynamics (MHDs) and utilizing a normal mode approach (NMA) [M. S. Chu et al., Nucl. Fusion 43, 441 (2003)]. It is found that poloidal sensors are more effective than radial sensors and coils inside of the vacuum vessel more effective than outside. For plasmas with non-negligible rotation, a comprehensive linear nonideal MHD code, the MARS-F has been found to be suitable. MARS-F [Y. Q. Liu et al., Phys. Plasmas 7, 3681 (2000)] has been benchmarked in the ideal MHD limit against the NMA. The effect of rotation stabilization of the plasma depends on the plasma dissipation model. Broad qualitative features of the experiment are reproduced. Rotation reduces the feedback gain required for RWM stabilization. Reduction is significant when rotation is near the critical rotation speed needed for stabilization. The International Thermonuclear Experimental Reactor (ITER) [R. Aymar et al., Plasma Phys. Controlled Fusion 44, 519 (2002)] (scenario IV for advanced tokamak operation) may be feedback stabilized with β above the no wall limit and up to an increment of ∼50% towards the ideal limit. Rotation further improves the stability.
High-beta steady-state research and future directions on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade and the Japan Atomic Energy Research Institute Fusion Torus-2 Modified
Authors: Ishida S. ,   JT-60 Team,   JFT-2M Group
Published: April 2004

In the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U), a high-βp ELMy H-mode (high-poloidal-beta high-confinement-mode with edge localized mode) plasma was sustained with βN∼2.7 for 7.4 s. Real-time neoclassical tearing mode (NTM) stabilization system was established and effective NTM suppression by early electron cyclotron (EC) wave injection was demonstrated. High fusion triple product of ni(0)τETi(0)=3.1×1020 keV⋅s⋅m−3 was achieved using the negative-ion based neutral beam current drive with βN∼2.5 and the bootstrap current fraction fBS∼50%. In a hot electron regime, a high electron cyclotron current drive efficiency of 4.2×1018 A/W/m2 was achieved at Te∼21 keV. Innovative current start-up scenario produced a current hole plasma with a very high fBS∼90%. No accumulation of helium and carbon impurities was observed for internal transport barrier (ITB) plasmas. While argon impurity was accumulated, EC injection effectively exhausted it across ITB. In a regime of ELM disappearance, a clear correlation between the ELM frequency and the toroidal velocity at pedestal was observed. In the Japan Atomic Energy Research Institute Fusion Torus-2 Modified (JFT-2M), high beta plasmas were produced with full ferritic inside wall up to βN=3.3, where high recycling steady H-mode discharges were developed up to βNH89P∼6 at ne/nGW∼0.7–1.0 with ITB. JT-60U started long pulse experiment in late 2003 and JFT-2M will conduct wall stabilization experiment in early 2004. The modification of JT-60 to a fully superconducting coil tokamak is regarded as the national centralized tokamak facility program to accomplish the high beta steady-state research in a collisionless regime.
Beam experiments in the extreme space-charge limit on the University of Maryland Electron Ring
Authors: Bernal S. , Li H. , Godlove T. , Haber I. , Kishek R. A. , Quinn B. , Reiser M. , Walter M. , Zou Y. , O’Shea P. G.
Published: April 2004

The University of Maryland Electron Ring (UMER), designed for transport studies of space-charge dominated beams in a strong focusing lattice, is nearing completion. UMER models, for example, the recirculator accelerator envisioned as a possible driver for heavy-ion inertial fusion. The UMER lattice will consist of 36 alternating-focusing (FODO) periods over an 11.5 m circumference. The main diagnostics are phosphor screens and capacitive beam position monitors placed at the center of each 20° bending section. In addition, pepper-pot and slit-wire emittance meters are in operation. We present experimental results for three cases of strong space-charge dominated transport (7.2, 24, and 85 mA, at 10 keV) and contrast them with one case in the emittance-dominated regime (0.6 mA at 10 keV). With focusing given by σ0=76°, the zero-current betatron phase advance per period, the range of currents corresponds to a space-charge tune depression of 0.2 to 0.8. This range is unprecedented for a circular machine. The beam physics over three transport distances is considered: at or near the source, over the length of the matching section (about 1 m), and single turn (10 m). Issues associated with beam characterization, scaling of various parameters, alignment, and envelope matching are discussed.
Compressible dynamics of magnetic field lines for incompressible magnetohydrodynamic flows
Authors: Kuznetsov E. A. , Passot T. , Sulem P. L.
Published: April 2004

It is demonstrated that the deformation of magnetic field lines in incompressible magnetohydrodynamic flows results from a compressible mapping associated with the transverse motion of fluid particles. Appearance of zeros for the Jacobian of this mapping corresponds to the breaking of magnetic field lines and the local blowup of the magnetic field intensity. The occurrence of such events is found to be unlikely in two dimensions but possible in three dimensions.
On ion temperature gradient and parallel velocity shear instabilities
Authors: Rogister Andre L. , Singh Raghvendra , Kaw Predhiman K.
Published: April 2004

The local dispersion relation for waves with frequencies in the range of the diamagnetic frequencies ωj* and parallel wave numbers satisfying the conditions k∥cs/ωe*∼1 and qRk∥≫1 has been obtained in the framework of kinetic theory keeping the equilibrium density, temperature, and parallel velocity gradients into account (j is the species index, qR the connection length, and cs the speed of sound). The analysis applies to the cases where the radial scale of the oscillations is comparable to or smaller than the equilibrium length scale. As the velocity-space integral appearing in the dispersion relation can be calculated only in asymptotic limits, exact instability criteria are obtained by means of the Nyquist diagram. Defining τi=Ti/Te, ηi=∂r ln Ti/∂r ln Ni, and ζ =∂rU∥,i/cs∂r ln Ni, it is found that unstable modes appear for ηi>1+1−ζ2/(1+τi) (which agrees with the standard ion temperature gradient instability condition ηi>2 if ζ=0) and 0<ηi<1−1−ζ2/(1+τi) (the case ηi<0 has not been analyzed), i.e., for ζ2⩾ ηi(2−ηi)(1+τi) (which does not agree with the standard parallel velocity shear instability condition |ζ|>√ if ηi=0). The center of the unstable range is characterized by the relation k∥cs/ωe*=−ζ/2(1+τi) from which it follows that qRk∥≫1 is verified if [kβas/2(1+τi)]qR∂rU∥,i/cs≫1 (kβ is the wave vector component in the direction of the binormal). The oscillations are not tied, under those conditions, to any particular rational surface; the roles of magnetic shear, trapped electrons, ion gyroradius and torus curvature are moreover negligible. The growth/decay rate of the oscillations has been calculated in the neighborhood of marginal (in)stability; the excitation/damping mechanism is (inverse) ion Landau damping. The wave frequency is a function of position so that localization of a wave packet results from a competition between linear growth and distortion (wave breaking in smaller eddies). Applications of the theory include the transition from the edge localized mode-free to enhanced D alpha high confinement regime and intermittency.
Multi-dimensional high energy density physics modeling and simulation of wire array Z-pinch physics
Authors: Garasi C. J. , Bliss D. E. , Mehlhorn T. A. , Oliver B. V. , Robinson A. C. , Sarkisov G. S.
Published: April 2004

The two- and three-dimensional (2D and 3D) versions of ALEGRA-HEDP [A. C. Robinson and C. J. Garasi, “Three-dimensional Z-pinch wire array modeling,” Computer Physics Communications, submitted] have been utilized to simulate discrete wire effects including precursor formation in 2D (r-θ plane) and nonuniform axial ablation (3D). Comparisons made between 2D and 3D simulations indicate that 2D simulations overestimate the mass ablation rate by a factor of 10–100 with respect to the 3D case, causing pre-mature motion of the array with respect to experimental data. Additionally, the 2D case advects a factor of 5 more current to axis than the 3D case. The integrity of the simulations is assessed by comparing the results to laser imaging of wire ablation and array trajectory information inferred from visible and x-ray imaging. Comparisons to previously proposed ablation models are also presented. These simulations represent the first high-fidelity three-dimensional calculations of wire-array pinch geometries.
An experimental study of the gridded electromagnet probe
Authors: Shadman K. , Egedal J. , Childs R.
Published: April 2004

A gridded electromagnet probe has been built and its viability tested for the measurement of the electron distribution function in a low-density, magnetized plasma. The probe uses a grid, a collector, and an electromagnet to provide a combination of electric and magnetic barriers to the electron flow. The combination makes the current collected sensitive to the speed distribution parallel and perpendicular to the magnetic field lines. The ability to draw current to the collector is analyzed first, where optimal settings are found for the electrodes on the sidewall. The current-voltage characteristics of the collector are then examined. These characteristics demonstrate ideal behavior for the high-energy electrons, thereby establishing the basic principles of the probe. However, they also show anomalous effects of the grid’s discrete structure and of space and surface charge inside the probe cavity. The deficiencies in the electric barrier at the grid and the poor level of signal-to-noise prevent the measurement of the distribution of the high-energy population. Improvements to the probe that address the nonideal effects are identified for future implementation.
An empirical investigation of compressibility in magnetohydrodynamic turbulence
Authors: Spangler Steven R. , Spitler Laura G.
Published: April 2004

The density fluctuations which occur in magnetohydrodynamic (MHD) turbulence are an important diagnostic of the turbulent dynamics, and serve as the basis of astrophysical remote sensing measurements. This paper is concerned with the relation between density fluctuations and fluctuations of the magnetic field and velocity. The approach is empirical, utilizing spacecraft observations of slow solar wind turbulence. Sixty-six data intervals of 1 h duration were chosen, in which the solar wind speed was less than 450 km/s, and in which the fluctuations in density and vector magnetic field appeared to be approximately stationary. The parameters of interest were the root-mean-square fluctuations of density and magnetic field, normalized by the respective mean values, εN≡〈(δn)2〉0.5/n0 and εB≡〈(δb)2〉0.5/B0, respectively, where n0 and B0 are the mean plasma number density and magnetic field strength. The conclusions of this study are as follows: (1) Consistent with previous investigations, the dependence of the normalized density fluctuation on the normalized magnetic field fluctuation is found to be between linear (εN=εB) and quadratic (εN=εB2). (2) The value of R≡εN/εB shows a wide range from <0.1 to >4; the median value is 0.46 and the mean is 0.72. (3) Typical normalized fluctuation amplitudes (εN and εB) for records of one hour length (maximum scale size of ≃1.6×106 km) are 0.03–0.08 for the density, and 0.04–0.21 for the magnetic field. (4) For most intervals, the magnitude of the perpendicular (to the large scale magnetic field) magnetic field fluctuations exceeds that of the parallel fluctuations by a factor of 3–4. This indicates that the turbulent magnetic field fluctuations are primarily transverse fluctuations. The implications of these results for theories of MHD turbulence, and for the remote sensing of turbulent plasmas such as the corona, the near-Sun solar wind, and the interstellar medium, are discussed.
The role of flow shear in the ballooning stability of tokamak transport barriers
Authors: Webster A. J. , Wilson H. R. , Scaife A. M. M.
Published: April 2004

A tokamak’s economic performance is strongly affected by the plasma pressure that it may sustain, which in turn is limited by the maximum pressure gradients that may be supported. Ballooning modes are typically driven unstable by increasing the pressure gradient, and because they can radially extend across many rational surfaces, they can seriously reduce a plasma’s energy confinement. Here an eigenmode formulation is used to study the stability of ballooning modes in internal transport barriers (“ITBs”), in which very strong pressure gradients and flow shears may be found. This extends previous studies that used an “eikonal” formulation, as it enables the study of: ballooning modes with a finite toroidal mode-number n (finite wavelength perpendicular to the magnetic field), to find new solution branches, to obtain the eigenmode structures, and to investigate the effects of a radially varying equilibrium. The structure of a finite n ballooning mode in flow shear is found to be significantly affected by a radially varying equilibrium, and at low flow shears the growth rates are increased above those of modes studied in the limit of n→∞. The different solution branches can couple as the flow shear is increased, leading to a pair of asymmetric mode structures with complex conjugate growth rates. These effects are shown to be a consequence of the mode trying to localize at the most unstable radial location, and its desire to rotate with the flow. In addition, closer to marginal stability a sufficiently strong flow-shear can (at least for some cases), destabilize a previously stable mode.
Experimental evidence of fast reconnection via trapped electron motion
Authors: Egedal J. , Fox W. , Porkolab M. , Fasoli A.
Published: April 2004

Magnetic reconnection in collisionless magnetohydrodynamic plasmas is studied under controlled conditions in toroidal X-line configurations formed in the center of the Versatile Toroidal Facility [J. Egedal et al., Rev. Sci. Instrum. 71, 3351 (2000)]. Profiles of key parameters are measured during driven reconnection pulses in the reproducible plasma discharges. The experimental results prove to be highly sensitive to the boundary conditions applied. In the configuration with “open” boundaries reconnection is fast; the measured current densities are about three orders of magnitude below those predicted based on classical Spitzer resistivity. These small current densities are consistent with the electrons following toroidally trapped orbits. Further, the size of the electron diffusion region scales with the drift orbit width of the trapped electrons. Experiments conducted with “closed” boundaries yield significantly different results. In this configuration the electrons are not trapped toroidally and large current densities are permitted to develop. These currents reduce significantly the reconnection rate in the closed configurations.
Some electrostatic modes based on non-Maxwellian distribution functions
Authors: Zaheer S. , Murtaza G. , Shah H. A.
Published: April 2004

A comparative study of fundamental modes such as Langmuir waves, dust ion acoustic waves, and dust-acoustic waves using non-Maxwellian distribution functions is presented. The real frequency and the growth rate of the modes are calculated by using kappa and generalized (r,q) distribution functions and results are compared with those of Maxwellian distribution. It is noted that in the limit (i) r=0, q→∞ for generalized (r,q) distributions and (ii) κ→∞ for kappa distributions, the non-Maxwellian functions reduce to Maxwellian.
Nonlinear and three-dimensional theory for cross-magnetic field propagation of short-pulse lasers in underdense plasmas
Authors: Ren C. , Mori W. B.
Published: April 2004

The nonlinear and finite spot size effects for short laser pulses propagating in a plasma across a constant magnetic field (ordinary and extraordinary modes) have been studied. Starting from a fluid Lagrangian for magnetized plasmas with immobile ions, we derive the envelope equation for the laser and also the equation for the plasma wake in a three-dimensional geometry. The derived equations reveal that the external magnetic field reduces the strength of ponderomotive self-focusing, causes astigmatic self-focusing, and leads to the possibility of deflecting a short and narrow laser pulse in a magnetized plasma.
Plasma-enhanced combustion of propane using a silent discharge
Authors: Rosocha L. A. , Coates D. M. , Platts D. , Stange S.
Published: April 2004

It is well known that applying an electric field to a flame can affect its propagation speed, stability, and combustion chemistry. External electrodes, arc discharges, plasma jets, and corona discharges have been employed to allow combustible gas mixtures to operate outside their flammability limits or to increase combustion speed. Previously reported experiments have involved silent electrical discharges applied to propagating flames. These demonstrated that the flame propagation velocity can be increased when the discharge is applied to the unburned gas mixture upstream of a flame. In contrast, the work reported here used a coaxial-cylinder, nonthermal, silent discharge plasma reactor to activate a propane gas stream before it was mixed with air and ignited. With the plasma, the physical appearance of the flame changes (increased stability) and substantial changes in mass spectrometer peaks are observed, indicating that the combustion process is enhanced with the application of the plasma.
Experimental demonstration of plasma startup by coaxial helicity injection
Authors: Raman R. , Jarboe T. R. , Nelson B. A. , Hamp W. T. , Izzo V. A. , O’Neill R. G. , Redd A. J. , Sieck P. E. , Smith R. J.
Published: April 2004

Experimental results on the transfer of a coaxial-helicity-injection (CHI) produced discharge to inductive operation are reported. CHI assisted plasma startup is more robust than inductive only operation and reduces volt-seconds consumption. After handoff to inductive operation, the initial 100 kA of CHI produced current drops to 50 kA, then ramps up to 180 kA, using only 30 mVs, about 40% higher than that produced by induction alone. Results show that initiation of CHI discharges at lower densities produce higher levels of coupling current. Coupling a CHI produced discharge to induction from a precharged central solenoid has produced record currents of 290 kA using only 52 mWb of central solenoid flux. CHI discharges can also be generated while the central transformer is in the process of being precharged, during which period it induces a negative loop voltage on the CHI discharge. These significant results were obtained on the Helicity Injected Torus-II (HIT-II) [T.R. Jarboe, Fusion Technol. 15, 7 (1989)] spherical torus experiment (major/minor radius of 0.3/0.2 m and elongation of 1.5).
Environmental effects of H2O on fracture initiation in silicon: A hybrid electronic-density-functional/molecular-dynamics study
Authors: Ogata Shuji , Shimojo Fuyuki , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya
Published: April 2004

A hybrid quantum-mechanical/molecular-dynamics simulation is performed to study the effects of environmental molecules on fracture initiation in silicon. A (110) crack under tension (mode-I opening) is simulated with multiple H2O molecules around the crack front. Electronic structure near the crack front is calculated with density functional theory. To accurately model the long-range stress field, the quantum-mechanical description is embedded in a large classical molecular-dynamics simulation. The hybrid simulation results show that the reaction of H2O molecules at a silicon crack tip is sensitive to the stress intensity factor K. For K=0.4 MPa⋅m, an H2O molecule either decomposes and adheres to dangling-bond sites on the crack surface or oxidizes Si, resulting in the formation of a Si–O–Si structure. For a higher K value of 0.5 MPa⋅m, an H2O molecule either oxidizes or breaks a Si–Si bond.
Transitions from electrostatic to electromagnetic whistler wave excitation
Authors: Degeling A. W. , Borg G. G. , Boswell R. W.
Published: April 2004

At frequencies below the electron cyclotron and above the lower hybrid frequency in a magnetoplasma, the refractive index is anisotropic and shows resonances at certain angles caused by electron inertia. For a radiating antenna in a plasma, the short wavelengths near this resonance angle may contribute to the radiation pattern of the antenna. A series of experiments is reported, in which waves were excited using a small 1-cm-diam electrostatically coupled antenna into a preformed plasma, densities (ne) from 1015 to 1018 m−3 and magnetic fields from 30 to 60 G. Maps of the wave amplitude and phase were made within the plasma by scanning the position of a b-dot probe. At low densities (e.g., ne<5×1016 m−3 at 50 G and 3 mTorr), a single amplitude maximum along the group velocity resonance cone angle was measured that decayed as the distance from the antenna increased. The observed radiation pattern in all cases was consistent with that of a point source in an unbounded plasma, and no global eigenmode resonances were found. As the density was increased, the apparent attenuation of the resonance cone waves increased and they appeared to withdraw into the antenna. At high densities (ne>5×1016 m−3) the radiation pattern was characterized by a monotonic increase in wave phase in the axial direction, a central maximum for Bz, and off-axis maxima for Bx and By which are consistent with the propagation of m=0 helicon waves, and no evidence of resonance cone structure. This change in the radiation pattern is reproduced numerically in a homogeneous plasma model including an electrostatically coupled antenna with the same geometry as that used in the experiment.
Observation of tearing mode deceleration and locking due to eddy currents induced in a conducting shell
Authors: Chapman B. E. , Fitzpatrick R. , Craig D. , Martin P. , Spizzo G.
Published: April 2004

Growth to large amplitude of a single core-resonant tearing mode in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch is accompanied by braking and eventual cessation of mode rotation. There is also a concurrent deceleration of bulk plasma rotation. The mode deceleration is shown to be well described by a time-dependent version of a magnetohydrodynamical model [R. Fitzpatrick et al., Phys. Plasmas 6, 3878 (1999)] in which a braking torque originates from eddy currents induced by the rotating mode in the conducting shell surrounding the plasma. According to the model, the electromagnetic braking torque is localized to the plasma in the immediate vicinity of the mode’s resonant surface, but viscosity transfers the torque to the rest of the plasma. Parametrizing the plasma viscous momentum diffusivity in terms of the global momentum confinement time, the model is used to predict both the momentum confinement time and the time evolution of the decelerating mode velocity. In both respects, the model is quite consistent with experimental data.
Polar direct drive on the National Ignition Facility
Authors: Skupsky S. , Marozas J. A. , Craxton R. S. , Betti R. , Collins T. J. B. , Delettrez J. A. , Goncharov V. N. , McKenty P. W. , Radha P. B. , Boehly T. R. , Knauer J. P. , Marshall F. J. , Harding D. R. , Kilkenny J. D. , Meyerhofer D. D. , Sangster T. C. , McCrory R. L.
Published: April 2004

Three recent developments in direct-drive target design have enhanced the possibility of achieving high target gain on the National Ignition Facility (NIF): (1) Laser absorption was increased by almost 50% using wetted-foam targets. (2) Adiabat shaping significantly increased the hydrodynamic stability of the target during the acceleration phase of the implosion without sacrificing target gain. (3) Techniques to reduce laser imprint using pulse shaping and radiation preheat were developed. These design features can be employed for direct-drive-ignition experiments while the NIF is in the x-ray-drive configuration. This involves repointing some of the beams toward the equator of the target to improve uniformity of target drive. This approach, known as polar direct drive (PDD), will enhance the capability of the NIF to explore ignition conditions. PDD will couple more energy to the fuel than x-ray drive. The compressed fuel core can be more easily accessed for high-ρR diagnostic development and for fast-ignitor studies. Polar direct drive is examined in this manuscript using two-dimensional hydrodynamic simulations to determine the level of target performance that can be achieved.
Beam ion driven instabilities in the National Spherical Tokamak Experiment
Authors: Gorelenkov N. N. , Belova E. , Berk H. L. , Cheng C. Z. , Fredrickson E. , Heidbrink W. W. , Kaye S. , Kramer G. J.
Published: April 2004

Recent progress in the analysis of the low and high frequency beam ion driven instabilities in the National Spherical Tokamak Experiment (NSTX) [S. Kaye et al., Fusion Technol. 36, 16 (1999)] plasma is reported. The low Alfvén speed with respect to the beam ion injection velocity in NSTX offers a window in the plasma parameter space to study instabilities driven by super-Alfvénic fusion alphas, which are expected in the International Tokamak Experimental Reactor—ITER [D. J. Campbell, Phys. Plasmas 8, 2041 (2001)]. Low frequency magnetic field activities identified as an instability of toroidicity-induced Alfvén eigenmodes (TAEs) have been observed in NSTX and analyzed with the linear hybrid kinetic magnetohydrodynamic stability code NOVA-K [C. Z. Cheng, Phys. Rep. 1, 211 (1992)]. The comparison between the TAE analysis and observations in NSTX and DIII–D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] similarity experiments confirms that the toroidal mode number of the most unstable TAE modes scales with q−2 and is independent of plasma major radius, where q is the safety factor. This scaling helps validate the predictive capability of the NOVA-K code for studying TAE stability in future burning plasma devices. The subion cyclotron frequency magnetic activities in NSTX are identified as compressional and global shear Alfvén eigenmodes (AEs) (CAEs and GAEs). CAE and GAE instabilities are driven by beam ions via the Doppler shifted cyclotron resonance by the velocity space bump-on-tail distribution function in the perpendicular velocity. Results of the GAE/CAE theoretical and numerical analysis are presented.
Electromagnetic gyrokinetic simulations
Authors: Parker S. E. , Chen Y. , Wan W. , Cohen B. I. , Nevins W. M.
Published: April 2004

A new electromagnetic kinetic electron δf particle simulation model has been demonstrated to work well at large values of plasma β times the ion-to-electron mass ratio [Y. Chen and S. E. Parker, J. Comput. Phys. 198, 463 (2003)]. The simulation is three-dimensional using toroidal flux-tube geometry and includes electron-ion collisions. The model shows accurate shear Alfvén wave damping and microtearing physics. Zonal flows with kinetic electrons are found to be turbulent with the spectrum peaking at zero and having a width in the frequency range of the driving turbulence. This is in contrast with adiabatic electron cases where the zonal flows are near stationary, even though the linear behavior of the zonal flow is not significantly affected by kinetic electrons. Zonal fields are found to be very weak, consistent with theoretical predictions for β below the kinetic ballooning limit. Detailed spectral analysis of the turbulence data is presented in the various limits.
New resonance and cut-off for low-frequency electromagnetic waves in dusty magnetoplasmas
Authors: Mamun A. A. , Shukla P. K. , Morfill G. E.
Published: April 2004

The dispersion properties of low-frequency (in comparison with the ion gyrofrequency) dust-electromagnetic waves in a four-component magnetized dusty plasma have been investigated. It has been shown that the simultaneous presence of positive and negative inertial dust grains introduces new resonance and cut-off for dust-electromagnetic waves, and a sharp forbidden- frequency-band in between the two branches of the propagating dust-electromagnetic waves. The two wave branches have been found to have frequencies of the order of negative and positive dust gyrofrequencies, respectively. The effects of space and laboratory dusty plasma parameters on these newly found resonance and cut-off and on the dispersion properties of the dust-electromagnetic wave branches have also been examined.
A kinetic model for resonant heating of ions by Alfvén waves in laboratory plasmas
Authors: Zhang T. X. , Li Bing
Published: April 2004

A kinetic model for the resonant heating of ions by Alfvén waves that propagate along the magnetic field lines and wave frequencies in the ion-cyclotron frequency range is developed. In terms of the quasi-linear approach, particle–Alfvén wave interactions are examined from the Vlasov equation in a hot, magnetized, and multi-ion plasma. Consequently, a new heating-rate expression for any species of ions that are heated by the Alfvén waves is derived. Based on the heating model and the dispersion relation of the magnetic field-aligned left-hand polarized EM ion-cyclotron-Alfvén (EMICA) waves, the resonant heating of H, H2, H3, H3, and He4 by the EMICA waves in a typical preheated laboratory plasma is numerically studied. The results indicate that the EMICA waves can efficiently heat ions through the cyclotron resonances. The temperatures of H, H2, H3, He2, and He4 can be increased by a factor of tens within hundreds of μ-seconds. The parameters that affect the EMICA-wave heating include the plasma density, the plasma temperature, the background magnetic field, and the wave-energy density. It is also dependent on the ion species. Each species of ions resonates with the EMICA waves in a narrow frequency range that is close to its ion-cyclotron frequency.
Wire array implosion characteristics from determination of load inductance on the Z pulsed-power accelerator
Authors: Waisman Eduardo M. , Cuneo M. E. , Stygar W. A. , Lemke R. W. , Struve K. W. , Wagoner T. C.
Published: April 2004

The time-dependent inductance of Z pinches and other loads on the Z pulsed-power accelerator at Sandia National Laboratories [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] is determined by using electrical measurements and a lumped-circuit analysis. One finds that ∑kαkVk−Lİ=d(LlIl)/dt, where ∑kαkVk is the weighted sum of the Z-insulator-stack voltages, L is the equivalent inductance of the magnetically insulated transmission lines connecting the stack to the load, I is the added current for those lines, Ll is the load inductance, and Il is the load current. Ll obtained from this expression is used to reconstruct the motion of the outer edge of wire-array Z-pinch loads, providing an estimate of the time at which the cores start moving significantly from their initial position. Results are consistent with previous optical measurements suggesting that core motion is delayed with respect to a zero-dimensional thin-shell model of the implosion. These results provide useful insights and constraints in experimental and theoretical research of wire arrays precursor and imploding plasma properties.
Nonlinear fluxes and forces from radio-frequency waves with application to driven flows in tokamaks
Authors: Myra J. R. , Berry L. A. , D’Ippolito D. A. , Jaeger E. F.
Published: April 2004

Nonlinear rf-driven sheared flows are of interest for turbulence control and basic physics experiments. Short-wavelength slow modes are required for efficient coupling of wave momentum to the plasma, requiring a kinetic hot-plasma theory. Here, a guiding-center formulation is developed which calculates the nonlinear particle and energy fluxes, energy absorption, and nonlinear forces on the plasma using a kinetic moment approach that is valid to first order in the ratio of the gyroradius compared to the wave envelope scale length and the plasma equilibrium scale length. Both the stress tensor and Lorentz force contribute to the net force on a fluid element. The forces driving flux-surface-averaged flows in a tokamak are extracted from the parallel and toroidal components. It is shown that flux-surface-averaged flows are driven by two classes of terms: direct absorption of wave momentum and dissipative stresses. Furthermore, the general kinetic expression for the force is shown to reduce to the standard cold-fluid ponderomotive force in an appropriate limit, but in this limit no flows are driven.
Heating and current drive by electron cyclotron waves
Authors: Prater R.
Published: April 2004

The physics model of electron cyclotron heating (ECH) and current drive (ECCD) is becoming well validated through systematic comparisons of theory and experiment. This work has shown that ECH and ECCD can be highly localized and robustly controlled in toroidal plasma confinement systems, leading to applications including stabilization of magnetohydrodynamic instabilities like neoclassical tearing modes, control and sustainment of desired profiles of current density and plasma pressure, and studies of localized transport in laboratory plasmas. The experimental work was supported by a broad base of theory based on first principles which is now well encapsulated in linear ray tracing codes describing wave propagation, absorption, and current drive and in fully relativistic quasilinear Fokker–Planck codes describing in detail the response of the electrons to the energy transferred from the wave. The subtle balance between wave-induced diffusion and Coulomb relaxation in velocity space provides an understanding of the effects of trapping of current-carrying electrons in the magnetic well. Strong quasilinear effects and radial transport of electrons, which may broaden the driven current profile, have also been observed under some conditions and appear to be consistent with theory, but in large devices these are usually insignificant. The agreement of theory and experiment, the wide range of established applications, and the technical advantages of ECH support the application of ECH in next-step tokamaks and stellarators.
Impact ionization in submicron silicon devices
Authors: Massey D. J. , David J. P. R. , Tan C. H. , Ng B. K. , Rees G. J. , Robbins D. J. , Herbert D. C.
Published: April 2004

Photomultiplication initiated by electrons and holes has been measured in submicron Si p+–i–n+ and n+–i–p+ diodes with nominal intrinsic region thicknesses between 0.8 and 0.1 μm. A local analysis of the thinner devices gives values of the electron and hole ionization coefficients (α and β, respectively) smaller than those in the literature, especially at low values of multiplication because of dead space effects. The dead space in Si appears to be less significant than in GaAs structures of similar dimensions.
Low-temperature formation of CoSi2 in the presence of Au
Authors: Detavernier C. , Lavoie C. , d’Heurle F. M. , Bender H. , Van Meirhaeghe R. L.
Published: April 2004

The formation of cobalt silicides was studied in the presence of interlayers and capping layers of Au, using both ex situ and in situ characterization techniques. The formation temperature of CoSi2 was found to be significantly lowered in the presence of Au. Two regimes can be discerned. For thin interlayers or capping layers, Au does not significantly affect the formation of Co2Si and CoSi, while it lowers the nucleation temperature of CoSi2. For thick interlayers, CoSi2 appears to form as the first phase at temperatures as low as 300 °C. The results are discussed within the context of classical nucleation theory and known models for phase selection during solid-state reactions.
High performance advanced tokamak regimes in DIII-D for next-step experiments
Authors: Greenfield C. M. , Murakami M. , Ferron J. R. , Wade M. R. , Luce T. C. , Petty C. C. , Menard J. E. , Petrie T. W. , Allen S. L. , Burrell K. H. , Casper T. A. , DeBoo J. C. , Doyle E. J. , Garofalo A. M. , Gorelov I. A. , Groebner R. J. , Hobirk J. , Hyatt A. W. , Jayakumar R. J. , Kessel C. E. , La Haye R. J. , Jackson G. L. , Lohr J. , Makowski M. A. , Pinsker R. I. , Politzer P. A. , Prater R. , Strait E. J. , Taylor T. S. , West W. P. , Team DIII-D
Published: April 2004

Advanced Tokamak (AT) research in DIII-D [K. H. Burrell for the DIII-D Team, in Proceedings of the 19th Fusion Energy Conference, Lyon, France, 2002 (International Atomic Energy Agency, Vienna, 2002) published on CD-ROM] seeks to provide a scientific basis for steady-state high performance operation in future devices. These regimes require high toroidal beta to maximize fusion output and poloidal beta to maximize the self-driven bootstrap current. Achieving these conditions requires integrated, simultaneous control of the current and pressure profiles, and active magnetohydrodynamic stability control. The building blocks for AT operation are in hand. Resistive wall mode stabilization via plasma rotation and active feedback with nonaxisymmetric coils allows routine operation above the no-wall beta limit. Neoclassical tearing modes are stabilized by active feedback control of localized electron cyclotron current drive (ECCD). Plasma shaping and profile control provide further improvements. Under these conditions, bootstrap supplies most of the current. Steady-state operation requires replacing the remaining Ohmic current, mostly located near the half radius, with noninductive external sources. In DIII-D this current is provided by ECCD, and nearly stationary AT discharges have been sustained with little remaining Ohmic current. Fast wave current drive is being developed to control the central magnetic shear. Density control, with divertor cryopumps, of AT discharges with edge localized moding H-mode edges facilitates high current drive efficiency at reactor relevant collisionalities. A sophisticated plasma control system allows integrated control of these elements. Close coupling between modeling and experiment is key to understanding the separate elements, their complex nonlinear interactions, and their integration into self-consistent high performance scenarios. Progress on this development, and its implications for next-step devices, will be illustrated by results of recent experiment and simulation efforts.
Inverse bremsstrahlung stabilization of noise in the generation of ultrashort intense pulses by backward Raman amplification
Authors: Berger Richard L. , Clark Daniel S. , Solodov Andrei A. , Valeo Ernest J. , Fisch Nathaniel J.
Published: April 2004

Inverse bremsstrahlung absorption of the pump laser beam in a backward Raman amplifier over the round-trip light transit time through the subcritical density plasma can more than double the electron temperature of the plasma and produce time-varying axial temperature gradients. The resulting increased Landau damping of the plasma wave and detuning of the resonance can act to stabilize the pump against unwanted amplification of Langmuir noise without disrupting nonlinear amplification of the femtosecond seed pulse. Because the heating rate increases with the charge state Z, only low-Z plasmas (hydrogen, helium, or helium–hydrogen mixtures) will maintain a low enough temperature for efficient operation.
Dynamo-free plasma in the reversed field pinch
Authors: Anderson J. K. , Biewer T. M. , Forest C. B. , O’Connell R. , Prager S. C. , Sarff J. S.
Published: April 2004

Transient application of a poloidal electric field to reversed field pinch (RFP) plasmas has led to a period in which dynamo activity (inherent in standard RFP plasmas) nearly vanishes. Measurements of the plasma resistivity, current density and electric field profiles show the edge-applied electric field accommodates Ohm’s law balance without a dynamo term over the entire cross section. Neoclassical theory accurately predicts the resistivity in the RFP, as the predicted resistivity profile (based on measurements of electron temperature, effective ionic charge, and two-dimensional equilibrium effects) is in agreement with the ratio of the parallel electric field and current density profiles.