bluuebunny/crossref_metadata_2025 huggingface dataset:
Power spectrum analysis for optical tweezers
Authors: Berg-Sørensen Kirstine , Flyvbjerg Henrik
Published: February 2004

The force exerted by an optical trap on a dielectric bead in a fluid is often found by fitting a Lorentzian to the power spectrum of Brownian motion of the bead in the trap. We present explicit functions of the experimental power spectrum that give the values of the parameters fitted, including error bars and correlations, for the best such χ2 fit in a given frequency range. We use these functions to determine the information content of various parts of the power spectrum, and find, at odds with lore, much information at relatively high frequencies. Applying the method to real data, we obtain perfect fits and calibrate tweezers with less than 1% error when the trapping force is not too strong. Relatively strong traps have power spectra that cannot be fitted properly with any Lorentzian, we find. This underscores the need for better understanding of the power spectrum than the Lorentzian provides. This is achieved using old and new theory for Brownian motion in an incompressible fluid, and new results for a popular photodetection system. The trap and photodetection system are then calibrated simultaneously in a manner that makes optical tweezers a tool of precision for force spectroscopy, local viscometry, and probably other applications.
Minimization of the acquisition time in energy dispersive x-ray diffraction measurements on disordered systems
Authors: Rossi Albertini V. , Paci B.
Published: February 2004

The energy dispersive x-ray diffraction (EDXD) technique proved to be a powerful tool for the structural investigations of disordered systems. Although EDXD fulfills all of the characteristics required to study this class of samples, the adopted measurement procedure is not standard. This article focused on the optimization of the experimental setup required in order to minimize the acquisition time. We demonstrate that, in the symmetrical transmission geometry (usually adopted) and under very general experimental conditions, the quantities influencing the total time of measurement depend on only two free parameters, namely the sample thickness and the width of the energy spectrum of the primary beam. For this reason, despite the large number of variables involved in the problem, the final result can be summarized in a three-dimensional plot of the acquisition time as a function of these two independent parameters. Therefore, the plots allow the most convenient choice of the two parameters to minimize the measurement time, at a parity of statistical accuracy. On the other hand, it shows how an inappropriate setting may expand this time by several orders of magnitude. The method is applied to the two systems that have been most thoroughly investigated by EDXD, namely CCl4 and Hg.
Investigation of volatile liquid surfaces by synchrotron x-ray spectroscopy of liquid microjets
Authors: Wilson Kevin R. , Rude Bruce S. , Smith Jared , Cappa Chris , Co D. T. , Schaller R. D. , Larsson M. , Catalano T. , Saykally R. J.
Published: February 2004

Soft x-ray absorption spectroscopy is a powerful probe of surface electronic and geometric structure in metals, semiconductors, and thin films. Because these techniques generally require ultrahigh vacuum, corresponding studies of volatile liquid surfaces have hitherto been precluded. We describe the design and implementation of an x-ray experiment based on the use of liquid microjets, permitting the study of volatile liquid surfaces under quasi-equilibrium conditions by synchrotron-based spectroscopy. The liquid microjet temperatures are also characterized by Raman spectroscopy, which connects our structural studies with those conducted on liquid samples under equilibrium conditions. In recent experiments, we have observed and quantified the intermolecular surface relaxation of liquid water and methanol and have identified a large population of “acceptor-only” molecules at the liquid water interface.
Viscous heating effect on the drag reduction of a bubble towards a near-contact wall
Authors: Chen Shing Bor
Published: February 2004

This work theoretically investigates the thermal effect of viscous dissipation on the drag force of a fast moving spherical bubble in a Newtonian liquid towards a near-contact planar wall. The bubble is small and is close enough to the wall so that creeping flow conditions are achieved in the thin gap. A lubrication analysis is employed to simplify the momentum and energy equations. For low wall thermal conductivity, simple governing equations can be derived and then solved numerically. At sufficiently high constant velocity, the drag force can be substantially reduced due to the viscosity decrease caused by viscous heating in the thin film. Under the action of a prescribed external force, the bubble velocity can be considerably increased at sufficient closeness to the wall, as compared with that if the viscosity were temperature independent.
24 kA solid state switch for plasma discharge experiments
Authors: Pribyl P. , Gekelman W.
Published: February 2004

We present the design of a modular, robust, high-current switch for controlling discharge current in the large plasma device (LAPD) at UCLA. As developed, 10 insulated gate bipolar transistor units are connected in parallel to provide 24 kA current-carrying capacity. Associated snubber and protection circuits are described in detail. The switch is typically operated at a 1 Hz repetition rate, with up to a 2% duty cycle. It has proven to be quite reliable, to date having pulsed approximately 5×107 times, with a net throughput of about 2 billion Coulombs. This type of switch not only has an application in such a plasma source, but can also be used to switch large currents for a variety of applications. The design is modular and the switch can be made smaller or larger depending on the desired current rating.
Ion temperature and plasma rotation profile effects in the neutron emission spectrum
Authors: Tardocchi M. , Gorini G. , Henriksson H. , Källne J.
Published: February 2004

The instrumental factors and measuring conditions affecting neutron emission spectrometry measurements of tokamak plasmas are described and analyzed. The measured energy broadening and shift of the neutron emission is used to deduce ion temperature (Ti) and toroidal plasma rotation velocity (Vt) representing average (effective) values for the nonuniform plasma volume viewed. Analytical expressions are derived for the relationship between the line-volume integrated effective temperature (Teff) and the radial profile Ti(r) for the case of thermal plasmas with isotropic neutron emission; effects on Teff due to spectral broadening from the radial dependence Vt(r) were also considered. The analysis method presented here is applied to high quality data obtained with the magnetic proton recoil neutron spectrometer installed at Joint European Torus for measurements of deuterium–tritium plasmas. Similarly, cases of anisotropic neutron emission were quantitatively assessed.
Respiratory monitoring system based on the nasal pressure technique for the analysis of sleep breathing disorders: Reduction of static and dynamic errors, and comparisons with thermistors and pneumotachographs
Authors: Alves de Mesquita Jayme , Lopes de Melo Pedro
Published: February 2004

Thermally sensitive devices—thermistors—have usually been used to monitor sleep-breathing disorders. However, because of their long time constant, these devices are not able to provide a good characterization of fast events, like hypopneas. Nasal pressure recording technique (NPR) has recently been suggested to quantify airflow during sleep. It is claimed that the short time constants of the devices used to implement this technique would allow an accurate analysis of fast abnormal respiratory events. However, these devices present errors associated with nonlinearities and acoustic resonance that could reduce the diagnostic value of the NPR. Moreover, in spite of the high scientific and clinical potential, there is no detailed description of a complete instrumentation system to implement this promising technique in sleep studies. In this context, the purpose of this work was twofold: (1) describe the development of a flexible NPR device and (2) evaluate the performance of this device when compared to pneumotachographs (PNTs) and thermistors. After the design details are described, the system static accuracy is evaluated by a comparative analysis with a PNT. This analysis revealed a significant reduction (p<0.001) of the static error when system nonlinearities were reduced. The dynamic performance of the NPR system was investigated by frequency response analysis and time constant evaluations and the results showed that the developed device response was as good as PNT and around 100 times faster (τ=5,3 ms) than thermistors (τ=512 ms). Experimental results obtained in simulated clinical conditions and in a patient are presented as examples, and confirmed the good features achieved in engineering tests. These results are in close agreement with physiological fundamentals, supplying substantial evidence that the improved dynamic and static characteristics of this device can contribute to a more accurate implementation of medical research projects and to improve the diagnoses of sleep-breathing disorders.
The plasma mirror—A subpicosecond optical switch for ultrahigh power lasers
Authors: Dromey B. , Kar S. , Zepf M. , Foster P.
Published: February 2004

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers (TW–PW). The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csΔt<λLaser must be met (cs: sound speed, Δt: time from plasma formation to the peak of the pulse). The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.
Instrument reflections and scene amplitude modulation in a polychromatic microwave quadrature interferometer
Authors: Dobson Chris C. , Jones Jonathan E. , Chavers D. Gregory
Published: February 2004

A polychromatic microwave quadrature interferometer has been characterized using several laboratory plasmas. Reflections between the transmitter and the receiver have been observed, and the effects of including reflection terms in the data reduction equation have been examined. An error analysis which includes the reflections, modulation of the scene beam amplitude by the plasma, and simultaneous measurements at two frequencies has been applied to the empirical database, and the results are summarized. For reflection amplitudes around 10%, the reflection terms were found to reduce the calculated error bars for electron density measurements by about a factor of 2. The impact of amplitude modulation is also quantified. In the complete analysis, the mean error bar for high-density measurements is 7.5%, and the mean phase shift error for low-density measurements is 1.2°.
Edge rotation and temperature diagnostic on the National Spherical Torus Experiment
Authors: Biewer T. M. , Bell R. E. , Feder R. , Johnson D. W. , Palladino R. W.
Published: February 2004

A new diagnostic for the National Spherical Torus Experiment is described whose function is to measure ion rotation and temperature at the plasma edge. The diagnostic is sensitive to C III, C IV, and He II intrinsic emission, covering a radial region of 15 cm at the extreme edge of the outboard midplane. Thirteen chords are distributed between toroidal and poloidal views, allowing the toroidal and poloidal rotation and temperature of the plasma edge to be simultaneously measured with 10 ms resolution. Combined with the local pressure gradient and EFIT reconstructed magnetic field profile, the edge flow gives a measure of the local radial electric field.
Synthesis of aluminum nitride films by plasma immersion ion implantation–deposition using hybrid gas–metal cathodic arc gun
Authors: Shen Liru , Fu Ricky K. Y. , Chu Paul K.
Published: February 2004

Aluminum nitride (AlN) is of interest in the industry because of its excellent electronic, optical, acoustic, thermal, and mechanical properties. In this work, aluminum nitride films are deposited on silicon wafers (100) by metal plasma immersion ion implantation and deposition (PIIID) using a modified hybrid gas–metal cathodic arc plasma source and with no intentional heating to the substrate. The mixed metal and gaseous plasma is generated by feeding the gas into the arc discharge region. The deposition rate is found to mainly depend on the Al ion flux from the cathodic arc source and is only slightly affected by the N2 flow rate. The AlN films fabricated by this method exhibit a cubic crystalline microstructure with stable and low internal stress. The surface of the AlN films is quite smooth with the surface roughness on the order of 1/2 nm as determined by atomic force microscopy, homogeneous, and continuous, and the dense granular microstructures give rise to good adhesion with the substrate. The N to Al ratio increases with the bias voltage applied to the substrates. A fairly large amount of O originating from the residual vacuum is found in the samples with low N:Al ratios, but a high bias reduces the oxygen concentration. The compositions, microstructures and crystal states of the deposited films are quite stable and remain unchanged after annealing at 800 °C for 1 h. Our hybrid gas–metal source cathodic arc source delivers better AlN thin films than conventional PIIID employing dual plasmas.
Low temperature scanning contact potentiometry
Authors: Reymond S. , Fischer O/.
Published: February 2004

We present a tool dedicated to potentiometric measurements over a length longer than the usual scanning length of local probes techniques, with a submicronic resolution, suitable for use in cryogenic environment. The measurement is carried out by bringing a conducting tip in contact with the sample surface. We describe in detail the connecting procedure and the inertial system used to move the sample. To illustrate the possibilities given by this system, we have measured the instantaneous voltage profile modified by Joule heating of a metallic bridge crossed by a high current density, and we have probed the potential profile across a normal-superconducting junction.
Effective suppression of fluorescence light in Raman measurements using ultrafast time gated charge coupled device camera
Authors: Martyshkin D. V. , Ahuja R. C. , Kudriavtsev A. , Mirov S. B.
Published: February 2004

A high level of fluorescence background signal rejection was achieved for solid and powder samples by using a combination of simple low-resolution spectrograph and ultrafast gated charge coupled device (CCD) camera. The unique timing characteristics of the CCD camera match exceptionally well to characteristics of a Ti:sapphire oscillator allowing fast gated light detection at a repetition rate of up to 110 MHz, making this approach superior in terms of the duty cycle in comparison with other time-resolved Raman techniques. The achieved temporal resolution was about 150 ps under 785 nm Ti: sapphire laser excitation. At an average excitation power up to 300 mW there was no noticeable sample damage observed. Hence, the demonstrated approach extends the capabilities of Raman spectroscopy regarding the investigation of samples with a short fluorescence lifetime. The combination of a spectrometer and a gated CCD camera allows simultaneous study of spectral and temporal characteristics of emitted light. This capability opens an exciting possibility to build a universal instrument for solving multitask problems in applied laser spectroscopy.
Intense narrow-bandwidth extreme ultraviolet laser system tunable up to 20 eV
Authors: Rupper P. , Merkt F.
Published: February 2004

Narrow-bandwidth, broadly tunable vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation has been generated up to 20 eV by resonance-enhanced four-wave mixing in argon using the five two-photon resonances of argon between 105 000 and 109 000 cm−1 corresponding to the optically accessible fine-structure components of the 3p54p electron configuration. These two-photon resonances were reached by using the output of an F2 excimer laser (157 nm) and the tripled output of a dye laser. The highest VUV/XUV intensities were obtained using the 3p54p′[1/2]0←3p6(1S0) two-photon resonance at 108 722.62 cm−1 in combination with the main excimer line. The conversion efficiency reaches an optimum for photon energies around 16 eV and slowly decreases when the photon energy rises to 20 eV. The use of the argon resonances also facilitates the generation of intense VUV laser radiation around 90 000 cm−1, a region that is not easily accessible by four-wave mixing with the commonly used two-photon resonances of krypton (202.3 and 212.5 nm) and xenon (222.6 and 249.6 nm). The bandwidth of the VUV/XUV laser radiation was measured to be less than 0.2 cm−1 over the entire range between 11 and 20 eV. The VUV/XUV laser system was used to measure the isotopic shift of the 2p54s[3/2]1←2p6(1S0) resonance in neon at 158 796 cm−1, and fully rotationally resolved photoelectron spectra of C2H2 around 92 000 cm−1 and CO2 between 140 000 and 146 000 cm−1. Improved values of the first adiabatic ionization energy of C2H2 (91 953.5±0.5 cm−1) and CO2 (111 112.3±0.8 cm−1) were determined.
Newly designed graded screen array for particle size measurements of unattached radon decay products
Authors: Fukutsu Kumiko , Yamada Yuji , Tokonami Shinji , Iida Takao
Published: February 2004

A new graded screen array was designed for measuring the particle size distribution of unattached radon decay products. Its validity was verified by experimental approaches using a radon/aerosol chamber. Use of fine wire mesh screens achieved both a high volumetric air flow rate and high alpha count rate. Consequently, this improvement produced both a high sensitivity and good precision in the particle size distribution measurement for unattached radon decay products with an activity median diameter around 1 nm. A special Mylar film cover was used to protect the contamination of detector surface. And shielding effects by wire of a screen were examined in our alpha counting system for precise measurements. As for the results, the particle size distribution of unattached radon decay products was observed at around 1 nm as a narrow peak with the geometrical standard deviation of 1.1. When carrier aerosols were injected into the chamber, typical bimodal distribution was clearly observed. This clear bimodal distribution was observed even in an atmosphere with a relatively low radon concentration of 150 Bq m−3. It indicates that this system is adequate for measurements of the particle size distribution of unattached radon decay products in the natural environment.
Turbulence spreading and transport scaling in global gyrokinetic particle simulations
Authors: Lin Z. , Hahm T. S.
Published: February 2004

An intriguing observation in magnetically confined plasma experiments and in global gyrokinetic particle simulations of toroidal ion temperature gradient turbulence is that the fluctuations are microscopic, while the resulting turbulent transport is not gyro-Bohm [Z. Lin et al., Phys. Rev. Lett. 88, 195004 (2002)]. A possible resolution to this puzzle is identified as turbulence spreading from the linearly active (unstable) region to the linearly inactive (stable) region. Large scale gyrokinetic simulations found that transport driven by microscopic fluctuations is diffusive and local, whereas the fluctuation intensity is determined by nonlocal effects. Fluctuations are found to spread from the linearly active region to the linearly inactive region. This turbulence spreading reduces the fluctuation intensity in the unstable region, especially for a smaller device size, and thus introduces a nonlocal dependence in the fluctuation intensity. The device size dependence of the fluctuation intensity, in turn, is responsible for the observed gradual transition from Bohm to gyro-Bohm transport scaling.
Instrument for the investigation of the nanostructure of Pu and other actinides
Authors: Trelenberg T. W. , Glade S. C. , Felter T. E. , Tobin J. G. , Hamza A. V.
Published: February 2004

An instrument for the synthesis and analysis of nanoparticles and ultrathin films of Pu and other actinides has been constructed. To facilitate the production of nanoscale specimens of these materials, pulsed laser ablation was chosen for the deposition process. The highly toxic and radioactive nature of these materials created a challenging safety environment that needed to be addressed before any work could begin. Particular attention has been paid in this respect towards our future work with plutonium, though the design structure presented here would work equally well with other nonradioactive toxic or reactive materials. The analytical capabilities of the instrument include in situ photoelectron spectroscopy, low energy electron diffraction, scanning tunneling microscopy, and scanning tunneling spectroscopy. The instrument design and first results will be discussed.
Application of microwave reflectometry to register Alfvén wave resonances in the TCABR tokamak
Authors: Ruchko L. F. , Valencia R. , Galvão R. M. O. , Lerche E. A. , Elfimov A. G. , Bellintani V. , Elizondo J. I. , Fagundes A. N. , Fonseca A. M. M. , Kuznetsov Yu. K. , Nascimento I. C. , de Sá W. P. , Sanada E. , da Silva R. P.
Published: February 2004

Application of microwave reflectometry to study Alfvén wave resonances in the TCABR tokamak is described. A microwave reflectometer was used to register plasma density oscillations driven by the excited Alfvén waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfvén resonance rA, which is defined by the relation ω=k∥(rA)CA(rA), is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the rf generator frequency increases. This method can give information about the localization of the rf power deposition zone in Alfvén wave plasma heating and current drive experiments. It also allows finding the plasma current profile in the region of the rf power deposition.
Comment on “Implementing of a precision fast thermoelectric cooler using a personal computer parallel port connection and ADN8830 controller” [Rev. Sci. Instrum. 74, 3862 (2003)]
Authors: Sloman A. W.
Published: February 2004

Eli Flaxer has described a feedback controlled circuit to drive a Peltier junction to control the temperature of a specimen in the range 0 to 50 °C in a room temperature environment [Rev. Sci. Instrum. 74, 3862 (2003)]. The amount of heat transferred per unit current by a Peltier junction varies substantially with the temperature difference across the Peltier junction. Flaxer’s circuit does not provide any mechanism to compensate for this variation. This means that if the proportional-integral-differential control loop he uses is optimized at any particular temperature differential, the control loop with be over-damped at lower specimen temperatures, and under-damped at higher specimen temperatures. A circuit using a second thermistor to monitor the temperature of the exhaust side of the Peltier junction, and a digital control loop, can minimize this problem [A. W. Sloman, Paul Buggs, James Molloy, and Douglas Stewart, Meas. Sci. Technol. 7, 1653 (1996)]. This circuit has the incidental advantage of offering ten times better temperature stability.
Optical communication components
Authors: Eldada Louay
Published: February 2004

We review and contrast key technologies developed to address the optical components market for communication applications. We first review the component requirements from a network perspective. We then look at different material systems, compare their properties, and describe the functions achieved to date in each of them. The material systems reviewed include silica fiber, silica on silicon, silicon on insulator, silicon oxynitride, sol-gels, polymers, thin-film dielectrics, lithium niobate, indium phosphide, gallium arsenide, magneto-optic materials, and birefringent crystals. We then describe the most commonly used classes of optical device technology and present their pros and cons as well as the functions achieved to date in each of them. The technologies reviewed include passive, actuation, and active technologies. The passive technologies described include fused fibers, dispersion-compensating fiber, beam steering, Bragg gratings, diffraction gratings, holographic elements, thin-film filters, photonic crystals, microrings, and birefringent elements. The actuation technologies include thermo-optics, electro-optics, acousto-optics, magneto-optics, electroabsorption, liquid crystals, total internal reflection technologies, and mechanical actuation. The active technologies include heterostructures, quantum wells, rare-earth doping, dye doping, Raman amplification, and semiconductor amplification. We also investigate the use of different material systems and device technologies to achieve building-block functions, including lasers, amplifiers, detectors, modulators, polarization controllers, couplers, filters, switches, attenuators, isolators, circulators, wavelength converters, chromatic dispersion compensators, and polarization mode dispersion compensators. Some of the technologies presented are well established in the industry and in some cases have reached the commodity stage, others have recently become ready for commercial introduction, while some others are still under development in research laboratories and require significant progress in fabrication and assembly processes before they become commercially viable.
Experimental Investigation of the Stability of Hagen-Poiseuille Flow
Authors: Fox John A. , Lessen Martin , Bhat Waman V.
Published: February 2004

The stability of pipe Poiseuille flow to azimuthally periodic disturbances is examined experimentally. The disturbance is generated in the fully developed flow region to avoid extraneous effects of possible entrance flow instability. A Reynolds number spectrum to above 3200 was covered with a frequency range from 13 to 1 kc/sec. The peak disturbance velocity amplitude was maintained at a value less than 2% of the maximum steady-state velocity in the pipe. Some measurements of spacewise changes in velocity and disturbance wavelength were made as well as radial surveys of disturbance amplitude and steady-state profiles. The results indicate that the stability is both frequency and Reynolds number dependent to the first mode of azimuthal periodicity, with a minimum critical Reynolds number occurring at about 2150.
Convective Instability in Fluids of High Thermal Diffusivity
Authors: Jakeman E.
Published: February 2004

The problem of the onset of thermal instability in heated layers of fluid is investigated in a linear approximation, assuming the confining planes to be surfaces of constant heat flux. It is shown that in the absence of time dependence a limiting point of the neutral stability curve can be obtained exactly by analytical methods. This point is evaluated for a variety of boundary conditions on the fluid motion, and it is verified that the results give criteria for the onset of instability as stationary convection. Similar calculations are made for an idealized fluid in the time-dependent case, and it is established that the combined effects of buoyancy and surface-tension gradients may lead to overstable oscillations in certain circumstances.
Quantum-Mechanical Theory of Nonlinear Plasma Phenomena in a Strong Magnetic Field
Authors: Walters G. M. , Harris E. G.
Published: February 2004

A quantum-mechanical description of a cold plasma in a strong magnetic field is developed from the hydrodynamic equations. The plasmon-particle and plasmon-plasmon interaction Hamiltonians are derived and expressed in terms of second quantized operators for plasmons and particles. The quantum-mechanical plasmon growth rate due to stimulated emission is obtained. The classical limit is taken and compared with the growth rate predicated by the linearized Vlasov equations. The possibility of a spacial gradient instability, that of Mikhailovskii and Timofeev, arises and is interpreted in terms of emission and absorption of plasmons by the energetic particles. The plasmon-particle interaction Hamiltonian is used to determine the time development of the particle distribution function. Finally, the plasmon-plasmon coupling is included in the time development of the plasmon distribution.
Excitation of Longitudinal Waves near Electron-Cyclotron Harmonics
Authors: Gruber S. , Bekefi G.
Published: February 2004

Longitudinal waves were excited radially across a plasma column subjected to an axial magnetic field. A strip line placed externally to the column was the source of excitation. The wavelength and propagation direction were measured by a movable antenna immersed in the plasma. The measurements are compared with theory and with observations made with an externally situated receiving antenna. The following conclusions are drawn. (a) Contrary to the commonly accepted notion, the longitudinal waves are propagating rather than standing waves. The propagation is radial towards the axis of the plasma column. Since the waves are backward waves, the energy propagates radially outwards. (b) The internal and external measurements are related to good accuracy through the geometrical optics, “phase quantization” condition, ∫ k dr ≈ 2mπ, where k is the propagation constant, r is the radial coordinate, and m is an integer. The left-hand side of the equation is deduced from measurements of k made with the movable antenna immersed in the plasma, and the right-hand side of the equation is obtained from measurements of the signal received by the external antenna.
Fast Randomization of an Electron Gas by Trapped Electroacoustic Waves
Authors: Kettani M. Ali , Hoyaux Max F.
Published: February 2004

A simplified theoretical model for a phenomenon of fast randomization of an electron gas which was investigated experimentally is presented. It is assumed that a certain region in constricted electrical discharges, called “plasma sac” can become the siege of trapped electroacoustic waves generated by a beam-plasma interaction. The interaction between those waves and the electron gas is treated by a pseudo-collisional method leading to a generalization of Boltzmann's H-theorem. The velocity distribution, which is essentially Maxwellian with a significant deficiency in the fast electron tail, is deduced.
Numerical Study of Density-Current Surges
Authors: Daly Bart J. , Pracht William E.
Published: February 2004

The marker and cell technique for the calculation of time-dependent incompressible flows is used to study density-current surges at density ratios between 1.05 and 3.0. The study is performed using the full two-fluid approach as well as a single-fluid technique involving solute transport and a Boussinesq approximation to the Navier-Stokes equation. A comparison of these results indicates that the Boussinesq approximation is reasonably accurate for density ratios less than 1.3. The results of free surface and confined flow calculations are compared and the effects of slope, viscosity, and surface tension on density current flows are examined. Complete descriptions are given for the first time of the techniques used for including solute transport and surface tension in marker and cell calculations, and an analysis of the calculational stability properties of the method is included. The numerical results, which are in good agreement with available experimental measurements, provide information about some previously univestigated aspects of density current flows. Of particular importance here is the variation of the front velocity and height with density ratio, for ratios in the range, 1.2-3.0. New analytical treatments of the initial current surge, the steady flow, and the effects of viscosity are presented and compared with computed results.
Ion Waves in a Collisionless Plasma
Authors: Weitzner H. , Dobrott D.
Published: February 2004

Longitudinal plane wave oscillations of electrons and ions described by the linearized Vlasov equation with no magnetic fields and coupled by Poisson's equation are examined for a wider class of distribution functions than previously analyzed. An expression is obtained for the time evolution of the electric field in terms of approximate wave contributions and a remainder. It is found that an electron plasma oscillation and an ion wave—either an ion acoustic wave or an ion plasma oscillation—are observable. For a short initial time interval coherent plasma oscillations are observable and persist if the ratio of Debye length to wavelength is small. For larger times, but not too large, ion waves are observable, and they persist for many periods if ρ−1 = {(T+/T−)[(kλ−)2 + γ0])12 ≪ 1, where γ0 is a positive constant of order unity determined by the electron distribution functions. The ion waves are thereafter exponentially damped into the incoherent background of the remaining contribution.
Propagation of Ion Waves in a Plasma Flow towards a Transonic Point
Authors: Cavaliere A. , Engelmann F. , Sestero A.
Published: February 2004

The propagation of ion-acoustic waves in a weakly ionized, bounded plasma is investigated, typically with reference to discharge tubes, where a flow of plasma towards the walls is required to ensure the balance between volume ionization and surface recombination. A macroscopic description is used which accounts for effects of ion-neutral collisions and finite ion temperature. Special attention is paid to the problem of the wave absorption at the plasma boundary. It results that the ion-acoustic waves are strongly absorbed at the plasma boundary whenever the electrostatic boundary conditions require the formation of a space-charge sheath near the walls (implying the existence of a transonic point of the plasma flow at the limit between the sheath and the plasma). The situation where no transonic point exists is also discussed.
Effects of Electron-Temperature Variation on Ion Acoustic Waves
Authors: Alexeff I. , Jones W. D. , Montgomery D.
Published: February 2004

It has recently been discovered how to vary the electron temperature in a simple discharge-tube plasma over large ranges in a controlled manner. Subsequent use of this technique to study the propagation characteristics of ion acoustic waves in the plasma, as a function of electron temperature, has made it possible to measure properties of both the waves and the plasma: (1) the theoretically predicted dependence of wave velocity on electron temperature has been verified; (2) good agreement was found between a strong damping of the waves occurring at low electron temperatures and a theoretically predicted Landau damping; and (3) an accurate means for measuring the plasma ion temperature has been found.
Interaction between an Electromagnetic Wave, Plasma Waves, and an Ion Acoustic Wave
Authors: Sugihara Ryo
Published: February 2004

A nonlinear process is considered where a tranverse wave with a frequency near the plasma frequency decays into a plasma oscillation and an ion acoustic wave. The inverse processes are investigated. Also, the interaction of four waves—one tranverse, one ion acoustic, and two plasma oscillations are investigated. The energy of the tranverse wave is partially converted to that of the ion acoustic wave in the presence of a plasma oscillation in both cases. A necessary condition for the process of four wave interaction is obtained. The phases of all interacting waves are assumed to be perfectly matched.
Nonlinear Oscillations in a Vlasov-Maxwell Plasma
Authors: Davidson R. C.
Published: February 2004

In the absence of spatial variations, the Vlasov-Maxwell equations are solved exactly allowing for an initial plasma current to generate large-amplitude, oscillatory, electric fields in the presence of a uniform external magnetic field. These results are extended to include the effects of weak spatial inhomogeneities for the case in which no external magnetic field is present. The analysis is carried out to first order in the small parameter, λ ∼(Debye length/length scale of spatial variations), by means of a multiple-time, multiple-space, perturbation scheme. Due to the presence of weak spatial variations, a magnetic field is generated oscillating at the plasma frequency, whereas the zero-order electric field is modified by oscillatory behavior at the second harmonic of the plasma frequency. The distribution function is obtained to order λ in situations where the zero-order electric field is initially curl free; under these circumstances no first-order magnetic field is generated and the problem remains electrostatic to order λ. In addition, a nonlinear differential equation is derived describing the long time (t ∼ 1/λ) behavior of the zero-order distribution function; the linear stability of this equation is discussed.
Negative Mass Instability with Bθ Field
Authors: Landau Ronald W.
Published: February 2004

Earlier negative mass instability calculations with a mirror magnetic field have been extended to also include a Bθfield. The plasma is contained in a torus, of elliptical cross section, which is radially thin. From a new dispersion relation it is found that: (a) for a cold plasma the negative mass instability persists even for large Bθ, although the growth rate goes as Bθ−1; (b) for a finite velocity spread plasma, the addition of a Bθ field allows much higher densities, because the stability criterion becomes independent of density. These results appear to explain the beam disruption observed on the Stevens Institute plasma betatron surprisingly well.
Investigation of the Positive Column above the Critical Field
Authors: Sheffield John
Published: February 2004

An experimental investigation, which includes detailed spatial correlation measurements, reveals that the helium positive column is dominated up to many times the critical magnetic field by a finite amplitude helical oscillation. The equations describing this situation are solved by introducing into them experimental values for some of the variables. With this technique it is possible to avoid the use of unrealistic boundary conditions. The calculated values of axial wavelength, helical rotational frequency, and the magnitude of the radial flux of ions are in fair agreement with experimental values. The calculated growth rate is essentially zero up to four times the critical field, in agreement with the theory of Holter and Johnson, but increases steadily with further increase in magnetic field. A possible explanation of the latter region is that the previously stable helical oscillation turns into a helical relaxation oscillation. While the results of this investigation are contrary to the requirements of the turbulence theory of Kadomstev, there is a faint indication that the system may become turbulent at very high magnetic fields.
Experimental Investigation of Axial Plasma Injection into a Magnetic Dipole Field
Authors: Jensen V. O.
Published: February 2004

A high-density helium plasma, accelerated from a conical pinch, is injected axially into a magnetic dipole field. Magnetic probe measurements show that, near the axis, a compression of the field is super-imposed on the standard diamagnetic depression. The compression starts downstream and moves towards the injector. Simultaneously with the compression, an increase in the electron temperature and reflection of a small amount of plasma are seen. The amount of plasma transmitted through the dipole field is found to be nearly independent of the field strength.
Simple Approximations to Turbulent Energy Transfer in the Universal Equilibrium Range
Authors: Tennekes H.
Published: February 2004

The spectral transfer of turbulent energy at high wave-numbers may be approximated by S/ε = (k5/3 E/α ε 2/3)n. The approximations of Pao (n = 1) and of Kovasznay (n = 32) are members of this family.
Precursor Ionization from Exploding Wires
Authors: Jones Donald L.
Published: February 2004

The precursor ionization in an exploding wire discharge is of the order of 1012 electrons cm-3. The quantity of ionization produced is a sensitive function of the gas type, distance from the wire axis, pressure, and capacitor energy.
Two-Pole Approximation for the Plasma Dispersion Function
Authors: Fried Burton D. , Hedrick C. L. , Mccune James
Published: February 2004

A simple algebraic approximation to the Hilbert transform of the Gaussian, valid over most of the complex plane, is given.
Energy Principle for Plasma Stability against Low-Frequency Interchanges
Authors: Rutherford Paul H. , Frieman Edward A.
Published: February 2004

An energy principle is constructed for the stability of a plasma equilibrium F ≡ F(μ, J, K) against low-frequency interchanges, including trapped particle instabilities.
Diffusion by Continuous Movements
Authors: Philip J. R.
Published: February 2004

The influence of velocity distribution on one-particle dispersion is studied. The ensemble of particle releases is subdivided into subensembles characterized by particle velocity at the instant of release. Under certain simplifying assumptions, the motion of the particles of each subensemble is a systematic drift, combined with dispersal about the subensemble drift position. The displacement variance due to subensemble drift is dominant for small time t since particle release, whereas that due to subensemble is the more important at large t. The assumption that subensemble dispersal is Gaussian yields an estimate of the probability distribution density function θ(x, t) governing particle position which is correct both in the limits of t small and t large. The estimate is thus better than that given by the usual assumption that θ is Gaussian, and appears useful for t values less than the Lagrangian integral scale. The subensemble approach offers some new insights into the problem of the instantaneous structure of the individual plume.
Formulation of the Theory of Turbulence
Authors: Orszag Steven A. , Kruskal Martin D.
Published: February 2004

A theory of homogeneous, isotropic turbulence in an incompressible fluid is formulated. This theory provides a basis for the dynamics of fully developed turbulence. A hierarchy of equations to determine the time evolution of single-time moments of the velocity field is derived. Various properties of this hierarchy are exhibited, such as positivity of the dynamically determined energy density. The theory is applied to the study of the inertial range, where the Kolmogoroff theory is justified. A generalized inertial-range theory is formulated. This “semilocal” theory is characterized by local energy transfer and nonlocal relaxation of triple moments. The generalized theory is applied to the study of hydromagnetic turbulence, which is shown to possess a semilocal inertial range.
Stability of a Plane Poiseuille Flow of a Second-Order Fluid
Authors: Chun D. H. , Schwarz W. H.
Published: February 2004

The effect of slight viscoelasticity on the hydrodynamic stability of a plane Poiseuille flow is investigated by the linearized method of small two-dimensional disturbances. The resulting equation is that of Orr-Sommerfeld, modified however by a non-Newtonian term. The constitutive equation used to represent the response of the fluid is that of a second-order fluid. The equations are solved numerically, and compared with the results of Thomas. The critical Reynolds number is found to be lowered as the magnitude of a non-Newtonian parameter increased.
Slip Coefficient of a Gas
Authors: Huang A. Ben
Published: February 2004

An analytic solution of the linearized Boltzmann-Bhatnagar-Gross-Krook equation for the slipvelocity problem is developed based on the discrete ordinate method. Analytic expressions of the perturbed distribution function, the velocity profile and the slip coefficient, particularly suitable to numerical calculations, are thus obtained. The numerical value of the slip coefficient is calculated for various orders of approximation. The result obtained for 6 discrete points is in excellent agreement with the numerically exact value.
Spectrographic Measurements of Electric Shock Tube Flow
Authors: Barach J. P. , Mayes T. W.
Published: February 2004

A Kerr-cell shuttered spectrograph has been used to make measurements of the state of the plasma produced by an electric shock tube operating at high pressure (100 Torr of CO2). Relative line intensities have been used to measure the temperature and Stark broading has been used to obtain the electron densities. The free-running plasma front is expected and found to be in local thermodynamic equilibrium. The complete state of the plasma flow is then deduced. Measured pressure, trajectory and density profiles approximate blast-wave calculations with, however, higher-pressure and lower-density values at the front than calculated from the Rankine-Hugoniot theory. The temperature profile does have a rising trend upstream, but the front temperature is much too high (a factor of four). Violent forward mixing of the arc gas, due to the deceleration of the front, accounts for this. An arc-produced photon precursor is assumed to dissociate the CO2.
Influence of Convection on Current Patterns in Nonequilibrium Plasmas
Authors: Sherman Arthur , Yeh Hsuan
Published: February 2004

The phenomenon of convection is investigated in nonequilibrium plasmas where there are substantial nonuniformities in the plasma state. It is shown that a boundary-value problem can be formulated to solve for the current stream function and the electron temperature including convective effects. The resulting equations are coupled and nonlinear so they are solved by an iteration technique. Two simple examples are chosen to illustrate the procedure. The first consists of a pair of electrodes in a straight channel, and the second is the entrance region of a parallel-walled solid electrode device. In each case, only an applied electric field was considered. The calculations show the current streamlines and the electron temperature distribution displaced downstream for both uniform and nonuniform flows. It is also shown that as the flow velocity increases the voltage necessary to pass a specified amount of current increases.
Stability of Parallel Flow between Concentric Cylinders
Authors: Mott J. E. , Joseph D. D.
Published: February 2004

The linear stability of parallel flow in a concentric annulus to infinitesimal, axially symmetric disturbances is considered. First, the Poiseuille flow in annular cylinders is studied with the ratio k of the outer to inner cylinder as a parameter. The critical Reynolds number is a monotone function of this radius ratio, increasing without bound as k → ∞ (Hagen-Poiseuille flow) from the plane Poiseuille flow limit (k = 1). Second, a one-parameter family of skewed (variable viscosity) flows in a fixed annulus is studied. The neutral curves for many of these skewed profiles have a second minimum, which for sufficiently skewed profiles, gives the lowest value of the Reynolds number. These two minima are associated with two very different distributions of Reynolds stress. Both distributions are such that on part of the channel the energy is transferred from the disturbance motion to the basic motion and both can be explained by analysis of the Reynolds stress jump condition and the known structure of Reynolds stress near a rigid wall.