bluuebunny/crossref_metadata_2025 huggingface dataset:
A high efficiency Smith–Purcel E×B drifting electron laser
Authors: Riyopoulos Spilios
Published: July 2002

A drifting electron laser (DEL) [S. Riyopoulos, Phys. Plasmas 3, 3828 (1996)] can by far exceed the 1/2Nw free electron laser (FEL) efficiency limit, provided the radiation strength exceeds that of the wiggler. It is shown here that a Smith–Purcel DEL, powered by an E×B drifting electron beam grazing a corrugated structure (grating), achieves high efficiency through unbound electron excursions at much lower radiation power; the radiation strength must only exceed the small strength of the dc potential ripple caused by the corrugation. Efficiency reduction, caused in a wiggler based DEL by a Gaussian radiation beam of small spot size, is also avoided. The nonlinear saturation efficiency of the grating DEL is obtained analytically for a sheet beam. The effects of the drift velocity shear caused by the space-charge field, as well as that of the thermal spreads in the injection velocity are found to be small and easier to overcome than in a FEL. It is also proven that, even for operation at low power, the efficiency remains higher than in a grating FEL due to comparatively larger height of the trapped particle island.
Control of plasma sheath and its potential by dielectric medium
Authors: Uhm Han S.
Published: July 2002

Influence of a dielectric medium on the properties of plasma sheaths is investigated. A theoretical model is developed under the assumption that collisionless plasma is confined by two planar electrodes. A planar dielectric medium with a finite thickness is inserted into the plasma, partitioning it into two pieces. Without a priori assumption about the plasma density, an analytical expression of potential distribution in the plasma-dielectric system is obtained. It is found that the electric potential at the dielectric center in high-density plasma is independent of its position and is a half-value of the applied voltage. The electric potential, field, and the charge density in high-density plasma are inversely proportional to the sum of 4+D/ελ, where D and ε are the thickness and dielectric constant, respectively, of the dielectric medium, and λ is the Debye length of high-density plasmas. In this context, the plasma sheaths may easily be controlled by an appropriate choice of dielectric material and its size.
Alfvén waves and wave-induced transport near an X point
Authors: Myra J. R. , D’Ippolito D. A.
Published: July 2002

The behavior of Alfvén waves and the corresponding variation of the wave-induced transport coefficients along a field line including the divertor X-point region are examined. It is shown that several competing effects exist and can be quantified using a quasilinear diffusion model that takes the magnetic geometry of the X point into account. To address the issue of mode behavior and the validity of the eikonal approximation near the X point, an exact analytical solution of an equation describing Alfvén waves in the X-point region is obtained. The results suggest that the X-point region can only dominate Alfvén wave-induced transport on flux surfaces that are very close to the separatrix.
Symmetric aluminum-wire arrays generate high-quality Z pinches at large array radii
Authors: Sanford T. W. L. , Mock R. C. , Spielman R. B. , Peterson D. L. , Mosher D. , Roderick N. F.
Published: July 2002

A Saturn-accelerator study of annular, aluminum-wire array, Z-pinch implosions, in the calculated high-wire-number plasma-shell regime [Phys. Rev. Lett. 77, 5063 (1996)], shows that the radiated x-ray pulse width increases from about 4 nsec to about 7 nsec, when the radius of the array is increased from 8.75 to 20 mm at a fixed array mass of 0.6 mg. Eulerian radiation- magnetohydrodynamic code (E-RMHC) simulations in the r-z plane suggest that this pulse-width increase with radius is due to the faster growth of the shell thickness (that arises from a two-stage development in the magnetic Rayleigh–Taylor instability) relative to the increase in the shell implosion velocity. Over the array radii explored, the measured peak total x-ray power of ∼40 TW and energy of ∼325 kJ show little change outside of a ±15% shot-to-shot fluctuation and are consistent with the E-RMHC simulations. Similarly, the measured peak K-shell (lines plus continuum) power of ∼8 TW and energy of ∼70 kJ show little change with radius. The minimal change in K-shell yield is in agreement with simple K-shell radiation scaling models that assume a fixed radial compression for all initial array radii. These results suggest that the improved uniformity provided by the large number of wires in the initial array reduces the disruptive effects of the Rayleigh–Taylor instability observed in small-wire-number imploding loads.
Prediction of Alfvén eigenmode dampings in the Joint European Torus
Authors: Jaun A. , Fasoli A. , Heidbrink W. W.
Published: July 2002

Predictions from a gyrokinetic toroidal plasma model reproduce for the first time the evolution of Alfvén eigenmode (AE) dampings over a range of discharges. The coupling between shear-and kinetic-Alfvén waves is responsible for the main source of damping through Landau interactions and can be an order of magnitude larger than fluid predictions neglecting global kinetic effects. Strong stabilization occurs when the wave field gets localized radially by a rise in the edge magnetic shear, explaining why global AEs have never been detected in the Joint European Torus [Rebut, Bickerton, and Keen, Nucl. Fusion 25, 1011 (1985)] in the presence of an X point and suggesting how global Alfvén instabilities could be avoided in future reactors.
On the dominant and subdominant behavior of stimulated Raman and Brillouin scattering driven by nonuniform laser beams
Authors: Berger R. L. , Still C. H. , Williams E. A. , Langdon A. B.
Published: July 2002

In a number of experiments, stimulated Brillouin (SBS) or Raman backscattering (SRS) has been observed to be much more vigorous than the other although the expectations based on linear gain exponents are that they should both be reflecting large amounts of incident light. Multidimensional fluid simulations of the growth and saturation of these two instabilities driven by a nonuniform incident laser beam are presented. On the fast time scale, the nonlinear saturation occurs via an anomalous damping inspired by fundamental studies of Langmuir turbulence [D. F. DuBois et al., Bull. Am. Phys. Soc. 41, 1531 (1996)] and acoustic wave turbulence [B. I. Cohen et al., Phys. Plasmas 4, 956 (1997)]. Over a longer time scale, SRS and SBS are limited by quasilinear processes such as flows induced by the transfer of momentum from the light to the plasma and ion temperature increases caused by a loss of light energy in SBS. The simulations show a reduction of the SBS reflectivity under conditions of strong SRS reflectivity even if the laser energy is not depleted. The recent observations of decreasing SBS reflectivity with increasing plasma density [D. S. Montgomery, Phys. Plasmas 5, 1973 (1998)] are shown to be consistent with linear theory and nonlinear simulations of SBS provided the increasing levels of SRS are included. Because the reflectivity is produced by scattering in intense hotspots, where the local reflectivity can be very large, the SBS and SRS can be anticorrelated even when the total scattering is quite modest.
Acoustic modes in a collisional dusty plasma
Authors: Ivlev A. V. , Samsonov D. , Goree J. , Morfill G. , Fortov V. E.
Published: July 2002

The acoustic modes in a collisional dusty plasma are studied, taking into account the influence of ion–neutral collisions, ion drag, and neutral friction. It is assumed that the frequency of ion–neutral collisions is greater than the frequency of ion–dust collisions. Two limiting cases of short-wavelength and long-wavelength modes are considered separately, depending on the ratio of the dust–ion acoustic frequency to the frequency of ion–neutral collisions. It is shown that for the long wavelengths coupling between the modes becomes strong, causing the appearance of the hybrid long-wavelength mode. It is also found that in the long-wavelength case there are two types of instability caused by ionization. The theoretical results are compared with the data obtained in experiments with growing particles.
Experimental study of the dynamics of conditionally averaged structures in weakly developed electrostatic turbulence
Authors: Grulke O. , Klinger T. , Piel A.
Published: July 2002

An experimental study of coherent structures in the electrostatic turbulence observed in a steady-state operated, magnetized, cylindrical low-β argon plasma is presented. The conditional averaging technique is used to detect and to characterize large-scale conditional structures. The results of conditional averaging of a single drift wave mode are consistent with theoretical predictions and the data obtained by an azimuthal array of 64 Langmuir probes. In the turbulent regime, large monopole structures in the density fluctuations are identified, which develop by particle trapping due to associated potential structures. These structures lead to transport of particles across magnetic field lines, as evident from the trajectory of the structures. The comparison of the dynamics of conditional structures and unaveraged spatiotemporal structures suggests that they are of the same origin.
Analytic solution of the Vlasov equation and closed fluid equations
Authors: Bendib K. , Bendib A.
Published: July 2002

Using the projection operators, the analytic solution of the Vlasov equation linearized with respect to the global Maxwellian distribution function is presented. The expressions of the generalized quasistatic collisionless transport coefficients are computed, and in particular, a strong plasma temperature anisotropy and a convective heat flux are pointed out. The results obtained have been used to close the fluid equations. The spectral range validity of the collisionless fluid equations is also studied with the use of the plasma response function.
Gyrokinetic description of the structural eccentricity influence on the starting current of a coaxial-cavity gyrotron
Authors: Zhang Shi-Chang , Thumm Manfred
Published: July 2002

Based on the gyrokinetics of free-electron masers, the structural eccentricity influence on the starting current of a 1.5 MW/140 GHz, TE28,16,1 coaxial-cavity gyrotron oscillator is studied. Results show that the structural eccentricity substantially increases the starting current. The stable term of the instability, which results from the contribution of the guiding-center distribution, is of significance in a coaxial-cavity gyrotron, although it has usually been neglected in a cylindrical-cavity gyrotron. It is found that the influence of the structural eccentricity on the performance of the coaxial cavity gyrotron is more serious than that of the beam misalignment. The influences of the operating magnetic field and the beam energy on the starting current are also taken into account.
Initial physics achievements of large helical device experiments
Authors: Motojima O. , Yamada H. , Komori A. , Ohyabu N. , Kawahata K. , Kaneko O. , Masuzaki S. , Ejiri A. , Emoto M. , Funaba H. , Goto M. , Ida K. , Idei H. , Inagaki S. , Inoue N. , Kado S. , Kubo S. , Kumazawa R. , Minami T. , Miyazawa J. , Morisaki T. , Morita S. , Murakami S. , Muto S. , Mutoh T. , Nagayama Y. , Nakamura Y. , Nakanishi H. , Narihara K. , Nishimura K. , Noda N. , Kobuchi T. , Ohdachi S. , Oka Y. , Osakabe M. , Ozaki T. , Peterson B. J. , Sagara A. , Sakakibara S. , Sakamoto R. , Sasao H. , Sasao M. , Sato K. , Sato M. , Seki T. , Shimozuma T. , Shoji M. , Suzuki H. , Takeiri Y. , Tanaka K. , Toi K. , Tokuzawa T. , Tsumori K. , Tsuzuki K. , Yamada I. , Yamaguchi S. , Yokoyama M. , Watanabe K. Y. , Watari T. , Hamada Y. , Matsuoka K. , Murai K. , Ohkubo K. , Ohtake I. , Okamoto M. , Satoh S. , Satow T. , Sudo S. , Tanahashi S. , Yamazaki K. , Fujiwara M. , Iiyoshi A.
Published: July 2002

The Large Helical Device (LHD) experiments [O. Motojima, et al., Proceedings, 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437] have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas (R=3.9 m, a=0.6 m), which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection (NBI) has produced plasmas with a fusion triple product of 8×1018 keV m−3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to 〈β〉=0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics (MHD) instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.
Neutrals’ effects on stability
Authors: Ödblom A. , Catto P. J. , Krasheninnikov S. I.
Published: July 2002

The physics of the neutral atoms is incorporated into a generalized description of edge instabilities in tokamaks and plasmas with a small neutral fraction. The description includes ideal and resistive ballooning modes; modes driven by a radial electron temperature gradient when the plasma is in contact with conducting material surfaces such as limitors or divertor target plates, the destabilizing effect of the parallel variation in the E×B drift frequency, and effects due to the flow of the neutral gas. The analysis considers the neutral dynamics in both the short and long neutral mean-free path limits (relative to the wavelength of the instability), since the perturbed ion–neutral coupling depends on collisionality. Moreover, parallel and cross-field variations in the equilibrium temperatures, densities, and potential are retained as well as the corresponding diamagnetic effects. In the short neutral mean-free path limit, the ion and neutral viscosities and heat fluxes must be retained, while in the long neutral mean-free path limit the neutrals are not perturbed, but the ion viscosity and heat flux must still be considered. The possible destabilizing impact of the new heat flux and viscous terms on stability is demonstrated.
Plasma fluctuations driven by a randomized laser beam
Authors: Brantov A. V. , Bychenkov V. Yu. , Tikhonchuk V. T. , Rozmus W. , Senecha V. K.
Published: July 2002

Nonthermal fluctuations produced by a randomized laser beam in an underdense plasma have been investigated in the regime of a nonlocal electron transport. A nonlocal closure to linearized fluid equations for fluctuating hydrodynamic variables properly accounts for inverse bremsstrahlung heating and ponderomotive force effects. It has been shown that a typical randomized laser beam as used in the inertial confinement fusion experiments can generate large amplitude density fluctuations well above thermal levels. An expression for the Thomson scattering cross-section from these fluctuations has been derived and discussed. The vortical velocity and magnetic field fluctuations can also be enhanced with the vortical plasma velocity reaching values comparable to the ion acoustic velocity.
Uniform-density, spherical electron focus
Authors: Barnes D. C.
Published: July 2002

An equilibrium electron distribution is exhibited which forms a uniform electron density focus within a spherical system. Such a focus may be used to form a spherical, harmonic well for ion focusing as previously discussed. A self-consistent density and space-charge potential are calculated and the optimum focus radius is determined. Nonideal effects on electron and ion motion in the resulting electrostatic well are briefly discussed and strategies for their minimization are derived.
Radial electric field required to suppress ion temperature gradient modes in the Electric Tokamak
Authors: Kissick M. W. , Leboeuf J.-N. , Cowley S. C. , Dawson J. M. , Decyk V. K. , Gourdain P.-A. , Gauvreau J.-L. , Pribyl P. A. , Schmitz L. W. , Sydora R. D. , Tynan G. R.
Published: July 2002

The Electric Tokamak (ET), currently under construction at the University of California–Los Angeles, is designed to rotate poloidally via a radial current induced by fast wave rf heating fast enough to bifurcate the plasma into a global “H mode” (“high confinement mode”). A global gyrokinetic code is used to explore and illustrate some of the effects on ion temperature gradient turbulence. The realistic radial electric field required to completely suppress these modes for ET parameters is demonstrated to be <−30 V/cm at its maximum near the half radius. The effects of both a poloidally supersonic bulk rotation threshold and the shear in this rotation near that supersonic threshold were shown to be important in reducing these modes.
The generalized hydrodynamic equations for arbitrary collision frequency in a weakly ionized plasma
Authors: Furkal E. , Smolyakov A.
Published: July 2002

Electron transport processes in a weakly ionized plasma with elastic electron-neutral collisions are studied by using the hybrid fluid/kinetic approach. The standard hierarchy of fluid moment equations is closed with expressions for higher hydrodynamic moments (heat flux and viscosity) in terms of the lower moments (temperature, density, and fluid velocity). The heat fluxes and viscosity moments are determined in the linear approximation from the kinetic equation in the Chapman–Enskog form. The obtained system of moment equations describe the transport processes in weakly ionized plasmas in the most general ordering, when the electron mean free path vTe/νe is arbitrary with respect to the characteristic length scale k−1 of the system’s inhomogeneity, and collision frequency νe is arbitrary with respect to the characteristic frequency ω. General expressions for the nonlocal (time and spatial dependent) transport coefficients are obtained. In the nonlocal limit, kvTe≫νe, the derived transport coefficients describe the wave–particle (Landau) interaction effects. Implications of nonlocal effects on plasma heating mechanisms are discussed.
Charged particle motion inside the retarding potential analyzer
Authors: Chao C. K. , Su S.-Y.
Published: July 2002

One of the Ionospheric Plasma and Electrodynamics Instrument (IPEI) payload onboard the Republic of China satellite 1 (ROCSAT-1) spacecraft is a Retarding Potential Analyzer (RPA) to measure the ionospheric plasma composition, temperature, and the ram velocity of the flow. A three-dimensional computer model has been developed to simulate the ion and electron motions inside the RPA of the IPEI payload. The dynamical changes in the charged particles’ motions corresponding to different electric potential profiles inside the RPA detector resulted in some distinctive measuring characteristics of the RPA detector, which show discrepancies in the measured of currents at the collector plate between the analytical model and the computer simulation. The important discoveries in the computer model are: (a) Ion transparency cannot be represented by the physical grid sizes in the detector. The alignments of the grid wires will also affect the ion transparency. The grid planes should be aligned for the maximum optical transparency to detect light ion fluxes. (b) Cut-off energy for protons is not as sharp as indicated in the analytical model. The contamination in the collector current from lighter ion species is possible. (c) Electron contamination at the collector plate is noticed if the mesh sizes are larger than the electron Debye length.
Fast particles-wave interaction in the Alfvén frequency range on the Joint European Torus tokamak
Authors: Fasoli A. , Borba D. , Breizman B. , Gormezano C. , Heeter R. F. , Juan A. , Mantsinen M. , Sharapov S. , Testa D.
Published: July 2002

Wave-particle interaction phenomena in the Alfvén Eigenmode (AE) frequency range are investigated at the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] using active and passive diagnostic methods. Fast particles are generated by neutral beam injection, ion cyclotron resonance heating, and fusion reactions. External antennas are used to excite stable AEs and measure fast particle drive and damping separately. Comparisons with numerical calculations lead to an identification of the different damping mechanisms. The use of the active AE diagnostic system to generate control signals based on the proximity to marginal stability limits for AE and low-frequency magnetohydrodynamic (MHD) modes is explored. Signatures of the different nonlinear regimes of fast particle driven AE instabilities predicted by theory are found in the measured spectra. The diagnostic use of AE measurements to get information both on the plasma bulk and the fast particle distribution is assessed.
Ablation front Rayleigh–Taylor growth experiments in spherically convergent geometry
Authors: Glendinning S. G. , Colvin J. , Haan S. , Kalantar D. H. , Landen O. L. , Marinak M. M. , Remington B. A. , Wallace R. , Cherfils C. , Dague N. , Divol L. , Galmiche D. , Richard A. L.
Published: July 2002

Experiments were performed on the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)], using indirectly driven capsules mounted in cylindrical gold hohlraums, to measure the Rayleigh–Taylor growth at the ablation front by time-resolved radiography. Modulations were preformed on the surface of Ge-doped plastic capsules. With initial modulation amplitude of 2–2.5 μm, growth factors of about six in optical depth were seen, in agreement with simulations using the radiation hydrocode FCI2 [G. Schurtz, La fusion thermonucleaire inertielle par laser, edited by R. Dautray et al. (Eyrolles, Paris, 1994), Vol. 2]. With initial modulation amplitude of 0.5 μm and a longer drive, growth factors of about 100–150 in optical depth were seen. Comparable planar experiments showed growth factors of about 40 in optical depth. Analytical models predict the observed growth factors.
Experimental evidence of improved confinement in a high-beta field-reversed configuration plasma by neutral beam injection
Authors: Asai T. , Suzuki Y. , Yoneda T. , Kodera F. , Okubo M. , Okada S. , Goto S.
Published: July 2002

The first experimental result of high power (14 kV, 23 A) neutral beam (NB) injection into a high-beta field-reversed configuration (FRC) is demonstrated. The result makes it clear that the NB injection improves the plasma performance, increasing the configuration lifetime more than 200% in comparison with the ordinary FRC under similar conditions. A novel NB injection system is presented for application to FRC plasmas. A set of three concave electrodes for beam extraction is used to focus the beam enabling to pass through a narrow port. The target of beam injection is a large bore FRC plasma contained in a mirror field with a mirror ratio of 2–9.
Predictive transport simulations of internal transport barriers using the Multi-Mode model
Authors: Zhu P. , Bateman G. , Kritz A. H. , Horton W.
Published: July 2002

The formation of internal transport barriers observed in both Joint European Torus (JET) [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)] and Doublet III-D Tokamak (DIII-D) [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] are reproduced in predictive transport simulations. These simulations are carried out for two JET-optimized shear discharges and two DIII-D negative central shear discharges using the Multi-Mode model in the time-dependent 1-1/2-D BALDUR transport code [C. E. Singer et al., Comput. Phys. Commun. 49, 275 (1988)]. The Weiland model is used for drift modes in the Multi-Mode model in combination with either Hahm–Burrell or Hamaguchi–Horton flow shear stabilization mechanisms, where the radial electric field is inferred from the measured toroidal velocity profile and the poloidal velocity profile computed using neoclassical theory. The transport barriers are apparent in both the ion temperature and thermal diffusivity profiles of the simulations. The timing and location of the internal transport barriers in the simulations and experimental data for the DIII-D cases are in good agreement, though some differences remain for the JET discharges. The formations of internal transport barriers are interpreted as resulting from a combination of E×B flow shear and weak magnetic shear mechanisms.
Anisotropic pressure stability of a plasma confined in a dipole magnetic field
Authors: Simakov Andrei N. , Hastie R. J. , Catto Peter J.
Published: July 2002

The interchange and ballooning stability of general anisotropic pressure plasma equilibria in a dipolar magnetic field are investigated. Starting with the Kruskal–Oberman form of the energy principle and using a Schwarz inequality, a fluid form of the anisotropic pressure energy principle is derived, which, after appropriate minimization, gives an interchange stability condition and an integro-differential ballooning equation. These results are applied to the case of an anisotropic pressure equilibrium having the perpendicular pressure equal to the parallel pressure times a constant and, in particular, to a model point dipole equilibrium. It is found that the model equilibrium is interchange stable for all plasma betas = (plasma pressure/magnetic pressure) and ballooning stable for all betas up to some critical value. The interesting planetary case of “tied” field lines is also considered.
Possible resolution of the “main intrigue” of the neoclassical tearing mode theory
Authors: Mikhailovskii A. B. , Tsypin V. S. , Nascimento I. C. , Galvão R. M. O.
Published: July 2002

The dilemma is discussed whether the polarization current effect on the neoclassical tearing modes is stabilizing or destabilizing. It is suggested that, if the island rotation frequency is formed due to the perpendicular viscosity and if one uses the correct expression for the viscosity allowing for the heat flux derivatives and takes into account the perpendicular heat flux in the ion heat balance equation, this effect should be stabilizing.
Subharmonic bifurcations and irregular pulsing behavior of modulated semiconductor lasers
Authors: Chen Y. C. , Winful H. G. , Liu J. M.
Published: July 2002

We report the observation of subharmonic bifurcations and irregular pulsing behavior in the output of directly modulated diode lasers. The bifurcation sequence differs from the predicted scenarios most noticeably in the presence of large intrinsic fluctuations which impede any stable period-two oscillations for non-self-pulsing lasers and period-four oscillations for self-pulsing lasers. The nondeterministic noise makes it difficult to observe high-period bifurcation and chaotic behavior in semiconductor lasers by direct current modulation.
Lateral impurity transport in silicon films on insulators during laser recrystallization
Authors: Sugahara K. , Nishimura T. , Akasaka Y. , Nakata H.
Published: July 2002

The lateral transport of dopants in silicon films on insulators during laser recrystallization is investigated. The dopants implanted locally in silicon films on insulators are found to be transported in the forward direction of the laser scan as well as the backward direction. Both transport lengths from the originally implanted region are measured as a function of the laser scan velocity. The transport mechanism is explained by taking into account a liquid phase diffusion and a segregation of impurities depending on the crystallization speed. The diffusion coefficients of (1.2±0.2)×10−4 and (1.3±0.4)×10−4 cm2/s for arsenic and boron, respectively, in molten silicon are obtained.
High efficiency XeCl laser with spiker and magnetic isolation
Authors: Fisher Charles H. , Kushner Mark J. , DeHart Terence E. , McDaniel John P. , Petr Rodney A. , Ewing J. J.
Published: July 2002

High efficiency x-ray preionized discharge XeCl laser excitation has been achieved by combining spiker/sustainer excitation techniques with a simple and reliable magnetic isolator. The troublesome rail-gap switch has been eliminated by using the laser discharge itself as the switch. An efficiency of 4% with 2.7 J optical energy was demonstrated.
Comparison of microwave and lower frequency discharges for plasma polymerization
Authors: Claude R. , Moisan M. , Wertheimer M. R. , Zakrzewski Z.
Published: July 2002

A plasma sustained by electromagnetic surface waves (SW’s) has been used to study the deposition rate R of hydrocarbon and fluorocarbon plasma polymer films as a function of the applied field frequency f in the range 12–400 MHz. The SW technique allows one to vary only f while keeping constant all other parameters known to influence R, for example, power density P in the plasma. A plot of R/P at a total pressure of 200 mTorr (27 Pa) displays two plateaus, that at f<30 MHz being about five times lower than that at f>100 MHz. This is attributed to the fact that electron energy distribution functions differ fundamentally at radio and microwave frequencies, for the gas pressure range considered.
Subpicosecond luminescence spectroscopy using sum frequency generation
Authors: Shah Jagdeep , Damen T. C. , Deveaud B. , Block Dominique
Published: July 2002

We report an improved time-resolved luminescence spectroscopy system using sum frequency generation. The system has the following attributes: high time resolution (<400 fs, currently limited by the laser), ability to determine absolute zero in time delay with high precision, wide spectral range, and large dynamic range which allows measurement of luminescence under weak photoexcitation. We illustrate these with examples of time-resolved luminescence spectra from GaAs.
Nonrandom alloying in In0.52Al0.48As/InP grown by molecular beam epitaxy
Authors: Hong Won-Pyo , Bhattacharya Pallab K. , Singh Jasprit
Published: July 2002

Anomalous behavior in Hall data of molecular beam epitaxial In0.52Al0.48As/InP for T≥400 K suggests the presence of alloy clustering. The relaxation time for alloy clustering has been determined and analysis of measured data gives an estimate of composition fluctuation and cluster size. Evidence of clustering in the same samples is also obtained from the measured velocity-field characteristics, low-temperature photoluminescence data, and carrier impact ionization coefficients.
Preparation of Y-Ba-Cu oxide superconductor thin films using pulsed laser evaporation from high T
c bulk material
Authors: Dijkkamp D. , Venkatesan T. , Wu X. D. , Shaheen S. A. , Jisrawi N. , Min-Lee Y. H. , McLean W. L. , Croft M.
Published: July 2002

We report the first successful preparation of thin films of Y-Ba-Cu-O superconductors using pulsed excimer laser evaporation of a single bulk material target in vacuum. Rutherford backscattering spectrometry showed the composition of these films to be close to that of the bulk material. Growth rates were typically 0.1 nm per laser shot. After an annealing treatment in oxygen the films exhibited superconductivity with an onset at 95 K and zero resistance at 85 and 75 K on SrTiO3 and Al2O3 substrates, respectively. This new deposition method is relatively simple, very versatile, and does not require the use of ultrahigh vacuum techniques.
Yttrium iron garnet single-mode buried channel waveguides for waveguide isolators
Authors: Pross E. , Tolksdorf W. , Dammann H.
Published: July 2002

Magneto-optic single-mode buried channel waveguides in substituted yttrium iron garnet suitable for waveguide isolators have been prepared. The channels are fabricated in a three-step procedure: starting with multiple-layer planar single-mode waveguides, rib waveguides are wet etched and then overgrown with top cladding. Such waveguides show mode profiles well matched to that of single mode fibers. In mode conversion experiments, an extinction ratio of 20 dB was obtained. Waveguides coupled to single-mode fibers on both sides showed an overall insertion loss of about 4 dB.
Low-temperature reactive ion etching and microwave plasma etching of silicon
Authors: Tachi Shinichi , Tsujimoto Kazunori , Okudaira Sadayuki
Published: July 2002

A new low-temperature reactive ion etching and microwave plasma etching method is described. Highly anisotropic silicon etching with extremely small width shifts has been performed with high selectivities of 30 for organic resist films. High etch rates of 500 and 1000 nm/min by reactive ion etching and microwave plasma etching, respectively, were achieved with a SF6 gas plasma at low wafer temperatures from −130 to −100 °C. It is concluded that lower temperatures during plasma treatment yield lower side etching and increase the dry etch resistance of organic masks.
Protein folding mediated by solvation: Water expulsion and formation of the hydrophobic core occur after the structural collapse
Authors: Cheung Margaret S. , García Angel E. , Onuchic José N.
Published: July 2002

The interplay between structure-search of the native structure and desolvation in protein folding has been explored using a minimalist model. These results support a folding mechanism where most of the structural formation of the protein is achieved before water is expelled from the hydrophobic core. This view integrates water expulsion effects into the funnel energy landscape theory of protein folding. Comparisons to experimental results are shown for the SH3 protein. After the folding transition, a near-native intermediate with partially solvated hydrophobic core is found. This transition is followed by a final step that cooperatively squeezes out water molecules from the partially hydrated protein core.
Anthropogenic modification of New England salt marsh landscapes
Authors: Bertness Mark D. , Ewanchuk Patrick J. , Silliman Brian Reed
Published: July 2002

Salt marshes play a critical role in the ecology and geology of wave-protected shorelines in the Western Atlantic, but as many as 80% of the marshes that once occurred in New England have already been lost to human development. Here we present data that suggest that the remaining salt marshes in southern New England are being rapidly degraded by shoreline development and eutrophication. On the seaward border of these marshes, nitrogen eutrophication stimulated by local shoreline development is shifting the competitive balance among marsh plants by releasing plants from nutrient competition. This shift is leading to the displacement of natural high marsh plants by low marsh cordgrass. On the terrestrial border of these same marshes, shoreline development is also precipitating the invasion of the common reed,
Phragmites,
by means of nitrogen eutrophication caused by the removal of the woody vegetation buffer between terrestrial and salt marsh communities. As a consequence of these human impacts, traditional salt marsh plant communities and the plants and animals that are dependent on these habitats are being displaced by monocultures of weedy species.
mtDNA from fossils reveals a radiation of Hawaiian geese recently derived from the Canada goose (Brantacanadensis)
Authors: Paxinos Ellen E. , James Helen F. , Olson Storrs L. , Sorenson Michael D. , Jackson Jennifer , Fleischer Robert C.
Published: July 2002

Phylogenetic analysis of 1.35 kb of mtDNA sequence from fossils revealed a previously unknown radiation of Hawaiian geese, of which only one representative remains alive (the endangered Hawaiian goose or nene,Branta sandvicensis). This radiation is nested phylogenetically within a living species, the Canada goose (Branta canadensis) and is related most closely to the large-bodied lineage within that species. The barnacle goose (Branta leucopsis) is also nested within the Canada goose species and is related most closely to the small-bodied lineage of Canada geese. The peripheral isolation of the barnacle goose in the Palearctic apparently allowed the evolution of its distinctive plumage pattern, whereas the two Nearctic lineages of Canada geese share a primitive plumage pattern. The Hawaiian lineage of Canada geese diverged more dramatically, splitting into at least three species that differ in body size, body proportions, and flight ability. One fossil species, limited to the island of Hawaii, was related closely to the nene but was over four times larger, flightless, heavy-bodied and had a much more robust cranium. Application of a rate calibration to levels of DNA divergence suggests that this species evolved on the island of Hawaii in less than 500,000 years. This date is consistent with the potassium/argon-based age of the island of Hawaii of 430,000–500,000 years. The giant Hawaii goose resembles the moa-nalos, a group of massive, extinct, flightless ducks that lived on older Hawaiian Islands and thus is an example of convergent evolution of similar morphologies in island ecosystems.
Protein topology and stability define the space of allowed sequences
Authors: Koehl Patrice , Levitt Michael
Published: July 2002

We describe a new approach to explore and quantify the sequence space associated with a given protein structure. A set of sequences are optimized for a given target structure, using all-atom models and a physical energy function. Specificity of the sequence for its target is ensured by using the random energy model, which keeps the amino acid composition of the sequence constant. The designed sequences provide a multiple sequence alignment that describes the sequence space compatible with the structure of interest; here the size of this space is estimated by using an information entropy measure. In parallel, multiple alignments of naturally occurring sequences can be derived by using either sequence or structure alignments. We compared these 3 independent multiple sequence alignments for 10 different proteins, ranging in size from 56 to 310 residues. We observed that the subset of the sequence space derived by using our design procedure is similar in size to the sequence spaces observed in nature. These results suggest that the volume of sequence space compatible with a given protein fold is defined by the length of the protein as well as by the topology (i.e., geometry of the polypeptide chain) and the stability (i.e., free energy of denaturation) of the fold.
An iron-regulated sortase anchors a class of surface protein during
Staphylococcus aureus
pathogenesis
Authors: Mazmanian Sarkis K. , Ton-That Hung , Su Kenneth , Schneewind Olaf
Published: July 2002

Sortase (SrtA), an enzyme that anchors surface proteins to the cell wall of Gram-positive bacteria, cleaves sorting signals at the LPXTG motif. We have identified a second sortase (SrtB) in the Gram-positive pathogen
Staphylococcus aureus
that is required for anchoring of a surface protein with a NPQTN motif. Purified SrtB cleaves NPQTN-bearing peptides
in vitro
, and a
srtB
mutant is defective in the persistence of animal infections.
srtB
is part of an iron-regulated locus called
i
ron-responsive
s
urface
d
eterminants (
isd
), which also contains a ferrichrome transporter and surface proteins with NPQTN and LPXTG motifs. Cell wall-anchored surface proteins and the
isd
locus seem involved in a novel mechanism of iron acquisition that is important for bacterial pathogenesis.
Generation of dopaminergic neurons and pigmented epithelia from primate ES cells by stromal cell-derived inducing activity
Authors: Kawasaki Hiroshi , Suemori Hirofumi , Mizuseki Kenji , Watanabe Kiichi , Urano Fumi , Ichinose Hiroshi , Haruta Masatoshi , Takahashi Masayo , Yoshikawa Kanako , Nishikawa Shin-Ichi , Nakatsuji Norio , Sasai Yoshiki
Published: July 2002

We previously identified a stromal cell-derived inducing activity (SDIA), which induces differentiation of neural cells, including midbrain tyrosine hydroxylase-positive (TH
+
) dopaminergic neurons, from mouse embryonic stem cells. We report here that SDIA induces efficient neural differentiation also in primate embryonic stem cells. Induced neurons contain TH
+
neurons at a frequency of 35% and produce a significant amount of dopamine. Interestingly, differentiation of TH
+
neurons from undifferentiated embryonic cells occurs much faster
in vitro
(10 days) than it does in the embryo (≈5 weeks). In addition, 8% of the colonies contain large patches of Pax6
+
-pigmented epithelium of the retina. The SDIA method provides an unlimited source of primate cells for the study of pathogenesis, drug development, and transplantation in degenerative diseases such as Parkinson's disease and retinitis pigmentosa.
Coupling between protein folding and allostery in the GroE chaperonin system
Authors: Yifrach Ofer , Horovitz Amnon
Published: July 2002

GroEL is an allosteric protein that facilitates protein folding in an ATP-dependent manner. Herein, the relationship between cooperative ATP binding by GroEL and the kinetics of GroE-assisted folding of two substrates with different GroES dependence, mouse dihydrofolate reductase (mDHFR) and mitochondrial malate dehydrogenase, is examined by using cooperativity mutants of GroEL. Strong intra-ring positive cooperativity in ATP binding by GroEL decreases the rate of GroEL-assisted mDHFR folding owing to a slow rate of the ATP-induced transition from the protein-acceptor state to the protein-release state. Inter-ring negative cooperativity in ATP binding by GroEL is found to affect the kinetic partitioning of mDHFR, but not of mitochondrial malate dehydrogenase, between folding in solution and folding in the cavity underneath GroES. Our results show that protein folding by this “two-stroke motor” is coupled to cooperative ATP binding.
Meiotic instability of CAG repeat tracts occurs by double-strand break repair in yeast
Authors: Jankowski Craig , Nasar Farooq , Nag Dilip K.
Published: July 2002

Expansion of trinucleotide repeats is associated with a growing number of human diseases. The mechanism and timing of expansion of the repeat tract are poorly understood. In humans, trinucleotide repeats show extreme meiotic instability, and expansion of the repeat tract has been suggested to occur in the germ-line mitotic divisions or postmeiotically during early divisions of the embryo. Studies in model organisms have indicated that polymerase slippage plays a major role in the repeat tract instability and meiotic instability is severalfold higher than the mitotic instability. We show here that meiotic instability of the CAG/CTG repeat tract in yeast is associated with double-strand break (DSB) formation within the repeated sequences, and that the DSB formation is dependent on the meiotic recombination machinery. The DSB repair results in both expansions and contractions of the CAG repeat tract.
p56Lckanchors CD4 to distinct microdomains on microvilli
Authors: Foti Michelangelo , Phelouzat Marie-Anne , Holm Åsa , Rasmusson Birgitta J. , Carpentier Jean-Louis
Published: July 2002

Cell-surface microvilli play a central role in adhesion, fusion, and signaling processes. Some adhesion and signaling receptors segregate on microvilli but the determinants of this localization remain mostly unknown. In this study, we considered CD4, a receptor involved in immune response and HIV infection, and p56Lck, a CD4-associated tyrosine kinase. Analysis of CD4 trafficking reveals that p56Lckbinds tightly to CD4 independently of its activation state and inhibits CD4 internalization. Electron microscopy analysis established that p56Lckmediates CD4 association with microvilli whereas biochemical data indicate that p56Lckexpression renders CD4 insoluble by the nonionic detergent Triton X-100. In addition, cytoskeleton-disrupting agent increased CD4 solubility, suggesting the involvement of cytoskeletal elements in CD4 anchoring to microvilli. This concept was supported further by the observation that the lateral mobility of CD4 within the plasma membrane was decreased in cells expressing p56Lck. Finally, isolation of detergent-resistant membranes revealed that the complex CD4-p56Lckis enriched within these domains as opposed to conditions in which CD4 does not interact with p56Lck. In conclusion, our results show that p56Lcktargets CD4 to specialized lipid microdomains preferentially localized on microvilli. This localization, which prevents CD4 internalization, might facilitate CD4-mediated adhesion processes and could correspond to the signaling site of the receptor.
Low dielectric response in enzyme active site
Authors: Mertz Edward L. , Krishtalik Lev I.
Published: July 2002

The kinetics of charge transfer depend crucially on the dielectric reorganization of the medium. In enzymatic reactions that involve charge transfer, atomic dielectric response of the active site and of its surroundings determines the efficiency of the protein as a catalyst. We report direct spectroscopic measurements of the reorganization energy associated with the dielectric response in the active site of α-chymotrypsin. A chromophoric inhibitor of the enzyme is used as a spectroscopic probe. We find that water strongly affects the dielectric reorganization in the active site of the enzyme in solution. The reorganization energy of the protein matrix in the vicinity of the active site is similar to that of low-polarity solvents. Surprisingly, water exhibits an anomalously high dielectric response that cannot be described in terms of the dielectric continuum theory. As a result, sequestering the active site from the aqueous environment inside low-dielectric enzyme body dramatically reduces the dielectric reorganization. This reduction is particularly important for controlling the rate of enzymatic reactions.
Inherited susceptibility to uterine leiomyomas and renal cell cancer
Authors: Launonen Virpi , Vierimaa Outi , Kiuru Maija , Isola Jorma , Roth Stina , Pukkala Eero , Sistonen Pertti , Herva Riitta , Aaltonen Lauri A.
Published: July 2002

Herein we report the clinical, histopathological, and molecular features of a cancer syndrome with predisposition to uterine leiomyomas and papillary renal cell carcinoma. The studied kindred included 11 family members with uterine leiomyomas and two with uterine leiomyosarcoma. Seven individuals had a history of cutaneous nodules, two of which were confirmed to be cutaneous leiomyomatosis. The four kidney cancer cases occurred in young (33- to 48-year-old) females and displayed a unique natural history. All these kidney cancers displayed a distinct papillary histology and presented as unilateral solitary lesions that had metastasized at the time of diagnosis. Genetic-marker analysis mapped the predisposition gene to chromosome 1q. Losses of the normal chromosome 1q were observed in tumors that had occurred in the kindred, including a uterine leiomyoma. Moreover, the observed histological features were used as a tool to diagnose a second kindred displaying the phenotype. We have shown that predisposition to uterine leiomyomas and papillary renal cell cancer can be inherited dominantly through the hereditary leiomyomatosis and renal cell cancer (HLRCC) gene. TheHLRCCgene maps to chromosome 1q and is likely to be a tumor suppressor. Clinical, histopathological, and molecular tools are now available for accurate detection and diagnosis of this cancer syndrome.