Electrostatic Potential Fluctuations Research Articles

Observation of geodesic acoustic modes (GAMs) and their radial propagation at the edge of the TEXTOR tokamak

The electrostatic potential and density fluctuations have been measured at the edge of the TEXTOR tokamak by two toroidally distant Langmuir probe arrays. The geodesic acoustic mode (GAM) zonal flows (ZFs) are observed in potential fluctuations with a toroidal and poloidal symmetric structure. The GAM frequency, fGAM, changes monotonically with the local temperature and is close to the frequency-dispersion predicted by theories. Bispectral analysis shows clear nonlinear coupling between the GAM and broadband ambient turbulence. The GAM packet has a narrow radial extent with kr ≃ 0.5–0.7 cm−1 and exhibits explicitly a radially outward propagation. Furthermore, the radial correlation structure of GAMs and their radial propagation have been investigated in a wide range of parameters by varying plasma density and edge safety factor (5.0 ⩽ q(a) ⩽ 5.9). It is found that the magnitude of the GAM correlations reduces remarkably with the increase in the plasma density approaching the density limit, while the radial wavelength of GAMs only decreases slightly in higher density and larger q(a) discharges. With increasing plasma density, the radial propagating phase speed of GAMs is strongly reduced along with the drop in the local temperature. The results provide new evidence on the propagation properties of GAM ZFs.

Potential fluctuation associated with the energetic-particle-induced geodesic acoustic mode in the Large Helical Device

Geodesic acoustic modes (GAM) driven by energetic particles are observed in the Large Helical Device (LHD) by a heavy ion beam probe. The GAM localizes near the magnetic axis. It is confirmed that the energetic-particle-induced GAM is accompanied by an electrostatic potential fluctuation and radial electric field fluctuation. The amplitude of the potential fluctuation is several hundred volts, and it is much larger than the potential fluctuation associated with turbulence-induced GAMs observed in the edge region in tokamak plasmas. The energetic-particle-induced GAM modulates the amplitude of the density fluctuation in a high-frequency range. The observed GAM frequency is constant at the predicted GAM frequency in plasmas with reversed magnetic shear. On the other hand, it shifts upwards from the predicted GAM frequency in plasmas with monotonic magnetic shear.

Heavy Ion Beam Probe Measurement in Turbulence Diagnostic Simulator

A numerical measurement module simulating a heavy ion beam probe was developed, and numerical measurements of electrostatic potential and density fluctuations are carried out for 3-D turbulent data generated by a global simulation of drift-interchange mode turbulence in helical plasmas. The deviation between measured and local values is estimated. It is found that the characteristic structures can be detected in spite of the screening effect due to the finite spatial resolution.

Surface Electrostatics Associated with Lipid Bilayer Curvature

Spontaneous lipid bilayer bending and related curvature are being recognized as an essential mechanism associated with many cellular functions. Experimental observations indicate that tubular vesicles of just 50 to 100 nm in diameter are commonly formed within the Golgi body and also between the endoplasmic reticulum and the Golgi. While changes in various conditions may promote the tubulation of the Golgi membrane, detailed understanding of basic biophysical processes beyond the bilayer bending are missing in the literature and so are convenient models of highly curved lipid tubules. Here we report a systematic study of a surface potential for both small unilamellar monodisperse lipid vesicles (SUMV) with diameters ranging from 30 to 100 nm and also lipid tubules that are stabilized by confining these structures within rigid homogeneous nanopores of similar diameters. For example, for SUMVs composed of negatively charged lipids the magnitude of the surface potential increased with bilayer bending from ca. −106 mV for 100 nm SUMV to −166 mV for 30 nm SUMV. These measurements were carried out by spin probe EPR method using recently synthesized lipids having pH-reporting nitroxides covalently tethered to the lipid polar head. Parallel differential scanning calorimetry experiments indicated the presence of at least two components within the lipid phase. These phase components were characterized by measurably different phase transition temperatures and correlation times of the lipid thermal relaxation. Overall, the data indicate that the bilayer bending affects the local electrostatic potential and lipid fluctuation properties in a rather large degree and is likely associated with a mechanism for cellular machinery function. Supported by U.S. DOE Contract DE-FG02-02ER15354.

Electrostatic Potential Fluctuations on Oxide-Passivated Si(111) Surfaces Measured Using Integrated Scanning Probe Microscopy

Variations of the electrostatic potential were investigated for oxide-passivated n-Si(111) surfaces with atomically flat terraces by measuring the force acting on an ultra-sharp tungsten probe that was attached to the quartz resonator of an atomic force microscope. When the probe-sample gap maintained a constant tunneling current, an enhancement of electrostatic force with a lateral extent of ∼5 nm was observed around underlying donor atoms and charged defects. Additional variations of the surface potential and the probe-sample capacitance across the surface steps were associated with excess electric charge at the step edge. [DOI: 10.1380/ejssnt.2011.117]

Experimental study of radial electric field and electrostatic potential fluctuation in the Large Helical Device

A heavy ion beam probe was installed on the Large Helical Device (LHD) to investigate the roles of radial electric fields (Er) in magnetically confined high-temperature plasmas. Two new observations are presented. One is the observation of electrostatic potential profiles during the formation of extremely hollow density profiles of impurities, called the impurity hole (Ida K et al 2009 Phys. Plasmas 16 056111), in the LHD plasmas. The measured Er is negative, and the Er determined by the ambipolarity condition of neoclassical particle fluxes is consistent with this observation. However, the transport analysis indicates that the formation of the extremely hollow profile is not attributable to the impurity fluxes driven by Er and the density and temperature gradients of the impurity. The other new observation is on the geodesic acoustic mode (GAM). The electrostatic potential fluctuation associated with the GAM, which is probably induced by energetic particles, in plasmas with the reversed or weak magnetic shear is identified. The GAM is localized in the core region of the plasma.

Low-frequency electrostatic waves in the ionospheric E region

Low-frequency electrostatic waves in the ionospheric E region are studied by analyzing data obtained by instrumented rockets. We identify the origin of the enhanced fluctuation level to be the Farley–Buneman instability. The basic information on instability, such as altitude varying spectra and speed of propagation are obtained. Comparison of power spectra for the fluctuations in plasma density and electrostatic potential, respectively, provides information on the electron dynamics. A bispectral analysis gives indications of phase-coherent couplings within the wave spectrum, while higher order structure functions indicate some intermittent features of the turbulence.

Preparation and properties of heavily doped and strongly compensated Ge films on GaAs

We present and generalize the preparation conditions and properties of heavily doped and strongly compensated (HDSC) Ge films obtained by deposition in the vacuum onto the semi-insulating GaAs (100) substrates. A possibility of formation of Ge films with various doping levels and compensation degrees (in particular, fully compensated) is demonstrated. Heavily doped and fully compensated Ge single-crystalline thin (∼0.1 μm) films obtained have high resistivity (up to 140 Ω cm), conductance activation energy as high as half the bandgap of Ge, low free charge carrier mobility (∼50 cm2/V s), and concentration (∼1014–1015 cm−3). The electrical and optical properties of the films are explained with allowance made for the presence of large-scale fluctuations of electrostatic potential in Ge. Under certain conditions, a two-dimensional potential relief may exist in thin HDSC Ge films, as well as two-dimensional percolation may occur.

Using randomly distributed charges to create quantum dots

Using a combination of modeling and tunneling spectroscopy, we investigate how electrostatic potential fluctuations generated by randomly distributed ionized donors close to a quantum well can produce deep and strongly confined quantum-dot-like potential minima with a rich spectrum of zero-dimensional electronic energy levels. We consider different types of random distribution of donors and how the electronic properties can be controlled and investigated in appropriately designed double barrier resonant tunneling diodes.

Electrostatics of the Protein-Water Interface

Electrostatic fluctuations within proteins are critical to their biological activity as carriers in electron transport chains typically requiring a significant number of single-electron hops. This mechanistic requirement poses the question of how a sufficient energetic efficiency is achieved. We present the results of numerical simulations of the statistics of electrostatic fluctuations at the protein/water interface. The statistics of the electrostatic potential fluctuations inside the protein is strongly non-Gaussian at high temperatures, but becomes consistent with the linear/Gaussian response below the temperature of the dynamical transition in proteins. At high temperatures the large, non-Gaussian electrostatic noise allows higher efficiency of electron transport chains which can be magnified by an order of magnitude compared to the predictions of Gaussian models. The appearance of non-Gaussian statistics is traced back to a significant polarization of the protein-water interface which slows its relaxation at lower temperatures and becomes kinetically frozen below the temperature of dynamical transition.

On the fluctuations that drive small ions toward, and away from, interfaces between polar liquids and their vapors

Contrary to the expectations from classic theories of ion solvation, spectroscopy and computer simulations of the liquid-vapor interface of aqueous electrolyte solutions suggest that ions little larger than a water molecule can prefer to reside near the liquid's surface. Here we advance the view that such affinity originates in a competition between strong opposing forces, primarily due to volume exclusion and dielectric polarization, that are common to all dense polar liquids. We present evidence for this generic mechanism from computer simulations of (i) water and (ii) a Stockmayer fluid near its triple point. In both cases, we show that strong surface enhancement of small ions, obtained by tuning solutes' size and charge, can be accentuated or suppressed by modest changes in either of those parameters. Statistics of solvent polarization, when the ion is held at and above the Gibbs dividing surface, highlight a basic deficiency in conventional models of dielectric response, namely, the neglect of interfacial flexibility. By distorting the solution's boundary, an ion experiences fluctuations in electrostatic potential and in electric field whose magnitudes attenuate much more gradually (as the ion is removed from the liquid phase) than for a quiescent planar interface. As one consequence, the collective responses that determine free energies of solvation can resolve very differently in nonuniform environments than in bulk. We show that this persistence of electric-field fluctuations additionally shapes the sensitivity of solute distributions to ion polarizability.

Spatiotemporal dynamics and transport reduction in helical magnetic configuration

Effects of multihelicity confinement magnetic fields on turbulent transport and zonal flows are investigated by means of spatiotemporal analysis of gyrokinetic Vlasov simulation results for the ion temperature gradient turbulence, where the standard and the inward-shifted configurations of the Large Helical Device are considered. The analysis of simulation results demonstrates that fluctuations of electrostatic potential for zonal flows exhibit spatiotemporal chaos in both configurations. However, the intensity of chaos found is considerably decreased in the inward-shifted configuration consistent with improved confinement. Enhanced zonal flow generation in the inward shifted case is accompanied by transport reduction which may be a direct consequence of chaos suppression.

Functional Analysis and Molecular Dynamics Simulation of LOX-1 K167N Polymorphism Reveal Alteration of Receptor Activity

The human lectin-like oxidized low density lipoprotein receptor 1 LOX-1, encoded by the ORL1 gene, is the major scavenger receptor for oxidized low density lipoprotein in endothelial cells. Here we report on the functional effects of a coding SNP, c.501G>C, which produces a single amino acid change (K>N at codon 167). Our study was aimed at elucidating whether the c.501G>C polymorphism changes the binding affinity of LOX-1 receptor altering its function. The presence of p.K167N mutation reduces ox-LDL binding and uptake. Ox-LDL activated extracellular signal-regulated kinases 1 and 2 (ERK 1/2) is inhibited. Furthermore, ox-LDL induced biosynthesis of LOX-1 receptors is dependent on the p.K167N variation. In human macrophages, derived from c.501G>C heterozygous individuals, the ox-LDL induced LOX-1 46 kDa band is markedly lower than in induced macrophages derived from c.501G>C controls. Investigation of p.K167N mutation through molecular dynamics simulation and electrostatic analysis suggests that the ox-LDL binding may be attributed to the coupling between the electrostatic potential distribution and the asymmetric flexibility of the basic spine residues. The N/N-LOX-1 mutant has either interrupted electrostatic potential and asymmetric fluctuations of the basic spine arginines.

Measurement of electrostatic potential fluctuation using heavy ion beam probe in large helical device

Heavy ion beam probe (HIBP) for large helical device (LHD) has been improved to measure the potential fluctuation in high-temperature plasmas. The spatial resolution is improved to about 10 mm by controlling the focus of a probe beam. The HIBP is applied to measure the potential fluctuation in plasmas where the rotational transform is controlled by electron cyclotron current drive. The fluctuations whose frequencies change with the time constant of a few hundreds of milliseconds and that with a constant frequency are observed. The characteristics of the latter fluctuation are similar to those of the geodesic acoustic mode oscillation. The spatial profiles of the fluctuations are also obtained.

Effect of potential fluctuations on the energy structure of GaAs/AlGaAs quantum wells with A+ centers

The transformation of photoluminescence spectra associated with A+ centers in GaAs/AlGaAs quantum wells due to changes in the pumping level and temperature is analyzed. It is shown that an important part in the formation of the energy structure of the system of GaAs/AlGaAs quantum wells is played by electrostatic potential fluctuations responsible for the spatial redistribution of charge and the appearance of free holes.

Electrostatic potential fluctuation induced by charge discreteness in a nanoscale trench

A simplified two-dimensional Monte Carlo simulation is performed to estimate the charging potential fluctuations caused by strong binary Coulomb interactions between discrete charged particles in nanometer scale trenches. It is found that the discrete charge effect can be an important part of the nanoscale trench research, inducing scattering of ion trajectories in a nanoscale trench by a fluctuating electric field. The effect can enhance the ion deposition on the side walls and disperse the material contact energy of the incident ions, among others.

Recurrence quantification analysis of electrostatic fluctuations in fusion plasmas

We have investigated plasma turbulence at the edge of a tokamak plasma using data from electrostatic potential fluctuations measured in the Brazilian tokamak TCABR. Recurrence quantification analysis has been used to provide diagnostics of the deterministic content of the series. We have focused our analysis on the radial dependence of potential fluctuations and their characterization by recurrence-based diagnostics. Our main result is that the deterministic content of the experimental signals is most pronounced at the external part of the plasma column just before the plasma radius. Since the chaoticity of the signals follows the same trend, we have concluded that the electrostatic plasma turbulence at the tokamak plasma edge can be partially explained by means of a deterministic nonlinear system.

Room temperature midinfrared electroluminescence from GaInAsSbP light emitting diodes

Room temperature electroluminescence in the midinfrared near 4μm is reported from GaInAsSbP light emitting diodes grown on GaSb by liquid phase epitaxy. Comparison of the electro- and photoluminescence revealed that light is generated on the p side of the diode. The energy shift (24meV) is consistent with band gap narrowing and recombination via band tail states due to the Zn doping (1×1018cm−3) in the p layer of the structure. The temperature dependent behavior of the luminescence and the improved emission intensity was attributed to recombination from localized states arising from electrostatic potential fluctuations due to compositional inhomogeneities in these alloys.

Optical and electrical properties of Te doped AlGaAsSb/AlAsSb Bragg mirrors on InP

We present a comparative study carried out on the optical and electrical characteristics of undoped and Te doped AlGaAsSb/AlAsSb Bragg mirrors with 6.5 pairs of layers and bulk undoped and Te doped AlGaAsSb epilayers alloys lattice matched on InP, grown by molecular beam epitaxy, using SIMS, photoluminescence, reflectivity and IxV techniques. The temperature dependence of PL transitions observed in the Bragg mirrors are similar to that observed in bulk samples and associated with the donor and acceptor recombinations in alloys with electrostatic potential fluctuations described by quasi-donor-acceptor-pair (QDAP) models. We verified by SIMS the presence of a macro fluctuation in the Te profile concentrations in the growth direction of the doped Bragg mirror. The influence of doping, alloy and macrofluctuation of incorporated Te on the optical and electrical properties of doped Bragg mirror is analyzed. The AlGaAs/AlAsSb seems a very attractive option for VCSEL technology.

Electrostatic potential fluctuations in a Maxwellian plasma

The spatial correlation function of a Maxwellian plasma with perturbations arising in the electrostatic potential due to random ion density fluctuations is examined. The entropy is found from the one-particle distribution function using the Shannon formula and then, using the Einstein method, the probability distribution for the electrostatic potential fluctuation is determined. This straightforward procedure is demonstrated to be a powerful tool in studying plasma correlation functions when the system entropy can be computed.