Electrostatic Potential Well Research Articles

Glass-based geometry-induced electrostatic trapping devices for improved scattering contrast imaging of nano-objects

Trapping of micro- and nano-objects in solution is of great scientific interest in various fields. One method of trapping and detecting objects smaller than 100nm is the combination of geometry-induced electrostatic (GIE) trapping devices and interferometric scattering detection (iSCAT). In GIE trapping, charged nano-objects are confined in a nanofluidic system that hosts topographically modified surfaces, resulting in electrostatic potential wells. We observe optical limits of detecting gold nanoparticles smaller than 60nm because of the high reflection of the strong background signal in current silicon-based GIE trapping chips. The high reflection rapidly leads to overexposure of the camera detector and thus limits the incident laser power. In this work, we introduce new functional geometry-induced electrostatic devices fabricated from glass substrates. Due to the reduced reflection at the water–glass interface compared to the silicon-based devices, higher incident laser power can be used to image the nano-objects resulting in higher contrast as well as signal-to-noise ratios (SNR) of the gold nanoparticles. Using glass-based GIE trapping devices, significant SNR increases are achieved in comparison to that of silicon-based devices. These improvements enable the detection of much smaller nanoparticles and thereby studies on their trapping, as well as further investigation in nanofluidic systems.

On stochastic heating of electrons by intense laser radiation in the presence of electrostatic potential well

A simple model developed by Paradkar et al. [Phys. Plasmas 19, 060703 (2012)] for the study of synergistic effects of electrostatic potential well and laser radiation is extended for the case where electric field of the well is accelerating electrons moving in the direction of the laser field propagation. It was found that in these cases, the rate of stochastic heating of energetic electrons remains virtually the same as in Paradkar et al. [Phys. Plasmas 19, 060703 (2012)], where electric field in electrostatic potential was slowing down electrons moving in the direction of the laser field propagation. However, the heating of electrons with relatively low energy can be sensitive to the orientation of the electrostatic potential well with respect to the direction of the laser radiation propagation.

Origins of electrostatic potential wells at dislocations in polycrystalline Cu(In,Ga)Se2 thin films

Thin-film solar cells based on Cu(In,Ga)Se2 (CIGSe) reach high power-conversion efficiencies in spite of large dislocation densities of up to 1010–1011 cm−2. The present work gives insight into the structural and compositional properties of dislocations in CIGSe thin films, which are embedded in a complete solar cell stack. These properties are related to the average electrical potential distributions obtained by means of inline electron holography. At a part of the dislocations studied, the average electrostatic potential shows local minima, all with depths of about −1.4 V. The measured average electrostatic potential distributions were modeled in order to reveal possible influences from strain fields, excess charge, and also compositional changes at the dislocation core. Cu depletion around the dislocation core, as evidenced by atom-probe tomography, explains best the measured potential wells. Their influences of the strain field around the dislocation core and of excess charge at the dislocation core are small. A structural model of dislocations in CIGSe thin films is provided which includes a Cu-depleted region around the dislocation core and gives a possible explanation for why decent photovoltaic performances are possible in the presence of rather large dislocation densities.

Electrostatic potential wells for on-demand drop manipulation in microchannels

Precise control and manipulation of individual drops are crucial in many lab-on-a-chip applications. We present a novel hybrid concept for channel-based discrete microfluidics with integrated electrowetting functionality by incorporating co-planar electrodes (separated by a narrow gap) in one of the microchannel walls. By combining the high throughput of channel-based microfluidics with the individual drop control achieved using electrical actuation, we acquire the strengths of both worlds. The tunable strength of the electrostatic forces enables a wide range of drop manipulations, such as on-demand trapping and release, guiding, and sorting of drops in the microchannel. In each of these scenarios, the retaining electrostatic force competes with the hydrodynamic drag force. The conditions for trapping can be predicted using a simple model that balances these forces.

Autoresonant-spectrometric determination of the residual gas composition in the ALPHA experiment apparatus

Knowledge of the residual gas composition in the ALPHA experiment apparatus is important in our studies of antihydrogen and nonneutral plasmas. A technique based on autoresonant ion extraction from an electrostatic potential well has been developed that enables the study of the vacuum in our trap. Computer simulations allow an interpretation of our measurements and provide the residual gas composition under operating conditions typical of those used in experiments to produce, trap, and study antihydrogen. The methods developed may also be applicable in a range of atomic and molecular trap experiments where Penning-Malmberg traps are used and where access is limited.

Motion of Finite-Sized Low-Density Photoion Bunch in Electrostatic Potential Wells

A finite-sized low-density photoplasma is produced by a two-step resonant photoionization method. Its density is varied in the range of ~(107-109) cm-3. The motion of photoplasma is studied in a linear electrostatic potential well that is created by plate-grid-plate geometry. To understand its dynamics, a 1-D particle-in-cell model has been developed. For density range ~(1 × 107-5 × 108) cm-3, an electric field ≤100 V/cm is sufficient to remove all the electrons from the photoplasma within a time of few nanoseconds, leaving behind an ion bunch. When a photoion bunch evolves in a potential well, a damped oscillation is observed on the current signal recorded on a grid electrode. The structure is explained by single-particle behavior of the photoion bunch. For densities>; 3 × 108 cm-3, the oscillation frequency depends on both externally applied electric field and internal field that is produced by space-charge interactions among charge particles. This is because, at higher densities, collective behavior dominates and the dynamics is governed by space-charge interactions.

Electrochemical Characterization of Thin Film Electrodes Toward Developing a DNA Transistor

The DNA-Transistor is a device designed to control the translocation of single-stranded DNA through a solid-state nanopore. Functionality of the device is enabled by three electrodes exposed to the DNA-containing electrolyte solution within the pore and the application of a dynamic electrostatic potential well between the electrodes to temporarily trap a DNA molecule. Optimizing the surface chemistry and electrochemical behavior of the device is a necessary (but by no means sufficient) step toward the development of a functional device. In particular, effects to be eliminated are (i) electrochemically induced surface alteration through corrosion or reduction of the electrode surface and (ii) formation of hydrogen or oxygen bubbles inside the pore through water decomposition. Even though our motivation is to solve problems encountered in DNA transistor technology, in this paper we report on generic surface chemistry results. We investigated a variety of electrode-electrolyte-solvent systems with respect to their capability of suppressing water decomposition and maintaining surface integrity. We employed cyclic voltammetry and long-term amperometry as electrochemical test schemes, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning, as well as transmission electron microscopy as analytical tools. Characterized electrode materials include thin films of Ru, Pt, nonstoichiometric TiN, and nonstoichiometric TiN carrying a custom-developed titanium oxide layer, as well as custom-oxidized nonstoichiometric TiN coated with a monolayer of hexadecylphosphonic acid (HDPA). We used distilled water as well as aqueous solutions of poly(ethylene glycol) (PEG-300) and glycerol as solvents. One millimolar KCl was employed as electrolyte in all solutions. Our results show that the HDPA-coated custom-developed titanium oxide layer effectively passivates the underlying TiN layer, eliminating any surface alterations through corrosion or reduction within a voltage window from -2 V to +2 V. Furthermore, we demonstrated that, by coating the custom-oxidized TiN samples with HDPA and increasing the concentration of PEG-300 or glycerol in aqueous 1 mM KCl solutions, water decomposition was suppressed within the same voltage window. Water dissociation was not detected when combining custom-oxidized HDPA-coated TiN electrodes with an aqueous 1 mM KCl-glycerol solution at a glycerol concentration of at least 90%. These results are applicable to any system that requires nanoelectrodes placed in aqueous solution at voltages that can activate electrochemical processes.

Measuring D(d,p)T fusion reactant energy spectra with Doppler shifted fusion products

Deuterium fusion reactant energy spectra have been measured using a diagnostic that records the Doppler shift imparted to charged particle fusion products of the D(d,p)T reaction by the center-of-mass velocity of the deuterium reactants. This diagnostic, known as the fusion ion Doppler shift diagnostic (FIDO) measures fast deuterium energy spectra in the inertial electrostatic confinement (IEC) experiment at the University of Wisconsin–Madison {Santarius et al. [Fusion Sci. Technol. 47, 1238 (2005)]}, a device to confine high energy light ions in a spherically symmetric, electrostatic potential well. This article details the first measurements of the fusion reactant energy spectra in an IEC device as well as the design and principles of operation of the FIDO diagnostic. Scaling of reactant energy spectra with a variety of experimental parameters have been explored.

A Proposed Test of Extended MHD Plasmas by Merging of Lithium Ion Plasmas with Pure Electron Plasmas

Recently, extended MHD plasmas have been proposed, especially in both theoretical and computational fields of plasma physics. To test those experimentally, we propose a new project using two nonneutral plasmas: pure ion and electron plasmas. The experiment is performed on a linear machine in which a pair of positive and negative electrostatic potential wells is externally produced. In the positive potential well, an 7Li+ plasma is confined, while an electron plasma in the negative potential one. Then, those nonneutral plasmas are merged into an extended MHD plasma. The most remarkable advantage to employing nonneutral plasmas is that the ion skin depth, that is a typical scale length of the extended MHD plasmas, can be extended to the plasma radius. This actually alleviates experimental difficulties in diagnosing properties of the extended MHD plasmas. With precisely controlling densities of both pure 7Li+ ion and electron plasmas, the proposed method may explore an unrevealed research field of extended MHD plasmas.

Experimental studies of the confinement of electron plasma in a multi-ring trap

Confinement of pure electron plasma has been studied in an electromagnetic multi-ring trap. The trap was housed inside a bore tube and surrounded by a superconducting solenoid. A magnetic field of 5 T was used to ensure radial confinement of electron plasma, while an electrostatic harmonic potential well was used for its axial confinement. The lifetime of the electron plasma was found to be strongly dependent on the temperature of the bore tube. In the case of electron plasma of 7×109 electrons and when the temperature of the bore tube changed from room temperature to 40 K, the lifetime also changed from 40 to 3700 s. A significant improvement in the electron plasma lifetime was observed when a rotating electric field with optimum parameters was applied. When the bore tube was operated at room temperature with the rotating electric field, the electron plasma lifetime increased up to 2200 s.

Deuterium anions in inertial electrostatic confinement devices

A magnetic deflection-energy analyzer and Faraday trap diagnostic have been used to make measurements of divergent deuterium anion flow in the inertial electrostatic confinement experiment at the University of Wisconsin-Madison (UW-IEC) [J. F. Santarius, G. L. Kulcinski, R. P. Ashley, D. R. Boris, B. B. Cipiti, S. K. Murali, G. R. Piefer, R. F. Radel, I. E. Radel, and A. L. Wehmeyer, Fusion Sci. Technol. 47, 1238 (2005)], a device to confine high-energy light ions in a spherically symmetric electrostatic potential well. Deuterium anion current densities as high as 8.5 microA/cm2 have been measured at the wall of the UW-IEC device, 40 cm from the surface of the device cathode with a detector assembly of admittance area 0.7 cm2. Energy spectra obtained using a magnetic deflection-energy analyzer diagnostic indicate the presence of D2(-), and D- ions produced through thermal electron attachment near the device cathode, as well as D- ions produced via charge-transfer processes between the anode and cathode of the device.

The bounce resonance heating of low-energy electrons in capacitively coupled discharges

Low-energy electrons are confined in the bulk plasma by the ambipolar potential and are typically heated inefficiently due to their low collisionality and the weak rf electric field present in the bulk when their mean-free path is much larger than the system length in capacitively coupled discharges. It is shown in this study, however, that electrons in the bulk that bounce inside the electrostatic potential well with a frequency equal to the rf excitation frequency are efficiently heated by coherent interaction with the rf field. This resonant heating manifests itself as a plateau in the electron energy probability function and is observed in a wide range of pressures from 25 mTorr to 1 Torr while decreasing the gap distance. The weak transverse magnetic field significantly influences the bounce frequency of the low-energy electrons. As a result, the electrons not in the resonant condition in the absence of a magnetic field can be led to satisfy the resonant condition, becoming effectively heated in the presence of a weak transverse magnetic field.

Feasibility Study of Axially- Extracted Virtual Cathode Oscillator

In this paper, we have analyzed the design parameters of the axially - extracted virtual cathode oscillator, which is high-power microwave source based on the concept of the virtual cathode associated with the intense relativistic electrons beam oscillations in the electrostatic potential well. The microwave emission by the virtual cathode oscillator results from both the space and time oscillations of virtual cathode and reflexing electrons trapped in the potential well between the virtual and real cathodes. In the X-band frequency spectrum 700 MW microwave peak power has been obtained analytically by the solid electron beam of 300 kV and 20 kA for feasible design parameters. The analysis has been performed by 2-dimensional, relativistic, electromagnetic particle-in-cell simulation code XOOPIC.

Electron surface acceleration on a solid capillary target inner wall irradiated with ultraintense laser pulses

When ultraintense laser pulses irradiate solid targets with a large incident angle, quasistatic magnetic and electric fields are induced, which confine electrons along the target surface in an electrostatic and vector potential well. In this case, electrons are resonantly accelerated along the surface by laser electric field inside the potential well. By this surface acceleration process, high energy electrons are effectively generated whose temperature well exceeds the ponderomotive energy. The optimum conditions for realizing surface acceleration and its energy scalings are given. Capillary type targets are shown to have an advantage in utilizing the surface acceleration process by increasing the interaction length.

Abnormal Heating of Low-Energy Electrons in Low-Pressure Capacitively Coupled Discharges

In low-pressure capacitively coupled plasmas, high-energy electrons are collisionlessly heated by large rf fields in the sheaths while low-energy electrons are confined in the bulk plasma by the ambipolar potential. Low-energy electrons are typically inefficiently heated due to their low collisionality and the weak rf electric field present in the bulk. It is shown, however, that as a result of the nonlinear interaction between the electron motion and the weak rf field present in the bulk, low-energy electrons can be efficiently heated. Electrons in the bulk that bounce inside the electrostatic potential well with a frequency equal to the rf excitation frequency are efficiently heated by the coherent interaction with the rf field. This resonant collisionless heating can be very efficient and manifest itself as a plateau in the electron energy probability function.

A generation mechanism of electrostatic waves and subsequent electron heating in the plasma sheet–lobe boundary region during magnetic reconnection

A generation mechanism of high‐energy electrons in the plasma sheet–lobe boundary region associated with magnetic reconnection is studied by using 2‐1/2‐dimensional kinetic simulations in a large system. Our simulation code employs the adaptive mesh refinement technique and the particle splitting algorithm to the conventional particle‐in‐cell code, which enable us to perform large‐scale kinetic simulations. It is shown that the electron two‐stream instability arising between the cold background electrons and the intense beam electrons with high temperature is responsible for the formation of the flat‐topped electrons, which have been often observed in the boundary region when magnetic reconnection has occurred. Electrons are quickly heated along the ambient magnetic field due to the trapping effect in the electrostatic potential wells, and the shoulder energy of the flat‐topped electron distribution reaches a few keV, consistent with satellite observations. The intense beam consists of high‐ and low‐energy electrons in the perpendicular direction. The former electrons come from the lobe region through the vicinity of the X line, while the latter originate from the opposite boundary region of the plasma sheet. The electron two‐stream instability in association with magnetic reconnection is also responsible for the generation of the electrostatic solitary waves that have been frequently detected in the boundary region. This indicates that the electrostatic turbulence should play an important role in strong electron energization in the plasma sheet–lobe boundary region during magnetic reconnection.

Condition for charged particle trapping in a three-dimensional electrostatic potential well in the presence of a magnetic field

Charged particle dynamics in a superimposed axially symmetric electrostatic potential well and a uniform magnetic field is studied within the framework of adiabatic theory. An approximate expression for the adiabatic invariant of the motion is derived. The condition for particle trapping in the potential well is established by taking into account finite gyroradius effects.

Electron surfing acceleration in magnetic reconnection

We discuss that energetic electrons are generated near the X‐type magnetic reconnection region due to a surfing acceleration mechanism. In a thin plasma sheet the polarization electric fields pointing toward the neutral sheet are induced around the boundary between the lobe and plasma sheet in association with the Hall electric current. By using a particle‐in‐cell simulation, we demonstrate that the polarization electric fields are strongly enhanced in an externally driven reconnection system, and some electrons can be trapped by the electrostatic potential well of the polarization field. During the trapping phase the electrons can gain their energies from the convection/inductive reconnection electric fields. We discuss that relativistic electrons with MeV energies are quickly generated in and around the X‐type neutral region by utilizing the surfing acceleration.

Spatial distribution of ion energies in an inertial electrostatic confinement device

The spatial distribution of ion energies was measured in an inertial electrostatic confinement device, which can be used as a small nuclear fusion source for the generation of neutrons. Using Doppler shift spectroscopy and a single ended Langmuir probe it was found that a virtual anode was established at the center of the electrostatic potential well resulting in much reduced ion energies at this point. Moreover, there was direct correlation between the ion energies and the spatial plasma potential showing the validity of the probe measurements. It was shown that the ion energy is a maximum at the point where the potential is the deepest, which occurs about halfway between the center and the cathode radius.

Comment on “A new class of exact solutions of the Vlasov equation” [Phys. Plasmas 8, 5081 (2001)

A recent paper obtained a class of exact one-dimensional solutions of the Vlasov–Poisson equations for a one-component plasma in a parabolic electrostatic potential well. Here it is shown that a significant class of these solutions has already been studied and shown to be chaotic.