High Electric Field Gradients Research Articles

Study of a magnetic alloy-loaded RF cavity for bunch compression at the CSR

The Heavy Ion Research Facility and Cooling Storage Ring (HIRFL-CSR) accelerator in Lanzhou offers a unique possibility for the generation of high density and short pulse heavy ion beams by non-adiabatic bunch compression longitudinally, which is implemented by a fast jump of the RF-voltage amplitude. For this purpose, an RF cavity with high electric field gradient loaded with Magnetic Alloy cores has been developed. The results show that the resonant frequency range of the single-gap RF cavity is from 1.13 MHz to 1.42 MHz, and a maximum RF voltage of 40 kV with a total length of 100 cm can be obtained, which can be used to compress heavy ion beams of 238U72+ with 250 MeV/u from the initial bunch length of 200 ns to 50 ns with the coaction of the two single-gap RF cavity mentioned above.

Experimental program for the Princeton Ion Source Test Facility

AbstractA 100 kV ion source test stand formerly operated at Lawrence Livermore National Laboratory has been relocated to Princeton Plasma Physics Laboratory, where it is being installed and prepared for operation. A variety of topics relevant to ion-beam-driven high energy density physics and heavy ion fusion will be explored at this facility. The practicality of magnetic insulation to improve the performance of electrostatic accelerators will be investigated by determining whether a pair of parallel plates forming a high-voltage gap can sustain higher electric field gradients, when an electric current is passed through the electrode at the cathode potential so as to produce a magnetic field, which is everywhere parallel to the surface. The facility will also be used to develop and characterize improved plasma sources for space charge neutralization of intense ion beam systems such as the Neutralized Drift Compression Experiment-II facility. The negative halogen ion beam and ion-ion plasma studies previously initiated when this test facility was located at Lawrence Livermore National Laboratory will be resumed, and other experimental topics are also under consideration.

A modular micro-fluidic platform for cells handling by dielectrophoresis

The physical manipulation of biological cells is of vital importance in the development of miniaturized systems for biological analyses. Dielectrophoresis (DEP) has been reported as a promising method for cell manipulation without physical contact, since it exploits the dielectric properties of cells suspended in a micro-fluidic sample, under the action of high-gradient electric fields. In view of a more extended use of DEP phenomena in lab-on-chip devices, we have worked on a silicon-based platform with microfabricated electrodes, which can offer integrated solutions for a wide variety of applications, customizable for specific user needs. The platform is composed of several functional units, organized in characterization modules for the dielectric analysis and in cell manipulation stages that can be rearranged on a single chip, depending on the target application. It, therefore, represents a complete and innovative research solution, suitable for industrial applications. The non-uniform electric field for cell manipulation is generated by micro-electrodes, patterned on the silicon substrate of micro-fluidic channels, using standard micro-fabrication techniques. Numerical and parametrical modelling using the finite element method was performed to simulate the electric field distribution, quantify the DEP force and, thus, to optimize the geometry of each functional module. In this paper, we report preliminary experimental results obtained by testing some fabricated units using Saccharomyces cerevisiae cells and sheep red blood cells. A system based on the combination of some selected modules is, finally, proposed for sorting cell subpopulations.

Red blood cell quantification microfluidic chip using polyelectrolytic gel electrodes

This paper reports on a novel microfluidic chip with polyelectrolytic gel electrodes (PGEs) used to rapidly count the number of red blood cells (RBCs) in diluted whole blood. The proposed microdevice is based on the principle that the impedance across a microchannel between two PGEs varies sensitively as RBCs pass through it. The number and amplitude of impedance peaks provide the information about the number and size of RBCs, respectively. This system features a low-voltage dc detection method and non-contact condition between cells and metal electrodes. Major advantages include stable detection under varying cellular flow rate and position in the microchannel, little chance of cell damage due to high electric field gradient and no surface fouling of the metal electrodes. The performance of this PGEs-based system was evaluated in three steps. First, in order to observe the size-only dependence of the impedance signal, three different sizes of fluorescent microbeads (7.2, 10.0, and 15.0 mum; Bangs laboratories, USA) were used in the experiment. Second, the cell counting performance was evaluated by using 7.2 mum fluorescent microbeads, similar in size to RBCs, in various concentrations and comparing the results with an animal hematoanalyzer (MS 9-5; Melet schloesing laboratories, France). Finally, in human blood sample tests, intravenously collected whole blood was just diluted in a PBS without centrifuge or other pretreatments. The PGE-based system produced almost identical number of RBCs in over 800-fold diluted samples to the results from a commercialized human hematoanalyzer (HST-N402XE; Sysmex, Japan).

Electron emission characterization of Mg photocathode grown by pulsed laser deposition within anS-band rf gun

Pulsed laser deposition (PLD) has been proposed several years ago as a suitable technique to deposit a pure Mg film over a radio frequency (rf) gun Cu backflange in order to obtain a high efficiency photocathode surface for the generation of high brightness electron beams. In this paper we report preliminary experimental results on the emission properties of a PLD grown Mg film within the high electric field gradients of a rf gun showing the effects of the rf conditioning process on the cathode surface. Even though a laser cleaning process should be performed on the sample surface in order to remove contaminated layers, the results presented here are very promising for the realization of a final Mg-based photocathode.

A compact linac for intensity modulated proton therapy based on a dielectric wall accelerator

A novel compact CT-guided intensity modulated proton radiotherapy (IMPT) system is described. The system is being designed to deliver fast IMPT so that larger target volumes and motion management can be accomplished. The system will be ideal for large and complex target volumes in young patients. The basis of the design is the dielectric wall accelerator (DWA) system being developed at the Lawrence Livermore National Laboratory (LLNL). The DWA uses fast switched high voltage transmission lines to generate pulsed electric fields on the inside of a high gradient insulating (HGI) acceleration tube. High electric field gradients are achieved by the use of alternating insulators and conductors and short pulse times. The system will produce individual pulses that can be varied in intensity, energy and spot width. The IMPT planning system will optimize delivery characteristics. The system will be capable of being sited in a conventional linac vault and provide intensity modulated rotational therapy. Feasibility tests of an optimization system for selecting the position, energy, intensity and spot size for a collection of spots comprising the treatment are underway. A prototype is being designed and concept designs of the envelope and environmental needs of the unit are beginning. The status of the developmental new technologies that make the compact system possible will be reviewed. These include, high gradient vacuum insulators, solid dielectric materials, SiC photoconductive switches and compact proton sources.

Study of Flexoelectric Phenomenon from Direct and from Inverse Flexoelectric Behavior of PMNT Ceramic

Flexoelectric phenomenon describes creation of electric polarization in non-conducting solids by applying of gradient of deformation. The inverse flexoelectric phenomenon describes the generation of a mechanical deformation in an insulating solid by an applied gradient of the electric field. Lead magnesium niobate-lead titanate (PMNT) ceramic permits study of the flexoelectric behavior because of its large flexoelectric coefficient. Special conical samples were prepared for the generation of a high electric field gradient and cylindrical samples with uniform field. Theoretical study of flexoelectric phenomenon seems to be very interesting in the last time because it was confirmed from direct (polarization caused by gradient of deformation) and from inverse behavior (deformation developed from gradient electric field) outgoing from thermodynamic theory.

TH‐C‐AUD‐09: A Proposal for a Novel Compact Intensity Modulated Proton Therapy System Using a Dielectric Wall Accelerator

Purpose: A novel compact CT‐guided intensity modulated proton radiotherapy (IMPT) system is introduced. The system is being designed to deliver motion‐managed IMPT to large target volumes. The system will be ideal for large and complex target volumes in young patients. Method and Materials: The basis of the design is the dielectric wall accelerator (DWA) system being developed at Lawrence Livermore National Laboratory (LLNL). The DWA will use fast switched high voltage transmission lines to generate pulsed electric fields on the inside of a high gradient insulating (HGI) acceleration tube. High electric field gradients are achieved alternating insulators and conductors and short pulse times. The system will produce individual pulses that can be varied in intensity, energy and spot width, all of which will be optimized in the IMPT planning system. It is anticipated that no magnets will be required and the neutron contamination will be very low. The system will be capable of being sited in a conventional linac vault. Results: The design specifications have been met in some component tests. Gradients of 100 MV/m have been achieved in small HGI samples. Optical switches based on fast laser switched SiC has been achieved. Feasibility tests of an optimization system for selecting the position, energy, intensity and spot size for a collection of spots comprising the treatment are underway. A prototype is being designed and concept designs of the envelope and environmental needs of the unit has commenced. Conclusion: The DWA accelerator represents breakthrough technology for intensity modulated proton therapy. The system is being designed from the ground up to be capable of CT‐guided intensity modulated proton therapy and to be housed in a conventional linac vault. Conflict of Interest:Some of the authors have financial interest in TomoTherapy Inc., which has licensed the DWA technology from LLNL.

Study of the Inverse Flexoelectric Phenomena in Ceramic Lead Magnesium Niobate-Lead Titanate

The inverse flexoelectric phenomenon describes the generation of a mechanical deformation in an insulating solid by an applied gradient of the electric field. Lead magnesium niobate-lead titanate (PMNT) ceramic permits study of the inverse flexoelectric behavior because of its large flexoelectric coefficient. Special conical samples were prepared for the generation of a high electric field gradient and cylindrical samples with uniform field. A strong deformation is caused by the electrostrictive behavior of PMNT ceramic. An asymmetric butterfly-shaped strain dependence on electric field is observed at wide temperature range on the conical samples. For evaluation of converse flexoelectric coefficient μ 33 (in matrix notation) it is necessary to know elastic properties of ceramic. The ultrasonic measurements on ceramic samples were performed for this purpose. In this letter we also report measurements of temperature dependence of flexoelectric coefficient μ 33 . The measurements confirmed a great enhancement of μ 33 coefficient in PMN ceramics, which cannot be explained exclusively by the high permittivity of the ceramic.

Environment of the Gating Charges in the Kv1.2 Shaker Potassium Channel

Recently, the structure of the Shaker channel Kv1.2 has been determined at a 2.9-Å resolution. This opens new possibilities in deciphering the mechanism underlying the function of voltage-gated potassium (Kv) channels. Molecular dynamics simulations of the channel, embedded in a membrane environment show that the channel is in its open state and that the gating charges carried by S4 are exposed to the solvent. The hydrated environment of S4 favors a local collapse of the electrostatic potential, which generates high electric-field gradients around the arginine gating charges. Comparison to experiments suggests furthermore that activation of the channel requires mainly a lateral displacement of S4. Overall, the results agree with the transporter model devised for Kv channels from electrophysiology experiments, and provide a possible pathway for the mechanistic response to membrane depolarization.

High-gradient electric field system for the dielectrophoretic separation of cells

A high-gradient electric field strength system, consisting of a chamber created from two concentric cylindrical electrodes and filled with glass beads, was constructed for the study of the dielectrophoretic separation of cells. The glass beads distort the electric field patterns and generate sites of high electric field gradients, which can capture cells by dielectrophoresis. In order to experimentally determine the efficacy of the system, yeast cells were injected into a constant flow through the system, and the number of cells trapped measured. The effects of changes in voltage, bead size and the size of the inner and outer electrodes on the trapping efficiency of the dielectrophoretic separation system were investigated. In addition, simulation and electric field analysis were carried out using FEMLAB. Results indicate that the trapping of cells occurs by a combination of mechanical trapping and dielectrophoresis. Analysis of the results allow predictions to be made for the optimum values of the voltage, bead size and size of inner and outer electrode for any high-gradient system, in order to generate sufficient electric field gradient for dielectrophoretic cell collection whilst reducing non-specific mechanical trapping.

New mechanism of cluster-field evaporation in rf breakdown

Using a simple field evaporation model and molecular dynamics simulations of nanoscale copper tip evolution in a high electric field gradient typical for linacs, we have studied a new mechanism for rf-field evaporation. The mechanism consists of simultaneous (collective) field evaporation of a large group of tip atoms in high-gradient fields. Thus, evaporation of large clusters is energetically more favorable when compared with the conventional, ``one-by-one'' mechanism. The studied mechanism could also be considered a new mechanism for the triggering of rf-vacuum breakdown. This paper discusses the mechanism and the experimental data available for electric field evaporation of field-emission microscopy tips.

Geometric and Material Determinants of Patterning Efficiency by Dielectrophoresis

Dielectrophoretic (DEP) forces have been used extensively to manipulate, separate, and localize biological cells and bioparticles via high-gradient electric fields. However, minimization of DEP exposure time is desirable, because of possible untoward effects on cell behavior. Toward this goal, this article investigates the geometric and material determinants of particle patterning kinetics and efficiency. In particular, the time required to achieve a steady-state pattern is theoretically modeled and experimentally validated for a planar, interdigitated bar electrode array energized in a standing-wave configuration. This measure of patterning efficiency is calculated from an improved Fourier series solution of DEP force, in which realistic boundary conditions and a finite chamber height are imposed to reflect typical microfluidic applications. The chamber height, electrode spacing, and fluid viscosity and conductivity are parameters that profoundly affect patterning efficiency, and optimization can reduce electric field exposure by orders of magnitude. Modeling strategies are generalizable to arbitrary electrode design as well as to conditions where DEP force may not act alone to cause particle motion. This improved understanding of DEP patterning kinetics provides a framework for new advances in the development of DEP-based biological devices and assays with minimal perturbation of cell behavior.

Surface analysis of oxygen free electrolytic-copperX-band accelerating structures and possible correlation to radio frequency breakdown events

X-band accelerator structures meeting the next linear collider design requirements have been found to suffer vacuum surface damage caused by rf breakdown, when processed to high electric-field gradients. Improved understanding of these breakdown events is desirable for the development of structure designs, fabrication procedures, and processing techniques that minimize structure damage. rf reflected wave analysis and acoustic sensor pickup have provided breakdowns localization in rf structures. Particle contaminations found following clean autopsy of four rf-processed traveling wave structures have been catalogued and analyzed. Their influence on rf breakdown, as well as that of several other material-based properties, are discussed.

Frequency and Voltage Dependence of the Dielectrophoretic Trapping of Short Lengths of DNA and dCTP in a Nanopipette

The study of the properties of DNA under high electric fields is of both fundamental and practical interest. We have exploited the high electric fields produced locally in the tip of a nanopipette to probe the motion of double- and single-stranded 40-mer DNA, a 1-kb single-stranded DNA, and a single-nucleotide triphosphate (dCTP) just inside and outside the pipette tip at different frequencies and amplitudes of applied voltages. We used dual laser excitation and dual color detection to simultaneously follow two fluorophore-labeled DNA sequences with millisecond time resolution, significantly faster than studies to date. A strong trapping effect was observed during the negative half cycle for all DNA samples and also the dCTP. This effect was maximum below 1 Hz and decreased with higher frequency. We assign this trapping to strong dielectrophoresis due to the high electric field and electric field gradient in the pipette tip. Dielectrophoresis in electrodeless tapered nanostructures has potential applications for controlled mixing and manipulation of short lengths of DNA and other biomolecules, opening new possibilities in miniaturized biological analysis.

Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results

Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques.In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly.In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor.In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence.On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field Er and confinement, limiter biasing acting mainly on the SOL.In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

Nanowire Arrays Electrodeposited from Liquid Crystalline Phases

Crystalline silver nanowire arrays (see Figure) with high aspect ratio are produced here by electrodeposition from a reverse hexagonal liquid-crystalline phase. Enhancement of the alignment is shown to be possible by applying a high electric field gradient during deposition. High-density bundles can be obtained by washing the deposited material and single wires can easily be retrieved by sonication of these bundles.

Dielectrophoresis with application to boiling heat transfer in microgravity. I. Numerical analysis

This article presents a numerical analysis for the use of a dielectrophoretic (DEP) force on vapor bubbles to sustain nucleate boiling heat transfer in space where the gravity-driven buoyancy force is absent. The analytic and numerical solution procedures for the DEP force are outlined for an infinite-plate and a finite-plate electrode geometry, respectively. A simple analysis is presented that describes the limits on the magnitude of the DEP force. A comparison is made between the DEP forces produced near the edges of the finite-plate electrodes (where there is a relatively high-gradient electric field) to the forces produced between the electrodes and away from the edges (where there is a relatively low-gradient electric field). The predicted DEP forces are then used to study the effectiveness of bubble transport for both high and low electric-field gradients in microgravity. To effectively maintain boiling in space with the DEP force, a knowledge of the electric field is required which addresses both the DEP force strength and the DEP force spatial distribution in order to appropriately balance between the short-distance vapor bubble detachment needs and the long-distance bubble transport requirements.

Novel lithography and signal processing with water vapor ions

A novel model is presented for the operation of field emission sources in air. Under this condition the cathode–anode distance is less than the mean free path for the particles emitted in air. Such conditions are encountered in scanning tunneling microscope lithography in air and in novel integrated microtriode tube devices designed to operate in air. In air, the presence of a high electric field gradient, produced at the tip, will cause the polar water molecules coating the surface to align with the field and move to the high field region. As a result, a Taylor cone of water, like that as it is observed with liquid metal ion sources, will form. The high electric field gradient at the Taylor cone, enhanced by the high dielectric constant, will produce a very finely focused beam of ions. We examine the case of water ion formation. With a positive tip bias, hydronium ions will form at electric fields of 1 V/nm. This is a factor of 4 smaller than that required to produce the field emission with electrons. The electric field distribution and the hydronium ion trajectories from a metal tip with a high permittivity material cone are calculated. The spot size of the ion beam on the substrate was 2.9 nm. A model for lithography, based on water ions, is presented. The model is applied to the design of a microtriode device for signal processing. The three-dimensional fabrication of such a device by e-beam induced chemical vapor deposition is demonstrated.

Effects of interparticle electric interactions on dielectrophoresis in colloidal suspensions.

Using our recently published statistical theory concerning the phase diagram of a low-conducting colloidal suspension subject to uniform electric fields [Phys. Rev. E 52, 1669 (1995)], we examine how the long-range electric-field-induced interparticle interactions affect the spatial arrangement of particles in such a suspension under the action of a nonuniform ac electric field. We find the conditions under which the resulting dielectrophoresis in nonuniform electric fields is accompanied by an electric-field-induced phase transition in the suspension. Moreover we predict that, in the case of positive dielectrophoresis, the particles will form chainlike aggregates aligned parallel to the electric field lines and attracted towards the higher electric-field region; whereas, for negative dielectrophoresis, the particles will form disklike aggregates aligned perpendicularly to the electric-field lines and repelled from the higher electric-field region. The theory also provides some insight regarding the dependence of the particle aggregation on the frequency of the applied nonuniform ac electric field. The predictions of the theory are consistent with the characteristic patterns of cell aggregation observed previously in high-gradient electric fields generated in microelectrode systems. \textcopyright{} 1996 The American Physical Society.