Electrostatic Transport Research Articles

Simulation of the pH Sensing Capability of an Open-Gate GaN-Based Transistor

Heterostructure transistors based on the AlGaN/GaN material structure are popular for chemical and biological sensors because of their nontoxicity and chemical inertness in biological or corrosive environments. They also offer very high sensitivity as a result of the very large gain and transconductance. Studies over the years show much research on this technology for sensing applications including pH sensing, prostate cancer detection, and breast cancer detection. However, the designs of these devices are not optimized because of nonexistent or rudimentary modeling and simulation support. Commercial software packages adept at simulating III-V-based transistors do not have applications built-in for chemical sensing capability. Current research relies on analytical models and in-house numerical programs to do specific and limited simulations. We build upon a TCAD simulation framework (FLOODS) specifically suited for III-V device simulation that is capable of self-consistent, fully-coupled simulation of reaction-dependent physics at the electrolyte/semiconductor interface. This paper outlines the simulation methodology for a pH sensor using an open-gate AlGaN/GaN High Electron Mobility Transistor (HEMT). The interfacial reactions are coupled to traditional device simulation equations, i.e. Poisson and carrier continuity. This enables surface charging reactions to be directly coupled to the device output terminal characteristics. We also solve for electrostatic field and ion transport in the electrolyte. This work will enable optimization of specific sensor technologies or exploration of the fundamental physics behind the sensing mechanism.

The HelCat basic plasma science device

TheHelicon-Cathode(HelCat) device is a medium-size linear experiment suitable for a wide range of basic plasma science experiments in areas such as electrostatic turbulence and transport, magnetic relaxation, and high power microwave (HPM)-plasma interactions. The HelCat device is based on dual plasma sources located at opposite ends of the 4 m long vacuum chamber – an RF helicon source at one end and a thermionic cathode at the other. Thirteen coils provide an axial magnetic fieldB⩾ 0.220 T that can be configured individually to give various magnetic configurations (e.g. solenoid, mirror, cusp). Additional plasma sources, such as a compact coaxial plasma gun, are also utilized in some experiments, and can be located either along the chamber for perpendicular (to the background magnetic field) plasma injection, or at one of the ends for parallel injection. Using the multiple plasma sources, a wide range of plasma parameters can be obtained. Here, the HelCat device is described in detail and some examples of results from previous and ongoing experiments are given. Additionally, examples of planned experiments and device modifications are also discussed.

Recent performance of the SNS H− ion source and low-energy beam transport system

Recent measurements of the H(-) beam current show that SNS is injecting about 55 mA into the RFQ compared to ∼45 mA in 2010. Since 2010, the H(-) beam exiting the RFQ dropped from ∼40 mA to ∼34 mA, which is sufficient for 1 MW of beam power. To minimize the impact of the RFQ degradation, the service cycle of the best performing source was extended to 6 weeks. The only degradation is fluctuations in the electron dump voltage towards the end of some service cycles, a problem that is being investigated. Very recently, the RFQ was retuned, which partly restored its transmission. In addition, the electrostatic low-energy beam transport system was reengineered to double its heat sinking and equipped with a thermocouple that monitors the temperature of the ground electrode between the two Einzel lenses. The recorded data show that emissions from the source at high voltage dominate the heat load. Emissions from the partly Cs-covered first lens cause the temperature to peak several hours after starting up. On rare occasions, the temperature can also peak due to corona discharges between the center ground electrode and one of the lenses.

Improvements to the internal and external antenna H− ion sources at the Spallation Neutron Source

The Spallation Neutron Source (SNS), a large scale neutron production facility, routinely operates with 30-40 mA peak current in the linac. Recent measurements have shown that our RF-driven internal antenna, Cs-enhanced, multi-cusp ion sources injects ∼55 mA of H(-) beam current (∼1 ms, 60 Hz) at 65-kV into a Radio Frequency Quadrupole (RFQ) accelerator through a closely coupled electrostatic Low-Energy Beam Transport system. Over the last several years a decrease in RFQ transmission and issues with internal antennas has stimulated source development at the SNS both for the internal and external antenna ion sources. This report discusses progress in improving internal antenna reliability, H(-) yield improvements which resulted from modifications to the outlet aperture assembly (applicable to both internal and external antenna sources) and studies made of the long standing problem of beam persistence with the external antenna source. The current status of the external antenna ion source will also be presented.

Development of High Power Electrostatic Accelerators for Nuclear and Medical Purposes in Argentina

We highlight some of the results of an ongoing program at the National Atomic Energy Commission (CNEA) in Argentina to develop low energy (0.2 to 2.5 MeV), high current (30-100mA) proton and deuteron electrostatic accelerators for nuclear and medical applications. This activity aims at developing: 1) Low-energy (200 keV) high-current (100mA) deuteron accelerators for D(d,n) and T(d,n) for neutron production for various nuclear applications, among them the injection of a subcritical reactor. 2) The development of a 700kV folded tandem for neutron production using the 9Be(d,n) reaction. 3) The development of a folded 1.2-1.4 MV tandem for epithermal neutron production through the 7Li(p, n) reaction. At the same time this machine can be operated as single-ended with a positive ion source at the terminal to produce 1.4 MeV deuteron beams for the 9Be(d, n) reaction. Development and progress have been made in the areas of mechanical, electromechanical and electronics components (structures, alternators,HV supplies, control systems, etc), ion sources, accelerator tubes, 3D electrostatic and selfconsistent beam transport simulations, high power neutron production targets.

Molecular Modeling of Hydronium Ion and Water Distribution in Water-Filled Pt Nanochannels with Corrugated Walls

Classical molecular dynamics was employed to study the impact of nanoconfinement and surface nanostructure on electrostatic phenomena and transport properties in catalyst layer pores of polymer electrolyte fuel cells. The porous structure of an ionomer-free ultrathin catalyst layer was represented by a water-filled nanochannel with corrugated and negatively charged Pt walls. The equilibrium hydronium ion distribution in the channel was used to assess local reaction conditions. We rationalized the effects of nanostructuring and metal surface charge on the electrostatic effectiveness factor of the channel. Furthermore, we investigated the water dynamics in the nanochannel, calculating the self-diffusion coefficients of surface and bulk-like water.

ISAC LEBT

The ISAC electrostatic Low Energy Beam Transport (LEBT) system is described.

Measurement and modelling of anomalous polarity pulses in a multi-electrode diamond detector

In multi-electrode detectors, the motion of excess carriers generated by ionizing radiation induces charge pulses at the electrodes, whose intensities and polarities depend on the geometrical, electrostatic and carriers' transport properties of the device. The resulting charge-sharing effects may lead to bipolar currents, pulse height defects and anomalous polarity signals affecting the response of the device to ionizing radiation. This latter effect has recently attracted attention in commonly used detector materials, but different interpretations have been suggested, depending on the material, the geometry of the device and the nature of the ionizing radiation. In this letter, we report on the investigation in the formation of anomalous polarity pulses in a multi-electrode diamond detector with buried graphitic electrodes. In particular, we propose a purely electrostatic model based on the Shockley-Ramo-Gunn theory, providing a satisfactory description of anomalous pulses observed in charge collection efficiency maps measured by means of Ion Beam Induced Charge (IBIC) microscopy, and suitable for a general application in multi-electrode devices and detectors.

Electrostatic properties of confluent Caco‐2 cell layer correlates to their microvilli growth and determines underlying transcellular flow

Recently, Rajapaksa et al. (2010) showed that the rate of uptake of potential vaccine delivery nanoparticles in the mucosal layer is a function of the electrostatic properties of the corresponding solvent. This fundamentally implies that the dominant driving forces that may be capitalized on for mucosal vaccine strategies are electrostatic in nature. We hypothesize that the driving force normal to the cell (in the direction from apical to basolateral across the cell) is of particular importance. In addition, it has been theoretically shown that the electrostatic properties of mucosal cells are directly related to their development of brush border. Here we correlate the development of brush border on a human mucosal epithelial model (Caco-2) cultured in DMEM on 3.0 µm pore sized polycarbonate membranes to their corresponding electrostatic properties characterized by measuring their normal zeta potential. Properties of normal streaming potential, hydraulic permeability, and brush border development (as determined by size and number) were monitored for 2, 6, and 16 days (after cells were confluent). Human endothelial cells (HECs), which lack brush border, were used as the control. Our results demonstrate that normal zeta potential of Caco-2 cells significantly changed from -5.7 ± 0.11 mV to -3.4 ± 0.11 mV for a period between 2 and 16 days, respectively. The zeta potential of the control cell line, HECs, stayed constant (statistically not different, P > 0.05) for the duration of the experiments. Our results show that the calculated increase in surface area of the Caco-2 cells with microvilli from 6 to 16 days was directly proportional to the corresponding measured zeta potential difference. These results imply that microvilli alter the electrostatic local environment around Caco-2 cells and, hence, enhance the normal electrostatic selective transport of solute across the mucosal barrier.

Plasma electron temperature variability in lunar surface potential and in electric field under average solar wind conditions

The Moon is immersed in plasma environment. The most interesting challenge of the lunar plasma– field environment is that it is alternatively dominated by the extended but variable outer atmosphere of the Earth – the magnetosphere – and by the extended but highly variable solar atmosphere – the solar wind. Understanding the plasma environment and its interaction with the lunar surface will be beneficial to both manned and robotic surface exploration activities and to scientific investigations. Presented is a preliminary map of variations of lunar surface electric potential over the day side and night side using probe equations and a discussion on dust dynamics in this E-field structure using the data from Electron Reflectometer in Lunar Prospector spacecraft during 1998–1999. On the day side, potential is around 5V and on the night side it reaches up to −82V. On the night side region, only highly energetic electrons can overcome this large negative potential. The variation at electron temperature (Te) strongly reflects in the surface potential. The potential reaches to a value of −82V for Te=58eV. Surface charging causes the electrostatic transport of charged dust grains. Dust grain size of 0.1μm shows a levitation height of 4.92m on lunar day side, 748m on terminator region and 3.7km on the night side. The radius of maximum sized grain to be lofted, Rmax, peaks at the terminator region (Rmax=0.83μm). At the transition region dust levitation is almost absent. This region is most suited for exploration activities as the region is free from hazards caused by lunar dust.

Charge and current transport in open field lines turbulence: Influence of plasma-surface boundary conditions

We investigate the impact of both parallel and transverse boundary conditions on the current and charge transport in open field line systems using the TOKAM2D code, which solves a minimal model for interchange turbulence. Various limit test cases are discussed and analyzed. In the parallel direction, the sheath conductivity is found to play an essential role in the stabilization of large-scale potential structures, leading to the formation of transport channel or transport barrier respectively for an insulating end wall or a wall with an enhanced sheath conductivity. On another hand, the addition of transverse boundary conditions intrinsically changes the transport characteristics, influencing both radial profiles and probability density functions. It underlines that in some cases a detailed description of the plasma-wall interaction process is required to get a proper description of the current loop pattern that determines electrostatic turbulent transport.

Magnetic stochasticity and transport due to nonlinearly excited subdominant microtearing modes

Subdominant, linearly stable microtearing modes are identified as the main mechanism for the development of magnetic stochasticity and transport in gyrokinetic simulations of electromagnetic ion temperature gradient driven plasma microturbulence. The linear eigenmode spectrum is examined in order to identify and characterize modes with tearing parity. Connections are demonstrated between microtearing modes and the nonlinear fluctuations that are responsible for the magnetic stochasticity and electromagnetic transport, and nonlinear coupling with zonal modes is identified as the salient nonlinear excitation mechanism. A simple model is presented, which relates the electromagnetic transport to the electrostatic transport. These results may provide a paradigm for the mechanisms responsible for electromagnetic stochasticity and transport, which can be examined in a broader range of scenarios and parameter regimes.

Experimental Studies of Electrostatic Fluctuations and Turbulent Transport in the Boundary of J-TEXT Tokamak Using Reciprocating Probe

As the basic of a deeper investigation on the turbulent transport, the fluctuation property in the boundary of the newly-reconstructed Joint Texas Experimental Tokamak (J-TEXT) is studied experimentally using the reciprocating Langmuir four-tip probe, which has been built and operated as the primary diagnostic tool in the boundary of J-TEXT tokamak. In this paper, spatial profiles of the plasma-edge parameters are obtained, such as electron temperature, plasma density, plasma potential, poloidal electric field and their fluctuations. The results indicate the existence of a Er×BT shear layer at the vicinity of last closed flux surface (LCFS), with the fluctuations suppressed in varying degrees. The turbulence-induced particle and energy fluxes can be calculated by the local plasma parameters above. Convection dominates the cross-field turbulent transport in boundary plasma. Electrostatic fluctuations properties are also studied in detail with the help of numerical analysis. Statistical analysis on density fluctuation shows that, the intermittency can affect the turbulence in the scrape-off layer (SOL).

Sampling of Small Regolith Particles from Asteroids Utilizing an Alternative Electrostatic Field and Electrostatic Traveling Wave

To realize reliable and autonomous regolith sampling from asteroids in space, the author has developed a new sampling system that uses electrostatic force. This system uses a combination of electrostatic capture and transport of particles. High voltage is applied between parallel screen electrodes of the sampling device. As a result of the electrostatic force, particles are captured by the screen electrodes. The captured particles are then transported by an electrostatic traveling wave and transferred to a collection capsule. It has been demonstrated that lunar soil simulant, iron particles, and crushed ice can be sampled if the end of the screen electrode is in contact with the regolith for a short period of time (within 1 s). Because gravity is extremely low on small asteroids, the process of sampling particles on asteroids will be easier than that on the Earth.

The effect of surface topography on the lunar photoelectron sheath and electrostatic dust transport

The dayside near-surface lunar plasma environment is electrostatically complex, due to the interaction between solar UV-induced photoemission, the collection of ambient ions and electrons, and the presence of micron and sub-micron sized dust grains. Further complicating this environment, although less well understood in effect, is the presence of surface relief, typically in the form of craters and/or boulders. It has been suggested that such non-trivial surface topography can lead to complex electrostatic potentials and fields, including “mini-wakes” behind small obstacles to the solar wind flow and “supercharging” near sunlit-shadowed boundaries (Criswell, D.R., De, B.R. [1977]. J. Geophys. Res. 82 (7); De, B.R., Criswell, D.R. [1977]. J. Geophys. Res. 82 (7); Farrell, W.M., Stubbs, T.J., Vondrak, R.R., Delory, G.T., Halekas, J.S. [2007]. Geophys. Res. Lett. 34; Wang, X., Horányi, M., Sternovsky, Z., Robertson, S., Morfill, G.E. [2007]. Geophys. Res. Lett. 34, L16104). In this paper, we present results from a three-dimensional, self-consistent, electrostatic particle-in-cell code used to model the dayside near-surface lunar plasma environment over a variety of local times with the presence of a crater. Additionally, we use the particle-in-cell model output to study the effect of surface topography on the dynamics of electrostatic dust transport, with the goal of understanding previous observations of dust dynamics on the Moon and dust ponding on various asteroids.

Electrostatic transport in L-mode scrape-off layer plasmas of Tore Supra tokamak. II. Transport by fluctuations

The turbulent transport of particles is investigated using rake probes at the top of the scrape-off layer (SOL) of circular ohmically heated L-mode plasmas in the Tore Supra tokamak [G. Giruzzi et al., Nucl. Fusion 49, 104010 (2009)]. Both radial and poloidal non-linear fluxes are estimated by means of two reciprocating arrays separated toroidally by 120°. The time average of the radial (poloidal) flux is positive (negative) through the whole SOL profile. The respective effective transport velocity are about 〈vr〉t≈30m s−1 and 〈vθ〉t≈−60m s−1 close to the last closed flux surface. Both components present a standard deviation about 10 times higher than their respective mean amplitude, and time-distributions highly skewed toward values of the same sign as their mean values. The existence of a nonlinear poloidal flux is associated with the local tilt of filament eddies due to electric and magnetic shear. At the last closed flux surface, where plasma filaments experience their early life, the orientation of the velocity field is consistent with structure dynamics which originate from the outboard midplane and spread along field lines toward the rest of the poloidal section. The localized tilt of the eddy structures corresponds to the effect of the magnetic shear. Further into the SOL, the orientation of the velocity field evolves along radius in agreement with a simple model of propagating filaments progressively tilted by an electric shear. The combined effects of electric and magnetic shear on the eddy tilting have potentially a crucial impact on the existence of a Reynolds stress 〈vrvθ〉t component, which is strongly poloidally asymmetric at the edge of tokamak plasmas.

Effect of poloidal asymmetries on impurity peaking in tokamaks

Poloidal impurity asymmetries are frequently observed in tokamaks. In this paper, the effect of poloidal asymmetry on electrostatic turbulent transport is studied, including the effect of the E×B drift. Collisions are modeled by a Lorentz operator, and the gyrokinetic equation is solved with a variational approach. The impurity transport is shown to be sensitive to the magnetic shear and changes sign for s≳0.5 in the presence of inboard accumulation. The zero-flux impurity density gradient (peaking factor) is shown to be rather insensitive to collisions in both ion temperature gradient and trapped electron mode driven cases. Our results suggest that the asymmetry (both the location of its maximum and its strength) and the magnetic shear are the two most important parameters that affect the impurity peaking.

Embedded-gate graphene transistors for high-mobility detachable flexible nanoelectronics

A high-mobility graphene field-effect transistor with embedded gate was fabricated on smooth spin-coated polyimide films. Electrostatic transport measurements reveal a maximum electron and hole mobility of 4930 cm2/V s and 1130 cm2/V s, respectively. Temperature dependent measurements indicate that carrier transport is not limited by intrinsic mechanisms but by charged impurities, surface roughness, and defects, suggesting that further increases in mobility are possible. The measured carrier mobilities are the highest reported for graphene transistors on polymeric substrates and hence enable high-speed devices for flexible electronics from graphene grown by size-scalable chemical vapor deposition.

Low-energy beam transport studies supporting the spallation neutron source 1-MW beam operation

The H(-) injector consisting of a cesium enhanced RF-driven ion source and a 2-lens electrostatic low-energy beam transport (LEBT) system supports the spallation neutron source 1 MW beam operation with ∼38 mA beam current in the linac at 60 Hz with a pulse length of up to ∼1.0 ms. In this work, two important issues associated with the low-energy beam transport are discussed: (1) inconsistent dependence of the post-radio frequency quadrupole accelerator beam current on the ion source tilt angle and (2) high power beam losses on the LEBT electrodes under some off-nominal conditions compromising their reliability.

H− radio frequency source development at the Spallation Neutron Source

The Spallation Neutron Source (SNS) now routinely operates nearly 1 MW of beam power on target with a highly persistent ∼38 mA peak current in the linac and an availability of ∼90%. H(-) beam pulses (∼1 ms, 60 Hz) are produced by a Cs-enhanced, multicusp ion source closely coupled with an electrostatic low energy beam transport (LEBT), which focuses the 65 kV beam into a radio frequency quadrupole accelerator. The source plasma is generated by RF excitation (2 MHz, ∼60 kW) of a copper antenna that has been encased with a thickness of ∼0.7 mm of porcelain enamel and immersed into the plasma chamber. The ion source and LEBT normally have a combined availability of ∼99%. Recent increases in duty-factor and RF power have made antenna failures a leading cause of downtime. This report first identifies the physical mechanism of antenna failure from a statistical inspection of ∼75 antennas which ran at the SNS, scanning electron microscopy studies of antenna surface, and cross sectional cuts and analysis of calorimetric heating measurements. Failure mitigation efforts are then described which include modifying the antenna geometry and our acceptance∕installation criteria. Progress and status of the development of the SNS external antenna source, a long-term solution to the internal antenna problem, are then discussed. Currently, this source is capable of delivering comparable beam currents to the baseline source to the SNS and, an earlier version, has briefly demonstrated unanalyzed currents up to ∼100 mA (1 ms, 60 Hz) on the test stand. In particular, this paper discusses plasma ignition (dc and RF plasma guns), antenna reliability, magnet overheating, and insufficient beam persistence.