Electrostatic Confinement Research Articles

Electromagnetic Design and Simulation of a New Fusion Device

A new inertial electrostatic confinement (IEC) fusion device is proposed. The device contains two ion guns which feed the plasma media by deuterium ions. The device is designed in three dimensions and the finite difference method is applied to satisfy the boundary values of the cylindrical chamber. It is the first time that both the ions and electrons are simulated in this geometry by particle in cell (PIC) technique. Ions and electrons can interact with six bar-sized cathodes of the chamber and each other as a result of many-body problem. The device has a central dc current-carrying rod, which generates a homogeneous magnetic field surrounding the six cathodes. Thus we expect a better confinement compared to the conventional devices. The simulation records the real-time position and momentum by using the electromagnetic equations together with the momentum equations. The dynamics of the particles is seen to be complicated with vibrations on the horizontal plane due to the field. DOI: http://dx.doi.org/10.5755/j01.eee.20.8.8435

Hydrogenated amorphous carbon thin films deposition by pulsed DC plasma enhanced by electrostatic confinement

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with attractive properties such as high hardness, low friction, chemical inertness and high wear resistance. In this work, hydrogenated amorphous carbon (a-C:H) thin films were deposited using a plasma enhanced chemical vapor deposition technique by pulsed DC plasma with a simple, low-cost and efficient arrangement of multi-cathodes and multi-anodes in order to enhance the plasma by electrostatic confinement. The samples were characterized by Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Elastic Recoil Detection Analysis, Raman Spectroscopy, and nanoindentation measurements. a-C:H thin films show an homogeneous hydrogen profile along the films deposited at −600V and −800V and variable working pressure. According to Raman spectra, both the ID/IG ratio and the G-peak position increase at higher voltages (more remarkable dependence) and lower working pressures (less remarkable dependence). Moreover, the hardness depends on working conditions such as power supply voltage and total working pressure. The electrostatic confinement enhances the deposition rates of a-C:H thin films up to values of 0.9μm·h−1, which is almost double than those previously published by pulsed DC plasma in similar conditions. The Raman spectra follow the stage 2 of an established model and this structure transition may explain the hardness behavior.

Suspended InAs nanowire gate-all-around field-effect transistors

Gate-all-around field-effect transistors are realized with thin, single-crystalline, pure-phase InAs nanowires grown by molecular beam epitaxy. At room temperature, the transistors show a desired high on-state current Ion of ∼10 μA and an on-off current ratio Ion/Ioff of as high as 106 at source-drain bias voltage of 50 mV and gate length of 1 μm with a gate underlap spacing of 1 μm from the source and from the drain. At low temperatures, the on-state current Ion is only slightly reduced, while the ratio Ion/Ioff is increased to 107. The field-effect mobility in the nanowire channels is also investigated and found to be ∼1500 cm2/V s at room temperature and ∼2000 cm2/V s at low temperatures. The excellent performance of the transistors is explained in terms of strong electrostatic and quantum confinements of carriers in the nanowires.

Neoclassical transport processes in weakly collisional plasmas with fractured velocity distribution functions

Ripples in magnetic or electrostatic confinement fields give rise to trapping separatrices, and conventional neoclassical transport theory describes the collisional trapping/detrapping of particles with fractured distribution function. Our experiments and novel theory have now characterized a new kind of neoclassical transport processes arising from chaotic (nominally collisionless) separatrix crossings, which occur due to E × B plasma rotation along θ−ruffled or wave-perturbed separatrices. This chaotic neoclassical transport becomes dominant at low collisionality when the collisional spreading of particle energy during the dynamical period is less than the separatrix energy ruffle.

Transport properties of overheated electrons trapped on a helium surface

An ultra-strong photovoltaic effect has recently been reported for electrons trapped on a liquid helium surface under a microwave excitation tuned at intersubband resonance [D. Konstantinov, A.D. Chepelianskii, K. Kono, J. Phys. Soc. Jpn 81, 093601 (2012)]. In this article, we analyze theoretically the redistribution of the electron density induced by an overheating of the surface electrons under irradiation, and obtain quantitative predictions for the photocurrent dependence on the effective electron temperature and confinement voltages. We show that the photo-current can change sign as a function of the parameters of the electrostatic confinement potential on the surface, while the photocurrent measurements reported so far have been performed only at a fixed confinement potential. The experimental observation of this sign reversal could provide a reliable estimation of the electron effective temperature in this new out of equilibrium state. Finally, we have also considered the effect of the temperature on the outcome of capacitive transport measurement techniques. These investigations led us to develop, numerical and analytical methods for solving the Poisson-Boltzmann equation in the limit of very low temperatures which could be useful for other systems.

Investigation on the feasibility of fusion in a compressed beam of ions subject to an electrostatic field

The paper reports a new electrostatic-confinement-based fusion approach, where a new non-equilibrium distribution function for an ion-beam compressed by an external electric force has been derived. This distribution function allows the system to possess appreciably low and insignificant thermal energy irrespective of the energy per particle. The spread of energy among particles in the non-equilibrium state is attributed to collisions in the presence of external force, whereas for equilibrium, the spreading of energy is due to the absence of force. The reactivity for a deuterium--deuterium fusion, using the proposed distribution function, has been computed. It is shown that for initiating fusion among the particles, the fusion time is comparable with the energy confinement time of ions for beam energy greater than 160 keV. The estimated energy gain factor Q (ratio of the output fusion power to the power consumed by the system) is around 12 for beam energy 170 keV and ion density 1015 cm−3. The energy loss due to particle scattering is estimated and is taken into consideration for the estimation of energy gain. An outline of a conceptual model of a device is proposed in accordance with the proposed theory and the device is not similar to the one used conventionally in Inertial Electrostatic Confinement systems based on collisions of a beam with a reflex beam or with background gas or plasma.

A prototype silicon double quantum dot with dispersive microwave readout

We present a unique design and fabrication process for a lateral, gate-confined double quantum dot in an accumulation mode metal-oxide-semiconductor (MOS) structure coupled to an integrated microwave resonator. All electrostatic gates for the double quantum dot are contained in a single metal layer, and use of the MOS structure allows for control of the location of the two-dimensional electron gas via the location of the accumulation gates. Numerical simulations of the electrostatic confinement potential are performed along with an estimate of the coupling of the double quantum dot to the microwave resonator. Prototype devices are fabricated and characterized by transport measurements of electron confinement and reflectometry measurements of the microwave resonator.

On the hydrogenated silicon carbide (SiCx:H) interlayer properties prompting adhesion of hydrogenated amorphous carbon (a-C:H) deposited on steel

This work reports a systematic study of physical–chemical properties of SiCx:H interlayers deposited by using tetramethlysilane on AISI 4140 at different temperatures (100 °C–550 °C) and its effects on the adhesion of a-C:H thin films. The bi-layers were obtained by pulsed-DC PECVD assisted by electrostatic confinement. The results show that the thickness of the SiCx:H interlayer exponentially decreases as the deposition temperature increases, i.e., a thermally activated kinetic process controls the global chemical reaction in the interlayer. There is a transition temperature (∼300 °C) for interlayer deposition where adhesion of a-C:H is reached. Above ∼300 °C, the a-C:H thin films show critical loads to wedge spallation from 298 (300 °C) to 478 mN (550 °C). At higher temperatures, H and Si contents decrease whereas C content increases in the interlayer. The improved adhesion is associated with the nature of chemical bonds formed in the interlayer.

Hydrogenated amorphous carbon thin films deposited by plasma-assisted chemical vapor deposition enhanced by electrostatic confinement: structure, properties, and modeling

Hydrogenated amorphous carbon (a-C:H) is a state-of-the-art material with established properties such as high mechanical resistance, low friction, and chemical inertness. In this work, a-C:H thin films were deposited by plasma-assisted chemical vapor deposition. The deposition process was enhanced by electrostatic confinement that leads to decrease the working pressure achieving relative high deposition rates. The a-C:H thin films were characterized by elastic recoil detection analysis, Rutherford backscattering spectroscopy, scanning electron microscopy, Raman spectroscopy, and nanoindentation measurements. The hydrogen content and hardness of a-C:H thin films vary from 30 to 45 at% and from 5 to 15 GPa, respectively. The hardness of a-C:H thin films shows a maximum as a function of the working pressure and is linearly increased with the shifting of the G-peak position and I D/I G ratio. The structure of a-C:H thin films suffers a clustering process at low working pressures. A physical model is proposed to estimate the mean ion energy of carbonaceous species arriving at the surface of a-C:H thin films as a function of processing parameters as pressure and voltage and by considering fundamentals scattering events between ion species and neutral molecules and atoms.

Density of states as a probe of electrostatic confinement in graphene

We theoretically analyze the possibility to confine electrons in single-layer graphene with the help of metallic gates, via the evaluation of the density of states of such a gate-defined quantum dot in the presence of a ring-shaped metallic contact. The possibility to electrostatically confine electrons in a gate-defined ``quantum dot'' with finite-carrier density, surrounded by an undoped graphene sheet, strongly depends on the integrability of the electron dynamics in the quantum dot. With the present calculations we can quantitatively compare confinement in dots with integrable and chaotic dynamics, and verify the prediction that the Berry phase associated with the pseudospin leads to partial confinement in situations where no confinement is expected according to the arguments relying on the classical dynamics only.

An accurate single-electron pump based on a highly tunable silicon quantum dot.

Nanoscale single-electron pumps can be used to generate accurate currents, and can potentially serve to realize a new standard of electrical current based on elementary charge. Here, we use a silicon-based quantum dot with tunable tunnel barriers as an accurate source of quantized current. The charge transfer accuracy of our pump can be dramatically enhanced by controlling the electrostatic confinement of the dot using purposely engineered gate electrodes. Improvements in the operational robustness, as well as suppression of nonadiabatic transitions that reduce pumping accuracy, are achieved via small adjustments of the gate voltages. We can produce an output current in excess of 80 pA with experimentally determined relative uncertainty below 50 parts per million.

Transition from non-monotonic to monotonic electrical diffuse layers: impact of confinement on ionic liquids

Intense investigations of room temperature ionic liquids have revealed not only their advantages in a wide range of technological applications but also triggered scientific debates about charge distribution properties within the bulk and near the solid-liquid interfaces. While many observations report on an alternating charge layering (i.e., spatially extended decaying charge density oscillations), there are recent conjectures that ionic liquids bear similarity to dilute electrolytes. Using a modified Poisson-Nernst-Planck model for ionic liquids (after Bazant et al., Phys. Rev. Lett. 2011, 106, 046102), we show that both behaviors are fundamental properties of ionic liquids. The transition from the non-monotonic (oscillatory) to the monotonic structure of electrical diffuse layers appears to non-trivially depend on ionic density in the bulk, electrostatic correlation length, confinement and surface properties. Consequently, the results not only reconcile the empirical results but also provide a powerful methodology to gain insights into the nonlinear aspects of concentrated electrolytes.

A theoretical study of Cu clusters in siliceous erionite

In terms of periodic density functional theory method, we present a theoretical study of the structure and electronic properties of different composites, obtained by the inclusion of distinct Cux (x=2,4,6,8) clusters within the C1 cage of siliceous erionite–zeolite. Results shows that for a periodic distribution of Cux clusters on zeolite, the hosting of those is trough electrostatic confinement which is governed by framework oxygen atoms located at particular equivalent sites. However, when the permanency of a cluster in a C1 cage is compromised, the cluster–cluster long range Coulomb interactions could rise to keep it inside. In addition, results also show that the electronic properties of free Cux clusters are largely transferred to the formed composite.

Doped semiconductor nanocrystal junctions

Semiconductor junctions are the basis of electronic and photovoltaic devices. Here, we investigate junctions formed from highly doped (ND≈1020−1021cm−3) silicon nanocrystals (NCs) in the 2–50 nm size range, using Kelvin probe force microscopy experiments with single charge sensitivity. We show that the charge transfer from doped NCs towards a two-dimensional layer experimentally follows a simple phenomenological law, corresponding to formation of an interface dipole linearly increasing with the NC diameter. This feature leads to analytically predictable junction properties down to quantum size regimes: NC depletion width independent of the NC size and varying as ND−1/3, and depleted charge linearly increasing with the NC diameter and varying as ND1/3. We thus establish a “nanocrystal counterpart” of conventional semiconductor planar junctions, here however valid in regimes of strong electrostatic and quantum confinements.

Scaling law of electron confinement in a zero beta polywell device

Orbital theory simulation was applied to an electron trap that uses a cube shaped magnetic cusp known as a Polywell device. The purpose of this device is to create a virtual cathode in order to achieve nuclear fusion using inertial electrostatic confinement. Analytical expressions of the electron confinement time and average position within the device were obtained in terms of the current in the field coils, the dimensions of the device, and the kinetic energy of the electrons. Comparisons with numerical simulations showed good agreement over a parameter range that spanned several orders of magnitude for the current. In addition, power loss from electrons exiting the trap was estimated in order to obtain minimum power requirement to maintain a virtual cathode within the device.

Non-Electric Applications of the Inertial Electrostatic Confinement Fusion Concept

Electrical energy is not the only commercial product that can be produced by the fusion of light elements. The reaction products from many fusion fuels can be used to provide products that can be of a near-term benefit to society well before practical fusion power plants are a reality. The use of fusion products (neutrons and protons) in Homeland Security applications to detect clandestine materials or the production of short half life Positron Emission Tomography isotopes for medical diagnostics of abnormalities (e.g. cancers) in the human body are but a few of the near term examples of the near term use of fusion energy. This paper shows how one of the many ways to promote fusion, namely the use of the Inertial Electrostatic Confinement concept, is uniquely suited to this task worldwide.

Recent Advances in IEC Physics and Technology at the University of Wisconsin

The University of Wisconsin-Madison has conducted research on gridded inertial electrostatic confinement (IEC) devices for the past 18 years. There are currently 4 experimental devices operating at voltages up to 180 kV and 60 mA. These devices have uncovered several new phenomena that have greatly improved our understanding of IEC devices. Recent advances include the discovery of a significant negative ion component of DD plasmas and spatial profiles of fusion reactions that did not conform to our prior understanding of these devices. The use of this technology has also contributed to our understanding of surface damage to high temperature in-vessel W components after even low exposures to energetic He ion fluences. Expansion of the voltage-ion current parameter space to 300 kV-200 mA in the near future will help our understanding of advanced fusion fuel cycles.

Spherical ion oscillations in a positive polarity gridded inertial-electrostatic confinement device

A pulsed, positive polarity gridded inertial electrostatic confinement device has been investigated experimentally, using a differential emissive probe and potential traces as primary diagnostics. Large amplitude oscillations in the plasma current and plasma potential were observed within a microsecond of the discharge onset, which are indicative of coherent ion oscillations about a temporarily confined excess of recirculating electron space charge. The magnitude of the depth of the potential well in the established virtual cathode was determined using a differential emissive Langmuir probe, which correlated well to the potential well inferred from the ion oscillation frequency for both hydrogen and argon experiments. It was found that the timescale for ion oscillation dispersion is strongly dependent on the neutral gas density, and weakly dependent on the peak anode voltage. The cessation of the oscillations was found to be due to charge exchange processes converting ions to high velocity neutrals, causing the abrupt de-coherence of the oscillations through an avalanche dispersion in phase space.

Preliminary Results of Experimental Studies from Low Pressure Inertial Electrostatic Confinement Device

In this study, Turkey’s first low pressure inertial electrostatic confinement (IEC) device, constructed at the Saraykoy Nuclear Research and Training Center (SNRTC-IEC), is introduced and the first results are reported. This device was designed for neutronic fusion studies in terms of D–D reaction. The SNRTC-IEC device consists of spherical chamber 300 mm in diameter and a grid-type spherical cathode in which high negative voltage is applied at the center of chamber. The outer surface of the device held at ground potential has 10 ports to connect the vacuum pump, high voltage load, residual gas analyzer, ion sources and other peripherals. Cathode voltage is 85 kV and it is particularly emphasized that the SNRTC-IEC device is studied at low pressure (1−10 × 10−4 mbar). The maximum total neutron production rate is measured at around 2.4 × 104 neutrons per second for the medium grid cathode.

Measuring time of flight of fusion products in an inertial electrostatic confinement fusion device for spatial profiling of fusion reactions

A new diagnostic has been developed that uses the time of flight (TOF) of the products from a nuclear fusion reaction to determine the location where the fusion reaction occurred. The TOF diagnostic uses charged particle detectors on opposing sides of the inertial electrostatic confinement (IEC) device that are coupled to high resolution timing electronics to measure the spatial profile of fusion reactions occurring between the two charged particle detectors. This diagnostic was constructed and tested by the University of Wisconsin-Madison Inertial Electrostatic Confinement Fusion Group in the IEC device, HOMER, which accelerates deuterium ions to fusion relevant energies in a high voltage (∼100 kV), spherically symmetric, electrostatic potential well [J. F. Santarius, G. L. Kulcinski, R. P. Ashley, D. R. Boris, B. B. Cipiti, S. K. Murali, G. R. Piefer, R. F. Radel, T. E. Radel, and A. L. Wehmeyer, Fusion Sci. Technol. 47, 1238 (2005)]. The TOF diagnostic detects the products of D(d,p)T reactions and determines where along a chord through the device the fusion event occurred. The diagnostic is also capable of using charged particle spectroscopy to determine the Doppler shift imparted to the fusion products by the center of mass energy of the fusion reactants. The TOF diagnostic is thus able to collect spatial profiles of the fusion reaction density along a chord through the device, coupled with the center of mass energy of the reactions occurring at each location. This provides levels of diagnostic detail never before achieved on an IEC device.