Space Potential Distribution Research Articles

Analysis of electron behaviour around a spring-shape filament inside a low-energy electron gun

Electron behaviour inside a low-energy electron gun based on a spring-shape filament were studied with experiment and Particle-In-Cell simulation. The energy range of the electron beam which we expected was from 1 to 20 eV. The analysis told that smaller size of filament is more useful to improve electron density in front of an extraction hole in the gun to enhance beam current. Relation between a space potential distribution in the gun and electron transport was also studied. A heater voltage to drive a filament for thermionic electron emissions has another role to form a spatial potential distribution in the gun. The potential guides electrons, which are at a distant area from the beam extraction hole, toward the hole. It can be a significant help to realize efficient electron extraction by a beam extraction electric field induced near the hole.

Accurate space potential measurements using an emissive probe in high pressure plasma and neutral gas

The accurate measurement of plasma potentials at high pressure is investigated with an emissive probe. The emissive probe I-V characteristic of argon plasma obtained at 100 Pa is interpreted in detail, showing two inflection points in the I-V trace. Accurate plasma potentials at high pressure can be determined from the first inflection point potential (the higher potential) by using the improved inflection point method of the emissive probe, while the second inflection point is a result of the additional ionizing phenomenon caused by the collision between the emitted electrons and the neutral argon atoms. Additionally, the accuracy of the plasma potential measurement at high pressure is verified to be within 0.3 V, confirmed by the measurement of space potential distribution between two parallel plates in neutral gas at 100 Pa. The accurate measurement of space potentials in high pressure plasma and neutral gas indicates that the improved inflection point method of the emissive probe approach is promising in the study of collisional sheath structures and electrostatic probe diagnostics of high pressure plasmas.

Measurement of Space Potential Distribution Around Overhead HVDC Transmission Lines Based on Potential Compensation on Suspended Conductor

An ion-flow field is very important for the design of an HVDC transmission line. Due to the space ions resulting from corona discharge, there are few practical methods to measure the space electric fields around HVDC transmission lines. In this paper, a compensation method is presented to measure the space potential in the ion-flow field. The space potential can be obtained by applying an adjustable voltage source to a suspended conductor, increasing the voltage step-by-step and measuring the current flowing through the voltage source or the ground-level ion current density under the suspended conductor. When the current is equal to zero or the ground-level ion current density is equal to that without the suspended conductor, the output voltage of the voltage source is the space potential to be measured. The paper first introduces the principle of the measurement method. Then, the feasibility of the method is testified on unipolar and bipolar reduced-scale models. Finally, the measured results are discussed and are compared with the results by the numerical method.

Recent advances in perovskite solar cells: Space potential and optoelectronic conversion mechanism

Perovskite solar cells, as a promising next-generation photovoltaic technology for large-scale application, have demonstrated the advantages of high absorption coefficient, tunable bandgap, considerable photoelectric conversion efficiency and low-cost fabrication. However, the photoelectric conversion process within the device is still not understood clearly. One of the major reasons is that it is difficult to directly observe the space potential inside the device and its effect on the photogenerated charge carriers. The direct measurement and analysis of the space potential inside the device and the clarification of the intrinsic relationship between the space potential and the charge carrier micro-process under illumination and different electric field conditions can reveal the photoelectric conversion mechanism in depth, and thus providing the scientific research basis for the further development. Kelvin probe force microscopy (KPFM), a testing technology that is non-contact, can detect the space potential distribution without any damage to the device, demonstrating the great potential to unveil the working mechanism of perovskite solar cells accurately. Such a characterization method can work under vacuum condition. The KPFM combines Kelvin method of measuring contact potential difference with the scan probe microscopy to characterize internal carrier dynamic behavior with high resolution on a nanometer scale. The study of the spatial potential distribution of semiconductor device plays an important role in understanding the working mechanism of new perovskite solar cells. For example, under an open-circuit condition, the intensity and width of the electric field and space charge region can be obtained from the spatial potential distribution, and the bending direction of the energy band can be judged according to the increase or decrease of the potential. While in a short-circuit case, the generation and transport of charge carriers can be obtained. In this review, we mainly introduce the research progress of the space potential distribution and optoelectronic conversion mechanism in perovskite solar cells. The key mechanism of charge carrier generation, separation, transport and recombination are revealed by using KPFM to directly observe the space potential variations caused by light or electric field. We also prospect the issues and challenges in the future research.

Self-Sustained Plasma-Beam Discharge at High Energy Density

This paper is related to the study of the formation conditions and dynamics of self-sustained plasma-beam discharge at high energy density in the plasma of multiply ionized atoms. The efficiency of energy input into the discharge is shown by using a high-current pulsed plasma diode of low pressure. Based on the original calculation method, the dynamics of active voltage and active power inputted into the discharge are given. In such a system, the level of inputted active power can reach the value of 100 MW at the initial stored energy of 140 J. It is pointed out that the main mechanism of energy input into the discharge is a double electric layer of the space charge that is formed in the discharge gap. The intense electron beam accelerates in the double layer and heats the plasma to a high temperature in a short period of time. By controlling the arrangement of double-layer formation, one can input the energy locally into a certain discharge region. The distinctive features of the double-layer formation near the high-voltage electrode with a limited working surface for the even and odd half periods of the discharge current are indicated. The distribution of space potential in the discharge gap confirms that the double-layer formation is the result of not only the current instability but also the discharge transition to a new form of the self-sustained electric discharge in gas environment.

Modeling of Lightning Exposure of Buildings and Massive Structures

A dynamic model for exposure of grounded objects to direct lightning strokes has been generalized to apply to buildings and massive structures. The model also applies to the mixed situation where a slender air terminal is installed on the top of a building to provide lightning protection. The basic elements of the model comprise a general criterion for continuous upward leader inception the validity of which is independent of the building or structure topology. This is followed by initiation of an upward leader trajectory which would either lead to a successful encounter with the negative downward leader through a final jump or to an aborted positive leader. Based on recent work published by the author, the model does not require any assumption regarding the ratio between the negative and positive leader speeds as has often been done previously. The modeling procedure has been facilitated through the use of dimensionless variables relating to the proximity effect of the building on space potential distribution, leader inception space potential, critical mean ambient ground field, and, finally, the attractive radius. A slender rod of the same height as the building is used as basis for comparison.

Propagation Behavior of Uranium Plasma from Electron Beam Metal Evaporator

When uranium vapor is generated with an electron beam evaporator, a uranium plasma is formed on the evaporating surface. This plasma rises and expands with the vapor. Propagation behavior of this plasma was investigated by measuring plasma parameters, drift energy of ions and vapor flux along the propagation path. Over the range of 20-50 cm from the evaporation surface, the plasma density decreased from 3 × 109 cm−3 to 3 × 108 cm−3, while the electron temperature had a constant value of 0.29 eV. When the space potential was lowered from 1.48 to 0.80 V, the plasma ions were accelerated to increase the drift energy from 1.50 to 2.14 eV. Validity of the Boltzmann electron distribution was checked by comparing the space potential distribution with the plasma density distribution, and also the floating potential distribution with the ion flux distribution. These results confirm that the ambipolar diffusion governs the plasma propagation behavior. The change in the plasma density during its propagation occurred...

The TMX heavy ion beam probe

A heavy ion beam probe has been used to measure the radial space potential distribution in the central cell of TMX. This was the first beam probe system to utilize computer control, CAMAC instrumentation, and fast time response for broadband fluctuation capabilities. The fast time response was obtained using off-line processing of the energy analyzer detector signals and wideband transimpedance amplifiers. The on-axis space potential was found to be 300-400 V, with /spl phi//sub e//T/sub ec//spl sim/8. The radial potential profile is parabolic when gas box fueling is used. The frequency of observed fluctuations was found to agree with the E/spl times/B plasma rotation frequency during the discharge. The measured Tl/sup ++/ secondary ion current level is consistent with calculations, given reasonable assumptions for beam attenuation. >

Space and time evolution of plasma potential in T-10 under variation of main gas influx

The HIBP method was used in the experiments on the T-10 tokamak to measure the space potential distribution as a function of gas feed. A high electric field was observed in the region where the ionization of neutrals takes place. The correlation of the potential with the gas flux dynamics and the MHD activity was also found.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electrical measurements in a 13.56 MHz radio-frequency discharge

In a planar radio-frequency (rf) plasma, the distribution of dc space potential between both electrodes and the dc surface potential of a substrate placed on a cathode electrode were measured using an electrostatic probe. The distribution of the space potential for a stainless steel (sus) cathode electrode greatly differs from that for a sus/quartz/sus cathode electrode. This is associated with a large voltage drop at the insulating quartz part. The surface potential of a semiconducting Si substrate, having a much smaller area than the electrode area, is quite different from a self-bias voltage appearing on the cathode electrode. It increased with the increase of substrate area, and then saturated at the same value as the self-bias voltage. The distribution of the substrate surface potential in a radial direction was also measured. The relationship between the surface potential of the substrates and the results of plasma processings are discussed using plasma deposition of amorphous carbon film. The deposition rate of the carbon film was exceedingly influenced by the surface potential of the substrate.

Energy distributions of oxygen and argon ions generated in a special glow-discharge source

The energy distributions of ions generated in a special glow-discharge source were measured by the suppressor grids method and calculated according to the Davis model. The suppressor grids method was used in order to eliminate the influence of ion beam divergence on the distribution shape. Space potential distribution, necessary in the Davis model, was determined with an electrostatic probe. The theoretical and experimental ion distributions obtained were very similar and closer than those of corresponding distributions described in a previous report [Z Wroński, Vacuum, 36, 329 (1986)], where distributions were measured by mass spectrometry.

Ion energy distributions in a special glow-discharge ion source

The ion energy distributions for a special glow-discharge source were obtained by two means: theoretical and experimental. The experimental energy distributions, determined by mass spectrometry, differ significantly from the theoretical energy distributions calculated according to the Davis model in which the charge transfer process plays an important part. Space potential distributions necessary in the Davis theory, were determined with an electrostatic probe. The differences between the distributions, calculated and measured, can be caused by some inaccuracy of the ‘simple’ Davis model.

Space potential distribution in the ISX-B tokamak.

Plasma potentials have been measured for the first time in neutral-beam-heated tokamak discharges, with a heavy-ion beam probe. Radial profiles of these potentials are presented for coinjection, counterinjection, and balanced injection, and for plasmas having only Ohmic heating. They are found to be very dependent on the direction of rotation with respect to the plasma current. They are also found qualitatively consistent with those inferred from radial momentum balance with use of the measured toroidal rotation velocity.

Comparison of the radial potential profile measured in a tokamak to predictions of stochastic magnetic field theory.

A space potential distribution which is positive at all radii has been measured in the research tokamak RENTOR. This is in sharp contrast to negative potential wells observed in other devices. The positive potential distribution is in qualitative agreement with the predictions of stochastic magnetic field theory.

Measurement of Space Potential Distribution in Hot Plasma by Fast Neutral Beam Method

Measurement of Space Potential Distribution in Hot Plasma by Fast Neutral Beam Method

Direct Measurement of Plasma Space Potential on Elmo Bumpy Torus

A heavy-ion--beam probe has been used to measure the space-potential distribution on the Elmo bumpy torus. The results indicate that under optimum confinement conditions the ambipolar electric field is nearly axisymmetric, points radially inward, and is of sufficient magnitude to dominate the ion poloidal drifts.

Comments on the Eichenbaum-Hernqvist Model of a Low-Pressure Diode

Using a special model of a collisionless plasma diode, Eichenbaum and Hernqvist have shown the interesting circumstance that two different interelectrode space-potential distributions may exist for the same boundary conditions and the same output voltage and current. It is shown here, using the same boundary conditions, that there are at least several additional and probably an infinite number of possible solutions.

Theory and Probe Measurements in a Magnetic Ion Source

The ``superstate'' in a magnetic ion source has been investigated experimentally and theoretically. The space-charge density is approximately constant throughout the source. The radial plasma density does not differ significantly from a Gaussian distribution. The radial distribution of space potential is approximately parabolic. The theoretical results have been tested by probe measurements, and good agreement has been found.

Space-Charge Effects in Electron Beams

The effects of space charge in long, magnetically focused electron beams directed parallel to positively charged sheath electrodes are determined from a simple analysis. The main effects of space charge are: (a) to introduce departure from the potential distribution of the electrostatic case; (b) to set an upper limit for the beam current; and (c) to introduce instabilities and hysteresis phenomena in the behavior of the tube. It is shown that in a long beam the longitudinal potential gradient is negligible throughout the major portion of the beam length so that the upper limit for beam current is independent of beam length and of potentials of end electrodes, but depends only upon the relative transverse dimensions of the beam and of the surrounding sheath electrode and upon the sheath-electrode potential. The analysis gives expressions for maximum beam current, for space-potential distribution and for the minimum value of the focusing magnetic field, in terms of the beam and sheath-electrode dimensions and the sheath-electrode potential. The use of multicellular sheath electrodes to minimize the effects of space charge is discussed. Thin "sheet" beams and "thin-walled" tubular beams are treated separately by a simplified "capacitance" method and expressions for maximum current, space potential, and for minimum-focusing magnetic field are derived. The effect of space-charge interaction between multiple beams inside a common sheath electrode is considered and a method of control of current in one beam by the current in the adjacent beam is analyzed and also illustrated by experimental data.