Emergence of transverse size in electric streamers

2021 ◽  
Author(s):  
Nikolai Lehtinen
Keyword(s):  
Accurate Determination ◽  
Practical Interest ◽  
Narrow Channel ◽  
Infinite Length ◽  
Uniform Field ◽  
X Rays ◽  
Transverse Size ◽  
Simpler Algorithm ◽  
Streamer Propagation ◽  
Streamer Length

<p>The accurate determination of parameters of electric streamer propagation in air, such as their velocity, transverse size (radius) and the maximum field at the tip, is extremely important, e.g., for the studies of further lightning development and acceleration of electrons at the tip, which may lead to generation of x-rays. Relations between these parameters produce a family of streamer-shaped solutions, while the radius remains undetermined. We hypothesize that all these solutions are, in fact, valid solutions of hydrodynamic equations, but the physical radius emerges when one solution is selected by the condition of being maximally unstable, i.e., having the highest velocity.</p><p>Direct verification of this hypothesis by hydrodynamic simulations is complicated by the fact that the streamer length is one of the background conditions which determine its parameters, and in a propagating streamer the length is constantly changing. To circumvent this, we simulate a `steady-state' streamer, such that its length is kept constant by synchronizing the motion of the electrode to which it is attached. We show that the predicted maximally-unstable selected solution does, in fact, emerge in the infinite time limit of the simulation. We note, however, that we were yet unable to test the first part of the hypothesis, i.e. to produce the non-selected solutions in the predicted family, as they are quickly replaced by the selected one.</p><p>We present the calculated streamer parameter dependence on external uniform field and streamer length for an isolated streamer and streamers propagating parallel to each other. In the latter case, the field of neighboring streamers makes the streamer propagation independent of its length when it exceeds the inter-streamer distance. We draw parallels of this situation to the selected solution for a viscous Saffman-Taylor finger of infinite length in a narrow channel [Luque et al, 2008, doi:10.1103/PhysRevE.78.016206].</p><p>The practical interest of this work lies in reducing the complexity of streamer propagation modeling, by avoiding detailed simulation of the streamer head, if we can calculate the parameters by a simpler algorithm.</p>

Radio emission from fast streamers

2020 ◽  
Author(s):  
Nikolai Lehtinen
Keyword(s):  
Electric Field ◽  
External Electric Field ◽  
Electron Attachment ◽  
Computational Method ◽  
Radio Emissions ◽  
Streamer Propagation ◽  
Streamer Length ◽  
Calculation Results ◽  
External Conditions ◽  
Lightning Initiation

<p>A new computational approach, based on treating an electric streamer as a nonlinear instability, allows to determine unambiguously its parameters, for a given streamer length and external electric field, which may be nonuniform. Among the determined parameters are the speed, current and conductivity inside the streamer. These parameters may vary over orders of magnitude, depending on external conditions.</p><p>We use these parameters to calculate the radio emissions which would be observed on the ground from fast discharges produced in lightning, in which streamer velocities approach a significant fraction of the speed of light. Fast discharges play an important role in lightning initiation and may be responsible for production of Terrestrial Gamma Flashes (TGF). They manifest themselves in ground-based radio observations as Narrow Bipolar Events (NBE), to which the calculation results are compared.</p><p>We will discuss conditions, the effect of which on streamer propagation (and therefore electromagnetic radiation) may be quantified with the used computational method. These include (i) the external electric field modification due to charges deposited by previous streamers; and (ii) electron attachment inside the streamer channel, which is strongly affected by cloud humidity.</p>

scholarly journals Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals

2020 ◽  
Author(s):  
Stefano Toso ◽  
Dmitry Baranov ◽  
Davide Altamura ◽  
Francesco Scattarella ◽  
Jakob Dahl ◽  
...  
Keyword(s):  
Structural Information ◽  
Accurate Determination ◽  
Diffraction Method ◽  
Atomic Displacement ◽  
Interparticle Spacing ◽  
X Rays ◽  
Structural Perfection ◽  
Fitting Algorithm ◽  
Atomic Displacement Parameters

Colloidal superlattices are fascinating materials made of ordered nanocrystals, yet they are rarely called “atomically precise.” That is unsurprising, given how challenging it is to quantify the degree of structural order in these materials. However, once that order crosses a certain threshold, constructive interference of X-rays diffracted by the nanocrystals dominates the diffraction pattern, offering a wealth of structural information. By treating nanocrystals as scattering sources forming a self-probing interferometer, we developed a multilayer diffraction method that enabled the accurate determination of nanocrystal size, interparticle spacing, and their fluctuations for samples of self-assembled CsPbBr<sub>3</sub> and PbS nanomaterials. The average nanocrystal displacement of 0.32-1.4 Å in the studied superlattices provides a figure of merit for their structural perfection and approaches the atomic displacement parameters found in traditional crystals. The method requires a laboratory-grade diffractometer and an open-source fitting algorithm for data analysis, providing a competitive alternative to resource-intensive synchrotron experiments.

Oxford HEXameter: Laboratory High Energy X-Ray Diffractometer for Bulk Residual Stress Analysis

2006 ◽  
Vol 524-525 ◽  
pp. 743-748 ◽  
Author(s):  
Alexander M. Korsunsky ◽  
Shu Yan Zhang ◽  
Daniele Dini ◽  
Willem J.J. Vorster ◽  
Jian Liu
Keyword(s):  
Accurate Determination ◽  
High Energy ◽  
Energy Dispersive ◽  
Detector Response ◽  
X Rays ◽  
X Ray Diffraction ◽  
X Ray ◽  
New Instrument ◽  
Synchrotron Beamlines

Diffraction of penetrating radiation such as neutrons or high energy X-rays provides a powerful non-destructive method for the evaluation of residual stresses in engineering components. In particular, strain scanning using synchrotron energy-dispersive X-ray diffraction has been shown to offer a fast and highly spatially resolving measurement technique. Synchrotron beamlines provide best available instruments in terms of flux and low beam divergence, and hence spatial and measurement resolution and data collection rate. However, despite the rapidly growing number of facilities becoming available in Europe and across the world, access to synchrotron beamlines for routine industrial and research use remains regulated, comparatively slow and expensive. A laboratory high energy X-ray diffractometer for bulk residual strain evaluation (HEXameter) has been developed and built at Oxford University. It uses a twin-detector setup first proposed by one of the authors in the energy dispersive X-ray diffraction mode and allows simultaneous determination of macroscopic and microscopic strains in two mutually orthogonal directions that lie approximately within the plane normal to the incident beam. A careful procedure for detector response calibration is used in order to facilitate accurate determination of lattice parameters by pattern refinement. The results of HEXameter measurements are compared with synchrotron X-ray data for several samples e.g. made from a titanium alloy and a particulate composite with an aluminium alloy matrix. Experimental results are found to be consistent with synchrotron measurements and strain resolution close to 2×10-4 is routinely achieved by the new instrument.

scholarly journals Diamond beamline I07: a beamline for surface and interface diffraction

2016 ◽  
Vol 23 (5) ◽  
pp. 1245-1253 ◽  
Author(s):  
Chris Nicklin ◽  
Tom Arnold ◽  
Jonathan Rawle ◽  
Adam Warne
Keyword(s):  
Photoelectron Spectroscopy ◽  
Accurate Determination ◽  
Ultrahigh Vacuum ◽  
Grazing Incidence ◽  
X Rays ◽  
Scanning Tunnelling Microscope ◽  
X Ray Diffraction ◽  
Double Crystal ◽  
X Ray ◽  
Wide Range

Beamline I07 at Diamond Light Source is dedicated to the study of the structure of surfaces and interfaces for a wide range of sample types, from soft matter to ultrahigh vacuum. The beamline operates in the energy range 8–30 keV and has two endstations. The first houses a 2+3 diffractometer, which acts as a versatile platform for grazing-incidence techniques including surface X-ray diffraction, grazing-incidence small- (and wide-) angle X-ray scattering, X-ray reflectivity and grazing-incidence X-ray diffraction. A method for deflecting the X-rays (a double-crystal deflector) has been designed and incorporated into this endstation, extending the surfaces that can be studied to include structures formed on liquid surfaces or at liquid–liquid interfaces. The second experimental hutch contains a similar diffractometer with a large environmental chamber mounted on it, dedicated toin situultrahigh-vacuum studies. It houses a range of complementary surface science equipment including a scanning tunnelling microscope, low-energy electron diffraction and X-ray photoelectron spectroscopy ensuring that correlations between the different techniques can be performed on the same sample, in the same chamber. This endstation allows accurate determination of well ordered structures, measurement of growth behaviour during molecular beam epitaxy and has also been used to measure coherent X-ray diffraction from nanoparticles during alloying.

Energy-Dispersive X-Ray Techniques for Accurate Heavy Element Assay

1986 ◽  
Vol 30 ◽  
pp. 285-292 ◽  
Author(s):  
H. Ottmar ◽  
H. Eberle ◽  
P. Matussek ◽  
I. Michel-Piper
Keyword(s):  
Absorption Edge ◽  
Heavy Element ◽  
Accurate Determination ◽  
Energy Dispersive ◽  
X Rays ◽  
Specific Absorption ◽  
X Ray ◽  
Xrf Analysis ◽  
Analysis Technique

Energy-dispersive X-ray techniques can be employed in two different ways for the accurate determination of element concentrations in specimens: (1) spectrometry of fluoresced characteristic X-rays as widely applied in the various modes of the traditional XRF analysis technique, and (2) spectrometry of the energy-differential transmittance of an X-ray continuum at the element-specific absorption-edge energies.

scholarly journals Discharge Enhancement Phenomenon and Streamer Control in Dielectric Barrier Discharge with Many Pores

Catalysts ◽  
2020 ◽  
Vol 10 (1) ◽  
pp. 68
Author(s):  
Jian-Guo Gu ◽  
Pan Zhao ◽  
Ya Zhang ◽  
Hong-Yu Wang ◽  
Wei Jiang
Keyword(s):  
Narrow Channel ◽  
Bottom Plate ◽  
Driving Voltage ◽  
Volume Discharge ◽  
Particle In Cell ◽  
Direction Control ◽  
Streamer Propagation ◽  
Surface Discharges ◽  
Left And Right ◽  
The Right

The surface and volume discharge enhancement phenomena and streamer propagation direction control in catalytic pores are significant for the plasma catalytic degradation of pollutants. In this work, we use a two-dimensional particle-in-cell with Monte Carlo collisions model to explore the effect of lateral voltage on streamer enhancement and streamer propagation control for different driving voltages in pores of various shapes, sizes, and numbers. The driving voltage is applied to the top of the device, while the lateral voltages are applied at the left and right sides of the device. The surface and volume discharge enhancement phenomena become more significant and streamer propagation is more restricted within a narrow channel as the lateral voltage (with the same values on the left and right sides) increases from −5 kV to −30 kV for a fixed driving voltage of −20 kV. In this case, both the volume and surface discharges are intensive, leading to highly concentrated plasma species in a narrow channel. Moreover, the streamer propagates in a straight direction, from top to the bottom plate, with the lateral voltage added on both sides. The streamer propagation, however, deviates from the center and is directed to the right side when the lateral voltage is applied to the left. Our calculations also indicate that increasing the number or size of the pores enhances both the volume and surface discharges.

scholarly journals X-ray study of phase boundaries in thermal diagrams of alloy systems—cu-zn system

1932 ◽  
Vol 137 (832) ◽  
pp. 397-417 ◽  
Keyword(s):  
Accurate Determination ◽  
Large Mass ◽  
Second Phase ◽  
X Rays ◽  
Phase Boundaries ◽  
Ray Method ◽  
X Ray ◽  
Method Of Measurement ◽  
Ray Methods ◽  
Alloy Systems

Within the last 10 years, the study of the diffraction of X-rays by crystal units in metals and alloys has shown that X-ray analysis, apart from giving data on the form and dimensions of the crystalline units, is a powerful method with which to examine the thermal diagrams of alloy systems. Rosenhain has pointed out, however, that although the X-ray analysis of alloys yields valuable results as to structure and so on, the constitutional diagrams arrived at by this means must necessarily be incomplete, firstly because a very small amount of a second phase in a large mass of another phase cannot be detected by X-ray methods, and secondly, because the parameter measurements have not been accurate enough to determine definitely the boundaries of solid solubility. In view of this statement we felt it would perhaps serve a useful purpose to carry out a careful investigation into the reliability of accurate X-ray methods for establishing thermal equilibrium diagrams of alloy systems. An account of such an investigation is given in this paper, and it may be stated at the outset that, where X-ray data could be compared with data reliably established by other methods, the agreement is as good as, if not better than, that between the data of the various methods amongst themselves. It will be shown that a phase boundary can be determined by the X-ray method from parameter measurements alone, and that an accurate determination can be made irrespective of the amount of the second phase present. It will be necessary to go into the method of measurement and the heat treatment of the alloys in some detail. Precautions have to be taken in the preparation and analysis of the alloys to ensure that the parameter measured is that of the alloy in the equilibrium state, and that its composition is that corresponding to the alloy actually used in the measurement. The paper is therefore divided into two parts; the first part deals with the method of measurement and the preparation of the alloys, and the second part refers to the X-ray examination of the alloys with a view to determining the phase boundaries.

Sign in / Sign up

Export Citation Format

Share Document