Average Expansion Velocity Research Articles

Effects of anode material on the evolution of anode plasma and characteristics of intense electron beam diode

In this paper, three kinds of materials including graphite, titanium (Ti) and molybdenum (Mo) are used as anodes to figure out the influence factors of anode material on the characteristics of the intense electron beam diode. The results show that the characteristics of diode are mainly determined by the cathode plasma motion under a 15 mm diode gap, in which the typical electron beam parameters are 280 kV, 3.5 kA. When the diode gap is reduced to 5 mm, the voltage of the electron beam reduces to about 200 kV, and its current increases to more than 8.2 kA. It is calculated that the surface temperatures of Ti and Mo anodes are higher than their melting points. The diode plasma luminescence images show that Ti and Mo anodes produce plasmas soon after the bombardment of electron beams. Ti and Mo lines are respectively found in the plasma composition of Ti and Mo anode diodes. Surface melting traces are also observed on Ti and Mo anodes by comparing the micromorphologies before and after bombardment of the electron beam. These results suggest that the time of anode plasma generation is closely related to the anode material. Compared with graphite, metal Ti and Mo anodes are more likely to produce large amounts of plasma due to their more significant temperature rise effect. According to the moment that anode plasma begins to generate, the average expansion velocities of cathode and anode plasma are estimated by fitting the improved space-charge limited flow model. This reveals that generation and motion of the anode plasma significantly affect the characteristics of intense electron beam diode.

Effect of heterogeneity size on three-phase contact line formation kinetics

Flotation relies on the differences in surface wettability. Although solids with the same hydrophobicity are generally regarded as having the same capture probability, the heterogeneities found on particle surfaces could complicate bubble–solid interactions. For surfaces with the same hydrophobicity and different sizes of inclusions, the heterogeneity size is expected to affect bubble–solid interactions. In this work, Microelectromechanical system techniques were used to fabricate chemically heterogeneous samples with the same hydrophobicity on which discrete hydrophilic dots with different diameters were distributed. The collision and adhesion processes between free-rising bubbles and these chemically heterogeneous surfaces were studied using a high-speed video camera. For all surfaces, the time intervals between successive collisions and the bubble trajectories were similar before bubble rupture. In contrast, the time elapsed between maximum air bubble deformation and rupture varied, with the rupture time appearing random. Nevertheless, the terminal diameter and average expansion velocity after bubble rupture were negatively correlated with the hydrophilic dot diameter. Thus, for heterogeneous surface with discrete hydrophilic dots, the bouncing behaviour of air bubbles before rupture is independent of heterogeneity size, whereas the three-phase contact line expansion behaviour after rupture is related to heterogeneity size. These insights can provide guiding principles for improving the flotation.

Towards a multitracer timeline of star formation in the LMC – II. The formation and destruction of molecular clouds

ABSTRACT The time-scales associated with various stages of the star formation process represent major unknowns in our understanding of galactic evolution, as well as of star and planet formation. This is the second paper in a series aiming to establish a multitracer timeline of star formation in the Large Magellanic Cloud (LMC), focusing on the life cycle of molecular clouds. We use a statistical method to determine a molecular cloud lifetime in the LMC of $t_{\text{CO}}=11.8^{+2.7}_{-2.2}$ Myr. This short time-scale is similar to the cloud dynamical time, and suggests that molecular clouds in the LMC are largely decoupled from the effects of galactic dynamics and have lifetimes set by internal processes. This provides a clear contrast to atomic clouds in the LMC, of which the lifetimes are correlated with galactic dynamical time-scales. We additionally derive the time-scale for which molecular clouds and H ii regions co-exist as $t_{\text{fb}}=1.2^{+0.3}_{-0.2}$ Myr, implying an average feedback front expansion velocity of 12 km s−1, consistent with expansion velocities of H ii regions in the LMC observed directly using optical spectroscopy. Taken together, these results imply that the molecular cloud life cycle in the LMC proceeds rapidly and is regulated by internal dynamics and stellar feedback. We conclude by discussing our measurements in the context of previous work in the literature, which reported considerably longer lifetimes for molecular clouds in the LMC, and find that these previous findings resulted from a subjective choice in timeline calibration that is avoided by our statistical methodology.

Multiepoch Optical Images of IRC+10216 Tell about the Central Star and the Adjacent Environment

Abstract Six images of IRC+10216 taken by the Hubble Space Telescope at three epochs in 2001, 2011, and 2016 are compared in the rest frame of the central carbon star. An accurate astrometry has been achieved with the help of Gaia Data Release 2. The positions of the carbon star in the individual epochs are determined using its known proper motion, defining the rest frame of the star. In 2016, a local brightness peak with compact and red nature is detected at the stellar position. A comparison of the color maps between 2016 and 2011 epochs reveals that the reddest spot moved along with the star, suggesting a possibility of it being the dusty material surrounding the carbon star. The relatively red, ambient region is distributed in an Ω shape and corresponds well to the dusty disk previously suggested based on near-infrared polarization observations. In a larger scale, differential proper motion of multiple ring-like patterns in the rest frame of the star is used to derive the average expansion velocity of transverse wind components, resulting in ∼12.5 km s−1(d/123 pc), where d is the distance to IRC+10216. Three-dimensional geometry is implied from its comparison with the line-of-sight wind velocity determined from half widths of submillimeter emission line profiles of abundant molecules. Uneven temporal variations in brightness for different searchlight beams and anisotropic distribution of the extended halo are revisited in the context of the stellar light illumination through a porous envelope with postulated longer-term variations for a period of ≲10 yr.

Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells

Aims.We study the growth of dust in oxygen-rich stellar outflows in order to find out to which extent dust growth models can quantitatively reconcile with the quantities and nature of dust as derived from observations of the infrared emission from circumstellar dust shells.Methods.We use a set of nine well-observed massive supergiants with optically thin dust shells as testbeds because of the relatively simple properties of the outflows from massive supergiants, contrary to the case of AGB stars. Models of the infrared emission from their circumstellar dust shells are compared to their observed infrared spectra to derive the essential parameters that rule dust formation in the extended envelope of these stars. The results are compared with a model for silicate dust condensation.Results.For all objects, the infrared emission in the studied wavelength range, between 6 and 25μm, can be reproduced rather well by a mixture of non-stoichiometric iron-bearing silicates, alumina, and metallic iron dust particles. For three objects (μCep, RW Cyg, and RS Per), the observed spectra can be sufficiently well reproduced by a stationary and (essentially) spherically symmetric outflow in the instantaneous condensation approximation. For these objects, the temperature at the onset of massive silicate dust growth is of the order of 920 K and the corresponding outflow velocity of the order of the sound velocity. This condensation temperature is only somewhat below the vapourisation temperature of the silicate dust and suggests that the silicate dust grows on the corundum dust grains that formed well inside of the silicate dust shell at a much higher temperature. The low expansion velocity at the inner edge of the silicate dust shell further suggests that, for these supergiants, the region inside the silicate dust shell has an only subsonic average expansion velocity, though a high degree of supersonic turbulence is indicated by the widths of spectral lines.Conclusions.Our results suggest that for the two major problems of dust formation in stellar outflows, that is (i) formation of seed nuclei and (ii) their growth to macroscopic dust grains, we are gradually coming close to a quantitative understanding of the second item.

The effect of inter-pulse delay on the spectral emission and expansion dynamics of plasma in dual-pulse fiber-optic laser-induced breakdown spectroscopy

The role of inter-pulse delay on plasma dynamics and spectral emission in dual-pulse fiber-delivery plasma has been investigated using fast imaging, optical emission spectroscopy, and laser shadowgraphy. The detection on the return spectrum in dual-pulse fiber-optic laser-induced breakdown spectroscopy showed that the self-reversal and self-absorption were reduced as the inter-pulse delay increased from 50 to 1000 ns. Using scanning electron microscopy, the ablation depth showed a changing trend of increasing first and then decreasing, and a maximum of ∼2.8 μm was achieved at 250-ns inter-pulse delay. Experimental results confirmed that the improvement was due to the reduction of the plasma thickness from 1.161 mm to 0.964 mm, and the calculation of electronic excitation temperature along the photon collection path showed that the excited atom densities became more spread, which both contributed to the self-absorption reduction. At a long inter-pulse delay, the plasma trailing edge would gradually separate from the target surface with a little further expansion distance of the leading edge, resulting in the reduction of plasma thickness after a time delay of several hundred nanoseconds. The heating of the peripheral cold particles by the second-generation plasma in the later expansion led to the rapid reduction of the peak temperature. Also, the early expansion trajectory of the second-generation plasma was tracked by laser shadowgraphy, which started to appear at an inter-pulse delay of 100 ns, and the average expansion velocity reached its maximum of ∼3.8 km/s.

Three-dimensional Kinematic Reconstruction of the Optically Emitting, High-velocity, Oxygen-rich Ejecta of Supernova Remnant N132D

Abstract We present a three-dimensional kinematic reconstruction of the optically emitting, oxygen-rich ejecta of supernova remnant N132D in the Large Magellanic Cloud (LMC). Data were obtained with the 6.5 m Magellan telescope in combination with the IMACS+GISMO instrument and survey [O iii] λλ4959, 5007 line emission in a ∼3′ × 3′ region centered on N132D. The spatial and spectral resolutions of our data enable detailed examination of the optical ejecta structure. The majority of N132D’s optically bright oxygen ejecta are arranged in a torus-like geometry tilted approximately 28° with respect to the plane of the sky. The torus has a radius of 4.4 pc (D LMC/50 kpc), exhibits a blueshifted radial velocity asymmetry of −3000 to +2300 km s−1, and has a conspicuous break in its circumference. Assuming homologous expansion from the geometric center of O-rich filaments, the average expansion velocity of 1745 km s−1 translates to an age since explosion of 2450 ± 195 yr. A faint, spatially separated “runaway knot” (RK) with total space velocity of 3650 km s−1 is nearly perpendicular to the torus plane and coincident with X-ray emission that is substantially enhanced in Si relative to the LMC and N132D’s bulk ejecta. These kinematic and chemical signatures suggest that the RK may have had its origin deep within the progenitor star. Overall, the main-shell morphology and high-velocity, Si-enriched components of N132D have remarkable similarity with those of Cassiopeia A, which was the result of a Type IIb supernova explosion. Our results underscore the need for further observations and simulations that can robustly reconcile whether the observed morphology is dominated by explosion dynamics or shaped by interaction with the environment.

Average transverse expansion velocities and global freeze-out temperatures in central Cu + Cu, Au + Au, and Pb + Pb collisions at high energies at RHIC and LHC

The experimental invariant transverse momentum [Formula: see text] spectra of the charged pions and kaons, protons and antiprotons, produced at midrapidity in central (0–10%) Au[Formula: see text]Au collisions at [Formula: see text], central (0–10%) Cu[Formula: see text]Cu collisions at [Formula: see text], central (0–10%) Au[Formula: see text]Au collisions at [Formula: see text], and central (0–5%) Pb[Formula: see text]Pb collisions at [Formula: see text], measured by BRAHMS, STAR and ALICE collaborations, were analyzed using three different transverse expansion (blast-wave) models: Siemens–Rasmussen blast-wave model, Simple transverse flow model, and Simplified (hydro-inspired) blast-wave model of Schnedermann et al. Combined (simultaneous) minimum [Formula: see text] fits of the experimental invariant [Formula: see text] spectra of the charged pions and kaons, protons and antiprotons with the above three model functions were conducted, using the identical selected optimal fitting ranges in [Formula: see text] in each studied collision system, and the values of the average transverse expansion velocity [Formula: see text] and global kinetic freeze-out temperature [Formula: see text] and their dependencies on the collision system [Formula: see text] and [Formula: see text] were extracted. The combined (simultaneous) fits using Hagedorn formula with the (embedded) simple transverse flow describe well the experimental invariant [Formula: see text] spectra of the charged pions, kaons, protons and antiprotons in the whole measured range in region [Formula: see text] in the analyzed central heavy ion collisions at RHIC and LHC, reproducing qualitatively well all the established dependencies of the parameters [Formula: see text] and [Formula: see text] on the collision system [Formula: see text] and [Formula: see text]. The obtained results were compared with those of the previous analyses of high energy heavy ion collisions.

Comparative visualized investigation of impact-driven high-speed liquid jets injected in submerged water and in ambient air

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.

Kinetic freeze-out in central heavy-ion collisions between 7.7 and 2760 GeV per nucleon pair

We fit the single-particle pt spectra of identified pions, kaons, and (anti)protons from central collisions of gold or lead nuclei at energies between 7.7 and 2760 GeV per nucleon pair. The blast wave model with included resonance production and with an assumption of partial chemical equilibrium is used and the fits are performed with the help of a Gaussian emulator process. A kinetic freeze-out temperature is found about 100 MeV for the lowest collision energies and 80 MeV at the LHC. The average transverse expansion velocity grows with increasing from 0.45 to 0.65. Due to partial chemical equilibrium, the influence of resonance decays on the shape of the pt spectra for above 27 GeV is small.

Effect of ambient pressure on spatial structures of electrical exploding aluminum wires in argon gas

The experiments demonstrated a strong dependence of the spatial structures of exploding aluminum wires on ambient pressures. Depending on whether having a current pause, electrical explosion of wires processes was classified into two types and analyzed by stage division. Through shadow, schlieren, and interference images, the spatial structures of exploding wires in two breakdown modes (internal and shunting) were observed. It was indicated that the basic difference is the relative positions of the wire core and the plasma channel. The breakdown mode makes a transition from the shunting mode to the internal mode with increasing pressures. Radial positions of the wire core boundary and the shockwave front were determined by comparing shadowgraphs and multi-frame photographs at different moments. The results demonstrated a gradual separation of the wire core and the shockwave, as well as their parabolic expansion trajectories. Furthermore, the average expansion velocity of wire cores decreases from ∼1.41 km/s to ∼0.78 km/s with the pressure varying from 50 kPa to 400 kPa.

AT 2018cow VLBI: no long-lived relativistic outflow

ABSTRACT We report on Very Long Baseline Interferometry (VLBI) observations of the fast and blue optical transient (FBOT), AT 2018cow. At ∼62 Mpc, AT 2018cow is the first relatively nearby FBOT. The nature of AT 2018cow is not clear, although various hypotheses from a tidal disruption event to different kinds of supernovae have been suggested. It had a very fast rise time (3.5 d) and an almost featureless blue spectrum, although high photospheric velocities (40 000 km s−1) were suggested early on. The X-ray luminosity was very high, ∼1.4 × 1043 erg s−1, larger than those of ordinary supernovae (SNe), and more consistent with those of SNe associated with gamma-ray bursts. Variable hard X-ray emission hints at a long-lived ‘central engine.’ It was also fairly radio luminous, with a peak 8.4-GHz spectral luminosity of ∼4 × 1028 erg s−1 Hz−1, allowing us to make VLBI observations at ages between 22 and 287 d. We do not resolve AT 2018cow. Assuming a circularly symmetric source, our observations constrain the average apparent expansion velocity to be ${\lt}0.49\, c$ by t = 98 d (3σ limit). We also constrain the proper motion of AT 2018cow to be ${\lt}0.51\, c$. Since the radio emission generally traces the fastest ejecta, our observations make the presence of a long-lived relativistic jet with a lifetime of more than 1 month very unlikely.

Resolving the Decades-long Transient FIRST J141918.9+394036: An Orphan Long Gamma-Ray Burst or a Young Magnetar Nebula?

Abstract Ofek identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as a radio source sharing similar properties and host galaxy type to the compact, persistent radio source associated with the first known repeating fast radio burst, FRB 121102. Law et al. showed that FIRST J1419+3940 is a transient source decaying in brightness over the last few decades. One possible interpretation is that FIRST J1419+3940 is a nearby analog to FRB 121102 and that the radio emission represents a young magnetar nebula (as several scenarios assume for FRB 121102). Another interpretation is that FIRST J1419+3940 is the afterglow of an “orphan” long gamma-ray burst (GRB). The environment is similar to where most such events are produced. To distinguish between these hypotheses, we conducted very long baseline interferometric (VLBI) radio observations using the European VLBI Network (EVN) at 1.6 GHz to spatially resolve the emission and to search for millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact radio source with a flux density of 620 ± 20 μJy (on 2018 September 18) and a source size of 3.9 ± 0.7 mas (i.e., 1.6 ± 0.3 pc given the angular diameter distance of 83 Mpc). These results confirm that the radio emission is nonthermal and imply an average expansion velocity of (0.10 ± 0.02)c. Contemporaneous high-time-resolution observations using the 100 m Effelsberg telescope detected no millisecond-duration bursts of astrophysical origin. The source properties and lack of short-duration bursts are consistent with a GRB jet expansion, whereas they disfavor a magnetar birth nebula.

SN 2014C: VLBI images of a supernova interacting with a circumstellar shell

We report on VLBI measurements of supernova 2014C at several epochs between $t = 384$ and 1057 days after the explosion. SN 2014C was an unusual supernova that initially had Type Ib optical spectrum, but after $t = 130$ d it developed a Type IIn spectrum with prominent H$\alpha$ lines, suggesting the onset of strong circumstellar interaction. Our first VLBI observation was at $t = 384$ d, and we find that the outer radius of SN 2014C was $(6.40 \pm 0.26) \times 10^{16}$ cm (for a distance of 15.1 Mpc), implying an average expansion velocity of $19300 \pm 790$ \kms\ up to that time. At our last epoch, SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side. The outer radius of the radio emission at $t = 1057$ d is $(14.9 \pm 0.6) \times 10^{16}$ cm. We find that the expansion between $t = 384$ and 1057 d is well described by a constant velocity expansion with $v = 13600 \pm 650$ \kms. SN 2014C had clearly been substantially decelerated by $t = 384$ d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year, as suggested by the observations at other wavelengths, but had progressed through the dense shell by the time of the VLBI observations.

Kinematics of OB-associations in Gaia epoch

We use stellar proper motions from the TGAS catalog to study the kinematics of OB-associations. The TGAS proper motions of OB-associations generally agree well with the Hipparcos proper motions. The parameters of the Galactic rotation curve obtained with TGAS and Hipparcos proper motions agree within the errors. The average one-dimensional velocity dispersion inside 18 OB-associations with more than 10 TGAS stars is sigma_v=3.9 km s-1, which is considerably smaller, by a factor of 0.4, than the velocity dispersions derived from Hipparcos data. The effective contribution from orbital motions of binary OB-stars into the velocity dispersion sigma_v inside OB-associations is sigma_b=1.2 km s-1. The median virial and stellar masses of OB-associations are equal to 7.1 10^5 and 9.0 10^3 Ms, respectively. Thus OB-associations must be unbound objects provided they do not include a lot of dense gas. The median star-formation efficiency is epsilon=2.1 percent. Nearly one third of stars of OB-associations must lie outside their tidal radius. We found that the Per OB1 and Car OB1 associations are expanding with the expansion started in a small region of 11--27 pc 7--10 Myr ago. The average expansion velocity is 6.3 km s-1.

SN 1978K: An evolved supernova outside our Local Group detected at millimetre wavelengths

Supernova 1978K is one of the oldest-known examples of the class of Type IIn supernovae that show evidence for strong interaction between the blast wave and a dense, pre-existing circumstellar medium. Here we report detections of SN 1978K at both 34 GHz and 94 GHz, making it only the third extragalactic supernova (after SN 1987A and SN 1996cr) to be detected at late-times at these frequencies. We find SN 1978K to be >400 times more luminous than SN 1987A at millimetre wavelengths in spite of the roughly nine year difference in ages, highlighting the risk in adopting SN 1987A as a template for the evolution of core-collapse supernovae in general. Additionally, from new VLBI observations at 8.4 GHz, we measure a deconvolved diameter for SN 1978K of ~5 milli-arcsec, and a corresponding average expansion velocity of <1500 km/s. These observations provide independent evidence of an extremely dense circumstellar medium surrounding the progenitor star.

New radio observations of the Type IIn Supernova 1978K

AbstractSN 1978K is the oldest-known Type IIn supernova, and one of the closest. We report new radio observations at high frequency and spatial resolution. SN 1978K has been detected at 34 and 94 GHz with the Australia Telescope Compact Array, while Very Long Baseline Interferometry at 8.4 GHz has allowed us to derive the past average expansion velocity, which indicates significant deceleration as the blast wave interacts with the dense circumstellar medium.

Radio evolution of supernova SN 2008iz in M 82

We report on multi-frequency Very Large Array (VLA) and Very Long Baseline Interferometry (VLBI) radio observations for a monitoring campaign of supernova SN$\,$2008iz in the nearby irregular galaxy M$\,$82. We fit two models to the data, a simple time power-law, S$\propto$ t$^\beta$, and a simplified Weiler model, yielding decline indices, $\beta$ of -1.22$\pm$0.07 (days 100-1500) and -1.41$\pm$0.02 (days 76-2167), respectively. The late-time radio light curve evolution shows flux-density flares at $\sim\,$970 and $\sim\,$1400 days which are a factor of $\sim\,$2 and $\sim\,$4 higher than the expected flux, respectively. The later flare, besides being brighter, does not show signs of decline at least from results examined so far (2014 January 23, day 2167). We derive the spectral index, $\alpha$, S$\propto$ $\nu^\alpha$ for frequencies 1.4 to 43$\,$GHz for SN$\,$2008iz during the period from $\sim\,$430 to 2167 days after the supernova explosion. The value of $\alpha$ shows no signs of evolution and remains steep $\approx$$-$1 throughout the period, unlike that of SN$\,$1993J which started flattening at $\sim\,$day 970. From the 4.8 and 8.4$\,$GHz VLBI images, the supernova expansion is seen to start with shell like structure that gets more and more asymmetric, then breaks up in the later epochs with bright structures dominating the southern part of the ring. This structural evolution differs significantly from SN$\,$1993J, which remains circularly symmetric over 4000 days after the explosion. The VLBI 4.8 and 8.4$\,$GHz images are used to derive a deceleration index, $m$, for SN$\,$2008iz, of 0.86$\,\pm\,$0.02, and the average expansion velocity between days 73 and 1400 as (12.1$\pm$0.2)$\times$10$^{3}\,$km$\,$s$^{-1}$.

Imaging the expanding shell of SN 2011dh

We report on third epoch VLBI observations of the radio-bright supernova SN 2011dh located in the nearby (7.8 Mpc) galaxy M51. The observations took place at $t=453$ d after the explosion and at a frequency of 8.4 GHz. We obtained a fairly well resolved image of the shell of SN 2011dh, making it one of only six recent supernovae for which resolved images of the ejecta are available. SN 2011dh has a relatively clear shell morphology, being almost circular in outline, although there may be some asymmetry in brightness around the ridge. By fitting a spherical shell model directly to the visibility measurements we determine the angular radius of SN 2011dh's radio emission to be $636 \pm 29$ $\mu$as. At a distance of 7.8 Mpc, this angular radius corresponds to a linear radius of $(7.4 \pm 0.3) \times 10^{16}$ cm and an average expansion velocity since the explosion of $19000^{+2800}_{-2400}$ kms$^{-1}$. We combine our VLBI measurements of SN 2011dh's radius with values determined from the radio spectral energy distribution under the assumption of a synchrotron-self-absorbed spectrum, and find all the radii are consistent with a power-law evolution, with $R \sim t^{0.97\pm0.01}$, implying almost free expansion over the period $t=4$ d to 453 d.

Experimental study on thermal characteristics of positive leader discharges using Mach-Zehnder interferometry

Leader discharge is one of the main phases in long air gap breakdown, which is characterized by high temperature and high conductivity. It is of great importance to determine thermal characteristics of leader discharges. In this paper, a long-optical-path Mach-Zehnder interferometer was set up to measure the thermal parameters (thermal diameter, gas density, and gas temperature) of positive leader discharges in atmospheric air. IEC standard positive switching impulse voltages were applied to a near-one-meter point-plane air gap. Filamentary channels with high gas temperature and low density corresponding to leader discharges were observed as significant distortions in the interference fringe images. Typical diameters of the entire heated channel range from 1.5 mm to 3.5 mm with an average expansion velocity of 6.7 m/s. In contrast, typical diameters of the intensely heated region with a sharp gas density reduction range from 0.4 mm to 1.1 mm, about one third of the entire heated channel. The radial distribution of the gas density is calculated from the fringe displacements by performing an Abel inverse transform. The typical calculated gas density reduction in the center of a propagating leader channel is 80% to 90%, corresponding to a gas temperature of 1500 K to 3000 K based on the ideal gas law. Leaders tend to terminate if the central temperature is below 1500 K.