Onset Of Ablation Research Articles

Rapid Ablation for Atrial Flutter by Targeting Maximum Voltage—Factors Associated with Short Ablation Times

The maximum voltage-guided (MVG) approach to ablation for atrial flutter targets high-amplitude signals along the cavotricuspid isthmus (CTI). It is based on the observation that the isthmus is often composed of bundles of conducting tissue and the hypothesis that these bundles manifest as high-amplitude electrograms, providing targets for selective ablation. We aim to identify patient and procedural factors that correlate with rapid isthmus ablation. All patients undergoing CTI ablation at our center from January 2005 to May 2006 were included. Patients were divided into outcome groups relative to the median value for total ablation time. The two groups were compared according to patient and procedural variables, using multivariate regression methods. Seventy-six patients were assessed with mean age 60.2 +/- 10.6 years; 63 (82.9%) were male. Mean ablation time to bidirectional block across the CTI was 6.85 +/- 5.87 min (range 0.68-28.7); median 4.77 min. Seventy-six percent of patients required less than 5 min total ablation time until bidirectional block was achieved. Variables independently associated with a short ablation time were the presence of sinus rhythm at start of ablation (P = 0.0050, odds ratio (OR) 8.03), high mean temperature among all ablations (P = 0.019, OR 17.81), and low variability of mean power among all ablations (P = 0.0048, OR 19.26). Using the MVG approach to atrial flutter ablation, shorter total ablation times are observed among patients in sinus rhythm at the onset of ablation, with higher mean temperature among ablation lesions, and less variability of power between ablations.

Computational investigation into the mechanisms of UV ablation of poly(methyl methacrylate)

Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study UV ablation of poly(methyl methacrylate) (PMMA) at 157 nm. We discuss the onset of ablation, the formation and distribution of products in the plume and stress relaxation of the polymer matrix. Laser induced heating and bond-breaks are considered as ablation pathways. We show here that depending on the nature of energy deposition the evolution of ablation plume and yield composition can be quite different. If all of photon energy is converted to heat it can set off ablation via mechanical failure of the material in the heated region. Alternatively, if the photon energy goes towards breaking bonds first, it initiates chemical reactions, polymer unzipping and formation of gaseous products inside the substrate. The ejection of these molecules has a hollowing out effect on the substrate which can lead to ejection of larger chunks. No excessive pressure buildup due to creation of gaseous molecules or entrainment of larger polymer chunks is observed in this case.

Trends in snow ablation over North America

AbstractA substantial decrease in snow cover extent (SCE) and snow depth over North America has been observed over the 1960–2000 period. One explanation for the changes in North American snow cover is a change in the frequency and/or intensity of snow ablation. This study uses a gridded dataset of United States and Canadian surface observations from 1960 to 2000 to examine patterns of snow ablation over North America. An ablation event is defined as an interdiurnal snow depth change exceeding a critical value. Results show a significant positive trend in the frequency of ablation events during March (p < 0.05) and a significant negative trend in May (p < 0.05), indicating an earlier onset of ablation. This pattern is consistent for ablation of varying intensity. Surface energy budget components and air mass frequencies are examined in relation to the observed trends in snow ablation. Changes in March ablation frequency were shown to be dominated by increases in the sensible heat flux. A higher frequency of dry moderate instead of moist polar air masses during high ablation years may explain the increase in sensible heat flux and ablation over the study period. Copyright © 2006 Royal Meteorological Society

Onset of crater formation during short pulse laser ablation

We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out.

Nanoprocessing of subcellular targets using femtosecond laser pulses

Abstract In this paper we review the work done in our laboratory on femtosecond laser dissection within single cells and living organisms. Precise dissection of biological material with ultrashort laser pulses requires a clear understanding of the pulse-energy dependence of the onset and extent of plasma-mediated ablation (i.e., the removal of material). We carried out a systematic study of the energy dependence of the plasma-mediated ablation of fluorescently-labeled subcellular structures in the cytoskeleton and in nuclei of fixed endothelial cells using femtosecond, near-infrared laser pulses focused through a high-numerical aperture objective lens (1.4 NA). We performed laser nanosurgery in live cells, where we ablated a single mitochondria and severed cytoskeletal filaments without compromising the cell membrane or the cell's viability. We also cut dendrites in living C. elegans without affecting the neighboring neurons. This nanoprocessing technique enables non-invasive manipulation of the structural machinery of cells and tissues down to several hundred nanometer resolution.

Coarse-Grained Model of the Interaction of Light with Polymeric Material: Onset of Ablation

A coarse-grained model has been developed for molecular dynamics simulations of the interaction of light with polymeric materials. The photon energy can result in a vibrational excitation (photothermal process) or disruption of a chemical bond (photochemical process) in a polymer. In the latter case, the formation of active radial sites and the occurrence of chemical reactions have to be taken into consideration. The novel feature of this model is the incorporation of chemical reactions into the united atom approximate representation of the polymer structure, which permits the study of laser ablation, degradation, or the effect of various chemical reactions on large time and length scales. The chemical reactions are included in the model in a probabilistic manner as in the kinetic Monte Carlo method. This model adopts physically and experimentally known quantities such as enthalpies and probabilities of reactions. Properties such as laser irradiation time, laser fluence, and wavelength are explicitly included. Moreover, no chemically correct interaction potential is required to incorporate the effects of chemical reactions on the dynamics of the system after energy deposition. We find that the model provides a plausible description of the essential processes. The laser-induced pressure relaxation is the main mechanism responsible for the onset of polymer ablation. Since the pressure relaxation processes are slow, there is a delay in the onset of ablation after the end of the laser pulse as is observed experimentally. The vaporization processes are not efficient for material removal, and the effect is minimal for both photochemical and photothermal processes. A lower fluence is needed for the onset of ablation with photochemical processes than photothermal processes.

Use of an axial nose-tip cavity for delaying ablation onset in hypersonic flow

A forward-facing cavity is examined as a means of reducing the severe heating and delaying ablation onset at the nose-tip of a hypersonic vehicle. Whereas previous studies have concentrated on the nature of flow-induced Hartmann-whistle oscillations or on heating rates alone, the present study addresses the effect of the cavity on ablation onset times through experiments and coupled flow field/heat conduction simulation. Using our previously developed experimental technique, a parametric study is undertaken to optimize the forward-facing cavity geometry for the most delayed ablation onset. The geometric parameters of cavity length, lip radius and diameter are independently optimized for a given nose-tip diameter. Then using benchmarked linked flow field/heat conduction simulations, numerical simulations are conducted for each parametrically optimized configuration in order to investigate the flow physics. The impact of the forward-facing cavity on aerodynamic drag is also considered.

Pulse energy dependence of subcellular dissection by femtosecond laser pulses

Precise dissection of cells with ultrashort laser pulses requires a clear understanding of how the onset and extent of ablation (i.e., the removal of material) depends on pulse energy. We carried out a systematic study of the energy dependence of the plasma-mediated ablation of fluorescently-labeled subcellular structures in the cytoskeleton and nuclei of fixed endothelial cells using femtosecond, near-infrared laser pulses focused through a high-numerical aperture objective lens (1.4 NA). We find that the energy threshold for photobleaching lies between 0.9 and 1.7 nJ. By comparing the changes in fluorescence with the actual material loss determined by electron microscopy, we find that the threshold for true material ablation is about 20% higher than the photobleaching threshold. This information makes it possible to use the fluorescence to determine the onset of true material ablation without resorting to electron microscopy. We confirm the precision of this technique by severing a single microtubule without disrupting the neighboring microtubules, less than 1 micrometer away.

Computational fluid dynamics simulation of laser-ablated carbon plume propagation in varying background gases for single-walled nanotube synthesis.

A computational fluid dynamics study was conducted to model the plume resulting from the laser ablation of a carbon target in a laser ablation oven for the production of carbon single-walled nanotubes (SWNTs). The goal is to gain understanding into the fluid dynamics and thermodynamics of the plume to ultimately improve SWNT production techniques. The simulations were carried out with a 12-species, 14-reaction chemistry model that included carbon species from C to C6. Metal catalysts in the carbon target were ignored for these simulations. Simulation times ranged from immediate ablation onset to 8 ms past the initial onset of ablation. A secondary goal of the study was to compare computational results with experimental results for three different background gases in the laser ablation oven-argon, helium, and nitrogen. Computational results indicated that lighter carbon species were more quickly diffused into the background gas for helium and nitrogen, resulting in lower localized mass fractions of carbon nanotube "feedstock." The expectation is that this effect will reduce the production of carbon nanotubes, which has been confirmed by experimental evidence. From this investigation, a possible "indicator species" for the production of SWNT appears to be C5.

Simplified model to account for dependence of ablation parameters on temperature and phase of the ablated material

A simple theoretical model involving heat conduction equation that includes the temperature and phase dependence of material properties is presented. It describes the heating, vaporization, and physics of laser created plasma for aluminum targets in ambient gas. One-dimensional (1D) heat conduction equation is numerically solved to calculate ablation threshold, ablation depth and ablation onset time and to investigate the effect of energy, nature of surface and laser pulse shape. Results show a marked difference in ablation characteristics between with and without the temperature and phase dependence. The temperature profiles inside the target and ablated vapors at various irradiance and pulse shapes are generated. Depending on the pulse shape and irradiance a certain region inside the plume becomes critically dense and reflects the incoming laser. The distance at which this shielding occurs varies with laser intensity, with the increase in intensity the time of occurrence of shielding decreases.

Ray Method of Solution of the Ablation Problem

Consideration is given to a three‐dimensional problem of heating of a half‐space by the Gaussian energy flux with allowance made for the heat‐flux inertia. It is assumed that the ablation front is formed at the points at which the temperature is equal to the melting temperature and the Stefan condition is satisfied. The mechanism of removal of mass is not considered. Using the ray method and taking into account three expansion terms, an ablation‐surface equation and temperature have been obtained. Account for the inertia of the heat flux leads to the fact that the ablation‐front velocity at the initial instant of time is finite. The condition for the onset of ablation of the material at the initial instant of time has been obtained. The solutions for the fronts of ablation and temperature are illustrated by the graphs.

Photochemical and Photothermal Model for Pulsed-Laser Ablation

A model of the interaction of UV laser pulses with organic polymers is presented. The three distinct features of this model are as follows: 1 ) It combines photochemical and photothermal processes for breaking bonds. 2 ) It tracks the percentage of bonds broken where the higher dissociative states are not allowed to relax back to the lower states. 3 ) The model does not need an experimentally inferred value of threshold e uence to determine the onset of ablation. The mathematical model presented here is based on the system of a two-photon absorption model, where sets of rate equations that include radiative transport and energy absorption are solved. Solutions of this model are discussed, and the results are shown to compare well with experimental etch depth vs e uence curves from the literature for a wide range of pulse widths from 7 to 300 ns. Parametric results are also presented for 193- and 308-nm UV laser wavelengths for different laser and polymer parameters. The dependence of the temperatures and ablation depths for different laser e uences and widths of the laser pulses are obtained.

Mathematical modeling of laser ablation in liquids with application to laser ultrasonics

The use of laser ablation as a means of generating ultrasonic waves in liquid metals is studied in this paper. A mathematical model for predicting the onset of ablation is developed, as is a model of the ablation process based on steady state, one-dimensional gas dynamics in which the vapor phase is treated as an ideal gas. The results of this model are then used in a quasi-two-dimensional model of laser ablation that accounts for the spatial distribution of intensity in the laser beam. Model predictions are compared with experiments conducted on liquid mercury and excellent agreement is obtained. Based on these results, a simplified model is developed that shows excellent agreement with both the theory and the experiments.

Laser etching processes : Towards sub-picosecond X-UV irradiation

Since the first observation of high order harmonic generation (HOHG) in a rare gas jet, the creation of ultrashort laser pulses in the extreme UV (XUV) has been a field of continuous development. The increasing accessibility of femtosecond lasers and chirped pulsed amplifiers (CPA) has helped to fuel research into the fundamentals of the HOHG process [1], its practical implementation and tentative uses of such a source in spectroscopy. One of our particular interests in the use of this facility is to study the ability of these ultrashort pulses to initiate photochemical processes on clean or adsorbate-covered (SF 6 or CCl 4 ) silicon surfaces. More recently, femtosecond or even atto-second laser pulses has been evoked as being the best tools to photo-ablate polymeric materials [2]. VUV studies [3] also demonstrate that higher photon energies applied on the same kind of matter can decrease dramatically the threshold fluence for the onset of ablation. We seek to determine exactly what happens when both conditions (ultrafast and VUV pulses) are combined during irradiation.

Image‐intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure

Abstract— Two‐station electro‐optical observations of the 1998 Leonid shower are presented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astronomical magnitude. The mean photometric mass of the data sample was 1.4 × 10−6 kg. The dependence of astronomical magnitude at peak luminosity on photometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of ∼1.0 × 10‐7 kg corresponds to a peak meteor luminosity of about +4.5 astronomical magnitudes. The mean beginning height of the Leonid meteors in this sample was 112.6 km and the mean ending height was 95.3 km. The highest beginning height observed was 144.3 km. There is relatively little dependence of either the first or last heights on mass, which is indicative of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with numerical modelling of the ablation of the meteoroids, suggests that silicate‐like materials are not the principal component of Leonid meteoroids and hints at the presence of a more volatile component. Light curves of many Leonid meteors were examined for evidence of the physical structure of the associated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.

Are meteoroids really dustballs?

Analysis of light curves of faint meteors seem to suggest that most meteoroids are collections of hundreds to thousands of fundamental grains at least some of which are released prior to the onset of intensive ablation. We would expect these grains, unless extremely uniform in physical properties, to be aerodynamically separated during flight, and therefore to produce wake, which is defined as instantaneous meteor light production from an extended spatial region. We present here theoretical results for wake production as a function of grain mass distribution, height of separation, zenith angle and velocity. In addition, we have obtained observational results from a new study which used short duration intensified CCD detectors to search for wake in sporadic meteors. The system employed coaxial intensified CCD cameras at each of two separated stations, one camera utilizing a rotating shutter and one not at each station. The majority of these nonshower meteors showed no statistically significant wake. However, several examples of apparent transverse separation of the light production regions were found. We also present results of two interesting Leonid meteors from the Mount Allison light curve experiment in NASA's 1998 Leonid Multi-instrument Aircraft Campaign (MAC) program. One of these provide support for the idea of transverse spread in the light production region, up to hundreds of meters, while the other provides a clear case of extreme wake in one Leonid meteor which can only be successfully modeled with very small (≈10 −16– 10 −17 kg ) constituent grains.

Microscopic mechanisms of laser ablation of organic solids in the thermal and stress confinement irradiation regimes

The results of large-scale molecular dynamics simulations demonstrate that the mechanisms responsible for material ejection as well as most of the parameters of the ejection process have a strong dependence on the rate of the laser energy deposition. For longer laser pulses, in the regime of thermal confinement, a phase explosion of the overheated material is responsible for the collective material ejection at laser fluences above the ablation threshold. This phase explosion leads to a homogeneous decomposition of the expanding plume into a mixture of liquid droplets and gas phase molecules. The decomposition proceeds through the formation of a transient structure of interconnected liquid clusters and individual molecules and leads to the fast cooling of the ejected plume. For shorter laser pulses, in the regime of stress confinement, a lower threshold fluence for the onset of ablation is observed and attributed to photomechanical effects driven by the relaxation of the laser-induced pressure. Larger and more numerous clusters with higher ejection velocities are produced in the regime of stress confinement as compared to the regime of thermal confinement. For monomer molecules, the ejection in the stress confinement regime results in broader velocity distributions in the direction normal to the irradiated surface, higher maximum velocities, and stronger forward peaking of the angular distributions. The acoustic waves propagating from the absorption region are much stronger in the regime of stress confinement and the wave profiles can be related to the ejection mechanisms.

Ablation Onset in Unsteady Hypersonic Flow About Nose Tip with Cavity

A forward-facing cavity is examined as a means of reducing the severe heating and ablation at the nose tip of a hypersonic vehicle. Whereas previous studies have concentrated on heating rates alone, the present study addresses the effects of the cavity on ablation onset times through experiments and joined flowfield/heat conduction simulations. A viable experimental technique has been developed to study ablation in the Mach 5 wind tunnel at the University of Texas at Austin J.J. Pickle Research Center. Using this technique, the time to ablation onset for a nonoptimized, rounded-lip, deep-cavity model has been found to be similar to that of the baseline hemisphere-cylinder model. Additionally, the computational technique predicts ablation onset times that agree surprisingly well with the Mach 5 experimental data. Thus, a benchmark is achieved in the computational techniques for future use for determining the experimental flowfield physics of this complex, unsteady hypersonic flow problem

Temperature distribution under irradiation of pathologically changed tissue by an excimer laser

A theoretical model of the temperature distribution of biological tissue under pulsed laser illumination is developed. With the help of the elaborated miniature thermometer based on a shieldless silicon transistor, the spatial-temporal distribution of temperatures of the artery inner wall with atherosclerotic inclusions (in vitro) under excimer-laser illumination as a function of laser power and the distance between the thermometer and laser spot on the tissue surface was experimentally measured. From a comparison of experimental data with the results of the model developed, the thermal diffusivity of tissue and the size of the “region of damage” (i.e., the region where the temperature exceeds 43–45°C) under laser removal (ablation) of the tissue was determined, as well as the threshold energy density of the ablation onset.

In-situ measurements of excimer laser irradiated zinc sulphide films on silicon

Time-resolved reflectivity at 675 nm of thin films of zinc sulphide on silicon during excimer laser irradiation is contrasted with theoretical calculations based on known data for the temperature dependence of the refractive index of both film and substrate and temperature profiles generated using an analytical solution of the heat diffusion equation for a two-layer system. The calculated rates of heating and cooling are found to be in excellent agreement with the experimental measurements and the maximum surface temperature at the ablation threshold is found to be 1150°C. We conclude therefore that the zinc sulphide does not reach the melting point (∼1800°C) before the onset of ablation and that sublimation plays a significant part in the ablation process.