In the present work, a numerical analysis is performed to investigate the comparative contributio... more In the present work, a numerical analysis is performed to investigate the comparative contribution of the mechanisms responsible for electron gain and losses in laser spark ignition and plasma formation of H2. The analysis considered H2 over pressure range 150 -3000 torr irradiated by a Nd:YAG laser radiation at wavelengths 1064 and 532 nm with pulse length 5.5 ns. The study based on a modified electron cascade model by one of the authors which solves numerically the time dependent Boltzmann equation as well as a set of rate equations that describe the rate of change of the excited states population. The model includes most of the physical processes that might take place during the interaction. Computations of The threshold intensity are performed for the combined and separate contribution of each of the gain and loss processes. Reasonable agreement with the measured values over the tested pressure range is obtained only for the case of the combined contribution. Basing on the calculation of the electron ...
A study is performed to investigate the dependency of threshold intensity on gas pressure observe... more A study is performed to investigate the dependency of threshold intensity on gas pressure observed in the measurements of the breakdown of molecular oxygen that carried out by Phuoc (2000) [1]. In this experiment, the breakdown was induced by 532 nm laser radiation of pulse width 5.5 ns and spot size of 8.5 µm, in oxygen over a wide pressure range (190-3000 Torr). The analysis aimed to explore the electron kinetic reliance on gas pressure for the separate contribution of each of the gain and loss processes encountered in this study. The investigation is based on an electron cascade model applied previously in Gamal and Omar (2001) [2] and Gaabour et al. (2013) [3]. This model solves numerically a differential equation designates the time evolution of the electron energy distribution, and a set of rate equations that describe the change of excited states population. The numerical examination of the electron energy distribution function and its parameters revealed that photo-ionization of the excited molecules plays a significant role in enhancing the electron density growth rate over the whole tested gas pressure range. This process is off set by diffusion of electrons out of the focal volume in the low-pressure regime. At atmospheric pressure electron, collisional processes dominate and act mainly to populate the excited states. Hence photo-ionization becomes efficient and compete with the encountered loss processes (electron diffusion, vibrational excitation of the ground state molecules as well as two body attachments). At high pressures (~3000 Torr) three body attachments are found to be the primary cause of losses which deplete the electron density and hence results in the slow decrease of the threshold intensity.
A numerical investigation of laser wavelength dependence on the threshold intensity of spark igni... more A numerical investigation of laser wavelength dependence on the threshold intensity of spark ignition in molecular hydrogen over a wide pressure range is presented. A modified electron cascade model (Gamal et al., 1993) is applied under the experimental conditions that carried out by Phuoc (2000) to determine the threshold intensity dependence on gas pressure for spark ignition in hydrogen combustion using two laser wavelengths namely; 1064 nm and 532 nm. The model involves the solution of the time dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations that describe the change of the formed excited molecules population. The model takes into account most of the physical processes that expected to occur in the interaction region. The results showed good agreement between the calculated thresholds for spark ignition and those measured ones for both wavelengths, where the threshold intensities corresponding to the short wavelength (532 nm) are found to be higher than those calculated for the longer one (1064 nm). This result indicates the depletion of the high density of low energy electrons generated through multi-photon ionization at the short wavelength via electron diffusion and vibrational excitation. The study of the EEDF and its parameters (viz, the temporal evolution of: the electron density, ionization rate electron mean en-ergy…) revealed the important role played by each physical process to the spark ignition as a function of both laser wavelength and gas pressure. More over the study of the time variation of the EEDF explains the characteristics of the ignited spark at the two wavelengths for the tested pressure values.
An electron cascade model adapted to provide a thorough understanding of the physical mechanisms ... more An electron cascade model adapted to provide a thorough understanding of the physical mechanisms involved in CO 2 laser-induced helium plasma is presented. The model combines a time-dependent calculation of the electron energy distribution with rate equations, describing how the population of excited states changes [Y. E. E.-D. Gamal and G. Abdellatif, Appl. Phys. B 117(1), 103 (2014)]. It encountered the possible elastic and inelastic electron collisional processes that enhance the electrons' growth, leading to gas breakdown. The analysis explores the experimental threshold intensity dependency on gas pressure [J. J. Camacho et al., Spectrochim. Acta, Part B 66(1), 57 (2011)]. The measurements are carried out using 9.621 lm over pressure in the range from 12.0 to 87.0 kPa. Since multiphoton ionization is improbable, ionization proceeds via the inverse bremsstrahlung absorption. In this experiment, the ignition of this process is initiated by the experimentally assumed pre-breakdown approach. No experimental estimation was given for the initial electron density. The electron diffusion and the loss of electron energy through elastic collisions have no contribution to this experiment. The calculations of the threshold intensity are performed to determine the initial electron density. The model's validity is assured by the reasonable agreement between the calculated thresholds, and the measured ones are only achieved at a specific initial electron value for each gas pressure. Over pressure exceeding 30.0 kPa, the agreement was reasonable in the presence of recombination losses. The threshold intensity is controlled by the initial electron density for lower pressures. The analysis showed how the gain and loss of electrons control the breakdown threshold for helium concerning the determined initial electron density for the tested pressures.
Solar filaments are commonly thought to be supported in magnetic dips, in particular, in those of... more Solar filaments are commonly thought to be supported in magnetic dips, in particular, in those of magnetic flux ropes (FRs). In this Letter, based on the observed photospheric vector magnetogram, we implement a nonlinear force-free field (NLFFF) extrapolation of a coronal magnetic FR that supports a large-scale intermediate filament between an active region and a weak polarity region. This result is a first, in the sense that current NLFFF extrapolations including the presence of FRs are limited to relatively small-scale filaments that are close to sunspots and along main polarity inversion lines (PILs) with strong transverse field and magnetic shear, and the existence of an FR is usually predictable. In contrast, the present filament lies along the weak-field region (photospheric field strength 100 G), where the PIL is very fragmented due to small parasitic polarities on both sides of the PIL and the transverse field has a low signal-to-noise ratio. Thus, extrapolating a large-scale FR in such a case represents a far more difficult challenge. We demonstrate that our CESE-MHD-NLFFF code is sufficient for the challenge. The numerically reproduced magnetic dips of the extrapolated FR match observations of the filament and its barbs very well, which strongly supports the FR-dip model for filaments. The filament is stably sustained because the FR is weakly twisted and strongly confined by the overlying closed arcades.
Considers the simultaneous effect of the processes of multiphoton and cascade ionisation in produ... more Considers the simultaneous effect of the processes of multiphoton and cascade ionisation in producing ionisation and breakdown molecular gases under the influence of three different short-flash lasers of wavelengths 0.6943 mu m, 1.06 mu m and 0.53 mu m with pulse durations 18 ps, 7 ps and 25 ps respectively. The gas was studied over a pressure range of 1-105
A theoretical investigation is made into the ionisation of helium, at pressures of 1800-17000 Tor... more A theoretical investigation is made into the ionisation of helium, at pressures of 1800-17000 Torr (0.24-2.27 MPa) by ruby laser radiation of pulse-length 40 ns. It is shown that, when the variation of electron energy gain in elastic collisions is taken into account, the agreement between computed and experimental breakdown threshold intensities is improved by a factor of 10 compared with calculations in which only the mean energy gain is computed. The agreement is also improved significantly by incorporating the collisional and multiphoton ionisation of excited helium atoms.
This paper presents a numerical analysis on the electrical breakdown of neon over a pressure rang... more This paper presents a numerical analysis on the electrical breakdown of neon over a pressure range 100-3000 Torr irradiated with 248 nm laser radiation of pulse length 18 ns operated at threshold irradiance varying between {4×1011} and 2×1013 W cm-2. The investigations are based on a previously developed electron cascade model which solves numerically the time dependent Boltzmann equation simultaneously with a set of rate equations describing the rate of change of the excited states population. The result of computations showed a good agreement between the calculated threshold irradiance and the measured ones over the whole range of gas pressure examined experimentally. A study of the effect of loss processes on the breakdown threshold revealed that diffusion losses are pronounced over pressures <1000 Torr. No evidence for recombination losses is observed at pressures up to 3000 Torr. Analysis of the calculated electron energy distribution function and its parameters reflected the importance of photoionization of the excited states as a physical mechanism responsible for the breakdown of neon under the experimental condition used in this work.
Electron-hole pairs in semiconductors can be stimulated by a laser beam with energy larger than t... more Electron-hole pairs in semiconductors can be stimulated by a laser beam with energy larger than the energy gap of the semiconductor. The interaction between an electron-hole plasma with a laser beam can be a source of instability. The dependence of the instability on the electron and hole temperatures and the unperturbed potential of the incident laser are examined. Using Maxwell's equations along with electron-hole fluid equations, an evolution equation describing the system is obtained. The latter is reduced to an energy equation that characterizes localized pulse propagation.
Arabian journal for science and engineering, Nov 5, 2020
The current study is dedicated to investigating the electron dynamics of the breakdown and plasma... more The current study is dedicated to investigating the electron dynamics of the breakdown and plasma generation in molecular oxygen at a pressure extended from 4.6 to 75 kPa. The breakdown is motivated by far-infrared laser source operational at λ 10.591 μm, (hν~0.12 eV) with pulse width 2τ 64 ns (FWHM) (Camacho et al. in J Phys D Appl Phys 41(10):105201, 2008). This experiment presumed the presence of a new electron ignition mechanism to produce a specific density of seed electrons in the interaction region as a substitution of the negligible involvement of the photoionization process. The analysis is grounded on adapting the electron cascade model given in our past paper (Evans and Gamal in J Phys D Appl Phys 13(8):1447, 1980). This model well thought out the determination of the threshold intensity as a function of gas pressure taken into account the possible physical processes which may take place in the interaction volume. In doing so, the governing differential equation that defines the variation of the energy of electrons during the laser pulse is solved numerically together with a set of rate equations presenting the change of population of the excited states. The calculated breakdown threshold intensity showed a reasonable agreement with the measured ones, where both indicated weak dependence over the tested pressure range. This behavior is resolved by studying the individual effect of each loss processes involved in the model on the threshold intensity concerning the experimentally assumed density of the initial electrons corresponding to the tested gas pressure range. Besides, to evaluate the precise involvement of the action of the single loss process to the mechanism responsible for plasma production, computations of the temporal variation of the density of electrons are carried out in the presence and absence of the individual loss process. Keywords Oxygen breakdown • CO 2 laser • Vibrational and rotational excitations • Inverse bremsstrahlung absorption • Stepwise collisional ionization • Dissociative attachment B Khaled A.
A study of the spectral emission in laser-induced breakdown spectroscopy of gases was performed. ... more A study of the spectral emission in laser-induced breakdown spectroscopy of gases was performed. The measurements were carried out on helium, argon, nitrogen, and air irradiated with ruby laser radiation at a wavelength of 694.3 nm and a pulse width of 40 ns ...
In the present work, a numerical analysis is performed to investigate the comparative contributio... more In the present work, a numerical analysis is performed to investigate the comparative contribution of the mechanisms responsible for electron gain and losses in laser spark ignition and plasma formation of H2. The analysis considered H2 over pressure range 150 -3000 torr irradiated by a Nd:YAG laser radiation at wavelengths 1064 and 532 nm with pulse length 5.5 ns. The study based on a modified electron cascade model by one of the authors which solves numerically the time dependent Boltzmann equation as well as a set of rate equations that describe the rate of change of the excited states population. The model includes most of the physical processes that might take place during the interaction. Computations of The threshold intensity are performed for the combined and separate contribution of each of the gain and loss processes. Reasonable agreement with the measured values over the tested pressure range is obtained only for the case of the combined contribution. Basing on the calculation of the electron ...
A study is performed to investigate the dependency of threshold intensity on gas pressure observe... more A study is performed to investigate the dependency of threshold intensity on gas pressure observed in the measurements of the breakdown of molecular oxygen that carried out by Phuoc (2000) [1]. In this experiment, the breakdown was induced by 532 nm laser radiation of pulse width 5.5 ns and spot size of 8.5 µm, in oxygen over a wide pressure range (190-3000 Torr). The analysis aimed to explore the electron kinetic reliance on gas pressure for the separate contribution of each of the gain and loss processes encountered in this study. The investigation is based on an electron cascade model applied previously in Gamal and Omar (2001) [2] and Gaabour et al. (2013) [3]. This model solves numerically a differential equation designates the time evolution of the electron energy distribution, and a set of rate equations that describe the change of excited states population. The numerical examination of the electron energy distribution function and its parameters revealed that photo-ionization of the excited molecules plays a significant role in enhancing the electron density growth rate over the whole tested gas pressure range. This process is off set by diffusion of electrons out of the focal volume in the low-pressure regime. At atmospheric pressure electron, collisional processes dominate and act mainly to populate the excited states. Hence photo-ionization becomes efficient and compete with the encountered loss processes (electron diffusion, vibrational excitation of the ground state molecules as well as two body attachments). At high pressures (~3000 Torr) three body attachments are found to be the primary cause of losses which deplete the electron density and hence results in the slow decrease of the threshold intensity.
A numerical investigation of laser wavelength dependence on the threshold intensity of spark igni... more A numerical investigation of laser wavelength dependence on the threshold intensity of spark ignition in molecular hydrogen over a wide pressure range is presented. A modified electron cascade model (Gamal et al., 1993) is applied under the experimental conditions that carried out by Phuoc (2000) to determine the threshold intensity dependence on gas pressure for spark ignition in hydrogen combustion using two laser wavelengths namely; 1064 nm and 532 nm. The model involves the solution of the time dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations that describe the change of the formed excited molecules population. The model takes into account most of the physical processes that expected to occur in the interaction region. The results showed good agreement between the calculated thresholds for spark ignition and those measured ones for both wavelengths, where the threshold intensities corresponding to the short wavelength (532 nm) are found to be higher than those calculated for the longer one (1064 nm). This result indicates the depletion of the high density of low energy electrons generated through multi-photon ionization at the short wavelength via electron diffusion and vibrational excitation. The study of the EEDF and its parameters (viz, the temporal evolution of: the electron density, ionization rate electron mean en-ergy…) revealed the important role played by each physical process to the spark ignition as a function of both laser wavelength and gas pressure. More over the study of the time variation of the EEDF explains the characteristics of the ignited spark at the two wavelengths for the tested pressure values.
An electron cascade model adapted to provide a thorough understanding of the physical mechanisms ... more An electron cascade model adapted to provide a thorough understanding of the physical mechanisms involved in CO 2 laser-induced helium plasma is presented. The model combines a time-dependent calculation of the electron energy distribution with rate equations, describing how the population of excited states changes [Y. E. E.-D. Gamal and G. Abdellatif, Appl. Phys. B 117(1), 103 (2014)]. It encountered the possible elastic and inelastic electron collisional processes that enhance the electrons' growth, leading to gas breakdown. The analysis explores the experimental threshold intensity dependency on gas pressure [J. J. Camacho et al., Spectrochim. Acta, Part B 66(1), 57 (2011)]. The measurements are carried out using 9.621 lm over pressure in the range from 12.0 to 87.0 kPa. Since multiphoton ionization is improbable, ionization proceeds via the inverse bremsstrahlung absorption. In this experiment, the ignition of this process is initiated by the experimentally assumed pre-breakdown approach. No experimental estimation was given for the initial electron density. The electron diffusion and the loss of electron energy through elastic collisions have no contribution to this experiment. The calculations of the threshold intensity are performed to determine the initial electron density. The model's validity is assured by the reasonable agreement between the calculated thresholds, and the measured ones are only achieved at a specific initial electron value for each gas pressure. Over pressure exceeding 30.0 kPa, the agreement was reasonable in the presence of recombination losses. The threshold intensity is controlled by the initial electron density for lower pressures. The analysis showed how the gain and loss of electrons control the breakdown threshold for helium concerning the determined initial electron density for the tested pressures.
Solar filaments are commonly thought to be supported in magnetic dips, in particular, in those of... more Solar filaments are commonly thought to be supported in magnetic dips, in particular, in those of magnetic flux ropes (FRs). In this Letter, based on the observed photospheric vector magnetogram, we implement a nonlinear force-free field (NLFFF) extrapolation of a coronal magnetic FR that supports a large-scale intermediate filament between an active region and a weak polarity region. This result is a first, in the sense that current NLFFF extrapolations including the presence of FRs are limited to relatively small-scale filaments that are close to sunspots and along main polarity inversion lines (PILs) with strong transverse field and magnetic shear, and the existence of an FR is usually predictable. In contrast, the present filament lies along the weak-field region (photospheric field strength 100 G), where the PIL is very fragmented due to small parasitic polarities on both sides of the PIL and the transverse field has a low signal-to-noise ratio. Thus, extrapolating a large-scale FR in such a case represents a far more difficult challenge. We demonstrate that our CESE-MHD-NLFFF code is sufficient for the challenge. The numerically reproduced magnetic dips of the extrapolated FR match observations of the filament and its barbs very well, which strongly supports the FR-dip model for filaments. The filament is stably sustained because the FR is weakly twisted and strongly confined by the overlying closed arcades.
Considers the simultaneous effect of the processes of multiphoton and cascade ionisation in produ... more Considers the simultaneous effect of the processes of multiphoton and cascade ionisation in producing ionisation and breakdown molecular gases under the influence of three different short-flash lasers of wavelengths 0.6943 mu m, 1.06 mu m and 0.53 mu m with pulse durations 18 ps, 7 ps and 25 ps respectively. The gas was studied over a pressure range of 1-105
A theoretical investigation is made into the ionisation of helium, at pressures of 1800-17000 Tor... more A theoretical investigation is made into the ionisation of helium, at pressures of 1800-17000 Torr (0.24-2.27 MPa) by ruby laser radiation of pulse-length 40 ns. It is shown that, when the variation of electron energy gain in elastic collisions is taken into account, the agreement between computed and experimental breakdown threshold intensities is improved by a factor of 10 compared with calculations in which only the mean energy gain is computed. The agreement is also improved significantly by incorporating the collisional and multiphoton ionisation of excited helium atoms.
This paper presents a numerical analysis on the electrical breakdown of neon over a pressure rang... more This paper presents a numerical analysis on the electrical breakdown of neon over a pressure range 100-3000 Torr irradiated with 248 nm laser radiation of pulse length 18 ns operated at threshold irradiance varying between {4×1011} and 2×1013 W cm-2. The investigations are based on a previously developed electron cascade model which solves numerically the time dependent Boltzmann equation simultaneously with a set of rate equations describing the rate of change of the excited states population. The result of computations showed a good agreement between the calculated threshold irradiance and the measured ones over the whole range of gas pressure examined experimentally. A study of the effect of loss processes on the breakdown threshold revealed that diffusion losses are pronounced over pressures <1000 Torr. No evidence for recombination losses is observed at pressures up to 3000 Torr. Analysis of the calculated electron energy distribution function and its parameters reflected the importance of photoionization of the excited states as a physical mechanism responsible for the breakdown of neon under the experimental condition used in this work.
Electron-hole pairs in semiconductors can be stimulated by a laser beam with energy larger than t... more Electron-hole pairs in semiconductors can be stimulated by a laser beam with energy larger than the energy gap of the semiconductor. The interaction between an electron-hole plasma with a laser beam can be a source of instability. The dependence of the instability on the electron and hole temperatures and the unperturbed potential of the incident laser are examined. Using Maxwell's equations along with electron-hole fluid equations, an evolution equation describing the system is obtained. The latter is reduced to an energy equation that characterizes localized pulse propagation.
Arabian journal for science and engineering, Nov 5, 2020
The current study is dedicated to investigating the electron dynamics of the breakdown and plasma... more The current study is dedicated to investigating the electron dynamics of the breakdown and plasma generation in molecular oxygen at a pressure extended from 4.6 to 75 kPa. The breakdown is motivated by far-infrared laser source operational at λ 10.591 μm, (hν~0.12 eV) with pulse width 2τ 64 ns (FWHM) (Camacho et al. in J Phys D Appl Phys 41(10):105201, 2008). This experiment presumed the presence of a new electron ignition mechanism to produce a specific density of seed electrons in the interaction region as a substitution of the negligible involvement of the photoionization process. The analysis is grounded on adapting the electron cascade model given in our past paper (Evans and Gamal in J Phys D Appl Phys 13(8):1447, 1980). This model well thought out the determination of the threshold intensity as a function of gas pressure taken into account the possible physical processes which may take place in the interaction volume. In doing so, the governing differential equation that defines the variation of the energy of electrons during the laser pulse is solved numerically together with a set of rate equations presenting the change of population of the excited states. The calculated breakdown threshold intensity showed a reasonable agreement with the measured ones, where both indicated weak dependence over the tested pressure range. This behavior is resolved by studying the individual effect of each loss processes involved in the model on the threshold intensity concerning the experimentally assumed density of the initial electrons corresponding to the tested gas pressure range. Besides, to evaluate the precise involvement of the action of the single loss process to the mechanism responsible for plasma production, computations of the temporal variation of the density of electrons are carried out in the presence and absence of the individual loss process. Keywords Oxygen breakdown • CO 2 laser • Vibrational and rotational excitations • Inverse bremsstrahlung absorption • Stepwise collisional ionization • Dissociative attachment B Khaled A.
A study of the spectral emission in laser-induced breakdown spectroscopy of gases was performed. ... more A study of the spectral emission in laser-induced breakdown spectroscopy of gases was performed. The measurements were carried out on helium, argon, nitrogen, and air irradiated with ruby laser radiation at a wavelength of 694.3 nm and a pulse width of 40 ns ...
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Papers by Dr. Yosr Gamal