Kuan-Ren Chen - Academia.edu (original) (raw)
Papers by Kuan-Ren Chen
MRS Proceedings, 1995
Several physics and computational approaches have been developed to globally characterize phenome... more Several physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed laser deposition of materials. these include thermal models of laser-solid target interactions that initiate the vapor plume; plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms; gas dynamic, hydrodynamic, and collisional descriptions of plume transport; and molecular dynamics models of the interaction of plume particles with the deposition substrate. the complexity of the phenomena involved in the laser ablation process is matched by the diversity of the modeling task, which combines materials science, atomic physics, and plasma physics.
AIP Conference Proceedings, 1994
The growth of an unstable electrostatic ion Bernstein mode in a uniformly magnetized, homogeneous... more The growth of an unstable electrostatic ion Bernstein mode in a uniformly magnetized, homogeneous plasma is simulated by time averaging the equations of motion over a cyclotron period. (AIP)
The relativistic instability of the electromagnetic ion cyclotron wave propagating across the mag... more The relativistic instability of the electromagnetic ion cyclotron wave propagating across the magnetic field [B. Coppi, S. Cowley and R. Kulsrud, Phys. Fluid 29, 4060 (1986)] is investigated. The relativistic instability is reactive, as in contrast to the inversed Landau instability. Due to the wave magnetic field is much larger than the electric field, the dielectric constant of the cold
Physical Review Letters, 1995
A dynamic source effect that accelerates the expansion of laser-ablated material in the direction... more A dynamic source effect that accelerates the expansion of laser-ablated material in the direction perpendicular to the target is demonstrated. A self-similar theory shows that the maximum expansion velocity is proportional to c s ͞a, where 1 2 a is the slope of the velocity profile and c s is the sound speed. Numerical hydrodynamic modeling is in good agreement with the theory. A dynamic partial ionization effect is also studied. With these effects, a is reduced and the maximum expansion velocity is significantly increased over that found from conventional models.
Caused by wave magnetic field and in sharp contrast to ES case, Alfvénic behavior and instability... more Caused by wave magnetic field and in sharp contrast to ES case, Alfvénic behavior and instability transition as the novel characteristics of the EM instabilities are illuminated in detail. The cubic (square) root scaling of the peak growth rate makes the relativistic effect more important than inversed Landau damping, especially for low fast ion density. For the cubic instability, there is a threshold (ceiling) on the slow ion temperature and density (the external magnetic field and the fast ion energy); the Alfvén velocity is required to be low. This Alfvénic behavior is important for its applications. The case of fast protons in thermal deuterons is numerically studied and compared with the analytical results. When the slow ion temperature or density (the external magnetic field or the fast ion energy) is increased (reduced) to be about twice (half) the threshold (ceiling), the cubic instability peak transits to the coupled quadratic instability and a new cubic instability branch ...
Plasma Physics and Controlled Fusion, 2005
The harmonic relativistic ion cyclotron instability driven by Mev α-particles in thermal deuteriu... more The harmonic relativistic ion cyclotron instability driven by Mev α-particles in thermal deuterium plasmas confined by non-uniform magnetic fields is studied with gyrokinetic theory. An integral dispersion equation is derived and solved numerically. The results provide clear evidence for the existence of the instability of α-particle cyclotron high harmonics in burning plasmas of one fast and one slow ion species. The characteristics of the modes are found to depend on magnetic non-uniformity parameter and α-particle density rather strongly.
Physics of Plasmas, 2000
A perturbation theory and a kinetic theory are developed to investigate the novel physics of rela... more A perturbation theory and a kinetic theory are developed to investigate the novel physics of relativistic ion cyclotron instabilities. The existence of the instabilities is determined by the normalized mass deficits per nucleon of fast and slow ions (δmf and δms, respectively), and by their Lorentz factors (γf and γs, respectively); while the ion bunching is caused by the relativistic variation of ion mass. If δmf−δms−γf+γs>0, only a quadratic instability can occur at high cyclotron harmonics of the fast ion in the lower-hybrid frequency regime and above; the threshold on the harmonic number is determined by the dielectric constant of the slow ion. The peak growth rate is higher at the harmonics just above the threshold. If it is negative, both a cubic instability (or instead a coupled quadratic instability if the resonant slow ion cyclotron harmonic is the first harmonic) and the high harmonic quadratic instability can be driven. The cubic instability is due to the harmonic inte...
Physical Review Letters, 1992
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1996
A dynamic source effect was found to accelerate the plume expansion velocity much higher than tha... more A dynamic source effect was found to accelerate the plume expansion velocity much higher than that from a conventional free-expansion model. A self-similar theory and a hydrodynamic model are developed to study this effect, which may help to explain experimentally observed high front expansion velocity. Background gas can also affect the kinetic energies. High background gas may cause the ablated materials to go backward. Experimentally observed plume splitting is also discussed.
IEEE Transactions on Plasma Science, 1990
... An attractive feature of the ICL is that the beam raises the dielectric constant of the ionch... more ... An attractive feature of the ICL is that the beam raises the dielectric constant of the ionchannel relative to its value outside the channel, and thus provides guiding of the ICL radiation. ... Wave guiding by the ion channel is also an interesting effect. ...
Physics of Plasmas, 1994
An explanation, supported by numerical simulations and analytical theory, is given for the harmon... more An explanation, supported by numerical simulations and analytical theory, is given for the harmonic cyclotron emission induced by fast ions in tokamak plasmas—in particular, for the emission observed at low harmonics in deuterium–deuterium and deuterium–tritium experiments in the Joint European Torus [e.g., Phys. Rev. Lett. 60, 33 (1988)]. It is shown that the first proton harmonic, whose field energy amplitude scales as the 0.84 power of the proton density, is one of the highest spectral peaks, whereas the first alpha harmonic is weak. The relative spectral amplitudes of different harmonics are compared. The results are consistent with the experimental observations. The simulations verify that the instabilities are caused by a weak relativistic mass effect. Simulation also shows that a nonuniform magnetic field leads to no appreciable change in the growth rate and saturation amplitude of the waves.
IEEE Conference Record - Abstracts. 1997 IEEE International Conference on Plasma Science
Summary form only given, as follows. A gyrobroadening process causing the perpendicular energy of... more Summary form only given, as follows. A gyrobroadening process causing the perpendicular energy of fast ions to be broadened by cyclotron instabilities due to the weak relativistic mass variation of the fast ions in magnetized plasma is reported. At low cyclotron harmonics, a two-gyro-stream instability can be excited with the involvement of thermal ions if the condition lfωcf <ω<lsωcs is satisfied. The fast ions can be anomalously thermalized. Their lost energy goes to bulk ion heating. While the fast proton can satisfy the condition, the fast alphas cannot. The low harmonics are linearly stable for the alphas. Therefore, for low harmonics one has to inject waves to affect the alpha dynamics. However, the alphas can excite high harmonic cyclotron waves in the lower hybrid regime, in which thermal ions respond as cold plasma. The interaction becomes selective. Only alphas with high pitch angle are involved. This is presented as an interesting collective thermal dynamics issue while the implications for application are also assessed
This is a preprint of a paper being submitted for publication. Contents of this paper should not ... more This is a preprint of a paper being submitted for publication. Contents of this paper should not be quoted or referred to without permission of the author(s) .
Physical Review B, 1999
A self-similar theory and numerical hydrodynamic modeling is developed to investigate the effects... more A self-similar theory and numerical hydrodynamic modeling is developed to investigate the effects of dynamic source and partial ionization on the acceleration of the unsteady expansion of laser-ablated material near a solid target surface. The dynamic source effect accelerates the expansion in the direction perpendicular to the target surface, while the dynamic partial ionization effect accelerates the expansion in all directions. The vaporized material during laser ablation provides a nonadiabatic dynamic source at the target surface into the unsteady expanding fluid. For studying the dynamic source effect, the self-similar theory begins with an assumed profile of plume velocity, uϭv/v m ϭ␣ϩ(1Ϫ␣), where v m is the maximum expansion velocity, ␣ is a constant, and ϭx/v m t. The resultant profiles of plume density and plume temperature are derived. The relations obtained from the conservations of mass, momentum, and energy, respectively, all show that the maximum expansion velocity is inversely proportional to ␣, where 1Ϫ␣ is the slope of plume velocity profile. The numerical hydrodynamic simulation is performed with the Rusanov method and the Newton Raphson method. The profiles and scalings obtained from numerical hydrodynamic modeling are in good agreement with the theory. The dynamic partial ionization requires ionization energy from the heat at the expansion front, and thus reduces the increase of front temperature. The reduction of thermal motion would increase the flow velocity to conserve the momentum. This dynamic partial ionization effect is studied with the numerical hydrodynamic simulation including the Saha equation. With these effects, ␣ is reduced from its value of conventional free expansion. This reduction on ␣ increases the flow velocity slope, decreases the flow velocity near the surface, and reduces the thermal motion of plume, such that the maximum expansion velocity is significantly increased over that found from conventional models. The result may provide an explanation for experimental observations of high-expansion front velocities even at low-laser fluence. ͓S0163-1829͑99͒08835-9͔
Plasma Physics and Controlled Fusion, 2009
Localized electrostatic wave modes due to relativistic ion cyclotron instabilities are observed i... more Localized electrostatic wave modes due to relativistic ion cyclotron instabilities are observed in a hybrid particle-in-cell simulation under a magnetic field with sinusoidal nonuniformity. In order to investigate the physical mechanism of the simulation results, an analytical theory, adopting a parabolic approximation around a magnetic field minimum, has been developed based on an absolute instability condition along with systematic expansion procedures. The spatial structure, growth rate and frequency of the localized wave modes predicted by the theory are found to agree well with those of simulation. In this work, we demonstrate that the instabilities can survive even when the magnetic variation is much larger than the Lorentz factor minus one. Furthermore, both the simulation and the theory show that the wave mode can exist where its frequency is smaller than the local harmonic cyclotron frequency, violating the well-known requirement for driving relativistic cyclotron instabilities.
Physics of Plasmas, 2000
The novel physics of relativistic ion cyclotron instabilities is numerically investigated. The gr... more The novel physics of relativistic ion cyclotron instabilities is numerically investigated. The growth rate spectrums and the possibility being absolute instability of two fast ion cases (that the fast ions are energetic proton and alpha particle, respectively) are numerically studied and compared with the analytical theory. The fundamental difference in the characteristics of the instabilities due to a slight change in fast ion mass per nucleon is emphasized; it is determined by the relative normalized mass deficit per nucleon of fast and slow ions, and by the difference of their Lorentz factors. For the energetic proton case, both a cubic instability and a high harmonic quadratic instability can be driven; while, for the energetic alpha particle case, only the quadratic instability can occur at the high alpha cyclotron harmonics in the lower hybrid frequency regime and above; the threshold is determined by the dielectric constant of the slow ion. The peak growth rate is highest at ...
Physics of Plasmas, 1996
The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for fil... more The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for film growth by pulsed-laser deposition of materials, since the magnitude and kinetic energy of the species arriving at the deposition substrate are key processing parameters. Dynamical calculations of plume propagation in vacuum and in background gas have been performed using particle-in-cell hydrodynamics, continuum gasdynamics, and scattering models. Results from these calculations are presented and compared with experimental observations.
Physical Review Letters, 1997
A modeling approach for calculating the expansion of a laser-generated plasma into a background g... more A modeling approach for calculating the expansion of a laser-generated plasma into a background gas has been developed. Although relatively simple in structure, the model gives excellent fits to various experimental data for Si in background gases of He and Ar, including the previously unexplained ''splitting'' of the ablated plume. The model is based on a combination of multiple-scattering and hydrodynamic approaches. It allows the plume to be broken up into components, or scattering orders, whose particles undergo 0, 1, 2,. .. collisions with the background. Particles can only be transferred from one order to the next higher order by collisions. The densities in the individual orders propagate according to the usual conservation equations to give the overall plume expansion. When Ar is the background gas, there is a non-negligible probability that Si plume atoms will reach the detector without undergoing any collisions. This gives rise to a flux component that is undisplaced from that obtained when no background gas is present in addition to the delayed peak from the scattered flux. In Ar only a few orders are necessary for convergence. The behavior in the light gas He is more complex because of the relatively small effect of any one-scattering event and the calculations must be carried out in some cases to as high as the 12th scattering order to find agreement with the experiments.
Physical Review Letters, 1994
A novel anomalous process causing the perpendicular energy of fast ions to be thermalized and los... more A novel anomalous process causing the perpendicular energy of fast ions to be thermalized and lost on average to bulk ion heating, instead of classical slowing down and bulk electron heating, is investigated with PIC simulations. More than half of the fast ions are slowed down to the thermal ion level, although some axe heated to twice their birth energy. The fast ion density perturbation is large. This process is excited by a new two-gyro-stream instability and may continually occur in a burning plasma. The implications for fusion ignition and fast ion confinement are assessed. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States G¢vernment. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or proces,_ disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Physical Review E, 2010
Resonance requires precise synchronization. Surprisingly, relativistic cyclotron instability can ... more Resonance requires precise synchronization. Surprisingly, relativistic cyclotron instability can survive under a magnetic field with its nonuniformity larger than the requirement of synchronism. Localized eigenmode observed in a hybrid simulation is found to be consistent with that predicted by an analytical theory including both profile and eigenvalue. Half of the spatial area of the wave profile is located where the frequency mismatch is negative as against to the positive requirement generally believed. The consequence on the ␣ dynamics is also demonstrated.
MRS Proceedings, 1995
Several physics and computational approaches have been developed to globally characterize phenome... more Several physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed laser deposition of materials. these include thermal models of laser-solid target interactions that initiate the vapor plume; plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms; gas dynamic, hydrodynamic, and collisional descriptions of plume transport; and molecular dynamics models of the interaction of plume particles with the deposition substrate. the complexity of the phenomena involved in the laser ablation process is matched by the diversity of the modeling task, which combines materials science, atomic physics, and plasma physics.
AIP Conference Proceedings, 1994
The growth of an unstable electrostatic ion Bernstein mode in a uniformly magnetized, homogeneous... more The growth of an unstable electrostatic ion Bernstein mode in a uniformly magnetized, homogeneous plasma is simulated by time averaging the equations of motion over a cyclotron period. (AIP)
The relativistic instability of the electromagnetic ion cyclotron wave propagating across the mag... more The relativistic instability of the electromagnetic ion cyclotron wave propagating across the magnetic field [B. Coppi, S. Cowley and R. Kulsrud, Phys. Fluid 29, 4060 (1986)] is investigated. The relativistic instability is reactive, as in contrast to the inversed Landau instability. Due to the wave magnetic field is much larger than the electric field, the dielectric constant of the cold
Physical Review Letters, 1995
A dynamic source effect that accelerates the expansion of laser-ablated material in the direction... more A dynamic source effect that accelerates the expansion of laser-ablated material in the direction perpendicular to the target is demonstrated. A self-similar theory shows that the maximum expansion velocity is proportional to c s ͞a, where 1 2 a is the slope of the velocity profile and c s is the sound speed. Numerical hydrodynamic modeling is in good agreement with the theory. A dynamic partial ionization effect is also studied. With these effects, a is reduced and the maximum expansion velocity is significantly increased over that found from conventional models.
Caused by wave magnetic field and in sharp contrast to ES case, Alfvénic behavior and instability... more Caused by wave magnetic field and in sharp contrast to ES case, Alfvénic behavior and instability transition as the novel characteristics of the EM instabilities are illuminated in detail. The cubic (square) root scaling of the peak growth rate makes the relativistic effect more important than inversed Landau damping, especially for low fast ion density. For the cubic instability, there is a threshold (ceiling) on the slow ion temperature and density (the external magnetic field and the fast ion energy); the Alfvén velocity is required to be low. This Alfvénic behavior is important for its applications. The case of fast protons in thermal deuterons is numerically studied and compared with the analytical results. When the slow ion temperature or density (the external magnetic field or the fast ion energy) is increased (reduced) to be about twice (half) the threshold (ceiling), the cubic instability peak transits to the coupled quadratic instability and a new cubic instability branch ...
Plasma Physics and Controlled Fusion, 2005
The harmonic relativistic ion cyclotron instability driven by Mev α-particles in thermal deuteriu... more The harmonic relativistic ion cyclotron instability driven by Mev α-particles in thermal deuterium plasmas confined by non-uniform magnetic fields is studied with gyrokinetic theory. An integral dispersion equation is derived and solved numerically. The results provide clear evidence for the existence of the instability of α-particle cyclotron high harmonics in burning plasmas of one fast and one slow ion species. The characteristics of the modes are found to depend on magnetic non-uniformity parameter and α-particle density rather strongly.
Physics of Plasmas, 2000
A perturbation theory and a kinetic theory are developed to investigate the novel physics of rela... more A perturbation theory and a kinetic theory are developed to investigate the novel physics of relativistic ion cyclotron instabilities. The existence of the instabilities is determined by the normalized mass deficits per nucleon of fast and slow ions (δmf and δms, respectively), and by their Lorentz factors (γf and γs, respectively); while the ion bunching is caused by the relativistic variation of ion mass. If δmf−δms−γf+γs>0, only a quadratic instability can occur at high cyclotron harmonics of the fast ion in the lower-hybrid frequency regime and above; the threshold on the harmonic number is determined by the dielectric constant of the slow ion. The peak growth rate is higher at the harmonics just above the threshold. If it is negative, both a cubic instability (or instead a coupled quadratic instability if the resonant slow ion cyclotron harmonic is the first harmonic) and the high harmonic quadratic instability can be driven. The cubic instability is due to the harmonic inte...
Physical Review Letters, 1992
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1996
A dynamic source effect was found to accelerate the plume expansion velocity much higher than tha... more A dynamic source effect was found to accelerate the plume expansion velocity much higher than that from a conventional free-expansion model. A self-similar theory and a hydrodynamic model are developed to study this effect, which may help to explain experimentally observed high front expansion velocity. Background gas can also affect the kinetic energies. High background gas may cause the ablated materials to go backward. Experimentally observed plume splitting is also discussed.
IEEE Transactions on Plasma Science, 1990
... An attractive feature of the ICL is that the beam raises the dielectric constant of the ionch... more ... An attractive feature of the ICL is that the beam raises the dielectric constant of the ionchannel relative to its value outside the channel, and thus provides guiding of the ICL radiation. ... Wave guiding by the ion channel is also an interesting effect. ...
Physics of Plasmas, 1994
An explanation, supported by numerical simulations and analytical theory, is given for the harmon... more An explanation, supported by numerical simulations and analytical theory, is given for the harmonic cyclotron emission induced by fast ions in tokamak plasmas—in particular, for the emission observed at low harmonics in deuterium–deuterium and deuterium–tritium experiments in the Joint European Torus [e.g., Phys. Rev. Lett. 60, 33 (1988)]. It is shown that the first proton harmonic, whose field energy amplitude scales as the 0.84 power of the proton density, is one of the highest spectral peaks, whereas the first alpha harmonic is weak. The relative spectral amplitudes of different harmonics are compared. The results are consistent with the experimental observations. The simulations verify that the instabilities are caused by a weak relativistic mass effect. Simulation also shows that a nonuniform magnetic field leads to no appreciable change in the growth rate and saturation amplitude of the waves.
IEEE Conference Record - Abstracts. 1997 IEEE International Conference on Plasma Science
Summary form only given, as follows. A gyrobroadening process causing the perpendicular energy of... more Summary form only given, as follows. A gyrobroadening process causing the perpendicular energy of fast ions to be broadened by cyclotron instabilities due to the weak relativistic mass variation of the fast ions in magnetized plasma is reported. At low cyclotron harmonics, a two-gyro-stream instability can be excited with the involvement of thermal ions if the condition lfωcf <ω<lsωcs is satisfied. The fast ions can be anomalously thermalized. Their lost energy goes to bulk ion heating. While the fast proton can satisfy the condition, the fast alphas cannot. The low harmonics are linearly stable for the alphas. Therefore, for low harmonics one has to inject waves to affect the alpha dynamics. However, the alphas can excite high harmonic cyclotron waves in the lower hybrid regime, in which thermal ions respond as cold plasma. The interaction becomes selective. Only alphas with high pitch angle are involved. This is presented as an interesting collective thermal dynamics issue while the implications for application are also assessed
This is a preprint of a paper being submitted for publication. Contents of this paper should not ... more This is a preprint of a paper being submitted for publication. Contents of this paper should not be quoted or referred to without permission of the author(s) .
Physical Review B, 1999
A self-similar theory and numerical hydrodynamic modeling is developed to investigate the effects... more A self-similar theory and numerical hydrodynamic modeling is developed to investigate the effects of dynamic source and partial ionization on the acceleration of the unsteady expansion of laser-ablated material near a solid target surface. The dynamic source effect accelerates the expansion in the direction perpendicular to the target surface, while the dynamic partial ionization effect accelerates the expansion in all directions. The vaporized material during laser ablation provides a nonadiabatic dynamic source at the target surface into the unsteady expanding fluid. For studying the dynamic source effect, the self-similar theory begins with an assumed profile of plume velocity, uϭv/v m ϭ␣ϩ(1Ϫ␣), where v m is the maximum expansion velocity, ␣ is a constant, and ϭx/v m t. The resultant profiles of plume density and plume temperature are derived. The relations obtained from the conservations of mass, momentum, and energy, respectively, all show that the maximum expansion velocity is inversely proportional to ␣, where 1Ϫ␣ is the slope of plume velocity profile. The numerical hydrodynamic simulation is performed with the Rusanov method and the Newton Raphson method. The profiles and scalings obtained from numerical hydrodynamic modeling are in good agreement with the theory. The dynamic partial ionization requires ionization energy from the heat at the expansion front, and thus reduces the increase of front temperature. The reduction of thermal motion would increase the flow velocity to conserve the momentum. This dynamic partial ionization effect is studied with the numerical hydrodynamic simulation including the Saha equation. With these effects, ␣ is reduced from its value of conventional free expansion. This reduction on ␣ increases the flow velocity slope, decreases the flow velocity near the surface, and reduces the thermal motion of plume, such that the maximum expansion velocity is significantly increased over that found from conventional models. The result may provide an explanation for experimental observations of high-expansion front velocities even at low-laser fluence. ͓S0163-1829͑99͒08835-9͔
Plasma Physics and Controlled Fusion, 2009
Localized electrostatic wave modes due to relativistic ion cyclotron instabilities are observed i... more Localized electrostatic wave modes due to relativistic ion cyclotron instabilities are observed in a hybrid particle-in-cell simulation under a magnetic field with sinusoidal nonuniformity. In order to investigate the physical mechanism of the simulation results, an analytical theory, adopting a parabolic approximation around a magnetic field minimum, has been developed based on an absolute instability condition along with systematic expansion procedures. The spatial structure, growth rate and frequency of the localized wave modes predicted by the theory are found to agree well with those of simulation. In this work, we demonstrate that the instabilities can survive even when the magnetic variation is much larger than the Lorentz factor minus one. Furthermore, both the simulation and the theory show that the wave mode can exist where its frequency is smaller than the local harmonic cyclotron frequency, violating the well-known requirement for driving relativistic cyclotron instabilities.
Physics of Plasmas, 2000
The novel physics of relativistic ion cyclotron instabilities is numerically investigated. The gr... more The novel physics of relativistic ion cyclotron instabilities is numerically investigated. The growth rate spectrums and the possibility being absolute instability of two fast ion cases (that the fast ions are energetic proton and alpha particle, respectively) are numerically studied and compared with the analytical theory. The fundamental difference in the characteristics of the instabilities due to a slight change in fast ion mass per nucleon is emphasized; it is determined by the relative normalized mass deficit per nucleon of fast and slow ions, and by the difference of their Lorentz factors. For the energetic proton case, both a cubic instability and a high harmonic quadratic instability can be driven; while, for the energetic alpha particle case, only the quadratic instability can occur at the high alpha cyclotron harmonics in the lower hybrid frequency regime and above; the threshold is determined by the dielectric constant of the slow ion. The peak growth rate is highest at ...
Physics of Plasmas, 1996
The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for fil... more The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for film growth by pulsed-laser deposition of materials, since the magnitude and kinetic energy of the species arriving at the deposition substrate are key processing parameters. Dynamical calculations of plume propagation in vacuum and in background gas have been performed using particle-in-cell hydrodynamics, continuum gasdynamics, and scattering models. Results from these calculations are presented and compared with experimental observations.
Physical Review Letters, 1997
A modeling approach for calculating the expansion of a laser-generated plasma into a background g... more A modeling approach for calculating the expansion of a laser-generated plasma into a background gas has been developed. Although relatively simple in structure, the model gives excellent fits to various experimental data for Si in background gases of He and Ar, including the previously unexplained ''splitting'' of the ablated plume. The model is based on a combination of multiple-scattering and hydrodynamic approaches. It allows the plume to be broken up into components, or scattering orders, whose particles undergo 0, 1, 2,. .. collisions with the background. Particles can only be transferred from one order to the next higher order by collisions. The densities in the individual orders propagate according to the usual conservation equations to give the overall plume expansion. When Ar is the background gas, there is a non-negligible probability that Si plume atoms will reach the detector without undergoing any collisions. This gives rise to a flux component that is undisplaced from that obtained when no background gas is present in addition to the delayed peak from the scattered flux. In Ar only a few orders are necessary for convergence. The behavior in the light gas He is more complex because of the relatively small effect of any one-scattering event and the calculations must be carried out in some cases to as high as the 12th scattering order to find agreement with the experiments.
Physical Review Letters, 1994
A novel anomalous process causing the perpendicular energy of fast ions to be thermalized and los... more A novel anomalous process causing the perpendicular energy of fast ions to be thermalized and lost on average to bulk ion heating, instead of classical slowing down and bulk electron heating, is investigated with PIC simulations. More than half of the fast ions are slowed down to the thermal ion level, although some axe heated to twice their birth energy. The fast ion density perturbation is large. This process is excited by a new two-gyro-stream instability and may continually occur in a burning plasma. The implications for fusion ignition and fast ion confinement are assessed. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States G¢vernment. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or proces,_ disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Physical Review E, 2010
Resonance requires precise synchronization. Surprisingly, relativistic cyclotron instability can ... more Resonance requires precise synchronization. Surprisingly, relativistic cyclotron instability can survive under a magnetic field with its nonuniformity larger than the requirement of synchronism. Localized eigenmode observed in a hybrid simulation is found to be consistent with that predicted by an analytical theory including both profile and eigenvalue. Half of the spatial area of the wave profile is located where the frequency mismatch is negative as against to the positive requirement generally believed. The consequence on the ␣ dynamics is also demonstrated.