The discharge operation regime of pulse modulated atmospheric radio frequency （RF） glow discharge in helium is investigated on the duty cycle and frequency of modulation pulses. The characteristics of radio frequenc...The discharge operation regime of pulse modulated atmospheric radio frequency （RF） glow discharge in helium is investigated on the duty cycle and frequency of modulation pulses. The characteristics of radio frequency discharge burst in terms of breakdown voltage, alpha（s）- gamma（γ） mode transition voltage and current are demonstrated by the discharge current voltage characteristics. The minimum breakdown voltage of RF discharge burst was obtained at the duty cycle of 20% and frequency of 400 kHz, respectively. The α-γ mode transition of RF discharge burst occurs at higher voltage and current by reducing the duty cycle and elevating the modulation frequency before the RF discharge burst evolving into the ignition phase, in which the RF discharge burst can operate stably in the γ mode. It proposes that the intensity and stability of RF discharge burst can be improved by manipulating the duty cycle and modulation frequency in pulse modulated atmospheric RF glow discharge.展开更多
The linear and nonlinear characteristics of time-resolved photoluminescence （PL） of n-type bulk semiconductor GaAs modulated with terahertz （THz） pulse are studied by using an ensemble Monte Carlo （EMC） method. ...The linear and nonlinear characteristics of time-resolved photoluminescence （PL） of n-type bulk semiconductor GaAs modulated with terahertz （THz） pulse are studied by using an ensemble Monte Carlo （EMC） method. In this paper the center energy valley （Г valley） electron concentration changes with the pulse delay time, sampling time and the outfield are mainly discussed. The results show that the sampling time and the THz field should exceed certain thresholds to effectively excite photoluminescence quenching （PLQ）. Adopting a direct current （DC） field makes the sampling time threshold shortened and the linear range of THz field-modulation PL expanded. Moreover, controlling the sampling time and the outfield intensity can improve the linear quality： with forward time, the larger outfield is used; with backward time, the smaller outfield is used. This study can provide a theoretical basis of THz field linear modulation in a larger range for new light emitting devices.展开更多
The generation of a very strong peak current in the first period(PCFP)in a pulse-modulated microwave discharge has been discussed in previous studies.In this paper we focus on the transition process from a pulsed disc...The generation of a very strong peak current in the first period(PCFP)in a pulse-modulated microwave discharge has been discussed in previous studies.In this paper we focus on the transition process from a pulsed discharge to a fully continuous one driven by the pulsed microwave power source by means of a kinetic model.The computational results show that by increasing the duty cycle or voltage modulation rate(VMR).the discharge eventually becomes fully continuous and PCFP can no longer he observed.In the transition process,the distributions of the electric field,electron energy probability function(EEPF)and plasma density are discussed according to the simulation data,showing different discharge structures.The simulations indicate that many high-energy electrons with electron energy larger than 20eV and low-energy electrons with electron energy less than 3eV could he generated in a pulsed microwave discharge,together with a reversal electric field formed in the anode sheath when PCFP occurs.However,only medium-energy electrons could be observed in a fully continuous discharge.Therefore,by investigating the transition process the pulse-modulated microwave discharges can be further optimized for plasma applications at atmospheric pressure.展开更多
A novel concept of collision avoidance single-photon light detection and ranging（LIDAR） for vehicles has been demonstrated, in which chaotic pulse position modulation is applied on the transmitted laser pulses for r...A novel concept of collision avoidance single-photon light detection and ranging（LIDAR） for vehicles has been demonstrated, in which chaotic pulse position modulation is applied on the transmitted laser pulses for robust anti-crosstalk purposes. Besides, single-photon detectors（SPD） and time correlated single photon counting techniques are adapted, to sense the ultra-low power used for the consideration of compact structure and eye safety. Parameters including pulse rate, discrimination threshold, and number of accumulated pulses have been thoroughly analyzed based on the detection requirements, resulting in specified receiver operating characteristics curves. Both simulation and indoor experiments were performed to verify the excellent anti-crosstalk capability of the presented collision avoidance LIDAR despite ultra-low transmitting power.展开更多
The state space average model and the large signal models of Pulse Skip Modulation （PSM） mode are given in this paper. Farther more, based on these models and simulations of PSM converter circuits, the analysis of t...The state space average model and the large signal models of Pulse Skip Modulation （PSM） mode are given in this paper. Farther more, based on these models and simulations of PSM converter circuits, the analysis of the characteristics of PSM converter is described in this paper, of which include efficiency, frequency spectrum analysis, output voltage ripple, response speed and interference rejection capability. Compared with PWM control mode, PSM converter has high efficiency, especially with fight loads, quick response, good interference rejection and good EMC characteristic. Improved PSM slightly, it could be a kind of good independent regulating mode during the whole operating process for a DC-DC converter. Finally, some experimental results are also presented in this paper.展开更多
By deriving the discrete-time models of a digitally controlled H-bridge inverter system modulated by bipolar sinu- soidal pulse width modulation （BSPWM） and unipolar double-frequency sinusoidal pulse width modulati...By deriving the discrete-time models of a digitally controlled H-bridge inverter system modulated by bipolar sinu- soidal pulse width modulation （BSPWM） and unipolar double-frequency sinusoidal pulse width modulation （UDFSPWM） respectively, the performances of the two modulation strategies are analyzed in detail. The circuit parameters, used in this paper, are fixed. When the systems, modulated by BSPWM and UDFSPWM, have the same switching frequency, the stabil- ity boundaries of the two systems are the same. However, when the equivalent switching frequencies of the two systems are the same, the BSPWM modulated system is more stable than the UDFSPWM modulated system. In addition, a convenient method of establishing the discrete-time model of piecewise smooth system is presented. Finally, the analytical results are confirmed by circuit simulations and experimental measurements.展开更多
From Maxwell＇s equations and Post＇s formalism, a generalized chiral nonlinear Schr6dinger equation （CNLSE） is obtained for the nonlinear chiral fiber. This equation governs light transmission through a dispersive ...From Maxwell＇s equations and Post＇s formalism, a generalized chiral nonlinear Schr6dinger equation （CNLSE） is obtained for the nonlinear chiral fiber. This equation governs light transmission through a dispersive nonlinear chiral fiber with joint action of chirality in linear and nonlinear ways. The generalized CNLSE shows a modu- lation of chirality to the effect of attenuation and nonlinearity compared with the case for a conventional fiber. Simulations based on the split-step beam propagation method reveal the role of nonlinearity with cooperation to chirality playing in the pulse evolution. By adjusting its strength the role of chirality in forming solitons is demonstrated for a given circularly polarized component. The application of nonlinear optical rotation is also discussed in an all-optical switch.展开更多
A new combined soft switching technique and a novel topology of aeronautical static inverter with high frequency pulse dc link are proposed in this paper, namely an electrical isolated converter can realize soft swi...A new combined soft switching technique and a novel topology of aeronautical static inverter with high frequency pulse dc link are proposed in this paper, namely an electrical isolated converter can realize soft switching of two stage cascade converters. It lays technical foundation for high power density, high efficiency and low cost aeronautical static inverter. The operation and design approach of this topology are carefully analyzed and studied. The validity of this topology is verified by simulation and test.展开更多
Vibrations or dither's are features of the PWM servo control system in their steady outputs. On the grounds of analyses and experiments of a PWM pneumatic servo control system, the paper puts forward four varietie...Vibrations or dither's are features of the PWM servo control system in their steady outputs. On the grounds of analyses and experiments of a PWM pneumatic servo control system, the paper puts forward four varieties of PWM modulation methods, and concludes on the relationship between dithers and the different methods, and then discusses the influence of friction to the dithers. Results from experiments regarding the dynamic and static responses on the given system support the theories presented.展开更多
High-power intense optical sources in the mid-to-long wavelength infrared region are very attractive for a wide range of fields from fundamental research to materials science and biology applications.However,there are...High-power intense optical sources in the mid-to-long wavelength infrared region are very attractive for a wide range of fields from fundamental research to materials science and biology applications.However,there are still significant challenges in extending long-wavelength infrared pulses into the relativistic regime using conventional optical techniques.Here,based upon a new type of plasma-based optical method,we present an efficient scheme capable of combining several high-power long-wavelength infrared laser pulses into one single,more intense pulse,thus bringing the intensity of the output pulse to the relativistic regime.Such intense infrared pulses will open up new possibilities for strong-field physics and ultrafast applications.Furthermore,this is beneficial to understand the underlying physics and nonlinear processes of modulation,propagation and energy transfer of high-power intense laser pulses in plasmas.展开更多
A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulsc-tmxlulated(PM)radio-frequency(RF)glow discharge in atmospheric helium assisted by a sub-microsecon...A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulsc-tmxlulated(PM)radio-frequency(RF)glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge.The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge.Under the assistance of pulsed discharge,the electron density in RF discharge burst reaches the magnitude of 1.87 x 1017-3m within 10 RF cycles,accompanied by the formation of sheath structure.It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst.Furthermore,the dynamics of PM RF glow discharge arc demonstrated by the spatiotcmporal evolution of the election density with and without pulsed discharge.The spatial profiles of electron density,electron energy and electric field at specific time instants arc given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.展开更多
Atmospheric pressure pulse-modulated radio-frequency(rf) plasmas have drawn growing attention due to their potential in applications.By selecting appropriate modulation parameters,the diffused and large-volume plasma ...Atmospheric pressure pulse-modulated radio-frequency(rf) plasmas have drawn growing attention due to their potential in applications.By selecting appropriate modulation parameters,the diffused and large-volume plasma can be generated in the pulse-modulated rf plasma with plenty of reactive oxygen species,which is essential for the biomedical application of helium–oxygen plasmas.In this paper,by means of a fluid model,the formation of the peak current in the first period(PCFP) in a pulse-modulated rf helium–oxygen discharge driven by a sinusoidal voltage is discussed,the existence of a reverse field near the anode caused by the negative and positive charges contributes greatly to the mechanism of PCFP.In the simulation,as oxygen admixture increases,the negative ions of O~- and O~-become dominative anions in the sheath region,which can’t be driven to the anode very quickly to build a reverse field,thus the PCFP eventually disappears.This study can effectively enhance the understanding of different transportation behavior of heavy negative ions and electrons,and further optimize pulsemodulated rf discharges with helium–oxygen mixtures in various applications.展开更多
A one-dimensional fluid/Monte-Carlo（MC）hybrid model is developed to describe capacitively coupled SiH_4/Ar discharge,in which the lower electrode is applied by a RF source and pulse modulated by a square-wave,to inv...A one-dimensional fluid/Monte-Carlo（MC）hybrid model is developed to describe capacitively coupled SiH_4/Ar discharge,in which the lower electrode is applied by a RF source and pulse modulated by a square-wave,to investigate the modulation effects of the pulse duty cycle on the discharge mechanism.An electron Monte Carlo simulation is used to calculate the electron energy distribution as a function of position and time phase.Rate coefficients in chemical reactions can then be obtained and transferred to the fluid model for the calculation of electron temperature and densities of different species,such as electrons,ions,and radicals.The simulation results show that,the electron energy distribution f（ε）is modulated evidently within a pulse cycle,with its tail extending to higher energies during the power-on period,while shrinking back promptly in the afterglow period.Thus,the rate coefficients could be controlled during the discharge,resulting in modulation of the species composition on the substrate compared with continuous excitation.Meanwhile,more negative ions,like Si H_3^-and Si H_2^-,may escape to the electrodes owing to the collapse of ambipolar electric fields,which is beneficial to films deposition.Pulse modulation is thus expected to provide additional methods to customize the plasma densities and components.展开更多
As a combination device for a step-up pulse transformer and a magnetic switch,the saturable pulse transformer is widely used in pulsed-power and plasma technology.A fractional-turn ratio saturable pulse transformer is...As a combination device for a step-up pulse transformer and a magnetic switch,the saturable pulse transformer is widely used in pulsed-power and plasma technology.A fractional-turn ratio saturable pulse transformer is constructed and analyzed in this paper.Preliminary experimental results show that if the primary energy storage capacitors are charged to 300 V,an output voltage of about 19 kV can be obtained across the capacitor connected to the secondary windings of a fractional-tum ratio saturable pulse transformer.Theoretical and experimental results reveal that this kind of pulse transformer is not only able to integrate a step-up transformer and a magnetic switch into one device,but can also lower the saturable inductance of its secondary windings,thus leading to the relatively high step-up ratio of the pulse transformer.Meanwhile,the application of the fractional-turn ratio saturable pulse transformer in a μs range pulse modulator as a voltage step-up device and main switch is also included in this paper.The demonstrated experiments display that an output voltage with an amplitude of about 29 kV,and a 1.6 μs pulse width can be obtained across a 3500 Ω resistive load,based on a pulse modulator,if the primary energy storage capacitors are charged to 300 V.This compact fractional-turn ratio saturable pulse transformer can be applied in many other fields such as surface treatment,corona plasma generation and dielectric barrier discharge.展开更多
A retarding field energy analyzer（RFEA） is used to measure the time-averaged ion energy distributions（IEDs） on the substrate in both continuous wave（CW） and synchronous pulse modulated radio-frequency（RF） indu...A retarding field energy analyzer（RFEA） is used to measure the time-averaged ion energy distributions（IEDs） on the substrate in both continuous wave（CW） and synchronous pulse modulated radio-frequency（RF） inductively coupled Ar plasmas（ICPs）.The effects of the phase shift θ between the RF bias voltage and the RF source on the IED is investigated under various discharge conditions.It is found that as θ increases from 0 to π,the IED moves towards the low-energy side,and its energy width becomes narrower.In order to figure out the physical mechanism,the voltage waveforms on the substrate are also measured.The results show that as θ increases from 0 to π,the amplitude of the voltage waveform decreases and,meanwhile,the average sheath potential decreases as well.Specifically,the potential drop in the sheath on the substrate exhibits a maximum value at the same phase（i.e.,θ = 0） and a minimum value at the opposite phase（i.e.,θ = π）.Therefore,when ions traverse across the sheath region above the substrate,they obtain less energies at lower sheath potential drop,leading to lower ion energy.Besides,as θ increases from π to 2π,the IEDs and their energy widths change reversely.展开更多
Driven by pulse modulated radio-frequency plasma in capacitively coupled discharge are studied by source, the behavior of SiH4/N2/02 using a one-dimensional fluid model. Totally, 48 different species （electrons, ions...Driven by pulse modulated radio-frequency plasma in capacitively coupled discharge are studied by source, the behavior of SiH4/N2/02 using a one-dimensional fluid model. Totally, 48 different species （electrons, ions, neutrals, radicals and excited species） are involved in this simulation. Time evolution of the particle densities and electron temperature with different duty cycles are obtained, as well as the electronegativity nsiH-3 /ne of the main negative ion （Sill3 ）. The results show that, by reducing the duty cycle, higher electron temperature and particle density can be achieved for the same average dissipated power, and the ion energy can also be effectively reduced, which will offer evident improvement in plasma deposition processes compared with the case of continuous wave discharge.展开更多
We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation（SPM）, we give the extensive study of the SPM influence on...We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation（SPM）, we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation（MPM） effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.展开更多
In this study,we explored a one-step direct synthesis of NH3 under mild experimental conditions utilizing pulse-modulated microwave plasma technology at atmospheric pressure.At a substantial gas flow rate,a microwave ...In this study,we explored a one-step direct synthesis of NH3 under mild experimental conditions utilizing pulse-modulated microwave plasma technology at atmospheric pressure.At a substantial gas flow rate,a microwave plasma jet was formed and the microwave-assisted ammonia synthesis can be realized.Impacts of various parameters including the gas flow rate,gas component,microwave absorbed power,pulse modulation frequency,and pulse duty cycle on ammonia synthesis were systematically investigated.To indicate the reaction path of ammonia synthesis,the distributions of both the gas temperature and active species were also studied using optical emission spectra technology.It is found that a considerable amount of ammonia was directly synthesized without involvement of any catalysts,the highest ammonia production rate and energy efficiency(EE),up to 2.93μmol·min^(-1) and 6.64×10^(-2)g·(k W·h)^(-1),respectively,were achieved under low microwave power of 84.42 W.The duty cycle has obvious influences on the synthesis efficiency,compared to a duty cycle of 80%,the ammonia synthesis rate,EE and nitrogen conversion decreased by about 22%at a duty cycle of 100%.This finding underscores the significance of incorporating pulse modulation in the microwave discharge process for ammonia synthesis.Furthermore,it was found that vibrational excitation of microwave plasma has a significant driving effect on ammonia synthesis.展开更多
The properties and the design of PWM systems are discussed. The relations among the orifice areas of the valves, initial times and the rate of piston areas are deduced. Also a PWM system is designed and some experimen...The properties and the design of PWM systems are discussed. The relations among the orifice areas of the valves, initial times and the rate of piston areas are deduced. Also a PWM system is designed and some experiments are done. The experiment results agree with those of theory analyses. It can be shown that the relations are correct and the conclusion of theory analyses is reasonable.展开更多
基金supported by National Natural Science Foundation of China(Nos.11475043 and 11375042)
文摘The discharge operation regime of pulse modulated atmospheric radio frequency （RF） glow discharge in helium is investigated on the duty cycle and frequency of modulation pulses. The characteristics of radio frequency discharge burst in terms of breakdown voltage, alpha（s）- gamma（γ） mode transition voltage and current are demonstrated by the discharge current voltage characteristics. The minimum breakdown voltage of RF discharge burst was obtained at the duty cycle of 20% and frequency of 400 kHz, respectively. The α-γ mode transition of RF discharge burst occurs at higher voltage and current by reducing the duty cycle and elevating the modulation frequency before the RF discharge burst evolving into the ignition phase, in which the RF discharge burst can operate stably in the γ mode. It proposes that the intensity and stability of RF discharge burst can be improved by manipulating the duty cycle and modulation frequency in pulse modulated atmospheric RF glow discharge.
基金supported by the National Natural Science Foundation of China(Grant Nos.11574105,61475054,61405063,and 61177095)the Hubei Science and Technology Agency Project,China(Grant No.2015BCE052)the Fundamental Research Funds for the Central Universities,China(Grant No.2017KFYXJJ029)
文摘The linear and nonlinear characteristics of time-resolved photoluminescence （PL） of n-type bulk semiconductor GaAs modulated with terahertz （THz） pulse are studied by using an ensemble Monte Carlo （EMC） method. In this paper the center energy valley （Г valley） electron concentration changes with the pulse delay time, sampling time and the outfield are mainly discussed. The results show that the sampling time and the THz field should exceed certain thresholds to effectively excite photoluminescence quenching （PLQ）. Adopting a direct current （DC） field makes the sampling time threshold shortened and the linear range of THz field-modulation PL expanded. Moreover, controlling the sampling time and the outfield intensity can improve the linear quality： with forward time, the larger outfield is used; with backward time, the smaller outfield is used. This study can provide a theoretical basis of THz field linear modulation in a larger range for new light emitting devices.
文摘The generation of a very strong peak current in the first period(PCFP)in a pulse-modulated microwave discharge has been discussed in previous studies.In this paper we focus on the transition process from a pulsed discharge to a fully continuous one driven by the pulsed microwave power source by means of a kinetic model.The computational results show that by increasing the duty cycle or voltage modulation rate(VMR).the discharge eventually becomes fully continuous and PCFP can no longer he observed.In the transition process,the distributions of the electric field,electron energy probability function(EEPF)and plasma density are discussed according to the simulation data,showing different discharge structures.The simulations indicate that many high-energy electrons with electron energy larger than 20eV and low-energy electrons with electron energy less than 3eV could he generated in a pulsed microwave discharge,together with a reversal electric field formed in the anode sheath when PCFP occurs.However,only medium-energy electrons could be observed in a fully continuous discharge.Therefore,by investigating the transition process the pulse-modulated microwave discharges can be further optimized for plasma applications at atmospheric pressure.
基金Project supported by Tsinghua University Initiative Scientific Research Program,China(Grant No.2014z21035)
文摘A novel concept of collision avoidance single-photon light detection and ranging（LIDAR） for vehicles has been demonstrated, in which chaotic pulse position modulation is applied on the transmitted laser pulses for robust anti-crosstalk purposes. Besides, single-photon detectors（SPD） and time correlated single photon counting techniques are adapted, to sense the ultra-low power used for the consideration of compact structure and eye safety. Parameters including pulse rate, discrimination threshold, and number of accumulated pulses have been thoroughly analyzed based on the detection requirements, resulting in specified receiver operating characteristics curves. Both simulation and indoor experiments were performed to verify the excellent anti-crosstalk capability of the presented collision avoidance LIDAR despite ultra-low transmitting power.
基金Supported by the National Natural Science Foundation of China (No.60436030)
文摘The state space average model and the large signal models of Pulse Skip Modulation （PSM） mode are given in this paper. Farther more, based on these models and simulations of PSM converter circuits, the analysis of the characteristics of PSM converter is described in this paper, of which include efficiency, frequency spectrum analysis, output voltage ripple, response speed and interference rejection capability. Compared with PWM control mode, PSM converter has high efficiency, especially with fight loads, quick response, good interference rejection and good EMC characteristic. Improved PSM slightly, it could be a kind of good independent regulating mode during the whole operating process for a DC-DC converter. Finally, some experimental results are also presented in this paper.
基金supported by the National Natural Science Foundation of China (Grant No. 51277146)the Foundation of Delta Science,Technologythe Education Development Program for Power Electronics (Grant No. DREG2011003)
文摘By deriving the discrete-time models of a digitally controlled H-bridge inverter system modulated by bipolar sinu- soidal pulse width modulation （BSPWM） and unipolar double-frequency sinusoidal pulse width modulation （UDFSPWM） respectively, the performances of the two modulation strategies are analyzed in detail. The circuit parameters, used in this paper, are fixed. When the systems, modulated by BSPWM and UDFSPWM, have the same switching frequency, the stabil- ity boundaries of the two systems are the same. However, when the equivalent switching frequencies of the two systems are the same, the BSPWM modulated system is more stable than the UDFSPWM modulated system. In addition, a convenient method of establishing the discrete-time model of piecewise smooth system is presented. Finally, the analytical results are confirmed by circuit simulations and experimental measurements.
基金Supported by the National Natural Science Foundation of China under Grant No 60977032the Program for Innovation Research of Science of Harbin Institute of Technology(PIRS-HIT)under Grant No T201407
文摘From Maxwell＇s equations and Post＇s formalism, a generalized chiral nonlinear Schr6dinger equation （CNLSE） is obtained for the nonlinear chiral fiber. This equation governs light transmission through a dispersive nonlinear chiral fiber with joint action of chirality in linear and nonlinear ways. The generalized CNLSE shows a modu- lation of chirality to the effect of attenuation and nonlinearity compared with the case for a conventional fiber. Simulations based on the split-step beam propagation method reveal the role of nonlinearity with cooperation to chirality playing in the pulse evolution. By adjusting its strength the role of chirality in forming solitons is demonstrated for a given circularly polarized component. The application of nonlinear optical rotation is also discussed in an all-optical switch.
文摘A new combined soft switching technique and a novel topology of aeronautical static inverter with high frequency pulse dc link are proposed in this paper, namely an electrical isolated converter can realize soft switching of two stage cascade converters. It lays technical foundation for high power density, high efficiency and low cost aeronautical static inverter. The operation and design approach of this topology are carefully analyzed and studied. The validity of this topology is verified by simulation and test.
文摘Vibrations or dither's are features of the PWM servo control system in their steady outputs. On the grounds of analyses and experiments of a PWM pneumatic servo control system, the paper puts forward four varieties of PWM modulation methods, and concludes on the relationship between dithers and the different methods, and then discusses the influence of friction to the dithers. Results from experiments regarding the dynamic and static responses on the given system support the theories presented.
基金the National Postdoctoral Program for Innovative Talents of China(No.BX20220206)。
文摘High-power intense optical sources in the mid-to-long wavelength infrared region are very attractive for a wide range of fields from fundamental research to materials science and biology applications.However,there are still significant challenges in extending long-wavelength infrared pulses into the relativistic regime using conventional optical techniques.Here,based upon a new type of plasma-based optical method,we present an efficient scheme capable of combining several high-power long-wavelength infrared laser pulses into one single,more intense pulse,thus bringing the intensity of the output pulse to the relativistic regime.Such intense infrared pulses will open up new possibilities for strong-field physics and ultrafast applications.Furthermore,this is beneficial to understand the underlying physics and nonlinear processes of modulation,propagation and energy transfer of high-power intense laser pulses in plasmas.
文摘A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulsc-tmxlulated(PM)radio-frequency(RF)glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge.The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge.Under the assistance of pulsed discharge,the electron density in RF discharge burst reaches the magnitude of 1.87 x 1017-3m within 10 RF cycles,accompanied by the formation of sheath structure.It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst.Furthermore,the dynamics of PM RF glow discharge arc demonstrated by the spatiotcmporal evolution of the election density with and without pulsed discharge.The spatial profiles of electron density,electron energy and electric field at specific time instants arc given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.
基金supported by National Natural Science Foundation of China(No.11975142)。
文摘Atmospheric pressure pulse-modulated radio-frequency(rf) plasmas have drawn growing attention due to their potential in applications.By selecting appropriate modulation parameters,the diffused and large-volume plasma can be generated in the pulse-modulated rf plasma with plenty of reactive oxygen species,which is essential for the biomedical application of helium–oxygen plasmas.In this paper,by means of a fluid model,the formation of the peak current in the first period(PCFP) in a pulse-modulated rf helium–oxygen discharge driven by a sinusoidal voltage is discussed,the existence of a reverse field near the anode caused by the negative and positive charges contributes greatly to the mechanism of PCFP.In the simulation,as oxygen admixture increases,the negative ions of O~- and O~-become dominative anions in the sheath region,which can’t be driven to the anode very quickly to build a reverse field,thus the PCFP eventually disappears.This study can effectively enhance the understanding of different transportation behavior of heavy negative ions and electrons,and further optimize pulsemodulated rf discharges with helium–oxygen mixtures in various applications.
基金supported by National Natural Science Foundation of China(No.11275038)
文摘A one-dimensional fluid/Monte-Carlo（MC）hybrid model is developed to describe capacitively coupled SiH_4/Ar discharge,in which the lower electrode is applied by a RF source and pulse modulated by a square-wave,to investigate the modulation effects of the pulse duty cycle on the discharge mechanism.An electron Monte Carlo simulation is used to calculate the electron energy distribution as a function of position and time phase.Rate coefficients in chemical reactions can then be obtained and transferred to the fluid model for the calculation of electron temperature and densities of different species,such as electrons,ions,and radicals.The simulation results show that,the electron energy distribution f（ε）is modulated evidently within a pulse cycle,with its tail extending to higher energies during the power-on period,while shrinking back promptly in the afterglow period.Thus,the rate coefficients could be controlled during the discharge,resulting in modulation of the species composition on the substrate compared with continuous excitation.Meanwhile,more negative ions,like Si H_3^-and Si H_2^-,may escape to the electrodes owing to the collapse of ambipolar electric fields,which is beneficial to films deposition.Pulse modulation is thus expected to provide additional methods to customize the plasma densities and components.
基金supported by National Natural Science Foundation of China under Grant Nos.51477177 and 51641707
文摘As a combination device for a step-up pulse transformer and a magnetic switch,the saturable pulse transformer is widely used in pulsed-power and plasma technology.A fractional-turn ratio saturable pulse transformer is constructed and analyzed in this paper.Preliminary experimental results show that if the primary energy storage capacitors are charged to 300 V,an output voltage of about 19 kV can be obtained across the capacitor connected to the secondary windings of a fractional-tum ratio saturable pulse transformer.Theoretical and experimental results reveal that this kind of pulse transformer is not only able to integrate a step-up transformer and a magnetic switch into one device,but can also lower the saturable inductance of its secondary windings,thus leading to the relatively high step-up ratio of the pulse transformer.Meanwhile,the application of the fractional-turn ratio saturable pulse transformer in a μs range pulse modulator as a voltage step-up device and main switch is also included in this paper.The demonstrated experiments display that an output voltage with an amplitude of about 29 kV,and a 1.6 μs pulse width can be obtained across a 3500 Ω resistive load,based on a pulse modulator,if the primary energy storage capacitors are charged to 300 V.This compact fractional-turn ratio saturable pulse transformer can be applied in many other fields such as surface treatment,corona plasma generation and dielectric barrier discharge.
基金Project supported by the Important National Science and Technology Specific Project,China(Grant No.2011ZX02403-001)the National Natural Science Foundation of China(Grand No.11675039)the Fundamental Research Funds for the Central Universities,China(Grand No.DUT16LK06)
文摘A retarding field energy analyzer（RFEA） is used to measure the time-averaged ion energy distributions（IEDs） on the substrate in both continuous wave（CW） and synchronous pulse modulated radio-frequency（RF） inductively coupled Ar plasmas（ICPs）.The effects of the phase shift θ between the RF bias voltage and the RF source on the IED is investigated under various discharge conditions.It is found that as θ increases from 0 to π,the IED moves towards the low-energy side,and its energy width becomes narrower.In order to figure out the physical mechanism,the voltage waveforms on the substrate are also measured.The results show that as θ increases from 0 to π,the amplitude of the voltage waveform decreases and,meanwhile,the average sheath potential decreases as well.Specifically,the potential drop in the sheath on the substrate exhibits a maximum value at the same phase（i.e.,θ = 0） and a minimum value at the opposite phase（i.e.,θ = π）.Therefore,when ions traverse across the sheath region above the substrate,they obtain less energies at lower sheath potential drop,leading to lower ion energy.Besides,as θ increases from π to 2π,the IEDs and their energy widths change reversely.
基金supported by National Natural Science Foundation of China (No.10775025)Important National Science & Technology Specific Project of China (No.2011ZX02403-001)Program for New Century Excellent Talents in University of China (NCET-08-0073)
文摘Driven by pulse modulated radio-frequency plasma in capacitively coupled discharge are studied by source, the behavior of SiH4/N2/02 using a one-dimensional fluid model. Totally, 48 different species （electrons, ions, neutrals, radicals and excited species） are involved in this simulation. Time evolution of the particle densities and electron temperature with different duty cycles are obtained, as well as the electronegativity nsiH-3 /ne of the main negative ion （Sill3 ）. The results show that, by reducing the duty cycle, higher electron temperature and particle density can be achieved for the same average dissipated power, and the ion energy can also be effectively reduced, which will offer evident improvement in plasma deposition processes compared with the case of continuous wave discharge.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11204328,61221064,61078037,11127901,11134010,and 61205208)the National Basic Research Program of China(Grant No.2011CB808101)the Natural Science Foundation of Shanghai,China(Grant No.13ZR1414800)
文摘We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation（SPM）, we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation（MPM） effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.
基金supported by National Natural Science Foundation of China(Nos.52077026,51977023,52177126 and 12475253)the Fundamental Research Funds for the Central Universities(No.DUT23YG227)the Dalian Life&Health Guiding Project(No.2023ZXYG34)。
文摘In this study,we explored a one-step direct synthesis of NH3 under mild experimental conditions utilizing pulse-modulated microwave plasma technology at atmospheric pressure.At a substantial gas flow rate,a microwave plasma jet was formed and the microwave-assisted ammonia synthesis can be realized.Impacts of various parameters including the gas flow rate,gas component,microwave absorbed power,pulse modulation frequency,and pulse duty cycle on ammonia synthesis were systematically investigated.To indicate the reaction path of ammonia synthesis,the distributions of both the gas temperature and active species were also studied using optical emission spectra technology.It is found that a considerable amount of ammonia was directly synthesized without involvement of any catalysts,the highest ammonia production rate and energy efficiency(EE),up to 2.93μmol·min^(-1) and 6.64×10^(-2)g·(k W·h)^(-1),respectively,were achieved under low microwave power of 84.42 W.The duty cycle has obvious influences on the synthesis efficiency,compared to a duty cycle of 80%,the ammonia synthesis rate,EE and nitrogen conversion decreased by about 22%at a duty cycle of 100%.This finding underscores the significance of incorporating pulse modulation in the microwave discharge process for ammonia synthesis.Furthermore,it was found that vibrational excitation of microwave plasma has a significant driving effect on ammonia synthesis.
文摘The properties and the design of PWM systems are discussed. The relations among the orifice areas of the valves, initial times and the rate of piston areas are deduced. Also a PWM system is designed and some experiments are done. The experiment results agree with those of theory analyses. It can be shown that the relations are correct and the conclusion of theory analyses is reasonable.
