2019 Vol. 21 No. 11
2019, 21(11): 115101.
DOI: 10.1088/2058-6272/ab2ff6
Abstract：
The advanced tokamak scenario is a promising operation scenario for ITER and fusion neutron sources. In this scenario the minimum value of the safety factor in the center of the plasma exceeds unity. In the compact spherical tokamak Globus-M, the formation of such conditions is possible with neutral beam injection at the current ramp-up phase. Due to the slower diffusion of current inside the plasma, a zone is formed with reduced heat and particle transport across the magnetic field, which affects the temperature and density profiles of the plasma. This leads to the peaked density profile formation and improvement of the energy confinement time. To achieve a high fraction of the bootstrap current, it is necessary to increase the plasma pressure. At the same time, the maximum allowable pressure is limited to the normalized beta limit.
The advanced tokamak scenario is a promising operation scenario for ITER and fusion neutron sources. In this scenario the minimum value of the safety factor in the center of the plasma exceeds unity. In the compact spherical tokamak Globus-M, the formation of such conditions is possible with neutral beam injection at the current ramp-up phase. Due to the slower diffusion of current inside the plasma, a zone is formed with reduced heat and particle transport across the magnetic field, which affects the temperature and density profiles of the plasma. This leads to the peaked density profile formation and improvement of the energy confinement time. To achieve a high fraction of the bootstrap current, it is necessary to increase the plasma pressure. At the same time, the maximum allowable pressure is limited to the normalized beta limit.
2019, 21(11): 115102.
DOI: 10.1088/2058-6272/ab3c83
Abstract：
A conservative scheme of kinetic electrons for gyrokinetic simulations in the presence of magnetic islands has been implemented and verified in the gyrokinetic toroidal code, where zonal and nonzonal components of all perturbed quantities are solved together. Using this new conservative scheme, linear simulation of kinetic ballooning mode has been successfully benchmarked with the electromagnetic hybrid model. Simulations of nonlinear interactions between magnetic islands and the ion temperature gradient (ITG) mode in a tokamak show that the islands rotate at the electron diamagnetic drift velocity. The linear ITG structure shifts from the island O-point toward the X-point due to the pressure flattening effect inside the islands, and the nonlinear ITG structure peaks along the magnetic island separatrix because of the increased pressure gradient there.
A conservative scheme of kinetic electrons for gyrokinetic simulations in the presence of magnetic islands has been implemented and verified in the gyrokinetic toroidal code, where zonal and nonzonal components of all perturbed quantities are solved together. Using this new conservative scheme, linear simulation of kinetic ballooning mode has been successfully benchmarked with the electromagnetic hybrid model. Simulations of nonlinear interactions between magnetic islands and the ion temperature gradient (ITG) mode in a tokamak show that the islands rotate at the electron diamagnetic drift velocity. The linear ITG structure shifts from the island O-point toward the X-point due to the pressure flattening effect inside the islands, and the nonlinear ITG structure peaks along the magnetic island separatrix because of the increased pressure gradient there.
2019, 21(11): 115401.
DOI: 10.1088/2058-6272/ab3248
Abstract：
UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X) (0–0) band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source. The effect of the addition of molecular gases N2 and O2 to He plasma jets on OH generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species. Stark broadening of the hydrogen Balmer emission line (Hβ) was used to estimate the electron density ne in the jets. For both He/N2 and He/O2 jets, ne was estimated to be on the order of 1015 cm−3. The effects of plasma power and gas flow rate were also studied. With increase in N2 and O2 flow rates, ne tended to decrease. Gas temperature in the He/O2 plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets. The highest OH densities in the He/N2 and He/O2 plasma jets were determined to be 1.0× 1016 molecules/cm3 at x=4 mm (from the jet orifice) and 1.8×1016 molecules/cm3 at x=3 mm, respectively. Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions. The presence of strong emissions of the N+2 bands in both He/N2 and He/O2 plasma jets, as against the absence of the N+2 emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of N+2 in these He plasma jets.
UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X) (0–0) band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source. The effect of the addition of molecular gases N2 and O2 to He plasma jets on OH generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species. Stark broadening of the hydrogen Balmer emission line (Hβ) was used to estimate the electron density ne in the jets. For both He/N2 and He/O2 jets, ne was estimated to be on the order of 1015 cm−3. The effects of plasma power and gas flow rate were also studied. With increase in N2 and O2 flow rates, ne tended to decrease. Gas temperature in the He/O2 plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets. The highest OH densities in the He/N2 and He/O2 plasma jets were determined to be 1.0× 1016 molecules/cm3 at x=4 mm (from the jet orifice) and 1.8×1016 molecules/cm3 at x=3 mm, respectively. Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions. The presence of strong emissions of the N+2 bands in both He/N2 and He/O2 plasma jets, as against the absence of the N+2 emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of N+2 in these He plasma jets.
2019, 21(11): 115402.
DOI: 10.1088/2058-6272/ab2ef2
Abstract：
Floating potential fluctuations of glow discharge magnetized plasma are found to expose mixed mode oscillations (MMOs) in the existence of plasma bubble. Plasma bubble has been formed by emerging density gradient in the form of a sheath around a cylindrical and spherical grid to a critical value of applied potential. Two Langmuir probes, LP1 and LP2, are retained in the ambient plasma to collect the plasma floating potential fluctuations at two different locations of the plasma system. The perceived instability pattern shows regular-irregular-regular MMOs under various imposed conditions. Furthermore, various nonlinear techniques such as phase space plot, recurrence plot and Hurst exponent have been executed to understand the underlying dynamical behavior of the system. Low-frequency (∼200–1200 Hz) oscillations are also supposed and are inferred as ion-acoustic waves excited by ionization instability. The observed results are then validated with the theory of the instability based on a fluid hydrodynamic approach.
Floating potential fluctuations of glow discharge magnetized plasma are found to expose mixed mode oscillations (MMOs) in the existence of plasma bubble. Plasma bubble has been formed by emerging density gradient in the form of a sheath around a cylindrical and spherical grid to a critical value of applied potential. Two Langmuir probes, LP1 and LP2, are retained in the ambient plasma to collect the plasma floating potential fluctuations at two different locations of the plasma system. The perceived instability pattern shows regular-irregular-regular MMOs under various imposed conditions. Furthermore, various nonlinear techniques such as phase space plot, recurrence plot and Hurst exponent have been executed to understand the underlying dynamical behavior of the system. Low-frequency (∼200–1200 Hz) oscillations are also supposed and are inferred as ion-acoustic waves excited by ionization instability. The observed results are then validated with the theory of the instability based on a fluid hydrodynamic approach.
2019, 21(11): 115403.
DOI: 10.1088/2058-6272/ab2b5a
Abstract：
In this paper, a honeycomb structure jet array with seven jet units was adopted to generate plasmas. Both the average discharge power and the emission intensity of the main excited species increase with increasing applied voltage. There are three stages of discharge evolution at different applied voltages: initial discharge, uniform discharge and strong coupling discharge. The spatial distribution of the emission intensity of the excited species can be divided into three categories: growth class, weakening class and variation class. The gas temperature along the whole plasma plume at different applied voltages is maintained at around 320 K and can be widely used in heat-labile applications.
In this paper, a honeycomb structure jet array with seven jet units was adopted to generate plasmas. Both the average discharge power and the emission intensity of the main excited species increase with increasing applied voltage. There are three stages of discharge evolution at different applied voltages: initial discharge, uniform discharge and strong coupling discharge. The spatial distribution of the emission intensity of the excited species can be divided into three categories: growth class, weakening class and variation class. The gas temperature along the whole plasma plume at different applied voltages is maintained at around 320 K and can be widely used in heat-labile applications.
2019, 21(11): 115404.
DOI: 10.1088/2058-6272/ab3275
Abstract：
The research herein examined the results of numerical simulations of the positive column of a glow discharge in argon dusty plasma using COMSOL Multiphysics software under conditions similar to the project known as PK-4. Various scenarios dealing with formations of spatial distributions of densities and fluxes for charged particles were studied, and evaluations of the influence of dust particles on the discharge were obtained in a wide range of dust densities. Two extreme cases were distinguished: weak dust influence when the densities, fluxes and electric field profiles are not perturbed, and strong dust influence when all three density profiles (electrons, ions and charged dust) in the dust cloud are similar (parallel) to each other, resulting in all created charges in the dust cloud being lost inside the cloud. In such a case, the ambipolar field and the transport of charged particles are decreased in the dust cloud, and any ambipolar flux is almost absent within the cloud.
The research herein examined the results of numerical simulations of the positive column of a glow discharge in argon dusty plasma using COMSOL Multiphysics software under conditions similar to the project known as PK-4. Various scenarios dealing with formations of spatial distributions of densities and fluxes for charged particles were studied, and evaluations of the influence of dust particles on the discharge were obtained in a wide range of dust densities. Two extreme cases were distinguished: weak dust influence when the densities, fluxes and electric field profiles are not perturbed, and strong dust influence when all three density profiles (electrons, ions and charged dust) in the dust cloud are similar (parallel) to each other, resulting in all created charges in the dust cloud being lost inside the cloud. In such a case, the ambipolar field and the transport of charged particles are decreased in the dust cloud, and any ambipolar flux is almost absent within the cloud.
2019, 21(11): 115501.
DOI: 10.1088/2058-6272/ab21a2
Abstract：
The use of atmospheric rotating gliding arc (RGA) plasma is proposed as a facile, scalable and catalyst-free approach to synthesizing hydrogen (H2) and graphene sheets from coalbed methane (CBM). CH4 is used as a CBM surrogate. Based on a previous investigation of discharge properties, product distribution and energy efficiency, the operating parameters such as CH4 concentration, applied voltage and gas flow rate can effectively affect the CH4 conversion rate, the selectivity of H2 and the properties of solid generated carbon. Nevertheless, the basic properties of RGA plasma and its role in CH4 conversion are scarcely mentioned. In the present work, a 3D RGA model, with a detailed nonequilibrium CH4/Ar plasma chemistry, is developed to validate the previous experiments on CBM conversion, aiming in particular at the distribution of H2 and other gas products. Our results demonstrate that the dynamics of RGA is derived from the joint effects of electron convection, electron migration and electron diffusion, and is prominently determined by the variation of the gas flow rate and applied voltage. Subsequently, a combined experimental and chemical kinetical simulation is performed to analyze the selectivity of gas products in an RGA reaction, taking into consideration the formation and loss pathways of crucial targeted substances (such as CH4, C2H2, H2 and H radicals) and corresponding contribution rates. Additionally, the effects of operating conditions on the properties of solid products are investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results show that increasing the applied voltage and decreasing CH4 concentration will change the solid carbon from its initial spherical structure into folded multilayer graphene sheets, while the size of the graphene sheets is slightly affected by the change in gas flow rate.
The use of atmospheric rotating gliding arc (RGA) plasma is proposed as a facile, scalable and catalyst-free approach to synthesizing hydrogen (H2) and graphene sheets from coalbed methane (CBM). CH4 is used as a CBM surrogate. Based on a previous investigation of discharge properties, product distribution and energy efficiency, the operating parameters such as CH4 concentration, applied voltage and gas flow rate can effectively affect the CH4 conversion rate, the selectivity of H2 and the properties of solid generated carbon. Nevertheless, the basic properties of RGA plasma and its role in CH4 conversion are scarcely mentioned. In the present work, a 3D RGA model, with a detailed nonequilibrium CH4/Ar plasma chemistry, is developed to validate the previous experiments on CBM conversion, aiming in particular at the distribution of H2 and other gas products. Our results demonstrate that the dynamics of RGA is derived from the joint effects of electron convection, electron migration and electron diffusion, and is prominently determined by the variation of the gas flow rate and applied voltage. Subsequently, a combined experimental and chemical kinetical simulation is performed to analyze the selectivity of gas products in an RGA reaction, taking into consideration the formation and loss pathways of crucial targeted substances (such as CH4, C2H2, H2 and H radicals) and corresponding contribution rates. Additionally, the effects of operating conditions on the properties of solid products are investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results show that increasing the applied voltage and decreasing CH4 concentration will change the solid carbon from its initial spherical structure into folded multilayer graphene sheets, while the size of the graphene sheets is slightly affected by the change in gas flow rate.
2019, 21(11): 115502.
DOI: 10.1088/2058-6272/ab3938
Abstract：
In this paper, the influences of gas doping (O2, N2, Air) on the concentrations of reactive species and bactericidal effects induced by a He plasma jet are studied. Firstly, results show that gas doping causes an increase in voltage and a decrease in current compared with the pure He discharge under the same discharge power, which might be attributed to the different chemical characteristics of O2 and N2 and verified by the changes in the gaseous reactive species shown in the optical emission spectroscopy (OES) and Fourier transform infrared (FTIR) spectroscopy. Secondly, the concentrations of aqueous reactive oxygen species (ROS) and reactive nitrogen species (RNS) are tightly related to the addition of O2 and N2 into the working gas. The concentrations of aqueous NO2- and NO3- significantly increase while the concentrations of aqueous ROS decrease with the admixture of N2. The addition of O2 has little effect on the concentrations of NO2- and NO3- and pH values; however, the addition of O2 increases the concentration of O2- and deceases the concentrations of H2O2 and OH. Finally, the results of bactericidal experiments demonstrate that the inactivation efficiency of the four types of plasma jets is He+O2>He+Air>He>He+N2, which is in accordance with the changing trend of the concentration of aqueous O2-. Simultaneously to the better understanding of the formation and removal mechanisms of reactive species in the plasma–liquid interaction, these results also prove the effectiveness of regulating the concentrations of aqueous reactive species and the bacteria inactivation effects by gas doping.
In this paper, the influences of gas doping (O2, N2, Air) on the concentrations of reactive species and bactericidal effects induced by a He plasma jet are studied. Firstly, results show that gas doping causes an increase in voltage and a decrease in current compared with the pure He discharge under the same discharge power, which might be attributed to the different chemical characteristics of O2 and N2 and verified by the changes in the gaseous reactive species shown in the optical emission spectroscopy (OES) and Fourier transform infrared (FTIR) spectroscopy. Secondly, the concentrations of aqueous reactive oxygen species (ROS) and reactive nitrogen species (RNS) are tightly related to the addition of O2 and N2 into the working gas. The concentrations of aqueous NO2- and NO3- significantly increase while the concentrations of aqueous ROS decrease with the admixture of N2. The addition of O2 has little effect on the concentrations of NO2- and NO3- and pH values; however, the addition of O2 increases the concentration of O2- and deceases the concentrations of H2O2 and OH. Finally, the results of bactericidal experiments demonstrate that the inactivation efficiency of the four types of plasma jets is He+O2>He+Air>He>He+N2, which is in accordance with the changing trend of the concentration of aqueous O2-. Simultaneously to the better understanding of the formation and removal mechanisms of reactive species in the plasma–liquid interaction, these results also prove the effectiveness of regulating the concentrations of aqueous reactive species and the bacteria inactivation effects by gas doping.
2019, 21(11): 115503.
DOI: 10.1088/2058-6272/ab3668
Abstract：
An efficient toluene removal in air using a plasma photocatalytic system (PPS) not only needs favorable surface reactions over photocatalysts under the action of plasma, but also requires the photocatalysts to efficiently absorb light emitted from the discharge for driving the photocatalytic reactions. We report here that the PPS constructed by integrating a black titania (B-TiO2) photocatalyst with a dielectric barrier discharge (DBD) can effectively remove toluene with above 70% CO2 selectivity and remarkably reduced the concentration of secondary pollutants of ozone and nitrogen oxides at a specific energy input of 1500 J·l−1, while exhibiting good stability. Photocatalyst characterizations suggest that the B-TiO2 provides a high concentration of oxygen vacancies for the surface oxidation of toluene in DBD, and efficiently absorbs ultraviolet–visible light emitted from the discharge to induce plasma photocatalytic oxidation of toluene. The presence of B-TiO2 in the plasma region also results in a high discharge efficiency, facilitating the generation of large numbers of reactive species and thus the oxidation of toluene towards CO2. The greatly enhanced performance of the PPS integrated with B-TiO2 in toluene removal offers a promising approach to efficiently remove refractory volatile organic compounds from air at low temperatures.
An efficient toluene removal in air using a plasma photocatalytic system (PPS) not only needs favorable surface reactions over photocatalysts under the action of plasma, but also requires the photocatalysts to efficiently absorb light emitted from the discharge for driving the photocatalytic reactions. We report here that the PPS constructed by integrating a black titania (B-TiO2) photocatalyst with a dielectric barrier discharge (DBD) can effectively remove toluene with above 70% CO2 selectivity and remarkably reduced the concentration of secondary pollutants of ozone and nitrogen oxides at a specific energy input of 1500 J·l−1, while exhibiting good stability. Photocatalyst characterizations suggest that the B-TiO2 provides a high concentration of oxygen vacancies for the surface oxidation of toluene in DBD, and efficiently absorbs ultraviolet–visible light emitted from the discharge to induce plasma photocatalytic oxidation of toluene. The presence of B-TiO2 in the plasma region also results in a high discharge efficiency, facilitating the generation of large numbers of reactive species and thus the oxidation of toluene towards CO2. The greatly enhanced performance of the PPS integrated with B-TiO2 in toluene removal offers a promising approach to efficiently remove refractory volatile organic compounds from air at low temperatures.
2019, 21(11): 115504.
DOI: 10.1088/2058-6272/ab373d
Abstract：
A new method for liquefying coal using dielectric barrier discharge plasma has been studied. By utilizing waste oil as the solvent and processing coal nanopowder in the plasma for 10 min, we have attained a liquid yield of more than 80%. The experiment shows that not only the coal nanopowder promoted the liquefaction process, but hydrogen radicals improved the liquid yield effectively. In the plasma processing, the phenomenon of the changing color of the nanopowder solution and not producing a solid residue has been obviously observed. The rational parameters that affected the liquefaction of coal nanopowder have been achieved through the experiment, and the liquefied products have been analyzed.
A new method for liquefying coal using dielectric barrier discharge plasma has been studied. By utilizing waste oil as the solvent and processing coal nanopowder in the plasma for 10 min, we have attained a liquid yield of more than 80%. The experiment shows that not only the coal nanopowder promoted the liquefaction process, but hydrogen radicals improved the liquid yield effectively. In the plasma processing, the phenomenon of the changing color of the nanopowder solution and not producing a solid residue has been obviously observed. The rational parameters that affected the liquefaction of coal nanopowder have been achieved through the experiment, and the liquefied products have been analyzed.
2019, 21(11): 115601.
DOI: 10.1088/2058-6272/ab341d
Abstract：
A digital pulse analysis system is an important diagnostic system in nuclear physics experimental research. In response to the demand for reflecting the particle state in a nuclear physics experiment, we have designed and developed a real-time digital pulse analysis system and applied it to the digital nuclear pulse waveform discrimination of different detectors in the HL- 2M tokamak. The system is based on the peripheral component interconnect extensions for instrumentation (PXI) platform, while its software was written in LABVIEW. The key technologies involved in the system implementation include digital pulse analysis technology, digital discrimination technology, pulse height analysis technology, etc. The system has been applied to the plastic scintillator detector at the Neutron Source Lab of the University of Science and Technology of China. And the experimental results indicate that the system can discriminate between neutron (n) particles and gamma (γ) particles well when used to measure the plastic scintillator detector.
A digital pulse analysis system is an important diagnostic system in nuclear physics experimental research. In response to the demand for reflecting the particle state in a nuclear physics experiment, we have designed and developed a real-time digital pulse analysis system and applied it to the digital nuclear pulse waveform discrimination of different detectors in the HL- 2M tokamak. The system is based on the peripheral component interconnect extensions for instrumentation (PXI) platform, while its software was written in LABVIEW. The key technologies involved in the system implementation include digital pulse analysis technology, digital discrimination technology, pulse height analysis technology, etc. The system has been applied to the plastic scintillator detector at the Neutron Source Lab of the University of Science and Technology of China. And the experimental results indicate that the system can discriminate between neutron (n) particles and gamma (γ) particles well when used to measure the plastic scintillator detector.
