In a tokamak fusion reactor operated at steady state,the equilibrium magnetic field is likely to have reversed shear in the core region,as the noninductive bootstrap current profile generally peaks off-axis.The revers...In a tokamak fusion reactor operated at steady state,the equilibrium magnetic field is likely to have reversed shear in the core region,as the noninductive bootstrap current profile generally peaks off-axis.The reversed shear Alfvén eigenmode(RSAE)as a unique branch of the shear Alfvén wave in this equilibrium,can exist with a broad spectrum in wavenumber and frequency,and be resonantly driven unstable by energetic particles(EP).After briefly discussing the RSAE linear properties in burning plasma condition,we review several key topics of the nonlinear dynamics for the RSAE through both wave-EP resonance and wave-wave coupling channels,and illustrate their potentially important role in reactor-scale fusion plasmas.By means of simplified hybrid MHD-kinetic simulations,the RSAEs are shown to have typically broad phase space resonance structure with both circulating and trapped EP,as results of weak/vanishing magnetic shear and relatively low frequency.Through the route of wave-EP nonlinearity,the dominant saturation mechanism is mainly due to the transported resonant EP radially decoupling with the localized RSAE mode structure,and the resultant EP transport generally has a convective feature.The saturated RSAEs also undergo various nonlinear couplings with other collective oscillations.Two typical routes as parametric decay and modulational instability are studied using nonlinear gyrokinetic theory,and applied to the scenario of spontaneous excitation by a finite amplitude pump RSAE.Multiple RSAEs could naturally couple and induce the spectral energy cascade into a low frequency Alfvénic mode,which may effectively transfer the EP energy to fuel ions via collisionless Landau damping.Moreover,zero frequency zonal field structure could be spontaneously excited by modulation of the pump RSAE envelope,and may also lead to saturation of the pump RSAE by both scattering into stable domain and local distortion of the continuum structure.展开更多
The shallow-water temperature profile is typically parameterized using a few empirical orthogonal function(EOF)coefficients.However,when the experimental area is poorly known or highly variable,the adaptability of the...The shallow-water temperature profile is typically parameterized using a few empirical orthogonal function(EOF)coefficients.However,when the experimental area is poorly known or highly variable,the adaptability of the EOFs will be significantly reduced.In this study,a new set of basis functions,generated by combining the internal-wave eigenmodes with the average temperature gradient,is developed for characterizing the temperature perturbations.Temperature profiles recorded by a thermistor chain in the South China Sea in 2015 are processed and analyzed.Compared to the EOFs,the new set of basis functions has higher reconstruction accuracy and adaptability;it is also more stable in ocean regions that have internal waves.展开更多
Reverse-sheared Alfv6n eigenmodes （RSAEs） have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency swee...Reverse-sheared Alfv6n eigenmodes （RSAEs） have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from -80 kHz to -110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about ρ = 0.37-0.46, just around the position of qmin with ρ -0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called ＇grand cascades＇, was also clearly observed on the ECE signal at ρ = 0.46. During the phase, a transient internal transport barrier （ITB） in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of qmin. A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease or qmin increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation.展开更多
Nonlinear evolution of multiple toroidal Alfven eigenmodes(TAEs) driven by fast ions is self-consistently investigated by kinetic simulations in toroidal plasmas.To clearly identify the effect of nonlinear coupling on...Nonlinear evolution of multiple toroidal Alfven eigenmodes(TAEs) driven by fast ions is self-consistently investigated by kinetic simulations in toroidal plasmas.To clearly identify the effect of nonlinear coupling on the beam ion loss,simulations over single-n modes are also carried out and compared with those over multiple-n modes,and the wave-particle resonance and particle trajectory of lost ions in phase space are analyzed in detail.It is found that in the multiple-n case,the resonance overlap occurs so that the fast ion loss level is rather higher than the sum loss level that represents the summation of loss over all single-n modes in the single-n case.Moreover,increasing fast ion beta β_h can not only significantly increase the loss level in the multiple-n case but also significantly increase the loss level increment between the single-n and multiple-n cases.For example,the loss level in the multiple-n case for β_h=6.0% can even reach 13% of the beam ions and is 44% higher than the sum loss level calculated from all individual single-n modes in the single-n case.On the other hand,when the closely spaced resonance overlap occurs in the multiple-n case,the release of mode energy is increased so that the widely spaced resonances can also take place.In addition,phase space characterization is obtained in both single-n and multiple-n cases.展开更多
Discrete Alfven eigenmodes in steady-state operation scenarios with negative magnetic shear in the international thermonuclear experimental reactor are investigated in this paper. These magnetohydrodynamic eigenmodes ...Discrete Alfven eigenmodes in steady-state operation scenarios with negative magnetic shear in the international thermonuclear experimental reactor are investigated in this paper. These magnetohydrodynamic eigenmodes are trapped by the s-induced potential wells along the magnetic field line. Here α = -q2Rdβ/dr with q being the safety factor, the ratio between plasma and magnetic pressures, and R the major radius, and r the minor radius. Due to negligible continuum damping via wave energy tunneling, these Alfven eigenmodes could be readily destabilized by energetic particles.展开更多
In this paper, NOVA/NOVA-K codes are used to investigate the stability of Alfvén eigenmodes(AEs) in the China Fusion Engineering Test Reactor(CFETR). Firstly, the stability of AEs excited by energetic alpha parti...In this paper, NOVA/NOVA-K codes are used to investigate the stability of Alfvén eigenmodes(AEs) in the China Fusion Engineering Test Reactor(CFETR). Firstly, the stability of AEs excited by energetic alpha particles is investigated. For the fully non-inductive scenario, it is found that all AEs are stable, and the least stable toroidal mode number is n= 8. However, for the hybrid mode scenario, it is found that many AEs are unstable, and the least stable toroidal mode numbers are n= 7, 8. Secondly, the effect of energetic alpha-particle parameters and beam ions on AE stability is also presented. The threshold of the least stable AE is about βcrit,α = 1.12%,crit,less than the value of alpha-particle beta(βα=1.34%). The result demonstrates that the AEs excited by alpha particles are weakly unstable. The effect of the beam ions on AE stability is found to be very weak in CFETR.展开更多
In the presence of energetic particles(EPs)from auxiliary heating and burning plasmas,fishbone instability and Alfvén modes can be excited and their transition can take place in certain overlapping regimes.Using ...In the presence of energetic particles(EPs)from auxiliary heating and burning plasmas,fishbone instability and Alfvén modes can be excited and their transition can take place in certain overlapping regimes.Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code,we have identified such a transition between the fishbone instability and theβ-induced Alfvén eigenmode(BAE)for the NBI heated plasmas on HL-2 A.When the safety factor at magnetic axis is well below one,typical kink-fishbone transition occurs as the EP fraction increases.When q0 is raised to approaching one,the fishbone mode is replaced with BAE for sufficient amount of EPs.When q0 is slightly above one,the toroidicity-induced Alfvén eigenmode dominates at lower EP pressure,whereas BAE dominates at higher EP pressure.展开更多
The hybrid scenario is a projection for CFETR operation with high plasma current and density.Therefore, the energetic particles(EPs) generated by fusion reactions can destabilize Alfvén eigenmodes(AEs), which cou...The hybrid scenario is a projection for CFETR operation with high plasma current and density.Therefore, the energetic particles(EPs) generated by fusion reactions can destabilize Alfvén eigenmodes(AEs), which could result in significant EPs loss and redistribution. Both the eigenvalue code NOVA-K and the wrapped local stability code TGLFEP are used to analyze AE stability. The simulation indicates the beta-induced Alfvén eigenmodes with n?>?5 in the core region are the most unstable. The NOVA-K code is used to benchmark the critical density gradient calculated by TGLFEP. In addition, the EPtran code is employed to predict EP transport induced by unstable AEs and turbulence, which reduce EP density in the core and drive approximately 30% EP transport from the core to the edge, thus the EP density profile flattens and EPs with lower energy deposit near the edge.展开更多
Bulk ion heating rate from nonlinear Landau damping of high mode number Toroidal Alfven Eigenmodes （TAEs） is calculated in the frame work of weak turbulence theory. The heating rate is lower than the nonlinear spect...Bulk ion heating rate from nonlinear Landau damping of high mode number Toroidal Alfven Eigenmodes （TAEs） is calculated in the frame work of weak turbulence theory. The heating rate is lower than the nonlinear spectral transfer rate to more stable modes, but relatively insensitive to the details of linear damping mechanisms.展开更多
In the large helical device （LHD） having three dimensional configuration, Alfven eigenmodes （AEs） destabilized by energetic ions are widely investigated using neutral beam heated plasmas with monotonic and non-mon...In the large helical device （LHD） having three dimensional configuration, Alfven eigenmodes （AEs） destabilized by energetic ions are widely investigated using neutral beam heated plasmas with monotonic and non-monotonic rotational transform （l/2π） profiles. In a plasma with monotonic l/2π-profile, core-localized toroidicity-induced Alfven eigenmode （TAE） as well as global one are often observed. With the increase in the averaged toroidal beta value, defined as the ratio of total plasma pressure to toroidal magnetic pressure, core-localized TAE with low toroidal mode number becomes global. In a relatively high beta plasma with monotonic l/2π-profile, two TAEs with different toroidal mode number often interact nonlinearly and generate another modes through three wave coupling. In a plasma with non-monotonic l/2π-profile generated by intense counter neutral beam current drive, reversed shear Alfven eigenmode （RSAE） and geodesic acoustic mode （CAM） excited by energetic ions were observed for the first time in a helical plasma. Nonlinear coupling was also observed between RSAE and GAM.展开更多
It was found that there are multiplicity of low shear toroidicity-induced Alfv′en eigenmodes in a zero beta limit if the inverse aspect ratio is larger than the magnetic shear at the mode location(Candy 1996 Phys. Le...It was found that there are multiplicity of low shear toroidicity-induced Alfv′en eigenmodes in a zero beta limit if the inverse aspect ratio is larger than the magnetic shear at the mode location(Candy 1996 Phys. Lett. A 215 299). Because the reversed shear Alfv′en eigenmode(RSAE) and even the RSAE associated with the non-circular triangularity-induced Alfv′en eigenmode(NAE) gap(NAE–RSAE) usually reside near the shear-reversal point, the condition that the inverse aspect ratio is larger than the magnetic shear is naturally satisfied. For this reason, we numerically investigate the existence of multiplicity of core-localized NAE–RSAEs and mode characteristics in the present work. We firstly verify the existence of the multiplicity for zero beta plasma by using a D-shaped equilibrium. It is pointed out that, for a given toroidal mode number, the Alfv′en cascade spectrum accommodates down-sweeping and up-sweeping modes above and below the NAE range of frequencies. An analytical model for the existence of multiple RSAE modes is in good agreement with the simulation results. One notices that the triangularity has a greater effect on the odd-type modes than that on the even-type modes: the odd-type modes come into existence because of the plasma triangularity.展开更多
Characterizing the gap eigenmode of shear Alfv′en waves(SAWs) and its interaction with energetic ions is important to the success of magnetically confined fusion. Previous studies have reported an experimental observ...Characterizing the gap eigenmode of shear Alfv′en waves(SAWs) and its interaction with energetic ions is important to the success of magnetically confined fusion. Previous studies have reported an experimental observation of the spectral gap of SAW on the on Large Plasma Device(LAPD)(Zhang et al. 2008 Phys. Plasmas 15 012103), a linear large plasma device(Gekelman et al. 1991 Rev. Sci. Instrum. 62 2875) possessing easier diagnostic access and lower cost compared with traditional fusion devices, and analytical theory and numerical gap eigenmode using ideal conditions(Chang 2014 Ph.D Thesis at Australian National University). To guide experimental implementation, the present work models the gap eigenmode of SAWs using exact LAPD parameters. A full picture of the wave field for previous experiment reveals that the previously observed spectral gap is not global but an axially local result. To form a global spectral gap, the number of magnetic mirrors has to be increased and stronger static magnetic field makes it clearer. Such a spectral gap is obtained for the magnetic field of B0(z) = 1.2 + 0.6 cos[2π(z-33.68)/3.63] with 7.74-m magnetic beach. By introducing two types of local defects(corresponding to Eθ(z0) = 0 and E’θ(z0) = 0 respectively), odd-parity and even-parity discrete eigenmodes are formed clearly inside the gap. The strength of these gap eigenmodes decreases significantly with collision frequency, which is consistent with previous studies. Parameter scans show that these gap eigenmodes can be even formed successfully for the field strength of B0(z) = 0.2 + 0.1 cos[2π(z-33.68)/3.63] and with only four magnetic mirrors, which are achievable by the LAPD at its present status. This work can serve as a strong motivation and direct reference for the experimental implementation of the gap eigenmode of SAWs on the LAPD and other linear plasma devices.展开更多
In this work,the gyrokinetic eigenvalue code LIGKA,the drift-kinetic/MHD hybrid code HMGC and the gyrokinetic full-f code TRIMEG-GKX are employed to study the mode structure details of reversed shear Alfvén eigen...In this work,the gyrokinetic eigenvalue code LIGKA,the drift-kinetic/MHD hybrid code HMGC and the gyrokinetic full-f code TRIMEG-GKX are employed to study the mode structure details of reversed shear Alfvén eigenmodes(RSAEs).Using the parameters from an ASDEXUpgrade plasma,a benchmark with the three different physical models for RSAE without and with energetic particles(EPs)is carried out.Reasonable agreement has been found for the mode frequency and the growth rate.Mode structure symmetry breaking(MSSB)is observed when EPs are included,due to the EPs’non-perturbative effects.It is found that the MSSB properties are featured by a finite radial wave phase velocity,and the linear mode structure can be well described by an analytical complex Gaussian expressionФ(s)=e^(-σ(s-s_(0))^(2))with complex parametersσand s_(0),where s is the normalized radial coordinate.The mode structure is distorted in opposite manners when the EP drive shifted from one side of qminto the other side,and specifically,a non-zero average radial wave number with opposite signs is generated.The initial EP density profiles and the corresponding mode structures have been used as the input of HAGIS code to study the EP transport.The parallel velocity of EPs is generated in opposite directions,due to different values of the average radial wave number,corresponding to different initial EP density profiles with EP drive shifted away from the qmin.展开更多
Alfvnic gap eigenmode(AGE) can eject energetic particles from confinement and thereby threaten the success of magnetically controlled fusion. A low-temperature plasma cylinder is a promising candidate to study this ei...Alfvnic gap eigenmode(AGE) can eject energetic particles from confinement and thereby threaten the success of magnetically controlled fusion. A low-temperature plasma cylinder is a promising candidate to study this eigenmode, due to easy diagnostic access and simple geometry, and the idea is to arrange a periodic array of magnetic mirrors along the plasma cylinder and introduce a local defect to break the field periodicity. The present work validates this idea by reproducing a clear AGE inside a spectral gap, and more importantly details the influence of the number and depth(or modulation factor)of magnetic mirror on the characteristics of AGE. Results show that AGE is suppressed by other modes inside the spectral gap when the number of magnetic mirrors is below a certain value, which leads to a weakened Bragg’s effect. The structure and frequency of AGE remain unchanged for a decreased number of magnetic mirrors, as long as this number is enough for the AGE formation. The width of spectral gap and decay constant(inverse of decay length) of AGE are linearly proportional to the depth of magnetic mirror, implying easier observation of AGE through a bigger mirror depth. The frequency of AGE shifts to a lower range with the depth increased, possibly due to the unfrozen plasma with field line and the invalidity of small-perturbation analysis. Nevertheless, it is exciting to find that the depth of field modulation can be increased to form AGE for a very limited number of magnetic mirrors. This is of particular interest for the experimental implementation of AGE on a low-temperature plasma cylinder with limited length.展开更多
Global linear gyrokinetic simulations using realistic DIII-D tokamak geometry and plasma profiles find co-existence of unstable reversed shear Alfvén eigenmodes(RSAE)with low toroidal mode number n and electromag...Global linear gyrokinetic simulations using realistic DIII-D tokamak geometry and plasma profiles find co-existence of unstable reversed shear Alfvén eigenmodes(RSAE)with low toroidal mode number n and electromagnetic ion temperature gradient(ITG)instabilities with higher toroidal mode number n.For intermediate n?=?[10,12],RSAE and ITG co-exist and overlap weakly in the radial domain with similar growth rates but different real frequencies.Both RSAE and ITG growth rates decrease less than 5%when compressible magnetic perturbations are neglected in the simulations.The ITG growth rates increase less than 7%when fast ions are not included in the simulations.Finally,the effects of trapped electrons on the RSAE are negligible.展开更多
Beam-ion losses induced by fast-ion-driven toroidal Alfven eigenmodes （TAE） were measured with a scintillator-based lost fast-ion probe （SLIP） in the large helical device （LHD）. The SLIP gave simultaneously the ...Beam-ion losses induced by fast-ion-driven toroidal Alfven eigenmodes （TAE） were measured with a scintillator-based lost fast-ion probe （SLIP） in the large helical device （LHD）. The SLIP gave simultaneously the energy E and the pitch angle X=arccos（v///v） distribution of the lost fast ions. The loss fluxes were investigated for three typical magnetic configurations of Rax-vac=3.60 m, 3.75 m. and 3.90 m, where Rax-vac is the magnetic axis position of the vacuum field. Dominant losses induced by TAEs in these three configurations were observed in the E/X regions of 50-190 keV/40°, 40-170 keV/25°, and 30-190 keV/30°, respectively. Lost-ion fluxes induced by TAEs depend clearly on the amplitude of TAE magnetic fluctuations, Rax-vac and the toroidal field strength Bt. The increment of the loss fluxes has the dependence of （bTAE/Bt）s. The power s increases from s = 1 to 3 with the increase of the magnetic axis position in finite beta plasmas.展开更多
The cross section of photonic crystal fiber (PCF) is a two dimensional photonic crystal. The rods formed in PCF are not exact cylinders, the shape of rods will affect the eigenmode formed in two dimensional photonic c...The cross section of photonic crystal fiber (PCF) is a two dimensional photonic crystal. The rods formed in PCF are not exact cylinders, the shape of rods will affect the eigenmode formed in two dimensional photonic crystals around a defect. Based on the relations between the defect eigenmodes and the radius of dielectric cylinders, the defect eigenmodes in photonic crystals in which the ellipse rods take the place of cylinders are studied by numerical calculation. The analysis of the relation between the eigenfrequency and the minor axis radius of ellipse rods show that the defect eigenfrequency is controlled by the cross section area of rods and the distribution of electromagnetic field around the defect is also affected by the cross section shape of rods. It provides a better way to modify the distribution of electromagnetic fields in photonic crystal and keeps the eigenfrequency unchanged.展开更多
Spatial modes have received substantial attention over the last decades and are used in optical communication applications.In fiber-optic communications,the employed linearly polarized modes and phase vortex modes car...Spatial modes have received substantial attention over the last decades and are used in optical communication applications.In fiber-optic communications,the employed linearly polarized modes and phase vortex modes carrying orbital angular momentum can be synthesized by fiber vector eigenmodes.To improve the transmission capacity and miniaturize the communication system,straightforward fiber vector eigenmode multiplexing and generation of fiber-eigenmode-like polarization vortices(vector vortex modes)using photonic integrated devices are of substantial interest.Here,we propose and demonstrate direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters.By exploiting vector vortex modes(radially and azimuthally polarized beams)generated from silicon microring resonators etched with angular gratings,we report data-carrying fiber vector eigenmode multiplexing transmission through a 2-km large-core fiber,showing low-level mode crosstalk and favorable link performance.These demonstrations may open up added capacity scaling opportunities by directly accessing multiple vector eigenmodes in the fiber and provide compact solutions to replace bulky diffractive optical elements for generating various optical vector beams.展开更多
The behavior of self-polarization emission in Nd∶Y_(3)Al_(5)O_(12)(YAG)=Cr^(4+)∶YAG lasers has been proved in some cases.However,the degree and direction of polarization were often sensitive and unstable.We experime...The behavior of self-polarization emission in Nd∶Y_(3)Al_(5)O_(12)(YAG)=Cr^(4+)∶YAG lasers has been proved in some cases.However,the degree and direction of polarization were often sensitive and unstable.We experimentally observed different beam profiles versus the angle of the polarizer relative to the polarization direction of the laser.In order to explore the polarization mechanism,the dynamics of intracavity polarized eigenmodes was analyzed theoretically.Simulative results were well consistent with our experimental observations.It indicated that the linear self-polarization emission was a composite state rather than an intrinsic state.This study contributed to the improvement of the polarization stability in Nd∶YAG=Cr^(4+)∶YAG passively Q-switched lasers.展开更多
This paper investigates the eigenmode optimization problem governed by the scalar Helmholtz equation in continuum system in which the computed eigenmode approaches the prescribed eigenmode in the whole domain.The firs...This paper investigates the eigenmode optimization problem governed by the scalar Helmholtz equation in continuum system in which the computed eigenmode approaches the prescribed eigenmode in the whole domain.The first variation for the eigenmode optimization problem is evaluated by the quadratic penalty method,the adjoint variable method,and the formula based on sensitivity analysis.A penalty optimization algorithm is proposed,in which the density evolution is accomplished by introducing an artificial time term and solving an additional ordinary differential equation.The validity of the presented algorithm is confirmed by numerical results of the first and second eigenmode optimizations in 1D and 2D problems.展开更多
基金supported by National Natural Science Foundation of China (Nos. 12205251, 12275236 and 12261131622)Italian Ministry for Foreign Affairs and International Cooperation Project (No. CN23GR02)+2 种基金the National Key Research and Development Program of China (Nos. 2019YFE03020003 and 2017YFE0301900)Users of Excellence program of Hefei Science Center CAS (No. 2021HSC-UE016)funded by the European Union via the Euratom Research and Training Programme (No. 101052200–EUROfusion)
文摘In a tokamak fusion reactor operated at steady state,the equilibrium magnetic field is likely to have reversed shear in the core region,as the noninductive bootstrap current profile generally peaks off-axis.The reversed shear Alfvén eigenmode(RSAE)as a unique branch of the shear Alfvén wave in this equilibrium,can exist with a broad spectrum in wavenumber and frequency,and be resonantly driven unstable by energetic particles(EP).After briefly discussing the RSAE linear properties in burning plasma condition,we review several key topics of the nonlinear dynamics for the RSAE through both wave-EP resonance and wave-wave coupling channels,and illustrate their potentially important role in reactor-scale fusion plasmas.By means of simplified hybrid MHD-kinetic simulations,the RSAEs are shown to have typically broad phase space resonance structure with both circulating and trapped EP,as results of weak/vanishing magnetic shear and relatively low frequency.Through the route of wave-EP nonlinearity,the dominant saturation mechanism is mainly due to the transported resonant EP radially decoupling with the localized RSAE mode structure,and the resultant EP transport generally has a convective feature.The saturated RSAEs also undergo various nonlinear couplings with other collective oscillations.Two typical routes as parametric decay and modulational instability are studied using nonlinear gyrokinetic theory,and applied to the scenario of spontaneous excitation by a finite amplitude pump RSAE.Multiple RSAEs could naturally couple and induce the spectral energy cascade into a low frequency Alfvénic mode,which may effectively transfer the EP energy to fuel ions via collisionless Landau damping.Moreover,zero frequency zonal field structure could be spontaneously excited by modulation of the pump RSAE envelope,and may also lead to saturation of the pump RSAE by both scattering into stable domain and local distortion of the continuum structure.
基金The Natural Science Foundation of Shandong Province of China under contract Nos ZR2022MA051 and ZR2020MA090the Fund of China Postdoctoral Science Foundation under contract No.2020M670891+1 种基金the Shandong University of Science and Technology Research Fund under contract No.2019TDJH103the Talent Introduction Plan for Youth Innovation Team in Universities of Shandong Province(Innovation Team of Satellite Positioning and Navigation).
文摘The shallow-water temperature profile is typically parameterized using a few empirical orthogonal function(EOF)coefficients.However,when the experimental area is poorly known or highly variable,the adaptability of the EOFs will be significantly reduced.In this study,a new set of basis functions,generated by combining the internal-wave eigenmodes with the average temperature gradient,is developed for characterizing the temperature perturbations.Temperature profiles recorded by a thermistor chain in the South China Sea in 2015 are processed and analyzed.Compared to the EOFs,the new set of basis functions has higher reconstruction accuracy and adaptability;it is also more stable in ocean regions that have internal waves.
基金supported by the National Key R&D Program of China (No. 2014GB106004)National Natural Science Foundation of China (Nos. 11605235, 11675211, 11505221)Scientific Research Grant of Hefei Science Center of CAS (No. 2015SRG-HSC010)
文摘Reverse-sheared Alfv6n eigenmodes （RSAEs） have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from -80 kHz to -110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about ρ = 0.37-0.46, just around the position of qmin with ρ -0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called ＇grand cascades＇, was also clearly observed on the ECE signal at ρ = 0.46. During the phase, a transient internal transport barrier （ITB） in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of qmin. A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease or qmin increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation.
基金Project supported by the National Key R&D Program of China(Grant No.2017YFE0301900)the National Natural Science Foundation of China(Grant No.11675083)the Fundamental Research Funds for the Central Universities of China(Grant No.DUT18ZD101).
文摘Nonlinear evolution of multiple toroidal Alfven eigenmodes(TAEs) driven by fast ions is self-consistently investigated by kinetic simulations in toroidal plasmas.To clearly identify the effect of nonlinear coupling on the beam ion loss,simulations over single-n modes are also carried out and compared with those over multiple-n modes,and the wave-particle resonance and particle trajectory of lost ions in phase space are analyzed in detail.It is found that in the multiple-n case,the resonance overlap occurs so that the fast ion loss level is rather higher than the sum loss level that represents the summation of loss over all single-n modes in the single-n case.Moreover,increasing fast ion beta β_h can not only significantly increase the loss level in the multiple-n case but also significantly increase the loss level increment between the single-n and multiple-n cases.For example,the loss level in the multiple-n case for β_h=6.0% can even reach 13% of the beam ions and is 44% higher than the sum loss level calculated from all individual single-n modes in the single-n case.On the other hand,when the closely spaced resonance overlap occurs in the multiple-n case,the release of mode energy is increased so that the widely spaced resonances can also take place.In addition,phase space characterization is obtained in both single-n and multiple-n cases.
基金supported by the National Natural Science Foundation of China (Grant No.10675037)
文摘Discrete Alfven eigenmodes in steady-state operation scenarios with negative magnetic shear in the international thermonuclear experimental reactor are investigated in this paper. These magnetohydrodynamic eigenmodes are trapped by the s-induced potential wells along the magnetic field line. Here α = -q2Rdβ/dr with q being the safety factor, the ratio between plasma and magnetic pressures, and R the major radius, and r the minor radius. Due to negligible continuum damping via wave energy tunneling, these Alfven eigenmodes could be readily destabilized by energetic particles.
基金supported by National Natural Science Foundation of China(Nos.11947056 and 12005100)the Natural Science Foundation of Hunan Province(No.2020JJ5468)+2 种基金the Opening Project of Cooperative Innovation Center for Nuclear Fuel Cycle Technology and Equipment,University of South China(No.2019KFY15)the Hunan Nuclear Fusion International Science and Technology Innovation Cooperation Base(No.2018WK4009)the Key Scientific Research Program of Education Department of Hunan Province(No.20A417).
文摘In this paper, NOVA/NOVA-K codes are used to investigate the stability of Alfvén eigenmodes(AEs) in the China Fusion Engineering Test Reactor(CFETR). Firstly, the stability of AEs excited by energetic alpha particles is investigated. For the fully non-inductive scenario, it is found that all AEs are stable, and the least stable toroidal mode number is n= 8. However, for the hybrid mode scenario, it is found that many AEs are unstable, and the least stable toroidal mode numbers are n= 7, 8. Secondly, the effect of energetic alpha-particle parameters and beam ions on AE stability is also presented. The threshold of the least stable AE is about βcrit,α = 1.12%,crit,less than the value of alpha-particle beta(βα=1.34%). The result demonstrates that the AEs excited by alpha particles are weakly unstable. The effect of the beam ions on AE stability is found to be very weak in CFETR.
基金supported by the National Magnetic Confinement Fusion Science Program of China(No.2019YFE03050004)National Natural Science Foundation of China(Nos.11875253,11775221,51821005)+3 种基金the Fundamental Research Funds for the Central Universities at University of Science and Technology of China(No.WK3420000004)Huazhong University of Science and Technology(No.2019kfy XJJS193)the Collaborative Innovation Program of Hefei Science Center,CAS(No.2019HSC-CIP015)the U.S.Department of Energy(Nos.DE-FG02-86ER53218 and DE-SC0018001)。
文摘In the presence of energetic particles(EPs)from auxiliary heating and burning plasmas,fishbone instability and Alfvén modes can be excited and their transition can take place in certain overlapping regimes.Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code,we have identified such a transition between the fishbone instability and theβ-induced Alfvén eigenmode(BAE)for the NBI heated plasmas on HL-2 A.When the safety factor at magnetic axis is well below one,typical kink-fishbone transition occurs as the EP fraction increases.When q0 is raised to approaching one,the fishbone mode is replaced with BAE for sufficient amount of EPs.When q0 is slightly above one,the toroidicity-induced Alfvén eigenmode dominates at lower EP pressure,whereas BAE dominates at higher EP pressure.
基金supported by National Natural Science Foundation of China (Grant No. 11535013)the National Key Research and Development Program of China (Grant Nos. 2017YFA0402500 and 2018YFE0302101)
文摘The hybrid scenario is a projection for CFETR operation with high plasma current and density.Therefore, the energetic particles(EPs) generated by fusion reactions can destabilize Alfvén eigenmodes(AEs), which could result in significant EPs loss and redistribution. Both the eigenvalue code NOVA-K and the wrapped local stability code TGLFEP are used to analyze AE stability. The simulation indicates the beta-induced Alfvén eigenmodes with n?>?5 in the core region are the most unstable. The NOVA-K code is used to benchmark the critical density gradient calculated by TGLFEP. In addition, the EPtran code is employed to predict EP transport induced by unstable AEs and turbulence, which reduce EP density in the core and drive approximately 30% EP transport from the core to the edge, thus the EP density profile flattens and EPs with lower energy deposit near the edge.
基金supported by the Seoul National University Research GrantR&D Program through the National Fusion Research Institute of Korea(NFRI) Funded by the Government Funds
文摘Bulk ion heating rate from nonlinear Landau damping of high mode number Toroidal Alfven Eigenmodes （TAEs） is calculated in the frame work of weak turbulence theory. The heating rate is lower than the nonlinear spectral transfer rate to more stable modes, but relatively insensitive to the details of linear damping mechanisms.
基金LHD project budget of Japan (NIFS08ULHH508)the Grant-in-aid for Scientific Research from MEST of Japan (No.16082209)the JSPS-CAS Core-University Program in the field of Plasma and Nuclear Fusion
文摘In the large helical device （LHD） having three dimensional configuration, Alfven eigenmodes （AEs） destabilized by energetic ions are widely investigated using neutral beam heated plasmas with monotonic and non-monotonic rotational transform （l/2π） profiles. In a plasma with monotonic l/2π-profile, core-localized toroidicity-induced Alfven eigenmode （TAE） as well as global one are often observed. With the increase in the averaged toroidal beta value, defined as the ratio of total plasma pressure to toroidal magnetic pressure, core-localized TAE with low toroidal mode number becomes global. In a relatively high beta plasma with monotonic l/2π-profile, two TAEs with different toroidal mode number often interact nonlinearly and generate another modes through three wave coupling. In a plasma with non-monotonic l/2π-profile generated by intense counter neutral beam current drive, reversed shear Alfven eigenmode （RSAE） and geodesic acoustic mode （CAM） excited by energetic ions were observed for the first time in a helical plasma. Nonlinear coupling was also observed between RSAE and GAM.
基金Project supported by the National Natural Science Foundation of China(Grant No.11675222)
文摘It was found that there are multiplicity of low shear toroidicity-induced Alfv′en eigenmodes in a zero beta limit if the inverse aspect ratio is larger than the magnetic shear at the mode location(Candy 1996 Phys. Lett. A 215 299). Because the reversed shear Alfv′en eigenmode(RSAE) and even the RSAE associated with the non-circular triangularity-induced Alfv′en eigenmode(NAE) gap(NAE–RSAE) usually reside near the shear-reversal point, the condition that the inverse aspect ratio is larger than the magnetic shear is naturally satisfied. For this reason, we numerically investigate the existence of multiplicity of core-localized NAE–RSAEs and mode characteristics in the present work. We firstly verify the existence of the multiplicity for zero beta plasma by using a D-shaped equilibrium. It is pointed out that, for a given toroidal mode number, the Alfv′en cascade spectrum accommodates down-sweeping and up-sweeping modes above and below the NAE range of frequencies. An analytical model for the existence of multiple RSAE modes is in good agreement with the simulation results. One notices that the triangularity has a greater effect on the odd-type modes than that on the even-type modes: the odd-type modes come into existence because of the plasma triangularity.
基金Project supported by the National Natural Science Foundation of China(Grant No.11405271)the China Postdoctoral Science Foundation(Grant No.2017M612901)+4 种基金the Fund from Chongqing Science and Technology Commission(Grant No.cstc2017jcyjAX0047)Chongqing Postdoctoral Special Foundation(Grant No.Xm2017109)the Fundamental Research Funds for Central Universities,China(Grant No.YJ201796)the Pre-research Key Laboratory Fund for Equipment(Grant No.61422070306)the Fund from the Laboratory of Advanced Space Propulsion(Grant No.LabASP-2017-10)
文摘Characterizing the gap eigenmode of shear Alfv′en waves(SAWs) and its interaction with energetic ions is important to the success of magnetically confined fusion. Previous studies have reported an experimental observation of the spectral gap of SAW on the on Large Plasma Device(LAPD)(Zhang et al. 2008 Phys. Plasmas 15 012103), a linear large plasma device(Gekelman et al. 1991 Rev. Sci. Instrum. 62 2875) possessing easier diagnostic access and lower cost compared with traditional fusion devices, and analytical theory and numerical gap eigenmode using ideal conditions(Chang 2014 Ph.D Thesis at Australian National University). To guide experimental implementation, the present work models the gap eigenmode of SAWs using exact LAPD parameters. A full picture of the wave field for previous experiment reveals that the previously observed spectral gap is not global but an axially local result. To form a global spectral gap, the number of magnetic mirrors has to be increased and stronger static magnetic field makes it clearer. Such a spectral gap is obtained for the magnetic field of B0(z) = 1.2 + 0.6 cos[2π(z-33.68)/3.63] with 7.74-m magnetic beach. By introducing two types of local defects(corresponding to Eθ(z0) = 0 and E’θ(z0) = 0 respectively), odd-parity and even-parity discrete eigenmodes are formed clearly inside the gap. The strength of these gap eigenmodes decreases significantly with collision frequency, which is consistent with previous studies. Parameter scans show that these gap eigenmodes can be even formed successfully for the field strength of B0(z) = 0.2 + 0.1 cos[2π(z-33.68)/3.63] and with only four magnetic mirrors, which are achievable by the LAPD at its present status. This work can serve as a strong motivation and direct reference for the experimental implementation of the gap eigenmode of SAWs on the LAPD and other linear plasma devices.
基金partially within the EUROFUSION Enabling Research Projects Projects‘NLED’(ER15-ENEA-03)‘NAT’(Cf P-AWP17-ENRMPG-01)+2 种基金‘MET’(ENR-MFE19-ENEA-05)‘ATEP’(ENR-MOD.01.MPG)carried out within the framework of the Eurofusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No.633053。
文摘In this work,the gyrokinetic eigenvalue code LIGKA,the drift-kinetic/MHD hybrid code HMGC and the gyrokinetic full-f code TRIMEG-GKX are employed to study the mode structure details of reversed shear Alfvén eigenmodes(RSAEs).Using the parameters from an ASDEXUpgrade plasma,a benchmark with the three different physical models for RSAE without and with energetic particles(EPs)is carried out.Reasonable agreement has been found for the mode frequency and the growth rate.Mode structure symmetry breaking(MSSB)is observed when EPs are included,due to the EPs’non-perturbative effects.It is found that the MSSB properties are featured by a finite radial wave phase velocity,and the linear mode structure can be well described by an analytical complex Gaussian expressionФ(s)=e^(-σ(s-s_(0))^(2))with complex parametersσand s_(0),where s is the normalized radial coordinate.The mode structure is distorted in opposite manners when the EP drive shifted from one side of qminto the other side,and specifically,a non-zero average radial wave number with opposite signs is generated.The initial EP density profiles and the corresponding mode structures have been used as the input of HAGIS code to study the EP transport.The parallel velocity of EPs is generated in opposite directions,due to different values of the average radial wave number,corresponding to different initial EP density profiles with EP drive shifted away from the qmin.
基金supported by the National Natural Science Foundation of China(Grant Nos.11405271,11372104,75121543,11332013,11372363,and 11502037)
文摘Alfvnic gap eigenmode(AGE) can eject energetic particles from confinement and thereby threaten the success of magnetically controlled fusion. A low-temperature plasma cylinder is a promising candidate to study this eigenmode, due to easy diagnostic access and simple geometry, and the idea is to arrange a periodic array of magnetic mirrors along the plasma cylinder and introduce a local defect to break the field periodicity. The present work validates this idea by reproducing a clear AGE inside a spectral gap, and more importantly details the influence of the number and depth(or modulation factor)of magnetic mirror on the characteristics of AGE. Results show that AGE is suppressed by other modes inside the spectral gap when the number of magnetic mirrors is below a certain value, which leads to a weakened Bragg’s effect. The structure and frequency of AGE remain unchanged for a decreased number of magnetic mirrors, as long as this number is enough for the AGE formation. The width of spectral gap and decay constant(inverse of decay length) of AGE are linearly proportional to the depth of magnetic mirror, implying easier observation of AGE through a bigger mirror depth. The frequency of AGE shifts to a lower range with the depth increased, possibly due to the unfrozen plasma with field line and the invalidity of small-perturbation analysis. Nevertheless, it is exciting to find that the depth of field modulation can be increased to form AGE for a very limited number of magnetic mirrors. This is of particular interest for the experimental implementation of AGE on a low-temperature plasma cylinder with limited length.
基金supported by the China National Magnetic Confinement Fusion Science Program(Grant No.2018YFE0304100)the US Department of Energy,Office of Science,Office of Advanced Scientific Computing Research and Office of Fusion Energy Sciences,and the Scientific Discovery through Advanced Computing(Sci DAC)program under Award No.DE-SC0018270(Sci DAC ISEP Center)+2 种基金the China Scholarship Council(Grant No.201806010067)used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory(DOE Contract No.DEAC05-00OR22725)the National Energy Research Scientific Computing Center(DOE Contract No.DE-AC02-05CH11231)
文摘Global linear gyrokinetic simulations using realistic DIII-D tokamak geometry and plasma profiles find co-existence of unstable reversed shear Alfvén eigenmodes(RSAE)with low toroidal mode number n and electromagnetic ion temperature gradient(ITG)instabilities with higher toroidal mode number n.For intermediate n?=?[10,12],RSAE and ITG co-exist and overlap weakly in the radial domain with similar growth rates but different real frequencies.Both RSAE and ITG growth rates decrease less than 5%when compressible magnetic perturbations are neglected in the simulations.The ITG growth rates increase less than 7%when fast ions are not included in the simulations.Finally,the effects of trapped electrons on the RSAE are negligible.
基金supported in part by the Grant-in-Aid for Scientific Research from MEXT (No. 16082209)from JSPS (Nos. 21360457, 21340175,and 22-7912)+1 种基金the LHD project budget (NIFS09ULHH508)partially supported by the JSPS-CAS Core-University program in thefield of‘Plasma and Nuclear Fusion’
文摘Beam-ion losses induced by fast-ion-driven toroidal Alfven eigenmodes （TAE） were measured with a scintillator-based lost fast-ion probe （SLIP） in the large helical device （LHD）. The SLIP gave simultaneously the energy E and the pitch angle X=arccos（v///v） distribution of the lost fast ions. The loss fluxes were investigated for three typical magnetic configurations of Rax-vac=3.60 m, 3.75 m. and 3.90 m, where Rax-vac is the magnetic axis position of the vacuum field. Dominant losses induced by TAEs in these three configurations were observed in the E/X regions of 50-190 keV/40°, 40-170 keV/25°, and 30-190 keV/30°, respectively. Lost-ion fluxes induced by TAEs depend clearly on the amplitude of TAE magnetic fluctuations, Rax-vac and the toroidal field strength Bt. The increment of the loss fluxes has the dependence of （bTAE/Bt）s. The power s increases from s = 1 to 3 with the increase of the magnetic axis position in finite beta plasmas.
文摘The cross section of photonic crystal fiber (PCF) is a two dimensional photonic crystal. The rods formed in PCF are not exact cylinders, the shape of rods will affect the eigenmode formed in two dimensional photonic crystals around a defect. Based on the relations between the defect eigenmodes and the radius of dielectric cylinders, the defect eigenmodes in photonic crystals in which the ellipse rods take the place of cylinders are studied by numerical calculation. The analysis of the relation between the eigenfrequency and the minor axis radius of ellipse rods show that the defect eigenfrequency is controlled by the cross section area of rods and the distribution of electromagnetic field around the defect is also affected by the cross section shape of rods. It provides a better way to modify the distribution of electromagnetic fields in photonic crystal and keeps the eigenfrequency unchanged.
基金supported by the National Basic Research Program of China(973 Program)under grants 2014CB340004,2014CB340001 and 2014CB340003the National Natural Science Foundation of China(NSFC)under grants 11690031,61761130082,11574001,11774116,11274131,61222502,61575224 and 61622510+4 种基金the Royal Society-Newton Advanced Fellowshipthe National Program for Support of Top-notch Young Professionalsthe Program for New Century Excellent Talents in University(NCET-11-0182)the Program for HUST Academic Frontier Youth Team,the Project ROAM(H2020-ICT-2014-1—Contract Number:645361)the Project Cornerstone(EPSRC-EP/L021129/1)。
文摘Spatial modes have received substantial attention over the last decades and are used in optical communication applications.In fiber-optic communications,the employed linearly polarized modes and phase vortex modes carrying orbital angular momentum can be synthesized by fiber vector eigenmodes.To improve the transmission capacity and miniaturize the communication system,straightforward fiber vector eigenmode multiplexing and generation of fiber-eigenmode-like polarization vortices(vector vortex modes)using photonic integrated devices are of substantial interest.Here,we propose and demonstrate direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters.By exploiting vector vortex modes(radially and azimuthally polarized beams)generated from silicon microring resonators etched with angular gratings,we report data-carrying fiber vector eigenmode multiplexing transmission through a 2-km large-core fiber,showing low-level mode crosstalk and favorable link performance.These demonstrations may open up added capacity scaling opportunities by directly accessing multiple vector eigenmodes in the fiber and provide compact solutions to replace bulky diffractive optical elements for generating various optical vector beams.
基金supported by the National Natural Science Foundation of China(Nos.61575199,61875199,6197520851761135115)+1 种基金Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB20000000)Science Foundation of Fujian Province(No.2019J02015)。
文摘The behavior of self-polarization emission in Nd∶Y_(3)Al_(5)O_(12)(YAG)=Cr^(4+)∶YAG lasers has been proved in some cases.However,the degree and direction of polarization were often sensitive and unstable.We experimentally observed different beam profiles versus the angle of the polarizer relative to the polarization direction of the laser.In order to explore the polarization mechanism,the dynamics of intracavity polarized eigenmodes was analyzed theoretically.Simulative results were well consistent with our experimental observations.It indicated that the linear self-polarization emission was a composite state rather than an intrinsic state.This study contributed to the improvement of the polarization stability in Nd∶YAG=Cr^(4+)∶YAG passively Q-switched lasers.
基金supported by National Natural Science Foundation of China(Nos.11201106 and 61303134)Natural Science Foundation of Zhejiang Province,China(No.LQ12A01001)Key Project of the Major Research Plan of NSFC(No.91130004).
文摘This paper investigates the eigenmode optimization problem governed by the scalar Helmholtz equation in continuum system in which the computed eigenmode approaches the prescribed eigenmode in the whole domain.The first variation for the eigenmode optimization problem is evaluated by the quadratic penalty method,the adjoint variable method,and the formula based on sensitivity analysis.A penalty optimization algorithm is proposed,in which the density evolution is accomplished by introducing an artificial time term and solving an additional ordinary differential equation.The validity of the presented algorithm is confirmed by numerical results of the first and second eigenmode optimizations in 1D and 2D problems.
