Transition from backward to sideward stimulated Raman scattering with broadband lasers in plasmas

Broadband lasers have been proposed as future drivers of inertial confined fusion(ICF)to enhance the laser-target coupling efficiency via the mitigation of various parametric instabilities.The physical mechanisms involved have been explored recently,but are not yet fully understood.Here,stimulated Raman scattering(SRS)as one of the key parametric instabilities is investigated theoretically and numerically for a broadband laser propagating in homogeneous plasma in multidimensional geometry.The linear SRS growth rate is derived as a function of scattering angles for two monochromatic laser beams with a fixed frequency differenceδω.Ifδω/ω_(0)∼1%,withω0 the laser frequency,these two laser beams may be decoupled in stimulating backward SRS while remaining coupled for sideward SRS at the laser intensities typical for ICF.Consequently,side-scattering may dominate over backward SRS for two-color laser light.This finding of SRS transition from backward to sideward SRS is then generalized for a broadband laser with a few-percent bandwidth.Particle-in-cell simulations demonstrate that with increasing laser bandwidth,the sideward SRS gradually becomes dominant over the backward SRS.Since sideward SRS is very efficient in producing harmful hot electrons,attention needs to be paid on this effect if ultra-broadband lasers are considered as next-generation ICF drivers.

Mitigating parametric instabilities in plasmas by sunlight-like lasers

Sunlight-like lasers that have a continuous broad frequency spectrum,random phase spectrum,and random polarization are formulated theoretically.With a sunlight-like laser beam consisting of a sequence of temporal speckles,the resonant three-wave coupling that underlies parametric instabilities in laser–plasma interactions can be greatly degraded owing to the limited duration of each speckle and the frequency shift between two adjacent speckles.The wave coupling can be further weakened by the random polarization of such beams.Numerical simulations demonstrate that the intensity threshold of stimulated Raman scattering in homogeneous plasmas can be doubled by using a sunlight-like laser beam with a relative bandwidth of∼1%as compared with a monochromatic laser beam.Consequently,the hot-electron generation harmful to inertial confinement fusion can be effectively controlled by using sunlight-like laser drivers.Such drivers may be realized in the next generation of broadband lasers by combining two or more broadband beams with independent phase spectra or by applying polarization smoothing to a single broadband beam.

基于太阳爆发灾变模型的数值实验

在Isenberg等人发展的灾变模型基础上根据接近真实的日冕环境,通过数值实验,对磁通量绳的平衡高度对光球磁场变化的响应开展了研究.利用NIRVANA程序进行了计算.日冕的等离子密度分布采用了一个半经验的模型,模拟中包含了物理耗散.考察了:磁通量绳的平衡位置及其演化特征;参考半径的变化对磁通量绳平衡位置的影响;磁通量绳内部平衡的性质以及在磁通量绳失去平衡之后一段时间内的动力学与运动学特征.结果表明:数值实验中得到的磁通量绳的平衡态位置与Isenberg等人的理论结果有微小的偏离,但是演化特征基本一致,在临界点处系统迅速失去平衡,向爆发态演化;参考半径的变化对磁通量绳平衡位置的影响与灾变模型给出的结果基本一致;磁通量绳在随着宏观磁结构演化的同时,还通过自身的调节达到内部平衡,当磁通量绳的内部和外部平衡都实现之后,系统整体也就达到了平衡状态;在爆发态下,磁通量绳的运动特征与Lin-Forbes模型和观测给出的结果一致,并且在通量绳的前方有快模激波出现;由于数值实验中包括了耗散,爆发过程中的磁能向其他形式能量的转换非常明显.

MHD Seismology of a loop-like filament tube by observed kink waves

We report and analyze observational evidence of global kink oscillations in a solar filament as observed in Ha by instruments administered by National Solar Observatory （NSO）/Global Oscillation Network Group （GONG）. An M1.1-class flare in active region （AR） 11692 occurred on 2013 March 15 and induced a global kink mode in the filament lying towards the southwest of AR 11692. We find periods of about 61-67 minutes and damping times of 92-117 minutes at positions of three ver- tical slices chosen in and around the filament apex. We find that the waves are damped. From the observed period of the global kink mode and damping timescale using the theory of resonant absorption, we perform prominence seismology. We estimate a lower cut-off value for the inhomogeneity length scale to be around 0.34-0.44 times the radius of the filament cross-section.

Signature of high-order azimuthal MHD body modes in sunspot's low atmosphere

The five-minute oscillations inside sunspots appear to be the absorption of the solar p-mode. It is a potential tool to probe a sunspot＇s sub-structure. We studied the collective properties of five-minute oscillations in the power and phase distribution at the sunspot＇s umbra-penumbra boundary. The azimuthal distributions of the power and phase of five-minute oscillations enclosing a sunspot＇s umbra were obtained with images taken with the Solar Dynamics Observatory/Atmospheric Imaging Assembly （SDO/AIA）. The azimuthal modes were quantified with periodogram analysis and justified with significance tests. The azimuthal nodal structures in an approximately ax- ially symmetric sunspot AR 11131 （2010 Dec 08） were investigated. Mode numbers ra = 2, 3, 4, 7, 10 were obtained in both 1700 A and 1600A bandpasses. The 1600A channel also revealed an extra mode at m = 9. In the upper atmosphere （304 A）, fewer modes were detected at m = 3, 4, 7. The azimuthal modes in the sunspot＇s low atmo- sphere could be interpreted as high-order azimuthal MHD body modes. They were detected in the power and phase of the five-minute oscillations in sunspot AR 11131 with SDO/AIA data. Fewer modes were detected in the sunspot＇s upper atmosphere.

太阳活动预测:太阳黑子数和太阳综合磁图

太阳磁场的演化对于了解和预测太阳活动至关重要,而目前对于太阳磁场的认识很大程度依赖于光球磁场.本文基于威尔科克斯天文台观测的光球磁场及其球谐展开形式,对光球磁场球谐系数的时间序列进行了分析,并对第25太阳活动周的太阳黑子数和综合磁图进行了预测.结果发现,光球磁场在太阳高年存在着复杂的短周期扰动.此外,光球磁场球谐系数g_(7)^(0)与太阳黑子数的时间序列接近同相位,这可能与太阳的子午环流有关.利用长短时记忆网络(LSTM),本文预测第25活动周的太阳黑子数峰值可能出现在2024年6月前后8个月内,峰值黑子数预计为166.9±22.6,下一个黑子数最低值可能出现在2031年1月左右.结合经验模态分解方法,进一步预测了截断到5阶的太阳综合磁图,并验证了预测结果具有一定的可靠性.此次预测不仅弥补了太阳磁场全球分布预测的空白,而且对未来的太阳观测计划具有潜在的参考价值.

Prediction of solar activities:Sunspot numbers and solar magnetic synoptic maps

The evolution of solar magnetic fields is significant for understanding and predicting solar activities.And our knowledge of solar magnetic fields largely depends on the photospheric magnetic field.In this paper,based on the spherical harmonic expansion of the photospheric magnetic field observed by Wilcox Solar Observatory,we analyze the time series of spherical harmonic coefficients and predict Sunspot Number as well as synoptic maps for Solar Cycle 25.We find that solar maximum years have complex short-period disturbances,and the time series of coefficient g_(7)~0 is nearly in-phase with Sunspot Number,which may be related to solar meridional circulation.Utilizing Long Short-Term Memory networks(LSTM),our prediction suggests that the maximum of Solar Cycle 25 is likely to occur in June 2024 with an error of 8 months,the peak sunspot number may be 166.9±22.6,and the next solar minimum may occur around January 2031.By incorporating Empirical Mode Decomposition,we enhance our forecast of synoptic maps truncated to Order 5,validating their relative reliability.This prediction not only addresses a gap in forecasting the global distribution of the solar magnetic field but also holds potential reference value for forthcoming solar observation plans.