用于温密物质研究的强流电子束加载技术

能量20 MeV、流强2.5 kA的电子束脉冲可以在数十ns的时间内将靶材料加载至温密物质状态，进而可以开展材料状态方程、电导率以及不透明度等的实验研究工作。介绍了在神龙一号加速器上开展温密物质实验研究的束靶作用方式以及相应的测试技术。对电子束在直径0.3 mm、长1 mm的金属靶丝内的能量沉积和流体动力学响应进行了数值模拟。结果表明:靶丝的温度随着靶材料原子序数的增加而上升，而靶丝内温度分布的均匀性随着原子序数的增加而降低；在电子束加载后40 ns时刻Ta丝内的最高温度可以达到约1.6 eV。     

用于温密物质研究的强流电子束加载技术

能量20 MeV、流强2.5 kA的电子束脉冲可以在数十ns的时间内将靶材料加载至温密物质状态,进而可以开展材料状态方程、电导率以及不透明度等的实验研究工作。介绍了在神龙一号加速器上开展温密物质实验研究的束靶作用方式以及相应的测试技术。对电子束在直径0.3 mm、长1 mm的金属靶丝内的能量沉积和流体动力学响应进行了数值模拟。结果表明:靶丝的温度随着靶材料原子序数的增加而上升,而靶丝内温度分布的均匀性随着原子序数的增加而降低;在电子束加载后40 ns时刻Ta丝内的最高温度可以达到约1.6 eV。     

Conductivities in Hot Aluminium Plasma

We perform model calculations for the electrical and thermal conductivity of aluminium plasma within the generalized linear response method of Zubarev for temperatures of (5–25) eV and densities of (0.01–10) g/cm³. The composition in the expanded plasma region is determined by considering higher ionization states up to 5+ and solving the respective system of coupled mass action laws. Besides this chemical picture, a generalized Thomas‐Fermi model is applied to calculate the equation of state and the average charge state of the ions for densities near and above solid state density. Interactions between the various species are treated on T matrix level. Numerical results for the electrical and thermal conductivity of aluminium plasma are compared with experimental data and, for high densities, also with results of a Born approximation with respect to a weak electron‐ion pseudopotential.     

双温度氩-氮等离子体热力学和输运性质计算

获得覆盖较宽温度和压力范围内的等离子体热力学和输运性质是开展等离子体传热和流动过程数值模拟的必要条件.本文通过联立Saha方程、道尔顿分压定律以及电荷准中性条件求解等离子体组分;采用理想气体动力学理论计算等离子体热力学性质;基于Chapman-Enskog方法求解等离子体输运性质.利用上述方法计算了压力为0.1, 1.0和10.0 atm (1 atm=101325 Pa),电子温度在300—30000 K范围内,非局域热力学平衡(电子温度不等于重粒子温度)条件下氩-氮等离子体的热力学和输运性质.结果表明压力和非平衡度会影响等离子体中各化学反应过程,从而对氩-氮等离子体的热力学及输运性质有较大的影响.在局域热力学平衡条件下,计算获得的氩-氮等离子体输运性质和文献报道的数据符合良好.     

强激光照射对6H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一性原理赝势法, 模拟研究了纤锌矿6H-SiC晶体在强激光照射下电子特性的变化. 研究结果表明, 电子温度T_e在升高到3.89 eV及以上后, 6H-SiC由间接带隙的晶体变为直接带隙的晶体; 带隙值随电子温度T_e升高先是增大后又快速减小, 当电子温度T_e大于4.25 eV以后, 带隙已经消失而呈现出金属特性.     

Investigation of warm dense matter using time-resolved X-ray absorption spectroscopy with third- and fourth- generation light sources

The advent of high-power lasers has provided insights into laboratory high energy density (>10¹¹ J/m³) physics. In particular, the properties of warm dense matter (WDM) with temperatures of 10⁴–10⁶ K and near-solid densities is a research area that has garnered significant interest recently. However, owing to the high temperatures and pressures associated with WDM, the measurement of fundamental properties is difficult, and insufficient data has been a significant setback in WDM research. Herein, we review recent developments in time-resolved X-ray absorption spectroscopy with synchrotron and X-ray free electron lasers for WDM research. Various physical properties, such as atomic bonding, electronic structures, electron–phonon coupling, and thermal conductivity of various elements in WDM conditions are investigated via this noble X-ray technique at various time scales from 100 ps to 100 fs.     

First principle investigation of the electronic and thermoelectric properties of Mg_2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties(Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor(PF) increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.     

Temperature dependences of the electron-phonon coupling, electron heat capacity and thermal conductivity in Ni under femtosecond laser irradiation

The electron temperature dependences of the electron-phonon coupling factor, electron heat capacity and thermal conductivity are investigated for Ni in a range of temperatures typically realized in femtosecond laser material processing applications, from room temperature up to temperatures of the order of 10⁴ K. The analysis is based on the electronic density of states obtained through the electronic structure calculations. Thermal excitation of d band electrons is found to result in a significant decrease in the strength of the electron-phonon coupling, as well as large deviations of the electron heat capacity and the electron thermal conductivity from the commonly used linear temperature dependences on the electron temperature. Results of the simulations performed with the two-temperature model demonstrate that the temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons leads to higher maximum lattice and electron temperatures achieved at the surface of an irradiated Ni target and brings the threshold fluences for surface melting closer to the experimentally measured values as compared to the predictions obtained with commonly used approximations of the thermophysical parameters.     

Thermal properties of amorphous materials at low temperatures

Recent investigations of X-ray diffraction and electron micrograph studies reveal high density clusters separated by density deficient regions (voids) in amorphous materials. The low temperature specific heat and the thermal conductivity anomalies are explained on the basis of such a structure for amorphous materials. It is a generalisation of Debye's theory applied to most of the amorphous solids in the temperature range from 0 to 10 K. The anharmonic effects lead to the observed temperature dependence of the sound velocity. The thermal conductivity between 0 and 2 K is due to thermal diffusion, the plateau observed between 2 and 20 K is a consequence of the decrease in thermal conductivity due to three phonon processes compensated by intercluster diffusion, while beyond this range it is due to excitations within a cluster limited by the size of a cluster. Further the model predicts the coefficient of expansion about 100 times that found in the corresponding crystalline solids. An experimental verification of this result can be a good test for the model.     

Temperature dependence of the conductivity of a silicon inversion layer at low temperatures

The temperature dependence of the conductivity of a high-mobility silicon MOSFET is reported for temperatures from 0.2 to 23 K and electron densities from 1 to 20 × 10¹¹cm⁻². The results show a strong increase in conductivity with decreasing temperature. However, this rate of increase was observed to rapidly diminish outside of a restricted range of temperature for a given density. The decreasing temperature dependence at the lower temperatures is attributed to the saturation of screening as the effects of broadening become comparable to thermal effects. The diminishing temperature dependence at the higher temperatures puts limits on the applicability of recent calculations for the conductivity.     

Thermally excited modes in a pure electron plasma

Thermally excited plasma modes are observed in near-thermal-equilibrium pure electron plasmas over a temperature range of 0.05相似文献   

Angular distribution of electron temperature and density in a laser-ablation plume

The angular distribution of electron temperature and density in a laser-ablation plume has been studied for the first time. The electron temperature ranges from 0.1 to 0.5 eV and is only weakly dependent on the angle in the low-intensity range studied here. In contrast, the typical ion energy is about 2 orders of magnitude larger, and its angular distribution is more peaked about the target normal. The derived values of the electron density are in agreement with the measured values of ion density.     

CoSb_3/C_(60)复合材料的固相反应合成和热电性能

用固相反应法和脉冲电流直接通电烧结法制备了CoSb3 C6 0 复合材料 ,其组分通过粉末x射线衍射法确定 ,SEM分析表明C6 0 颗粒是均匀地分布在CoSb3基体中 .在 30 0— 80 0K范围内测量了材料的电导率、赛贝克系数和热导率 ,研究了纳米颗粒的尺寸和分布状态对复合材料热电性能的影响 .外加的C6 0 纳米颗粒在高温时降低了复合材料的晶格热导率 ,而对电传输性能影响较小 ,从而有效地提高了复合材料的热电性能 .与CoSb3相比 ,CoSb3 6 5 4 ? 0复合材料的ZT值提高了 4 0 % .     

低覆盖度CO分子在 Ni（110）面的吸附研究

利用密度泛函理论研究了低覆盖度下CO分子在Ni（110）表面的吸附结构和电子态。研究结果表明：在低覆盖度情况下, CO分子优先垂直吸附在短桥位,其次是顶位和长桥位。垂直短桥位吸附、顶位吸附相应的振动频率分别是1850.52 cm-1、1998.08cm-1。态密度的研究结果表明：CO分子和Ni原子在-10 eV -8 eV,-8 eV—-6 eV及1 eV -5 eV能量范围内发生了杂化作用。-10ev -8ev能量范围内的杂化主要来源于C、O原子的s轨道、pz轨道与Ni原子s、p、d轨道的杂化作用。-8ev—-6ev能量范围内的杂化作用主要来源于C、O原子的py、 px轨道与Ni原子d、s轨道的杂化作用。轨道间的杂化作用是吸附作用的主要来源。 我们计算的吸附位置与相应的振动频率与相关实验结果基本一致。     

VO_2薄膜Vis-NIR及NIR-MIR椭圆偏振光谱分析

采用射频磁控溅射在石英玻璃基底上反应溅射制备单斜相(M相)VO_2薄膜.利用V-VASE和IR-VASE椭圆偏振仪及变温附件分别在0.5—3.5 eV(350—2500 nm)和0.083—0.87 eV(1400—15000 nm)入射光能量范围内对相变前后的VO_2薄膜进行光谱测试,运用逐点拟合的方式,并通过薄膜的吸收峰的特征,在0.5—3.5 eV范围内添加3个Lorentz谐振子色散模型和0.083—0.87 eV范围内添加4个Gaussion振子模型对低温态半导体态的薄膜椭偏参数进行拟合,再对高温金属态的薄膜添加7个Lorentz谐振子色散模型对进行椭偏参数的拟合,得到了较为理想的拟合结果.结果发现:半导体态的VO_2薄膜的折射率在近红外-中红外基本保持在最大值3.27不变,且消光系数k在此波段接近于零,这是由于半导体态薄膜在可见光-近红外光范围内的吸收主要是自由载流子吸收,而半导体态薄膜的d//轨道内的电子态密度较小.高温金属态的VO_2薄膜的折射率n在近红外-中红外波段具有明显的增大趋势,且在入射光能量为0.45 eV时大于半导体态的折射率;消光系数k在近红外波段迅速增大,原因是在0.5—1.62 eV范围内,能带内的自由载流子浓度增加及电子在V_(3d)能带内发生带内的跃迁吸收,使k值迅速增加;当能量小于0.5 eV时k值变化平缓,是由于薄膜内自由载流子浓度和电子跃迁率趋于稳定所致.     

Ab Initio simulations of dense helium plasmas

We study the thermophysical properties of dense helium plasmas by using quantum molecular dynamics and orbital-free molecular dynamics simulations, where densities are considered from 400 to 800 g/cm3 and temperatures up to 800 eV. Results are presented for the equation of state. From the Kubo-Greenwood formula, we derive the electrical conductivity and electronic thermal conductivity. In particular, with the increase in temperature, we discuss the change in the Lorenz number, which indicates a transition from strong coupling and degenerate state to moderate coupling and partial degeneracy regime for dense helium.     

"Ultracold" neutral plasmas at room temperature

We report a measurement of the electron temperature in a plasma generated by a high-intensity laser focused into a jet of neon. The 15?eV electron temperature is determined using an analytic solution of the plasma equations assuming local thermodynamic equilibrium, initially developed for ultracold neutral plasmas. We show that this analysis method accurately reproduces more sophisticated plasma simulations in our temperature and density range. While our plasma temperatures are far outside the typical "ultracold" regime, the ion temperature is determined by the plasma density through disorder-induced heating just as in ultracold neutral plasma experiments. Based on our results, we outline a pathway for achieving a strongly coupled neutral laser-produced plasma that even more closely resembles ultracold neutral plasma conditions.     

Characteristics of the NASA Lewis Bumpy Torus Plasma Generated with Positive Applied Potentials

Experimental observations have been made during steady-state operation of the NASA Lewis Bumpy Torus experiment at input powers up to 150 kW in deuterium and helium gas, and with positive potentials applied to the midplane electrodes. This steady-state ion heating method utilizes a modified Penning discharge operated in a bumpy torus confinement geometry such that the plasma is acted upon by a combination of strong electric and magnetic fields. Experimental investigation of a deuterium plasma revealed electron temperatures from 14 to 140 eV and ion kinetic temperatures from 160 to 1785 eV. At least two distinct modes of operation exist, each of which is associated with a characteristic range of background pressure and electron temperature. Experimental data show that the average ion residence time in the plasma is virtually independent of the magnetic field strength.     

Lattice thermal conductivity of polyvinyl acetate at low temperatures

The lattice thermal conductivity of a non-crystalline polymer has been studied at low temperatures in the frame of the density fluctuation model by calculating the total lattice thermal conductivity of polyvinyl acetate in the temperature range 0.1–4K as an example and a very good agreement has been reported between the calculated and the experimental values of the lattice thermal conductivity in the entire temperature range of study. It is also found that at low temperatures, the lattice thermal resistivity of a non-crystalline polymer is mainly due to scattering of phonons by empty spaces.