电子束能量沉积的相对论Fokker-Planck方程的计算方法

针对相对论电子束在高密度等离子体中的能量沉积过程,建立三维动量空间中快电子能量沉积的相对论Fokker-Planck方程的可计算物理模型,构造数值算法并研制数值模拟程序.通过与解析模型和蒙特卡罗模拟相比较,验证数值方法和程序的可靠性.在二维动量空间的模拟基础上,通过计算能量区间为0.5 MeV~3.5 MeV的快电子在背景密度为300 g·cm^-3的氘氚等离子体中的能量沉积过程,发现由于碰撞效应使平均散射角趋近平衡,三维动量空间计算快电子连续射程和穿透深度与二维结果基本一致.     

Time-resolved radial uniformity of pulse-modulated inductively coupled O_2/Ar plasmas

Time-resolved radial uniformity of pulse-modulated inductively coupled O_2/Ar plasma has been investigated by means of a Langmuir probe as well as an optical probe in this paper. The radial uniformity of plasma has been discussed through analyzing the nonuniformity factor β(calculated by the measured n_e, lower β means higher plasma radial uniformity). The results show that during the active-glow period, the radial distribution of ne exhibits an almost flat profile at the beginning phase, but it converts into a parabola-like profile during the steady state. The consequent evolution for β is that when the power is turned on, it declines to a minimum at first, and then it increases to a maximum, after that, it decays until it keeps constant. This phenomenon can be explained by the fact that the ionization gradually becomes stronger at the plasma center and meanwhile the rebuilt electric field(plasma potential and ambipolar potential) will confine the electrons at the plasma center as well. Besides, the mean electron energy( ε_(on)) at the pulse beginning decreases with the increasing duty cycle. This will postpone the plasma ignition after the power is turned on. This phenomenon has been verified by the emission intensity of Ar(λ = 750.4 nm). During the after-glow period, it is interesting to find that the electrons have a large depletion rate at the plasma center. Consequently, ne forms a hollow distribution in the radial direction at the late stage of after-glow. Therefore, β exhibits a maximum at the same time. This can be attributed to the formation of negative oxygen ion(O~-) at the plasma center when the power has been turned off.     

甚高频激发容性耦合氩等离子体的电子能量分布函数的演变

甚高频激发的容性耦合等离子体由于离子通量和能量的相对独立可控而受到人们的关注. 本文采用朗缪尔探针诊断技术测量了40.68 MHz激发的容性耦合Ar等离子体的特性(如电子能量概率分布、电子温度和密度等)随宏观参量的演变情况. 实验结果表明, 电子能量概率分布随着气压的增加从双麦克斯韦分布逐步转变为单麦克斯韦分布并最终演变为Druyvesteyn分布, 而射频激发功率的增加促进了低能电子布居数的增强; 在从等离子体放电中心移向边界的过程中, 低能电子的布居数显著下降, 而高能电子的布居则有所上升; 放电极板间距的变化直接导致了等离子体中电子加热模式的转变. 另外, 我们也对等离子体中的高低能电子密度和温度的分配情况进行了讨论.     

Electronic transport of Lorentz plasma with collision and magnetic field effects

The electronic transverse transport of Lorentz plasma with collision and magnetic field effects is studied by solving the Boltzmann equation for different electron density distributions. For the Maxwellian distribution, it is shown that transport coefficients decrease as ? increases, ? is the ratio of an electron's magneto-cyclotron frequency to plasma collision frequency. It means that the electrons are possible to be highly collimated by a strong magnetic field. For the quasimonoenergetic distribution with different widths, it is found that the transport coefficients decrease greatly as εˉ decreases.In particular when the width approaches to zero the transverse transport coefficients are hardly affected by the magnetic field and the minimal one is obtained. Results imply that the strong magnetic field and quasi-monoenergetic distribution are both beneficial to reduce the electronic transverse transport. This study is also helpful to understand the relevant problems of plasma transport in the background of the inertial confinement fusion.     

Electronic density of states of high pressure liquid uranium from shock vaporization

Strong shock waves were used to compress and heat initially porous uranium plates to high pressures (≈10⁶atm) and temperatures high enough (some 10⁴K), that one should expect a dominant contribution of the electrons to the thermal state. Information about this contribution, which depends on the electronic density of states distribution D(E), is obtained by measuring the entropy in the shocked state on the basis of the shock vaporization technique. The results indicate that a considerable redistribution of the six valence electrons occurs when uranium is compressed and heated.     

二氧化铀电子结构和弹性性质的第一性原理研究

本文采用第一性原理的方法系统研究了UO₂的晶体结构、电子结构和弹性性质. 在计算中采用广义梯度近似结合Hubbard U项描述电子的局域强关联效应. 首先通过计算能带带隙大小并与理论值比较的方法, 得到了合理的有效库仑相关作用能(U_eff)的取值, 同时通过态密度的计算, 进一步验证了U_eff取值的合理性. 计算得到UO₂中U原子的U_eff值为3.30 eV (U_eff=U-J, U=3.70 eV, J=0.40 eV). 应用此参数计算得到的UO₂晶格常数为5.54 Å, 带隙宽度为2.17 eV. 该结果优于目前现有的研究结果, 同时在同样的U_eff值条件下计算所得到的弹性常数与实验值也符合得较好. 相较于之前的基于实验测量并分析得到的U_eff值, 我们所采用的方法在对UO₂性质描述上更为准确. 不同的有效库仑相关作用能取值下的态密度结果表明, 有效库仑相关作用能的大小可以影响铀原子5f电子轨道的分布.     

ESR in GdxRe1-xAl2 (Re=La, Lu, Y) - the experimental and the theoretical studies

The ESR measurements in Gd_xRe_1-xAl₂ (Re=La, Lu, Y) in high concentration range are presented. The theoretical analysis of ESR data in the whole concentration range x ε (0,1), which explains consistently the observed positive g shifts in unbottlenecked region and negative g shifts in bottlenecked region exhibits unambigously the d-character of band electrons in these compounds.     

Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

Ultrabright γ-ray emission from the interaction of an intense laser pulse with a near-critical-density plasma

An efficient scheme for generating ultrabright γ-rays from the interaction of an intense laser pulse with a near-critical-density plasma is studied by using the two-dimensional particle-in-cell simulation including quantum electrodynamic effects. We investigate the effects of target shape on γ-ray generation efficiency using three configurations of the solid foils attached behind the near-critical-density plasma: a flat foil without a channel (target 1), a flat foil with a channel (target 2), and a convex foil with a channel (target 3). When an intense laser propagates in a near-critical-density plasma, a large number of electrons are trapped and accelerated to GeV energy, and emit γ-rays via nonlinear betatron oscillation in the first stage. In the second stage, the accelerated electrons collide with the laser pulse reflected from the foil and emit high-energy, high-density γ-rays via nonlinear Compton scattering. The simulation results show that compared with the other two targets, target 3 affords better focusing of the laser field and electrons, which decreases the divergence angle of γ-photons. Consequently, denser and brighter γ-rays are emitted when target 3 is used. Specifically, a dense γ-ray pulse with a peak brightness of 4.6×10²⁶ photons/s/mm²/mrad²/0.1%BW (at 100 MeV) and 1.8×10²³ photons/s/mm²/mrad²/0.1%BW (at 2 GeV) are obtained at a laser intensity of 8.5×10²² W/cm² when the plasma density is equal to the critical plasma density n_c. In addition, for target 3, the effects of plasma channel length, foil curvature radius, laser polarization, and laser intensity on the γ-ray emission are discussed, and optimal values based on a series of simulations are proposed.     

Itinerant to relocalized transition of f electrons in the Kondo insulator CeRu4Sn6

The three-dimensional electronic structure and the nature of Ce 4f electrons of the Kondo insulator CeRu₄Sn₆ are investigated by angle-resolved photoemission spectroscopy, utilizing tunable photon energies. Our results reveal (i) the three-dimensional k-space nature of the Fermi surface, (ii) the localized-to-itinerant transition of f electrons occurs at a much high temperature than the hybridization gap opening temperature, and (iii) the “relocalization” of itinerant f-electrons below 25 K, which could be the precursor to the establishment of magnetic order.     

Study of the chemical bonding of titanium with silicon and oxygen by AES and SEELS

Auger electron Spectroscopy (AES) and slow electron energy loss Spectroscopy (SEELS) have been employed to study the electronic structure of Ti, TiSi₂ and TiO₂. The changes in the Auger and loss spectra when Ti chemically binds with silicon to form TiSi₂ and with oxygen to form TiO₂ have been understood as manifestations of changes in electronic participation. AES spectra show distinct changes in line shapes of transitions involving the Ti valence electrons. The SEELS spectra provide information regarding shallow core levels, valence band and the collective excitation energies of the volume and surface plasmons. By monitoring the changes in the Auger peak at 387 eV and the 3p→ 3d quasiatomic transition (at about 45 eV), the role of d-orbital occupancies are studied in Ti and its compounds. The SEELS studies in the 0-80 eV range have enabled the authors to observe the behaviour of the 3p → 3d quasiatomic transition in Ti, which persists after oxidation but almost disappears during TiSi₂ formation. The values of the plasmon losses are related to the collective behaviour of conduction electrons.     

Lower hybrid waves in plasmas with negative ions

The ion-ion hybrid mode, with frequencies ω≈(ω_c+ω_c-)^1/2, is briefly analyzed for a warm plasma containing positive ions (gyrofrequency ω_c+), negative ions (gyrofrequency ω_c-), and electrons. Experiments with SF₆ ^- as the negative ion are proposed     

Enhancement of subgap conductance in a graphene superconductor junction by valley polarization

We theoretically study the differential conductance of a graphene/graphene superconductor junction, where the valley polarization of Dirac electrons is considered in the nonsuperconducting region. It is shown that the subgap conductance will increase monotonically with the valley-polarization strength when the chemical potential μ is near the Dirac point μ≤ 3?(? is the superconducting gap), whereas it will decrease monotonically when μ is far away from the Dirac point, μ≥ 5?.The former case is induced by the specular Andreev reflection while the retro-reflection accounts for the later result. Our findings may shed light on the control of conductance of a graphene superconductor junction by valley polarization.     

Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed     

HL-2A装置SDD软X射线PHA阵列实验结果

用软X射线脉冲高度分析(PHA)阵列系统获得了等离子体的电子温度剖面和电子速率分布的时间演化。测量结果表明,电子温度剖面在OH阶段较平缓,接近抛物线1.0×[1-(r/a)²]²分布;而在ECRH(功率0.8MW)阶段,等离子体中心(z＝0)电子温度上升了0.6keV,边缘(z＝30cm)处只上升了0.1keV,反映出ECRH功率沉积在等离子体中心区域;在ECRH期间有大量的高能电子产生,因而电子速率分布在ECRH期间显著改变;等离子体中心的高能电子的数量和能量都比等离子体边缘的增加更大,ECRH(～0.8MW)期间等离子体中心(z＝0)产生的高能电子的能量可达17keV。分析表明：在ECRH(纵场B_t=1.3T)放电期间,ECRH加热效果显著,ECRH的功率主要沉积在等离子体中心附近;电子温度剖面在ECRH阶段较OH阶段峰化;ECRH期间有大量的高能电子产生,电子速率分布被改变成为非麦克斯韦分布。     

High temperature transport processes in lithium niobate

The electrical conductivity of single crystal lithium niobate (LiNbO₃) was determined as a function of temperature for various oxygen partial pressures. The electrical conductivity is proportional to P_o2^−1/4 which can be explained by a defect equilibrium involving singly ionized oxygen vacancies and electrons.

Measurements of electrical transport numbers at 1000°K show the electrical conductivity of LiNbO₃ to be ionic at one atmosphere of oxygen and electronic at low oxygen partial pressures.

Thermoelectric measurements indicate that LiNbO₃ at low oxygen partial pressures is n-type and that the concentration of electrons at 1000°K and in an atmosphere of 50% C0/50% CO₂a is 4 × 10¹⁷cm³ with a mobility of 1.7 cm²V sec.

The diffusion of oxygen in LiNbO₃ was determined as a function of temperature at an oxygen partial pressure of 70 Torr. by measuring O¹⁸/O¹⁶ isotope exchange with the gas phase as a function of time. The diffusion data may be represented by D = 3.03 × 10⁻⁶ exp (−29.4 kcal mole⁻¹/RT)cm²sec. Consideration of the Nernst-Einstein relation for oxygen and the variation in conductivity with Li₂O activity indicate that the ionic conduction is caused by transport of lithium ions.     

Low-field magnetoresistance of Ag-substituted perovskite-type manganites

A series of bulk polycrystalline Nd_1−xAg_xMnO₃ samples were prepared by conventional solid-state reaction processing, for x between 0.1 and 0.5. The structure, magnetism, and magnetoresistance (MR) of the samples are investigated. The X-ray diffraction patterns show that the x=0.1 sample is a single perovskite structure, while x0.167, samples consist of a ferromagnetic perovskite phase and two nonmagnetic phases, Ag and Mn₃O₄. The MR of Nd_1−xAg_xMnO₃ is enhanced by increasing the composition of Ag. Compared with the sample of x=0.1, the x=0.5 sample has a significantly larger value of low-field MR. The enhanced low-field MR is related to the spin-dependent scattering of spin-polarized electrons at the interfaces between the perovskite grains and silver granules.     

激光照射中Z薄靶等离子体状态的计算-激光等离子体相互作用的理论模拟(Ⅰ)

本文叙述用一维非平衡辐射流体力学激光打靶程序模拟计算高功率密度(~10¹³W/cm²)激光照射中Z介质薄靶形成的等离子体状态。考虑的物理过程有轫致、光电离、电子碰撞电离及它们的逆过程,Compton散射过程等。Compton散射采用Fokker-Planck近似;电子和离子热传导采用限流扩散近拟;光子方程采用多群限流扩散近似;用平均原子模型计算布居数。激光的吸收主要考虑逆轫致吸收。用功率密度分别为5×10¹³W/cm²和1×10¹⁴W/cm²,波长0.53μm,脉宽450ps的激光从两面和单面打se薄靶,模拟计算结果与国外的实验结果^[1]一致。     

A generalized Vlasov equation

A two-special dimension electronic system characterized by a plasma parameter Γ 1 is analyzed; then, by using a rigorous non-equilibrium statistical mechanical theory, the evolution of distribution function is considered. A generalized Vlasov equation (GVE) is derived. Compared to the usual Vlasov equation, GVE presents an additional velocity-dependent correlation term. Taking as a starting point the GVE, the phenomenological approximation to two-particles function, ƒ₂(r₁r₂p₁p₂; t) = ƒ₁(r₁p₁;t)f₁(r₂p₂;t)g(r₁−r₂), proposed by Singwi, Tosi, Landi and Sjolander is analyzed.     

Effects of low-LET radiations on CR-39 track detector

The effects of Co-60 gamma-rays and about 25-MeV pulsed electrons have been investigated on CR-39 plastic nuclear track detector. The bulk etch rate was measured as a function of the absorbed dose under three different irradiation conditions; gamma-ray exposure in air at the dose rate of 6 kGy/h, that in vacuum and electron shower in air at 126 kGy/h on average. It was found that the bulk etch rate (V) increased nearly exponentially with the dose (D); V=V₀exp(kD), and the characteristic parameter (k) was 35, 9 and 12 MGy⁻¹, respectively. It is considered that a lack of oxygen in damage formation processes could occur momentarily during a short electron pulse, just similarly to the vacuum effect of reduction in track etch rates in outgassed CR-39 plastic.