高频放电CO2激光器电子能量分布及输运系数

利用文献中给出的截面数据，引入高频放电激励ＣＯ２激光器的Ｂｏｌｔｚｍａｎｎ方程，对其进行数值求解，研究了频率对电子能量分布及输运系数的影响。     

磁场对四端量子波导中电子输运性质的影响

采用模匹配方法，研究了非均匀磁场下开放的四端量子波导中的电子输运性质. 结果表明，从一端入射的电子可以透射到两个与之垂直的输出端和一个与之平行的输出端. 在没有外加磁场的情况下，两个垂直输出端的输运概率是相同的，但垂直端与水平端的输运概率不同；在外加磁场下，由于磁边缘态效应，两个垂直输出端的输运概率也有着相当大的差别. 通过施加不同的磁场，我们能获得丰富的电子输运结构，如台阶，宽谷，尖峰等；通过调节磁场的大小和比例以及结构参数可控制该量子结构在各输出端的输运概率. 关键词： 电子输运 介观体系 磁效应     

磁场下非对称T型量子波导的电子输运行为

运用模匹配方法和求解单电子薛定谔方程,来演示非对称T型磁量子结构的电子输运性质.结果表明,结构因子和磁势垒都能改变电子散射模数,电子输运谱因此变得复杂而丰富,散射区域出现了完全局域态和磁边缘态.在特定的结构参数和磁场强度下,能观测到宽谷、尖峰、共振透射和共振反射等电子输运现象,即可以通过调节磁场大小和结构参数来实现波矢过滤. 关键词： 介观体系 电子输运 磁效应     

GaN／A1xGal-xN异质结二维电子气的磁电阻研究

通过对GaN／A1xGa1-xN异质结中二维电子气磁输运结果的分析,研究了磁电阻的起因．结果表明,整个磁场范围的负磁电阻是由电子一电子相互作用引起的,而高场下的正磁电阻来源于平行电导的进一步修正．用拟合的方法得到了电子一电子相互作用项以及平行电导层的载流子浓度和迁移率,并用不同的计算方法对拟合结果进行了验证．     

二维Bloch电子在电场与磁场中的准经典输运行为

分别从周期场中电子准经典运动方程和作者之一最近提出的输运平衡方程出发，分析了二维Bloch电子在电场和磁场作用下的准经典输运，给出了不同电场下电子纵向漂移速度v_x和霍耳漂移速度v_y随磁场的变化趋势.结果显示，v_x和v_y均随磁场连续变化，在任何给定电场下都没有发现纵向漂移速度或霍耳漂移速度随磁感应强度发生突变.这一结论与其他作者的预言相左.分析了造成分歧的可能原因，并指出：在电子准经典运动方程中直接引入阻尼力来描述准经典 关键词：     

单层碳纳米管的磁输运特性

依据磁场中Boltzmann输运方程及单层磁纳米管(SWNTs)的能量色散关系,对单个SWNTs中轴向磁场诱发的低温磁阻进行了数值计算.分析表明:当电子以低能输运时,SWNTs的磁阻有明显的Aharonov-Bohn(A-B)效应,与并SWNTs的能隙相对应.     

负磁剪切环形等离子体中的电子温度梯度不稳定性研究

本文用本征模积分方程研究了环形等离子体中磁剪切为负时由电子温度梯度驱动的不稳定性。考虑了电子的全部动力学效应，强调了稳定性阈值附近的模式行为及相应的湍性输运，给出了磁剪切为负的临界梯度的拟合公式。     

平衡方程输运理论与电子关联

１９３４年丁秦生和雷啸霖提出的热载流子输运平衡方程理论把电场作用下多电子运动化为一个单粒子（质心）的力学运动加上相对电子的统计运动，这使得非线性输运得到简洁描述，成为实际系统输运研究中很有用的方法。它的成功也重新引发了学术界的与之争。争论使人们认识到当前存在两类不同的输运理论。通常的玻耳兹曼方程、久保公式等代表弱电子耦合极限，得出的线性电阻率正比于；而雷－丁平衡方程理论、力关联函数公式等则代表强电子耦合极限，得出的线性电阻率正比于．两种理论都在广泛使用，各有自己的适用范围和局限。     

AlxGa1-xN/GaN异质结二维电子气磁输运性质研究

在1.5K低温和0～9T的高磁场下研究了AlGaG/GaN异质结二维电子气的磁输运性质.实验结果在4块样品中都观察到了Shubnikov-da HaSS振荡的双周期行为.表明异质结的三角势阱中有两个子带被电子占据.通过电子子带占据时电子浓度分配的线形行为得到第二子带被占据的阈值浓度为7.2×1012cm-2.通过对不同样品量子散射时间和输运迁移率的研究,说明在1.5K下远程离化施主散射在量子散射时间中起主要作用.     

中子星中简并电子气体的临界磁化

中子星内部的电子处于高度简并或完全简并的状态,电子磁矩(包括内禀磁矩和朗道反磁矩)的取向不是随机的,而是呈现出极强的磁化行为.考虑了磁化后的磁诱导方程要改写,改写后的方程添加了新的磁场生成项,更重要的改变是等效磁扩散系数变小了(顺磁情况),在临界情况(等效扩散系数等于零),磁场在磁生成项的作用下增加直到抑制机理出现,朗道反磁矩就是在这个时候变得越来越重要.磁场增加的最终结果使中子星局域磁场成为振荡的,对外看来有可能成为磁星. 关键词： 中子星 简并 磁化     

横流放电CO2激光的理论分析

本文提出了横流放电CO₂激光器的理论分析模型,导出了稳态发射的基本方程式,得到与熟知的非流动稳态增益饱和及功率理论相同的简单解析关系式。关系式包含了流动效应及光腔上游泵浦效应的修正项。根据强度沿流动方向变化的分析,求得饱和强度等参量在提高流速时的极限值。本文分析为文献[1—3]建议的选择平均增益系数并利用非流动稳态关系计算流动激光器特性的半经验方案提供了理论证明,同时将文献[4]提出的流动激光器的定性分析发展为定量计算。由于本文公式的简单性与非流动情况一样,因而本理论能够取代半经验方案。 关键词：     

Subsonic non-steady-state gas flows in channels with inner cavities

A set of gasdynamic equations is given in the general form for matter with an arbitrary equation of state in the case when the entropy equation is used instead of the energy equation. In the ideal gas approximation in view of viscosity, a numerical investigation is performed of non-steady-state two-dimensional flows in a channel with a cavity. The calculation results have demonstrated that, given the flow velocity and the geometry of channel and cavity, pressure pulsations arise that are due to the departure of vortices from the cavity into the main flow. The values of the amplitude and frequency of pressure pulsations are determined. If measures are taken aimed at limiting the departure of vortices from the cavity, for example, a baffle is installed to restrict the interaction between the main flow and gas in the cavity, one can considerably increase the flow velocity in the channel, unaffected by the cavity. Such non-steady-state flows may be realized in MHD-generator channels, resonators of gas flow lasers, gas ducts for ventilation and gas transport systems, mufflers, whistles, etc.     

Theoretical modeling of microwave-pumped high-pressure gas lasers

The electron avalanche and laser excitation processes in high-pressure discharges at microwave frequencies are investigated. In our model, the applied electromagnetic field is treated classically and assumed to be monochromatic. The Boltzmann equation for the electron velocity distribution function under the influence of an alternating electric field is numerically solved for a typical XeCl laser gas mixture. All relevant elastic, inelastic and electron-electron collisions are included in solving the Boltzmann equation. The theoretical modeling of microwwave-pumped high-pressure gas lasers are developed based on the first law of thermodynamics in order to determineE _rms/n (root-mean-square field strength/total number density of gas molecules) which is required by the Boltzmann equation to calculate the electron kinetics rates and microwave-power absorption by the plasma. A sample calculation of the microwave-pumped XeCl laser is presented, and a fair agreement between theory and experiment is seen.Paper partially presented at the 10th Int. Conf. on Lasers and Applications, Lake Tahoe, Nevada, USA (1987)     

Efficient cw sodium dimer Raman laser operation in a high-temperature sapphire cell

cw Raman lasing of Na₂ molecules generated in a heated, sealed-off, all-sapphire cell is demonstrated. Being not damaged by highly corrosive alkaline vapours, this type of cell enables operation without buffer gas in contrast to the normal heatpipe operation of these lasers. This allows us to study Raman lasers in alkaline vapours in new regimes and under ideal conditions. With an argon ion pump laser at 488 nm, Raman laser operation at 525 nm with more than 10% efficiency and thresholds below 0.2 mW for a cell without buffer gas (length 9 cm) have been obtained so far. The low thresholds, being a factor of 10 less than for comparable heatpipe operation, gives us the chance to use low-power diode lasers as pump sources and to realize compact reliable Raman laser systems. Received: 17 May 1999 / Published online: 25 August 1999     

Numerical solution of Boltzmann tranport equation for TEA CO2 laser having nitrogen-lean gas mixtures to predict laser characteristics and gas lifetime

Selective laser isotope separation by TEA CO₂ laser often needs short tail-free pulses. Using laser mixtures having very little nitrogen almost tail free laser pulses can be generated. The laser pulse characteristics and its gas lifetime is an important issue for long-term laser operation. Boltzmann transport equation is therefore solved numerically for TEA CO₂ laser gas mixtures having very little nitrogen to predict electron energy distribution function (EEDF). The distribution function is used to calculate various excitation and dissociation rate of CO₂ to predict laser pulse characteristics and laser gas lifetime, respectively.Laser rate equations have been solved with the calculated excitation rates for numerically evaluated discharge current and voltage profiles to calculate laser pulse shape. The calculated laser pulse shape and duration are in good agreement with the measured laser characteristics. The gas lifetime is estimated by integrating the equation governing the dissociation of CO₂. An experimental study of gas lifetime was carried out using quadrapole mass analyzer for such mixtures to estimate the O₂ being produced due to dissociation of CO₂ in the pulse discharge. The theoretically calculated O₂ concentration in the laser gas mixture matches with experimentally observed value. In the present TEA CO₂ laser system, for stable discharge the O₂ concentration should be below 0.2%.     

The stability of the active medium of RF-exited CO2 lasers with gold as catalyst

Using mass-spectrometric investigations the gas composition of the active medium of sealed-off cw RF-excited CO₂ waveguide lasers have been studied. It has been found that a low degree of CO₂ dissociation and a laser power improvement can be achieved by means of a gold catalyst in the laser discharge volume. The conditions for long operational lifetimes of these lasers are described. Received: 21 December 1999 / Revised version: 1 May 2000 / Published online: 6 September 2000     

Carrier transport and its effect on the turn-on delay time in strained GalnAsP/InP multiple quantum well lasers

The effects of carrier transport on turn-on delay time in multiple quantum well lasers were investigated both theoretically and experimentally. By using rate equation analysis with two components of the carrier density inside and outside of the quantum wells, we found that carrier transport caused two important effects: one is the stationary effect of a significant reduction in carrier density in quantum wells; the other is an increase in differential carrier lifetime.As an experimental investigation, compressively strained 1.3 m GalnAsP/InP multiple quantum well (MQW) lasers were fabricated and their turn-on delay times were measured and investigated. The short-cavity buried-heterostructure lasers showed low-threshold current (2 to 3 mA) and small turn-on delay time (<200 ps) at biasless 30 mA pulse current. Although these performances are suitable for high-speed digital transmission, it was found that the carrier lifetimes derived from the turn-on delay measurement were larger for strained quantum well lasers than for conventional quantum well lasers and double heterostructure lasers. These phenomena are explained using the carrier transport model and are discussed. The solutions for further reduction in carrier lifetime and turn-on delay are discussed.     

气体激光器中的类透镜效应分析

本文讨论了类负透镜介质一般曲面镜光学谐振腔,并用来分析气体激光器中的热透镜效应。对平凹腔连续波二氧化碳激光器进行了模式分析。结果表明,热透镜效应对输出功率和频率特性影响相当大,对基模输出激光器的影响最为突出。最后,简单地讨论了离子气体激光器和某些其它激光器中的可能热透镜效应。     

13C-separation by a continuous discharge CO2 laser q-switched at 10 kHz

Compared to atmospheric pressure pulsed CO₂ (TEA) lasers, continuous-discharge CO₂ lasers can deliver photons at much lower costs. By a Q-switched version of such a laser we demonstrated by multiphoton dissociation of CHClF₂ the production of 1 g of C₂F₄ enriched to 50% ¹³C. This is a larger quantity of high enrichment than has been produced so far using TEA lasers. The process is already automated to a large extent. Scaling up seems feasible. These results have been achieved, although the process turned out to be much more nonlinear for our pulses (length 200 ns) than for TEA laser pulses. This difference is attributed to intensity effects, caused by direct multiphoton transitions in the excitation ladder. To avoid that selectivity breaks down, the gas has to be exchanged more than once per pulse. The required speed is much less, if ~250 mbar of Ar are added to the working gas.     

Kinetic simulations of fuel ion transport in ICF target implosions

A numerical code solving the ion Vlasov-Fokker-Planck kinetic equation is used to compute the hydrodynamics of the thermonuclear fuel in inertial confinement fusion pellets. Compared with standard hydrodynamics calculations, the kinetic results show enhanced ion transport between the core and the outer part of the target. Consequences are discussed in the case of plastic shells filled with deuterium gas and cryogenic deuterium-tritium targets envisioned for achieving ignition with megajoule-class lasers.Received: 25 April 2003, Published online: 12 August 2003PACS: 52.25.Fi Transport properties - 52.65.Ff Fokker-Planck and Vlasov equation