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We have carried out Monte Carlo simulation of the motion of Ar+ ions in the space charge sheath surrounding a cylindrical Langmuir probe. The ion currents to the probe have been calculated from these simulations and the percentages of ions crossing the sheath boundary that are collected by the probe have been determined. It has been shown that the collisions of ions with neutral helium gas atoms in the sheath increase the percentage of ions collected by the probe above that predicted by collisionless orbital motion limited current (OMLC) theory at lower helium pressure and decrease this percentage below the OMLC theory prediction at higher helium pressure. It has been shown also that the ion current almost does not depend on probe radius at higher helium pressures. The results of the simulations have been compared with recent Langmuir probe measurements made in flowing afterglow plasmas and with other probe theories. 相似文献
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The current/voltage characteristics of a cylindrical Langmuir probe have been studied in Ar+/electron afterglow plasmas in helium carrier gas under truly thermal conditions at 300 K using our flowing afterglow/Langmuir probe (FALP) apparatus. The orbital motion limited (oml) ion and electron current regions of the probe characteristics have been explored over a wide range of the reduced probe voltage (up to ~ 100) and over a wide range of electron (ne) and ion (n+) number densities (1.6 × 107 to 1.5 × 1010 cm?3) at a constant pressure of the He carrier gas of 1.2 Torr. The observed increase of the probe ion currents above those predicted by collisionless oml theory, resulting in an apparent increase of the measured ion number density above ne in the plasma, is explained by the enhancement in the ion current collection efficiency due to collisions of ions with neutral gas atoms in the space charge sheath surrounding the probe. The continuous change in the exponent, χ, of the power-law dependence,i+ ∞ V of the ion current, i+, on the probe voltage, Vp from 0.5 at the highest n+ (smallest sheath) towards 1.0 at the lowest n+ (large sheath) indicates that the ion current collection from the plasma changes from the oml current regime at the high n+ to the continuum regime at the low n+ when the ions undergo multiple collisions with the helium atoms in the space charge sheath and thus “drift” towards the probe. 相似文献
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利用AMPT蒙特卡罗产生器中的强子化程序模块ART1.0, 得到了强子化过程的演化图形, 并基于强子在不同时刻的空间分布图, 估算出了不同时刻的反应区域半径. 将所得结果与根据HBT关联得到的结果相比较, 定出了冻结为强子气体的时刻, 得到了合理的结果. 相似文献
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为阐述分野照射治疗方式应用于中国科学院近代物理研究所(IMP) 肿瘤重离子临床治疗试验研究的理论基础,利用Monte Carlo (MC) 软件包Gate/Geant4 模拟计算了碳离子束分野照射分衔接处的横向剂量分布,得到了患者摆位误差±1.0 mm范围内的横向剂量分布和分野衔接处中心剂量随分野间距的变化关系,计算了分野间距5.0 mm的横向剂量分布。将分野间距5.0 mm的分野照射与整野照射的MC结果进行比较,发现:在靶区范围内两者模拟的剂量偏差在6.8% 以内,符合程度较好,并提出了在患者定位精确度较高的情况下(即患者治疗体位摆位误差在±1.0 mm范围) 解决分野衔接处剂量热点问题的可行方法,从而较大幅度地提高了分野照射时靶区范围内的剂量均匀性。To characterize the theoretical basis of eld patching technique in carbon ion beam therapy, Monte Carlo software package Gate/Geant4 was used to simulate the lateral dose distribution of matched elds. The lateral dose distribution of the matched elds within 1.0 millimeter of patient positioning error and the relationship between the central dose of the matched elds and the eld patching gap were obtained. Moreover, the lateral dose pro le of eld patching irradiation with 5.0 mm gap was simulated. While comparing the lateral dose distribution of eld patching irradiation with 5.0 mm gap with the undivided eld irradiation, a good agreement between both results with a maximum dose deviation of 6.8% was observed within the target volume.Hence a feasible method for solving the issue of dose hotspots during eld patching under the condition of higher patient positioning accuracies (patient positioning error within 1.0 millimeter) was proposed and the dose homogeneit within the target volume could be greatly improved when eld patching technique is adopted in carbon ion beam therapy. 相似文献
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利用蒙特卡罗(Monte-Carlo)方法模拟了激光等离子体相互作用中所产生的超热电子在固体物质中的输运过程,模拟运算采用连续慢化(CSDA)和玻恩(Born)近似的单一散射模型。文中给出了具有单能,束状分布的超热电子在双层固体靶中产生的Kα射线强度随表面靶层厚度的变化曲线。 相似文献
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Monte Carlo simulation within the grand canonical ensemble, the histogram reweighting technique, and finite size scaling analysis are used to explore the phase behaviour of heteronuclear dimers, composed of A and B type atoms, on a square lattice. We have found that for the models with attractive BB and AB nearest-neighbour energy, uBB=uAB=−1, and for non-repulsive energy between AA nearest-neighbour sites, uAA<0, the system belongs to the universality class of the two-dimensional Ising model. However, when uAA>0, the system exhibits a non-universal critical behaviour. We have evaluated the dependences of the critical point characteristics on the value of uAA. 相似文献
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In biological suspensions, the forward light scattering is done mainly by the contribution of the suspended cells. The multiple scattering is almost always unavoidable, its contribution being described by Monte Carlo simulations or by approximate analytical formula. The Monte Carlo approach we propose moves one photon at a time and checks all scattering centers to find, at each simulation step, which one will scatter the photon. The validation of the simulation results is performed by comparing the obtained angular distribution with the predictions of the effective phase function theory reported recently in the literature and with the angle resolved experimental measurements performed on human red blood cells (RBCs) in suspensions at different hematocrits. The results show a good agreement in the small concentration range. 相似文献
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根据量子力学原理,氢原子中电子的几率分布由电子的定态波函数可以唯一地确定.在大多数教科书中,通常是将电子沿径向和角度方向的几率分布分别进行讨论,因此很难给出电子在整个空间几率分布的一个完整图象.我们使用蒙特卡罗随机抽样的方法,通过计算机作图形象而逼真地绘制了氢原子中电子的几率分布图,即所谓的电子云.下面介绍所使用的方法. 相似文献
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Michael Betancourt 《Annalen der Physik》2019,531(3)
From its inception in the 1950s to the modern frontiers of applied statistics, Markov chain Monte Carlo has been one of the most ubiquitous and successful methods in statistical computing. The development of the method in that time has been fueled by not only increasingly difficult problems but also novel techniques adopted from physics. Here, the history of Markov chain Monte Carlo is reviewed from its inception with the Metropolis method to the contemporary state‐of‐the‐art in Hamiltonian Monte Carlo, focusing on the evolving interplay between the statistical and physical perspectives of the method. 相似文献
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Finding a Probe for Extracting Information on the Momentum Dependent-Interaction in Heavy Ion Collisions 下载免费PDF全文
The effect of momentum-dependent interaction on the kinetic energy spectrum of the neutron-proton ratio ( (n/p)gas)b( Ek ) for 64Zn +64Zn is studied. It is found that ( (n/p)gas)b( Ek ) sensitively depends on the momentumdependent interaction and weakly on the in-medium nucleon-nucleon cross section and symmetry potential. Therefore ( (n/p)gas)b( Ek ) is a possible probe for extracting information on the momentum-dependent interaction in heavy ion collisions. 相似文献
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Technical Physics - We investigate the structure of the wall sheath of a gas discharge near a flat surface at a negative potential for high mean electron energy. It is shown that in the conditions... 相似文献
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Monte Carlo Simulation on Measurements of Branching Fractions for Some Exclusive Semileptonic Decays of Neutral D Mesons on BES-III 下载免费PDF全文
LIU Hui-Hui CHEN Jiang-Chuan RONG Gang MA Hai-Long ZHAO Ming-Gang LIU Jian 《中国物理快报》2008,25(4):1231-1234
Using the Monte Carlo samples of about 800 pb-1 and 500pb-1 generated at and around 3.773GeV with BOSS (BESIII Offline Software System) 6.0.2 and BOSS 6.1.0, we have studied the branching fractions for D0 →K-l+νl and D0 →πi-l+νl (l=e, μ). According to the simulation results, the expected precision of these measurements for 20fb-1 ψ(3770) data which will be collected on BESIII are well estimated. 相似文献
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This article reports about the ion sheath thickness variation occurring in front of a negatively biased plate immersed in the target plasma region of a double plasma device. The target plasma is produced due to the local ionization of neutral gas by the high energetic electrons coming from the source region (main discharge region). It is observed that for an increase in cathode voltage (filament bias voltage) in the source region, the ion flux into the plate increases. As a result, the sheath at the plate contracts. Again, for an increase in source anode voltage (magnetic cage bias), the ion flux to the plate decreases. As a result, the sheath expands at the plate. The ion sheath formed at the separation grid of the device is found to expand for an increase in cathode voltage and it contracts for an increase in the anode voltage of the main discharge region. One important observation is that the applied anode bias can control the Bohm speed of the ions towards the separation grid. Furthermore, it is observed that the ion current collected by the separation grid is independent of changes in plasma density in the diffusion region but is highly dependent on the source plasma parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Effect of Discharge Voltage on an Ion Sheath Formed at a Grid in a Multi-Dipole Discharge Plasma 下载免费PDF全文
M. K. Mishra A. Phukan 《中国物理快报》2008,25(3):1011-1014
It is experimentally demonstrated that a relatively strong ion-rich sheath formed at a fixed negative bias of the grid can be changed to a rather weak ion sheath (sheath potential weakly retards electrons) only by increasing the discharge voltage in the system. At sufficiently high negative grid bias, an increase of discharge voltage enhances the ion collection current at the grid. An explanation is put forward in support of this experimental observation. A slight density enhancement with a fall in plasma electron temperature is also observed with the increasing negative grid bias. 相似文献
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Langmuir Probe in Non‐Maxwellian Plasma: Correlation Between the Electron Energy Distribution Function,the Ion Energy at the Sheath Edge and the Floating Voltage 下载免费PDF全文
This work is devoted to the study of the Langmuir probe in non‐Maxwellian plasma, assuming mono‐energetic singly charged ions and a collisionless sheath. Using a general analytical equation for the Electron Energy Distribution Function (EEDF), we study the effect of the EEDF profile on: a) The ion energy at the sheath edge of a negatively biased collector, b) the I‐V probe characteristic and c) the floating voltage (Vp‐Vf). Different methods are used and compared to determine these parameters or characteristics. A correlation is given between the floating voltage, the ion energy at the sheath edge and the EEDF profile. The study is also extended to distribution functions with several components. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献