Modelling of Carbon Erosion and Transport at Divertor Target Plate Using a Suite of Codes

To investigate carbon‐based material erosion and ensuing eroded carbon species density distribution, a Particle‐In‐Cell Monte Carlo collision code plus a plasma‐surface interaction code were combined with a Monte Carlo impurity transport code ITCD. It is found that a “density hill” forms at a distance of 3‐4 mm from divertor target plate under certain circumstances: high plasma temperature, small incident angle and strength of the magnetic field. By means of the study with the combined code on several scenarios with different plasma temperatures, the incident angles and strengths of the magnetic field, the simulation results reveal that they have a significant influence on the chemical erosion of carbon‐based material and the spatial distribution of eroded carbon species density above divertor target plate: the eroded carbon species density is smaller at a lower plasma temperature; the scenario with smaller incident angle and greater strength of the magnetic field can reduce the chemical erosion of carbon‐based material and the eroded carbon species density above divertor target plate evidently (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transport and Energy Deposition of hot Electrons through the Shock Ignition pre Compressed Target

Shock ignition as an alternative scheme of the laser fusion has the potential of achieving efficient implosion. However, hot electrons produced in result of ignitor‐corona interaction may penetrate deep into the fuel making the compression less effective. Transport and energy deposition of hot electron beam into the dense pre compressed of HiPER target by means of Monte Carlo approach are discussed considering the influence of real density and electron beam characteristics. The target parameters before igniting the hot spot have been extracted from a fluid code and used as the initial profile for Monte Carlo simulations. In comparison with simplified step like density profile, electrons penetrate slightly deeper in the case of real shaped density profile. In addition, deposition zone of a broad spectrum electron beam is wider while, monoenergetic electrons depose their energy locally resulting more maximum energy deposition value. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Deuterium and Impurity Flux Measurements in the Boundary Plasma of T-10

Time resolved measurements of deuterium and impurity fluxes in the boundary layer of T-10 tokamak were carried out using carbon strips as deposition probes. Analysing these probes by RBS and ERD deuterium impact energies of 25–40 eV could be estimated from the saturation behaviour of deuterium in carbon. The high impurity concentration level and it's time behaviour give indications for local impurity producing sources playing an important role for the impurity transport in the near wall plasma.     

Deuterium-tritium fuel impact ignition in the presence of degenerate plasma

The impact ignition model is proposed based on the collision of a deuterium-tritium (DT) layer accelerated to high velocities in a conical target. Simple mechanism, low cost, high coupling efficiency, and lack of the need for Petawatt laser pulses are the prominent advantages of this model. However, an increase in the productivity of this ignition mechanism is an important issue. In this regard, in this paper, the idea of impact ignition using the plasma degeneracy mechanism has been investigated. For this purpose, first, the ignition energy gain and stopping power of the DT beam in pure and impure fuels, by employing both degenerate and non-degenerate plasmas, have been examined numerically. Then, in order to assess the penetration depth and range of the incident beam, simulations have been carried out using a three-dimensional (3D) Monte Carlo code for two states of degenerate and non-degenerate pre-compressed pure fuel. The results imply that the state of degeneracy causes an increase by about 63% in the energy gain of impact ignition. In addition, the degeneracy condition leads to an approximate enhancement of 60% in the energy deposition of the pure fuel and about 67% for the impure fuel, with a mixed density ratio of 1.5%; therefore, the range and penetration depth decrease significantly in comparison to the non-degenerate one. This can be indicative of the increasing efficiency of impact ignition conditions in the presence of degenerate plasma. The results of the range for the pure fuel have also been confirmed by a 3D Monte Carlo simulation code.     

Particle in Cell/Monte Carlo Collision Method for Simulation of RF Glow Discharges: Effect of Super Particle Weighting

A parallel Particle in Cell/Monte Carlo Collision (PIC/MCC) numerical code for glow discharge plasma simulations is developed and verified. This method is based on simultaneous solution of the Lorentz equations of motion of super particles, coupled with the Poisson's equation for electric field. Collisions between the particles are modelled by the Monte Carlo method. Proper choice of particle weighting is critically important in order to perform adequate and efficient PIC simulations of plasma. Herein, effects of particle weighting on the simulations of capacitive radio‐frequency argon plasma discharges are studied in details. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Equivalent detector models for the simulation of efficiency response of an HPGe detector with PENELOPE code

ABSTRACT

In this work, some ‘equivalent’ models for the simulation of efficiency response of a High-Purity Germanium (HPGe) detector, installed inside a ‘low background’ bunker in the Engineering Department of the University of Palermo, were developed. The main feature was to attribute the uncertainties of the model to only one of the parameters, the dead layer of the detector, keeping unchanged the other data provided by the manufacturer. With this technique, using the Monte Carlo PENELOPE code in the 2011 version, the efficiency response was evaluated and compared with the previous one performed with MCNP5 code.

The validation of equivalent models is performed by comparing the simulation results with that of experimental spectrometric measurements of calibrated point sources and characterized volumetric sources such as a Marinelli beaker and an air filter reduced to a ‘packet-sample’.

The use of equivalent models makes the evaluation of efficiency curves with a Monte Carlo code easier and faster, and requires only a few experimental values for validation.     

Numerical Modelling of Impurity Retention in the ITER Tokamak Scrape-off Layer Plasma

The hydrogen plasma and carbon ions flows generated at the target plates in the ITER tokamak scrape-off layer are numerically investigated. A 2D model of hydrogen plasma and impurity ions is presented. The model is based on the electron-ion Braginski fluid equations [1] for hydrogen plasma and rate equations for impurities. Arbitrary level of impurity ions concentration is assumed. Recycling of hydrogen, sputtering and self sputtering of carbon atoms at the target plates are taken into account. Equations of the model are solved in the slab geometry using 2D-multifluid numerical code EPIT [2]. Problems of impurity ions retention and radiation in the divertor volume are analyzed. Results of calculations for ITER tokamak boundary plasma are presented, showing that poor retention is likely at high impurity concentration in the divertor volume. The radiation power can be a significant part of ingoing energy.     

Mechanisms for Power Deposition in Ar/SiH4 Capacitively Coupled RF Discharges

In low-pressure capacitively coupled parallel-plate radiofrequency (RF) discharges, such as those used in plasma processing of semiconductor materials, power deposition and the rate of electron-impact excitation collisions depend upon time during the RF cycle and position in the discharge. Power is coupled into the discharge in at least two ways: by way of a high-energy "e-beam" component of the electron distribution resulting from electrons falling through or being accelerated by the oscillating sheaths, and by "joule heating" in the body of plasma. This paper will discuss the method of power deposition by electrons and the spatial dependence of electron-impact excitation rates in low-pressure capacitively coupled RF discharges with results from a Monte Carlo plasma simulation code. Mixtures of argon and silane will be examined as typical examples of discharges used for the plasma deposition of amorphous silicon.     

Magnetic properties of diluted magnetic semiconductors: Quantum Monte Carlo approach

The magnetic correlation between magnetic impurities in semiconductors is investigated by performing the quantum Monte Carlo (QMC) simulation. The Anderson Hamiltonian with the realistic parameters obtained by the local density approximation (LDA) calculation is employed. The LDA calculation gives a dispersion of the host (GaAs) electron and the mixing energy between host and magnetic impurity (Mn). The mixing between host and impurity electrons generates the impurity bound state in the energy gap of semiconductors. The long range ferromagnetic coupling is observed when the Fermi energy locates between the band edge and the impurity bound state. The ferromagnetic coupling is enhanced by decreasing temperature.     

An efficient numerical tool for dose deposition prediction applied to synchrotron medical imaging and radiation therapy

Medical imaging and radiation therapy are widely used synchrotron‐based techniques which have one thing in common: a significant dose delivery to typically biological samples. Among the ways to provide the experimenters with image guidance techniques indicating optimization strategies, Monte Carlo simulation has become the gold standard for accurately predicting radiation dose levels under specific irradiation conditions. A highly important hampering factor of this method is, however, its slow statistical convergence. A track length estimator (TLE) module has been coded and implemented for the first time in the open‐source Monte Carlo code GATE/Geant4. Results obtained with the module and the procedures used to validate them are presented. A database of energy‐absorption coefficients was also generated, which is used by the TLE calculations and is now also included in GATE/Geant4. The validation was carried out by comparing the TLE‐simulated doses with experimental data in a synchrotron radiation computed tomography experiment. The TLE technique shows good agreement versus both experimental measurements and the results of a classical Monte Carlo simulation. Compared with the latter, it is possible to reach a pre‐defined statistical uncertainty in about two to three orders of magnitude less time for complex geometries without loss of accuracy.     

高能铁离子在水介质中核反应过程所导致的能量沉积

在银河宇宙射线中,400MeV/nucleon的铁离子通量相对来讲是较高的,400MeV/nucleon的铁离子对空间辐射引起的损伤和辐射剂量有重要的贡献.本文以Geant4为基础,对400MeV/nucleon Fe离子与物质相互作用后通过核反应过程产生的次级碎片进行模拟分析.对铁离子在水中产生的能量沉积和铁离子与水介质发生核反应后产生的次级碎片的能量沉积进行了模拟研究,得到了通过核反应过程产生次级粒子所导致的剂量贡献. 关键词： 能量沉积 剂量 碎片     

Proton Crystallization in a Dense Hydrogen Plasma

We perform a first‐principle analysis of Coulomb crystallization in neutral two‐component mass asymmetric plasmas by large‐scale path integral Monte Carlo simulations for a dense hydrogen plasma in a broad density range. We observe two large jumps in the relative distance fluctuations of protons that are connected with qualitative changes in the behavior of the proton pair distribution function and are attributed to the formation of a spatially ordered state. A third smaller jump inside the ordered phase indicates a structural transition (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

高能质子在散裂靶中的能量沉积计算与实验验证

高能质子在散裂靶中的能量沉积是散裂靶中子学研究的重要内容之一,准确掌握高能质子在散裂靶中引起的能量沉积分布与瞬态变化是开展散裂靶热工流体设计的重要前提.本文采用MCNPX,PHITS与FLUKA三种蒙特卡罗模拟程序,计算并比较了高能质子入射重金属铅靶、钨靶的能量沉积分布及不同粒子对总能量沉积的占比贡献;针对高能质子入射...     

Etchant anisotropy controls the step bunching instability in KOH etching of silicon

STM investigations of vicinal Si(111) surfaces etched in KOH solutions under controlled flow conditions show that step bunching instability is due to inhomogeneities that develop in the etchant as the result of highly step-site-specific etching reactions. Other previously postulated mechanisms for step bunching, including anisotropic surface diffusion, surface strain, and impurity deposition, are conclusively ruled out. The inhomogeneities locally accelerate etching near surface steps. Kinetic Monte Carlo simulations of this process qualitatively reproduce the observed morphologies.     

Investigation of optical emission spectroscopy in diamond chemical vapor deposition by Monte Carlo simulation

The optical emission spectra(atomic hydrogen(Hα,Hβ,Hγ),atomic carbon C(2p3s→2p2:λ=165.7 nm) and radical CH(A2△→X2П:λ=420-440 nm))in the gas phase process of the diamond film growth from a gas mixture of CH4 and H2 by the technology of electron-assisted chemical vapor deposition (EACVD)have been investigated by using Monte Carlo simulation.The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons.And a method of determining the mean temperature of electrons in the plasma in-situ iS given.The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.     

Pseudospinodal in the Monte Carlo simulation of the decomposition of an alloy

A hypothesis about the existence of a pseudospinodal separating the homogeneous and heterogeneous nucleation regions in the phase diagram of a binary alloy has been proposed on the basis of the results obtained from the Monte Carlo simulation of the decomposition in a simple system with a short-range attractive potential of the impurity atoms.     

液体闪烁体探测器γ射线测量研究

利用几种单能γ源对BC501A型液体闪烁体探测器进行能量刻度, 以得到探测器对电子的光输出响应函数。 介绍了用蒙特卡罗模拟法确定康普顿边缘所对应的电子能量的方法, 得到的结果与用半高点确定康普顿边缘方法所得的结果做了比较。 利用德国PTB开发的PHRESP蒙特卡罗程序计算出液体闪烁体探测器对各种单能γ射线的光响应矩阵, 阐述了利用γ射线响应函数矩阵和γ射线的反冲电子谱求解待测γ能谱的原理, 对解谱的误差来源也进行了简要的分析。 The light output of the BC501A liquid scintillation detector for γ rays was calibrated by serial monoenergetic γ ray sources. The Monte Carlo simulation method to confirm Compton edge was described briefly. The result was compared with that obtained by the semi height method. The γ ray response matrix of BC501A liquid scintillation detector was calculated by the Monte Carlo code PHRESP from PTB. The method of using response function matrix and recoil electron spectrum to unfold γ ray energy spectrum was introduced and the error of unfolding spectrum was also briefly discussed.     

蒙卡中子输运程序JMCT和子通道热工水力程序COBRA-EN耦合计算

介绍了中子输运蒙特卡罗方法与热工水力耦合计算的流程。开发了一套蒙卡中子输运程序JMCT和子通道分析程序COBRA-EN耦合接口。通过33棒束模型的计算展示了考虑耦合计算和不考虑耦合计算的差异,论证了耦合计算在反应堆分析中的重要性。通过对反应堆组件的模拟计算,测试了耦合计算的正确性。最后分析了蒙卡计算的统计涨落和迭代计算过程中收敛标准的关系,讨论了蒙卡中子输运和热工水力耦合过程中收敛标准设置的方案和可行性。     

Particle-in-cell plus direct simulation Monte Carlo (PIC-DSMC)approach for self-consistent plasma-gas simulations

The particle-in-cell (PIC) and direct simulation Monte Carlo (DSMC) approaches have been combined into a PIC-DSMC model for self-consistent simulations of low-temperature collisional plasmas and the background gas. This novel approach is based on the weighting collision simulation scheme allowing for disparate number densities and time scales of different species. The applicability of the developed algorithm is illustrated by simulations of one-dimensional direct current and two-dimensional magnetron sputtering discharges in argon. An appreciable effect of the energetic discharge species on the density, temperature, and flow field of the background gas shows the importance of the coupled plasma-gas simulation for such technologies as sputtering, dry etching, plasma enhanced vapor deposition, etc