Effect of Equilibrium Current Profiles on External Kink Modes in Tokamaks

Based on a linearized MHD model, the effect of equilibrium current profiles on external kink modes in tokamaks is studied by MARS code. Three types of equilibrium current profiles are adopted in this work. Firstly, a set of parabolic equilibrium current profiles are chosen. In these profiles the maximum current values in the center of the plasma are fixed, and the currents have different gradient and jump at the plasma boundary. The effects of the current gradient and jump on the growth rate of external kink mode are investigated. It is found that the current jump which causes the q profiles to change plays an important role in the externM kink modes in tokamaks. Secondly, a set of step equilibrium current profiles with different jump positions are chosen. The effect of jump position on external kink modes is discussed. Thirdly, a set of parabolic equilibrium current profiles with current bumps are chosen for the case of off-axis heating. The effects of height~ width and position of the current bumps on external kink modes are analyzed. The fiat equilibrium current profiles are disadvantageous for the MHD stabilities of tokamaks, because of the large current jump at the plasma edge. The peaked equilibrium current profiles and a large and localized current bump near the plasma edge benefit the MHD stabilities of tokamaks.     

Fission product migration in the coatings of irradiated uranium dioxide HTR fuel particles

Electron probe microanalysis has been used to determine quantitative profiles of the fission products caesium, xenon and barium in the coatings of UO₂ HTR fuel particles irradiated for up to 140 days to burn-ups of between 5% and 11% heavy atom fission at temperatures of 1100–1500°C. The xenon profiles were typical of recoil. Barium profiles showed a peak due to recoil but also a low level ‘tail’ extending to the silicon carbide layer indicating a fast diffusing barium species. In contrast to barium, strontium profiles were not observed. Caesium was the most abundant fission product in the inner pyrocarbon coating. The profiles often extended to the silicon carbide layer where a marked fall in caesium concentration was observed. Some profiles displayed zones of fairly constant caesium concentration which it is suggested correspond to caesium/pyrocarbon ‘compound’ formation as opposed to physical aspects of the manufacturing process.     

A genetic algorithm used for the intensity level discretization in MLC leaf sequencing for step and shoot IMRT

The inverse planning for a step-and-shoot plan in intensity-modulated radiotherapy （IMRT） is usually a multiple step process. Before being converted into the MLC segments, the optimum intensity profiles of beams, which are generated by an optimization algorithm, shall be discretized into a few intensity levels. The discretization process of the optimum intensity profiles can induce deviations in the final dose distribution from the original optimum dose distribution. This paper describes a genetic algorithm for the discretization of given optimum intensity profiles. The algorithm minimizes an objective function written in terms of the intensity levels. Both the dose-based objective function, which is defined by the deviation between the dose distributions before and after the discretization, and the intensity-based objective function, which is defined by the deviation between the optimum intensity profiles and the discretization intensity profiles, have been adopted. To evaluate this algorithm, a series of simulation calculations had been carried out using the present algorithm, the even-spaced discretization and the k-means clustering algorithm respectively. By comparing the resultant discretization-induced deviations （D!Ds） in intensity profiles and in dose distributions, we have found that the genetic algorithm induced less DIDs in comparison with that induced in the even-spaced discretization or the k-means clustering algorithm. Additionally, it has been found that the DIDs created in the genetic algorithm correlate with the complexity of the intensity profiles that is measured by the ＂fluence map complexity＂.     

Saturation of deuterium retention in carbon a new calibration for plasma edge probes

Depth profiles were measured for deuterium implanted in carbon at 530, 1500, and 3000 eV. Measured profiles were in good agreement with profiles calculated using the TRIM code. Comparison between profiles for high and low fluence implants show a scaling of the local deuterium concentration with incident deuterium fluence up to a saturation concentration of 0.44 ± 0.08 D/C ratio. New measurements of retention versus fluence for monoenergetic deuterium implanted in carbon for incident energies from 300 to 1500 eV are in very good agreement with the retention behavior predicted by a saturation model using the observed depth profiles and saturation concentrations. The retention is calculated for deuterium with a Maxwellian velocity distribution to provide a new calibration for the determination of the energy and flux of deuterium incident on carbon probes exposed to the edge of magnetically confined plasmas. This new calibration gives deuterium energies and fluxes significantly different from those obtained using earlier calibrations. A simple analytic representation is also presented for retention of deuterium with a Maxwellian velocity distribution which provides a systematic approach for analyzing probe data.     

Image processing methods for noise reduction in the TJ-II Thomson Scattering diagnostic

The Thomsom Scattering diagnostic of the TJ-II stellarator provides temperature and density profiles. The CCD camera acquires images corrupted with noise that, in some cases, can produce unreliable profiles. The main source of noise is the so-called stray-light. In this paper we describe an approach that allows mitigation of the effects that stray-light has on the images: extraction regions with connected-components. In addition, the robustness and effectiveness of the noise reduction technique is validated in two ways: (1) supervised classification and (2) comparison of electron temperature profiles.     

Measurement of Liquid Turbulent Structure in Bubbly Flow at Low Void Fraction Using Ultrasonic Doppler Method

Microscopic structure in bubbly flows has been a topic of interest in the study of fluid dynamics. In the present paper, the ultrasonic Doppler method was applied to the measurement of bubbly. The experiments were carried out for an air-water dispersed bubbly flow in a 20 mm × 100 mm vertical rectangular channel having a void fraction smaller than 3%. Two ultrasonic transducers were installed on the outer surface of the test section with a contact angle of 45^° off the vertical axis, one facing upward and the other facing downward. By applying statistical methods to the two directional velocity profiles, Reynolds stress profiles were calculated. Furthermore, to clarify the wake effect induced by the leading bubbles, the velocity profiles were divided into two types of data. The first one is for all of the liquid data and the other is the data which did not include the wake effect. For Re_m ≥1,593, it was observed that the bubbles suppressed the liquid turbulence. Furthermore, comparing with the Reynolds stress profiles in bubbly flow, it was found that Reynolds stress profiles varied with the amount of bubbles present in the flow and the effect of wake causes turbulence in the liquid.     

N~+和Bi~+离子注入InP反冲原子P和In深度分布偏离的研究

本文给出了用背散射和沟道技术所分析的N~+和Bi~+注入InP中In和P原子位移的面密度分布。发现移位原子In和P面密度分布峰的深度随注入剂量的增大而加深。在相同的剂量下磷原子分布峰与表面的距离比In原子峰的大。从移位原子面密度分布峰的移动和分布的展宽与注入剂量的关系出发,研究了离子注入过程中所存在的反冲效应。并讨论了移位原子的反冲机理。     

Experimental study on a steam-driven turbulent jet in subcooled water

This study was designed to investigate the behaviour of a steam-driven turbulent jet in subcooled water. The total pressures along the axis of the jet were measured under various steam mass fluxes, water temperatures and nozzle sizes. The results indicated that the lifting-pressure coefficient decreased with inlet steam pressure and water temperature, and an optimal lifting-pressure coefficient could be obtained at pressure ratio 0.228. Then an empirical correlation based on the influencing factors was given to fit the position of the maximum pressure. Moreover, the velocity profiles in the turbulent jet region were obtained, and the results showed the self-similarity property of the velocity profiles. An empirical correlation was suggested to correlate the jet half width obtained from the velocity profiles.     

The submicroscopic track kinetic theory and the variational principle

The variational principle was used to obtain the general equations for chemical etching track profiles inside and outside the physical damage region produced by energetic ions in Makrofol E. The track profiles developed at very short etching times were used to obtain the velocity profiles in order to perform computer determinations of track profiles for longer etching times. Track profiles obtained by means of the variational principle and the submicroscopic track kinetic theory are in good agreement, and compare favorably with experimental results.     

Two-point model analysis of SOL plasma in EAST

A two-point model is used to investigate the characteristics of scrape-off layer(SOL) plasma with the field line tracing method in the experimental advanced superconducting tokamak. The profiles of plasma density, temperature and particle flux on the divertor target calculated by the model are in reasonable agreement with experimental observation. Moreover, the profiles of plasma parameters on the divertor target strongly depend on the SOL magnetic topology or the equilibrium configuration from the modeling.     

Temperature dependence of trapping and depth profiles of 6 to 15 kev deuterium in carbon

Depth profiles of 30 keV D⁺₂ and 20 keV D⁺₂ implanted into edge and basal-oriented pyrolytic graphite have been measured by means of the D(³He,α)H nuclear reaction in the temperature range of 300 to 800 K. At room temperature deuterium concentrations up to 30 at.% are found in a surface layer corresponding to the range of the ions. The measured depth profiles do not fully agree either with calculated range profiles or with the damage profiles, but are determined by the two together. At higher temperatures the deuterium concentrations decrease and the profiles broaden. At room temperature the amount of trapped deuterium increases linearly with dose below 10¹⁸ deuterons/cm². The trapping coefficient is roughly 60%. At 5 × 10¹⁸ deuterons/cm² the amount of trapped deuterium in the probed layer (~4000 Å) reaches saturation and the trapping coefficient becomes zero. The saturation value decreases with increasing temperature and increases with increasing energy.     

中压沸腾工况相径向分布实验研究

采用高温高压单探头光学探针．在中压沸腾工况下进行了局部空泡率与汽泡频率径向分布特性实验研究,并根据探针信号对两相流流型进行了识别,分析了中压沸腾工况下空泡率径向分布与流型的关系。研究结果表明：随热平衡含汽率增加,整个直径方向上空泡率分布从近U形向鞍形和弧形发展;汽泡频率则以近U形分布为主;泡状流工况下,空泡率呈U形或近壁区显著高于中心区的鞍形分布,弹状流工况下,中心区空泡率略低于近壁区。整个直径上空泡率呈弧形分布。     

Three-dimensional elastic recoil detection analysis of hydrogen in materials for use in the Yucca Mountain nuclear waste repository

An original system has been developed capable of performing three-dimensional Energy Recoil Detection Analysis (ERDA) of hydrogen in materials with scanned and finely focused heavy-ion beams. The technique is being used at the Lawrence Livermore National Laborotory Multi-user Tandem Laboratory to measure the hydrogen content in materials under consideration for use in the Yucca Mountain nuclear waste repository. From the measurement of hydrogen concentration profiles we can extrapolate reaction rates. A critical problem is the rate of dissolution of the glass being used. The HI-ERDA (Heavy Ion-ERDA) technique can provide this information which is needed in order to predict the overall rate of nuclear waste glass degradation in a waste repository. The technique is calibrated using a silicon wafer implanted with a known concentration of hydrogen. The sample is illuminated by a ³⁵Cl ion beam that is micro-scanned across the sample. From these measurements we reconstruct three dimensional profiles of hydrogen content which can then be used to obtain spatially-resolved hydrogen depth profiles.     

Numerical Research of Mode Conversion in an Inhomogeneous Plasma

The linear mode conversion of electromagnetic waves in the hot, unmagnetized inhomogeneous plasma is studied numerically for different density profiles, and the dependence of the absorption coefficient on the incident angles and the wave frequencies are obtained for different electrons＇ temperature. The results show that the shapes of the density profiles and the electron＇s temperature create a certain effect on the coefficients of absorption, which reaches its peak value （about 50%） for appropriate parameters. Effective absorption occurs in a limited range of parameter q.     

Damage distributions in LiNbO3 crystal induced by MeV F tilted implantations

X-cut LiNbO₃ crystals have been implanted by 0.8, 1.0 and 1.2 MeV F⁺ tilted at angles of 15°, 45° and 60° with doses of 5 × 10¹⁴, 7 × 10¹⁴ and 5 × 10¹⁴ ions/cm², respectively. The Rutherford backscattering (RBS)/channeling technique was used to investigate the induced damage distributions. The damage profiles were deconvoluted from the measured spectra after considering the energy spread due to the different stopping power of channeled and nonchanneled ions. Good agreements were obtained between the measured damage profiles and the calculated defect profiles by TRIM'90 (transport of ions in matter, version 1990), except that the measured damage concentration was enhanced in the near-surface region. Information on the lateral and longitudinal damage spread in LiNbO₃ crystals was obtained from the damage profiles induced by tilted ion implantations and compared with TRIM'90 calculation.     

A new approach to plasma profile control in ITER

Advanced tokamak operation in ITER, such as the steady-state and hybrid modes, requires an active real-time feedback control of plasma profiles to achieve the advanced regimes for sustained operation. In this work, we have explored a potentially robust control technique that simplifies the active real-time control of electron temperature and safety factor profiles in ITER. As a new and simple approach, static responses of the plasma profiles to power changes of auxiliary heating and current drive are modelled and updated in real-time, differing from the techniques which use a dynamic model deduced from identification experiments, or even a simplified explicit model. To allow real-time update of the plasma profile response model, the underlying physics is simplified with several assumptions. The electron temperature profile response is modelled by simplifying the electron heat transport equation. The safety factor profile response is modelled by directly relating it to the changes of source current density profiles. The required actuator power changes are calculated using the singular value decomposition technique, taking the saturation of the actuator powers into account. The potential of this control technique has been tested by applying it to simulations of the ITER hybrid mode operation using CRONOS. In these simulations, the electron temperature and safety factor profiles were well controlled either independently or simultaneously.     

Reconstruction of submicron size spatial distributions of implanted particles in solids by computer tomography

By means of computer tomography, distributions of implanted ions in solids can be reconstructed in two or three dimensions. For this purpose, a set of depth profiles, measured under various implantation angles but otherwise identical conditions has to be provided. This approach is independent of the measuring technique by which the depth profiles are obtained, and therefore allows even the determination of submicron structures. For amorphous targets with rotationally symmetric implanted particle distributions, the evaluation is especially easy, as only two-dimensional concentration distributions need to be determined. Further, spatial distributions of radiation damage by transferred nuclear or electronic energy might be obtained, and the examination of implanted particle distributions after modification by diffusional processes is also possible. The mathematical problem to convert the set of angularly dependent depth profiles to a spatial particle distribution is treated by solving the corresponding matrix equation in an iterative way, as described in the literature on medical tomography. Special attention has to be given to the near-surface depletion in the depth profiles due to backscattering, which might lead to some distortion of the reconstructed distributions. To perform tomograpic measurements at an accelerator on a routine basis, it is advisable to modify the conventional experimental setup slightly, as described here. As a first example, the spatial distribution of 100 keV B in amorphized Si is derived from depth profiles which were measured previously by a nuclear reaction analysis technique with thermal neutrons. Those depth distributions showed slight deviations (about 20%) from the theoretically predicted ones (TRIM) in directions between about 30° and 60°. This results in a somewhat different distribution than predicted.     

Effects of kinetic profiles on neutron wall loading distribution in CFETR

The China Fusion Engineering Test Reactor (CFETR) is under design,which aims to bridge the gaps between ITER and the future fusion power plant.The neutron wall loading (NWL) depends on the neutron source distribution,which depends on the density and temperature profiles.In this paper,we calculate the NWL of CFETR and study the effects of density and temperature profiles on the NWL distribution along the first wall.Our calculations show that for a 200 MW fusion power,the maximum NWL is at the outer midplane and the vaule is about 0.4 MW m-2.The density and temperature profiles have little effect on the NWL distribution.The value of NWL is determined by the total fusion power.     

Derivation of Finite Medium Age-Diffusion Kernels in Source-Sink Theory

Based on a simple adsorption theory, a mathematical model was proposed to predict axial iodine profiles of the column of silica gel impregnated with silver nitrate (hereinafter referred to as Ag-S) in an off-gas treatment system for spent fuel dissolution. The unknown parameters of the model: the effective diffusion coefficient D_ea and the Langmuir coefficient K were determined by curve fitting of iodine profile experimentally obtained. At 423 K, D_ea and K were found to be 5.60×10^-7 m².s^-1 and 1.0×10⁵ m³.kg^-1, respectively. Using the parameter values, the model could well predict the iodine profiles obtained at 423 K in the previous works under different experimental conditions. Furthermore, the effect of silver contents on the iodine profiles was reasonably predicted. It was suggested that the proposed model is simple and would be useful to predict the iodine profiles in Ag-S adsorbent columns.