Assessment of ionization-assisted damage processes in CTR first-wall materials

Assessments have been made of the potential significance for CTR first-wall radiation damage of ionization-assisted processes. Mechanisms are examined in which the energy available for atomic displacements is supplied by transient Coulomb interactions attending ionization and charge transfers induced in relevant materials by primary plasma radiation or by recoils. An energy-deposition cross section appropriate to such mechanisms is defined, compared numerically to the usual Lindhard nuclear stopping cross section and used to estimate maximal possible effects. We find that, for materials suggested for first-wall applications and under anticipated fluxes of plasma components, the calculated energy-deposition cross sections for the mechanisms considered are insignificant in comparison to the nuclear stopping cross sections except in some cases of He⁺⁺ projectiles in non-metals.     

Tritium management in the first-wall materials of A-DC and TAURO blankets

The tritium management in the first wall of two European breeding blanket options, A-DC and TAURO, has been simulated numerically to analyse the influence of the material selected: ODS-RAFM steel for the Advanced Dual-Coolant (A-DC) and SiC_f/SiC composite for TAURO options. The SRIM code has been used to simulate triton implantation and define the tritium source in each kind of material as a function of the depth. The TMAP4 code was used to analyse the posterior transitory gas transport process within the material, while taking into account the tritium transport properties of each material and the temperature variation through material thickness and operating time. Both the transient evolution and the final steady-state tritium transport behaviour have been characterised. The tritium transient flux to the coolant, the recycling flux and the absorbed tritium transient inventories have been simulated. Main conclusions have been drawn about the tritium performance of each first wall.     

Dependence of blister-“deckeldicke” and of depth profiles of implanted He ions in Nb on angle of incidence

The dependence of blister deckeldicke and of depth profiles of 30 and 100 keV ³He ions incident on niobium single crystals at different angles of incidence has been measured by Rutherford backscattering (double alignment), scanning electron microscope, and nuclear reaction techniques. A strong dependence of deckeldicke and blister size on the angle of incidence is found. Deckeldicke is found to be larger than the most probable ion range. For small angles of incidence, the result agrees with the depth of the half maximum value of the ³He distribution.     

Measurement of He and H depth profiles in tungsten using ERDA with medium heavy ion beams

Elastic recoil detection analysis method (ERDA) with medium heavy analyzing ion beam and its application for the simultaneous measurement of light elements in a very heavy substrate is presented. The availability of cross section data and the method of cross section calculation for recoiled particles are discussed. Different ion species for analyzing beam are discussed with respect to the cross-section data availability, sensitivity of the method, and the depth resolution. Calculations of depth resolution for each element and maximum depth of analysis for tungsten substrate are presented. The influence of the detector geometry and multiple scattering effects on the depth resolution is discussed. An example spectrum measured on tungsten implanted with He seeded D plasma is shown.     

Comparison of reconstruction methods of depth distribution of tritium in materials based on BIXS

Three reconstruction methods based on Tikhonov regularization are used to solve the ill-posed inverse problem when incorporating Monte Carlo calculations into the β-ray induced X-ray spectrometry (BIXS) for analyzing the tritium content and depth distribution in the material surface. These methods are implemented by the GENEREG, CONTIN and REGUTOOLS codes, respectively. The tritium sample is assumed to be a molybdenum slab of 5 μm thickness, and an argon gas of 5 mm thickness is filled in front of the molybdenum slab. The Monte Carlo code PENELOPE is used to simulate the X-ray energy spectra induced by tritium’s β-decays. The reconstruction results for tritium depth distributions by these three codes are compared and the comparison shows that the code GENEREG can give satisfactory results. Some discussions are made.     

A direct and fast computing method for damage energy depth distribution in ion implanted materials

A theoretically simple and computationally fast method for calculating damage energy density distributions for ions implanted into noninsulating materials is presented. The effect of the energy transported by the recoiling target atoms is taken into account in order to extend the calculation to lower energies and the near-surface target region. It is found that the results obtained over energy and mass ranges of interest for ion implantation experiments are in good agreement with previously reported experimental and calculated “damage” distributions.Special emphasis is placed upon applications to surface studies, sputtering and amorphization of Si(111) surfaces, as recently reported in a REM study.     

Measurement of nitrogen depth profile in steel

The characterization of nitridated steel samples, in special the depth profile of nitrogen, aims to help improving the quality of the surface and to increase the durability of the steel pieces. In this work we used ERDA and NRA to determinate the profile of nitrogen in different sets of stainless steel samples. An incident beam of ³⁵Cl of 56 MeV was used for ERDA analysis of a first set of samples. Results indicated an homogeneous distribution for most of the identified elements, with atomic nitrogen concentrations around 2% in the analyzed depth range (0.2 μm) and the presence of thin films on the surface (about 50 × 10¹⁵ at/cm²), one of C and the other of iron oxide. In a second set of samples, 4.43 MeV gamma rays produced from ¹⁵N(H,αγ)¹²C reaction, using an external proton beam of 1.3 MeV, were used to quantify nitrogen concentration. N concentrations of about 0.47% were obtained comparing the gamma production rate of the samples with a referenced material (Stainless steel CRM298 - 0.236% of N in mass) irradiated in the same conditions. Also PIXE analyses were done on both sets of samples in order to identify main elements in the matrix.     

Measurement of IGSCC depth by SAFT-UT

In order to examine the ability of SAFT-UT of measuring the IGSCC depth, two preparatory experiments on artificial defects and one on a real IGSCC were performed.In the first, the capabilities of SAFT-UT of determining crack depth were assessed for an artificial defect in a carbon steel. It was found that crack depth could be measured by obtaining the crack tip images.In the second, influences of large grain size were investigated by testing a slot made by EDM in the weld heat affected zone of austenitic stainless steel piping. It was found that a narrow beam spread probe, i.e., a large F number probe, was better for measurement of crack depth in an austenitic stainless steel.Finally, a real IGSCC was tested as introduced in two different test blocks. The positions of tip and the opening of the IGSCC deduced by SAFT-UT were in a fairly good agreement with those revealed afterwards.     

The relative importance of surface phenomena in CTR first-wall designs

Plasma-wall interactions are modeled using physical sputtering theory. Sputtering yields are calculated for incoming D-T ions that have a monoenergetic or Maxwellian distribution in energy. The yield curves assuming the Maxwellian energy distribution are flatter than the monoenergetic curves. The consequences of sputtering considered here are surface composition and microtopography changes of the first-wall surface. For the iron-carbon system, the surface becomes enriched with carbon when deuterons are incident on the target. The number of cycles for crack initiation at the first-wall surface is linearly proportional to differences in sputtering yield of two adjacent areas. These effects are of first order and appear to be dominant in limiting the life of the first wall.     

Tritium implantation and depth profiling in fusion related materials

Tritium behaviour in solids and particularly its permeation and inventory in the first wall, limiters, breeding blanket materials and in other structural elements of fusion reactors is a subject of great concern in all projects aiming at D + T fusion. In the present work elastic recoil detection (ERD) under ⁴He bombardment and the T(d, α)n nuclear reaction analysis (NRA) in the forward detection geometry were applied to the depth profiling of tritium at submicron distances below the surface of selected fusion related materials. Experimental results obtained for tritium implanted in titanium, graphite and lithium aluminate LiAlO₂ are presented as the examples.     

Depth distribution profiling of deuterium and 3He

A technique using nuclear microanalysis has been developed to determine ²D an ³He concentrations versus depth profiles in the near surface regions (～2 μm) of solids. Ultimate near-surface depth resolutions of (≈100 Å) should be attainable by these techniques. A probing beam of ³He ions is used to analyze for the deuterium. By energy analyzing the emitted ⁴He ions from the reaction d(³He,p)⁴He and by simple computer analysis, deuterium concentration versus depth profiles are obtained. The same methods are applicable for ³He profiling except in that case a ²D probing beam is used. The techniques is currently being developed using Er hydride and Sc hydride films. It is particularly suited for the study of the transition metals with high hydrogen solubility such as Nb and V which are of interest for CTR reactor applications.     

3He质谱测定技术研究

通过质谱分析方法测定氢同位素气体中单原子离子的离解系数，提高对氢同位素气体质谱重叠峰的分辨能力，解决了氢同位素气体中微量^3He测定技术问题。研究结果表明：当氢同位素气体中氚丰度大于5％时，该方法可以准确测定氢同位素气体中的^3He含量，^3He含量相对标准偏差小于0．6％。     

Control of first-wall surface conditions in the 2XIIB magnetic mirror plasma confinement experiment

The control of first-wall surface conditions in the 2XIIB Magnetic Mirror Plasma Confinement experiment is described. Before each plasma shot, the first wall is covered with a freshly gettered titanium surface. Up to 5 MW of neutral beam power has been injected into 2XIIB, resulting in first-wall bombardment fluxes of 10¹⁷ atoms · cm^?2 · s^?1 of 13-keV mean energy deuterium atoms for several ms. The background gas flux is measured with a calibrated, 11-channel, fast-atom detector. Background gas levels are found to depend on surface conditions, injected beam current, and beam pulse duration. For our best operating conditions, an efective reflex coefficient of 0.3 can be inferred from the measurements. Experiments with long-duration and high-current beam injection are limited by charge exchange; however, experiments with shorter beam duration are not limited by first-wall surface conditions. We conclude that surface effects will be reduced further with smoother walls.     

用飞行时间法测量氚的深度分布

氚在材料中的浓度、深度分布及其在材料中的扩散行为,是从事核聚变研究者最为关心的问题之一。氚(钛)靶是14MeV中子发生器的主要部件,精确地分析氚在靶中的行为,可以为合理地设计氚靶的厚度提供依据,并使靶的寿命和密封的中子发生器的寿命得以延长。因此,研究一些可行的方法来分析氚在基质材料中的行为有着应用的价值。用离子背散射     

Visible light emission from silicon: a quantum effect in highlyporous materials

The authors discuss progress in the control of the luminescent properties of porous silicon and in the understanding of the basic mechanisms which govern the light emission. The main features of porous silicon formation and properties are briefly recalled. The photoluminescence characteristics are reported. It is shown that anodic oxidation of porous silicon is a technique which provides photoluminescent layers with good mechanical properties and enhanced emission efficiency. A model accounting for the quite long measured carrier lifetimes is outlined. The electroluminescence which appears during the anodic oxidation of porous silicon in pure water was studied     

High temperature surface effects of He implantation in ICF fusion first wall materials

The first wall armor of the inertial confinement fusion reactor chambers must withstand high temperatures and significant radiation damage from target debris and neutrons. The resilience of multiple materials to one component of the target debris has been investigated using energetic (20-40 keV) helium ions generated in the inertial electrostatic confinement device at the University of Wisconsin. The materials studied include: single-crystalline, and polycrystalline tungsten, tungsten-coated tantalum-carbide ‘foams’, tungsten-rhenium alloy, silicon carbide, carbon-carbon velvet, and tungsten-coated carbon-carbon velvet. Steady-state irradiation temperatures ranged from 750 to 1250 °C with helium fluences between 5 × 10¹⁷ and 1 × 10²⁰ He⁺/cm². The crystalline, rhenium alloyed, carbide foam, and powder metallurgical tungsten specimens each experienced extensive pore formation after He⁺ irradiation. Flaking and pore formation occurred on silicon carbide samples. Individual fibers of carbon-carbon velvet specimens sustained erosion and corrugation, in addition to the roughening and rupturing of tungsten coatings after helium ion implantation.