Physical sputtering model for fusion reactor first-wall materials

A model has been developed to provide energy-dependent physical sputter yields for various plasma particles (ions and neutrals) incident on candidate first-wall materials. The physical sputter yield is expressed in terms of the atomic and mass numbers of the projectile and target atoms, the surface binding energy of the targets and the energy of the incident particle. The general shapes of the yield curves are based on theoretical models, whereas the magnitudes of the yields are derived primarily from experimental data. The model applies to both high- and low-Z incident particles bombarding high- and low-Z wall materials. Although the model was developed for metal first-wall materials, it has been extended, with minor modifications, to predict physical sputter yields of several stable compound wall materials. A comparison of predicted yields with available experimental data has been made for a number of candidate first-wall materials.     

Angular distribution of sputtered atoms induced by low-energy heavy ion bombardment

The sputtering yield angular distributions have been calculated based on the ion energy dependence of tohal sputtering yields for Ni and Mo targets bombarded by low-energy Hg^ ion. The calculated curves show excellent agreement with the corresponding Wehner‘s experimental results of sputtering yield angular distribution. The fact clearly demonstrated the intrinsic relation between the ion energy dependence of total sputtering yields and the sputtering yield angular distribution. This intrinsic relation had been ignored in Yamamura‘s papers (1981,1982) due to some obvious mistakes.     

Liquid target generation techniques in molecular dynamics studies of sputtering

Techniques of generating liquid targets for molecular dynamics studies of sputtering were investigated in systems consisting of 603 Cu atoms. The effects of different boundary conditions on sputtering yield, layer yield ratio, energy distribution, and polar angular distribution of ejected particles were found to be important. A box boundary condition and a semiperiodic boundary condition were considered. The box boundary conditions required the particles to experience pure reflection at the boundaries, while semiperodic boundary conditions demanded position and momentum periodicity in the two dimensions defined by the surface of the target. Sputtering from the target generated with a box boundary condition resulted in a 60% higher total yield, a slightly higher first layer yield ratio, a 60% lower surface binding energy, and a sharper polar angular distribution than from the target generated under a semiperodic boundary condition at the same temperature. Since the results obtained with the semiperiodic boundary conditions are in better agreement with experimental sputtering results, we have concluded that the semiperiodic boundary conditions produces a target that better represents the free surface of real liquid systems.     

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.     

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.     

Effect of Internal Antenna Coil Power on the Plasma Parameters in13.56 MHz/60 MHz Dual-Frequency Sputtering

The plasma property of a hybrid ICP/sputtering discharge driven by 13.56 MHz/60 MHz power sources was investigated by Langmuir probe measurement. For the pure sputtering discharge, the low electron density and ion flux, the rise of floating potential and plasma potential with increasing power, as well as the bi-Maxwellian distribution of electron en- ergy distributions （EEDFs） were obtained. The assistance of ICP discharge led to the effective increases of electron density and ion flux, the suppression of rise of floating potential and plasma potential, as well as the change of EEDFs from bi-Maxwellian distribution into Maxwellian dis- tribution. The increase of electron density and ion flux, and the EEDFs evolution were related to the effective electron heating by the induced electric field.     

MD simulation of cluster formation during sputtering

The cluster ejection due to cluster impact on a solid surface is studied through molecular dynamics (MD) simulations. Simulations are performed for Cu cluster impacts on the Cu(1 1 1) surface for cluster energy 100 eV/atom, and for clusters of 6, 13, 28 and 55 atoms. Interatomic interactions are described by the AMLJ–EAM potential. The vibration energy spectrum is independent of the incident cluster size and energy. This comes from the fact that sputtered clusters become stable through the successive fragmentation of nascent large sputtered clusters. The vibration energy spectra for large sputtered clusters have a peak, whose energy corresponds to the melting temperature of Cu. The exponent of the power-law fit of the abundance distribution and the total sputtering yield for the cluster impacts are higher than that for the monatomic ion impacts with the same total energy, where the exponent δ is given by Y_n∝n^δ and Y_n is the yield of sputtered n-atom cluster. The exponent δ follows a unified function of the total sputtering yield, which is a monotonic increase function, and it is nearly equal to δ −3 for larger yield.     

Effect of Frequency and Power of Bias Applied to Substrate on Plasma Property of Very-High-Frequency Magnetron Sputtering

The effect of the frequency and power of the bias applied to the substrate on plasma properties in 60 MHz(VHF) magnetron sputtering was investigated.The plasma properties include the ion velocity distribution function(IVDF),electron energy probability function(EEPF),electron density n_e,ion flux Γ_i,and effective electron temperature T_(eff).These parameters were measured by a retarding field energy analyzer and a Langmuir probe in the 60 MHz magnetron sputtering,assisted with 13.56 MHz or 27.12 MHz substrate bias.The 13.56 MHz substrate bias led to broadening and multi-peaks IVDFs,Maxwellian EEPFs,as well as high electron density,ion flux,and low electron temperature.The 27.12 MHz substrate bias led to a further increase of electron density and ion flux,but made the IVDFs narrow.Therefore,the frequency of the substrate bias was a possible way to control the plasma properties in VHF magnetron sputtering.     

Temperature dependence of sputtering behavior of Cu-Li alloys

Three different Cu-Li alloys (4, 10, and 16% Li) have been sputtered by 1 keV and 100 eV D⁺ ions. The Cu sputtering yield and the total weight loss was measured as a function of the target temperature between 25 and 700°C. The yield was measured by the catcher foil technique and by the weight loss method in order to differentiate between the total weight loss and the Cu sputtering yield (i.e., Rutherford backscattering analysis (RBS) of the catcher foil). Targets with lower Li concentration (4 and 10%) did not show a significant change of the Cu sputtering yield [1] as found by other authors [2] and this is probably due to the higher current density (10¹⁵ cm⁻² s⁻¹) in this experiment. An increase in weight loss at temepratures above 550°C was caused by Li evaporation. The target with high Li concentration (16% Li) showed a reduction of the Cu sputtering yield by more than a factor of 50 for both the 1 keV and the 100 eV D⁺ ions. This reduction occurs in a small temperature range around 550°C, which coincides with the transition on the Cu-Li phase diagram from the a-phase to α + liquid. For temperatures above 550°C the sputtering yield increases again, most probaby due to an enhanced evaporation of Li. At optimum temperature conditions, the evaporation rate of Li for the Cu-Li alloy is many orders of magnitude lower than the rate for pure Li. According to the phase diagram, the Cu-Li alloy with even higher Li concentration could reach optimum conditions at lower temperatures and, therefore, would be a promising first-wall candidate.     

ATLAS: Accelerated torus-like angular source for fusion neutronics applications

There is increasing demand for fast and accurate neutronics analysis to be conducted on specific ITER components. The ATLAS (accelerated torus-like angular source) algorithm has been developed to accelerate such neutronics calculations by approximating the 3-D plasma volume source as a surface source at the first-wall region of interest. The ATLAS routine rapidly calculates the angular distribution of the unscattered neutron flux at the chosen first wall location, on a discrete angular grid, and writes a surface source for both MCNP and Attila. This discretised source can then be used to accurately reproduce the full 3-D toroidal volume source in angle and energy, and to quickly simulate the irradiation of individual components or regions in the ITER model. While designed for ITER, the method is general in implementation and can be applied to any tokamak.     

Fusion reactor first-wall cooling for very high energy fluxes

A study has been made of the feasibility of a protective first-wall shield between the plasma and the containment vessel for early experimental controlled thermonuclear fusion machines. The proposed first-wall shield is a water-cooled array of thin-walled tubes designed to take very high local energy fluxes originating from the neutral beam injectors. Detailed computer calculations reveal that heat flux capabilities of 3300 W/cm² are possible with first-wall shield sections made up of tubes 1 m long of Ta-10W alloy (with tubes of 10 mm i.d. and tube wall thickness of 0.5 mm) with a structural safety factor of about four. Required pumping powers on the order of 1 MW/m² of first-wall area exposed to these high energy fluxes are predicted for flow in the non-boiling regime. If operation in the subcooled nucleate boiling regime can be achieved without oscillations or instabilities, the required pumping power is shown to decrease by about an order of magnitude.     

Neutron damage calculations in Cu,Nb and Au to 32 MeV: Application to sputtering and deuteron-breakup neutron sources

Primary recoil distributions and specific damage energies have been computed for high energy deuteron-breakup neutrons in Cu, Nb and Au. The calculations are based on theoretical neutron cross sections and consider in particular a d-Be spectrum broadly peaked at 15 MeV with some neutrons above 30 MeV. The theoretical results are similar to corresponding calculations for monoenergetic 15-MeV neutrons and are in good agreement with range measurements of (n, 2n) recoils generated by high energy d-Be neutrons in Nb and Au. The calculations are also consistent with recent d-Be neutron sputtering experiments in Nb and Au and demonstrate the usefulness of deuteron-breakup neutron sources for simulating fusion neutron effects.     

First wall environment in the reference theta-pinch reactor (RTPR)

A review of the RTPR first-wall environment is given, with emphasis on the physical processes which determine the first wall insult. Recent results of calculations of the transient phenomena which occur when a neutral gas blanket is set up for first wall protection are given. New first-wall designs are described, and their responses to the first wall fluxes are discussed.     

The angular distribution of particles sputtered from Cu,Zr, and Au surfaces by ion bombardment at grazing incidence

The angular distribution of particles sputtered from polycrystalline Cu, Zr, and Au targets has been measured for bombardment with Ar⁺- and Xe⁺-ions at perpendicular and grazing ion incidence (80° and 85° with respect to the surface normal). The ion energy was varied between 100 keV and 900 keV. The measured distributions follow approximately a cosine-squared curve rather than the cosine function, they are found to be symmetric with respect to the surface normal and almost independent of the ion species, the ion energy and the angle of incidence. Values of sputtering yield of Cu, Au and Zr are also presented.     

Atomistic self-sputtering mechanisms of rf breakdown in high-gradient linacs

Molecular dynamics (MD) models of sputtering solid and liquid surfaces - including the surfaces charged by interaction with plasma, Coulomb explosion, and Taylor cone formation - were developed. MD simulations of self-sputtering of a crystalline (1 0 0) copper surface by Cu⁺ ions in a wide range of ion energies (50 eV-50 keV) were performed. In order to accommodate energetic ion impacts on a target, a computational model was developed that utilizes MD to simulate rapid atomic collisions in the central impact zone, and a finite-difference method to absorb the energy and shock wave for the collisional processes occurring at a longer time scales. The sputtering yield increases if the surface temperature rises and the surface melts as a result of heat from plasma. Electrostatic charging of the surface under bombardment with plasma ions is another mechanism that can dramatically increase the sputtering yield because it reduces the surface binding energy and the surface tension. An MD model of Taylor cone formation at a sharp tip placed in a high electric field was developed, and the model was used to simulate Taylor cone formation for the first time. Good agreement was obtained between the calculated Taylor cone angle (104.3°) and the experimental one (98.6°). A Coulomb explosion (CE) was proposed as the main surface failure mechanism triggering breakdown, and the dynamics of CE was studied by MD.     

Cluster size effect on reactive sputtering by fluorine cluster impact using molecular dynamics simulation

The mechanism of high-yield sputtering induced by reactive cluster impact was investigated using molecular dynamics (MD) simulations. Various sizes of fluorine clusters were radiated on clean silicon surface. At an incident energy of 1 eV/atom, F atom and F₂ molecule are only adsorbed on the surface and sputtering of Si atom does not occur. However, fluorine cluster, which consists of more than several tens molecules causes sputtering. In this case, most of Si atoms are sputtered as fluorinated material such as SiF_x. This effect is due to the fact that cluster impact induces high-density particle and energy deposition, which enhances both formation of precursors and desorption of etching products. The deposition of atoms and energy becomes denser as the incident cluster size increases, so that larger clusters have shown higher sputtering yield.     

Measurements of Time-Dependent Neutron Spectra from Low-Temperature Ice Block

The time-dependent neutron spectra from the surface of an externally pulsed finite block (156 mm diameter, 160 mm high) of ice were measured at 86, 35, 26 and 16 K over an energy range of 0.0006～0.02 eV and for durations of time extending from 100 to beyond 600 μS following fast neutron injection.

It was found that, at 86 and 38 K, asymptotic equilibrium distribution was established already around 400μs after neutron pulse injection, whereas at 26 K, even after 800 μs, the average neutron energy of the measured spectrum was still slightly higher than that of Maxwellian distribution corresponding to the moderator temperature, and further at 16 K, 900 μs after neutron pulses, the difference between the averages of measured and Maxwellian distribution remained at about 50%.

These results should usefully serve test in neutron scattering kernels, as well as studies for improving methods of calculation.     

聚变堆实验包层模块第一壁温度场及热结构分析

铍相对于众多聚变反应堆的第一壁护甲材料,有着许多优点,这些优点使它和钨及碳基材料一起被选作国际热核聚变实验堆（ITER）第一壁的候选防护材料。对中国氦冷固态增殖剂实验包层模块（CHHCSBTBM）第一壁进行多场耦合模拟分析结果表明,使用表面热负荷模拟分析时,未考虑中子负载情况下,模拟分析结果与其它结果有较大出入,故使用表面热负荷模拟分析时必须考虑中子负载情况。而对第一壁热结构分析表明,铍保护板的应力超过了其许用应力,可以寻找其它铍合金或第一壁护甲材料以满足第一壁护甲材料热结构应力要求。     

Measurements of Total Neutron Cross Sections at 24-keV by means of Iron-Filter Method

Sputtering of the solid hydrogens by light ions has shown isotope effects which are greater than for any other solid targets. These hydrogenic solids are unique because of the extreme volatility and because the first step of the electronic sputtering process is identical for all hydrogenic solids. The sputtering for protons in the energy range from 5 to 10 keV can be qualitatively described by an electronic spike of cylindrical geometry. The sputtering yield of solid tritium has been evaluated on the basis of results for solid H₂, D₂ and HD.     

keV能区单能中子参考辐射场核反应靶设计

质子轰击中等质量靶核是产生keV能区单能中子的一种常用方法。选择⁴⁵Sc(p,n)⁴⁵Ti反应中子源作为keV能区单能中子参考辐射场的中子源,利用FloWizard软件模拟大束流条件下核反应靶的温度分布,分析了影响靶温度分布的主要因素。利用Target程序模拟核反应靶出射中子能谱,分析了不同材料的散射中子本底。同时精细调节5SDH 2加速器端电压,测量了薄靶（Sc）的激发曲线,测量结果与NPL和PTB的接近。