Hydrogen Re-emission Research Articles

Isotopic Effect in Ion-Induced Re-Emission of Hydrogen Implanted into Graphite

Isotopic Effect in Ion-Induced Re-Emission of Hydrogen Implanted into Graphite

Interaction of a two-component hydrogen-containing glow discharge plasma with Pd and stainless steel

The interaction of a two-component hydrogen-containing glow discharge plasma ( q = 3 × 10 16ion cm −2 s −1, U = 250−450 V) with Ar, He, N 2 and O 2 impurities ( P imp / P H = 0−1.0) with Pd and stainless steel has been studied using TPD, DRS and hydrogen permeability techniques. It was shown that for Pd the rate limiting step for hydrogen penetration is surface processes while for stainless steel the penetration is limited by bulk diffusion. Activation of penetration processes by glow discharge plasma causes an increase in the penetrating flow from 4.2 × 10 14 to 5.6 × 10 18 atom cm −2 s −1 ( T mem = 400 K) and from 1.2 × 10 15 to 4.1 × 10 16 atom cm −2 s −1 ( T mem = 470 K), respectively. The addition of impurities into the hydrogen plasma results in a decrease of the penetrating flow by a factor of 2–3 for inert gases and more than an order of magnitude for impurities of chemically active gases. Changes in the hydrogen re-emission rate constant. S R, depending on plasma composition was shown for stainless steel surfaces. The re-emission rate constant ranged from 1.58 to 2.63 × 10 −2 cm s −1 for He impurities, from 1.58 to 3.05 × 10 −2 cm s −1 for Ar impurities and from 1.58 to 10.5 × 10 −2 cm s −1 for N 2 impurities.

Isothermal Re-Emission of Hydrogen Implanted into Isotropic Graphite.

Isothermal re-emission of hydrogen from graphite implanted with 5 keV H2 + ion beam up to saturation (3×1018/cm2) at room temperature has been studied by means of the elastic recoil detection (ERD) technique at temperatures of 450, 500, 550 and 600°C. It is found that the concentration of retained hydrogen decreases rapidly in the beginning and then decreases very gradually as the annealing time increases. The re-emission profiles have been analyzed taking into account local molecular recombination between activated hydrogen atoms and that between an activated hydrogen atom and a trapped one together with retrapping of the activated hydrogen atom. It is shown that the re-emission of hydrogen by isothermal annealing occurs mainly due to the former type of local molecular recombination and that the activation energy of the thermally activated detrapping rate constant is 0.50±0.04eV. Moreover, it is shown that an analytical expression for the re-emission profile reproduces reasonably well the observed therma...

Isotopic Effect in Ion-Induced Re-Emission of Hydrogen Implanted into Graphite.

The re-emission of H and D atoms, implanted into graphite, induced by 1.5 MeV He+bombardment has been studied by means of the elastic recoil detection (ERD) technique by changing the initial peak concentration of H and D, the temperature and the He+ ion flux. The experimental re-emission profiles were analyzed solving analytically the mass balance equations. The whole re-emission profiles are shown to be excellently reproduced in terms of the ion- induced detrapping (σd), trapping (or retrapping) (ΣT ) and local molecular recombination (k) between an activated (free) hydrogen atom and a trapped one and that the effective detrapping cross sections ΣdK/ΣT for H and D are determined as the best fitting parameter to be 2.9xl0-18 and 7.7x 10-19 cm2, respectively. It is also shown from the experimental data on little dependence of the re-emission profile on the He+ ion flux that the local molecular recombination plays a major role in the ion-induced re-emission. Furthermore, the difference between the effective...

Large-scale effects of H 2O and O 2 on the absorption and permeation in Nb of energetic hydrogen particles

The influence of chemically active gases (O 2 and H 2O) on hydrogen re-emission was investigated by and permeation through a Nb membrane acted upon by 20 keV H 2 + ions. Both the initial retention of ions after switching on the ion beam and the build-up of re-emission with time can be directly monitored with the new UHV apparatus by the changes of hydrogen pressure in the vacuum chamber in front of the membrane, since this pressure is determined by the incoming beam. In the absence of O 2 or H 2O, only a small portion of the flux absorbed penetrates the membrane, and the re-emission flux virtually equal to the flux of incident ions is established following the absorption of a relatively small dose. The admittance of O 2 or H 2O on the inlet side of the membrane continuously acted upon by the H 2 +-ion beam results in a dramatic increase of both the retention and the permeation of implanted hydrogen. With the pressures of O 2 and H 2O corresponding to about 0.03 O-atoms supplied to the sample surface per impinging proton, the time to reach the steady-state re-emission sharply rises, indicating a drastic increase of the dose absorbed. The steady-state re-emission flux is not changed appreciably, while the permeation flux, though increased by orders of magnitude, remains small compared to the incident ion flux. With greater pressures of the admitted gases (more than 0.1 O-atom supplied to the surface per proton), not only the sorptional capacity is increased, but also the permeation flux reaches up to the magnitude comprising the major part of the absorbed flux (i.e. superpermeability develops), and the re-emission flux correspondingly falls.

Implantation and re-emission of hydrogen and helium in the beam-stopping panels of a 10 MW ion beam line

Significant amounts of hydrogen and helium are found to be implanted into the beam-stopping panels during beam operation. The amount implanted appears to exceed the particle flux of the beam in the early stages of the pulse. Gas is released during the pulse, probably due to the warming up of the beam-stopping panels. The gas concentration in the beam-stopping panels is ~ 10 22 cm −3. The concentration and release of hydrogen and helium are comparable, indicating a common process.

Hydrogen recycling constant during ion bombardment

The validity of a simple diffusion model based on the assumptions of small ion range compared with diffusion length, negligible hydrogen-defect interaction and second order thermal desorption is analysed in its application to hydrogen re-emission from, and accumulation in, materials during ion implantation. Comparison with experiment have been made for different metals (Mo, Ni, SS) and nonmetals (carbon, pyrographite, uglesitall, BN, TiC, TiO 2, SiO 2, TiB 2). The model calculations disagree with experimental measurements of re-emission during the transient stage of implantation, when re-emission is less than 100%, but give very good agreement with experiments on the accumulation of hydrogen in materials at high ion fluences. The recycling parameter R = D/K may be derived from the high-dose experiments and used for further predictions of hydrogen retention in the first wall.

Particle balance and wall pumping in tokamaks

The properties of carbon, with respect to its ability to absorb and release hydrogen, are reviewedand applied to the interpretation of density behaviour in Ohmic-, NBI- and ICRF-heated tokamakdischarges. Based on the experimental observations, improvement of H-mode parameters in JET dueto reduced hydrogen re-emission from the walls is predicted with a numerical model. The particleremoval rates required for Q = 1 in JET are estimated.

Materials surface modification by plasma bombardment under simultaneous erosion and redeposition conditions

The first in-depth investigation of surface modification of materials by continuous, high-flux argon plasma bombardment under simultaneous erosion and redeposition conditions have been carried out for copper and 304 stainless steel using the PISCES facility. The plasma bombardment conditions are: incident ion flux range from 10 17 to 10 19 ions s −1 cm −2, total ion fluence between 10 19 and 10 22 ions cm −2, electron temperature range from 5 to 15 eV, and plasma density range from 10 11 to 10 13 cm −3. The incident ion energy is 100 eV. The sample temperature is between 300 and 650 K. Under redeposition-dominated conditions, the material erosion rate due to the plasma bombardment is significantly smaller (by a factor up to 10) than expected from the classical ion beam sputtering yield data. It is found that surface morphologies of redeposited materials strongly depend on the plasma bombardment condition. The effect of impurities on surface morphology is elucidated in detail. First-order models are implemented to interpret the reduced erosion rate and the surface morphology evolution. Also, fusion related surface properties of redeposited materials such as hydrogen reemission and plasma driven permeation have been characterized.

Dynamical calculation of low energy hydrogen reemission off hydrogen covered surfaces

The reemission of low energy ( < 100 eV) hydrogen is a critical issue in plasma/first wall recycling. Previous molecular dynamics calculations, using the Embedded Atom Method (EAM), showed a decreasing reflection coefficient at energies below 10 eV for clean nickel surfaces with no surface defects. These calculations have been extended to investigate the influence of chemisorbed hydrogen atoms on the reemission coefficient for typical fusion operating conditions. A hybrid EAM method has been developed to include molecular hydrogen formation. The results of these calculations indicate that, in addition to atomic reflection and atomic penetration into the bulk which occur on a clean surface, hydrogen atom induced atomic and molecular desorption can compete, thereby affecting the reemission coefficient. The results are dependent on the incident hydrogen energy as well as surface coverage.

Quasi-thermodynamic prediction of hydrogen reemission behavior from titanium films

A new approach is developed to describe hydrogen reemission behavior from a reactive metal film in which the implanted hydrogen concentration exceeds the dilute solution range. In this approach, conventional thermodynamic information for the metal-hydrogen system, namely, the phase diagram and the P-C-T (pressure-concentration-temperature) relation is used extensively to determine the reemitted hydrogen flux. As a specific case, this method has been applied to hydrogen reemission from a titanium film. The predicted reemission curves show characteristic discontinuities in time corresponding to phase changes in the solid titanium-hydrogen system as the system proceeds to a final stationary state. Such discontinuities have been found in existing experimental data on deuterium reemission from titanium oxide and carbide coatings but were neglected as being exceptional. The corresponding hydrogen concentration build-up in the titanium film does smoothly approach the stationary state value. The effects of temperature (300–800 K), titanium film thickness (10–3000 Å), and incoming hydrogen flux ( 1.0 × 10 16 to 1.0 × 10 18 H atoms/cm 2.s) on the hydrogen reemission behavior are investigated. The time required to reach a stationary state changes reasonably with incoming hydrogen flux and film thickness. This characteristic transient time minimizes at ~ 600 K and maximizes at ~ 700 K. The stationary state hydrogen concentration is found to be independent of film thickness. An F-C-T (flux-concentration-temperature) relation is derived and used to estimate the stationary state hydrogen concentration for a given flux and temperature.

The influence of plasma-induced heating on re-emission of hydrogen from graphite targets

Graphite is a useful low-Z material for limiters and divertor plates. Hydrogen bombardment results in trapping or, at higher temperatures, in beam-enhanced detrapping. The amount trapped is a function of the fluence φ, target temperature θ, and energy of the impinging ions E0. By means of the usual sheath model and trapping data from the literature the particle re-emission coefficient, r, is determined as a function of the power density at the target, p, plasma temperature in front of the target, T, and time. Largely irrespective of p and T, r has a maximum value when the surface temperature is around 350 ° C. The maximum value of r rises as p and T. For T = 20 eV (normal incidence) and p = 3.5 MW/cm2, for instance, it is r(max)−1 = 3.6 × 10−3. Application of the results to ASDEX Upgrade indicates that the use of graphite target plates is compatible with density control requirements.

Investigation of neutral hydrogen transport by means of H α-Resonance fluorescence measurements at the tokamak UNITOR

The investigations reported here were performed at the tokamak device UNITOR. The plasma is under normal operational conditions limited by a a toroidal limiter. A poloidal limiter can be installed, too. Space and time resolved H α-fluorescence measurements enabled us to determine the local density of excited H-atoms. Using a collisional radiative model the ground state neutral density varying from 10 12cm −3 at the wall to 10 9 cm −3 in the plasma core could be deduced. A Monte Carlo code for neutrals taking into account reflection, trapping and re-emission of neutral hydrogen at the walls was used to interpret the measurements. The local fluxes from and to the wall needed to fit the measured density are of the order of 10 18 cm −2s −1.

Helium and hydrogen re-emission during implantation of molybdenum, vanadium and stainless steel

He re-emission has been measured during 300 keV He + implantation from 400°C to 1200°C in V and Mo between −170°C and 700°C in 316 SS. H re-emission has been measured during 150 keV H + implantation of Mo, V, and 316 SS between −90°C and 115 °C. In general the He re-emission is low until a critical dose is reached, at which time the re-emission abruptly rises to higher values. Both the critical dose and re-emission are strongly temperature dependent. The H re-emission generally rises smoothly to an equilibrium value which is both temperature and material dependent. Only at the lowest temperature does the H re-emission in Mo bear a qualitative resemblance to He re-emission.

A Room-Temperature Diffusion Constant for Hydrogen in Proton-irradiated Steel

RECENT measurements of hydrogen diffusion-rates in α-iron by Johnson and Hill1 gave approximately 2 × 10−7 cm.2/sec. for the value of D at room temperature; and in steel by Frank, Lee, and Williams2 gave approximately 1 × 10−7 cm.2/sec. Johnson and Hill measured re-emission of hydrogen from cylinders previously soaked in hydrogen at high temperatures and pressures, while Frank et al. used a mass spectrometer to measure the emission of hydrogen and deuterium from thin sheets. Frank, Swets and Fry3, observing the build-up of hydrogen permeating through thin sheets of steel, obtained for D at room temperature values about an order of magnitude higher.