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For a next-step fusion device the erosion at the vessel walls during normal operation caused by the bombardment with different species from the plasma is estimated. The wall fluxes are calculated for a given ‘burning’ plasma with the computer code B2-EIRENE. With the known sputtering yields the erosion rates for vessel walls made out of Be, C, Fe, Mo or W, are calculated. The erosion by ions is found to be of the same order of magnitude as the erosion by energetic neutrals. The erosion has minima around the area of gas feed and maxima close by and at the bottom near the baffles of the divertor. The calculated total erosion is of the order of 1021 to 1022 atom/s for low Z materials, and a factor 10–20 lower for high Z materials, such as W, resulting in about 5 kg per full-day of operation or 1 t/y.

The possibilities for optimization of doped fine grain graphites with high thermal conductivity and high thermal shock resistance are demonstrated at laboratory scale. A mixture of MCMB powder and different carbides (B4C, TiC, VC, ZrC and WC) was used as starting material. VC acts as catalyst of the graphitization at the lowest temperature, and ZrC is the most effective catalyst of all investigated carbides. A direct proportionality between the mean crystallite height, Lc, and the thermal conductivity at room temperature was found for all materials except for the B4C- and the ZrC-doped graphites. With increasing graphitization temperature the open porosity of all doped materials becomes gradually closed, suggesting the existence of a diffusion mechanism responsible for both the catalytic effect and the closing of the open porosity. The addition of carbides does not strongly influence the mechanical properties of pure graphite. A high ratio flexural strength to Young’s modulus was achieved.

At the Accelerator Laboratory of Ludwig Maximilian University (LMU) and Technical University Munich (TUM) Garching, Germany, the ultra-clean injector of the accelerator mass spectrometry facility was upgraded in order to permit depth profiling of sample materials. The facility was tested and then routinely used in measurements of depth profiles of tritium and deuterium in vessel wall samples from the fusion experiments Joint European Torus (JET) in Culham, Great Britain and from the Axial Symmetric Divertor Experiment (ASDEX-Upgrade) in Garching, Germany.

In high temperature plasma experiments several processes contribute to erosion and loss of material from the vessel walls. This material may enter the plasma edge and the central plasma where it acts as impurities. It will finally be re-deposited at other wall areas. These erosion processes are: ○ evaporation due to heating of wall areas. At very high power deposition evaporation may become very large, which has been named blooming. Large evaporation and melting at some areas of the vessel wall surface may occur during heat pulses, as observed in plasma devices during plasma disruptions. ○ At tips on the vessel walls and/or hot spots on the plasma exposed solid surfaces electrical arcs between the plasma and the vessel wall may ignite. They cause the release of ions, atoms and small metal droplets, or of carbon dust particles. ○ Finally, atoms from the vessel walls are removed by physical and chemical sputtering caused by the bombardment of the vessel walls with ions as well as energetic neutral hydrogen atoms from the boundary plasma. All these processes have been, and are, observed in today's plasma experiments. Evaporation can in principle be controlled by very effective cooling of the wall tiles, arcing is reduced by very stable plasma operation, and sputtering by ions can be reduced by operating with a cold plasma in front of the vessel walls. However, sputtering by energetic neutrals, which impinge on all areas of the vessel walls, is likely to be the most critical process because ions lost from the plasma recycle as neutrals or have to be refuelled by neutrals leading to the charge exchange processes in the plasma. In order to quantify the wall erosion, Materials Factors (MF) have been introduced in the following for the different erosion processes.

Doping of carbon leads to a reduction of chemical erosion. This reduction is more effective if dopants are in the form of sub-μm precipitates with a very homogeneous distribution. However, dopants should not degrade but rather improve the thermal conductivity of graphite. Taking into account the catalytic effect of some metals and carbides on the graphitization, the potential for a development of improved doped carbon materials with reduced chemical erosion and optimized thermomechanical properties becomes evident. First results on the development of doped carbon materials starting from mesophase carbon powder and with different fine grain carbides as dopants show that VC acts as an effective catalyst for the graphitization. The materials obtained show high densities, a relatively low open porosity and good mechanical properties.

Several carbide-doped (SiC, TiC, V 8 C 7 , WC, ZrC) graphites have been produced. The erosion of these materials at low-energy (eV) hydrogen ion bombardment has been investigated using the weight-loss method, mass spectroscopy, ion beam analysis, and scanning electron microscopy (SEM). The erosion yields of the WC- and V 8 C 7 -doped graphites are reduced by a factor of 2 for 30 eV D at 300 K compared to pure graphite. This observed reduction is partly attributed to surface enrichment of carbide due to preferential C erosion. The other part is assigned to changes in the chemical erosion process (Y surf ) as well as at elevated temperatures in the thermal activated process (Y therm ). The reduction of both erosion processes is determined for all dopants to be more than 25% of the erosion yield of the undoped graphite.

The interaction of nitrogen ions with carbon films is investigated in view of the nitrogen recycling behavior on carbon divertor targets during plasma operation with nitrogen gas puffing. Nitrogen ion implantation is carried out in the energy range between 0.75 and 20 keV and at irradiation temperatures between room temperature and 800°C. Nitrogen retention and release are analyzed in situ, using elastic recoil detection and time-resolved residual gas analysis, respectively. It is found that the nitrogen contents inside the implanted near-surface layers saturate at large fluences, and that the temperature and energy dependence of the saturation level is qualitatively different from the behavior found for the bombardment of graphite surfaces with hydrogen ions. The different behavior is mainly attributed to the formation of N2 bubbles in the implanted near-surface layers. The N2 formation is also identified as the driving force for nitrogen saturation and release at large fluences. It is concluded that nitrogen interacting with carbon surfaces cannot be regarded as a non-recycling gas under high-dosage irradiation conditions.

An experimental setup that allows quantitative measurements of the temperature programmed desorption of different gases from any sample is described. The samples to be analyzed are uniformly heated to temperatures up to 2100 K by electron bombardment from the rear side at a rate of about 10 K/s. The temperature is controlled and can be measured on the front side by a pyrometer. The gases released from a sample are measured with a quadrupole mass spectrometer according to the partial pressure change in the residual gas. In order to get quantitative results the equipment is calibrated for different gases. The facility was tested with measurements of the hydrogen inventory and desorption temperatures for graphite samples from the vessel walls of the fusion experiment ASDEX Upgrade.

The total hydrogen isotope inventories in the tungsten coated divertor tiles of ASDEX-Upgrade were measured by thermal desorption spectroscopy (TDS) and - in the near surface region - by nuclear reaction analysis (NRA). The highest amounts of deuterium (≤ 5 x 10 22 D/m 2 ) are measured in the inner divertor region outside the separatrix due to the formation of a C:H layer on the tungsten by codeposition of deuterium with carbon. The inventories in the outer divertor - where erosion dominates - are generally smaller by an order of magnitude.

Erosion and deposition at the vessel walls of the main chamber of JET were measured with long term samples during the whole operation period of the Mark I carbon divertor from April 1994 until March 1995. Assuming toroidal symmetry, about 70 g Ni + Cr + Fe was found to be eroded from the inner torus wall and about 55 g Be from the outer torus wall due to sputtering by energetic charge-exchange neutrals. Deposition has been measured on a poloidal section of the Mark I C divertor and on tiles from the inner and outer wall limiters. Eroded material is redeposited in the divertor and in the main chamber where it is found predominantly at the sides of the poloidal limiters and at the outer vessel wall. At these deposition-dominated areas in total about 25 g Ni + Cr + Fe, 28 g Be and 370 g C are found. The carbon originates mostly from the carbon limiters; additionally some may be originating from the divertor plates.

The neutron induced transmutations, gas production, activation, atomic displacements and afterheat have been calculated for several potential low- and high-Z first wall materials in a DT fusion reactor. The neutron spectra taken are those calculated for the first wall of the shielding blanket proposed for the International Thermonuclear Experimental Reactor (ITER). For low Z materials the production of hydrogen and helium isotopes dominate, while for high Z materials activation and afterheat dominate. For a total flux of 14.1 MeV neutrons corresponding to a wall load of 1 MW a m −2 , the tritium production in a 1 cm thick low Z material layer of the first wall is of the order of 10 16 – 10 19 cm −2 , while the calculated He concentrations are of the order of ∼10 −3 per wall atom or 10 20 cm −2 . These gases may modify the thermophysical properties of the wall material.

The retention of 1 keV and 8 keV deuterium implanted in B, Si, and Ti doped graphites and in the carbides B 4 C, SiC and TiC up to 6.7 × 10 19 D atoms/cm 2 has been studied at room temperature with thermal desorption spectroscopy (TDS) and ion beam analysis. Compared to pyrolytic graphite all doped graphites showed a tendency to increased retention of deuterium, especially the materials with high porosity. For the carbides the saturation concentration in the implantation zone in B 4 C is about 0.4 D per target atom and therefore comparable to the saturation concentration of D in graphite. SiC exhibits a significant higher saturation concentration of 0.6 D per target atom, whereas TiC shows a significant lower saturation concentration of 0.15 D per target atom.

The hydrogen and deuterium inventories of the ASDEX Upgrade divertor tiles were measured after the experimental period from December 1994 to July 1995 by thermal desorption spectroscopy (TDS) of samples cut out of the divertor tiles. The samples were heated by electron bombardment up to 2100 K; the released gases were measured by means of a calibrated quadrupole mass spectrometer. The measured hydrogen or deuterium inventories are of the order of 1023 m-2. They are larger for samples of the inner divertor than of the outer divertor by a factor of about 2. The largest inventory was found at the separatrix position of the inner divertor. Most of the released hydrogen (H) can be attributed to water adsorbed in the near surface region during the air exposure prior to the TDS measurements. The total inventories measured by TDS exceed the inventories in the near surface region (

Elastic Recoil Detection Analysis (ERDA) with primary ion beams having energies in the MeV/nucleon range allows the measurement of absolute concentrations and depth profiles especially of low Z elements in the surface layers of a solid. In the present work, we use simple mathematical relations for defining the optimal measurement geometry with respect to measuring large depths with high intensities. Results obtained by the straightforward “surface approximation” are shown and compared to calculations with a more complete model. A parameter discussion is given examining optimal geometries for different experimental set-ups. When using detectors sensitive to very low energies in order to be able to analyse large depths, the intensity loss due to the typically smaller solid angles of these detectors can be compensated by the higher intensity of the recoils in the corresponding geometry.

The accelerator mass spectrometry (AMS) facility at the 3 MV Tandetron in Rossendorf has been applied for quantitative depth profiling of deuterium and tritium in samples cut from graphite protection tiles at the vessel walls of the fusion experiment ASDEX-Upgrade at the Max-Planck-Institut fur Plasmaphysik in Garching. The tritium originates from D(d,p)T fusion reactions in the plasma and it is implanted in the vessel walls together with deuterium atoms and ions from the plasma. The T concentrations in the surface layers down to the analyzing depth of about 25 μm are in the range of 1011 to 5 × 1015 T-atoms/cm3 corresponding to a tritium retention of 3 × 1010 to 3.5 × 1012 T-atoms/cm2. The much higher deuterium concentrations in the samples were simultaneously measured by calibrated conventional SIMS. In the surface layers down to the analyzing depth of about 25 μm the deuterium concentrations are between 3 × 1010 and 8 × 1021 atoms/cm3, corresponding to a deuterium retention of 2.5 × 1016 to 2.5 × 1018 atoms/cm2 The estimated total amount of tritium in the vessel walls is of the same order of magnitude as the total number of neutrons produced in D(d,n)3He reactions.

Tritium depth profiling measurements by accelerator mass spectrometry have been performed at the facility installed at the Rossendorf 3 MV Tandetron. In order to achieve a uniform erosion at the target surface inside of a commercial Cs ion sputtering source and to avoid edge effects, the samples were mechanically scanned and the signals were recorded only during sputtering at the centre of the sputtered area. The sputtered negative ions were mass analysed by the injection magnet of the Tandetron. Hydrogen and deuterium profiles are measured with the Faraday cup between the injection magnet and the accelerator, while the tritium is counted after the accelerator with semiconductor detectors. Depth profiles have been measured for carbon samples which had been exposed to the plasma at the first wall of the Garching fusion experiment ASDEX-Upgrade and from the European fusion experiment JET, Culham/UK. The tritium contents of the samples from JET were one up to five orders higher compared to the samples from ASDEX-Upgrade. The problem of the detector overloading during measurements of samples with a high tritium content has been solved by installation of a scanning system in the AMS beamline, which enables defined scanning of the tritium beam over an aperture in front of the detector.

During the 1994/1995 discharge period two graphite tiles at the outer and the inner lower divertor plates of the tokamak ASDEX-Upgrade have been replaced by tiles of RG-Ti-91 which is a graphite, doped with about 2 at% of Ti and subsequently treated at high temperatures and pressures to achieve a high thermal conductivity perpendicular to one direction. The RG-Ti-91 tiles have been installed with the directions of high thermal conductivity normal to the plasma exposed surface. The plates have been exposed to about 670 tokamak discharges each lasting about 2 to 3 s. The performance of the discharges has not been influenced by the presence of the RG-Ti-91 tiles, i.e., no additional impurities, such as Ti, have been observed in the plasma. The temperature increase of the RG-Ti-91 tiles during high power loads was about a factor 2 lower than the temperature increase of fine grain graphite plates. The surfaces of the tiles showed preferential erosion of the carbon, and TiC crystallites staying out on the surfaces. This is similar to the surface structure which develops at D-ion bombarded surfaces of this RG-Ti-91 material in the laboratory.

For depth profiling of hydrogen isotopes by elastic recoil detection analysis (ERDA) with 2.6 MeV 4He ions a special ΔE - E telescope type detector system was tested. It allows the separation of hydrogen and deuterium recoils as well as backscattered helium particles. For particle identification from the energy loss ΔE a thin 3.7 μm silicon transmission detector was used while the residual energy was measured with a standard 100 μm PIPS silicon surface barrier detector. The depth profiles of deuterium and hydrogen were measured at plasma deposited a-C:H:D layers as well as at samples exposed to plasma discharges in the tokamak experiment ASDEX-Upgrade.

In experiments with magnetically confined hot plasmas in respect to controlled thermonuclear fusion, such as tokamaks or stellerators, the surface layers of the vessel walls are modified by the plasma by erosion, redeposition, hydrogen isotope implantation and heating due to the power load from the plasma. Thus, the composition and structure of the surface layers are finally different to those of the material initially installed. This new material at the surface layers of tokamak experiments is sometimes also named ‘Tokamakium’. Generally, all elements ever introduced into the vessel can be found in the surface layers of the plasma-facing wall tiles. At all areas of the plasma-facing components both erosion and deposition have been observed including the deposition of metal droplets. Some areas are erosion dominated, while at others deposition dominates with atomic depositions of up to several μm, which partly flake. The elements of dust particles introduced into the vessel by in-vessel works get mostly incorporated as impurities into the surface layers of the plasma facing material.

The modification of the surface layer composition during Elastic Recoil Detection Analysis (ERDA) of different carbon materials which had been loaded with hydrogen has been determined experimentally. For an amorphous C:H layer there is an initial overall loss of about 7 × 10 6 hydrogen atoms and about 2.3 × 10 6 carbon atoms per incident 200 MeV iodine ion from the analysing beam. Besides erosion of the amorphous C:H layer there is a depletion of the hydrogen in the carbon layer. For ERDA analysis with 130 MeV Ni ions the hydrogen release is about a factor of 3 lower compared to the ERDA analysis with 200 MeV I-ions. However the Rutherford cross section is more than a factor of 7 larger for 200 MeV I-ions compared to 130 MeV Ni ions, resulting in a higher sensitivity for the ERDA analysis with the heavier ions.

In analysis of surface layers using ΔE − E rest telescopes for the detection of energetic particles, the energy of these particles may exceed the maximum energy for full stopping in the telescope. At these energies the two-dimensional ΔE − E rest spectra shows a discontinuity, since for higher energies the ΔE value is correctly measured, but the measured rest energy decreases for increasing incident energy. Both an analytical procedure and a numerical routine have been developed to calculate the spectrum of particles not fully stopped. The analytical procedure depends only on the measured data of the curve for the fully stopped particles, as well as an extrapolation of this curve to higher energies corresponding to a telescope measuring the full rest energy for all possible incident energies, but with the same ΔE stage. The numerical routine uses stopping power coefficients to calculate a corrected spectrum. Examples are given for the correction of spectra from carbon samples exposed in the JET tokamak, where a range of elements from Be to Ni was measured in single experimental runs.

The trapping of energetic deuterium codeposited with beryllium, carbon and tungsten has been measured on a silicon collector probe at room temperature. The subsequent release of the trapped deuterium at elevated temperatures was determined by thermal desorption spectroscopy. At room temperature, deuterium codeposits both with carbon and BeO with a ration of 0.41 D-atorns/C-atom and 0.38 D-atoms/BeO, respectively. No codeposition of deuterium with tungsten is observed. The thermal release of codeposited hydrogen from BeO begins at 400 K. All hydrogen is released at temperatures above 800 K.

The plasma induced erosion and redeposition at the vessel walls in today's fusion devices have been investigated both with the computer simulation code ERO, and in experiments. Well prepared carbon probes with implanted and evaporated markers in the surface layers have been exposed in the scrape-off layer (SOL) of several tokamaks such as JET, TEXTOR and ASDEX-Upgrade. The main plasma parameters (electron density and temperature, impurity concentration in the SOL) are simultaneously determined. After exposure to single plasma discharges, erosion and redeposition of the marker material were measured by surface layer analysis with MeV ion beam techniques. The experimental results were compared with the results from the ERO code. The measured erosion/redeposition could be described with ERO, which takes into account the impurity concentration in the SOL, the dynamical change of the surface composition (causing a modification of the sputtering yield during the exposure) and E × B drift effects.

After the 1990/91 discharge period several samples have been cut from the JET vessel walls, the carbon and the beryllium limiters, and the carbon and the beryllium X-point divertor tiles. The surface layers of these samples have been analysed in detail by ion beam techniques, such as PIXE, HIERDA, RBS, SIMS and Sputter-Auger for the absolute amounts as well as for the depth distributions of the different deposits. The major deposits are H, D, Be, C and O, as well as other impurities, such as Ni, Cr, Fe, and C. From the measurements on the carbon samples it was found that the oxygen deposits correlate with the Be deposits, while the measurements on the Be samples show that the C deposits correlate with the D deposits, indicating a codeposition of these elements. In addition H concentrations mostly larger than the D concentrations are measured. These may partly originate also from adsorption during the exposure to air.

A HIERD setup (HI-ERD=heavy ion elastic recoil detection) is introduced including a semiconductor Δ-E-E telescope for the detection of hydrogen and helium isotopes and a Bragg Ionization Chamber (BIC) for the identification of heavier recoils. An optimum forward scattering geometry with respect to maximum analyzable depth and sensitivity is determined experimentally as well as by calculations for the analysis of oxygen and hydrogen using 35 MeV 35Cl incident ions. As a relevant application of the method, the Na content of borosilicate glass is investigated in samples subject to anodic bonding procedures.

Plasma induced erosion and redeposition at the vessel walls has been measured by surface layer analysis of a carbon probe with evaporated markers before and after exposure in the boundary plasma of the TEXTOR tokamak. The results are compared with those calculated with the computer simulation program ERO. The erosion is dominated by carbon and oxygen impurity fluxes in the boundary plasma. Carbon is deposited on the marker surfaces and has to be resputtered for the marker material to be sputtered. Mo atoms sputtered from the marker are found to be redeposited in the slit of the probe. The redeposition pattern can be explained by direct deposition of sputtered Mo neutrals.

After plasma operation in 1991 samples were cut from C and Be tiles of the JET vessel walls, limiters, and X-point tiles along a poloidal cross section in octant 4W, and the surface layers were analysed by the MeV ion beam methods NRA, PIXE, and RBS to determine the D and metal impurity distributions. The amount of trapped deuterium ranges from 5 × 10 15 to 5 × 10 17 D/cm 2 , metal impurities such as Ni, Cr, and Fe are in the range of 10 15 to 3 × 10 16 /cm 2 , Cl in the range of 10 16 to 10 18 /cm 2 , K and Ca in the range of 10 15 to 10 16 /cm 2 and Ti about 10 15 /cm 2 . Except for Cl, all measured concentrations are a factor of up to about ten lower than those observed on JET walls tiles before Be evaporation and Be wall components were used. This observation is in agreement with the reduced impurity concentrations measured in the JET plasma. The D and impurity distributions along the X-point tiles were analysed in some detail. Generally, the deuterium trapping and impurity deposition show minima in areas where erosion dominates and amounts a factor of about ten larger in between.

Surface composition changes, sputtering yields, and hydrogen trapping and release at D-ion bombardment with energies between 50 eV and 1 keV at temperatures up to 2000 K have been investigated with well defined ion beams for the Ti doped graphite RG-Ti (1.7 at % Ti). Due to its high thermal conductivity and thermal shock resistance this material represents a promising candidate for use at high heat load plasma facing areas, such as limiters and divertor plates.

The most straightforward method to measure simultaneously the amount and the depth distribution of different light atoms trapped in the surface layers of a solid, such as needed in investigations of the plasma-wall interaction in controlled thermonuclear fusion research, is the heavy ion elastic recoil detection (HIERD) technique. This technique has been applied using 30 MeV 35Cl ions to measure the light ions collected in a Ti target and in C targets. The recoiled atoms have been identified and detected by a ΔE − E telescope consisting of a 14 μm transmission detector and a SSB detector with a 12 μm Al foil at the entrance to stop all heavy ions. The hydrogen and helium isotopes are clearly identified. The sensitivity of this technique is the largest for H and decreases proportional to Mr3 i.e. proportional to 8 and 27 for the two heavier hydrogen isotopes, while the depth resolution is the best for tritium. Recoils heavier than He up to sodium are simultaneously detected by a Bragg ionization chamber.

A ΔE-E telescope for mass separation of the isotopes of hydrogen is described for a 4He bombarding energy of 4.0 MeV. The use of a 13.6 μm Si transmission detector as ΔE detector enabled the separation of ERDA signals from the three isotopes of hydrogen in thick samples containing hydrogen (H), deuterium (D) and tritium (T). Whereas a ΔE detector is normally used as an atomic charge (Z) separator in heavy ion spectroscopy, in this study it was used to separate the different masses of the hydrogen isotopes. H events were fully separated from the other isotopes and the separation between D and T events was such that more than 95% of the signals corresponding to the two isotopes could be extracted for analytical purposes, with the maximum depth information obtainable for a single isotope being about 4.1 × 1018 Ti at./cm2 for H, 5.3 × 1018 Ti at./cm2 for D and 4.8 × 1018 Ti at./cm2 for T. The technique, which is especially useful in targets containing all the isotopes of hydrogen, is illustrated by means of a study of the depth distribution of hydrogen isotopes in solid thick TiDx and TiTx targets.

The main atomic processes occuring at the solid wall during plasma wall interaction are presented. The data are based on experimental results obtained under laboratory and plasma conditions and/or by computer simulations.

Collision cascade sputtering may be influenced by the target temperature due to a modification of the collision cascade as well as by changes of the surface binding energy or heat of sublimation. Calculations with the computer simulation program TRIM.SP have shown that already a small decrease of the surface binding energy has a strong nonlinear effect on the sputtering yield, especially at energies near the threshold energy for sputtering. For silver the heat of sublimation near the melting point is about 2.7% lower than at room temperature. However, an increase of the sputtering yield of about 36% for 60 eV deuterium ion bombardment at normal incidence was found. Due to the large sublimation rate of Ag at temperatures above about 850 K sputtering yields measured at these temperatures show large uncertainties, and it will be difficult to experimentally confirm the increase of the collisional sputtering yield with temperature.

A criterion is derived for selecting materials capable of withstanding the very large steady state power depositions, that occur on some areas of the vessel walls of present high temperature plasma experiments and are expected in a fusion reactor. To allow the deposited heat to be removed, the material must have a high thermal conductivity k(T) . Due to the temperature gradient across the material, the removable heat flux is determined by the integral fk(T) d T , taken between the temperature T 1 on the cooling side and the maximum temperature T 2 in the heat deposition area. This integral was calculated for several relevant materials. With 1 cm thick materials, the maximum steady state power flux density which can be carried off by conduction is 5 to 20 MW/m2, except for carbon materials, where values up to 90 MW/m2 can be achieved. However, the thermal conductivity of carbon materials is degraded by radiation damage.

Two possible techniques for the measurement of erosion/redeposition in the new JET divertor have been assessed; colour fringe analysis (CFA) and speckle interferometry. CFA is a simple technique in which the target is viewed with a colour camera, and can study optically transparent deposited films up to 1 μm in thickness. Speckle interferometry is a powerful technique for following changes in surface topology, but its sensitivity of >1 μm makes it more ideally suited to the next generation of tokamaks.

The material transport by erosion and redeposition at the plasma facing wall areas in high temperature plasma experiments has been studied using limiter-like carbon probes with well defined surface deposits and depth markers. The probes have been exposed in the scrape-off layer (SOL) of the Joint European Torus (JET) during single discharges. For the evaluation of these experiments a computer program, ERO, has been developed. The calculated erosion-deposition rates for carbon as a function of the distance to the last closed flux surface (LCFS) agree well with the experimental results. For a single 4He JET discharge, erosion yields of 530 A for the silicon deposit and 80 A for vanadium have been measured near the LCFS. A large amount of redeposited silicon (about 17% of the sputtered atoms) has been found on the probe surface in co-deposition with carbon on an area not favoured by the proposed model. This observation can be explained by an additional force on the impurity ions (e.g. the existence of a local electric field), which may cause the deposition

From a tile of the JET toroidal belt limiter installed during the 1990 operation period, samples were cut from the plasma-facing side along the poloidal direction and the surface layers were analysed by SEM and by the ion beam techniques NRA, PIXE, RBS and HIERD. The poloidal distribution of D is fairly uniform with values of up to 2×1017 cm−2. The contamination of the Be by metallic impurities from the vessel wall is typically up to 2×1016 Ni cm−2, 5×1015 Cr cm−2 and 2×1016 Fe cm−2, giving totally about 0.2 g of metals deposited on the upper limiter. Other impurities amount up to 3×1017 C cm−2, 7×1017 O cm−2, 2×1017 Cl cm−2, 2×1016 K cm−2, 4×1015 Ca cm−2, 3×1015 Ti cm−2 and 1×1015 Zn cm−2. The D deposition is about a factor of 4 smaller than on the C limiter of the previous discharge period, while the deposition of the Inconel components is about one order of magnitude smaller than on the C limiter. O is found to be trapped in the Be at depths up to several micrometers, which may be caused by segregation of pure Be through the oxide to the surface of oxide layers on the limiter. This would improve the O gettering of the Be. The lower D concentration is consistent with the observed degassing of D in the time between discharges.

In the modular advanced stellarator W7-AS, the plasma performance and the main characteristics of the plasma-wall interaction are strongly affected by the three-dimensional edge topology. Both limiter-and separatrix-dominated configurations are possible. TiC and bulk-boronized limiter materials have been used. The impurity behaviour and the accessible plasma parameter ranges are compared for different limiter and wall conditions. With limiters, optimum plasma performance in currentless ECRF-or NBI-heated discharges was achieved with bulk-boronized graphite limiter material and boronized walls. Solid target sputter boronization, however, was found to be ineffective in comparison with boronization by He/B 2 H 6 glow discharges. For separatrix-dominated discharges, conditioning by wall coating has short-term effects only. Enhanced, localized plasma outflow to the wall due to islands at the boundary quickly erodes the layers. The possibility to develop a divertor concept is discussed. Basic properties of the plasma edge edge as derived from Langmuir probes and limiter calorimetry are described. Modeling is complicated by three-dimensionality. In a first approach, a 1D edge transport model on the basis of distinct flux bundles is applied.

Plasma facing materials in fusion experiments are generally made from low-Z materials due to the relatively high tolerable concentration of eroded surface atoms in the plasma. The high energy flux to limiter or divertor structures requires operation at elevated temperatures. As pure graphite shows regimes of enhanced erosion at elevated temperatures, doping of graphites with small concentrations of impurities, such as B and Si has been proposed. A series of measurements using 1 keV D + ions has shown considerable improvement in the erosion behaviour of boronated graphite. Chemical sputtering around 900 K can be completely suppressed at boron contents above 3 at% and the onset of radiation enhanced sublimation can be shifted to surface temperatures above 1600 K. The erosion behaviour of pure and doped graphites is finally compared to high-Z materials, such as W, taking erosion rates and plasma impurity radiation into account. While W shows favourable behaviour at edge plasma temperatures below 50 eV, doped graphites can be used at high plasma temperatures but only at surface temperatures below 1600 K.

For studying deuterium depth profiles in surfaces of JET (Joint European Torus) limiters, a new deuterium profiling method was developed and used. A smooth 10-degrees slope plane on the specimen was produced by ion beam slope cutting using a 10 keV Kr ion beam. To detect deuterium the nuclear reaction H-2(C-12, p)C-13 was used at E(C-12(3+) = 7.5 MeV. The very narrow proton peak could be clearly separated from the background of the spectra. For measuring the lateral and vertical deuterium distribution the C-12 beam of the Rossendorf 5 MV tandem accelerator was focussed to about 6-mu-m and scanned across the specimen. The preliminary results show a deuterium distribution up to a depth of about 5-mu-m in a carbon limiter

The potential of the elastic recoil detection technique using heavy ions (HIERD) for the analysis of tiles from the vessel walls of high temperature plasma experiments which have deposits of low-Z elements, such as Be, B, C, N and O, is shown. The analysis has been performed with the 13 MV MP-tandem accelerator in Garching and the 5 MV tandem accelerator in Rossendorf. The recoils have been detected with Bragg ionization chambers. The deposits of low-Z elements and their depth distributions could be determined up to a thickness of about 1 μm.

Depth distributions for deuterium trapped within the Be/C layer on a JET limiter after long-term operation have been measured up to a depth of 7 μm using the D(3He, p)4He reaction in a resonance-like technique. For several points along a JET limiter tile, depth profiles and the total amount of trapped deuterium have been determined.

Backscattering spectrometry of heavy ions with MeV energies offers a higher mass resolution compared to 4He ions while the applicability of the Rutherford cross sections formula is maintained. This feature is used for the accurate composition analysis of compounds, especially of high Tc superconducting films. Heavy incident ions, applied in combination with particle identifying recoil atom spectrometers are also useful for light element depth profiling. Here examples are given for the simultaneous analysis of Be, B, C, N, O, and F in thin films or surface layers.

Helium, as the ash of burning D-T plasma, is an unavoidable impurity component necessarily present already in near future tokamak experiments with significant alpha particle heating. Its efficient removal from the burning zone of a D-T fusion reactor plays a key role in the path towards achievement of economic fusion power production. A survey is given of the issues related to this question. Since there is as yet no experimental experience with thermonuclear plasmas significantly heated by fusion products, the review is based on results from simulation experiments of helium injection into hydrogen or deuterium tokamak plasmas, and from numerical transport code work. Both kinds of results are discussed with reference to handy ignition criteria obtained for steady D-T burning, which have been reformulated in terms relevant for the ash removal problem.

During 1989, following a brief all carbon phase, JET was operated with an evaporated beryllium film on all the plasma facing surfaces and then, in a third phase, with bulk beryllium used for the belt limiters and one set of antenna protection tiles. Subsequent analysis of wall components and long term samples (LTS) using Beta Backscatter and Ion Beam Analysis has been conducted to determine the condition of the First Wall and its deuterium inventory after beryllium operation. Ex-situ analysis of components where deposition occurs during operation shows mixed carbon and beryllium layers with an approximate ratio 1:1 and some localised higher Z components. The deuterium inventory situation is littled changed by the introduction of beryllium. Similar surface levels of deuterium, ~1 × 1022 atoms m−2, have been seen for both carbon and beryllium belt limiter tiles.

For a continuously burning D-T fusion plasma the particle and energy confinement times have to be appropriate since otherwise the produced 4He and the energy cannot be exhausted. The upper limits for the 4He particle confinement parameter neτp(He) are discussed. Comparison with the energy confinement parameter neτE given by the burn condition shows that there is only a limited operational window for a fusion reactor. For a steady state burning D-T plasma, only small concentrations of impurities are tolerable. With increasing impurity concentration, the window becomes smaller and finally closes.

The surface layer of beryllium-coated graphite was analysed by backscattering and nuclear reaction methods using a 2.5 MeV accelerator. For the Be analysis the nuclear reactions 9 Be(p, d) 8 Be, 9 Be(p, α) 6 Li, 9 Be(d, p) 10 Be, 9 Be(d, t) 8 Be, 9 Be(d, α) 7 Li and 9 Be( 3 He, p) 11 B are compared with backscattering of 4 He + and protons. For the analysis of carbon, which provides complementary information to the depth profiling of Be, and for the analysis of oxygen, which is the third element of importance in the system, elastic backscattering of 4 He + and protons is applied. It turns out that the backscattering analysis with protons of 1.3 MeV presents the best compromise for a quick and straightforward measurement of all three elements. The other methods have their merits for a more detailed analysis.

In the search for 'cold fusion' reactions in solids loaded with deuterium atoms at high concentration, ion implantation has been used as a means to reach the required high deuterium concentration in the lattice. In the present investigations, 300 eV to 6 keV D+ ions were injected into 3 μm thick titanium foils at room temperature and at 140 K. The protons from the D(d,p)T reaction were monitored using a solid state detector with a large solid angle, both during and after implantation. After implantation of deuterium in the titanium foil up to saturation, i.e. with about 1.8 deuterium atoms per titanium atom at room temperature, the possible fusion reaction rate at equilibrium was determined over a period of 65 h. The measured count rate was within the limits of the natural background, and an upper limit of 1 × 10−23 per deuterium pair per second was established. During implantation of deuterium at energies between 3 and 6 keV, the count rate is dominated by reactions of the incident energetic deuterons with deuterons already implanted in the foil. The D-D reaction cross-sections, evaluated at 140 K, follow closely those obtained by extrapolation of the Gamow function to low energies. During implantation of 300 eV D+ ions, no counts from the D(d,p)T reactions were monitored, showing that even under extreme conditions of non-equilibrium the possible 'cold fusion' reaction rate is smaller than the background count rate.

Carbon probes were exposed in the center of the Faraday screen of the ASDEX ICRH antenna during single ICRH discharges. The subsequent analysis of the probes by ion beam techniques gave the depth, as a measure for the particle energy, and the total amount of the implanted hydrogen and deuterium. The results indicate, that during the RF fast ions hit both, the antenna surface and the wall around the torus. This can be an important reason for the production of heavy impurities by ICRH. A comparison with CX neutral fluxes shows, that these cannot explain the measured implantations.

We report on a method to measure the depth profile of deuterium up to a depth of 7 μm in a carbon/beryllium layer using the D( 3 He, p) 4 He nuclear reaction in a resonance-like technique. The deuterium depth profile is determined from a measurement of the proton yield as a function of incident 3 He ion energy for a limiter tile from the Joint European Torus (JET) tokamak.