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The heaviest elements are synthesized in heavy-ion induced hot fusion reactions with various actinide targets. Because the actinide material is often available only in very limited amounts, a deposition method with high yields (~90 %) is needed. We report on the production of 244Pu, 243Am, 248Cm, 249Bk, and 249Cf targets on thin Ti backings by molecular plating. Different chemical purification steps using ion chromatographic techniques were applied for the purification of 249Cf and 244Pu. The deposition procedure applied for the production of ~0.4–0.8 mg/cm2 thick targets is described. The deposition yield was determined either by α-particle or γ-ray spectroscopy. Furthermore, neutron activation analysis has been applied in the case of 244Pu, 243Am, and 248Cm. Information about the spatial distribution and homogeneity of the target layer was obtained by radiographic imaging.

Working gas composition and edge impurity levels have been monitored during H-D changeover for non-gettered, ohmic discharges in the ISX-B tokamak using optical spectroscopy, mass spectrometry, and surface probe analysis. The isotopic exchange in the plasma is ~80% complete within the first 15 ms of the first discharge in the new isotope, but requires ~10 discharges to reach 98% completion. Fe edge densities increase by a factor of ~3 for D as compared to H plasmas, whereas 0 levels remain comparable for D and H operation. The changeover results indicate that the plasma isotopic composition in ISX-B is dominated by the fill gas, and that most of the smaller contribution from isotopic exchange with exposed surfaces is completed early in the discharge. The impurity data suggest a hydrogen sputtering mechanism for heavy impurity introduction and a chemical process for 0 release.

The influence of near-surface impurities on the enhanced sputtering of graphite near 600°C and above 1000° Chas been investigated under various doping conditions. The chemical sputtering yield of methane molecules for 2 keV D + ions on graphite at 600°C has been determined for several coverages of Si, Fe, Ni, Ti, Mo, and Au. Graphite samples were pre-deposited with ~ 500 A of a given impurity, and weight loss was monitored in-situ using a vacuum balance while sputtering through the deposited layer. Small amounts of impurities remaining on the surface after prolonged sputtering reduced chemical erosion by 30–50%. The radiation-enhanced sublimation of graphite observed for energetic ion bombardment above 1000°C was investigated during 50 keV Ar sputtering and simultaneous Ti evaporation. For very low surface concentrations of Ti, graphite sputtering increased while for concentrations above 10 at% the yields were significantly reduced. A graphite sample containing 4 at% bulk Si was also investigated and showed greatly reduced chemical erosion at 600° Cbut no reduction in radiation-enhanced sublimation at 1500°C due to surface enrichment and depletion of Si for the low- and high-temperature irradiation, respectively.

Primary recoil spectra and specific damage energies have been computed for neutron interactions in Cu and Nb at neutron energies up to 32 MeV. The calculations are based on theoretical neutron cross sections and are in good agreement with recent radiation damage experiments using high energy neutrons from the Be(d,n) reaction. The results are particularly relevant to the understanding of radiation effects from high energy deuteron-breakup neutron sources.

The enhanced sputtering of graphite at temperatures above 1000 K has been investigated for a variety of incident ion species and energies. Papyex graphite strips were irradiated with 35 to 150 keV H, C, O, and Ar ions in the temperature range from 300 to 1800 K, and with 0.13 to 8 keV H ions at 1800 K. Sputtering yields were determined by weight change measurements, and were compared in many cases with yields determined by in-situ ion beam analysis of collector probes. For temperatures above 1000 K, enhanced sputtering yields are found which increase with temperature to 6 to 20 times room temperature values at 1800 K. At a given temperature, measured yields for all species and energy combinations scale with the nuclear deposited energy at the surface and do not correlate with electronic losses. The low energy H measurements indicate a transferred energy threshold of ∼ 5 eV for the enhanced erosion process, significantly lower than the displacement threshold energy in graphite of 30 eV. The enhanced erosion is characterized by an activation energy which varies from 0.5 to 1.1 eV with decreasing incident particle mass. The overall results, together with recently reported velocity and mass spectra of the sputtered particles, suggest a radiation-enhanced sublimation mechanism.

The possible synergistic effects which can contribute to plasma-surface interaction phenomena in fusion devices are reviewed. These effects include the influence of reactive ions, surface modification, temperature, radiation damage, and external forces and fields on erosion yields, hydrogen retention and release, and other surface processes. The important synergistic effects are described in terms of surface and edge conditions encountered in present fusion devices and expected in future reactors. Priority data needs include the chemical erosion of graphite at high particle fluxes, melt-layer stability under disruption-induced eddy-current forces, the influence of bulk neutron damage on hydrogen retention, and in-situ measurements of synergistic effects in operating fusion devices.

The density and velocity distribution of neutral Fe atoms in the plasma-edge of the ISX-B tokamak have been measured by laser fluorescence spectroscopy. Fe neutrals were excited at 302 nm using a flashlamp-pumped dye laser. The fluorescence light was detected at 382 nm with an interference filter and photomultiplier system. The frequency-doubled laser output (700 W) and pulse length (500 ns) allowed saturation pumping of the Fe transition thereby enabling absolute measurement of densities. Insertion of an additional Fabry-Perot etalon into the laser cavity narrowed the laser bandwidth to 0.6 pm corresponding to a velocity resolution of 600 m/s (0.1 eV for Fe). Neutral densities were measured for two locations in the plasma-edge as a function of discharge time. Results show a detection sensitivity of 1012 atoms/m3 with a neutral Fe density of a few 1012 atoms/m3 in the first 20 ms of the discharge and 1014 atoms/m3 30 ms after the discharge. The velocity distribution of the neutral Fe at the end of the discharge is thermal (400–1000 K) while the smaller peak at the beginning of the discharge shows evidence of higher velocities.

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.

The erosion of carbon and the deposition of carbon and impurities have been determined for graphite samples exposed at the wall of JET during the six months of operation in 1985. Graphite erosion was determined using SIMS analysis of the depth change of implanted 13C marker layers in the samples. Impurity deposition was measured using SIMS and Rutherford backscattering. The samples showed high erosion ranging from 0.2 to 0.6 μm at various locations, with significant poloidal variation in both erosion and deposition. The deposition of substantial quantities of metal impurities and carbon was related to a probe accident and a deliberate heavy carbonization procedure during the operation period. The highest observed erosion exceeds the expected sputter erosion due to charge exchange neutrals by more than an order of magnitude, unless neutral fluxes are enhanced at the inner wall due to ions arriving along open field lines during current ramp-down.

An isotopic marker technique has been developed and used to measure near-surface erosion and carbon and impurity deposition on graphite samples in the ASDEX divertor chamber. Papyex graphite strips were enriched to approx. 20% /sup 13/C over the first 1000 A by ion implantation. The implanted /sup 13/C can be clearly distinguished from /sup 12/C of the host by Rutherford backscattering using 2.7 MeV /sup 4/He. Carbon erosion and deposition were determined from changes in the depth distribution of the /sup 13/C, and impurity accumulation was also monitored in the backscattering spectrum. The /sup 13/C profiles remained sharp in spite of high heat fluxes to the samples (sufficient to melt stainless steel), and the results showed clear evidence of simultaneous erosion and deposition of carbon and impurities. During 2.4 MW neutral injection, net carbon erosion exceeded 250 A/s in the divertor throat. High carbon erosion was also observed during periods of substantial Fe and Ti impurity accumulation on the samples. The results demonstrate the applicability of the /sup 13/C technique for measuring erosion/deposition at high-heat-flux surfaces such as limiters or divertor plates.

Quantitative radial profiles have been determined for deuterium and heavy impurities ( A ⩾ 12 ) trapped on deposition probes in the plasma edge of ISX-B. Silicon samples were exposed to 1, 4 and 12 tokamak discharges at distances of 10 to 50 mm behind the limiter radius for both ohmic and beam-heated plasmas. The species, quantities and saturation behavior of the principal impurities were determined by ion backscattering and channeling analysis, while nuclear reaction D( 3 He, p) 4 He and damage analyses were used for the detection and depth profiling of deuterium. The components of stainless steel (Fe, Cr, Ni) were found to increase linearly with the number of discharges, while radial profiles for oxygen and D showed saturation effects in a single discharge. Estimates of D energies and both impurity and D fluxes in the plasma-edge are given for ohmically heated and 350 kW neutral beam injected discharges.

Plasma-material interactions in the scrapeoff region of a tokamak nave important effects on the overall performance of the machine. In order to prudently select the most appropriate materials for walls, limiters and armor plate, it is necessary to characterize the plasma that interacts with these surfaces and to understand what effect different modes of operation of the tokamak have on plasma characteristics. We have made a series of measurements on the ISX tokamak using deposition probe techniques to identify and quantify the impurities (Z ≥ 8) in the limiter shadow and to determine the temporal behavior of both impurity and plasma particles in this region. These measurements have been made under a variety of tokamak operating conditions, including doth ohmic and neutral beam heated discharges. The results are interpreted in terms of edge conditions, impurity introduction, gas puffing and the relative importance of wall and limiter contributions.

Surface erosion of thin Au films has been directly observed for samples exposed to H2 and D2 discharges at various locations and orientations in the plasma-edge of ISX-B. The erosion of Au as well as the accumulation of D, O and Fe on the samples was determined using ion scattering, channeling and nuclear reaction techniques. Erosion rates varied from ~0.1 nm/discharge at 2 cm behind the limiter to near zero as the wall was approached. Comparable erosion rates were observed for H2 and D2 plasmas. The results are consistent with a dominant role for O sputtering (in the absence of arcing) for erosion at the limiter radius in ISX-B with negligible erosion at the wall.

The density and the velocity distribution of neutral Al released from the vacuum wall in the EBT-S fusion device have been measured using laser-induced fluorescence. Al I was excited at the 308 nm resonance transition using a frequency-doubled dye laser. Absolute Al-densities (~ 108/cm3) were determined at distances between 5 cm and 14 cm from the wall and for various plasma conditions, by comparison of the intensity of induced fluorescence with that obtained from Rayleigh scattering. Al-velocity distributions perpendicular to the wall were determined by scanning the laser through the Doppler-broadened absorption line. The influence of geometry and ionization on the shape of the distributions is given special attention to. From these distributions mean velocities were derived (~ 2.6 × 105cm/s), and along with the measured densities Al-fluxes were calculated. The results indicate sputtering, the possible mechanisms of which are discussed.

Laser-induced fluorescence has been used to measure the density and ionization length of neutral metal atoms released from a reference limiter during plasma discharges in the Impurity Study Experiment (ISX-B) tokamak. A tunable, pulsed laser beam is introduced into the tokamak at the midplane along a major radius and passes in front of a stainless steel reference limiter positioned between the wall and the main limiter radius. Neutral iron atoms released from the limiter are detected by saturation pumping of the ground-state transition at 302 nm, followed by off resonance detection of the resulting fluorescence radiation. The sampling rate is 15 Hz; additional time resolution is provided by overlaying similar discharges with different time synchronization. In ohmic discharges in ISX-B, a nearly constant impurity release rate of 8 × 1014 atom cm−2 s−1 occurs during the steady-state part of the discharge, with substantial peaks during the breakdown, rampup, and termination of the shot. Measured ionization lengths are suggestive of particle velocities typical of sputtering. These results are presented with similar data from neutral-beam-heated shots.

The isochronal recovery of high-energy d-Be neutron damage has been measured in Cu, Nb, and Pt from 8 to 400 K, using changes in the electrical resistivity. High-purity wire specimens were irradiated at 4.2 K with neutrons produced at the Oak Ridge Isochronous Cyclotron by the reaction of 40-MeV deuterons on a thick Be target. The resulting neutron energy spectrum was broadly peaked near 15 MeV. When compared with recent fission neutron results the recovery of d-Be neutron damage is slightly suppressed through stage II for Cu and Nb, but significantly reduced (about a factor of two) in stage I in Pt. This suggests that the primary damage state remains essentially unaltered in lighter elements as the mean neutron energy increases from 2 to 15 MeV, but that substantial configurational changes may occur over the same energy range for heavier elements. The results of recent damage-production measurements and room-temperature irradiations using d-Be neutrons are discussed in terms of the recovery spectra.

A description is given of an experiment that has been designed for the ISX-B tokamak at the Oak Ridge National Laboratory using beryllium as the primary plasma limiter.

Normal incidence sputtering yields have been measured for 200 – 2000 eV hydrogen ions on Au, Ni and 304 stainless steel films using a Rutherford backscattering technique. The measured yields agree well with recent results obtained at higher ion doses using the weight-loss method as well as with model calculations based on the binary collision approximation.