J.B. Roberto

Enhanced sputtering of graphite at high temperature

Abstract 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.

Measurement of the density and velocity distribution of neutral Fe in ISX-B by laser fluorescence spectroscopy

Abstract 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.

Measurement of the density and velocity distribution of sputtered Al in EBT-S by laser-induced fluorescence

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.

Isochronal recovery of high-energy d-Be neutron damage in Cu, Nb, and Pt from 8 to 400 K☆

Abstract 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.

Deuterium and impurity deposition profiles in the plasma edge of ISX-B

Abstract 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.

Surface erosion in the plasma-edge of ISX-B☆

Abstract 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 energy dependence of neutron damage in Cu and Nb

Abstract 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.

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

Abstract 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.

Design features and focal properties of simple three element two- or three-dimensional lens systems for decelerating intense ion beams to very low energies

Abstract This paper describes the design features and focal properties of a simple two- or three-dimensional lens system for decelerating ion beams of variable intensity to very low energies (⩽100 → ⩾2000 eV). The characteristics of the lenses were determined by numerically computing the electrostatic field distributions and particle trajectories through specified electrode configurations. The results of computer simulation studies are presented which illustrate the influence of space charge, angular distribution, and aperture size on the focal properties of the lenses. The measured focal properties of a lens designed according to such calculations agree well with the predicted values.

The ISX-JET beryllium limiter experiment☆

Abstract 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.