H. Hijazi

Flux threshold measurements of He-ion beam induced nanofuzz formation on hot tungsten surfaces

We report measurements of the energy dependence of flux thresholds and incubation fluences for He-ion induced nano-fuzz formation on hot tungsten surfaces at UHV conditions over a wide energy range using real-time sample imaging of tungsten target emissivity change to monitor the spatial extent of nano-fuzz growth, corroborated by ex situ SEM and FIB/SEM analysis, in conjunction with accurate ion-flux profile measurements. The measurements were carried out at the multicharged ion research facility (MIRF) at energies from 218 eV to 8.5 keV, using a high-flux deceleration module and beam flux monitor for optimizing the decel optics on the low energy MIRF beamline. The measurements suggest that nano-fuzz formation proceeds only if a critical rate of change of trapped He density in the W target is exceeded. To understand the energy dependence of the observed flux thresholds, the energy dependence of three contributing factors: ion reflection, ion range and target damage creation, were determined using the SRIM simulation code. The observed energy dependence can be well reproduced by the combined energy dependences of these three factors. The incubation fluences deduced from first visual appearance of surface emissivity change were (2–4) × 1023 m−2 at 218 eV, and roughly a factor of 10 less at the higher energies, which were all at or above the displacement energy threshold. The role of trapping at C impurity sites is discussed.

He-ion and self-atom induced damage and surface-morphology changes of a hot W target

We report results of measurements on the evolution of the surface morphology of a hot tungsten surface due to impacting low-energy (80?12?000?eV) He ions and of simulations of damage caused by cumulative bombardment of 1 and 10?keV W self-atoms. The measurements were performed at the ORNL Multicharged Ion Research Facility, while the simulations were done at the Kraken supercomputing facility of the University of Tennessee. At 1?keV, the simulations show strong defect-recombination effects that lead to a saturation of the total defect number after a few hundred impacts, while sputtering leads to an imbalance of the vacancy and interstitial number. On the experimental side, surface morphology changes were investigated over a broad range of fluences, energies and temperatures for both virgin and pre-damaged W-targets. At the lowest accumulated fluences, small surface-grain features and near-surface He bubbles are observed. At the largest fluences, individual grain characteristics disappear in focused ion beam/scanning electron microscopy (FIB/SEM)?scans, and the entire surface is covered by a multitude of near-surface bubbles with a broad range of sizes, and disordered whisker growth, while in top-down SEM imaging the surface is virtually indistinguishable from the nano-fuzz produced on linear plasma devices. These features are evident at progressively lower fluences as the He-ion energy is increased.

A large-acceptance beam-deceleration module for retrofitting into ion-source beam lines

We describe a large-acceptance deceleration module capable of decelerating large-emittance full-intensity ion beams typical of ECR ion sources to very low energies with high efficiency. The deceleration module is designed to permit convenient retrofitting into an existing beam line to replace, e.g., the first Faraday cup after magnetic analysis of the beam extracted from the ion source. For starting energies of 10 keV, and incident ion currents as large as 300 μA, deceleration efficiencies have been measured to be greater than 80% for final energies as low as 70 eV. The decelerated beam intensity can be monitored either by insertion of a beam catcher floating at the final deceleration voltage or from the current to the exit grid itself, with suitable correction applied for the grid transparency factor. The behavior of the deceleration optics was modeled using SIMION, incorporating the effects of intra-beam space charge repulsion. We describe a recent application of this deceleration module to study near-surface He bubble and blister formation of a W target heated to 1250 K and irradiated with a 98 eV He ion beam with a flux of ∼10(16) cm(-2) s(-1).

Anorthite sputtering by H+ and Arq+ (q = 1-9) at solar wind velocities

Here, we report sputtering measurements of anorthite-like material, taken to be representative of soils found in the lunar highlands, impacted by singly and multicharged ions representative of the solar wind. The ions investigated include protons, as well as singly and multicharged Ar ions (as proxies for the nonreactive heavy solar wind constituents), in the charge state range +1 to +9, at fixed solar wind-relevant impact velocities of 165 and 310 km/s (0.25 keV/amu and 0.5 keV/amu). A quartz microbalance approach (QCM) for determination of total sputtering yields was used. The goal of the measurements was to determine the sputtering contribution of the heavy, multicharged minority solar wind constituents in comparison to that due to the dominant H+ fraction. The QCM results show a yield increase of a factor of about 80 for Ar+ versus H+ sputtering and an enhancement by a factor of 1.67 between Ar9+ and Ar+, which is a clear indication of a potential sputtering effect.

Surface-morphology changes and damage in hot tungsten by impact of 80 eV – 12 keV He-ions and keV-energy self-atoms

We report results of measurements on the evolution of the surface morphology of a hot tungsten surface due to impacting low-energy (80 ? 12,000 eV) He ions, performed at the ORNL Multicharged Ion Research Facility (MIRF). Surface-morphology changes were investigated over a broad range of fluences, energies and temperatures for both virgin and pre-damaged W-targets. At low fluences, ordered coral-like and ridge-like surface structures are observed, with great grain-to-grain variability. At the largest fluences, individual grain characteristics disappear in FIB/SEM scans, and the entire surface is covered by a multitude of near-surface bubbles with a broad range of sizes, and disordered whisker growth, while in top-down SEM imaging the surface is virtually indistinguishable from the nanofuzz produced on linear plasma devices. These features are evident at progressively lower fluences as the He-ion energy is increased. In addition, simulations were carried out of damage caused by cumulative bombardment of 1 keV W self-atoms, using LAMMPS at the Kraken supercomputing facility of the University of Tennessee. The simulations show strong defect-recombination effects that lead to a saturation of the total defect number after a few hundred impacts, while sputtering and implantation lead to an imbalance of the vacancy and interstitial numbers.

Kinetic and potential sputtering of an anorthite-like glassy thin film

In this paper, we present measurements of He+ and He+2 ion-induced sputtering of an anorthite-like thin film at a fixed solar-wind-relevant impact energy of ~0.5 keV/amu using a quartz crystal microbalance approach (QCM) for determination of total absolute sputtering yields. He+2 ions are the most abundant multicharged ions in the solar wind and increased sputtering by these ions in comparison to equi-velocity He+ ions is expected to have the biggest effect on the overall sputtering efficiency of solar wind impact on the moon. Our measurements indicate an almost doubling of the sputtering yield for doubly charged incident He ions compared to same velocity He+ impact. Using a selective sputtering model, the new QCM results presented here, together with previously published results for Ar+q ions and SRIM results for the relevant kinetic sputtering yields, the effect due to multicharged solar-wind ion impact on local near-surface modification of lunar anorthite-like soil is explored. It is shown that the multicharged solar wind component leads to a more pronounced and significant differentiation of depleted and enriched surface elements as well as a shortening of the timescale over which such surface compositional modifications might occur in astrophysical settings. In addition, to validate previous and future determinations of multicharged-ion-induced sputtering enhancement for those cases where the QCM approach can’t be used, relative quadrupole-mass-spectrometry (QMS) based measurements are presented for the same anorthite-like thin film as were investigated by QCM, and their suitability and limitations for charge-state-enhanced yield measurements are discussed.

Kinetic And Potential Sputtering Of Lunar Regolith: The Contribution Of The Heavy (Minority) Solar Wind Ions

In this paper the sputtering of lunar regolith by protons and solar wind heavy ions is considered. From preliminary measurements of H + , Ar +1 , Ar +6 and Ar +9 ion sputtering of JSC-1A AGGL lunar regolith simulant at solar wind velocities, and TRIM simulations of kinetic sputtering yields, the relative contributions of kinetic and potential sputtering contributions are estimated. An 80-fold enhancement of oxygen sputtering by Ar + over same-velocity H + , and an additional x2 increase for Ar +9 over same-velocity Ar + was measured. This enhancement persisted to the maximum fluences investigated (~10 16 /cm 2 ). Modeling studies including the enhanced oxygen ejection by potential sputtering due to the minority heavy ion multicharged ion solar wind component, and the kinetic sputtering contribution of all solar wind constituents, as determined from TRIM sputtering simulations, indicate an overall 35% reduction of near-surface oxygen abundance. XPS analyses of simulant samples exposed to singly and multicharged Ar ions show the characteristic signature of reduced (metallic) Fe, consistent with the preferential ejection of oxygen atoms that can occur in potential sputtering of some metal oxides.