Abstract
Second ion experiment for sputtering and TDS analysis is a high-current ion source for erosion and retention studies with focus on wall materials for fusion devices. The system is composed of a DuoPIGatron type ion source, three consecutive grids for ion extraction, acceleration and beam focusing, a differential pumping stage, a dipole magnet for mass filtering, a quadrupole doublet lens, a target chamber, a load-lock, and a chamber for thermal desorption spectrometry. The acceleration potential of the source can be varied between 500 V and 10 kV. The target chamber has a base pressure of 10-8 mbar and an operating pressure of 5 × 10-7 mbar. The target can be rotated to study angle-dependent effects and can be heated via electron-impact heating up to 1300 K for high temperature erosion and implantation studies. The target chamber is equipped with an in situ magnetic suspension balance. The operating parameters of the ion source were mapped to achieve the maximum ion current at the target for various gas species and accelerating potentials. The beam emittance for a D3 + ion beam was measured after deflection in the dipole magnet. This was used for ion beam simulations, which were instrumental for the design of the quadrupole lenses. If the quadrupole doublet is used, the ion flux to the target is increased by up to a factor of 4. Additionally, the relative population of neutral particles present in the beam at the target was quantified and is equal to 0.8% when averaged over the measurement positions. The typical beam footprint at the target under normal incidence has an area of 0.5 cm2. The ion current reaching the target increases with the accelerating potential. Due to this effect, the ion flux density at the target in the low-ion-impact-energy range can be increased by operating the source at a higher extraction potential and by applying a (decelerating) potential to the target. Ion impact energies as low as 200 eV/D are achieved this way with a D3 + current of 100 μA when focusing the beam with the quadrupole doublet lens, equating to a D particle flux density of 3.7 × 1019 m-2 s-1. At ion impact energies of 2 keV/D, the maximum achievable flux density with D3 + is 6 × 1019 D m-2 s-1. Experimental determination of sputter yields was performed via ex situ weight loss measurement for bulk Au samples, showing reasonably good agreement with simulations and experimental data from the literature.
Highlights
The choice of plasma-facing material is highly significant for future fusion devices, as it imposes a series of safety, operational and economic constraints on the device
The spatially-resolved neutral population within the beam at the position of the target was measured by eroding a 50-nm-thick Au film on a Si substrate and measuring the change of the layer thickness by Rutherford Backscattering Spectrometry (RBS). These experiments resulted in a maximum neutral population of 1.5% and an average population over the measured locations of 0.8%
The results agree within 15% with the sputter yield calculated by the sputtering code SDTrimSP [14]
Summary
The choice of plasma-facing material is highly significant for future fusion devices, as it imposes a series of safety, operational and economic constraints on the device. Ion sources have a number of distinct advantages over plasma devices, producing a mono-energetic beam which can be mass-filtered with magnetic sector fields, allowing for well-defined experiments. These machines have achieved significantly lower particle flux densities than their plasma counterparts, leading to longer exposure times to reach the desired fluence. At SIESTA, a DuoPIGatron type ion source is employed to provide particle flux densities of several 1019 m−2s−1 to the target with a mono-energetic, mass-filtered ion beam (typically D3+), with a final operating energy range of 200 eV − 10 keV. The beam-stopper is equipped with a Faraday cup to measure the impinging ion current at this location
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