(a) SIMION ion trajectory simulation for a 10 keV, 400 μA ion beam with a 7 mm, 3.2° divergence angle waist at start of simulation and decelerated to a final energy of 95 eV, showing ∼100% transmission and a final spot size of ∼11.5 mm. Entrance slits are set at 12 × 12 mm and the grid/target distance is 1 mm; the electrical connection details are schematically indicated. (b) Equipotential plot of the optimized ion optics, with applied potentials indicated. The equipotential contour spacing up beam of E2 is 472 V, and about 40 V for regions including E2 and down beam thereof. Also shown in the figure are the grounded vacuum walls of beam line in which the decel module is mounted. (c) SIMION ion trajectory simulation for the initial conditions of (a) without space charge repulsion. Note final spot size is reduced to ∼4.5 mm.
(a) The mechanical design of the large acceptance decel module, identifying the different module components (mounting rings, apertures, lens elements, grid, and target); (b) photograph of the decel module showing the deceleration elements E0, E1, E2, and EG.
Schematic diagram of the decel module mounted in a 4-way 6-in. CF cross, showing target sample mounting, heating, and electrical connection details.
(a) Plot of target current as function of applied deceleration voltage on target T, EG and E2, with a fixed value of +4.0 kV on E1, for a 300 μA He ion beam extracted from the CAPRICE ECR ion source at 10 kV source potential. The straight line between data points is drawn to guide the eye. Note high target transmission up to ∼9.930 kV beam deceleration voltage. (b) Expanded region of plot shown in (a) showing the negatively biased sample current as a function of the beam deceleration voltage in the range 10.000 kV-10.030 kV, which includes the beam cut-off; the shown straight line extrapolation to the negative current baseline provides an estimate of the ECR plasma potential as described in the text.
Plot of the effective secondary electron emission coefficient γ eff, calculated from the measured positively and negatively biased target currents, as described in the text. The curve depicts the expected energy dependence of the “true” secondary electron emission coefficient for He ion impact on tungsten, and is drawn to guide the eye. The deviation of γ eff from the curve at low energies indicates the presence of additional slow electrons produced away from the target, which causes underestimation of the target ion current at energies below 90 eV.
Photograph of a tungsten target sample after irradiation at 1250 K by a 98 eV He+ ion beam with an accumulated fluence in excess of 1020 cm−2. The dimension of the final beam spot size is about 8.5 × 6.5 mm.
Surface SEM image of an (a) unirradiated and (b) irradiated heated tungsten target region. Note formation of pinholes and bubbles on the irradiated area.
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