Collisional and thermal effects on liquid lithium sputtering

The lithium sputtering yield from lithium and tin-lithium surfaces in the liquid state under bombardment by low-energy, singly charged particles as a function of target temperature is measured by using the Ion-surface Interaction Experiment facility. Total erosion exceeds that expected from conventional collisional sputtering after accounting for lithium evaporation for temperatures between 200 and $400°\mathrm{C}$. Lithium surfaces treated with high-fluence D atoms are bombarded by ${\mathrm{H}}^{+}$, ${\mathrm{D}}^{+}$, ${\mathrm{He}}^{+}$, and ${\mathrm{Li}}^{+}$ at energies between 200 and $1000\mathrm{eV}$ and 45° incidence. Erosion measurements account for temperature-dependent evaporation. For example, $700\mathrm{eV}$ ${\mathrm{He}}^{+}$ particles bombarding the D-treated liquid Li surface at room temperature result in a sputter yield of 0.12 Li/ion and at temperatures $\sim 2.0T_m$ (where $T_m$ is the melting temperature of the sample), a yield near and above unity. The enhancement of lithium sputtering is observed to be a strong function of temperature and moderately on particle energy. Bombardment of a low-vapor-pressure lithium alloy (0.8 Sn-Li), used for comparison, also results in nonlinear rise of lithium erosion as a function of temperature. Measurements on both pure liquid Li and the alloy indicate a weak dependence with surface temperature of the secondary ion-induced secondary ion emission. Treatment of liquid Li surfaces with D, yields reduced sputtering under ${\mathrm{He}}^{+}$ impact by a factor of 5–6 when measured at room temperature due to preferential sputtering effects.

Figure 1

The ion-surface interaction experiment (IIAX). The experimental device is shown with two differentially pumped chambers: on the right the ion gun chamber, and on the left the main chamber where the lithium target sample is located. The inset diagram shows the position of the QCO (quartz crystal oscillator) with respect to the lithium target. The distance between the lithium target and the QCO is $d$, the angle of ejected flux $\varphi$, the radius of the crystal, $R$ and the length from the edge of QCM to center of crystal is designated $L$.

Figure 2

(Color online) The data show a decrease in the crystal frequency as evaporated lithium is collected. As the liquid sample is exposed to the ion beam, the QCM (quartz crystal microbalance) measures both sputtering and evaporation fluxes.

Figure 3

Measurements of the sputtering yield of lithium bombarded by ${\mathrm{He}}^{+}$ ions at 45° incidence for a variety of incident particle energies and target temperatures by Allain et al. (Ref. 26). The data is plotted with the BSY (Bohdansky-Sigmund-Yamamura) model based on linear sputtering theory. At each incident particle energy the nonlinear behavior of the lithium-sputtering yield is shown as the temperature increases.

Figure 4

Lithium sputtering yield as function of sample temperature and deuterium fluence exposure. Also plotted is the BSY-thermal model for deuterium-treated and nondeuterium-treated lithium surfaces.

Figure 5

Measurements of temperature-dependent lithium sputtering from ${\mathrm{D}}^{+}$ bombardment for various incident particle energies at 45° incidence of liquid lithium.

Figure 6

Lithium-sputtering yield from singly charged deuterium and hydrogen ions on liquid lithium at $700\mathrm{eV}$ and oblique incidence as a function of target temperature.

Figure 7

Measurements of temperature-dependent lithium sputtering from ${\mathrm{Li}}^{+}$ bombardment for various incident particle energies at 45° incidence of liquid lithium.

Figure 8

Measurements of lithium self-sputtering yield as a function of incident particle energy for various temperatures at 45° incidence. Also plotted is the BSY linear sputtering yield model for comparison.

Figure 9

Lithium sputtering yield from ${\mathrm{He}}^{+}$ bombardment on liquid lithium for various incident particle energies and temperatures at oblique incidence. Also shown is the lithium sputtering yield from tin-lithium surfaces from both ${\mathrm{He}}^{+}$ and ${\mathrm{D}}^{+}$ bombardment.

Figure 10

Inverse-temperature dependence of the lithium sputter yield parametrized with incident particle energy and compared to a TRIM temperature-dependent model (Ref. 38).

Figure 11

Inverse-temperature dependence of the lithium sputter yield parametrized with (a) incident particle energy compared to a TRIM temperature-dependent model (Ref. 38) and incident flux, and (b) incident mass.

Figure 12

Measurements of the ion-induced secondary ion sputtering fraction as a function of temperature for $700\mathrm{eV}$ ${\mathrm{D}}^{+}$, ${\mathrm{He}}^{+}$, and ${\mathrm{Li}}^{+}$ bombardment at 45° incidence on lithium.

Figure 13

Measurements of the ion-induced secondary ion sputtering fraction as a function of temperature for $700\mathrm{eV}$ ${\mathrm{D}}^{+}$ and ${\mathrm{He}}^{+}$ bombardment at 45° incidence on 0.8 Sn-Li.