Ion-surface InterAction Experiment Research Articles

Chemical sputtering studies of lithiated ATJ graphite

Lithium evaporation treatments in the National Spherical Torus Experiment (NSTX) have shown dramatic improvements in plasma performance increasing the viability of lithium as a Plasma Facing Component (PFC) material. In order to understand the complex system of lithiated ATJ graphite, chemical sputtering measurements of plain and lithiated ATJ graphite are conducted in our upgraded IIAX (Ion Surface Interaction Experiment) facility with a differentially pumped Magnetic Sector Residual Gas Analyzer (MSRGA). Chemical sputtering of graphite is dependent on the ion energy and substrate temperature, hence the effects of treating ATJ graphite with lithium in hydrogen plasma is investigated in terms of different target temperatures and bias voltages. For this purpose, lithium was evaporated in situ onto ATJ graphite and chemically sputtered species in hydrogen plasma were measured using MSRGA. The dominant chemical sputtering product was CH4. Initial experiments show that lithium treatments have suppressed the chemical sputtering of ATJ graphite.

Physical and chemical erosion studies of lithiated ATJ graphite

Lithium evaporation treatments for ATJ graphite tiles in the National Spherical Torus Experiment (NSTX) have shown dramatic improvements in plasma performance increasing the viability of lithium as Plasma Facing Component (PFC) material. In order to understand the complex system of lithiated ATJ graphite, studies of physical and chemical erosion of plain and lithiated ATJ graphite are conducted in the Ion Surface Interaction Experiment (IIAX) facility. Since lithium is known to sputter also as ions, the ionization fraction was measured and found to be 30±6% for a 2000eV Li ion beam sputtering of lithiated graphite. For chemical erosion measurements, a new facility was developed wherein the target is irradiated with deuterium plasma and the resulting erosion products are detected using a residual gas analyzer. The initial results from this ongoing work are presented here. It is shown qualitatively that lithium treatments suppress the chemical erosion of ATJ graphite.

Lithium research as a plasma facing component material at the University of Illinois

Plasma facing component (PFC) materials are critical to the development of future fusion reactor. While several different PFC materials have been considered in the past, there is an increased interest in the scientific community on the use of lithium as a future PFC material, after its initial success demonstrated by the TFTR “supershots” and more recent work performed on the CDX-U and NSTX spherical tori. The worldwide usage of lithium in tokamaks has grown rapidly over the past few years because of these results. The Center for Plasma-Material Interactions (CPMI) at the University of Illinois at Urbana-Champaign (UIUC) has been actively involved in lithium research since its foundation and currently, there are three experimental facilities dedicated to studying lithium interactions relevant to fusion conditions namely: 1. Ion surface InterAction eXperiment (IIAX); 2. Divertor Erosion and Vapor Shielding eXperiment (DEVeX); and 3. Solid/Liquid Lithium Divertor Experiment (SLiDE). One of the significant advantages of these experimental facilities is the ability to study the basic physics of the phenomenon taking place. In this review paper, a brief description of the experimental facilities and their capabilities is provided and the interested reader is advised to look in future publications for the recent results.

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 $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. 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 $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and 45\ifmmode^\circ\else\textdegree\fi{} incidence. Erosion measurements account for temperature-dependent evaporation. For example, $700\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ ${\mathrm{He}}^{+}$ particles bombarding the D-treated liquid Li surface at room temperature result in a sputter yield of 0.12 Li/ion and at temperatures $\ensuremath{\sim}2.0{T}_{m}$ (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.

Temperature dependence of liquid Sn sputtering by low-energy He + and D + bombardment

Absolute sputtering yields of liquid tin from 240 to 420 °C due to irradiation by low-energy helium and deuterium have been measured. For ion energies ranging from 300 to 1000 eV, temperature enhancement of liquid tin sputtering was noted. These measurements were obtained by IIAX (the Ion-surface InterAction eXperiment) using a velocity-filtered ion beam at 45° incidence to sputter material from a liquid tin target onto deposition monitors. Sputtering yields from 500 eV ion bombardment at 45° incidence increase from 0.1 ± 0.03 and 0.019 ± 0.008 Sn particles/ion at room temperature, for He + and D + ions respectively, to 0.30 ± 0.12 and 0.125 ± 0.05 Sn particles/ion for 380 °C. Temperature enhanced sputtering has been seen in other liquid metals (namely lithium, tin–lithium, and gallium) using both ion beam and plasma irradiation.

Studies of liquid-metal erosion and free surface flowing liquid lithium retention of helium at the University of Illinois

The erosion of liquid-metals from low-energy particle bombardment at 45° incidence has been measured for a combination of species and target materials in the ion-surface interaction experiment (IIAX) at the University of Illinois Urbana-Champaign. Measurements include bombardment of liquid Li, Sn–Li and Sn by H + , D + , He + , and Li + particles at energies from 100 to 1000 eV and temperatures from 20 to 420 °C. Lithium sputtering near and just above the melting point shows little change compared to room temperature, solid-Li yields. When lithium is sputtered, about 2/3 of the sputtered flux is in the charged state. Temperature-dependent sputtering results show enhanced (up to an order-of-magnitude increase) sputter yields as the temperature of the sample is increased about a factor of two of the melting point for all liquid-metals studied (e.g., Li, Sn–Li, and Sn). The enhancement is explained by two mechanisms: near-surface binding of eroded atoms and the nature of the near-surface recoil energy and angular distribution as a function of temperature. The Flowing Liquid Retention Experiment (FLIRE) measured particle transport by flowing liquid films when exposed to energetic particles. Measurements of retention coefficient were performed for helium ions implanted by an ion beam into flowing liquid lithium at 230 °C in the FLIRE facility. A linear dependence of the retention coefficient with implanted particle energy is found, given by the expression R = (5.3 ± 0.2) × 10 −3 keV −1 . The ion flux level did not have an effect for the flux level used in this work (∼10 13 cm −2 s −1 ) and square root dependence with velocity is also observed, which is in agreement with existing particle transport models.

D +, He + and H + sputtering of solid and liquid phase tin

Absolute sputtering yields of Sn in the solid and liquid phases for incident H +, D +, and He + ions with energies of 300 to 1000 eV were determined via a series of experiments using the ion-surface interaction experiment (IIAX), a facility designed to measure such ion-surface interactions. IIAX incorporates an ion source with an array of ion beam filters and optics to bring a near mono-energetic beam of ions to the target, in this case at 45° to the surface normal. A pair of quartz crystal microbalances record the amount of mass collected from both sputtering and evaporation from the target. VFTRIM-3D modeling results of tin sputtering best fit the experimental data when the bulk of pure tin was modeled with 60% SnO coverage in the top three monolayers most likely indicating the presence of a very thin oxide layer during experimentation.

Temperature dependence of liquid-lithium sputtering from oblique 700 eV He ions

The lithium-sputtering yield of liquid lithium as a function of sample temperature has been measured in the ion-surface interaction experiment (IIAX). Lithium sputtering is measured for D +, He + and Li + bombardment at energies between 100 and 1000 eV at 45° incidence. In this work VFTRIM-3D is used to provide a qualitative physical picture of mechanisms responsible for the temperature dependence of liquid-lithium sputtering. The present study is done for 700 eV He + bombardment of liquid lithium at 45° incidence with respect to the target normal. The lithium-sputtering yield, after evaporation is taken into account, is found to increase almost an order of magnitude when the target temperature is increased from the melting point up to roughly 410 °C. The deposited energy distribution near the liquid-lithium surface is found to play a significant role in explaining the observed enhanced lithium sputtering as well as the temperature dependence of the surface binding energy.

Measurements and modelling of solid phase lithium sputtering

The absolute sputtering yields of D+, He+ and Li+ on deuterium saturated solid lithium have been measured and modelled at 45° incidence in the energy range 100-1000 eV. The Ion-surface InterAction Experiment (IIAX) was used to measure the absolute sputtering yield of lithium in the solid phase from bombardment with a Colutron ion source. The lithium sample was treated with a deuterium plasma from a hollow cathode source. Measurements also include bombardment of non-deuterium-saturated lithium surfaces. The results lead to the conclusion that the chemical state of the deuterium treated lithium surface plays a major role in the decrease of the lithium sputtering yield. Specifically, preferential sputtering of implanted deuterium atoms over lithium atoms in deuterium treated samples results in a decrease of at least 60% of the lithium sputtering yield, in the case of He+ bombardment. These results also demonstrate that lithium self-sputtering is well below unity and that the fraction of sputtered species in an ionic state ranges from 55 to 65% for incident particle energies between 100 and 1000 eV. Furthermore, correlation of Monte Carlo VFTRIM-3D simulations and IIAX experimental data demonstrate that the surface composition has a one to one ratio between deuterium and lithium components.

Measurements and modeling of D, He and Li sputtering of liquid lithium

The absolute sputtering yields of D +, He + and Li + on solid lithium have been measured and modeled at low energies in the ion-surface interaction experiment (IIAX). The experiment has been extended to measure physical sputtering from liquid lithium surfaces bombarded by D +, He + and Li +. A Colutron ion source is used to create and accelerate gaseous or metal ions onto the liquid metal target. A plasma cup removes any oxides and saturates the surface with deuterium. A small high-temperature, HV substrate heater is used to heat the 0.76 g lithium sample past its melting point to 200°C. Upon melting, a thin oxide layer is formed on the exposed lithium surface, which is cleaved by an in situ arm rotated in front of the target. Results suggest that the absolute sputtering yield of lithium is less than unity. In addition, the behavior of liquid lithium self-sputtering suggests stratification of the top liquid metal surface. This is consistent with VFTRIM-3D modeling, where D atoms migrate into the bulk while the first few monolayers remain mostly lithium.

D, He and Li sputtering of liquid eutectic Sn–Li

The absolute sputtering yields from bombardment of D +,He + and Li + on liquid tin–lithium eutectic have been measured and modeled at energies between 200 and 1000 eV. The Ion-surface InterAction Experiment (IIAX) has been optimized to reliably measure the absolute sputtering yield of many ion-target combinations including solid and liquid lithium. A Colutron ion source is used to create and accelerate gaseous or metal ions onto a liquid metal target. The bombarding ions are mass-selected through an E X B filter and decelerated near the target. The target can be rotated in order to provide variation in the angle of incidence. Deuterium plasma from a hollow cathode source is used to remove any remaining oxides. Upon melting of the sample a thin oxide layer forms and is cleaved by an in situ arm. Results show that sputtering yields from liquid tin–lithium are larger than pure lithium. In addition, modeling with VFTRIM-3D confirms that Li atoms segregate to the surface of liquid tin–lithium. This is consistent with results of ion fraction of sputtered atoms, which show a sputtered-atom ion fraction of 65% for liquid tin–lithium, equal to pure liquid lithium, and <10% for solid tin–lithium.