J. Nucl. Materials, 290-293, 180-184 (2001).
Allain, J. P., Ruzic, D. N., Hendricks, M. R.
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.
J. Nucl. Materials, 290-293, 33-37 (2001).
Allain, J. P., Ruzic, D. N., Hendricks, M. R.
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.
Fusion Energy and Design, 49-50, 127-134 (2000).
Mattas, R. F., Allain, J. P., Bastasz, R., Brooks, J. N., Evans, T., Hassanein, A., Luckhardt, S., McCarthy, K., Mioduszewski, P., Maingi, R., Mogahed, E., Moir, R., Molokov, S., Morley, N., Nygren, R., ROnglien, T., Reed, C., Ruzic, D. N., Svitoslavsky, I., Sze, D., Tillack, M., Ulrichson, M., Wade, P.M., Wooley, R., Wong, C.
The advanced limiter-divertor plasma-facing systems (ALPS) program was initiated in order to evaluate the potential for improved performance and lifetime for plasma-facing systems. The main goal of the program is to demonstrate the advantages of advanced limiter/divertor systems over conventional systems in terms of power density capability, component lifetime, and power conversion efficiency, while providing for safe operation and minimizing impurity concerns for the plasma. Most of the work to date has been applied to free surface liquids. A multi-disciplinary team from several institutions has been organized to address the key issues associated with these systems. The main performance goals for advanced limiters and divertors are a peak heat flux of >50 MW/m2, elimination of a lifetime limit for erosion, and the ability to extract useful heat at high power conversion efficiency (
40%). The evaluation of various options is being conducted through a combination of laboratory experiments, modeling of key processes, and conceptual design studies. The current emphasis for the work is on the effects of free surface liquids on plasma edge performance.
Physics of Plasmas, 7, 1421-1432 (2000).
Alman, D. A., Ruzic, D. N.
A model of collisional processes of hydrocarbons in hydrogen plasmas has been developed to aid in computer modeling efforts relevant to plasma–surface interactions. It includes 16 molecules (CH up to CH4, C2H to C2H6, and C3H to C3H6) and four reaction types (electron impact ionization/dissociative ionization, electron impact dissociation, proton impact charge exchange, and dissociative recombination). Experimental reaction rates or cross sections have been compiled, and estimates have been made for cases where these are not available. The proton impact charge exchange reaction rates are calculated from a theoretical model using molecular polarizabilities. Dissociative recombination rates are described by the equation A/TB where parameter A is fit using polarizabilities and B is estimated from known reaction rates. The electron impact ionization and dissociation cross sections are fit to known graphs using four parameters: threshold energy, maximum value of the cross section, energy at the maximum, and a constant for the exponential decay as energy increases. The model has recently been used in an analysis of the Joint European Torus [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)] MARK II carbon inner divertor using the WBC Monte Carlo impurity transport code. The updated version of WBC, which includes the full set of hydrocarbon reactions, helps to explain an observed asymmetry in carbon deposition near the divertor.
J. Vac. Sci. Technol., A, 13, 797-801 (2000).
Allain, M. M. C., Hayden, D. B., Juliano, D. R., Ruzic, D. N.
Conventional magnetron sputter deposition with a rf inductively coupled plasma (ICP) has demonstrated that ionized metal fluxes can be effectively utilized to fill trenches and vias with high aspect ratios. The ICP is created with a seven turn (1/2 wavelength), water cooled coil located between the magnetron cathode and the substrate. A large fraction of the metal atoms sputtered from the magnetron cathode are ionized by the ICP. These ions are accelerated across the sheath toward the substrate and deposited at normal incidence, by placing a negative bias on the substrate. A gridded energy analyzer configured with a quartz crystal microbalance is located in the center of the substrate plane to determine the ion and neutral deposition rates. While keeping the magnetron power, rf coil, target to substrate distance, pressure and diagnostic location constant, the ionization fraction was measured for two metal targets: Cu and Ti using three different working gases: Kr, Ar and Ne. Variations in target materials and working gases are shown to have an effect on ionization and deposition rates. The ionization rate is a sensitive function of the metal’s ionization potential. The electron energy distribution in the plasma is affected by the sputtered metal and the working gases’ ionization potential.
Phys Rev E., 61, 1904-1911 (2000).
Wang, Z., Cohen, S. A., Ruzic, D. N., Goeckner, M. J.
Energy distributions of nitrogen atoms (N) in a hollow-cathode planar sputtering magnetron were obtained by use of optical emission spectroscopy. A characteristic line, N i 8216.3 Å, well separated from molecular nitrogen emission bands, was identified. Jansson’s nonlinear spectral deconvolution method, refined by minimization of χw2, was used to obtain the optimal deconvolved spectra. These showed nitrogen atom energies from 1 eV to beyond 500 eV. Based on comparisons with vftrim computer code results, it is proposed that the energetic N’s are generated from N2+ ions after these ions are accelerated through the sheath and dissociatively reflect from the cathode.
J. Vac. Sci. Tech., A17, 3443-3448 (1999).
Smith, P. C., Ruzic, D. N.
An apparatus and analysis method to obtain both the angular distribution of sputtered atoms and the total sputtering yield for materials of interest to physical vapor deposition (PVD) has been created. Total yield is determined by collecting the sputtered material on a quartz crystal oscillator (QCO) microbalance. The sputtered material is also collected on a pyrolytic graphite witness plate. By mapping the concentrations of the sputtered material on this plate, both polar and azimuthal angular distributions of the sputtered material can be determined. Utilizing this setup, data have been obtained for (200–500 eV) Ar + normally incident on polycrystalline aluminum sputtering targets with strong (100) and (110) crystallographic orientations. The overall yields of these samples compare well to the available data as well as empirical formulas. Crystallographic effects in the angular distributions are clearly seen. The Al(100) sample shows 12% enhanced sputtering along the 
110
direction at all energies.
Surf. Coating Tech., 121, 401-404 (1999).
Hayden, D. B., Juliano, D. R., Neumann, M. N., Allain, M. M., Ruzic, D. N.
A helicon antenna that sits remotely outside the vacuum system is attached to a magnetron sputtering system. This increases the electron temperature, which increases the ionization of the sputter flux for achieving ionized physical vapor deposition (IPVD). There are no shadowing and contamination problems, unlike other IPVD devices with immersed coils, since the helicon antenna is outside the vacuum system. Furthermore, the target to substrate distance can be kept small. At 2 kW magnetron power, 4 kW helicon power, 45 mTorr argon gas, and with a copper target, ionization fractions to the substrate of 51±10% and a deposition rate of 847±42 Å/min are measured using a quartz crystal oscillator (QCO) and a multi-grid filter. Without the antenna, the ionization fraction to the QCO is 30±6% and the deposition rate is 815±41 Å/min. Multiple remote sources are envisioned to be positioned radially around a sputtering chamber, controlling uniformity while increasing the ionization further. Since 21% additional ionization is achieved using only one source, with no threat of contamination inside the vacuum chamber, the helicon source has good potential for a secondary plasma source in IPVD applications.
J. Nucl. Mater., 266-269, 1303-1308 (1999).
Ruzic, D. N., Allain, M. C., Budny, R. V.
Five TFTR deuterium supershots with increasing Li pellet injection are analyzed in detail. Five chords of experimental H-α measurements are compared to predictions from a series of computational models. First, experimental data from the discharge is used in the TRANSP plasma transport code to predict the ion flux to the wall. Then a modified version of the DEGAS neutral transport code which includes both reflection, desorption and sputtering of hydrogenic species from the wall is used to determine the neutral density profile across the machine. This data combined with the known density and temperature contours predicts values for the magnitude of H-α light observed for 16 viewing angles of the diagnostic. To match the experimental data, the wall reflection, desorption and sputtering coefficients were altered using data from VFTRIM-3D to include the effect of the added Li. In addition, the first 10 cm of the stainless steel wall adjoining the C inner bumper limiter was treated as C-covered; the highest flux area of the inner wall was treated as a sink; and the lower reflection coefficients for a Li-wall rather than a C-wall were used over an increasingly larger area of the inner wall as the Li concentration in the discharges increased.
J. Nuclear Materials, 266-269, 58-66 (1999).
Brooks, J. N., Alman, D., Federici, G., Ruzic, D. N., Whyte, D. G.
We are analyzing erosion and tritium codeposition for ITER, DIII-D, and other devices with a focus on carbon divertor and metallic wall sputtering, for detached and semi-detached edge plasmas. Carbon chemical-sputtering/hydrocarbon-transport is computed in detail using upgraded models for sputtering yields, species, and atomic and molecular processes. For the DIII-D analysis this includes proton impact and dissociative recombination for the full methane and higher hydrocarbon chains. Several mixed material (Si–C doping and Be/C) effects on erosion are examined. A semi-detached reactor plasma regime yields peak net wall erosion rates of 1.0 (Be), 0.3 (Fe), and 0.01 (W) cm/burn-yr, and 50 cm/burn-yr for a carbon divertor. Net carbon erosion is dominated by chemical sputtering in the 1–3 eV detached plasma zone. Tritium codeposition in divertor-sputtered redeposited carbon is high (10–20 g T/1000 s). Silicon and beryllium mixing tends to reduce carbon erosion. Initial hydrocarbon transport calculations for the DIII-D DiMES-73 detached plasma experiment show a broad spectrum of redeposited molecules with 90% redeposition fraction.
