The physics of magnetic shielding in Hall thrusters were validated through laboratory experiments demonstrating essentially erosionless, high-performance operation. The magnetic field near the walls of a laboratory Hall thruster was modified to effectively eliminate wall erosion while maintaining the magnetic field topology away from the walls necessary to retain efficient operation. Plasma measurements at the walls validate our understanding of magnetic shielding as derived from the theory. The plasma potential was maintained very near the anode potential, the electron temperature was reduced by a factor of two to three, and the ion current density was reduced by at least a factor of two. Measurements of the carbon backsputter rate, wall geometry, and direct measurement of plasma properties at the wall indicate that the wall erosion rate was reduced by a factor of 1000 relative to the unshielded thruster. These changes effectively eliminate wall erosion as a life limitation in Hall thrusters, enabling a new class of deep-space missions that could not previously be attempted.

Magnetic shielding of a laboratory Hall thruster. II. Experiments

An analytical formula is developed for the evolution of angular dependence of sputtering yields by extending the theory of sputtering yield proposed by Sigmund. We demonstrate that the peak of sputtering yield at oblique incidence can be attributed to a balance between the increased energy deposited on the surface by incident ion which enhances the sputtering yield and the decreased depth travelled by recoil atom which reduces the sputtering yield. The predicted dependence of sputtering yield on the incident angle is in good agreement with experimental observations.

/pdf/angular-dependence-of-sputtering-yield-of-amorphous-and-588mu9vaty.pdf

Angular dependence of sputtering yield of amorphous and polycrystalline materials

There is an increasing demand for thin films with tailored properties, which requires the use and control of adequate deposition techniques. Ion beam sputter deposition (IBSD) is a physical vapor deposition (PVD) technique that is capable of fulfilling the technological challenges. In contrast to other PVD techniques, IBSD offers a unique opportunity to tailor the properties of the film-forming particles (sputtered target and scattered primary particles) and, hence, thin film properties. This is related to the fact that the generation and acceleration of the primary particles from the ion beam source, the generation of film-forming particles at the target, and thin film growth on the substrate are spatially separated. Thus, by changing ion beam parameters (ion species and ion energy) and geometrical parameters (ion incidence angle and emission angle), the energy distributions of the film-forming particles are modified. Even though in use for several decades, IBSD was not investigated systematically until lately. Utilizing the full potential of IBSD requires a comprehensive understanding of the physical processes. This tutorial describes the systematics of IBSD: The correlation between process parameters, properties of the film-forming particles, and thin film properties. The most important process parameters are the scattering geometry and the primary particle species. Depending on the material, different film properties can be influenced. Examples are adhesion, structural properties, composition, surface roughness, mass density, optical properties, stress, and electrical resistivity. In addition to the experimental results, fundamental physical aspects, experimental setups, and techniques for thin film deposition and particle characterization are described.

Tutorial: The systematics of ion beam sputtering for deposition of thin films with tailored properties

Band effect on the charge exchange process at solid surfaces

: An axisymmetric hybrid-PIC model of the Hall thruster plasma discharge has been upgraded to simulate the erosion of the thruster acceleration channel, the degradation of which is the main life-limiting factor of the propulsion system. Evolution of the thruster geometry as a result of material removal due to sputtering is modeled by calculating wall erosion rates, stepping the grid boundary by a chosen time step and altering the computational mesh between simulation runs. The code is first tuned to predict the nose cone erosion of a 200 W Busek Hall thruster, the BHT-200. Simulated erosion profiles from the first 500 hours of operation compare favorably to experimental data. The thruster is then subjected to a virtual life test that predicts a lifetime of 1,330 hours, well within the empirically determined range of 1,287-1,519 hours. The model is then applied to the BHT-600, a higher power thruster, to reproduce wear of its exit ring configuration over 932 hours of firing. Though some optimized code features remain the same, others need adjustment to achieve comparable erosion results. Better understanding of the physics of anomalous plasma transport and low-energy sputtering are identified as the most pressing needs for improved lifetime models.

/pdf/modeling-of-hall-thruster-lifetime-and-erosion-mechanisms-4bkdrlnq5t.pdf

Modeling of Hall Thruster Lifetime and Erosion Mechanisms (Preprint)

An analytic empirical formula for the angular dependence of light-ion sputtering and heavy-ion sputtering has been proposed. The present empirical formula has two adjustable parameters which are determined by the least-squares method. One of the two parameters corresponds to Sigmundf, and the other is the angle of incidence at the maximum yield. The present empirical formula is found to be very useful for preliminary descriptions of the angular dependence of sputtering yield. This work was carried out under the collaborating Research Program at the Institute of Plasma Physics, Nagoya University. Lindhard's theory of the nuclear stopping power is developed to take into account the binding force between the lattice atoms of solids. It is shown that the nuclear stopping power is proportional to the velocity (not the energy) of the moving atom at the low velocity limit. Debye temperature dependence of the nuclear stopping power is discussed.

An empirical formula for angular dependence of sputtering yields

Quantum theory of charge exchange scattering of atoms at solid surfaces

A quantum-mechanical theory of positronium formation at surfaces is presented. The neutralization probability of positrons implanted into solids escaping from a surface is calculated. The theory of the resonant neutralization of ions at a surface is improved for positrons by taking into account the quantum effect of the motion of the positrons near the surface. The angular distributions and the energy distributions of the emitted positronium are calculated. We give the relationship of the positronium energy distribution and the density of states at the surface.

Quantum theory of positronium formation at surfaces.

The energy and angular distributions of positroniums (Ps) formed at surfaces are calculated numerically using the integro-differential equation which appears in the dynamical theory of Ps formation based on the semiclassical trajectory approximation. The wide-band approximation, which is well known in the theory of neutralization of surface-scattered ions, is not adequate for the Ps formation. Strong deviation from the Born approximation is pointed out in the case of narrow-band electrons.

Theory of positronium formation with surface electrons for realistic band structures

The two-dimensional momentum distribution of positronium formed and ejected from a surface is calculated and compared with experimental results. It is shown that the higher-order Born contribution is very important for explaining the experimental momentum distributions of positronium ejected from the surface.

Calculation of momentum distribution of positronium ejected from surfaces

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