Search

Diffusion and decomposition of SiHx species adsorbed on the clean Si(100) surface are processes of relevance for the growth of crystalline silicon by plasma-enhanced chemical vapor deposition. In this work, we report an extensive search of diffusion and decomposition pathways for SiH3, SiH2, and SiH by means of combined ab initio metadynamics simulations and optimization of minimum-energy reactions paths. We find that on the clean surface SiH3 undergoes stepwise decompositions into Si and H adatoms according to SiH3 → SiH2 +H→SiH+2H→Si+3H with an overall reaction barrier of the order of 0.8 eV, consistent with the scenario inferred from secondary ion mass spectroscopy data. The lifetime of SiH3 at room temperature calculated within transition state theory in the harmonic approximation is in agreement with experiments. The lifetime of SiH2 turns out to be similar to that of SiH3. Possible trap states for SiH2 are proposed, based on energetics and by comparing calculated scanning tunneling microscope images with experimental data. © 2009 The American Physical Society.

The oscillating sail is a novel solar sail configuration where a triangular sail is released at a deflected angle with respect to the Sun-direction. As a result, the sail will conduct an undamped oscillating motion around the Sun-line due to the offset between the centre-of-pressure and centre-of-mass. In this paper, the resulting oscillatory motion of the acceleration vector is exploited to design new families of periodic orbits in the Earth-Moon circular restricted three-body system. In particular, the effect of adding an oscillating sail to the family of Lyapunov orbits at the L1- and L2-points as well as the family of distant retrograde orbits (DROs) is investigated. Because the solar sail Earth-Moon system is non-autonomous (due to the apparent orbital motion of the Sun), the sail’s oscillating period, the orbital period and the period of the Sun around the Earth-Moon system all need to be commensurable in order for the orbits to be repeatable over time. Using a differential correction technique, orbits that satisfy these constraints can be obtained and the results comprise new families of periodic orbits that are parameterised by the required sail performance. In addition to exploiting the oscillating sail for generating new orbit families, this paper also investigates its potential for orbital transfers. By combining a systematic search method with a local optimiser, oscillating sail parameters and orbital parameters can be obtained that enable transfers between classical Lyapunov orbits at the L1-point, connections between classical Lyapunov orbits at different Lagrange points as well as transfers between orbits within the family of classical DROs.

By means of first-principles calculations we studied the decomposition pathways of SiH₃ on Ge(100) and of GeH₃ on Si(100), of interest for the growth of crystalline SiGe alloys and Si/Ge heterostructures by plasma-enhanced chemical vapor deposition. We also investigated H desorption via reaction of two adsorbed SiH₂/GeH₂ species (β₂ reaction) or via Eley-Rideal abstraction of surface H atoms from the impinging SiH₃ and GeH₃ species. The calculated activation energies for the different processes suggest that the rate-limiting step for the growth of Si/Ge systems is still the β₂ reaction of two SiH₂ as in the growth of crystalline Si.

A new scheme, sketch-map, for obtaining a low-dimensional representation of the region of phase space explored during an enhanced dynamics simulation is proposed. We show evidence, from an examination of the distribution of pairwise distances between frames, that some features of the free-energy surface are inherently high-dimensional. This makes dimensionality reduction problematic because the data does not satisfy the assumptions made in conventional manifold learning algorithms We therefore propose that when dimensionality reduction is performed on trajectory data one should think of the resultant embedding as a quickly sketched set of directions rather than a road map. In other words, the embedding tells one about the connectivity between states but does not provide the vectors that correspond to the slow degrees of freedom. This realization informs the development of sketch-map, which endeavors to reproduce the proximity information from the high-dimensionality description in a space of lower dimensionality even when a faithful embedding is not possible.

A very high probability (∼60%) for H abstraction induced by Si H3 thermal impacts on the Si (001) (2×1) hydrogenated surface is reported, as a consequence of the Eley-Rideal mechanism by which a silane molecule is formed. The reaction probability is computed within a fully dynamical approach. After running a limited set of ab initio Car-Parrinello simulations to validate a suitable empirical potential, an actual probability for the mechanism was estimated by averaging over thousands of classical molecular dynamics simulations. The probability of H abstraction is shown to be quite constant in the typical experimental range for plasma-enhanced chemical vapor deposition. Very low evidence for insertion of the radical into surface Si-Si bonds is found. © 2007 The American Physical Society.

A very high probability (similar to 60%) for H abstraction induced by SiH3 thermal impacts on the Si(001)(2x1) hydrogenated surface is reported, as a consequence of the Eley-Rideal mechanism by which a silane molecule is formed. The reaction probability is computed within a fully dynamical approach. After running a limited set of ab initio Car-Parrinello simulations to validate a suitable empirical potential, an actual probability for the mechanism was estimated by averaging over thousands of classical molecular dynamics simulations. The probability of H abstraction is shown to be quite constant in the typical experimental range for plasma-enhanced chemical vapor deposition. Very low evidence for insertion of the radical into surface Si-Si bonds is found.

This paper provides a detailed mission analysis and systems design of a near-term and far-term pole-sitter mission. The pole-sitter concept was previously introduced as a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high-latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the north or south pole and, as such, can provide real-time, continuous and hemispherical coverage of the polar regions. Being on a non-Keplerian orbit, a continuous thrust is required to maintain the pole-sitter position. For this, two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar electric propulsion (SEP) and a far-term pole-sitter mission where the SEP thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. In order to maximize the spacecraft mass at the start of the operations phase of the mission, the transfer from Earth to the pole-sitter orbit is designed and optimized assuming either a Soyuz or an Ariane 5 launch. The maximized mass upon injection into the pole-sitter orbit is subsequently used in a detailed mass budget analysis that will allow for a trade-off between mission lifetime and payload mass capacity. Also, candidate payloads for a range of applications are investigated. Finally, transfers between north and south pole-sitter orbits are considered to overcome the limitations in observations due to the tilt of the Earth's rotational axis that causes the poles to be alternately situated in darkness. It will be shown that in some cases these transfers allow for propellant savings, enabling a further extension of the pole-sitter mission.

Spacecraft formation flying has shown to be promising approach to enhance mission capabilities. Nevertheless, formation flying presents several control challenges which escalate as the numbers of elements in the formation is increased. The objective of this paper is to develop decentralised control algorithms to regulate the station-keeping, reconfiguration and collision avoidance of spacecraft in formation around eccentric reference orbits using the combination of a Linear Quadratic Regulator (LQR) and an Artificial Potential Function (APF). Within this control scheme, the LQR will provide station-keeping and reconfiguration capabilities toward desired positions, while optimizing fuel consumption and the APF will ensure collision free manoeuvres between the elements of the formation during manoeuvres. The controller is designed under the assumption of continuous thrust as a standard LQR problem using the Pontryagin minimum principle, an APF based in normalized Gaussian functions and the Tschauner and Hempel (TH) equations as the relative dynamics model.

The fascinating idea of shepherding asteroids for science and resource utilisation is being considered as a very\ud credible concept in a not too distant future. Past studies have identified asteroids which could be injected into\ud manifolds which wind onto periodic orbits around collinear Lagrangian points of the Sun-Earth system, by means of\ud a low-cost manoeuvre. However, the periodic orbits as well as the manifolds are highly unstable, and small errors in\ud the capture manoeuvre would bring to complete mission failure, with potential danger of collision with the Earth\ud itself. The main source of injection error in position and velocity is the epistemic uncertainty of the asteroid mass,\ud which cannot be measured directly. For this reason, asteroid orbit control will be a strict requirement for such\ud mission. This paper investigates the controllability of some asteroids during the transfer and along the period orbits,\ud assuming the use of a solar-electric low-thrust engine. The control scheme is based on a linear quadratic regulator. A\ud stochastic simulation with a Monte Carlo approach is used to simulate a range of different perturbed initial\ud conditions. Results show that only a small subset of the considered combinations of trajectories/asteroids are reliably\ud controllable, and therefore controllability must be taken into account in the selection of potential targets.

In order to increase the range of potential mission applications of solar sail technology, this paper introduces the\ud concepts of shape change and continuously variable optical properties to large gossamer spacecraft. Merging the two\ud concepts leads to the idea of solar sails as multi-functional platforms that can have potential benefits over conventional\ud solar sails by delivering additional key mission functions such as power collection, sensing and communications. To\ud this aim, the paper investigates the static deflection of a thin inelastic circular sail film with a variable surface reflectivity\ud distribution. The sail film is modelled as a single surface framed by a rigid supporting hoop structure. When changing\ud the reflectivity coefficient across the sail surface, the forces acting on the sail can be controlled without changing the\ud incidence angle relative to the Sun. In addition, by assigning an appropriate reflectivity function across the sail, the\ud load distribution due to solar radiation pressure can also be manipulated to control the billowing of the film. By an\ud appropriate choice of reflectivity across the sail, specific geometries can be generated, such as a parabolic reflector,\ud thus enabling a multi-functional sail. This novel concept of optical reconfiguration can potentially extend solar sail\ud mission applications.

Designs for geostationary SPS are extremely large in scale, more than an order of magnitude larger than\ud the International Space Station. The problem of how to control the orbital motion of such large structures,\ud accounting for various perturbing forces, is therefore a topic worthy of further study. The primary objective\ud of the proposed research is to perform a detailed study of SPS orbit dynamics, obtaining a comprehensive\ud understanding of the effect of perturbations on orbits of large SPS structures over a time-frame commensurate\ud with proposed SPS lifetimes (30-40 years). Analytical equations derived by the process of averaging of the\ud SPS equations of motion shall be used in determining the long-term orbital behaviour. Previous studies have\ud simply assumed a geostationary orbit (GEO) then designed control systems for maintaining it thus. It is\ud found that an alternative SPS orbital location known as the geosynchronous Laplace plane (GLP) is superior\ud to GEO. An SPS in GLP requires virtually no fuel to maintain its orbit, avoids the main orbital debris\ud population originating from GEO satellites and is extremely robust, i.e. loss of control is inconsequential.\ud The GLP SPS saves of order 104 to 105 kg per year in fuel compared to a GEO SPS for equivalent power\ud delivery compared to GEO.

The paper provides a survey of novel mission concepts for continuous, hemispheric polar observation and direct-link polar telecommunications. It is well known that these services cannot be provided by traditional platforms: geostationary satellites do not cover high-latitude regions, while low- and medium-orbit Sun-synchronous spacecraft only cover a narrow swath of the Earth at each passage. Concepts that are proposed in the literature are described, including the pole-sitter concept (in which a spacecraft is stationary above the pole), spacecraft in artificial equilibrium points in the Sun–Earth system and non-Keplerian polar Molniya orbits. Additionally, novel displaced eight-shaped orbits at Lagrangian points are presented. For many of these concepts, a continuous acceleration is required and propulsion systems include solar electric propulsion, solar sail and a hybridisation of the two. Advantages and drawbacks of each mission concept are assessed, and a comparison in terms of high-latitude coverage and distance, spacecraft mass, payload and lifetime is presented. Finally, the paper will describe a number of potential applications enabled by these concepts, focusing on polar Earth observation and telecommunications.

In this paper, displaced geostationary orbits using hybrid low-thrust propulsion, a complementary combination of Solar Electric Propulsion (SEP) and solar sailing, are investigated to increase the capacity of the geostationary ring that is starting to become congested. The SEP propellant consumption is minimized in order to maximize the mission lifetime by deriving semi-analytical formulae for the optimal steering laws for the SEP and solar sail accelerations. By considering the spacecraft mass budget, the performance is also expressed in terms of payload mass capacity. The analyses are performed both for the use of pure SEP and hybrid low-thrust propulsion to allow for a comparison. It is found that hybrid low-thrust control outperforms the pure SEP case both in terms of payload mass capacity and mission lifetime for all displacements considered. Hybrid low-thrust propulsion enables payloads of 255–487 kg to be maintained in a 35 km displaced orbit for 10–15 years. Adding the influence of the J2 and J2,2 terms of the Earth's gravity field has a small effect on this lifetime, which becomes almost negligible for small values of the sail lightness number. Finally, two SEP transfers that allow for an improvement in the performance of hybrid low-thrust control are optimized for the propellant consumption by solving the accompanying optimal control problem using a direct pseudospectral method. The first type of transfer enables a transit between orbits displaced above and below the equatorial plane, while the second type of transfer enables customized service for which a spacecraft is transferred to a Keplerian parking orbit when geostationary coverage is not needed. While the latter requires a modest propellant budget, the first type of transfer comes at the cost of an almost negligible SEP propellant consumption.

This work investigates the orbital and attitude dynamics of future reconfigurable multi-panel solar sails able to change\ud their shape during a mission. This can be enabled either by changing the relative position of the individual panels, or\ud by using articulated mechanisms and deployable, retractable and/or inflatable structures. Such a model introduces the\ud concept of modular spacecraft of variable morphology to large gossamer spacecraft. However, this joint concept is\ud complex in nature and requires equations for coupled orbit/attitude dynamics. Therefore, as a starting point, the system\ud is modelled as a rigid-body dumbbell consisting of two tip masses connected by a rigid, massless panel. The system\ud is subjected to a central gravitational force field under consideration of solar radiation pressure forces. Therefore, we\ud assign reflectivity coefficients to the tip masses and a high area-to-mass ratio. An analytical Hamiltonian approach\ud is used to describe the planar motion of the system in Sun-centred Keplerian and non-Keplerian circular orbits. The\ud stability and controllability of the system is enabled through changing the reflectivity coefficients, for example through\ud the use of electro-chromic coating on its surface. The creation of artificial unstable equilibria of the system due to the\ud presence of solar radiation pressure and heteroclinic connections between the equilibria are investigated. We further\ud derive a constraint for the solar radiation pressure forces to maintain the system on a circular Sun-centred orbit. It is\ud planned that the structure is eventually capable of reconfiguring between the equilibria by a minimum actuation effort.

Recent studies have shown the feasibility of an Earth pole-sitter mission using low-thrust propulsion. This mission concept involves a spacecraft following the Earth's polar axis to have a continuous, hemispherical view of one of the Earth's poles. Such a view will enhance future Earth observation and telecommunications for high latitude and polar regions. To assess the accessibility of the pole-sitter orbit, this paper investigates optimum Earth pole-sitter transfers employing low-thrust propulsion. A launch from low Earth orbit (LEO) by a Soyuz Fregat upper stage is assumed after which solar electric propulsion is used to transfer the spacecraft to the pole-sitter orbit. The objective is to minimize the mass in LEO for a given spacecraft mass to be inserted into the pole-sitter orbit. The results are compared with a ballistic transfer that exploits manifold-like trajectories that wind onto the pole-sitter orbit. It is shown that, with respect to the ballistic case, low-thrust propulsion can achieve significant mass savings in excess of 200 kg for a pole-sitter spacecraft of 1000 kg upon insertion. To finally obtain a full low-thrust transfer from LEO up to the pole-sitter orbit, the Fregat launch is replaced by a low-thrust, minimum time spiral, which provides further mass savings, but at the cost of an increased time of flight.

This paper presents the preliminary systems design of a pole-sitter. This is a spacecraft that hovers over an Earth pole, creating a platform for full hemispheric observation of the polar regions, as well as direct-link telecommunications. To provide the necessary thrust, a hybrid propulsion system combines a solar sail with a more mature solar electric propulsion (SEP) thruster. Previous work by the authors showed that the combination of the two allows lower propellant mass fractions, at the cost of increased system complexity. This paper compares the pure SEP spacecraft with the hybrid spacecraft in terms of the launch mass necessary to deliver a certain payload for a given mission duration. A mass budget is proposed, and the conditions investigated under which the hybrid sail saves on the initial spacecraft initial mass. It is found that the hybrid spacecraft with near- to mid-term sail technology has a lower initial mass than the SEP case if the mission duration is 7 years or more, with greater benefits for longer duration missions. The hybrid spacecraft with far-term sail technology outperforms the pure SEP case even for short missions.

Because of an increase in the number of geostationary spacecraft and the limits imposed by east–west spacing\ud requirements, the geostationary orbit is becoming congested. To increase its capacity, this paper proposes to create\ud new geostationary slots by displacing the geostationary orbit either out of or in the equatorial plane by means of\ud hybrid solar sail and solar electric propulsion. To minimize propellant consumption, optimal steering laws for the\ud solar sail and solar-electric-propulsion thrust vectors are derived and the performance in terms of mission lifetime is\ud assessed. For comparison, similar analyses are performed for conventional propulsion, including impulsive and pure\ud solar electric propulsion. It is shown that hybrid sails outperform these propulsion techniques and that out-of-plane\ud displacements outperform in-plane displacements. The out-of-plane case is therefore further investigated in a\ud spacecraft mass budget to determine the payload mass capacity. Finally, two transfers that enable a further\ud improvement of the performance of hybrid sails for the out-of-plane case are optimized using a direct pseudospectral\ud method: a seasonal transit between orbits displaced above and below the equatorial plane and a transit to a parking\ud orbit when geostationary coverage is not needed. Both transfers are shown to require only a modest propellant\ud budget, outweighing the improvements they can establish.

This paper investigates astrodynamics at extremes of length-scale, ranging from swarms of future ‘smart dust’ devices to the capture and utilisation of small near Earth asteroids. At the smallest length-scales families of orbits are found which balance the energy gain from solar radiation pressure with energy dissipation due to air drag. This results in long orbit lifetimes for high area-to-mass ratio ‘smart dust’ devices. High area-to-mass hybrid spacecraft, using both solar sail and electric propulsion, are then considered to enable ‘pole-sitter’ orbits providing a polar-stationary vantage point for Earth observation. These spacecraft are also considered to enable displaced geostationary orbits. Finally, the potential material resource available from captured near Earth asteroids is considered which can underpin future large-scale space engineering ventures. The use of such material for geoengineering is investigated using a cloud of unprocessed dust in the vicinity of the Earth-Sun L1 point to fractionally reduce solar insolation.

This paper investigates astrodynamics at extremes of length-scale, ranging from swarms of future `smart dust' devices to the capture and utilisation of small near Earth asteroids. At the smallest length-scales families of orbits are found which balance the energy gain from solar radiation pressure with energy dissipation due to air drag. This results in long orbit lifetimes for high area-to-mass ratio `smart dust' devices. High area-to-mass hybrid spacecraft, using both solar sail and electric propulsion, are then considered to enable `pole-sitter' orbits providing a polar-stationary vantage point for Earth observation. These spacecraft are also considered to enable displaced geostationary orbits. Finally, the potential material resource available from captured near Earth asteroids is considered which can underpin future large-scale space engineering ventures. The use of such material for geo-engineering is investigated using a cloud of unprocessed dust in the vicinity of the Earth-Sun $L_1$ point to fractionally reduce solar insolation.