Renewed interest in space exploration and aspirations for colonization of Mars, Moon and possibly other extra-terrestrial bodies puts pressure on the present-day space technology to become more efficient. Space engines, or thrusters, are the key element of any spacecraft and thus, continuous improvement in thruster design and performance is needed to realize the mankind’s goal of becoming a truly space faring civilization. Space micropropulsion systems that utilize plasma and electric fields to accelerate and expel mass to produce reactive thrust are advanced propulsion systems that can deliver very high specific impulse. In this review, we outline basic physical principles and design approaches for future direct-arc plasma propulsion systems. We then examine major obstacles and prospects for application of direct-arc plasmas in this type of space thrusters.

Direct current arc plasma thrusters for space applications: basic physics, design and perspectives

Nowadays, multi-element cathodes are frequently employed to grow multi-element thin films and coatings using cathodic arc deposition processes. During cathode erosion, the cathode spot sequentially ignites on the cathode surface and imposes melting–solidification cycles that lead to material intermixing and the formation of a modified layer on the cathode surface. To allow us to study these surface modifications, a 10 μm thick Mo/Al multilayer coating was sputter-deposited onto a standard Ti arc cathode. This cathode was eroded by a dc steered arc discharge for a short duration enabling the observation of single craters formed by type 1 and 2 cathode spots. Furthermore, separated clusters of overlapping craters and a fully eroded surface caused by different stages of erosion were differentiated when scanning the erosion track in the lateral direction. Cross sections of single craters were prepared by focused ion beam techniques while metallographic methods were applied to obtain cross sections of overlapping craters and the modified layer. The layers of the multilayer coating acted as trace markers providing new insights into the material intermixing within craters, the material displacements during crater formation, the plasma pressure acting on the craters, and the temperature gradient (heat-affected zone) below the craters. The observations are discussed within the framework of established arc crater formation models.

Insights into surface modification and erosion of multi-element arc cathodes using a novel multilayer cathode design

In this study, based on density functional theory, the formation enthalpy, density of states, elastic properties and elastic anisotropy of Al3M (M = Ti, Ta, V, Nb, Hf) compounds were calculated The calculation results of enthalpy formation indicate that Al3M (M = Ti, Ta, V, Nb, Hf) compounds are formed State density analysis of Al3M (M = Ti, Ta, V, Nb, Hf) compounds at the Fermi level is non-zero, showing metal properties The density of states show that Al3Ta has the highest structural stability The calculation results of mechanical properties show that Al3Ta has greater resistance to bulk deformation and shear deformation By calculating the anisotropic indexes, the sequence of anisotropy in elastic modulus is Al3Hf > Al3Ti > Al3V > Al3Nb > Al3Ta

First-principles study of stability, electronic properties and anisotropic elasticity of Al3M (M=Ti, Ta, V, Nb, Hf) intermetallic compounds

Anisotropic thermal expansion coefficients of tetragonal γ -TiAl and hexagonal α 2 -Ti3Al phases were calculated using first principles methods. Two approaches with different computational costs and degrees of freedom were proposed. The predicted values were compared with available experimental data showing that for γ -TiAl, the more computational demanding method with decoupled impact of volume and temperature effects on the cell shape leads to significantly better results than that with only ground-state optimised unit cell geometry. In the case of the α 2 -Ti3Al phase, both approaches yielded comparable results. Additionally, heat capacity and bulk modulus were evaluated as functions of temperature for both phases, and were fitted to provide an analytical formula which can be further used.

https://www.mdpi.com/1996-1944/12/8/1292/pdf

Thermal Expansion and Other Thermodynamic Properties of α2-Ti3Al and γ-TiAl Intermetallic Phases from First Principles Methods

In this study, properties of low-index L12-Al3Nb/Al such as structure stability, thermodynamic, strength, electron properties, unstable stacking fault energy, plasticity, and fracture behavior of interface (L12-Al3Nb(001)/Al(001), L12-Al3Nb(110)/Al(110), and L12-Al3Nb(111)/Al(111)) were systematically studied. The results showed that the structure of the strongest interfaces of L12-Al3Nb(001)/Al(001), L12-Al3Nb(110)/Al(110), and L12-Al3Nb(111)/Al(111) were stacked similar to bulk L12-Al3Nb or Al, with maximum work of adhesion of 2.80 J/m2, 2.99 J/m2, and 1.74 J/m2, respectively. The maximum computed theoretical tensile strength of L12-Al3Nb (001)/Al(001), L12-Al3Nb (110)/Al(110), and L12-Al3Nb (111)/Al(111) interfaces were 18.71 GPa, 16.85 and 11.88Gpa respectively, for the rigid scheme; and 8.91 GPa, 8.52 GPa and 6.98 GPa respectively, for the full-relaxed scheme. Meanwhile, the interfaces were virtually prone to break on the Al side on full-relaxed stretching. Moreover, the partial density of states of the interface indicated that hybridization occurs among the Al-s, Al-p, Nb-s, Nb-p, and Nb-d orbitals, forming s-p-d hybrid orbital. Finally, based on the unstable stacking fault energy and Rice ratio, the interfaces have high plasticity once subjected to shear stress in the  ,  ,  , or directions, which elucidate the interface properties and understand the underlying mechanism between L12-Al3Nb and Al alloys, thereby provide vital information to guide designing of novel Al alloys.

A comprehensive study of the L12-Al3Nb/Al interface properties using first-principles calculations

Author(s): Zohrer, S; Anders, A; Franz, R | Abstract: Cathodic arcs have been utilized in various applications including the deposition of thin films and coatings, ion implantation, and high current switching. Despite substantial progress in recent decades, the physical mechanisms responsible for the observed plasma properties are still a matter of dispute, particularly for multi-element cathodes, which can play an essential role in applications. The analysis of plasma properties is complicated by the generally occurring neutral background of metal atoms, which perturbs initial ion properties. By using a time-resolved method in combination with pulsed arcs and a comprehensive Nb-Al cathode model system, we investigate the influence of cathode composition on the plasma, while making the influence of neutrals visible for the observed time frame. The results visualize ion detections of 600 μs plasma pulses, extracted 0.27 m from the cathode, resolved in mass-per-charge, energy-per-charge and time. Ion properties are found to be strongly dependent on the cathode material in a way that cannot be deduced by simple linear extrapolation. Subsequently, current hypotheses in cathodic arc physics applying to multi-element cathodes, like the so-called 'velocity rule' or the 'cohesive energy rule', are tested for early and late stages of the pulse. Apart from their fundamental character, the findings could be useful in optimizing or designing plasma properties for applications, by actively utilizing effects on ion distributions caused by composite cathode materials and charge exchange with neutrals.

/pdf/time-resolved-ion-energy-and-charge-state-distributions-in-3lom88mbek.pdf

Time-resolved ion energy and charge state distributions in pulsed cathodic arc plasmas of Nb-Al cathodes in high vacuum

Author(s): Zohrer, S; Golizadeh, M; Koutna, N; Holec, D; Anders, A; Franz, R | Abstract: Many properties of cathodic arcs from single-element cathodes show a correlation to the cohesive energy of the cathode material. For example, the burning voltage, the erosion rate, or, to a lesser extent, plasma properties like electron temperatures or average ion energy and charge states. For multi-element cathodes, various phases with different cohesive energies can initially be present in the cathode, or form due to arc exposure, complicating the evaluation of such correlations. To test the influence of morphology and phase composition of multi-element cathodes on cathodic arc properties, a Nb-Al cathode model system was used that includes: pure Nb and Al cathodes; intermetallic Nb3Al, Nb2Al and NbAl3 cathodes; and three composite Nb-Al cathodes with atomic ratios corresponding to the stoichiometric ratios of the intermetallic phases. Pulsed cathodic arc plasmas from these cathodes were examined using a mass-per-charge and energy-per-charge analyzer, showing that charge-state-resolved ion energy distributions of plasmas from the intermetallic and corresponding composite cathodes are nearly identical. An examination of converted layers of eroded cathodes using x-ray diffraction and scanning electron microscopy indicates the formation of a surface layer with similar phase composition for intermetallic and their corresponding composite cathode types. The average arc voltages do not follow the trend of cohesive energies of Nb, Al and intermetallic Nb-Al phases, which have been calculated using density functional theory. Possible reasons for this effect are discussed based on the current knowledge of multi-element arc cathodes and their arc plasma available in literature.

/pdf/erosion-and-cathodic-arc-plasma-of-nb-al-cathodes-composite-1ir90dgnsd.pdf

Erosion and cathodic arc plasma of Nb-Al cathodes: composite versus intermetallic

First-principles evolutionary algorithms are employed to shed light on the phase stability of Al–Nb intermetallics. While the tetragonal Al3Nb and AlNb2 structures are correctly identified as stable, the experimentally reported Laves phase of AlNb3 yields soft phonon modes implying its dynamical instability at 0 K. The soft phonon modes do not disappear even upon elevating the temperature in the simulation up to 1500 K. X-Ray diffraction patterns recorded for our powder-metallurgically produced arc cathodes, however, clearly show that the AlNb3 phase exists. We propose that AlNb3 is dynamically stabilised by ordered antisite defects at the Al sublattice, leading also to a shift of the Nb content from 75 to ∼81 at.%. Unlike the defect-free AlNb3, the antisite-stabilised variant hence falls into the compositional range consistent with our CALPHAD-based phase diagram as well as with the previous reports.

https://www.mdpi.com/1996-1944/12/7/1104/pdf

Experimental Chemistry and Structural Stability of AlNb3 Enabled by Antisite Defects Formation

https://iopscience.iop.org/article/10.1088/1361-6463/aaeecc/pdf

Influence of Ar gas pressure on ion energy and charge state distributions in pulsed cathodic arc plasmas from Nb–Al cathodes studied with high time resolution

Despite substantial progress in the last decades, the physics responsible for the plasma properties of cathodic vacuum arcs are still a matter of dispute, particularly for multi-element cathodes. The plasma analysis is complicated by the generally occurring neutral background of metal atoms, which perturbs initial ion properties. By using a time-resolved method in combination with pulsed arcs and a Nb-Al cathode model system, we investigate the influence of cathode composition on ion properties, while making the influence of neutrals visible for the observed time frame. This model system consists out of two different Nb-Al compositions with the the atomic ratios 75/25 and 25/75, as well as pure Nb and Al cathodes. The results visualize average charge states of Nb and Al ions of $600\ \mu\mathrm{s}$ plasma pulses, extracted 0.27 m from the cathode, resolved in energy and time. In addition to high vacuum at a base pressure of 10–4 Pa, results at three elevated Ar gas pressures (0.04 Pa, 0.20 Pa and 0.40 Pa) are shown. For high vacuum, ion properties were generally found to be strongly dependent on the cathode material, but deviating from a simple linear interpolation between the ion properties from Nb and Al cathodes. The influence of an inert background gas is analyzed by comparing these results with those at increased pressure, which show reduced ion charge states, up to a situation where mostly Nb2+ and Al+ ions are detected.

Siegfried Zöhrer

Papers

Time-resolved ion energy and charge state distributions in pulsed cathodic arc plasmas of Nb-Al cathodes in high vacuum

Erosion and cathodic arc plasma of Nb-Al cathodes: composite versus intermetallic

Experimental Chemistry and Structural Stability of AlNb3 Enabled by Antisite Defects Formation

Influence of Ar gas pressure on ion energy and charge state distributions in pulsed cathodic arc plasmas from Nb–Al cathodes studied with high time resolution

Time and Energy-resolved Average Ion Charge States in Pulsed Cathodic Vacuum Arc Plasmas of Nb-A1 Cathodes as a Function of Ar Pressure