Review On Laser Lightcraft Research At DLR Stuttgart

A review on 15 years research on remote laser propulsion with a parabolic thruster at DLR is presented. Mission scenarios were analyzed for nanosatellite launch to Low Earth Orbit (LEO) using a ground-based high energy laser as energy supply significantly optimizing the mass-to-payload-ratio. Experimental work was carried out using a home-made, electron-beam sustained CO2 high energy laser in the 10 kW class with around 10 μs pulse length. The parabolic thruster was compared with the Lightcraft Technology Demonstrator in air-breathing mode as well as with Polyoxymethylene (POM) as an ablative propellant with respect to laser pulse energy and beam profile taking into account for standardization issues of ballistic pendula. Experiments showed good performance of pure air-breathing mode without propellant down to 200 mbar ambient pressure allowing for a drastic propellant reduction for the initial flight phase during dense atmosphere. The commonly used hydrodynamic point explosion model with a strong shock wave was analyzed with respect to the optimization of the impulse coupling coefficient in geometric scaling by the adaptation of nozzle diameter and length to the range of the applied laser pulse energy. The usage of ablative propellants like POM, inevitable in the vacuum of space, yields enhancement of impulse coupling under atmospheric conditions which can partly be attributed to combustion. Various polymer-metal composites were developed and analyzed in order to achieve a higher specific impulse, but failed due to material inhomogeneity. Started up with wire-guided flights, using only air as propellant in a laser-induced breakdown, the detonation reproducibility by means of an ignition pin on the axis of symmetry of the thruster was proven in the free flight experiments yielding an altitude up to 8 m, limited by the laboratory ceiling. Nevertheless, flight dynamic analysis of a tilted pin as steering gear and hovering experiments near ground level revealed crucial coupling of lateral and angular motion and the demand of spin-stabilization for a beam-riding flight. A review of related publications, in cooperation with US AFRL, University of Stuttgart and Nagoya University, is given as a compendium

Laserantriebe

Einführung in "Beamed Energy Propulsion": Funktionsprinzip des Lasers, Beispiele und Ausbreitung von Laserstrahlung. Überblick über Antriebskonzepte: Photon Propulsion, ablative Laserantriebe, Laser Lightcraft, Kollisionsvermeidung bei Weltraumschrott und Near-Earth Objects, Beseitigung von Weltraumschrott

Analyse des laser-ablativen Mikroantriebs

Die Leitidee des MICROLAS-Konzeptes für einen laser-ablativen Mikroantrieb besteht darin, im Sinne minimalen Schubrauschens bewegliche Komponenten zu vermeiden, so dass Treibstrahl, Photonen und Elektronen das Einzige sind, was sich bei dieser Art des Antriebs bewegt: Ein gepulster Festkörperlaser wird dazu verwendet, kleine Treibstoffmengen von einem metallischen Target abzutragen und zu beschleunigen. Auf diese Weise werden äußerst kleine Impulsbits erzeugt. Dafür wird der Laserstrahl mittels eines Objektivs fokussiert und über einen Umlenkspiegel auf das Treibstofftarget gerichtet. Da die Wiederholrate der Laserpulse über mehrere Größenordnungen, bis ca. 100 kHz, variiert werden kann, ist es möglich, einen großen Schubbereich abzudecken, indem die passende Repetitionsrate gewählt wird. Dabei bleibt das einzelne Impulsbit unverändert, es wird zuvor bei der Optimierung des Arbeitspunktes durch die Laserparameter, die sich für den jeweiligen Treibstoff am besten eignen, für die gesamte Betriebsdauer festgelegt. Durch die Verwendung relativ geringer Impulsbits im Bereich von 1 – 10 nNs kann im repetierenden Betrieb die erforderliche Schubpräzision von 0,1 μN erreicht werden. Dabei erlaubt die Möglichkeit, die Repetitionsrate des Lasersystems nahezu instantan zu verändern, eine schnelle Ansprechzeit auf gewünschte Änderungen des momentanen Schublevels. Die Parameterstudie für den Einsatz von Aluminium und Gold beim Einsatz im MICROLAS-Konzept hat maximale Impulskoppelkoeffizienten im Bereich von 25 μN/W bis 40 μN/W, ergeben. Unter der Annahme einer (realistischen) Energieeffizienz von 20% für Festkörperlaser ergibt dies eine maximale Schubleistung von 12 bis 16 μN bei 2 W elektrischer Anschlussleistung. Bei einer stärkeren Stromversorgung ist der Schub prinzipiell nach oben hin skalierbar. Erste Analysen zum Schubrauschen vermitteln einen vielversprechenden Eindruck vom MICROLAS-Konzept da bereits unter sub-optimalen Bedingungen ein Rauschlevel gemessen wurde, das nur noch eine Größenordnung über den Spezifikationen lag, obwohl es sich bei diesen Experimenten lediglich um einen Nachweis des laser-induzierten Schubs handelte, ohne jede Optimierung von Laser, Treibstoff oder Scanparametern. Für laser-ablative Schuberzeugung mit Pulsen im Nanosekunden-Bereich ergeben die Parameterstudien, dass ein hoher spezifischer Impuls oberhalb von 2000 s möglich ist. Orientierende Messungen zur Divergenz des Treibstrahls haben ergeben, dass der 50%-Wert der Winkelverteilung bereits innerhalb der geforderten Spezifikationen liegt, der 90%-Wert jedoch noch doppelt so groß ist. Generell müssen die Spezifikationen für Schub und spezifischen Impuls in Abhängigkeit von den Laserparametern noch im Hinblick auf den repetierenden flächenhaften Abtrag untersucht werden, da hier Abweichungen von den Ergebnissen der Einzelpulsexperimente und der numerischen Parameterstudie zu erwarten sind. Insgesamt gesehen besitzt der MICROLAS-Antrieb unter Berücksichtigung der Parameter von Laser und Ablationsschema sowie im Hinblick auf Regelungstechnologie ein vielversprechendes Optimierungspotential, mit dem sich die Anforderung nach minimalem Schubrauschen erfüllen lässt

Space debris removal by non-destructive orbit modification using ground-based high-power lasers

Accelerated deterioration of ecosystems naturally expands situational awareness from sustainability efforts towards emergency response. While this holds true for, e.g., climate change, the current evolution of Earth’s orbital environment develops into a status demanding for short-term action far beyond sustainability measures for space debris mitigation. Possibly not being the most relevant option for space sustainability efforts, high-power lasers might nonetheless play a significant role in response to the increasing number of known debris objects. Lasers, however, with a perception ranging from well-known everyday life applications via technology optimism and weaponization efforts up to visionary propulsion concepts, demand for a thoughtful assessment of their beneficial as well as their destructive potential regarding thermo-mechanical interaction with space debris. In our work we present a holistic approach to realistically assess conceivable contributions of ground-based highpower laser technology for mitigation of the space debris situation in the low Earth orbit. Departing from experimental work on laser-induced momentum coupling, our simulations cover aspects of beam transmission like atmospheric extinction, turbulence compensation, and beam pointing jitter. Laser-matter interaction is computed considering different generic target shapes, various target materials as well as the dependency of thermo-mechanical coupling on the incident laser fluence. Moreover, estimates are derived on the debris remediation performance of a repetitive 100 kJ laser system for perigee lowering to achieve atmospheric burnup after multiple laser station overpasses. The related laser irradiation constraints for operational safety are explored in terms of the target’s thermo-mechanical integrity throughout the entire orbit modification maneuver

Dynamic Material Parameters in Molecular Dynamics and Hydrodynamic Simulations on Ultrashort-Pulse Laser Ablation of Aluminum

Recent developments on the open-source Molecular Dynamics Code IMD from the Institute of Functional Materials and Quantum Technologies at Stuttgart University are reported. Whereas IMD supports laser ablation comprising the Two-Temperature Model by default, reliable simulation data mainly can be found for the femtosecond regime, since laser-matter interaction is implemented by the Lambert-Beer law solely. For laser-matter interaction in the picosecond regime, IMD algorithms were modified in order to enable the dynamic recalculation of the dielectric permittivity of each FD cell for every timestep following the corresponding implementation in the open-source hydrodynamic code Polly-2T from the Joint Institute of High Temperatures at the Russian Academy of Sciences, Moscow. Thus, optical changes in material and jet due the temporal evolution of temperature, density and mean charge are taken into account. Moreover, as implemented in Polly-2T as well, a dynamic model for the thermal conductivity of the electron gas is introduced in IMD to consider effects for a wide range of temperatures. Thus, hydrodynamic (HD) and Molecular Dynamics (MD) simulations are compared extracting the residual effects of the different model approaches which persist in a particle-based method (IMD) using an Embedded-Atom potential (EAM) and a finite cell based approach (Polly-2T) employing the semi-empirical equations of state (EOS) including ionization. Simulation results are compared with experimental results and literature data

Space Sustainability by Laser Propulsion - DLR Research from Launch to Post-Mission Disposal

An overview is given on laser propulsion studies carried in the recent decades at the Institute of Technical Physics at DLR in Stuttgart. Laser-based propulsion concepts for spacecraft range from laser-ablation micropropulsion via remotely powered orbital transfer for, e.g., post-mission disposal up to launch concepts for picosatellites powered from a ground-based high energy laser. These concepts offer great benefits in terms of space sustainability by choosing non-toxic propellants and, in the case of remotely powered propulsion, even dramatically reducing the amount of required propellant by orders of magnitude. In addition to that, the related laser technology can be employed for risk mitigation in and protection of the orbital environment by space debris collision avoidance and de-orbiting from the Low Earth Orbit. In conclusion, research and development should focus on propulsion-specific high-power lasers as a backbone for these promising space applications

Momentum Predictability and Heat Accumulation in Laser-based Space Debris Removal

Small-sized space debris objects pose a challenging threat for aerospace security: Within a size range of 1 to 10 centimeters they are still large enough to cause severe damage to space assets but, moreover, they are almost too small to be detected and monitored. Recent progresses in laser-based debris monitoring yield promising results in detection and tracking of even small-sized debris objects and enable for the short-term solution of obstacle avoidance maneuvers. For the long run, however, it has to be analyzed whether and how the usage of high energy lasers can provide for a reliable solution of the space debris problem in the abovementioned particle size range. In our simulations we explore both potential and limitations of laser-ablative momentum generation, in particular with respect to the predictability of its magnitude and direction as well as to the accumulation of residual heat from the ablation process. For this purpose, we use Monte Carlo simulations with a variety of debris target geometries in order to analyze how restrictions in laser pointing accuracy as well as random target orientation affect the predictability of laser-ablative momentum generation. Moreover, pulse number restrictions are discussed in order to avoid target melting and its compactification. A database on laser-matter interaction is established for various materials and laser parameters enabling us to account for the local fluence distribution on the target surface obtained by raytracing with respect to the laser beam profile. Accordingly, main similarities and differences between short-pulse and ultrashort-pulse laser ablation are highlighted and discussed with respect to their implications for debris removal

Momentum predictability and heat accumulation in laser-based space debris removal (Erratum)

The article [Opt. Eng. 58(1), 011004 (2018), doi: https://doi.org/10.1117/1.OE.58.1 .011004] was originally published with two errors concerning 1) the computation of the optimum fluence for momentum coupling, and 2) simulation results on laser pointing accuracy

Ablative collision avoidance for space debris in the Low Earth Orbit by a single multi-kJ pulse from a ground-based laser

We analyze the conceptual idea whether already a single high energy laser pulse, emitted from a laser station on ground, might cause material ablation at the surface of a debris object generating recoil for a sufficiently high velocity change to allow for space debris collision avoidance. In our simulations we assess laser beam propagation through the turbulent atmosphere as well as laser interaction with debris targets of unknown shape and material. The results are discussed in terms of debris displacement, momentum transfer uncertainty, success probability, and limitations due to debris size, mass, and the required laser fluence at the target

Laser impulse coupling measurements at 400 fs and 80 ps using the LULI facility at 1057 nm wavelength

At the École Polytechnique « LULI » facility, we have measured the impulse coupling coefficient Cm (target momentum per joule of incident laser light) with several target materials in vacuum, at 1057 nm and 400 fs and 80 ps pulse duration. A total of 64 laser shots were completed in a two-week experimental campaign, divided between the two pulse durations and among the materials. Our main purpose was to resolve wide discrepancies among reported values for Cm in the 100 ps region, where many applications exist. A secondary purpose was to compare Cm at 400 fs and 80 ps pulse duration. The 80 ps pulse was obtained by partial compression. Materials were Al, Ta, W, Au, and POM (polyoxymethylene, trade name Delrin). One application of these results is to pulsed laser ablation propulsion in space, including space debris re-entry, where narrow ranges in Cm and specific impulse Isp spell the difference between dramatic and uneconomical performance. We had difficulty measuring mass loss from single shots. Imparted momentum in single laser shots was determined using pendulum deflection and photonic Doppler velocimetry. Cm was smaller at the 400 fs pulse duration than at 80 ps. To our surprise, Cm for Al at 80 ps was at most 30 N/MW with 30 kJ/m2 incident fluence. On the other extreme, polyoxymethylene (POM, trade name Delrin) demonstrated 770 N/MW under these conditions. Together, these results offer the possibility of designing a Cm value suited to an application, by mixing the materials appropriately