When setting up a lunar station, technologies for the use of locally available materials are crucial. Such technologies drastically reduce the need for transportation from Earth. We aim to provide proof of a key technology, namely Mobile Selective Laser Melting (M-SLM) for terrain modelling i.e. for building large structures such as launch/landing pads, but also building infrastructures like shelters protecting astronauts or equipment against radiation and micrometeorites on the Moon. The M-SLM technology has the advantage that only electrical energy and a moving system are required. For M-SLM, a mobile high power laser beam is directed on lunar regolith leading to its melting. Subsequently, the melt cools down and solid structures are generated. The MOONRISE instrument should serve in a short-term mission as a proof-of-principle experiment for the M-SLM technology on the lunar surface. In a first step, an Engineering Model (EM) of our MOONRISE instrument with a volume of 10 cm x 10 cm x 15 cm and a mass of about 2.7 kg has been built and thoroughly tested on ground. It could be accommodated on a rover or a robotic arm to move the laser spot in order to create 1D, 2D and even 3D regolith structures on the Moon. Recently, three new projects have been initiated in order to (1) develop the MOONRISE payload towards a Flight Model (FM) with accommodation on a commercial lunar lander, in order to (2) apply 2D laser beam deflection techniques for process scaling on a potential follow-on payload and in order to (3) investigate the detailed process of regolith laser melting under lunar gravity conditions in the Einstein-Elevator.
In-Situ Resource Utilization (ISRU) technologies pave the way for a sustainable colony on the Moon. Above all, the construction of structures using only the available resources is an important factor in reducing costs and logistical effort. The MOONRISE project aims to melt lunar regolith using lasers on mobile platforms for the Additive Manufacturing of structures. This process is called Mobile Selective Laser Melting (M-SLM) and has the advantage that only electrical energy and a moving system are required. In order to validate the laser melting of lunar regolith simulants on ground, a vacuum chamber was designed to host powder material at pressures of around 10-2 mbar. Laser exposure and high speed monitoring were performed through a window. Prior to finalizing the payload design, the type of laser source, appropriate spot size, power, and duration of exposure were determined by means of experimentation. For reasons of cost-efficiency, the payload development approach is to profit as much as possible from components commercial off-the-shelf (COTS), i.e. commercially available components, which have no formal space qualification. These components, e.g. built for automotive application, often withstand harsh environments or even have space heritage without the costly and long-lasting process of formal space qualification. For MOONRISE, COTS parts – partly based on space heritage - have been screened in environmental tests and selected for the payload. A detailed preliminary design review of the MOONRISE payload was conducted in 2019. The payload mainly consists of a printed circuit board (PCB) for system communication, a fiber coupled diode laser, an electrical diode driver, a beam focusing optics, and an LED illumination. For baseline operation, a laser power of typically 70W will be applied for 6s to the lunar surface at a distance of about 25cm. The LED illumination is supporting visualization of the molten regolith by external cameras. The MOONRISE payload can be accommodated to a rover or a robotic arm to ensure mobility for the melting experiments. Following that, an Engineering Model (EM) has been assembled and tested for functionality. The dimension of the payload is 1.5U CubeSat and it has a mass of about 2.5kg with further reduction potential towards flight model (FM) development. In the following steps, environmental tests, such as vibration and thermal-vacuum cycling, will be carried out with the EM. As laser melting of regolith under vacuum conditions produced dense material, tests were continued under low gravity conditions in the large-scale research device Einstein-Elevator at the Hannover Institute of Technology (HITec) of the Leibniz University Hannover (Germany), which is a further development of a classical drop tower with which experiments are carried out under conditions of microgravity [3]. It allows experiments under zero gravity conditions for about four seconds. The flight can be repeated up to 300 times per day. The Einstein-Elevator also enables adjustment of the gravity level from 0 to 5g, a feature that was used to carry out melting experiments with the EM under lunar gravitation conditions.
With the development of touch panel display the need to process thinner glass using Ultra-Short Pulse (USP) laser has increased. Beam shaping improves the process yield and quality but requires specific precautions when applied to USP laser due to high peak power and dispersion.
Bessel beams improve the quality of glass drilling and cutting due to the extended depth of field. We present Bessel beam generation using a reflective off-axis axicon giving a more stable beam compatible with scanning system and with a profile closer to theory. The characteristics of the beam and of the processed glass are described.
In micro-material processing with ultrashort laser pulses (USP), the surface quality during drilling and thin film ablation varies with the scanning speed and the pulse repetition rate. However, while high pulse repetition rates tend to be desirable, local heat accumulation caused by increasing pulse overlap is counterproductive. Thus, the scanning speed must be scaled with the pulse repetition rate, preferably by supplementing the already existing setup.
In this work, we present a dynamic extension through the combination of an acousto-optical deflector (AOD) with a galvanometer scanner. This combines the best of two worlds: the dynamic beam deflection of the AOD and the large scanning field of the galvanometer scanner. The integrated AOD is able to deflect the laser beam pulse by pulse within its scanning field and to modulate the beam intensity simultaneously. The mechanical limitations and problems of the galvanometer scanner, such as vibrations and overshoots due to fast mirror rotations, can be specifically compensated by the high precision of the AOD. As a result, in addition to process time reduction, the surface and image quality improves significantly. In any case, the laser source needs synchronization with the AOD because the propagation of sound waves within the AOD crystal is slower than the laser pulse propagation through the medium. In the first step, a comparatively slow AOD based on tellurium dioxide with a transversal crystal alignment is used. The process time of a thin film ablation with 4 μJ at 1 MHz was reduced considerably by applying a USP laser system (Coherent Monaco).
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