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  1. Applying New Laser Interaction Models to the ORION Problem

    Previously, Phipps, et al. developed a model that permitted laser ablation impulse predictions within a factor of two over an extremely broad range of pulse durations and wavelengths in the plasma regime. This model lacked the ability to predict the intensity for optimum impulse generation. For the lower-intensity vapor regime, below the plasma transition, Sinko developed a useful, fluence-dependent model which predicts impulse delivered for pulsed lasers on polymers at a specific wavelength. Phipps subsequently developed an alternate model which treats elemental solids in the vapor regime, that only requires knowledge of basic material parameters and vapor pressure vs. temperaturemore » data. These data, except for optical absorptivity, are wavelength-independent. A simple technique combines either vapor model with the plasma model to form a complete model that moves smoothly through the vapor to plasma transition. In this paper, we apply these models to show the optimum momentum coupling fluence on target, at the transition from the vapor to the plasma regimes, for aluminum (a typical debris material) and polyoxymethylene (representing polymeric debris). The application of this work is the ORION laser space debris mitigation concept. This is an improvement over previous work, in which this optimum was only estimated from the plasma ignition threshold. We present calculations showing how impulse delivered to debris targets in the ORION concept varies with pulse duration, at an optimum fluence determined by nonlinear optical effects in the Earth's atmosphere.« less
  2. A Conceptual Tree of Laser Propulsion

    An original attempt to develop a conceptual tree for laser propulsion is offered. The tree provides a systematic view for practically all possible laser propulsion concepts and all inter-conceptual links, based on propellant phases and phase transfers. It also helps to see which fields of laser propulsion have been already thoroughly explored, where the next effort must be applied, and which paths should be taken with proper care or avoided entirely.
  3. Usage Of Polyacetal Powders As Laser Ablation Propulsion Propellants

    We examined impulse characteristics of polyoxymeythylene (POM) powders under irradiation by a TEA (Transversely-Excited at Atmospheric pressure)CO{sub 2} laser pulse. The impulse performance exhibited large scatter due to splashing particles. When the powder was hydraulically compacted to form a disk, the momentum coupling coefficient became comparable with that for bulk material, but the mass consumption was increased by several times.
  4. CO{sub 2} Laser Ablation Propulsion Tractor Beams

    Manipulation of objects at a distance has already been achieved with no small measure of success in the realm of microscopic objects on the scale size of nanometers to micrometers in applications including laser trapping and laser tweezers. However, there has been relatively little effort to apply such remote control to macroscopic systems. A space tractor beam could be applied to a wide range of applications, including removal of orbital debris, facilitation of spacecraft docking, adjustment of satellite attitude or orbital position, etc. In this paper, an ablative laser propulsion tractor beam is demonstrated based on radiation from a CO{submore » 2} laser. Cooperative, layered polymer targets were used for remote impulse generation using a CO{sub 2} laser. The use of a structured ablatant enabling switching between thrust directional parity (i.e., forward or reverse) and imparting torque to a remote target. Fluence-dependent results are presented in the context of polymer ablation modeling work and with consideration of confined ablation effects.« less
  5. From Shadowgraph to Monochromatic Schlieren: Time-Resolved Imaging of Dim Laser-Induced Phenomena in the Presence of Saturating Plasma Emission

    Plasma emission generated during a laser ablation event can saturate an imaging system, washing out valuable information about non-luminous features such as vapor plumes and shock waves. A step-by-step experimental history of development of imaging system from Shadowgraph to Monochromatic Schlieren is presented here as a method of resolving this issue. An ICCD system was used to image ablation events on {mu}s-timescales in combination with a fiber-coupled Nd:YAG laser operating at a primary output of 50 mW at 532 nm wavelength, which was used as the illumination source. A monochromatic band pass filter at 532{+-}10 nm was inserted into themore » Schlieren system between the focusing lens and the spatial filter, producing a minute shift in the Schlieren focus. With the filter, the saturation effects were eliminated and features such as shock waves and vapor plumes were clearly visible even when plasma was present. Issues with implementation of this system for application to the study of laser propulsion are discussed in detail.« less
  6. Sphere-Wall Impact Experiments with Piezoelectric Force Sensors

    Measurement of impulse imparted to a target from {mu}s-timescale laser ablation events is often performed with piezoelectric force sensors. For pulsed laser ablation with a target resting on the force sensors, an effect can occur for a vertical thrust stand in an exhaust-up configuration that results in measurement of about twice the actual imparted impulse. A CO{sub 2} laser operating at 10.6 {mu}m wavelength, 300 ns pulse length, and up to 20 J pulse energy single shots was used to ablate samples of PCTFE. Force sensor measurements of the imparted impulse were compared to tests with a ballistic pendulum overmore » a variety of fluences. The theoretical impulse delivered by the impacts of 6 mm diameter spheres of aluminum, steel, POM, and PTFE on the force sensor were studied, and the coefficients of restitution were measured for the targets. Practical issues for measurement of ablation-imparted impulse with piezoelectric sensors are discussed.« less
  7. Laser Propulsion with Liquid Propellants Part I: an Overview

    Despite years of research, laser propulsion on liquid propellants has yet to achieve specific impulses greater than tens of seconds. It is well established that liquids, when used as propellants, can provide coupling coefficient (C{sub m}) on the order of 100-1000 dyne/W. However, the specific impulse (I{sub sp}) has proven to be significantly inferior to that for solid propellants. This paper will examine the various laser propulsion schemes based on ablation of liquid propellants and intended primarily for space applications. The principal shortcomings associated with liquid propellants will be outlined.
  8. Constant-Fluence Area Scaling for Laser Propulsion

    A series of experiments was conducted on polyoxymethylene (POM, trade name Delrin registered ) propellants in air at atmospheric pressure. A TEA CO{sub 2} laser with maximum output power up to 20 J was used to deliver 300 ns pulses of 10.6 {mu}m radiation to POM targets. Ablation at a constant fluence and a range of spot areas was achieved by varying combinations of the laser energy and spot size. Relevant empirical scaling laws governing laser propulsion parameters such as the momentum coupling coefficient (C{sub m}) and specific impulse (I{sub sp}) for spot areas within a range of about 0.05-0.25more » cm{sup 2} are presented. Experimental measurements of imparted impulse, C{sub m}, I{sub sp}, and ablated mass per pulse were made using dynamic piezoelectric force sensors and a scientific balance. Finally, Schlieren ICCD imaging of shock waves and vapor plumes was performed and analyzed.« less
  9. Modeling CO{sub 2} Laser Ablative Impulse with Polymers

    Laser ablation vaporization models have usually ignored the spatial dependence of the laser beam. Here, we consider effects from modeling using a Gaussian beam for both photochemical and photothermal conditions. The modeling results are compared to experimental and literature data for CO{sub 2} laser ablation of the polymer polyoxymethylene under vacuum, and discussed in terms of the ablated mass areal density and momentum coupling coefficient. Extending the scope of discussion, laser ablative impulse generation research has lacked a cohesive strategy for linking the vaporization and plasma regimes. Existing models, mostly formulated for ultraviolet laser systems or metal targets, appear tomore » be inappropriate or impractical for applications requiring CO{sub 2} laser ablation of polymers. A recently proposed method for linking the vaporization and plasma regimes for analytical modeling is addressed here along with the implications of its use. Key control parameters are considered, along with the major propulsion parameters needed for laser ablation propulsion modeling.« less
  10. Measurement Issues In Pulsed Laser Propulsion

    Various measurement techniques have been used throughout the over 40-year history of laser propulsion. Often, these approaches suffered from inconsistencies in definitions of the key parameters that define the physics of laser ablation impulse generation. Such parameters include, but are not limited to the pulse energy, spot area, imparted impulse, and ablated mass. The limits and characteristics of common measurement techniques in each of these areas will be explored as they relate to laser propulsion. The idea of establishing some standardization system for laser propulsion data is introduced in this paper, so that reported results may be considered and studiedmore » by the general community with more certain understanding of particular merits and limitations. In particular, it is the intention to propose a minimum set of requirements a literature study should meet. Some international standards for measurements are already published, but modifications or revisions of such standards may be necessary for application to laser ablation propulsion. Issues relating to development of standards will be discussed, as well as some examples of specific experimental circumstances in which standardization would have prevented misinterpretation or misuse of past data.« less

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