- Email: [email protected]
An advanced optical system for laser ablation propulsion in space
Acta Astronautica 96 (2014) 97–105
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
An advanced optical system for laser ablation propulsion in space$ Grant Bergstue n, Richard Fork, Patrick Reardon The University of Alabama in Huntsville, United States of America
a r t i c l e i n f o
abstract
Article history: Received 5 March 2013 Received in revised form 18 September 2013 Accepted 16 November 2013 Available online 3 December 2013
We propose a novel space-based ablation driven propulsion engine concept utilizing transmitted energy in the form of a series of ultra-short optical pulses. Key differences are generating the pulses at the transmitting spacecraft and the safe delivery of that energy to the receiving spacecraft for propulsion. By expanding the beam diameter during transmission in space, the energy can propagate at relatively low intensity and then be refocused and redistributed to create an array of ablation sites at the receiver. The ablation array strategy allows greater control over flight dynamics and eases thermal management. Research efforts for this transmission and reception of ultra-short optical pulses include: (1) optical system design; (2) electrical system requirements; (3) thermal management; (4) structured energy transmission safety. Research has also been focused on developing an optical switch concept for the multiplexing of the ultra-short pulses. This optical switch strategy implements multiple reflectors polished into a rotating momentum wheel device to combine the pulses from different laser sources. The optical system design must minimize the thermal load on any one optical element. Initial specifications and modeling for the optical system are being produced using geometrical ray-tracing software to give a better understanding of the optical requirements. In regards to safety, we have advanced the retro-reflective beam locking strategy to include look-ahead capabilities for long propagation distances. Additional applications and missions utilizing multiplexed pulse transmission are also presented. Because the research is in early development, it provides an opportunity for new and valuable advances in the area of transmitted energy for propulsion as well as encourages joint international efforts. Researchers from different countries can cooperate in order to find constructive and safe uses of ordered pulse transmission for propulsion in future space-based missions. & 2013 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Ablation Propulsion Optics Ultrashort Pulse Space
1. Introduction 1.1. Multiplexed ultra-short pulses for space A growing area of research relevant to new missions in space is that of energy transmission, especially in regards to providing thrust. Transmitting energy between spacecrafts offers a remote power source. This strategy can reduce the
☆
This paper was presented during the 63rd IAC in Naples. Corresponding author. Tel.: 256 466 5647. E-mail addresses: [email protected] (G. Bergstue), [email protected] (R. Fork), [email protected] (P. Reardon). n
mass, complexity and cost of a satellite or spacecraft receiving the transmitted energy. The quantum process of stimulated emission enables configuration of optical energy in forms relevant to generation of propulsive thrust in space. Energetic ultrashort optical pulses generated as lowest order Gaussian modes can be both propagated over long distances in the vacuum of space and then simply and efficiently be transformed into propulsive thrust. The energetic pulses of interest, however, cannot typically be used in the atmosphere of Earth. Such pulses tend to collapse and self-focus if propagated over significant distance in Earth's atmosphere. A further constraint is that the quantum process that enables generation of these energetic ultrashort optical
0094-5765/$ - see front matter & 2013 IAA. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2013.11.021
98
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
pulses requires specifically designed optical resonator and amplifier systems. These systems are typically limited to generating average power from a given mode-locked laser system on the order of a kilowatt [1,2]. This amount of average power from an individual mode-locked laser system can probably be improved to some degree, but also will probably not be improved to a degree that will enable a single mode-locked laser system to provide the levels of average power typically required for many applications in space. As a means of accessing the higher average power required for many applications of interest in space, we are exploring multiplexing of mode-locked, ultra-short pulse lasers to advance propulsion capabilities for use in space. We currently see two types of such multiplexing. One approach is analogous to the time domain multiplexing used in terrestrial information oriented optical networks, but designed instead for applications delivering levels of power useful for propulsive thrust in space over relatively long distances. We describe some of that work in this current paper. We are also exploring a second type of multiplexing analogous to space division multiplexing in terrestrial optical networks. This latter strategy appears more relevant to shorter distance transmission of large average levels of power in space. An application of current interest is deflecting near Earth objects that may be discovered on collision course with Earth. We describe this latter work in a recent paper [3]. In this current paper we discuss primarily the time division multiplexing strategy for long distance transmission of ultrashort optical pulses to be used for propulsion. A recent focus in the space industry has been on developing new space-based propulsion engine designs. For example, strong emphasis has been placed on micro-propulsion for future NASA missions. The “NASA Space Technology Roadmaps and Priorities” gives micro-propulsion a high priority score as a potential solution to meet current technological challenges [4]. On the topic of micro-propulsion, the “NASA Space Technology Roadmaps and Priorities” report says, “The benefits of developing micro-propulsion concepts are not confined to small satellites, to NASA, or to the aerospace industry. For instance, micro-propulsion could be used by larger satellites for missions requiring accurate thrust delivery to counteract orbital perturbations … . They could also be used for precise formation flying of spacecraft clusters or as modular distributed propulsion for the control of large space structures” [4]. We propose a means of creating this modular distributed propulsion through beamed energy in the form of multiplexed ultra-short optical pulses in space (MUOPS). In order to implement this strategy using MUOPS, a key piece of technology that is needed is the multiplexing optical switch. Below it is shown that a rotating off-axis parabolic reflector (OAP) can be used to collect pulses from different laser sources and multiplex them together given the correct positioning. There are limiting issues with this design however, and a more practical and rugged device must be used. It is proposed that a modified momentum wheel with multiple reflectors polished into it can serve as the multiplexing optical switch.
1.1.1. Multiplexing concept review The authors previously proposed a multiplexing strategy to produce the needed amounts of energy for effective ablative thrust in space [4]. In brief summary, this concept described a strategy for reaching these high average power levels by utilizing an array of laser sources multiplexed together rather than a single, large laser source. Cooperation and safety were not only stressed but necessary for the effective transfer of the energy to be used for thrust generation. The pulses, once multiplexed, are expanded in time and space as they propagate as to not have sufficient intensity to cause ablation. Only at the cooperating spacecraft ready to receive the energy with the correct optical system to focus the pulses can ablative thrust be created. The strategy also included distributing the ablation events over an array at the receiver as to give more control to the thrust applied to the spacecraft [5]. While these methods were shown as possible options for the safe and effective transmission of ultra-short pulses in near-Earth space, specific optical system designs were left for future research. Proposed in authors' current work are new methods for achieving the multiplexing of the ultra-short optical pulses for transmission to a cooperating spacecraft.
2. New concepts for an ultra-short pulse multiplexing transmission system for propulsion in space 2.1. Multiplexing optical switch One of the most important elements needed for this multiplexing strategy is a modulating switch to combine the laser sources into useful amounts of energy. An example of the importance of a multiplexing switch can be seen in terrestrial fiber communication networks. Just as a higher modulation of light increases the amount of data that can be transmitted, a higher modulation of the ultra-short pulses increases the total average power sent between spacecrafts. The modulation of these energetic ultrashort pulses is significantly more difficult as the pulses have a much higher energy. The multiplexing device for the proposed system must combine and direct the pulses from an array of laser sources. One way of accomplishing this is by rotating an OAP about its local axis. If a point source is placed at the effective focal length of a parabolic reflector, collimated light of good quality will be produced. By moving the reflector off the optical axis, the source can be placed on axis while the collimated light propagates unblocked by the source. To verify this concept, a ray-trace simulation of a rotating OAP was done with Code V (Fig. 1). This shows that a point source placed at any point along the circular pattern traced out by the OAP will produce well collimated light. However this is only true if the OAP is in the correct angular and rotational position to reflect the light. There are other limiting issues with this particular optical switch design. The largest problem lies with the rotation of the OAP due to its size and asymmetric weight distribution. In order to transmit ultra-short pulse energy
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
99
Fig. 1. Four images showing a single collimated beam being reflected to four different locations depending on the rotational position of the OAP. The focal points form a circular pattern around the collimated beam when the OAP is fully rotated. The images above are technical ray-traces done with Code V, an optical system simulation program.
over megameter distances in near-Earth space, a large transmitting optic is required. The diameter of the reflector can be determined using Gaussian equations describing beam propagation as well as taking into account diffraction to maximize transmitted intensity. The desired transmission distance will be twice what is known as the Rayleigh Range, “zR”, which describes the distance light can be considered collimated. By knowing the Rayleigh Range, the maximum beam radius “wm” for the collimated energy can be calculated using rffiffiffiffiffiffiffiffiffiffi 2zR λ wm ¼ ð1Þ π where “λ” is the wavelength. In order to collect and transmit maximum intensity, the transmitting aperture must be sufficiently large as to not clip the power at the edge of the Gaussian beam. Even though only a small amount of power will be lost, this causes diffraction effects in the near and far fields that create ripples in the intensity profile and degradation in overall beam quality [6]. It has been found that in order to transmit 499% of the intensity and minimize rippling to 71% (Fig. 2), the aperture diameter “d” must be [6] d 4:6wm
ð2Þ
Due to the size requirements on the transmitting reflector, mechanically rotating the large OAP to collect pulses from the different laser sources becomes impractical. Also, the OAP will not have cylindrical symmetry or be symmetrically weighted. This makes rotating a large reflector even more demanding. Additionally, this strategy includes the constraining of the laser sources in a circular pattern around the transmitted multiplexed pulses and limits the possible number and orientation of the laser
Fig. 2. Shows the transmission distance as a function of aperture diameter calculated using d ¼ 4.6w. This aperture will transmit 499% intensity and minimize intensity pattern distortion due to rippling to 7 1% [6].
sources. Despite these flaws, the principles described above do serve as a basis for another more practical optical switch concept. 2.2. Momentum wheel optical switch To avoid rotating an OAP as a means of multiplexing the ultra-short pulses to create ablative thrust, a smaller, more agile switch is needed. Authors' current research involves the utilization of a reflecting, multi-faceted momentum wheel operating at high revolution speeds to act as the switching mechanism. Multiple reflectors can be polished
100
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
onto the face of the momentum wheel near the outer edges to allow for both the maximum amount of reflectors per area and the fastest switching speeds. Each reflector will be a flat with a different orientation as it will only transmit pulses from a specific laser source. Each laser source will have a corresponding reflector on the momentum wheel switch with the needed angular orientation to reflect the incoming pulses along a common optical path. This concept takes advantage of the well understood characteristics of a momentum wheel while adding the fast switching capabilities needed for ultra-short pulse multiplexing. Maximizing cylindrical symmetry will be critical so the momentum wheel rotates in a predictable manner. Even though the differently oriented reflectors on the momentum wheel optical switch (Fig. 3) may cause slight asymmetries in weight distribution, this can be managed and more easily manufactured compared to a large rotating OAP. These weighting asymmetries will cause precessional rotations which also need to be tested for and taken into account. The rotational speed of the momentum wheel optical switch will depend on certain system parameters. These include the number of laser sources, the pulse energies of the laser system, the average power of the transmitted energy, size of the momentum wheel, etc. The number of reflectors on the momentum wheel will depend on the total number of laser sources in the system as the reflectors correspond to individual sources for multiplexing (Fig. 4). Also, if a higher number of laser sources with lower pulses energies are required for this system, then higher rotational speeds will be needed. This effectively increases the repetition rate of the multiplexed ultra-short
C A
B Polished regions at a variety of angles, but configured to leave wheel with maximum cylindrical symmetry Fig. 3. A diagram showing the concept of the momentum wheel optical switch. As the momentum wheel rotates, each reflector will direct pulses from a corresponding laser source along a common path to the transmitting OAP. (A) Front view. (B) Angled view. (C) Side view. The red reflector illustrates how only a single reflector will be illuminated at a time. (For interpretation of the reference to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Simplistic diagram showing the ultra-short pulse multiplexing concept using rotating momentum wheel optical switch. The pulses are multiplexed into structured light with the combined average power of the individual laser sources.
pulses as to reach average power levels sufficient for generating ablative thrust. There are some limiting factors for the switching speeds that control the repetition rate of the combined ultra-short pulses. These are determined by the ablation process itself as well as optimizing the timing of the ablation events for maximum thrust generation. Ablation causes ionized particles and nano-particles to eject from the surface of the ablated material in an opaque plume [7]. If the ablative pulses are ordered too closely together in time, this plume will interfere with the following pulses. If an ultra-short pulse has to pass through an opaque plume before reaching the ablation material, the pulse will be disrupted and not cause maximum ablative thrust. In order to avoid this, the pulse spacing must be such that the plumes of ejected material are sufficiently dissipated so the subsequent pulses can most effectively ablate the desired material. These plumes can become optically transparent after approximately 5 ms[8]. Generating thrust by ablation in space also gives advantage in this area as the ablative plumes have the shortest lifetimes in a nearvacuum environment [6]. The time for the plume to become optically transparent will determine the repetition rate of the multiplexed pulses and the rotational speed of the momentum wheel switch. The combining of multiple laser sources to generate a single, higher average power, good quality laser is a current area of research being developed using different methods, such as the use of diffractive optics [9]. Although there has been success in these efforts, there is some error introduced by combining the lasers. The accuracy of the momentum wheel optical switch in multiplexing the different pulses will need to be fully characterized. Unlike terrestrial multiplexing devices utilizing fiber, the energy propagated during transmission in near-Earth space is not confined in a guiding medium. Although this allows higher energies to be sent, the effectiveness in transmitting the combined energy is more sensitive to accuracy in multiplexing, pointing accuracy, and propagation distances. One issue that will need to be considered is the reflection angle error introduced by the rotating motion
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
of momentum wheel optical switch. Because each reflector will have a slightly different angular orientation, as the wheel sweeps through its rotation the incoming pulses will be reflected over a certain angular range. This range of angular error will need to be minimized for each reflector in order to effectively combining the pulses along a common optical axis. The degree to which the reflected pulses are not perfectly directed depends on the geometry and optical path lengths of the system as well as the angular orientation of the reflectors. The more extreme the angle of the reflectors on the momentum wheel optical switch, the stronger the possible deviation will be. The angle at which the reflector directs the incoming pulses depends on the rotational position of the momentum wheel. This makes the timing of the pulse on the reflector important to multiplexing accuracy. The pulse timing can be adjusted to account for the rotational position of the momentum wheel optical switch. The ability to precisely time the pulses on the center of their corresponding reflectors will help to minimize this reflection error and maximize multiplexing efficiency. An advantage in this area is the ultra-short temporal profile of the pulses. Even relative to the high tip speeds of a rotating momentum wheel, the wheel will not move very far during the reflection time of a single pulse. This means the individual pulses will experience little angular reflection error while being combined. 2.3. Laser system In order to generate the multiplexed pulses for transmission, a reliable ultra-short pulse laser system is needed to produce the array of sources. As mentioned above, creating an ultra-short pulse laser system with high pulse energies, good beam quality, and high average power is non-trivial and will take new advancements to achieve. Size and weight are also important issues that must be taken into consideration which make solid state and fiber laser sources attractive for this system. The stability advantages of frequency comb lasers can also be utilized in the laser system. Frequency comb lasers provide the extreme precision in pulse timing that will be needed for accurate multiplexing on the momentum wheel optical switch as well as timing for the control system. As far as producing high enough average power for the source lasers, a master oscillator power amplifier (MOPA) laser design can be used [2]. The MOPA can seed multiple smaller ultra-short pulse amplifiers which can then act as the array of laser sources for the momentum wheel optical switch to combine. Individual pulses from the MOPA can be picked out to determine the repetition rate while multipass thin disk amplifiers have been shown to be useful in increasing ultrashort pulse energies to the needed millijoule level [10]. 2.4. Optical system design A more robust examination of the optical system requirements in the MUOPS ablation engine concept is also presented. The OAP mentioned above is a necessary
101
part of the optical system as it is needed to collimate the multiplexed ultra-short pulses in the lowest order Gaussian mode capable of propagating megameter distances. The parabolic shape of the reflector is important as is creates the least amount of optical aberration and therefore the best beam quality. While a parabolic reflector is more difficult to manufacture than a spherical reflector, a spherical mirror will introduce large amounts of spherical aberration and will compromise beam quality during propagation. An adaptive optical feedback control system will be needed. This system can take advantage of the ultra-short pulse nature and use them at lower energies to probe the multiplexing system as a whole. The ultra-short pulses give the ability to probe the system to produce extremely fast and accurate data due to the speed and precision of the pulse. Using interferometric strategies, the temporal and spatial alignment of the optical switch with regards to the laser sources can be measured. The ultra-short pulses will be sufficiently fast to retrieve real-time accurate data about the switch. The timing and position of the momentum wheel optical switch must be verified before any higher power pulses are delivered to the transmitting optics. This optical verification system can test for any rotational precessions in the momentum wheel and prepare the pulses to be sent for any predictable variation patterns. Even with the advantages of using the pulses as testing probes, this is a technically demanding task which will require high response time sensors and adjustment controls. Another important set of optical components for the transmission system are the optics which add and remove chirp to the ultra-short pulses. Adding chirp to the pulses is a means of managing the high pulse energies so damage is not done to system's optical elements. A chirped pulse has an increased temporal profile which decreases the peak power of the pulse. Chirping pulses is common practice in chirped pulse amplification which is a method for further increasing ultra-short pulse energy. The chirping of the pulse can be achieved through use of optical gratings for spreading the frequency profile. Another method for consideration is that of using chirped or double chirped mirrors. Double chirped mirrors provide high reflectivity and are able to compensate for dispersion as the ultra-short pulse propagates. These mirrors can also be made to work over a broad spectral range [11]. There are multiple methods for managing the energetic pulses as they propagate throughout the transmission system. If a chirp is applied, the temporal width of the ultra-short pulses is made larger as it propagates through the transmission system so pulses do not carry significantly high enough instantaneous powers to cause damage to the optics in that state. The multiplexed pulses can then be expanded for propagation, further reducing the average intensity and minimizing the ablative potential during transmission. The receiving spacecraft can then remove the chirp making the pulses capable of ablation and thrust generation only once they have been delivered. If the receiving spacecraft does not have the capability to remove the chirp, the transmitting spacecraft can still deliver the pulses with care taken not to damage any of
102
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
off with the ejected material. This is due to the intense energy of the pulse breaking the atomic bonds on the surface of the material, which creates the ejected plume, and not being absorbed into the bulk of the material. Another advantage of the distributed ablative propulsion technique is the spreading of the ablation events over a large, or multiples areas at the receiving spacecraft. This distributed thrust delivery system concept is discussed in further detail below. 2.6. Control system and electrical requirements
Fig. 5. A geometrical optics ray-trace showing, as an example, a pair of off-axis reflectors acting as a zoom system for the focusing the multiplexed ultra-short pulses. The model was designed and optimized in Code V.
the components in the system. This can be done by keeping the beam width of the pulses sufficiently large as to not reach ablation intensities throughout the system. An optical zooming system (Fig. 5) is also suggested for any focusing adjustments that must be made for effective transmission and thrust generation by the ultra-short optical pulses. Focusing the multiplexed ultra-short pulses may be needed in scenarios such as providing thrust for micro-satellite systems or other receivers with different sized receiving optics on board. The last optic in the system must be the directing reflector. This will be needed for fine adjustments in pointing and tracking. This last reflector will be a flat as to not add any additional power and change the intended propagation behavior of the multiplexed light. The pointing precision required is discussed in further detail below. 2.5. Thermal management Thermal management concerns at the transmitting spacecraft will consist of the laser system and the thermal loads on the optics. Traditional heat sink and distribution methods can be used on both the master oscillator as well as the ultra-short pulse amplifiers. Controlling thermal issues on the optics themselves is an issue of pulse power, average power, and mirror materials for avoiding the optical damage threshold of the reflectors. The momentum wheel optical switch also helps with thermal management as each the multiplexing work is shared over “N” number of reflectors. This method avoids a single, small optic handling the combined intensities of the multiplexed pulses. There is also a cooling time for each reflector as it rotates around the wheel until it reflects the next series of pulses from its corresponding laser source. At the receiving spacecraft, thermal management will again involve not only minimizing heat on the optical elements but also handling the heat from ablative thrust. The heat produced from ablative thrust is mostly carried
In order for the transmission optics to make the adjustments determined by feedback from the probing pulses, a robust governing control system is needed. This control system's electronics must have sufficient processing power to make the needed calculations and drive the adjustment mechanisms quickly and accurately. An interesting new distributed control system is being developed by NASA's Glenn Research Center [12]. This distributed control strategy is being designed for use on a single, large combustion engine that aims to advance new, high temperature electronics as well as increase computational efficiency for the control system. The controls for the MUOPS ablation engine strategy will also need to be distributed, but on a larger scale. Instead of over an individual engine, the distribution will span across the transmitting and receiving spacecraft as well as within the individual spacecrafts themselves. A control system that is distributed within the spacecrafts can more efficiently drive the optical and mechanical devices needed to multiplex and guide the pulses for thrust control. Each spacecraft's control system must also take into account other's in order to create a position lock for transmission accuracy. 3. Satellite tracking 3.1. Positional tracking/lock Before the multiplexed pulses can be safely transmitted, a positional lock must be made between both the sending and receiving spacecrafts. A low power tracking beam can be sent out to verify the location of the satellites and align the necessary optical and electrical components to receive the incoming pulses. Early research suggested a retro-reflective beam lock strategy be used to achieve the positional lock. This technique is effective for short distances where the fast propagation time of the light between the spacecrafts is nearly negligible in regards to the time to establish a positional lock. For long transmission distances however, this is not the case. As an example, a transmission distance of one megameter will take light approximately three milliseconds to propagate. During that time, satellites in low-Earth orbit with velocities around 7500 m/s will travel roughly 25 m. The position locking beam will need to lead the receiving spacecraft by the necessary amount for successful transmission of the multiplexed energy for propulsion. This also means in the time it takes the transmitting spacecraft to send the weak positioning beam and receive positional data back, the
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
sending spacecraft itself will have moved on the order of tens of meters. As a solution we propose an intelligent beam lock system to account for propagation times of the energy and provide look-ahead pointing. In addition to the initial weak tracking beam, radio contact can be utilized to continually transmit location information between the cooperating satellites. With sensors on board the receiving spacecraft, the accuracy of the initial beam can be determined. The control system can then calculate the needed pointing corrections and transmitted them back to the sending spacecraft until a fully accurate position lock is formed. High precision in pointing accuracy is required as well. For a receiving optic with a radius on the order of a few meters, transmitting the multiplexed pulses over megameter long distances will require a minimum pointing accuracy on the order of microradians. 3.1.1. Safety This position tracking/locking system used between the sending and receiving satellites will be necessary to ensure the safe transmission of the energy. Much like cooperation is required for unlocking the ablative potential of the transmitted ultra-short pulses, cooperation and communication between the satellites is needed to establish a positional lock. Constant information must be openly transmitted to all parties involved in the energy transmission process. Additionally, all desired energy transmissions can be required to receive verification and allowance by an overseeing international organization much like the Laser Clearing House at Vandenberg Air Force Base does for NASA. 3.2. Receiving spacecraft The receiving spacecraft must have the necessary equipment to guide the incoming tracking beam into position lock as well as collect the transmitted energy sent for propulsion. Sensors on the receiving spacecraft can relate the relative accuracy back to the transmitting spacecraft which can then make the according adjustments depending on satellite position and velocity. Walking in the tracking beam to a successful position lock will be necessary before the higher power structured pulses can be accurately delivered. A similar optical system used to generate the multiplexed ultra-short pulses can be used in reverse to collect the transmitted energy and guide it to its own demultiplexing system. The transmitted energy must be directed to distribute the thrust due to ablation to the needed areas of the spacecraft. The chirping of the pulses also has to be considered at the receiving spacecraft. In order to generate ablative thrust with chirped pulses, the chirp must be removed at the receiving spacecraft to increase pulses' instantaneous power. However, as with the transmitting system, care must be taken to not cause ablation before the intended thrust is created. The optical system distributing the pulses must take into account chirp and beam width to avoid this problem.
103
As a means to distribute the transmitted energy, a demultiplexing momentum wheel device can again be used to raster the incoming pulses over a predetermined ablation area. If a more advanced distribution of the pulses is needed throughout the spacecraft, additional fast steering mirrors can be used to direct pulse trains to the desired location on the spacecraft. Separating the ablation events and the ablation areas throughout the spacecraft creates a larger area affected by the thrust generated which allows for more precise control of flight dynamics. As mentioned above, a distributed control system can be used to guide the pulses at the receiving spacecraft. 4. Additional applications and missions The proposed MUOPS strategy (Fig. 6) can be used for a variety of future forward looking missions in near-Earth space and beyond. One such possibility that the authors also propose is that of small orbital debris mitigation by laser ablation. The same multiplexing strategy involving the momentum wheel optical switch can be implemented for much shorter transmission distances (1–10 km). The multiplexed pulses can also be focused to cause ablation at the target for delivering a deorbiting force on the debris. Further information on this topic can be seen in Dr. Richard Fork's 2012 IAC paper [13]. Another possible new mission utilizing the proposed ultra-short pulse multiplexing strategy is that of horizontal lift to orbit (HLO). With the ability to generate and transmit useful levels of energy in near-Earth space, the same energy can be directed to assist spacecraft in HLO strategies. The pulse must be prepared differently for this application as the ascending spacecraft may have different or limited optics on board to generate thrust. Also, chromatic dispersion must also be considered as the pulses will have to travel through the upper levels of the atmosphere to reach the spacecraft. This can be accounted for by knowing the distance of atmosphere the pulses will be propagating through and applying the needed corrections to pulse shape, chirping, etc. for effective transmission. Further research on both of these possible missions is desired by the authors. Finally, new missions for sustained exploration of the asteroid belt and further into the solar system could be enhanced by the use of ablative propulsion distribution. For example, forward-looking missions to develop a more significant human presence in the solar system would benefit from the presence of an energy infrastructure capable of providing ultra-short pulse ablative propulsion to either manned or unmanned spacecraft. The laser-based energy infrastructure described by authors' paper at the 2011 IAC could be applied to the exo-planet Ceres in the asteroid belt (Fig. 7) [5]. This would provide a means of establishing renewable energy for long-term missions and experiments on Ceres such as research for asteroid characterization and mining. The smaller size of Ceres compared to Earth means shorter transmission distances and smaller transmitting optics are required. Also due to its smaller size, Ceres has a much reduced gravitational field strength, approximately 2.8% that of Earth's. This could allow for more energy efficient lift to orbit missions using
104
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
Faceted Momentum Wheel
Chirping Optics
Beam Directing
Variable Focus
Verify/Correct Beam Alignment Amplifiers and Delay Ultrashort Pulse Laser Lines
Monitor/ Control/ Clock
Tracking Beam and Monitor
Not to Scale
TRANSMITTING SPACECRAFT
Fig. 6. Basic layout of a possible system design showing important elements in the MUOPS strategy for creating an ablation propulsion engine.
Fig. 7. A simulation of the energy infrastructure around the exo-planet Ceres. The cones of light represent the possible delivery areas from the different satellites. This satellite configuration allows for energy transmission anywhere in the near-planet space of Ceres at anytime. Ceres and the white lines representing the orthogonal orbits are to scale, however the markers representing the satellites are enlarged for visibility.
ablative propulsion on Ceres as the power required to reach escape velocity will be reduced as well. Ceres also has little to no atmosphere to interfere with the transmitted pulses which will help maintain pulse shape, beam quality, and transmission efficiency. The MUOPS strategy can also be applied to other important applications such as near Earth object (NEO) deflection. Multiplexed ultrashort pulses can be used to precisely apply thrust to an NEO on a collision course with Earth in order to change its trajectory. One advantage of this application of the MUOPS strategy is that the NEO material is both the receiving target as well as the propellant. Because the NEO material is the propellant, the thrust vector from ablation can be controlled and adjusted depending on where the focused pulses are delivered on the surface of the NEO. Additionally, multiple areas can be illuminated on the NEO simultaneously as to
not only provide additional thrust, but also to steer the NEO in the desired direction. For example, three areas can be illuminated to balance the thrust with respect to NEO's center of mass and to minimize unwanted rotation of the NEO. In order to implement this technique, a system of microsatellites could be used to map the surface of the NEO as to determine the desired location to apply the pulses as well to reflect and focus the pulses on the NEO surface. Multiple, physically separate, but well-synchronized laser systems on the transmitting spacecraft can illuminate these areas of the NEO simultaneously. This free-space multiplexing of optical pulses in vacuum allows scalable power applicable to NEO deflection [3]. The NEO deflection strategy together with the ultrashort pulse multiplexing and transmission technique proposed above provide possible means of both long and short distance transmission of ultrashort optical pulses for propulsion in space.
5. International cooperation For a new and innovative effort such as this, cooperation between multiple nations and space organizations is needed. Sharing information on energy transmission strategies, scheduled transmission times and purposes are critically important for furthering energy transmission for in-space propulsion research and development. In addition to the sharing of information, the sharing of the work in developing this technology can also take place. The ultra-short pulse multiplexing strategy described above is cross-cutting and will require advancements in multiple disciples of engineering such as optical, electrical, aerospace, mechanical, and control systems. Forward-looking projects like this with potential to create new jobs in the space industry are always needed and provide exciting opportunities for furthering technological advancements.
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
105
6. Current and future plans for research
References
The authors and students at the University of Alabama in Huntsville, both graduate and undergraduate alike, continue to pursue this research and development of the described MUOPS strategy for ablative propulsion. The authors and graduate students are currently involved in creating a more complete model and developing a control system to govern the necessary behavior of the different elements of the transmitting and receiving spacecrafts. Additional laboratory experiments and testing are desired by the authors. New optical elements, such as an OAP, have recently been acquired by the research team so that initial testing of the proposed multiplexing strategies can be done to further refine the understanding of the technique. Researchers and students at the University of Alabama in Huntsville are enthusiastic about this work and are well suited to help develop such strategies. The University of Alabama in Huntsville not only has experience with testing large optical systems, e.g. The James Webb Space Telescope, but also has the capabilities for precision manufacturing of the optical reflectors needed for the effort [14].
[1] T. Eidam, S. Hanf, E. Seise, T.V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, A. Tünnermann, Femtosecond fiber CPA system emitting 830 W average output power, Opt. Lett. 35 (2010) 94–96. [2] P. Russbueldt, T. Mans, J. Weitenberg, H.D. Hoffmann, R. Poprawe, Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier, Opt. Lett. 35 (2010) 4169–4171. [3] R. Fork, S. Cole, L. Burgess, G. Bergstue, Mode-locked Lasers Applied to Deflecting a Near Earth Object on Collision Course with Earth, 〈arXiv:1309.3238〉 [physics.space-ph]. [4] United States, National Aeronautics and Space Administration, NASA Space Technology Roadmaps and Priorities, 2012, 〈www.nap.edu〉. [5] G. Bergstue, R. Fork, Beamed energy for abalative propulsion in near Earth space, in: Proceedings of the 62nd International Astronautical Congress IAC-11-C4.8.2, Cape Town, South Africa, 2011. [6] Anthony E. Siegman, Lasers, University Science Books, Mill Valley, CA, 1986 (Print). [7] S. Amoruso, R. Bruzzese, X. Wang, J. Xia, Propagation of a femtosecond pulsed laser ablation plume into a background atmosphere, Appl. Phys. Lett. 92 (2008) 041503. [8] X. Wang, S. Amoruso, J. Xia, Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation, Appl. Surf. Sci. 225 (2009) 5211–5214. [9] S.M. Redmond, D.J. Ripin, C.X. Yu, S.J. Augst, T.Y. Fan, P.A. Thielen, J.E. Rothenberg, G.D. Goodno, Diffractive coherent combing of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam, Opt. Lett. 37 (2012) 2832–2834. [10] A. Giesen, J. Speiser, High-power thin disk lasers, Proc. SPIE 8547 (1– 8) (2012). [11] F.X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H.A. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch, T. Tschudi, Design and fabrication of double-chirped mirrors, Opt. Lett. 22 (1997) 831–833. [12] D. Cully, Tools and technology development for distributed engine control, in: Propulsion Control and Diagnostics Workshop, February 28, 2012, 〈http://www.grc.nasa.gov/WWW/cdtb/aboutus/workshop2012/ Presentations/Session%203.%20Distributed%20Engine%20Control/ DEC_01_Culley.pdf〉. [13] R. Fork, J. Rogers, L. Burgess, G. Bergstue, Active removal of small orbital debris using laser systems in space, in: Proceedings of the 63rd International Astronautical Congress IAC-12-A6.7.6, Naples, Italy, 2012. [14] D.M. Chaney, R.J. Brown, S.E. Kendrick, P.J. Reardon, J.B. Hadaway, J. Carpenter, R. Eng, Results of the Beryllium AMSD Mirror Cryogenic Optical Testing, Proceedings of SPIE, Optical, Infrared, and Millimeter Space Telescopes, 5487 (2004) 833. http://dx.doi.org/10.1117/12.549956.
Acknowledgments The authors would like to thank the University of Alabama in Huntsville's Office of the Vice President for Research, Lockheed Martin, and Lufthansa for supporting the trip to present this research at the 2012 IAC. Research funding was provided by NASA's Marshall Space Flight Center under grant number NNM11AA01A Supplement 5 from 08/17/2011 to 12/31/2011. The authors would also like to thank Mr. Luke Burgess, Mr. Ken Pitalo, Mr. David Pollock, and Mr. Arthur Palosz for valuable conversations pertaining to the development of this research.
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
An advanced optical system for laser ablation propulsion in space$ Grant Bergstue n, Richard Fork, Patrick Reardon The University of Alabama in Huntsville, United States of America
a r t i c l e i n f o
abstract
Article history: Received 5 March 2013 Received in revised form 18 September 2013 Accepted 16 November 2013 Available online 3 December 2013
We propose a novel space-based ablation driven propulsion engine concept utilizing transmitted energy in the form of a series of ultra-short optical pulses. Key differences are generating the pulses at the transmitting spacecraft and the safe delivery of that energy to the receiving spacecraft for propulsion. By expanding the beam diameter during transmission in space, the energy can propagate at relatively low intensity and then be refocused and redistributed to create an array of ablation sites at the receiver. The ablation array strategy allows greater control over flight dynamics and eases thermal management. Research efforts for this transmission and reception of ultra-short optical pulses include: (1) optical system design; (2) electrical system requirements; (3) thermal management; (4) structured energy transmission safety. Research has also been focused on developing an optical switch concept for the multiplexing of the ultra-short pulses. This optical switch strategy implements multiple reflectors polished into a rotating momentum wheel device to combine the pulses from different laser sources. The optical system design must minimize the thermal load on any one optical element. Initial specifications and modeling for the optical system are being produced using geometrical ray-tracing software to give a better understanding of the optical requirements. In regards to safety, we have advanced the retro-reflective beam locking strategy to include look-ahead capabilities for long propagation distances. Additional applications and missions utilizing multiplexed pulse transmission are also presented. Because the research is in early development, it provides an opportunity for new and valuable advances in the area of transmitted energy for propulsion as well as encourages joint international efforts. Researchers from different countries can cooperate in order to find constructive and safe uses of ordered pulse transmission for propulsion in future space-based missions. & 2013 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Ablation Propulsion Optics Ultrashort Pulse Space
1. Introduction 1.1. Multiplexed ultra-short pulses for space A growing area of research relevant to new missions in space is that of energy transmission, especially in regards to providing thrust. Transmitting energy between spacecrafts offers a remote power source. This strategy can reduce the
☆
This paper was presented during the 63rd IAC in Naples. Corresponding author. Tel.: 256 466 5647. E-mail addresses: [email protected] (G. Bergstue), [email protected] (R. Fork), [email protected] (P. Reardon). n
mass, complexity and cost of a satellite or spacecraft receiving the transmitted energy. The quantum process of stimulated emission enables configuration of optical energy in forms relevant to generation of propulsive thrust in space. Energetic ultrashort optical pulses generated as lowest order Gaussian modes can be both propagated over long distances in the vacuum of space and then simply and efficiently be transformed into propulsive thrust. The energetic pulses of interest, however, cannot typically be used in the atmosphere of Earth. Such pulses tend to collapse and self-focus if propagated over significant distance in Earth's atmosphere. A further constraint is that the quantum process that enables generation of these energetic ultrashort optical
0094-5765/$ - see front matter & 2013 IAA. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2013.11.021
98
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
pulses requires specifically designed optical resonator and amplifier systems. These systems are typically limited to generating average power from a given mode-locked laser system on the order of a kilowatt [1,2]. This amount of average power from an individual mode-locked laser system can probably be improved to some degree, but also will probably not be improved to a degree that will enable a single mode-locked laser system to provide the levels of average power typically required for many applications in space. As a means of accessing the higher average power required for many applications of interest in space, we are exploring multiplexing of mode-locked, ultra-short pulse lasers to advance propulsion capabilities for use in space. We currently see two types of such multiplexing. One approach is analogous to the time domain multiplexing used in terrestrial information oriented optical networks, but designed instead for applications delivering levels of power useful for propulsive thrust in space over relatively long distances. We describe some of that work in this current paper. We are also exploring a second type of multiplexing analogous to space division multiplexing in terrestrial optical networks. This latter strategy appears more relevant to shorter distance transmission of large average levels of power in space. An application of current interest is deflecting near Earth objects that may be discovered on collision course with Earth. We describe this latter work in a recent paper [3]. In this current paper we discuss primarily the time division multiplexing strategy for long distance transmission of ultrashort optical pulses to be used for propulsion. A recent focus in the space industry has been on developing new space-based propulsion engine designs. For example, strong emphasis has been placed on micro-propulsion for future NASA missions. The “NASA Space Technology Roadmaps and Priorities” gives micro-propulsion a high priority score as a potential solution to meet current technological challenges [4]. On the topic of micro-propulsion, the “NASA Space Technology Roadmaps and Priorities” report says, “The benefits of developing micro-propulsion concepts are not confined to small satellites, to NASA, or to the aerospace industry. For instance, micro-propulsion could be used by larger satellites for missions requiring accurate thrust delivery to counteract orbital perturbations … . They could also be used for precise formation flying of spacecraft clusters or as modular distributed propulsion for the control of large space structures” [4]. We propose a means of creating this modular distributed propulsion through beamed energy in the form of multiplexed ultra-short optical pulses in space (MUOPS). In order to implement this strategy using MUOPS, a key piece of technology that is needed is the multiplexing optical switch. Below it is shown that a rotating off-axis parabolic reflector (OAP) can be used to collect pulses from different laser sources and multiplex them together given the correct positioning. There are limiting issues with this design however, and a more practical and rugged device must be used. It is proposed that a modified momentum wheel with multiple reflectors polished into it can serve as the multiplexing optical switch.
1.1.1. Multiplexing concept review The authors previously proposed a multiplexing strategy to produce the needed amounts of energy for effective ablative thrust in space [4]. In brief summary, this concept described a strategy for reaching these high average power levels by utilizing an array of laser sources multiplexed together rather than a single, large laser source. Cooperation and safety were not only stressed but necessary for the effective transfer of the energy to be used for thrust generation. The pulses, once multiplexed, are expanded in time and space as they propagate as to not have sufficient intensity to cause ablation. Only at the cooperating spacecraft ready to receive the energy with the correct optical system to focus the pulses can ablative thrust be created. The strategy also included distributing the ablation events over an array at the receiver as to give more control to the thrust applied to the spacecraft [5]. While these methods were shown as possible options for the safe and effective transmission of ultra-short pulses in near-Earth space, specific optical system designs were left for future research. Proposed in authors' current work are new methods for achieving the multiplexing of the ultra-short optical pulses for transmission to a cooperating spacecraft.
2. New concepts for an ultra-short pulse multiplexing transmission system for propulsion in space 2.1. Multiplexing optical switch One of the most important elements needed for this multiplexing strategy is a modulating switch to combine the laser sources into useful amounts of energy. An example of the importance of a multiplexing switch can be seen in terrestrial fiber communication networks. Just as a higher modulation of light increases the amount of data that can be transmitted, a higher modulation of the ultra-short pulses increases the total average power sent between spacecrafts. The modulation of these energetic ultrashort pulses is significantly more difficult as the pulses have a much higher energy. The multiplexing device for the proposed system must combine and direct the pulses from an array of laser sources. One way of accomplishing this is by rotating an OAP about its local axis. If a point source is placed at the effective focal length of a parabolic reflector, collimated light of good quality will be produced. By moving the reflector off the optical axis, the source can be placed on axis while the collimated light propagates unblocked by the source. To verify this concept, a ray-trace simulation of a rotating OAP was done with Code V (Fig. 1). This shows that a point source placed at any point along the circular pattern traced out by the OAP will produce well collimated light. However this is only true if the OAP is in the correct angular and rotational position to reflect the light. There are other limiting issues with this particular optical switch design. The largest problem lies with the rotation of the OAP due to its size and asymmetric weight distribution. In order to transmit ultra-short pulse energy
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
99
Fig. 1. Four images showing a single collimated beam being reflected to four different locations depending on the rotational position of the OAP. The focal points form a circular pattern around the collimated beam when the OAP is fully rotated. The images above are technical ray-traces done with Code V, an optical system simulation program.
over megameter distances in near-Earth space, a large transmitting optic is required. The diameter of the reflector can be determined using Gaussian equations describing beam propagation as well as taking into account diffraction to maximize transmitted intensity. The desired transmission distance will be twice what is known as the Rayleigh Range, “zR”, which describes the distance light can be considered collimated. By knowing the Rayleigh Range, the maximum beam radius “wm” for the collimated energy can be calculated using rffiffiffiffiffiffiffiffiffiffi 2zR λ wm ¼ ð1Þ π where “λ” is the wavelength. In order to collect and transmit maximum intensity, the transmitting aperture must be sufficiently large as to not clip the power at the edge of the Gaussian beam. Even though only a small amount of power will be lost, this causes diffraction effects in the near and far fields that create ripples in the intensity profile and degradation in overall beam quality [6]. It has been found that in order to transmit 499% of the intensity and minimize rippling to 71% (Fig. 2), the aperture diameter “d” must be [6] d 4:6wm
ð2Þ
Due to the size requirements on the transmitting reflector, mechanically rotating the large OAP to collect pulses from the different laser sources becomes impractical. Also, the OAP will not have cylindrical symmetry or be symmetrically weighted. This makes rotating a large reflector even more demanding. Additionally, this strategy includes the constraining of the laser sources in a circular pattern around the transmitted multiplexed pulses and limits the possible number and orientation of the laser
Fig. 2. Shows the transmission distance as a function of aperture diameter calculated using d ¼ 4.6w. This aperture will transmit 499% intensity and minimize intensity pattern distortion due to rippling to 7 1% [6].
sources. Despite these flaws, the principles described above do serve as a basis for another more practical optical switch concept. 2.2. Momentum wheel optical switch To avoid rotating an OAP as a means of multiplexing the ultra-short pulses to create ablative thrust, a smaller, more agile switch is needed. Authors' current research involves the utilization of a reflecting, multi-faceted momentum wheel operating at high revolution speeds to act as the switching mechanism. Multiple reflectors can be polished
100
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
onto the face of the momentum wheel near the outer edges to allow for both the maximum amount of reflectors per area and the fastest switching speeds. Each reflector will be a flat with a different orientation as it will only transmit pulses from a specific laser source. Each laser source will have a corresponding reflector on the momentum wheel switch with the needed angular orientation to reflect the incoming pulses along a common optical path. This concept takes advantage of the well understood characteristics of a momentum wheel while adding the fast switching capabilities needed for ultra-short pulse multiplexing. Maximizing cylindrical symmetry will be critical so the momentum wheel rotates in a predictable manner. Even though the differently oriented reflectors on the momentum wheel optical switch (Fig. 3) may cause slight asymmetries in weight distribution, this can be managed and more easily manufactured compared to a large rotating OAP. These weighting asymmetries will cause precessional rotations which also need to be tested for and taken into account. The rotational speed of the momentum wheel optical switch will depend on certain system parameters. These include the number of laser sources, the pulse energies of the laser system, the average power of the transmitted energy, size of the momentum wheel, etc. The number of reflectors on the momentum wheel will depend on the total number of laser sources in the system as the reflectors correspond to individual sources for multiplexing (Fig. 4). Also, if a higher number of laser sources with lower pulses energies are required for this system, then higher rotational speeds will be needed. This effectively increases the repetition rate of the multiplexed ultra-short
C A
B Polished regions at a variety of angles, but configured to leave wheel with maximum cylindrical symmetry Fig. 3. A diagram showing the concept of the momentum wheel optical switch. As the momentum wheel rotates, each reflector will direct pulses from a corresponding laser source along a common path to the transmitting OAP. (A) Front view. (B) Angled view. (C) Side view. The red reflector illustrates how only a single reflector will be illuminated at a time. (For interpretation of the reference to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Simplistic diagram showing the ultra-short pulse multiplexing concept using rotating momentum wheel optical switch. The pulses are multiplexed into structured light with the combined average power of the individual laser sources.
pulses as to reach average power levels sufficient for generating ablative thrust. There are some limiting factors for the switching speeds that control the repetition rate of the combined ultra-short pulses. These are determined by the ablation process itself as well as optimizing the timing of the ablation events for maximum thrust generation. Ablation causes ionized particles and nano-particles to eject from the surface of the ablated material in an opaque plume [7]. If the ablative pulses are ordered too closely together in time, this plume will interfere with the following pulses. If an ultra-short pulse has to pass through an opaque plume before reaching the ablation material, the pulse will be disrupted and not cause maximum ablative thrust. In order to avoid this, the pulse spacing must be such that the plumes of ejected material are sufficiently dissipated so the subsequent pulses can most effectively ablate the desired material. These plumes can become optically transparent after approximately 5 ms[8]. Generating thrust by ablation in space also gives advantage in this area as the ablative plumes have the shortest lifetimes in a nearvacuum environment [6]. The time for the plume to become optically transparent will determine the repetition rate of the multiplexed pulses and the rotational speed of the momentum wheel switch. The combining of multiple laser sources to generate a single, higher average power, good quality laser is a current area of research being developed using different methods, such as the use of diffractive optics [9]. Although there has been success in these efforts, there is some error introduced by combining the lasers. The accuracy of the momentum wheel optical switch in multiplexing the different pulses will need to be fully characterized. Unlike terrestrial multiplexing devices utilizing fiber, the energy propagated during transmission in near-Earth space is not confined in a guiding medium. Although this allows higher energies to be sent, the effectiveness in transmitting the combined energy is more sensitive to accuracy in multiplexing, pointing accuracy, and propagation distances. One issue that will need to be considered is the reflection angle error introduced by the rotating motion
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
of momentum wheel optical switch. Because each reflector will have a slightly different angular orientation, as the wheel sweeps through its rotation the incoming pulses will be reflected over a certain angular range. This range of angular error will need to be minimized for each reflector in order to effectively combining the pulses along a common optical axis. The degree to which the reflected pulses are not perfectly directed depends on the geometry and optical path lengths of the system as well as the angular orientation of the reflectors. The more extreme the angle of the reflectors on the momentum wheel optical switch, the stronger the possible deviation will be. The angle at which the reflector directs the incoming pulses depends on the rotational position of the momentum wheel. This makes the timing of the pulse on the reflector important to multiplexing accuracy. The pulse timing can be adjusted to account for the rotational position of the momentum wheel optical switch. The ability to precisely time the pulses on the center of their corresponding reflectors will help to minimize this reflection error and maximize multiplexing efficiency. An advantage in this area is the ultra-short temporal profile of the pulses. Even relative to the high tip speeds of a rotating momentum wheel, the wheel will not move very far during the reflection time of a single pulse. This means the individual pulses will experience little angular reflection error while being combined. 2.3. Laser system In order to generate the multiplexed pulses for transmission, a reliable ultra-short pulse laser system is needed to produce the array of sources. As mentioned above, creating an ultra-short pulse laser system with high pulse energies, good beam quality, and high average power is non-trivial and will take new advancements to achieve. Size and weight are also important issues that must be taken into consideration which make solid state and fiber laser sources attractive for this system. The stability advantages of frequency comb lasers can also be utilized in the laser system. Frequency comb lasers provide the extreme precision in pulse timing that will be needed for accurate multiplexing on the momentum wheel optical switch as well as timing for the control system. As far as producing high enough average power for the source lasers, a master oscillator power amplifier (MOPA) laser design can be used [2]. The MOPA can seed multiple smaller ultra-short pulse amplifiers which can then act as the array of laser sources for the momentum wheel optical switch to combine. Individual pulses from the MOPA can be picked out to determine the repetition rate while multipass thin disk amplifiers have been shown to be useful in increasing ultrashort pulse energies to the needed millijoule level [10]. 2.4. Optical system design A more robust examination of the optical system requirements in the MUOPS ablation engine concept is also presented. The OAP mentioned above is a necessary
101
part of the optical system as it is needed to collimate the multiplexed ultra-short pulses in the lowest order Gaussian mode capable of propagating megameter distances. The parabolic shape of the reflector is important as is creates the least amount of optical aberration and therefore the best beam quality. While a parabolic reflector is more difficult to manufacture than a spherical reflector, a spherical mirror will introduce large amounts of spherical aberration and will compromise beam quality during propagation. An adaptive optical feedback control system will be needed. This system can take advantage of the ultra-short pulse nature and use them at lower energies to probe the multiplexing system as a whole. The ultra-short pulses give the ability to probe the system to produce extremely fast and accurate data due to the speed and precision of the pulse. Using interferometric strategies, the temporal and spatial alignment of the optical switch with regards to the laser sources can be measured. The ultra-short pulses will be sufficiently fast to retrieve real-time accurate data about the switch. The timing and position of the momentum wheel optical switch must be verified before any higher power pulses are delivered to the transmitting optics. This optical verification system can test for any rotational precessions in the momentum wheel and prepare the pulses to be sent for any predictable variation patterns. Even with the advantages of using the pulses as testing probes, this is a technically demanding task which will require high response time sensors and adjustment controls. Another important set of optical components for the transmission system are the optics which add and remove chirp to the ultra-short pulses. Adding chirp to the pulses is a means of managing the high pulse energies so damage is not done to system's optical elements. A chirped pulse has an increased temporal profile which decreases the peak power of the pulse. Chirping pulses is common practice in chirped pulse amplification which is a method for further increasing ultra-short pulse energy. The chirping of the pulse can be achieved through use of optical gratings for spreading the frequency profile. Another method for consideration is that of using chirped or double chirped mirrors. Double chirped mirrors provide high reflectivity and are able to compensate for dispersion as the ultra-short pulse propagates. These mirrors can also be made to work over a broad spectral range [11]. There are multiple methods for managing the energetic pulses as they propagate throughout the transmission system. If a chirp is applied, the temporal width of the ultra-short pulses is made larger as it propagates through the transmission system so pulses do not carry significantly high enough instantaneous powers to cause damage to the optics in that state. The multiplexed pulses can then be expanded for propagation, further reducing the average intensity and minimizing the ablative potential during transmission. The receiving spacecraft can then remove the chirp making the pulses capable of ablation and thrust generation only once they have been delivered. If the receiving spacecraft does not have the capability to remove the chirp, the transmitting spacecraft can still deliver the pulses with care taken not to damage any of
102
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
off with the ejected material. This is due to the intense energy of the pulse breaking the atomic bonds on the surface of the material, which creates the ejected plume, and not being absorbed into the bulk of the material. Another advantage of the distributed ablative propulsion technique is the spreading of the ablation events over a large, or multiples areas at the receiving spacecraft. This distributed thrust delivery system concept is discussed in further detail below. 2.6. Control system and electrical requirements
Fig. 5. A geometrical optics ray-trace showing, as an example, a pair of off-axis reflectors acting as a zoom system for the focusing the multiplexed ultra-short pulses. The model was designed and optimized in Code V.
the components in the system. This can be done by keeping the beam width of the pulses sufficiently large as to not reach ablation intensities throughout the system. An optical zooming system (Fig. 5) is also suggested for any focusing adjustments that must be made for effective transmission and thrust generation by the ultra-short optical pulses. Focusing the multiplexed ultra-short pulses may be needed in scenarios such as providing thrust for micro-satellite systems or other receivers with different sized receiving optics on board. The last optic in the system must be the directing reflector. This will be needed for fine adjustments in pointing and tracking. This last reflector will be a flat as to not add any additional power and change the intended propagation behavior of the multiplexed light. The pointing precision required is discussed in further detail below. 2.5. Thermal management Thermal management concerns at the transmitting spacecraft will consist of the laser system and the thermal loads on the optics. Traditional heat sink and distribution methods can be used on both the master oscillator as well as the ultra-short pulse amplifiers. Controlling thermal issues on the optics themselves is an issue of pulse power, average power, and mirror materials for avoiding the optical damage threshold of the reflectors. The momentum wheel optical switch also helps with thermal management as each the multiplexing work is shared over “N” number of reflectors. This method avoids a single, small optic handling the combined intensities of the multiplexed pulses. There is also a cooling time for each reflector as it rotates around the wheel until it reflects the next series of pulses from its corresponding laser source. At the receiving spacecraft, thermal management will again involve not only minimizing heat on the optical elements but also handling the heat from ablative thrust. The heat produced from ablative thrust is mostly carried
In order for the transmission optics to make the adjustments determined by feedback from the probing pulses, a robust governing control system is needed. This control system's electronics must have sufficient processing power to make the needed calculations and drive the adjustment mechanisms quickly and accurately. An interesting new distributed control system is being developed by NASA's Glenn Research Center [12]. This distributed control strategy is being designed for use on a single, large combustion engine that aims to advance new, high temperature electronics as well as increase computational efficiency for the control system. The controls for the MUOPS ablation engine strategy will also need to be distributed, but on a larger scale. Instead of over an individual engine, the distribution will span across the transmitting and receiving spacecraft as well as within the individual spacecrafts themselves. A control system that is distributed within the spacecrafts can more efficiently drive the optical and mechanical devices needed to multiplex and guide the pulses for thrust control. Each spacecraft's control system must also take into account other's in order to create a position lock for transmission accuracy. 3. Satellite tracking 3.1. Positional tracking/lock Before the multiplexed pulses can be safely transmitted, a positional lock must be made between both the sending and receiving spacecrafts. A low power tracking beam can be sent out to verify the location of the satellites and align the necessary optical and electrical components to receive the incoming pulses. Early research suggested a retro-reflective beam lock strategy be used to achieve the positional lock. This technique is effective for short distances where the fast propagation time of the light between the spacecrafts is nearly negligible in regards to the time to establish a positional lock. For long transmission distances however, this is not the case. As an example, a transmission distance of one megameter will take light approximately three milliseconds to propagate. During that time, satellites in low-Earth orbit with velocities around 7500 m/s will travel roughly 25 m. The position locking beam will need to lead the receiving spacecraft by the necessary amount for successful transmission of the multiplexed energy for propulsion. This also means in the time it takes the transmitting spacecraft to send the weak positioning beam and receive positional data back, the
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
sending spacecraft itself will have moved on the order of tens of meters. As a solution we propose an intelligent beam lock system to account for propagation times of the energy and provide look-ahead pointing. In addition to the initial weak tracking beam, radio contact can be utilized to continually transmit location information between the cooperating satellites. With sensors on board the receiving spacecraft, the accuracy of the initial beam can be determined. The control system can then calculate the needed pointing corrections and transmitted them back to the sending spacecraft until a fully accurate position lock is formed. High precision in pointing accuracy is required as well. For a receiving optic with a radius on the order of a few meters, transmitting the multiplexed pulses over megameter long distances will require a minimum pointing accuracy on the order of microradians. 3.1.1. Safety This position tracking/locking system used between the sending and receiving satellites will be necessary to ensure the safe transmission of the energy. Much like cooperation is required for unlocking the ablative potential of the transmitted ultra-short pulses, cooperation and communication between the satellites is needed to establish a positional lock. Constant information must be openly transmitted to all parties involved in the energy transmission process. Additionally, all desired energy transmissions can be required to receive verification and allowance by an overseeing international organization much like the Laser Clearing House at Vandenberg Air Force Base does for NASA. 3.2. Receiving spacecraft The receiving spacecraft must have the necessary equipment to guide the incoming tracking beam into position lock as well as collect the transmitted energy sent for propulsion. Sensors on the receiving spacecraft can relate the relative accuracy back to the transmitting spacecraft which can then make the according adjustments depending on satellite position and velocity. Walking in the tracking beam to a successful position lock will be necessary before the higher power structured pulses can be accurately delivered. A similar optical system used to generate the multiplexed ultra-short pulses can be used in reverse to collect the transmitted energy and guide it to its own demultiplexing system. The transmitted energy must be directed to distribute the thrust due to ablation to the needed areas of the spacecraft. The chirping of the pulses also has to be considered at the receiving spacecraft. In order to generate ablative thrust with chirped pulses, the chirp must be removed at the receiving spacecraft to increase pulses' instantaneous power. However, as with the transmitting system, care must be taken to not cause ablation before the intended thrust is created. The optical system distributing the pulses must take into account chirp and beam width to avoid this problem.
103
As a means to distribute the transmitted energy, a demultiplexing momentum wheel device can again be used to raster the incoming pulses over a predetermined ablation area. If a more advanced distribution of the pulses is needed throughout the spacecraft, additional fast steering mirrors can be used to direct pulse trains to the desired location on the spacecraft. Separating the ablation events and the ablation areas throughout the spacecraft creates a larger area affected by the thrust generated which allows for more precise control of flight dynamics. As mentioned above, a distributed control system can be used to guide the pulses at the receiving spacecraft. 4. Additional applications and missions The proposed MUOPS strategy (Fig. 6) can be used for a variety of future forward looking missions in near-Earth space and beyond. One such possibility that the authors also propose is that of small orbital debris mitigation by laser ablation. The same multiplexing strategy involving the momentum wheel optical switch can be implemented for much shorter transmission distances (1–10 km). The multiplexed pulses can also be focused to cause ablation at the target for delivering a deorbiting force on the debris. Further information on this topic can be seen in Dr. Richard Fork's 2012 IAC paper [13]. Another possible new mission utilizing the proposed ultra-short pulse multiplexing strategy is that of horizontal lift to orbit (HLO). With the ability to generate and transmit useful levels of energy in near-Earth space, the same energy can be directed to assist spacecraft in HLO strategies. The pulse must be prepared differently for this application as the ascending spacecraft may have different or limited optics on board to generate thrust. Also, chromatic dispersion must also be considered as the pulses will have to travel through the upper levels of the atmosphere to reach the spacecraft. This can be accounted for by knowing the distance of atmosphere the pulses will be propagating through and applying the needed corrections to pulse shape, chirping, etc. for effective transmission. Further research on both of these possible missions is desired by the authors. Finally, new missions for sustained exploration of the asteroid belt and further into the solar system could be enhanced by the use of ablative propulsion distribution. For example, forward-looking missions to develop a more significant human presence in the solar system would benefit from the presence of an energy infrastructure capable of providing ultra-short pulse ablative propulsion to either manned or unmanned spacecraft. The laser-based energy infrastructure described by authors' paper at the 2011 IAC could be applied to the exo-planet Ceres in the asteroid belt (Fig. 7) [5]. This would provide a means of establishing renewable energy for long-term missions and experiments on Ceres such as research for asteroid characterization and mining. The smaller size of Ceres compared to Earth means shorter transmission distances and smaller transmitting optics are required. Also due to its smaller size, Ceres has a much reduced gravitational field strength, approximately 2.8% that of Earth's. This could allow for more energy efficient lift to orbit missions using
104
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
Faceted Momentum Wheel
Chirping Optics
Beam Directing
Variable Focus
Verify/Correct Beam Alignment Amplifiers and Delay Ultrashort Pulse Laser Lines
Monitor/ Control/ Clock
Tracking Beam and Monitor
Not to Scale
TRANSMITTING SPACECRAFT
Fig. 6. Basic layout of a possible system design showing important elements in the MUOPS strategy for creating an ablation propulsion engine.
Fig. 7. A simulation of the energy infrastructure around the exo-planet Ceres. The cones of light represent the possible delivery areas from the different satellites. This satellite configuration allows for energy transmission anywhere in the near-planet space of Ceres at anytime. Ceres and the white lines representing the orthogonal orbits are to scale, however the markers representing the satellites are enlarged for visibility.
ablative propulsion on Ceres as the power required to reach escape velocity will be reduced as well. Ceres also has little to no atmosphere to interfere with the transmitted pulses which will help maintain pulse shape, beam quality, and transmission efficiency. The MUOPS strategy can also be applied to other important applications such as near Earth object (NEO) deflection. Multiplexed ultrashort pulses can be used to precisely apply thrust to an NEO on a collision course with Earth in order to change its trajectory. One advantage of this application of the MUOPS strategy is that the NEO material is both the receiving target as well as the propellant. Because the NEO material is the propellant, the thrust vector from ablation can be controlled and adjusted depending on where the focused pulses are delivered on the surface of the NEO. Additionally, multiple areas can be illuminated on the NEO simultaneously as to
not only provide additional thrust, but also to steer the NEO in the desired direction. For example, three areas can be illuminated to balance the thrust with respect to NEO's center of mass and to minimize unwanted rotation of the NEO. In order to implement this technique, a system of microsatellites could be used to map the surface of the NEO as to determine the desired location to apply the pulses as well to reflect and focus the pulses on the NEO surface. Multiple, physically separate, but well-synchronized laser systems on the transmitting spacecraft can illuminate these areas of the NEO simultaneously. This free-space multiplexing of optical pulses in vacuum allows scalable power applicable to NEO deflection [3]. The NEO deflection strategy together with the ultrashort pulse multiplexing and transmission technique proposed above provide possible means of both long and short distance transmission of ultrashort optical pulses for propulsion in space.
5. International cooperation For a new and innovative effort such as this, cooperation between multiple nations and space organizations is needed. Sharing information on energy transmission strategies, scheduled transmission times and purposes are critically important for furthering energy transmission for in-space propulsion research and development. In addition to the sharing of information, the sharing of the work in developing this technology can also take place. The ultra-short pulse multiplexing strategy described above is cross-cutting and will require advancements in multiple disciples of engineering such as optical, electrical, aerospace, mechanical, and control systems. Forward-looking projects like this with potential to create new jobs in the space industry are always needed and provide exciting opportunities for furthering technological advancements.
G. Bergstue et al. / Acta Astronautica 96 (2014) 97–105
105
6. Current and future plans for research
References
The authors and students at the University of Alabama in Huntsville, both graduate and undergraduate alike, continue to pursue this research and development of the described MUOPS strategy for ablative propulsion. The authors and graduate students are currently involved in creating a more complete model and developing a control system to govern the necessary behavior of the different elements of the transmitting and receiving spacecrafts. Additional laboratory experiments and testing are desired by the authors. New optical elements, such as an OAP, have recently been acquired by the research team so that initial testing of the proposed multiplexing strategies can be done to further refine the understanding of the technique. Researchers and students at the University of Alabama in Huntsville are enthusiastic about this work and are well suited to help develop such strategies. The University of Alabama in Huntsville not only has experience with testing large optical systems, e.g. The James Webb Space Telescope, but also has the capabilities for precision manufacturing of the optical reflectors needed for the effort [14].
[1] T. Eidam, S. Hanf, E. Seise, T.V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, A. Tünnermann, Femtosecond fiber CPA system emitting 830 W average output power, Opt. Lett. 35 (2010) 94–96. [2] P. Russbueldt, T. Mans, J. Weitenberg, H.D. Hoffmann, R. Poprawe, Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier, Opt. Lett. 35 (2010) 4169–4171. [3] R. Fork, S. Cole, L. Burgess, G. Bergstue, Mode-locked Lasers Applied to Deflecting a Near Earth Object on Collision Course with Earth, 〈arXiv:1309.3238〉 [physics.space-ph]. [4] United States, National Aeronautics and Space Administration, NASA Space Technology Roadmaps and Priorities, 2012, 〈www.nap.edu〉. [5] G. Bergstue, R. Fork, Beamed energy for abalative propulsion in near Earth space, in: Proceedings of the 62nd International Astronautical Congress IAC-11-C4.8.2, Cape Town, South Africa, 2011. [6] Anthony E. Siegman, Lasers, University Science Books, Mill Valley, CA, 1986 (Print). [7] S. Amoruso, R. Bruzzese, X. Wang, J. Xia, Propagation of a femtosecond pulsed laser ablation plume into a background atmosphere, Appl. Phys. Lett. 92 (2008) 041503. [8] X. Wang, S. Amoruso, J. Xia, Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation, Appl. Surf. Sci. 225 (2009) 5211–5214. [9] S.M. Redmond, D.J. Ripin, C.X. Yu, S.J. Augst, T.Y. Fan, P.A. Thielen, J.E. Rothenberg, G.D. Goodno, Diffractive coherent combing of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam, Opt. Lett. 37 (2012) 2832–2834. [10] A. Giesen, J. Speiser, High-power thin disk lasers, Proc. SPIE 8547 (1– 8) (2012). [11] F.X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H.A. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch, T. Tschudi, Design and fabrication of double-chirped mirrors, Opt. Lett. 22 (1997) 831–833. [12] D. Cully, Tools and technology development for distributed engine control, in: Propulsion Control and Diagnostics Workshop, February 28, 2012, 〈http://www.grc.nasa.gov/WWW/cdtb/aboutus/workshop2012/ Presentations/Session%203.%20Distributed%20Engine%20Control/ DEC_01_Culley.pdf〉. [13] R. Fork, J. Rogers, L. Burgess, G. Bergstue, Active removal of small orbital debris using laser systems in space, in: Proceedings of the 63rd International Astronautical Congress IAC-12-A6.7.6, Naples, Italy, 2012. [14] D.M. Chaney, R.J. Brown, S.E. Kendrick, P.J. Reardon, J.B. Hadaway, J. Carpenter, R. Eng, Results of the Beryllium AMSD Mirror Cryogenic Optical Testing, Proceedings of SPIE, Optical, Infrared, and Millimeter Space Telescopes, 5487 (2004) 833. http://dx.doi.org/10.1117/12.549956.
Acknowledgments The authors would like to thank the University of Alabama in Huntsville's Office of the Vice President for Research, Lockheed Martin, and Lufthansa for supporting the trip to present this research at the 2012 IAC. Research funding was provided by NASA's Marshall Space Flight Center under grant number NNM11AA01A Supplement 5 from 08/17/2011 to 12/31/2011. The authors would also like to thank Mr. Luke Burgess, Mr. Ken Pitalo, Mr. David Pollock, and Mr. Arthur Palosz for valuable conversations pertaining to the development of this research.