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Photonic laser thruster
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A photonic laser thruster (PLT) is an amplified photonic propulsion thruster for space propulsion that works on the principle of a photon-pushed sail, generating thrust directly from the momentum of a photon from a laser reflected from a mirror. The thruster, invented by Young K. Bae differs from other solar sail and laser propulsion thrusters in that an amplification process is used, in which the incident beam is re-used by being reflected by a stationary mirror, with an amplification stage at each reflection. Because of the recycling of energy, the photonic laser thruster has been proposed to be more energy efficient than other laser-pushed sail concepts. Possible uses of the photonic laser thruster for earth-orbit applications include propellant-free and thrust-plume-contamination-free spacecraft maneuvering for precision formation flying, large optical and RF synthetic aperture construction, and stationkeeping. The usage of the photonic laser thruster for main space propulsion would require scaling-up of the laser power and controlling laser diffraction over interplanetary and interstellar distances. Photonic laser thrusters have a very high specific impulse, and in principle could permit spacecraft to reach much higher speeds that approach a fraction of the light speed, unlike conventional rockets, which are limited by the Tsiolkovsky rocket equation. History Background The use of light for propulsion has been researched since the beginning of the 20th century, with the analysis of a sail pushed by the pressure of sunlight by Friedrich Tsander. Photon propulsion has been discussed for decades as a propulsion that could enable interstellar flight. In the traditional photonic propulsion, such as laser- or microwave-pushed lightsails, photons transfer their momentum to the sail by reflection. Since, for a sail moving slowly with respect to the speed of light, very little of the energy of the photon is lost on reflection, a theoretical way to increase the efficiency is by recycling photons, bouncing the reflected photons back to the sail by a mirror, Specifically, Forward introduced beamed laser propulsion, aiming at the goal of achieving roundtrip manned interstellar travel. Recycling photon propulsion Marx, Redding and Simmons and McInnes calculated that the energy conversion efficiency of terrestrial laser-driven propulsion is approximately proportional to v/c at low speeds (v<0.1c), thus is small at low speeds (v<<0.1c). However, at higher speeds (v>0.1c), owing to the favorable Doppler shift energy transfer, onboard photon propulsion becomes much more energy efficient. Photons transfer their energy to the spacecraft by redshifting due to Doppler shift upon reflection, thus the higher the spacecraft speed, the higher the efficiency. The figure shows the energy transfer efficiency from photons to the spacecraft's kinetic energy as a function of β=v/c (the spacecraft velocity divided by the light velocity) in photon propulsion. As the spacecraft velocity approaches the light velocity (v≈c), the efficiency of photon propulsion approaches 100%, as if the spacecraft acts like a black hole in the moving direction. proposed a multi-bounce lightsail craft, such that the beam is reflected back and forth between the lightsail and a source reflector. Advanced reflectors permit more than 1000 bounces, reducing power requirements by 1000× compared to single bounce proposals. Using 100 MW to 1 GW lasers, a sub-100 day Mars transit is possible. who obtained amplified photon thrust of ~0.4 µN with a 300-W laser and a photon thrust amplification factor of ~2.6. in which even one nanometer perturbation in cavity length destroys the resonance and nulls the photon thrust. The injection of laser power into the cavity remains challenging. Meyer et al. Photonic laser thruster After 2000 Bae began to investigate photon recycling for use in a nanometer accuracy formation flight, for a NASA-NIAC project called Photon Tether Formation Flight (PTFF). The goal was to sustain fixed-formation flight with a baseline distance between craft of over 10 km, for next generation NASA space missions. In 2006 Bae investigated active resonant optical cavities, in which the optical gain medium is located within the cavity, and coined the term "photonic laser thruster" (PLT) for such thrusters. ~ 0.06% of the wavelength. For an order of magnitude estimation, it can be assumed that a thruster utilizing the YAG laser system will be limited by the gain bandwidth to the first order, i.e., its theoretical maximum spacecraft velocity is ~ (180 km/s), 0.06% of the light velocity, c3x10 m/s. To overcome this redshift limitation, at high operation velocities, wide bandwidth lasers should be used. Lasers are often designed to maximize power outside the laser cavity, while PLT calls for maximizing laser power inside the cavity. A schematic diagram of this design is shown in Figure 8. A thin disk gain medium is coated with HR with a reflectivity of up to 99.999% and attached to a heat sink. The recycling photons between the gain medium and the HR mirror located in the mission platform deliver amplified thrust beaming from the resource platform. In 2014, Bae's group, working under a NASA program (NIAC, NASA Innovative Advanced Concepts), demonstrated intracavity power of 154 kW with a 0.6 cm diameter thin disk laser, which can be translated into a photon thrust of 1.03 mN. In April 2015, the group successfully measured photon thrust up to 1.1 mN with a digital scale. They were able to accelerate, slow and stop a 0.45 kg spacecraft-simulating platform along a 2m frictionless air track in a Class-1,000 cleanroom. In August 2015, under the NASA program the group successfully propelled a Cube satellite, and demonstrated and measured photon thrust up to 3.5 mN with the use of the newly developed NIST/Scientech/Navy radiation pressure sensor. An intracavity power of the photonic laser thruster over 500 kW was demonstrated with 500 W laser pumping. demonstrated a 1 km-long laser resonator similar to the PLT cavity, which is an active cavity, in 1995 and proposed that such resonators could scale to 100 km. Recently, 4-km Fabry-Pérot cavities, which are passive cavities, but share the same intracavity power multiplication principle with the PLT cavity, have been demonstrated in LIGO for gravitational wave detection with an intracavity multiplication factor of 280 and an intracavity laser power on the order of 100 kW. Based on these results and the state-of-the-art technologies in precision optics, the PLT cavity length over 1,000 km is promising. Further studies need to be performed to determine whether the PLT cavity could be scaled for astronomical distances. Such a problem can be solved by installing another photonic laser thruster at the destination as in photonic railway. Applications Orbit tuning PLTs are studied for maneuvering spacecraft in near earth orbit, propellantless operation, thrust and power beaming for "perpetual" stationkeeping, and ultra-precision spacecraft formation flying, with or without tethers. a group of spacecraft that can exploit relative positions and velocities so that differential gravity provides a force opposite to the photon thrust from a PLT. In such a scheme, with two orbiting platforms moving in formation, when the photon thrust changes, their positions relative to the center of mass change as well, until the "virtual gravitational tug" counterbalances it again.==== If the photonic laser thruster is scalable for the use in such main space propulsion, multiple photonic laser thrusters can be used to construct a photonic railway that has been proposed as a potential permanent transport infrastructure for interplanetary or interstellar commutes, allowing the transport craft themselves to carry very little or no fuel. The photonic railway Bae's investigation concluded that the development of interplanetary and interstellar photonic railway will require development of the ways to utilize Photon Bose Einstein Condensation or diffraction-minimized laser beam propagation with non-diffracting beams, such as Bessel beam, or x-ray lasers and future advanced material science and technologies. Bae further concluded that the realization of the interstellar photonic railway would require that the PLT technology developments ride on the Moore’s law as the 20th century silicon devices did. See Also
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