Post by execute13 on Aug 29, 2017 5:39:41 GMT
The two most important characteristics of a laser are its wavelength and its efficiency. The former determines the size of the aperture needed for a given performance. The latter determines the power required for a given output, thus the reactor power, and so the weight, cost, and cross-section of the reactors and radiators required.
This concept's goal is to massively improve efficiency.
A gas laser (likely IBr) is contained by a transparent envelope, surrounded by a cylindrical blackbody radiator, surrounded by a reflective insulator. The laser envelope is doped with up-converting particles and fluorescent particles that convert a wide set of bands in the blackbody spectrum to light in the pumping band of the laser. The output path of the laser includes an angled dichroic mirror that reflects thermal emissions from the laser back to the blackbody.
The only way energy will leave the system is as laser output, plus extremely small thermal leakage through the interfaces. The efficiency, then, is very close to 100% as any lost energy turns back into heat, which is what is powering the laser anyway. The blackbody, if at 2050 K, emits 1 MW per square meter of interior surface area. The main design puzzle is to efficiently convert this thermal radiation into the laser pumping wavelengths.
THE LASER MEDIUM
Iodine-Bromine photodissociation lasers, developed for direct solar-pumped lasers have a massive 150nm wide visible-light absorption bandwidth. They can achieve >10% efficiencies while being pumped directly from sunlight; that's how broad the pumping band is.
PHOTON UPCONVERSION
Semiconductor nanoparticles (Quantum dots) can upconvert from near infrared to visible light by absorbing two photons, then emitting one photon. This is reminiscent of frequency doubling crystals, but the process does not need a high intensity to operate efficiently. This component is crucial for having a high power output for the laser, as the thermal radiation pumping the laser is limited in power by the size of the laser and the safe operating temperatures of the components. Multiple sizes of quantum dots are used to upconvert multiple overlapping spectra into the pumping band.
There are a lot of different kinds of quantum dots, and I’ve yet to find anything prohibiting liquids from acting as quantum dots. These would likely be nanometer-scale liquid droplets embedded in a fused quartz, diamond or corundum envelope. If they still can’t work hot enough, there are other upconversion mechanisms available.
One option: Lanthanide-doped nanoparticles with wonderful names like “NaYF4:Yb/Tm”. A safe use temperature of 970K is the figure I found, but that may be the answer to a slightly wrong question.
Another option is, surprise surprise, carbon nanotubes, which can exhibit photon-phonon (yes, phonon) upconversion where thermal energy powers the frequency shift. The effect is greater at a higher temperature I have no idea of its safe operating temperatures, though.
Ordinary fluorescent particles convert the small amount of radiation that’s shorter wavelength than the pumping band back to the pumping band. This includes any radiation upconverted by a doubling mechanism like two-photon absorption from an original wavelength that’s not a subharmonic of the pumping band.
BLACKBODY PUMP
The blackbody will be, effectively, a cylindrical radiator with a high-emissivity coating on the inside and a low-emissivity surface on the outside. Standard radiator optimisation applies. The heat transferred to this radiator by the coolant circulating inside it is the power supplied to the laser from a heat source like a nuclear reactor. Electricity can be cogenerated if the reactor temperature is higher than the laser’s operating temperature.
To maximise efficiency, the outside of the blackbody pump is surrounded by a reflector that returns virtually all the outward-emitted light back to the blackbody. This can be layers of kapton-dacron-aluminized mylar laminate like used in multi-layer insulation for spacecraft today, or something more elaborate like aluminium films sandwiched between silica aerogel layers.
Either way, the mass of the reflector is approximately zero.
In story terms, this design is an excuse for the actual laser element to be very large, as the power available is limited by the size of the lasing medium. Ideally, the laser will be very long, to maximize surface area without decreasing beam quality.
Any thoughts?
References:
en.wikipedia.org/wiki/Photon_upconversion
sci-hub.cc/10.1038/nature13883
www.nature.com/articles/ncomms9920
link.springer.com/chapter/10.1007/978-3-642-71859-5_73 (I actually have this book available, if anyone wants me to look for something in it)
link.springer.com/article/10.1134/S0036023608110028
sci-hub.cc/10.1021/mz500803w
en.wikipedia.org/wiki/Multi-layer_insulation
This concept's goal is to massively improve efficiency.
A gas laser (likely IBr) is contained by a transparent envelope, surrounded by a cylindrical blackbody radiator, surrounded by a reflective insulator. The laser envelope is doped with up-converting particles and fluorescent particles that convert a wide set of bands in the blackbody spectrum to light in the pumping band of the laser. The output path of the laser includes an angled dichroic mirror that reflects thermal emissions from the laser back to the blackbody.
The only way energy will leave the system is as laser output, plus extremely small thermal leakage through the interfaces. The efficiency, then, is very close to 100% as any lost energy turns back into heat, which is what is powering the laser anyway. The blackbody, if at 2050 K, emits 1 MW per square meter of interior surface area. The main design puzzle is to efficiently convert this thermal radiation into the laser pumping wavelengths.
THE LASER MEDIUM
Iodine-Bromine photodissociation lasers, developed for direct solar-pumped lasers have a massive 150nm wide visible-light absorption bandwidth. They can achieve >10% efficiencies while being pumped directly from sunlight; that's how broad the pumping band is.
PHOTON UPCONVERSION
Semiconductor nanoparticles (Quantum dots) can upconvert from near infrared to visible light by absorbing two photons, then emitting one photon. This is reminiscent of frequency doubling crystals, but the process does not need a high intensity to operate efficiently. This component is crucial for having a high power output for the laser, as the thermal radiation pumping the laser is limited in power by the size of the laser and the safe operating temperatures of the components. Multiple sizes of quantum dots are used to upconvert multiple overlapping spectra into the pumping band.
There are a lot of different kinds of quantum dots, and I’ve yet to find anything prohibiting liquids from acting as quantum dots. These would likely be nanometer-scale liquid droplets embedded in a fused quartz, diamond or corundum envelope. If they still can’t work hot enough, there are other upconversion mechanisms available.
One option: Lanthanide-doped nanoparticles with wonderful names like “NaYF4:Yb/Tm”. A safe use temperature of 970K is the figure I found, but that may be the answer to a slightly wrong question.
Another option is, surprise surprise, carbon nanotubes, which can exhibit photon-phonon (yes, phonon) upconversion where thermal energy powers the frequency shift. The effect is greater at a higher temperature I have no idea of its safe operating temperatures, though.
Ordinary fluorescent particles convert the small amount of radiation that’s shorter wavelength than the pumping band back to the pumping band. This includes any radiation upconverted by a doubling mechanism like two-photon absorption from an original wavelength that’s not a subharmonic of the pumping band.
BLACKBODY PUMP
The blackbody will be, effectively, a cylindrical radiator with a high-emissivity coating on the inside and a low-emissivity surface on the outside. Standard radiator optimisation applies. The heat transferred to this radiator by the coolant circulating inside it is the power supplied to the laser from a heat source like a nuclear reactor. Electricity can be cogenerated if the reactor temperature is higher than the laser’s operating temperature.
To maximise efficiency, the outside of the blackbody pump is surrounded by a reflector that returns virtually all the outward-emitted light back to the blackbody. This can be layers of kapton-dacron-aluminized mylar laminate like used in multi-layer insulation for spacecraft today, or something more elaborate like aluminium films sandwiched between silica aerogel layers.
Either way, the mass of the reflector is approximately zero.
In story terms, this design is an excuse for the actual laser element to be very large, as the power available is limited by the size of the lasing medium. Ideally, the laser will be very long, to maximize surface area without decreasing beam quality.
Any thoughts?
References:
en.wikipedia.org/wiki/Photon_upconversion
sci-hub.cc/10.1038/nature13883
www.nature.com/articles/ncomms9920
link.springer.com/chapter/10.1007/978-3-642-71859-5_73 (I actually have this book available, if anyone wants me to look for something in it)
link.springer.com/article/10.1134/S0036023608110028
sci-hub.cc/10.1021/mz500803w
en.wikipedia.org/wiki/Multi-layer_insulation
