Beam-powered propulsion - keeping the beam focused

[schplade]

I've often read that beam-powered propulsion is the only basic interstellar propulsion concept without physics problems. To me, that doesn't seem far from the truth. However, for a long time I've felt that the biggest obstacle to overcome is the beam divergence problem. Most of the concepts I've read about use a single laser, or a laser array stationed at a single location. It seems to me that - unless your spacecraft can withstand enormous G-forces - you're going to need an acceleration path that's billions of kilometers long if you hope to reach 0.1 c or greater. Could you ever have that with a single laser? I've imagined that you would need many powerful space-based lasers along the acceleration path, but that would obviously introduce a lot of complication.

How small of a laser "spot size" could you realistically hope to achieve over a distance of a billion kilometers? Is there a theoretical best that you could shoot for?

 

[Simon Bridge]

I don't know what you mean by "without physics problems".

The idea is to make the laser tightly collimated rather than dynamically focussing it. ("focussing" usually refers to producing a clear image, or, in this context, making the beam converge to a waist at some specific location.)
The degree of collimation depends on things like the length of the laser cavity compared to it's diameter ... so you can, in principle, make it as collimated as you like. iirc the current best lasers divert from parallel to order of milliradians.

If you don't insist on a laser source - you can use sychrotron radiation for your beam.

Now - you wouldn't want to fly through the beam that is boosting something else...

 

[sophiecentaur]

How small of a laser "spot size" could you realistically hope to achieve over a distance of a billion kilometers? Is there a theoretical best that you could shoot for?

Why would you need such a good focus? A bit of beam spread would only produce a small amount of lateral momentum transfer (cancelling out for a symmetrical beam). It would only correspond to the Sin of the angle of spread, the main beam would only reduce as the Cos of the angle. Overall efficiency would presumably depend, not only on the beam spread but also on the efficiency of the projecting system. I have a feeling that would be lower for a narrow beam. Lasers are not that efficient, so they may not be the best way to produce your driving beam. This sort of Engineering has many facets.

 

[Filip Larsen]

If the beam can be modeled as a static, ideal Gaussian beam, then it seems that beam divergence [1] can be used to give estimate for the minimum expected reduction in beam power (equaling reduction in reflective force) as a function of range for a given beam waist (which relate to reflector size) and beam wavelength.

For instance, using microwaves λ ≈ 10^-2m and a waist of 100 km² the power received at the reflector is reduced to around 87% at 10 million km and around 14% at 1 AU relative to the power received when the reflector is in the waist [2].

[Edit: fixed wrong calculation, reworded example]

[1] https://en.wikipedia.org/wiki/Gaussian_beam#Beam_divergence
[2] https://en.wikipedia.org/wiki/Gaussian_beam#Power_through_an_aperture

 

[mfb]

Why would you need such a good focus?

You have to hit the spacecraft with as much power as possible. Over large distances the beam is wider than the spacecraft , so power received by the spacecraft is directly linked to the focusing quality.

iirc the current best lasers divert from parallel to order of milliradians.

A laser pointer achieves that (1 mm spread over 1 m distance = 1 millirad). Good lasers are diffraction limited, together with a telescope you can get microradians.

 

[sophiecentaur]

Ahh. I was assuming that the propulsion was by reaction from an on-board source. Would that not be better at great distances? The amount of wasted (spilled) energy, despite the apparent high quality of a beam would be enormous at a distance of a few AUs. Beam steering could also be a problem.
I guess there could be no real limit on the diameter of the receiving reflector, though. (many km, once deployed)

 

[mfb]

An on-board beam source needs an on-board power source, which makes the spacecraft heavier. Mass and energy are always limiting factors.

 

[sophiecentaur]

An on-board beam source needs an on-board power source, which makes the spacecraft heavier. Mass and energy are always limiting factors.

Yes. I realize that but there must be a limit to the range that a passive system can be expected to work. Beyond that range, an on board propulsion system would be essential. What sort of diameter would be reasonable for the sail, I wonder. It could be very flimsy in the benign conditions of deep space.
Also, when the craft gets to its destination, it cannot use a propelling beam to get it to manoeuvre to the intended destination - or even just to slow down. I suggest a hybrid system would probably be optimal.

 

[Filip Larsen]

Also, when the craft gets to its destination, it cannot use a propelling beam to get it to manoeuvre to the intended destination - or even just to slow down.

There exists light sail propulsion designs [1] that allows breaking maneuvers. Of course, the engineering challenges for such systems are presumably orders higher than for a simple "acceleration-only" light sail, but should still be within what is physical possible.

[1] Roundtrip Interstellar Travel Using Laser-Pushed Lightsails, R. L. Forward, Journal of Spacecraft and Rockets, Vol. 21, No. 2 (1984), pp. 187-195, http://www.lunarsail.com/LightSail/rit-1.pdf

 

[sophiecentaur]

There exists light sail propulsion designs [1] that allows breaking maneuvers.

That's a clever idea. Just a few practical problems to overcome, though! I would think that the propulsion source would be better placed away from the Earth but then there would be the problem of getting power to it. But all this stuff sort of assumes the existence of fusion power so that problem would not be so great.
The low efficiency of lasers is surely relevant here. Using lower frequencies would make it possible to use more efficient sources, would it not? Achievable beam width would be traded off against efficiency.

 

[Cutter Ketch]

Beam quality for laser beams is essentially the same as the diffraction limit for optics. The minimum possible beam divergence is inversely proportional to the aperture size.

Half angle >= wavelength / (pi * beam radius at waist)

However you don't have to completely fill the aperture to get close to the diffraction limit. A few small apertures around the edges of a large virtual aperture can achieve nearly the same divergence if the beams are coherent. A visible laser with an Earth diameter sized virtual aperture could achieve a 2e-14 divergence and so increase diameter by less than 1000x at Alpha Centauri.

 

[GTOM]

You have to hit the spacecraft with as much power as possible. Over large distances the beam is wider than the spacecraft , so power received by the spacecraft is directly linked to the focusing quality.A laser pointer achieves that (1 mm spread over 1 m distance = 1 millirad). Good lasers are diffraction limited, together with a telescope you can get microradians.

Could it become much better?
http://iopscience.iop.org/article/10.1088/1742-6596/425/5/052034
It talked about a nanoradian, but probably i missed something.

 

[sophiecentaur]

I would appreciate some (informed and numerical) comment about my comments about the efficiency of lasers compared with alternative sources. There seems to be a lot of information but I can't seem to find a source that will say what efficiency you could expect for a continuously running laser with many kW output - at any frequency. The 'wall plug' efficiency and the mean power are both very important factors in a project like this. We do know that, already, there are microwave sources that can produce several kW of continuous power. This link indicates that 750kW of continuous power can be achieved from a 'Gyroklystron' and, at 300kW, the efficiency is around 32%. Is there any laser source with the equivalent power? The disadvantage of a microwave source is the need for an eye wateringly large aperture for the beamwidths needed but the limit to achievable aperture would be enormous. The beauty of microwave devices is that they can be used in synchronised sets, to achieve as much power as you want.
I know that lasers are flavour of the month (decade) but the alternatives are always there. I wondered about a massive fresnel lens, used to direct the Sun's radiation in a useful direction. It would, I guess, need to be held on station somehow to avoid it being blown away by some of the same radiation pressure that it is directing.

 

[mfb]

Could it become much better?
http://iopscience.iop.org/article/10.1088/1742-6596/425/5/052034
It talked about a nanoradian, but probably i missed something.

That is the precision of the central position, the beam width doesn't seem to be given anywhere. Diffraction limits it to more than a nanoradian for realistic optical element sizes. X-ray lasers are not very efficient, however.

The wall plug efficiency of CO2 lasers can be as high as http://www.sematech.org/meetings/archives/litho/7739/presentations/23%20High%20Pwr%20CO2%20Lasers-Soumagne.pdf, fiber lasers reach http://www.messer-cs.com/de/us/processes/laser-cutting/vertical-cutting-laser/faser-laser/ but with a worse beam quality.

 

[sophiecentaur]

That is the precision of the central position, the beam width doesn't seem to be given anywhere. Diffraction limits it to more than a nanoradian for realistic optical element sizes. X-ray lasers are not very efficient, however.

The wall plug efficiency of CO2 lasers can be as high as http://www.sematech.org/meetings/archives/litho/7739/presentations/23%20High%20Pwr%20CO2%20Lasers-Soumagne.pdf, fiber lasers reach http://www.messer-cs.com/de/us/processes/laser-cutting/vertical-cutting-laser/faser-laser/ but with a worse beam quality.

What mean Power would a CO₂ laser be capable of?

 

[mfb]

Depends on the application. I have seen lasers up to tens of kW, I don't know about applications even higher powers could have.

 

[sophiecentaur]

Is that a continuous Power rating or short burst?

 

[mfb]

Average power of course. Peak power would be irrelevant.

 

[sophiecentaur]

I was just checking that meant continuous for a long time. The project would need to be powered for a long time (years) and without interrupted service. I guess a hot reserve would be called fur, whatever system were used.

 

[mfb]

Short breaks shouldn't harm, especially if with the current laser sizes, as you need a huge number of lasers in parallel anyway.

 

[GTOM]

That is the precision of the central position, the beam width doesn't seem to be given anywhere. Diffraction limits it to more than a nanoradian for realistic optical element sizes. X-ray lasers are not very efficient, however.

The wall plug efficiency of CO2 lasers can be as high as http://www.sematech.org/meetings/archives/litho/7739/presentations/23%20High%20Pwr%20CO2%20Lasers-Soumagne.pdf, fiber lasers reach http://www.messer-cs.com/de/us/processes/laser-cutting/vertical-cutting-laser/faser-laser/ but with a worse beam quality.

Thanks, but i would like to make other things clear.

https://en.wikipedia.org/wiki/Angul...tion_limit_diameter_vs_angular_resolution.svg

According to this one, angular resolution of miliarcsec is reachable. That is closer to nrad than microrad. Could we focus the laser this precisely if we know exact distance of rocket?

How inefficient are xasers? I read that an X-Fel needs to accelerate electrons to 0.998c. Does that mean, that due to relativistic mass, you get J output (beam power) from KJ input?

 

[mfb]

In principle you can use a telescope in reverse, but they are designed for low power (starlight) - making them efficient reflectors of high-powered laser beams might need some adjustments. On the positive side, you have a single wavelength, so the mirrors can be made much more reflective.

Electron acceleration is quite efficient, not sure about the conversion process afterwards. This article claims that 10% might be possible.

 

[schplade]

In this paper James Benford argues that microwaves are the most cost-effective for propelling a sailcraft, although they might be at a disadvantage for missions that require long acceleration times.
https://arxiv.org/ftp/arxiv/papers/1112/1112.3016.pdf

I guess this shows you where I got the "without physics problems" phrase from. He's definitely not the only physicist that describes it that way though. It's an oversimplification, but I can see some validity in it. Beamed energy propulsion allows you to leave the fuel behind.

With the speeds required for interstellar missions, carrying the fuel along in the spacecraft just isn't a good option. The propellant mass ratio becomes such a crippling problem that the idea is almost dead on arrival. Antimatter might offer the energy density you need, but that presents its own massive engineering challenges. At least, that's the impression I've had so far from reading about it.

 

[Cutter Ketch]

I would appreciate some (informed and numerical) comment about my comments about the efficiency of lasers compared with alternative sources. There seems to be a lot of information but I can't seem to find a source that will say what efficiency you could expect for a continuously running laser with many kW output - at any frequency. The 'wall plug' efficiency and the mean power are both very important factors in a project like this. We do know that, already, there are microwave sources that can produce several kW of continuous power. This link indicates that 750kW of continuous power can be achieved from a 'Gyroklystron' and, at 300kW, the efficiency is around 32%. Is there any laser source with the equivalent power? The disadvantage of a microwave source is the need for an eye wateringly large aperture for the beamwidths needed but the limit to achievable aperture would be enormous. The beauty of microwave devices is that they can be used in synchronised sets, to achieve as much power as you want.
I know that lasers are flavour of the month (decade) but the alternatives are always there. I wondered about a massive fresnel lens, used to direct the Sun's radiation in a useful direction. It would, I guess, need to be held on station somehow to avoid it being blown away by some of the same radiation pressure that it is directing.

Different lasers different answers, but they can be more efficient than you might think. Another issue in trying to find the information is that there are several stages to the efficiency and people will hide inefficiency by only quoting part of the efficiency.

Wall plug efficiency: the whole enchilada

But that includes the inefficiency of the electronics: power conversion, pulse forming, unavoidable poor efficiency when the current is high and the voltage is low. So to avoid talking about that inefficiency you will see

Electrical to optical efficiency: the optical power out over the electrical power delivered to the pump head.

But that includes the inefficiency of the first step. Particularly with diode lasers which are 10 - 60% efficient depending on the pump wavelength, that can be a big step. So to avoid talking about that people publish

Optical to optical efficiency: the optical power out divided by the optical power delivered to the pump head.

But that includes the energy used creating a population inversion and getting to threshold. Particularly in three level laser systems especially when they are pulses that can be a very large inefficiency, so people talk about

Slope efficiency: optical energy out divided by the optical energy delivered to the pump head over and above the threshold energy

Universally everybody ignores the often very large amount of power it takes to run cooling systems, which can often dominate the power consumption.

I know you didn't ask about any of that, but I just wanted to highlight why it can be tough to get a straight answer. The easiest way to get a real answer is not by looking at articles, but rather by looking at spec sheets of lasers you can buy.

Now for some answers.

Diode lasers around 800nm and in the 9xx range typically have > 50% electrical to optical. It varies with temperature and they can be 65% efficient in Alaska in the dead of winter and fall to 50% in Death Valley in summer. At room temperature running CW with a good power supply you can get close to 50% wall plug efficiency. Diode lasers at other wavelength vary in efficiency (different materials). 25% around 1.5um. 10-15% for blue, etc. One off champion results have achieved > 70%, but you can't actually buy that. The efficiency of diodes make them the perfect first step for other lasers.

Diode pumped solid state lasers (DPSSL) are pretty good at converting the diode light. Nd:YAG can achieve 30-40% O-O and so 15-20% E-O. For high power this often involves very high current circuits which are hard to make brilliantly efficient so the wall plug efficiency might only be 10. However that is for a single resonator. Amplifier stages can be more efficient and if you work hard you can push that up. Northrop Grumman's 100kW class directed energy weapon achieved 17% wall plug efficiency (not including the substantial and power hungry cooling system) with diffraction limited beam quality.

Gas lasers like CO2 can also achieve about 20% wall plug efficiency.

So for futuristic beam riding discussions 20% is a safe starting point, and you can imagine figuring out how to scale the diode laser efficiency up to high power and top 50%

On the other hand, for a beam rider who cares how efficient the ground laser is?

 

[GTOM]

In principle you can use a telescope in reverse, but they are designed for low power (starlight) - making them efficient reflectors of high-powered laser beams might need some adjustments. On the positive side, you have a single wavelength, so the mirrors can be made much more reflective.

Electron acceleration is quite efficient, not sure about the conversion process afterwards. This article claims that 10% might be possible.

Also X-Rays can be only focused efficiently with diffraction with perfect lenses. Which gives me the speculation, that if we want to reach Alpha Centaury, first send a lots of lenses, that keep refocusing the beam.

 

[sophiecentaur]

Any propulsion beam will need to be generated in a spacecraft and the Energy requirement will be another massive problem to deal with. A football field (or greater) size PV array would be called for and the efficiency of that would also be relevant. Hence my question / comment about using sunlight, direct, with a focussing mirror or lens to produce the beam. There would be a danger of such a beam, however generated, slicing its way through anyone who strayed into it. It would need to be marked on the 'charts' just like bridges, cables and wrecks on marine charts. Any source would also be occluded occasionally by other orbiting bodies and that would need to be dealt with.
It's so easy to get drawn into these sorts of projects without considering all sides of the problem. It reminds me of the Space Elevator idea, which is also very attractive - but also poses some possibly unsolvable snags.

 

[mfb]

You cannot focus sunlight in any realistic way. Its angular spread is just too large.

Any source would also be occluded occasionally by other orbiting bodies and that would need to be dealt with.

Completely negligible.
For an Earth-based beam, you would certainly check if there are crossing satellite orbits, but even that is unlikely.

 

[sophiecentaur]

You cannot focus sunlight in any realistic way. Its angular spread is just too large.
Completely negligible.
For an Earth-based beam, you would certainly check if there are crossing satellite orbits, but even that is unlikely.

What do you mean by "angular spread"? If it were a distant Point Source then it could be focussed on an Airy Disc, with angular spread according to the aperture. I agree that some sort of Relay Optics would be needed to achieve this for a large source like the Sun. Overall efficiency would not suffer from two energy conversion processes that an electrical device would involve.
The Earth is spinning so could the source be steered in a practical way? ( mirrors, perhaps) Also, you would need a number of sources to avoid a long daily break in transmission. How could you keep the relevant bit of sky clear at all times?
Have you ever observed the number of satellites up there on a clear night? It's like Piccadilly Circus up there and they are mostly in trans polar orbits, covering the whole of the sky, eventually and they are not all recorded and tracked. There is loads of junk up there which would be dangerously reflective, too. You could never eliminate the risk of damage to orbiting objects or to Earth by reflection. This beam would not be pencil thin, but metres wide (same requirements as for a Very Large Telescope).

 

[mfb]

The sun is not a distant point source, it has an angular diameter of about half a degree at 1 AU, and correspondingly your beam has the same minimal spread. Sunlight reflected by a 100 meter mirror will diverge notably after 10 kilometers. Do you want to place another huge mirror every 10 kilometers? Going to 2 AU reduces the spread by a factor 2, but then you need four times the mirror size for the same power. The same scaling applies to every other distance.

 

[schplade]

I used the Gaussian equation for evolving beam width to figure out how wide the beam would need to be for a given distance and wavelength.
https://en.wikipedia.org/wiki/Gaussian_beam#Evolving_beam_width

Hopefully I'm not misusing the equation. All of this assumes an ideal Gaussian beam.

For example, if you want to use the laser over a range of a billion kilometers, and you're using a visible violet wavelength (400 nm), then the following is the calculation if you assume a beam waist size of 253 meters at the midpoint along the beam. The beam diameter shown is the maximum over that range - this is the beam diameter at the source and at the 1 billion km mark:

Total Range: 1,000,000,000,000 m
Distance from focus: 500,000,000,000 m
Wavelength: 0.0000004 m (visible violet)
Waist Size: 253 m

Rayleigh range = (π * 253²) / 0.0000004 = 502,725,510,409.07

Spot size = 253 * √(1 + (500,000,000,000 / 502,725,510,409.07)²) = 356.83 m

I believe the spot size is the beam radius, so beam diameter = 713.65 m

So I presume this means you would have to run the laser through a 713.65-meter telescope?

I plugged the equation into a spreadsheet and fiddled with the numbers until I got what looked to be the narrowest possible beam for a few different wavelengths. All of these are for a total distance of a billion kilometers. The spot sizes and beam diameters shown describe the beam at its widest points over that range:

 

[sophiecentaur]

The sun is not a distant point source, it has an angular diameter of about half a degree at 1 AU, and correspondingly your beam has the same minimal spread. Sunlight reflected by a 100 meter mirror will diverge notably after 10 kilometers. Do you want to place another huge mirror every 10 kilometers? Going to 2 AU reduces the spread by a factor 2, but then you need four times the mirror size for the same power. The same scaling applies to every other distance.

Is that true? I'm assuming a concave mirror, probably with some other optics. I realize that optics tends not to be intuitive so can you help me with some basics about this?

 

[mfb]

Is that true?

Yes. It does not matter what you use as optics, you cannot reduce the size in phase space. For the same reason, a solar oven cannot exceed the temperature of the surface of sun: you cannot focus sunlight better.

What you can do: make a small hole in an absorber, then use only light shining through that pinhole, and focus that. You can focus it better, but only because you reduced the light with the pinhole. The intensity reaching the spacecraft doesn't go up from that process.

 

[sophiecentaur]

you cannot reduce the size in phase space.

Thanks. That makes sense.

 

[schplade]

Am I using the evolving beam width equation correctly? I have no idea if it's reasonable to assume something close to an ideal Gaussian beam in this case. If the math is right, then it suggests to me that you would need to focus the laser through a telescope with a 713.65-meter aperture to produce a beam with those characteristics.

 

[mfb]

The scaling looks right, the order of magnitude looks right, factors of 2 will depend on the precise engineering solution anyway.

 

[Cutter Ketch]

Am I using the evolving beam width equation correctly? I have no idea if it's reasonable to assume something close to an ideal Gaussian beam in this case. If the math is right, then it suggests to me that you would need to focus the laser through a telescope with a 713.65-meter aperture to produce a beam with those characteristics.

Real beams can with work be forced to behave very similar to the ideal Gaussian. How closely a real beam follows Gaussian behavior is beam quality and is parameterized as M^2. This is a simple scaling of the Gaussian behavior. For a given M^2 if the waist is made M times bigger than an ideal Gaussian then the divergence will also be M times bigger. The beam quality product of waist size times beam divergence is therefore M^2 bigger and that's the reason that the parameter is M^2 and not just M.

 

[schplade]

I've been wondering lately about the challenge of building a giant laser-projecting telescope, like the one mentioned earlier. How does it compare to the challenge of building a giant light-gathering telescope of the same size?

At our current level of technology it's a little ridiculous to talk about building a 700-meter telescope, but the 100-meter Overwhelmingly Large Telescope is something that has already been proposed. The projected cost was about 1.5 billion Euros.

https://en.wikipedia.org/wiki/Overwhelmingly_Large_Telescope


Segmented mirror technology is allowing us to build telescopes bigger and bigger. Could a segmented mirror telescope used "in reverse" produce something close to an ideal Gaussian laser beam?

Without knowing much about the subject, I'm guessing that there would be certain advantages to building a telescope like this vs. building a light-gathering one. As mfb said, the mirrors could be made much more reflective if you're only dealing with a single wavelength. Also, I assume you would never have to refocus it. You could adjust it to the beam divergence angle that works best and leave it at that. You'd still have to aim it at the spacecraft , of course, but at these distances the spacecraft would have to be capable of keeping itself on the beam no matter what.

One complication that I could imagine is the "jitter" problem. At extreme distances, every tiny vibration at the laser's source is probably going to have a big effect when you're hundreds of millions of kilometers away. That's why I've wondered if it wouldn't be best to have multiple space-based lasers along the spacecraft 's flight path, but I don't think I've seen any beamed propulsion concepts where the beam isn't directly in line with the acceleration path of the spacecraft .

 

[sophiecentaur]

Segmented mirror technology is allowing us to build telescopes bigger and bigger. Could a segmented mirror telescope used "in reverse" produce something close to an ideal Gaussian laser beam?

Such a telescope would need to be in space and it would need to be pretty massive to maintain such a stable optical performance. (I do mean massive and not just a large area) That would be a very high cost construction.

 

[mfb]

Such a telescope would need to be in space

Why?
Breakthrough Starshot plans ground-based lasers, for example.

OWL was canceled with estimated project costs of € 1.5 billions, but that was too low. E-ELT with a 42 m mirror (instead of 100) has construction costs of about € 1 billion. Increasing the mirror area by a factor 7 would certainly raise the price tag by more than 50%.

Also, I assume you would never have to refocus it. You could adjust it to the beam divergence angle that works best and leave it at that.

You would need adaptive optics, correcting atmospheric turbulence. You need active optics to position the mirrors precisely as well. You have to aim precisely at the spacecraft and constantly change the orientation of the telescope (as the Earth rotates).

 

[schplade]

You would need adaptive optics, correcting atmospheric turbulence. You need active optics to position the mirrors precisely as well. You have to aim precisely at the spacecraft and constantly change the orientation of the telescope (as the Earth rotates).

That's part of why I think it might actually be easier to build the telescope in space. This would definitely NOT be a short-term project. What if the beam needs to be aimed at the spacecraft for days, or even weeks at a time? It's just hard for me to imagine how you could ever aim a laser so precisely that it hits a departing spacecraft light seconds, minutes, or hours away. How could you make the necessary adjustments in time? Even at the distance of the moon, any signal from the spacecraft would take more than a second to reach the earth.

The only way that I can conceive of this working is if the spacecraft has the capability of keeping itself centered on the beam. As far as the laser source itself is concerned, the goal would be to keep it stable and to keep it aimed at the destination star system at all times. That's obviously not going to be possible on a rotating body like the earth. In addition to that, it's kind of scary to think of the dangers that would be posed by such a powerful laser being reflected back at the earth.

I agree that you would need some sort of active optics system to position the mirrors so precisely, so in that sense you would be constantly refocusing the laser. I don't see why you would necessarily have to change the distance to the focus though. That could definitely be my ignorance talking.

How would you keep the beam stable, and aimed at the destination star system? I have no idea. Maybe you could build it into a large asteroid, and correct the rotation of the asteroid somehow so that one side is constantly facing the destination. That's just guesswork, of course. I'm still not convinced that something on this scale would ever be feasible.

 

[GTOM]

Is there any estimation, what if they used particle beams instead of lasers? Any plausible ways to have neutralized particle beams don't diverge for a long time?
(Maybe speed particles very near to c, so length contraction applies?)

 

[mfb]

It's just hard for me to imagine how you could ever aim a laser so precisely that it hits a departing spacecraft light seconds, minutes, or hours away.

You don't. You have to accelerate it quickly to keep the distance reasonable.

The JWST has a primary mirror with 6.5 meters, and estimated costs of $8 billions. For 1/8 of that price you get the E-ELT with 42 meters diameter, 40 times the area. Or 10-15 E-ELTs for the same price, with hundreds of times the mirror area. Ground-based telescopes are so much easier to build. They also have easier access to electricity, and you don't have to place a weapon of mass destruction in orbit.

Aiming telescopes is necessary both in space and on the ground, and it is easier on the ground (where you have a solid surface as reference). It is not an issue at all, every telescope does that routinely for operation. Forget spacecraft steering - the telescope has to aim. You need several laser "telescopes" anyway: Even a magic spacecraft with perfect steering from nowhere would still need multiple telescope beams that aim at the same spot.

In addition to that, it's kind of scary to think of the dangers that would be posed by such a powerful laser being reflected back at the earth.

The reflection would be scattered and spread out enough to be not dangerous.

Is there any estimation, what if they used particle beams instead of lasers? Any plausible ways to have neutralized particle beams don't diverge for a long time?

You cannot reflect a particle beam with >99.99% efficiency, not even with 10% efficiency. The sail would evaporate.

 

[GTOM]

You cannot reflect a particle beam with >99.99% efficiency, not even with 10% efficiency. The sail would evaporate.

How about harnessing the particle beams power with a magnetic field instead of a sail?

 

[mfb]

Any plausible ideas how to generate a strong magnetic field with a gram-sized spacecraft ?

~30 microrad beam divergence has been demonstrated at the LHC, although that is not optimized for that parameter. That is still orders of magnitude more than an optical beam. And you cannot shoot it through the atmosphere.

 

[GTOM]

~30 microrad beam divergence has been demonstrated at the LHC, although that is not optimized for that parameter. That is still orders of magnitude more than an optical beam. And you cannot shoot it through the atmosphere.

Is it an all same charge, or a neutralized beam?

 

[sophiecentaur]

Why? (not Earth based)
Breakthrough Starshot plans ground-based lasers, for example.

If the source is on Earth, you have only a very few hours per day to fire it in the direction you want. Half the time, it's the other side of the Earth and low elevation beams are very much disturbed by the atmosphere. That would have the effect of reducing the propulsive power by a factor of around 25%, maximum. Also, the Earth is a very wobbly platform to operate from. Initial development and proof of concept work would make sense on Earth - and the Starshot project is described as such. When you get down to it, it's swings and roundabouts and the optimum choice can only be made after a lot of work. A space platform would be a major investment and that could only be justified way down the line. Atmospheric absorption will always be a factor in any link budget and you can't avoid clouds in most parts of the Earth.
Has anyone considered the problem of backscatter from a high power Earthbound laser source? I would have to be significant. Passing satellites and planes would also be pretty vulnerable if this beam were turned on continuously. I'm sold on an eventual location in Space.

 

[mfb]

Is it an all same charge, or a neutralized beam?

It is a charged beam, but neutralizing it will at best stop a further increase in divergence, it won't reduce the existing divergence.

If the source is on Earth, you have only a very few hours per day to fire it in the direction you want.

That is more than sufficient with a launch process shorter than an hour. You can't launch that many probes into the same direction, but I expect power consumption to limit that number anyway. If it is not, just launch probes in different directions to different stars. The line of sight requirement does not reduce the power to launch a spacecraft because the lasers can be close together.

There are places with a very low probability of clouds. If the lasers are powerful enough, you might even be able to evaporate the water droplets in the clouds.

Has anyone considered the problem of backscatter from a high power Earthbound laser source?

100 GW over millions of square kilometers? Completely irrelevant.
You don't want a satellite to fly through the beam, sure, but the beams are narrow. It would rarely be an issue, and you can switch off a laser for a second if necessary.

 

[sophiecentaur]

100 GW over millions of square kilometers? Completely irrelevant.

I was thinking that the 'spot size' through the atmosphere would be small. Coming back from a single cloud, that spot would surely appear pretty bright but at least the reflection wouldn't be specular.
Edit - but not all that small, I guess as it's still converging.

 

[mfb]

Well, let's assume we build OWL and use it in reverse with a 1 GW laser. That is an area density of 150 times the solar irradiation, with a nearly constant cross section across the whole atmosphere. It will burn every bird flying directly through the beam - but the beam will be visible, birds can avoid them.
If you scatter 10% of that at 3 km height uniformly in all directions, that leads to a peak ground irradiation of 0.9 W/m². Clearly visible, but nothing compared to sunlight.

 

[sophiecentaur]

I was reading more about this star shot project. Something I don't understand is what sort of link budget is working for getting a useful signal back from the remote star from a 'nano-sized' transmitter. The (reversed) receiving dish would have loads of gain but power circuits need to be a reasonable size in order to produce significant transmit power. Power dissipation doesn't scale and circuits producing tens (hundreds / millions) of Watts need to be pretty big or they have to operate at impractical temperatures. I know you can use narrower bandwidths to make up for this but eventually you can't carry any worthwhile amount of data . I could only find journalist-type articles on the project and they were very thin on technical details.