FeaturedI Beam-powered propulsion - keeping the beam focused

1. Dec 7, 2016

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?

2. Dec 7, 2016

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...

3. Dec 7, 2016

sophiecentaur

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.

4. Dec 7, 2016

Filip Larsen

If the beam can be modelled 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

Last edited: Dec 7, 2016
5. Dec 7, 2016

Staff: Mentor

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.

6. Dec 7, 2016

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)

7. Dec 7, 2016

Staff: Mentor

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

8. Dec 7, 2016

sophiecentaur

Yes. I realise 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.

9. Dec 7, 2016

Filip Larsen

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

10. Dec 7, 2016

sophiecentaur

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.

11. Dec 7, 2016

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.

12. Dec 9, 2016

GTOM

Could it become much better?
http://iopscience.iop.org/article/10.1088/1742-6596/425/5/052034

13. Dec 9, 2016

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.

14. Dec 9, 2016

Staff: Mentor

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 20%, fiber lasers reach http://www.messer-cs.com/de/us/processes/laser-cutting/vertical-cutting-laser/faser-laser/ [Broken] but with a worse beam quality.

Last edited by a moderator: May 8, 2017
15. Dec 9, 2016

sophiecentaur

What mean Power would a CO2 laser be capable of?

Last edited by a moderator: May 8, 2017
16. Dec 9, 2016

Staff: Mentor

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

17. Dec 9, 2016

sophiecentaur

Is that a continuous Power rating or short burst?

18. Dec 9, 2016

Staff: Mentor

Average power of course. Peak power would be irrelevant.

19. Dec 10, 2016

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.

20. Dec 10, 2016

Staff: Mentor

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