Light Beamer | Phase

In order to test the feasibility of the system, the case of a meter-scale sail was examined. For example, to focus the light beam on a 4mX4m sail across an acceleration distance of 2x106 km requires a focusing angle of 2 nano-radians (0.4 milliarcseconds), which is the diffraction limit for a kilometer-scale light beamer operating at a wavelength of 1 micron.

Interferometry with the Event Horizon Telescope has already been demonstrated across the Earth, achieving sub-nano-radian precision at a 1mm wavelength. This is an encouraging benchmark on the way to achieving the required precision of the optical system for a kilometer-scale array on the ground.

May 17, 2016 06:34 igrekov2@gmail.com Posted on: Breakthrough Initiatives

"Can greatly increase the efficiency of the laser system. The main part of the light energy, after reflection from the sails, scatters in space. The beam of light reflected from the sails-mirrors, should be directed to a mirror located on the Land (or on a massive space ship). Consistently running through the resonant system of mirrors and reflected light will give most of the energy the spacecraft. A description of such photonic engine is given in the patent for useful model No. 64298 (RU) F03H, author, Urmatckikh A. V."

May 18, 2016 10:28 igrekov2@gmail.com Posted on: Breakthrough Initiatives

PS Sail - mirror (reflector) may be a thickness of a few nano-meters. Problems: -the Scattering of the beam after reflection. Accurate focusing of the beam on the reference mirror... as in the paragraph on "Phase".

Jul 23, 2016 21:17 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Excellent questions. Thank you.

Millimeter wave phased arrays are commonly used in radio astronomy and a recent Event Horizon Telescope has phased up a number of millimeter wave telescopes over the world.
Laser phased arrays are much less common but have been successfully used in the laboratory. Analysis shows these are capable of relatively large baselines based on the coherence length and phase noise of the optical (laser) amplifiers. Starshot will be breaking new ground in building large baseline laser phased arrays. Our arrays are close packed (dense) vs sparce arrays as sparce arrays are not appropriate for use as the fraction of emitted power on the sail is roughly equal to the packing fraction which for Starshot is near unity (dense packed array).
Most radio astronomy arrays operate in a sparce array format as they are not trying to transmit efficiently but simply want to receive with good angular resolution but they have poor overall receiving efficiency since they are a sparce array.

Some of the answers below concern what we refer to as “photon recycling”. This is an old concept and used in virtually all interference filters and Fabry Perot filters in the lab and in systems like the LIGO gravity wave detector. The problem is not in the concept but in the implementation over the vast distances that we accelerate over. This concept is fine for short range systems but not for the kind of system we are building. See section 2.3 of the “roadmap and in Kulkarni and Lubin 2016.

For technical details see section 2.3 in the paper “A Roadmap to Interstellar Flight”:
http://arxiv.org/abs/1604.01356

– Prof. Philip Lubin, Breakthrough Starshot

Aug 24, 2016 22:26 Joe Hale Posted on: Centauri Dreams

The challenge of accurately hitting the light sail is further complicated by time. Near the end of the mission the laser beam will take around four years to hit a sail moving at 20% of C. Also adding to the complexity is the fact that the laser will be launched form a moving planet circling a moving star. But I think we can do it!

Aug 30, 2016 07:15 michael.million@sky.com Posted on: Centauri Dreams

It would seem free electron lasers is the way to go for the laser system, very powerful and can have a very low divergence spread (emittance), they are also quite efficient through electron recycling. If the electron power control system is synchronised with all the other laser system units a powerfully phase array becomes more likely.

Aug 31, 2016 10:06 michael.million@sky.com Posted on: Centauri Dreams

FEL's have the ability to change the emission wavelength allowing the Doppler/reflectance sail issue to be resolved, they can also be used to generate X-rays with nm rad emission, this allows us to have a more continuous power phase to the craft. X-ray emission though must take place in space due to the absorption in the atmosphere but if used will allow even greater velocities than 0.2 c in fact much, much higher >0.8 c.

Sep 01, 2016 14:54 michael.million@sky.com Posted on: Centauri Dreams

Perhaps if we had a number of FEL linacs/undulators arranged in a circle arrangement with a mirror at the end of each unit with a hole in it so electrons can make their way through to the next linac/undulator unit. The electrons can be kept in constant motion around the 'circle' reducing losses significantly as they don't need to be slowed down or stopped. Each mirror is then used to deflect the produced laser light upwards to propel the craft.

Sep 04, 2016 17:59 michael.million@sky.com Posted on: Centauri Dreams

We could build a test scalable FEL model of modest power built for a fairly low price.

I am thinking of three linac/undulators arranged in a triangle with a mirror/prism near towards the apex with a hole in them. If we bend the electron beam with large magnets before each mirror and into the next linac/undulator system and so on we can improve the efficiency quite a bit by recycling the electrons. With the hole in the mirror/prisms we can use some of laser light as a seed via a reflection prism into the next linac/undulator unit and so on which would again aid the efficiency of the system. This test model could be built in the Nevada desert (mentioned earlier) where the altitude is fairly high, but importantly it would have access to the power grid or at least some decent mobile electrical generators.

Nov 05, 2016 03:13 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE
"Aug 24, 2016 22:26Joe HalePosted on: Centauri Dreams
The challenge of accurately hitting the light sail is further complicated by time. Near the end of the mission the laser beam will take around four years to hit a sail moving at 20% of C. Also adding to the complexity is the fact that the laser will be launched form a moving planet circling a moving star. But I think we can do it!"

Answer:
We think it is also possible. Note that we are only illuminating the sail for the first two minutes.

- Avi Loeb, Breakthrough Starshot

Nov 05, 2016 03:15 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Aug 30, 2016 07:15michael.million@sky.comPosted on: Centauri Dreams
It would seem free electron lasers is the way to go for the laser system, very powerful and can have a very low divergence spread (emittance), they are also quite efficient through electron recycling. If the electron power control system is synchronised with all the other laser system units a powerfully phase array becomes more likely."

RE:
"Aug 31, 2016 10:06michael.million@sky.comPosted on: Centauri Dreams
FEL's can change the emission wavelength allowing the Doppler/reflectance sail issue to be resolved, they can also be used to generate X-rays with nm rad emission, this allows us to have a more continuous power phase to the craft. X-ray emission though must take place in space due to the absorption in the atmosphere but if used will allow even greater velocities than 0.2 c in fact much, much higher >0.8 c."

Answer:
It is thought that moving the beamer to space would increase the cost on the order of at least 5 times and maybe more so the faster speeds should wait until we lick the lift problem. Changing the laser wave lengths is a good idea but we are limited by the water in the atmosphere. Near one micron wavelength is in a part of the atmosphere that is very transparent if we move very far from this location we will begin to lose energy.

- Avi Loeb, Breakthrough Starshot

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