Lightsail | Integrity under thrust

To inform the study, a beamer in the 100 GW class was considered. If, for example, 10-5 of the energy is absorbed by a 4mx4m sail, it will be heated by about 60kW per m2, which is roughly 60 times more than sunlight illumination on Earth. This will heat the material but not melt it. Using fully dielectric sails, we can reduce the absorption to less than 10-9 for optimized materials.

Two possible approaches to mitigate the heating challenge have been identified:

1. High reflectivity

Use a material with better than 99.999% reflectivity. Usually, highly reflective surfaces are dielectric mirrors, which are composed of ‘sandwiches’ of material, with each layer reflecting back a modest fraction of the total. Each layer needs to be at least a quarter of a wavelength thick. The weight can be reduced by using a monolayer with high reflectivity at the correct wavelength. Based on recent research, this could be achieved by a ‘hole-pocked’ layer, highly reflective for very specific angles where reflectivity caustics arise. (These caustics occur for wavelengths of light that are actually longer than the sheet thickness.) Adding the holes serves a dual purpose; it reduces the weight of the sail and it could greatly increase reflectivity. This is but one possibility being explored. Modern materials research will explore new materials such as graphene; Breakthrough Starshot aims to take advantage of this rapidly advancing field. The basic Starshot system allows a wide range of options for nanocraft masses and capabilities, all using the same array. This gives it great flexibility in optimizing the science and technology roadmaps.

2. Low absorption

Use a material (such as glass) that has a very low absorption coefficient even when not highly reflective. Such materials are used in fiber optics systems with high power applications. Without the protection of a highly reflective sail, the StarChip electronics would need to be protected from the incoming flux. But this could be accomplished by a combination of geometry (orienting the electronics ‘sideways’ with a low cross-section) and placing a very highly reflective coating only on the sensitive components. These can use the multi-layer dielectric approaches mentioned above, which have already been demonstrated in the lab. Using low absorption sail material, together with a limited use of high-reflectivity shielding for critical electronics, would protect the StarChip without increasing its mass beyond the gram scale. There are a number of high-reflectivity, low absorption materials in existence. For possible fabrication and verification, a demonstrable design of silicon microcubes on a silicon dioxide substrate is under consideration.

As demonstrated by the Japanese IKAROS mission, spinning the sail can reduce wrinkles on its surface. Special attention is needed to avoid impurities and non- uniformities in the sail composition - for example, near mechanical attachments - or accumulation of dust particles on its surface, which could otherwise lead to a localized deposition of energy. There are a wide variety of options allowing optimization of the sail design.

Apr 20, 2016 07:07 Micky Badgero Posted on: Breakthrough Initiatives

OK, so Yuri Milner covers the sail material well in his paper. The sail will be dielectric and won't vaporize. The sail can be slightly conical for stability, like the dihedral of airplane wings, or a V-shaped hull for a boat, and the probe will act as a keel. The whole unit can be spun for more stability. And the probe will be covered with dielectric mirror to protect against the laser beam.

But what keeps the shroud lines from vaporizing? Adding dielectric mirror to them would multiply the weight many fold.

May 26, 2016 15:26 michael.million@sky.com Posted on: Breakthrough Initiatives

The material will have to be a dielectric one, low absorpion is the way to go. If we had a few of these independent dielectric sails in a sandwich configuration with the near laser sails allowing a fair amount of light through and be designed to concentrate the exiting light onto the next sail and so on to lower divergence issues. Each sail would act as a reflector in one direction and still allow laser light through to the next, it will also allow a photomultipling effect as light bounces between the sandwiched sails to occur generating more thrust than a simple reflection. The last sail will contain all the electronics for observations and perhaps the other slower sails could be used as relay transmitters to and from the observation sail.

Building the electronics in pyramids would help with 'g' forces and laser light heating.

Jun 28, 2016 07:02 michael.million@sky.com Posted on: Centauri Dreams

The dielectric mirror material may be better arranged in a pyramid or cone configuration that uses total internal reflection as a means of reflecting the laser light, a thin layer of dielectric material with these pyramid/cones etched into the back of the sail may allow greater reflection than a simple 1/4 wave surface reflection.

Aug 01, 2016 14:08 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Apr 13, 2016
Tim Peery
Posted on: Breakthrough Initiatives
"Did I miss the reasoning for such a short, intense time of acceleration? Why not take more time and decrease power and all related requirements?"

Apr 14, 2016 02:27
Karen Pease
Posted on: Breakthrough Initiatives
"@tim:
You have to accelerate quickly because your beam quickly loses intensity with distance, and your craft gets up to *very* fast speeds."

Answer:
Karen Pease is right: The trade-offs in the system are complex, but one conclusion is quite clear: If we go to lower accelerations the system will not only cost substantially more but the sail diameter will be smaller, the beam aperture on the ground will be far larger and the allowable sail mass will drop. For the most part, we have optimized the system to minimize the cost and the cost is a strong function of these variables.

– James Benford, Breakthrough Initiatives

Aug 24, 2016 12:41 peter@americanfalconinc.com Posted on: Breakthrough Initiatives

The concept of using metal or water etc. seems to be incorrect. The lighter, stronger and more thermally resisitant material would be a ceramic textile.

Aug 28, 2016 06:41 Markus Hlusiak Posted on: Breakthrough Initiatives

I haven’t seen this issue discussed yet:
Due to the Doppler effect, incoming laser light frequency will vary by about 20% during the acceleration phase. Do the mentioned values of 99.999% take into account that this reflectivity needs to be acheived over a 20%-wide spectrum and not just for a single wavelength? I’m not sure if quarter-wavelength stacks are up to that task.

Sep 04, 2016 14:10 Andrew Palfreyman Posted on: Centauri Dreams

When discussing different acceleration profiles, bear in mind that pointing accuracy coupled with sail size determines the practical acceleration distance. One nanoradian pointing accuracy is a tough but achievable figure. So for a 4 metre square sail (conservatively big), we can only guarantee sail illumination for 4 million Km, or 0.03 AU. To achieve a terminal speed of 0.2c over that distance, the acceleration needs to be 46,000 gee! (450,000 m/s2). For a 2.2 gm all-up mass that's a force of 1 KN, which in turn calls for a laser power level of 150 GW, continuously on for 135 seconds. That's 2 PJ of energy.

All a bit of a stretch, innit?

Sep 04, 2016 17:36 michael.million@sky.com Posted on: Centauri Dreams

'So for a 4 metre square sail (conservatively big), we can only guarantee sail illumination for 4 million Km, or 0.03 AU.'

If the beam is Gaussian in nature it will be slightly further out, as for the power yip it is certainly a lot, about 2-3 Saturn V first stages!

'I haven’t seen this issue discussed yet:
Due to the Doppler effect, incoming laser light frequency will vary by about 20% during the acceleration phase. Do the mentioned values of 99.999% take into account that this reflectivity needs to be achieved over a 20%-wide spectrum and not just for a single wavelength? I’m not sure if quarter-wavelength stacks are up to that task.'

If we use a FEL, free electron laser, which has a variable wavelength property it can be adjusted to take into account the Doppler effect provided we do not wonder into an absorption region of the atmosphere, 1.6 micron or thereabouts looks good.

Sep 05, 2016 07:21 michael.million@sky.com Posted on: Centauri Dreams

If we arranged the sail as a series of silicon pyramids to make use of the total internal reflection process and then truncated some of them we could have these truncated pyramids layered with dielectric mirror materials for reflection. We then coat the sides of these dielectric layers with a metal conductor so when an electric field is applied the transparency/reflectivity changes allowing us to control the surface response of the sail to the laser and therefore its attitude very fast to give active control of the sail. Silicon has a very low absorbance to certain infrared frequencies and we are well positioned to manufacture silicon to a high standard.

http://nanophotonics.spiedigitallibrary.org/data/Journals/NANOP/23544/053526_1_3.png

The pyramids can be closer than shown here.

Sep 08, 2016 11:45 michael.million@sky.com Posted on: Centauri Dreams

If we had these pyramids connected via nanotube/sheets we could deform the sail to form a mirror or comms dish.

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

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