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.

Sep 10, 2016 13:15 michael.million@sky.com Posted on: Centauri Dreams

Although silicon has a low co-efficient of absorption it may not be ideal, it however may do well for a mould to form the basic pyramids and then use the fibre optic quality glass to generate the pyramids that will use total internal reflection. Fibre optic glass is a remarkable material with properties well aligned with the sail design.

http://www.gcse.com/waves/images/reflector.gif

Sep 14, 2016 20:40 Nikita Kiselov Posted on: Breakthrough Initiatives

Why not considering making the light sail ablative? The absorbed energy would be released again to some extent by slowly vaporizing the light sail, and as a side effect, maybe even generating some thrust.

The vapor between the laser and the craft would be the major downside as I see it, it would require research to be either non-obstructing or be cleared easily.

On the plus side, the light sail could be heavier at launch due to the decreasing mass during acceleration, and perhaps double as a protection for the craft during the cruise.

Also, the light sail could be composed of multiple layers that would be exposed during different phases (early acceleration to late acceleration and so on).
This would also give you the possibility to "ditch" non-needed qualities of it by ablating the corresponding layer.

Oct 11, 2016 00:26 sda1950a@gmail.com Posted on: Breakthrough Initiatives

I would expect thrust would be improved by using a plasma sail instead of a solar sail. the plasma sail would consist of a panel of Mylar with 4 quadrants of aluminum coating. The aluminum would be charged negatively to attract positive ions which provide the thrust. Reducing the charge to one or two quadrants would provide steering for the S/C. Overall neutral charge could be maintained by a positively charged wire facing rearward.

Nov 05, 2016 03:09 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Sep 14, 2016 20:40Nikita KiselovPosted on: Breakthrough Initiatives
Why not considering making the light sail ablative? The absorbed energy would be released again to some extent by slowly vaporizing the light sail, and as a side effect, maybe even generating some thrust.
The vapor between the laser and the craft would be the major downside as I see it, it would require research to be either non-obstructing or be cleared easily. 
On the plus side, the light sail could be heavier at launch due to the decreasing mass during acceleration, and perhaps double as a protection for the craft during the cruise.
Also, the light sail could be composed of multiple layers that would be exposed during different phases (early acceleration to late acceleration and so on).
This would also give you the possibility to "ditch" non-needed qualities of it by ablating the corresponding layer."

Answer:
These are all great ideas and are in the mix for the final design. Although the challenge of controlling the ablation of the sail under such extreme conditions will be difficult.

- Avi Loeb, Breakthrough Starshot

Nov 05, 2016 03:10 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Oct 11, 2016 00:26sda1950a@gmail.comPosted on: Breakthrough Initiatives
I would expect thrust would be improved by using a plasma sail instead of a solar sail. the plasma sail would consist of a panel of Mylar with 4 quadrants of aluminum coating. The aluminum would be charged negatively to attract positive ions which provide the thrust. Reducing the charge to one or two quadrants would provide steering for the S/C. Overall neutral charge could be maintained by a positively charged wire facing rearward."

Answer:
Any approach that uses thrust from gas/plasma particles is limited to speeds that are at most an order of magnitude above the particle speed. Since we aim to reach a fraction of the speed of light, the proposed plasma engine is not practical.

- Avi Loeb, Breakthrough Starshot

Nov 30, 2016 20:49 Allen Hall Posted on: Breakthrough Initiatives

tuned reflectivity isn't likely to hold true with acceleration as you would see a 22% doppler shift in the laser frequency as you accelerate to 20% c. At some point you would fry the sail.

Dec 10, 2016 16:00 michael.million@sky.com Posted on: Centauri Dreams

'tuned reflectivity isn't likely to hold true with acceleration as you would see a 22% doppler shift in the laser frequency as you accelerate to 20% c. At some point you would fry the sail.'

A FEL can change its wavelength to accommodate the light shift due to the Doppler effect but we will need to be mindful of the atmospheric absorption properties over those wavelength changes.

Jan 05, 2017 02:22 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Nov 30, 2016 20:49 Allen Hall Posted on: Breakthrough Initiatives
tuned reflectivity isn't likely to hold true with acceleration as you would see a 22% doppler shift in the laser frequency as you accelerate to 20% c. At some point you would fry the sail."

Answer:
Thank you for thinking about this and contributing. Recent work is being performed by SRI which suggests that reflecting light in wavelengths that are 20% broad is indeed possible. Please see this recent work on perfect dielectric-metamaterial reflector by Brian Slovick, Zhi Gang Yu, Marcy Berding, and Srini Krishnamurthy Phys. Rev. B 88, 165116 – Published 9 October 2013.

- Pete Klupar, Breakthrough Starshot

Jan 05, 2017 02:25 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Dec 10, 2016 16:00 michael.million@sky.com Posted on: Centauri Dreams
'tuned reflectivity isn't likely to hold true with acceleration as you would see a 22% doppler shift in the laser frequency as you accelerate to 20% c. At some point you would fry the sail.'

A FEL can change its wavelength to accommodate the light shift due to the Doppler effect but we will need to be mindful of the atmospheric absorption properties over those wavelength changes."

Answer:
Thank you for your help in this matter. It is our assumptions because of the properties of the atmosphere it is clear we will need to keep the frequency of the laser fairly constant.

- Pete Klupar, Breakthrough Starshot

Jan 09, 2017 16:49 michael.million@sky.com Posted on: Centauri Dreams

If we use silicon we can go a way into the micron wavelength region before absorption due to the Doppler effect starts to become a problem.

http://www.pveducation.org/pvcdrom/materials/optical-properties-of-silicon

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