|Headline:||Generating Light Efficiently|
|Date:||Saturday, December 21, 2019|
|Posted By:||Plaid Hatter Games|
Something has been bugging me about how much power the Iliad needs just to keep the lights on. Literally keep the lights on. I mean, sure, assuming they have unlocked fusion immense amounts of power are at their finger tips. But immense amounts of power still require immense amounts of fuel. And this ship has to maintain that level of power for a long, long time.
Nuclear fusion actually produces a tremendous amount of light by itself. That's basically why stars are so bright we can see them light years away. That light is produced as gamma rays, and by the time the photon has made its way through the material of the star, enough energy has been lost that we see it much further down on the EM spectrum, a Ultraviolet, Visible Light, Infrared, and Radio waves.
As it turns out, humanity has already discovered how to use nuclear reactions to produce immense amounts of artificial light. One such project was the National Synchrotron Light Source. The https://en.wikipedia.org/wiki/Synchrotron_radiation is that a charged particle spinning around a magnetic ring produces light. The more energy driving the particle, the more intense (and the higher frequency) the photon.
Well... referring back to Sizing Infrastructure, note that the power generation system is already a massive ring. And that we spin our fuel particles up to fantastic speeds to focus beams of them into our reactor. That ring is going to be leaking light. Crazy amounts of light. As in, we just need to capture that light, and transport it via Light Pipes and/or Optical Fiber to where we need it on the ship.
If we go back to my calculations in Introducing the Iliad, LED based light can convert 1 watt of electricity into 266 lumens of light. And that is a science-fiction grade theoretical limit. The problem is to produce daylight levels of illumination (120000 lumens/meter) over the cultivated area of the ship (1.31E+06 meters), and even accounting for day/night cycles, consumes 3.50E+10 kw/h of power per day. That requires 1458333.3333333333 megawatts of power generation, which consumes 1.7675 kg of fuel per day.
Obviously, 1.76kg of fuel seems kind of tiny. But it takes a tremendous amount of additional energy (and thus fuel) to energize that 1.76kg into fusing. Our means of converting energy from fusion into useful electricity is also only about 40% efficient. At best. So if we can capture light directly from the system instead of converting fuel into energy into electricity into light was save ourselves the energy that is lost in each of those conversion processes. We also save ourselves the problem of dealing with the waste heat that is also produced by every one of those conversions.
There will still be plenty of LED lighting on board, though. I am anticipating that carrying light in bulk to places is like carrying any other utility. In volume it makes sense. Where you actually want rooms to be dark, say, bedrooms or movie theaters, you would prefer an electrically based light source that can be completely shut off. You also need LEDs as backup lighting for times when the reactor's synchrotron ring is shut off.
But unlike the LED lighting, light pipes never wear out. Well, at least on the scale of the expected life time of the ship.
Oddly enough, there is precious little downside to this approach. When designing the accellerator, you simply integrate a means to capture the photons of light emitted by the particles. Those photons are a side-effect of accelerating particles to near-relativistic speeds. The only hassle I can anticipate is that the lighting requirements of the ship may dictate a more powerful accelerator than would be strictly required to support the fusion reactors.
I had one of my customary number dumps prepared, with a segue into using Radiation Pressure to implement a Photon Rocket, but it occurred to me that I happen to know someone who happens to work with atom smashers, and it would be better to send him a quick email with a few questions before I make a complete idiot out of myself. Ok, more of a complete idiot.
Well, I was expecting the reply to take a few days, but by Physicist friend emailed me back almost immediately:
From: Ron Fox Re: Strange Question about synchrotrons
The power requirements for a synchrotron are pretty bad. Here's an faq for the advanced photon light source. Link
The problem is you spend all the energy accelerating electrons and only get a bit of it back in photons... And that's spread over a large range of wavelengths some visible, most not... To get decent intensities, you actually have to add more power in the form of magnetic "Wiggler's". Since the main ring magnets don't actually get you much.
Since this is science fiction, why not power everything from a fusion reactor. Theoretically cheap and virtually unlimited energy as long as you've got the hydrogen... You could theoretically scoop that up in space as well as there's a lot of it out there.... That's the premise for ramscoops in e.g. Larry Niven's work.
Long story short, synchrotrons aren't going to get me anywhere near the efficiencies I was thinking they would. And it's nice to hear from a physicist, This is Science Fiction, just say it's fusion powered.