Antimatter question

[Hellbound]

RecoveringYuppy:

That was me, but my efficiency comments were regarding the entire process, not just the steam engine. I didn't mean to imply heat was necessarily inefficient in all cases, simply in this case. And that is because only about 50% of the reaction is released as heat (if that much). So straight off the bat your efficiency is at 50%. I'm not thinking about the ideal engine efficiency and input reservoir/output resorvoir; that deals mainly with the efficiency of converting heat to work, whereas I was looking at total energy in and out.

In any case, it's moot due to production costs. Antimatter isn't useful for energy production because it takes far too much energy to produce. It is good for storage, if you need a very high-density storage system, which makes rocket engines a natural application for it.

 

[RecoveringYuppy]

How'd you arrive at that initial 50%?

 

[Hellbound]

How'd you arrive at that initial 50%?

Because it's my understanding that only 50% of the energy released in a matter/antimatter reaction is released as heat. Most of it is in gammas, which we'd have a real problem trying to capture for electricity (at this point, in any case). I'll double-check my figures on that, because that is off my recollection (which is far from perfect).

 

[RecoveringYuppy]

I think you'll find it's all gamma rays but I wouldn't make a distinction between unfocused gamma rays and heat.

 

[Hellbound]

Okay, well, it seems I was wrong.

But my conclusion was right

I knew that 50% figure came from somewhere It produces gamma rays and pions, ans well as neutrinos. From Wikipedia:

Not all of that energy can be utilized by any realistic technology, because as much as 50% of energy produced in reactions between nucleons and antinucleons is carried away by neutrinos, so, for all intents and purposes, it can be considered lost.[2]

So, we were both right and wrong, but we've both learned something today

ETA: Although, looking at more sources, it seems that electron-positron reactions produce mostly gammas, so maybe there's a way around that. But still, the loses in production are enormous. And I wouldn't know how you could get and maintain a usable mass of positrons and electrons without getting any other particles mixed in.

 

[RecoveringYuppy]

Yes. I had no idea that neutrinos/pions would carry away so much in such reactions.

 

[Big Al]

Antimatter isn't useful for energy production because it takes far too much energy to produce. It is good for storage.

You would also need some kind of evacuated electromagnetic containment system. The softer the vacuum, the more of your AM fuel you lose from interactions with normal matter inside the field. You therefore need power to evacuate the containment, and more to maintain the field. I don't even pretend to know the numbers, but my instinct tells me this sort of system is going to be more on an industrial scale than light and pocket-sized. Not exactly ideal for spacecraft. Look at the size of tokamaks - and they're only working with tiny pinches of plasma.

In a tokamak, if the magnetic bottle fails, the reaction is quickly snuffed out. If your antimatter containment fails... well, goodbye spaceship.

 

[Cuddles]

Yes. I had no idea that neutrinos/pions would carry away so much in such reactions.

Just the neutrinos. Pions are hadrons (made of quarks) and are much easier to stop than gamma rays, so any energy converted to them (or any other particles that aren't neutrinos) won't be lost.

 

[RecoveringYuppy]

Don't get it. Why not pions?

ETA: Oh wait, I get what you're saying. You aren't saying pions can't be produced, you're saying the energy they carry can be captured. Yes. I wasn't implying it was lost.