Author Topic: Fusion Thruster is 40 Times More Efficient Than Ion Engines  (Read 22662 times)

Offline sanman

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Here's an article in IEEE's Spectrum magazine, on a design for a fusion thruster which is 40 times more efficient than ion engines:

http://spectrum.ieee.org/aerospace/space-flight/a-fusion-thruster-for-space-travel/0

Offline Nomadd

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 When the article contains utter nonsense like "In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10'18 (couldn't figure out how to type the exponent right) watts per square centimeter" and then estimates ten years till we have engines going to Mars, I pretty much give up on it.
 I was just at Radio Shack today, and I'm pretty sure I didn't see any "commercially available" two quintillion watt lasers for sale.
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Offline ChewyOlive

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When the article contains utter nonsense like "In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10'18 (couldn't figure out how to type the exponent right) watts per square centimeter" and then estimates ten years till we have engines going to Mars, I pretty much give up on it.
 I was just at Radio Shack today, and I'm pretty sure I didn't see any "commercially available" two quintillion watt lasers for sale.

 They're pulsing the laser at that power for a very short amount of time. Thats easy to do.

Offline simonbp

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The trick is that they are talking about a pulsed laser focused onto a very small area. It is a standard lab procedure to turn a low-power continuous-wave (CW) laser in a pulsed one with very high pulse power (I've done it). The second half is the area of the beam is focused (with a mirror or lens) down to much less than one square cm.

So, let's take some reasonable numbers: a pulse length of 1 picosecond and focused spot size of 1 micron, with a rep rate of 75 MHz. That gives a pulse power of 7.8 mJ, and a CW power of 0.5 kilowatts. Large, but not unreasonable.

It's also (very) likely the reporter got confused as to which number corresponded where, as this sounds like a report on an oral presentation. An actual journal article on the subject would be more helpful.

Oh, and Radio Shack still exists? I though they went out of business years ago...
« Last Edit: 06/29/2011 01:23 am by simonbp »

Offline Eric Hedman

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Oh, and Radio Shack still exists? I though they went out of business years ago...
They mostly sell cell phones and practically nothing else.  I remember the days when you could buy all sorts of electronic components.  I must be old.

Offline Star-Drive

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Come on guys, Radio Shack still sells parts for the hobbyist in some of their stores and most definetly from their web site, See:

http://www.radioshack.com/category/index.jsp?categoryId=2032058 

That said, buying your parts from Allied, Digikey, Mouser and/or Newark mail order might just make more sense depending on what you are in need of.

Best,
Star-Drive

Offline Patchouli

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Come on guys, Radio Shack still sells parts for the hobbyist in some of their stores and most definetly from their web site, See:

http://www.radioshack.com/category/index.jsp?categoryId=2032058 

That said, buying your parts from Allied, Digikey, Mouser and/or Newark mail order might just make more sense depending on what you are in need of.

Best,

I sometimes still buy parts from radio shack if I need a simple part right away.

Online Malderi

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I don't get where the energy is coming from, here. There's two options:

1) This is positive-power fusion, i.e., you're really getting more energy out of the reaction than you're putting in, in which case this isn't really specific to propulsion at all and is merely a different kind of fusion reactor. The line in the article concerning converting some of the energy back into electricity seems to confirm this. I'm not sure what the propulsion implications of this are.

2) This is not positive-power fusion, and it's "just" a way to convert energy you've already got (from, say, solar arrays) into very high exhaust velocities and thus, efficient thrust. If you were to tap off energy from that into an electrical power system in this scheme, thermodynamics says you lose some of it, so that's kind of stupid.

So I'm not really sure what's going on here, but I'm no electrical or nuclear engineer!

Offline Nomadd

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When the article contains utter nonsense like "In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10'18 (couldn't figure out how to type the exponent right) watts per square centimeter" and then estimates ten years till we have engines going to Mars, I pretty much give up on it.
 I was just at Radio Shack today, and I'm pretty sure I didn't see any "commercially available" two quintillion watt lasers for sale.

 They're pulsing the laser at that power for a very short amount of time. Thats easy to do.
I might be a few years out of date, but that would be over a thousand times the power of the most powerful pulse a few years ago achieved at LLNL, or the rig at UT Austin.
 Maybe someone has come up with an "easy " way to do it since then.
« Last Edit: 06/29/2011 11:47 am by Nomadd »
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Offline sanman

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Clearly the thrust is produced directly as a result of a fusion reaction. Presumably that reaction could be energy-positive, thus producing more thrust. Just as with a chemical rocket, the fuel is being turned into energetic propellant via the laser:


Online Malderi

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #10 on: 06/30/2011 03:04 am »
So, this is actually a fusion reactor, just one in which the fuel gets accelerated very quickly and can therefore be also used as a thruster.

Offline madscientist197

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #11 on: 06/30/2011 03:55 am »
Note that they didn't actually say what the thrust is, just that 100,000 particles per pulse were accelerated. Considering that low thrust is generally associated with high ISP systems, the omission of a comparable thrust metric is suspicious.
John

Online aero

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #12 on: 06/30/2011 05:10 am »
Note that they didn't actually say what the thrust is, just that 100,000 particles per pulse were accelerated. Considering that low thrust is generally associated with high ISP systems, the omission of a comparable thrust metric is suspicious.
Just using some average numbers turned up in a web search, I think the alpha recoil would be something like 0.7 micro Newtons. That is, alpha particle mass = 4u (6.64 e-27 kg), V ~= 15,000 km/sec, pulse rate 75 MHz, 1e+5 alphas/pulse and operating for a full second. Someone care to calculate the electric power needed to neutralize the +2 charge on the alpha particles?
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Offline madscientist197

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #13 on: 06/30/2011 05:57 am »
So if the entire setup weighed 1 ton, it would take merely 42 years to accelerate to 1 metre per second assuming 700 nN. The thrust really needs to be scaled up by four orders of magnitude or so, without scaling up the mass similarly.

Not impossible, but ambitious.
John

Offline DLR

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #14 on: 06/30/2011 03:25 pm »
How is that different from other laser-based fusion propulsion schemes?

Does it promise to produce net power?

Offline simonbp

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #15 on: 06/30/2011 08:36 pm »
How is that different from other laser-based fusion propulsion schemes?

They (NIF and MegaJoule) use a spherical arrangement of beams to create a ball of plasma hot/high-pressure enough to ignite a capsule of Deuterium-Tritium. This in turn emits a large amount of gamma rays and neutrons, which are converted back in electricity.

This system uses a linear two-stage arrangement. The first stage is the laser hitting a CH2 target, which creates a beam of high-energy protons. These protons then hit the second boron target, which allows them to fuse, emitting three He nuclei per proton (via the inverse triple alpha reaction). The alpha particles are then directed out of the engine by a charged grid, just like any other ion engine.

The key advantage of this design is that it can produce lots of high-energy ions for much less input energy than a conventional RF-based system. It would still need an external power source, but would use that power much more effectively.

Quote
Does it promise to produce net power?

No. The purpose is rocket propulsion, not power. Some of the excess alphas can be captured and converted to electricity to increase the system efficiency, but net gain is not the goal.

Offline baldusi

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #16 on: 06/30/2011 09:56 pm »
Quote
Does it promise to produce net power?

No. The purpose is rocket propulsion, not power. Some of the excess alphas can be captured and converted to electricity to increase the system efficiency, but net gain is not the goal.

You convert electric power to very high thrust, then. Does it provides more energy (in thrust) than electricity put?

Offline simonbp

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #17 on: 07/01/2011 05:01 pm »
No, you are never going to get high thrust from such a setup; it's just not optimal for that.

What you will get is precisely what it says on the label, a very energy-efficient low-thrust engine. This makes Solar-Electric systems much more feasible, as they need much less solar area to work. Thus, you might be able to pull off a manned SEP Mars vehicle with a solar area comparable to ISS, rather than several square kilometers....

Offline neutrino78x

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #18 on: 07/18/2011 04:58 pm »
Seems to me that the ideal fusion rocket would use (straight) hydrogen as fuel, so that you could, in theory,  refuel at the target planet and/or solar system!! :)

--Brian

Offline qraal

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #19 on: 09/07/2011 12:18 pm »
Seems to me that the ideal fusion rocket would use (straight) hydrogen as fuel, so that you could, in theory,  refuel at the target planet and/or solar system!! :)

--Brian

Unfortunately fusing light hydrogen is near impossible and has too low a power density to be useful anyway. Even the CNO-catalytic fusion cycle is relatively low in power density compared to other options. Fusing deuterium is much easier, though it's roughly 1500-6000 times rarer than regular hydrogen. For interplanetary missions though, one can dilute the fusion product exhaust with regular hydrogen to get more thrust. The hydrogen isn't fuel, just reaction mass.

Offline simonbp

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #20 on: 09/09/2011 10:38 pm »
Deuterium is rare, but not that rare; $600/litre...

Offline KelvinZero

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #21 on: 01/05/2012 01:49 pm »
Hi, just had a random thought,
the article says that the initial protons have about 0.16Mev and this eventually produces 4 helium-4 nuclei each with almost 2.5Mev.

What if these alpha particles were themselves directed to collide with some hydrogen, wouldn't these collisions likely produce more protons which themselves would have more than enough energy to initiate another stage of proton-boron fusion in another further out film of boron, and so on?

(Presumably this cannot work because otherwise an arbitrarily small input could produce an arbitrarily large output and we would have actual fusion power, not just a non-contained source of thrust)

Offline Robotbeat

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #22 on: 01/05/2012 04:47 pm »
Produce net power from fusion (i.e. power the fusion plant with just its own power, plus light a light bulb) and then we can talk about a fusion thruster being available in "just 10 years."

I don't think net-power fusion is impossible (and actually, something like laser-induced inertial-confinement fusion seems like the most promising approach to me right now), but until it's demonstrated (won't even ask about the economics), talking about a fusion thruster in the short-term is ridiculous.

There's a spaceship with a very high specific power method of harnessing net-power fusion for propulsion right now orbiting a dwarf planet (arguably). Anyone have a guess of what I'm talking about?
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Offline go4mars

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #23 on: 01/05/2012 04:54 pm »
Not sure what the first method will be, and reports on the outcome of polywell have provided no information so far. 

But I think the big investment by oilsands giant Cenovous into an inertial confinement fusion company in Vancouver (General Fusion Inc) lends a degree of credibility. 

http://www.generalfusion.com/

I haven't read up on this enough to guess whether it would be useful for space applications other than making the cost of fuel (oxygen and hydrogen for example) nearly free.   

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Online aero

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #24 on: 01/05/2012 04:57 pm »
Forty times more efficient than ion thrusters. What would that amount to in terms of Isp?

Using VASIMR with ISP of 5000 seconds, gives about 200,000 seconds for this fusion thruster. That does not compare well with the fission fragment engine Ips of 1,500,000 seconds, as discussed on another thread
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Offline baldusi

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #25 on: 01/05/2012 05:02 pm »
There's a spaceship with a very high specific power method of harnessing net-power fusion for propulsion right now orbiting a dwarf planet (arguably). Anyone have a guess of what I'm talking about?
SEP :p But there's a discussion of its own if Vesta is, indeed, a dwarf planet. But yes, the Sun is the most successful fusion powered source around. The only problem is the square distance law.

Offline DLR

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #26 on: 01/05/2012 07:02 pm »
Produce net power from fusion (i.e. power the fusion plant with just its own power, plus light a light bulb) and then we can talk about a fusion thruster being available in "just 10 years."

I don't think net-power fusion is impossible (and actually, something like laser-induced inertial-confinement fusion seems like the most promising approach to me right now), but until it's demonstrated (won't even ask about the economics), talking about a fusion thruster in the short-term is ridiculous.

There's a spaceship with a very high specific power method of harnessing net-power fusion for propulsion right now orbiting a dwarf planet (arguably). Anyone have a guess of what I'm talking about?

Net power production from the fusion reaction is not required if you want to use a fusion thruster.

You could power the fusion reaction using an external power source (nuclear or solar) and simply use the fusion reaction to produce directed thrust with a very high exhaust velocity, much higher than with conventional electric thrusters. This is what this fusion thruster is about. It isn't about sustaining the fusion reaction on its own energy.

Offline Robotbeat

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #27 on: 01/05/2012 08:56 pm »
Produce net power from fusion (i.e. power the fusion plant with just its own power, plus light a light bulb) and then we can talk about a fusion thruster being available in "just 10 years."

I don't think net-power fusion is impossible (and actually, something like laser-induced inertial-confinement fusion seems like the most promising approach to me right now), but until it's demonstrated (won't even ask about the economics), talking about a fusion thruster in the short-term is ridiculous.

There's a spaceship with a very high specific power method of harnessing net-power fusion for propulsion right now orbiting a dwarf planet (arguably). Anyone have a guess of what I'm talking about?

Net power production from the fusion reaction is not required if you want to use a fusion thruster.

You could power the fusion reaction using an external power source (nuclear or solar) and simply use the fusion reaction to produce directed thrust with a very high exhaust velocity, much higher than with conventional electric thrusters. This is what this fusion thruster is about. It isn't about sustaining the fusion reaction on its own energy.
Conventional electric thrusters don't have a limit in exhaust velocity, really. Their limit is in the power source. So, if you aren't getting net power out, this fusion thruster is just making life difficult.

Optimal exhaust velocity (i.e. Isp*g0) depends on three things: "alpha" (basically, power per mass), efficiency of the propulsion system (that is ELECTRICAL, etc, efficiency not Isp!!!), what delta-v is needed, and how much time you have to do your mission. For the current "alpha" capabilities, the optimal Isp is usually well within current electric thruster capabilities... Increasing Isp 20-50 fold over "conventional" electric thrusters with the same power systems would actually dramatically reduce performance and be far from optimal.

Here's a neat introduction:
http://www.ippt.gov.pl/~sbarral/misc/EP_course.pdf
« Last Edit: 01/05/2012 09:11 pm by Robotbeat »
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Offline DLR

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #28 on: 01/05/2012 10:29 pm »
The conversion of electrical energy into exhaust jet energy needs to be more efficient than in a standard electric thruster for this to make sense. A self-sustaining fusion reaction is not required. To reduce ISP and raise thrust, hydrogen could be injected into the exhaust stream.


But anyway, aren't the NASA guys working on this thruster talking about satellite station keeping in higher orbits, where you want your ISP to be pretty high?

Offline KelvinZero

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #29 on: 01/06/2012 01:48 am »
Produce net power from fusion (i.e. power the fusion plant with just its own power, plus light a light bulb) and then we can talk about a fusion thruster being available in "just 10 years."

I don't think net-power fusion is impossible (and actually, something like laser-induced inertial-confinement fusion seems like the most promising approach to me right now), but until it's demonstrated (won't even ask about the economics), talking about a fusion thruster in the short-term is ridiculous.

There's a spaceship with a very high specific power method of harnessing net-power fusion for propulsion right now orbiting a dwarf planet (arguably). Anyone have a guess of what I'm talking about?

Give me a moment, it will probably dawn on me.

I certainly don't think net positive output is impossible either, it just always seems 50 years away and I doubt I just cracked it.

my understanding is that the thruster discussed in the OP was more immediately plausible than net positive power because it can gain a benefit even if the energy is not returned to the system. It is well known that off the shelf equipment can fuse a few atoms. We just cannot capture as much energy as it took to initiate it.

Thinking about it, is the flaw that I was assuming that a high enough energy proton is likely to initiate fusion? If 1000:1 it is just thrown out of the nuclei again then this is why you need containment.

Offline Robotbeat

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #30 on: 01/06/2012 02:08 am »
The fusion thruster in question is designed to return power. The laser that initiates the reaction would take a lot of power as well, and that has to come from somewhere (and unfortunately beyond the asteroid belt and Jupiter, solar isn't practical).

BTW, here's a great (free! from NASA) textbook on electric propulsion. It doesn't seem to go in depth for mission design using electric propulsion, but has just about everything else in depth.
http://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel__cmprsd_opt.pdf
(It also doesn't go much into detail about the very high performing and long-tested NEXT thruster.)

Still, very good resource.
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Offline KelvinZero

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #31 on: 01/06/2012 03:41 am »
Ah, well if so (and you mean power that is returned to the system, e.g. to power the laser) then you are right, this is just another fusion power scheme and I will believe it when they sell it to a power company.

The idea only seems plausible to me (as in plausible that it works and yet is not already exploited for power generation) if it were claiming to create fast moving particles that could be exploited for thrust but could not be efficiently converted to electrical power returned to the system.

I suppose, thinking about it, there is no reason why fast moving charged particles could not be efficiently converted to electrical power.

Offline scienceguy

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #32 on: 01/06/2012 04:10 am »
I suppose, thinking about it, there is no reason why fast moving charged particles could not be efficiently converted to electrical power.

What about passing the charged particles through spirals of wire?
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Offline Robotbeat

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #33 on: 01/06/2012 04:48 am »
There are similar ideas for harnessing fusion power without using a heat engine (i.e. turbine, etc). But none have demonstrated net power out.
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Offline KelvinZero

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #34 on: 01/06/2012 08:39 am »
I suppose, thinking about it, there is no reason why fast moving charged particles could not be efficiently converted to electrical power.

What about passing the charged particles through spirals of wire?

Absolutely you can generate electricity from a stream of charged particles. e.g. http://en.wikipedia.org/wiki/MHD_generator

I think it is easy to get confused following this conversation. The last few posts have left me unsure I have even communicated the direction of my argument. Im agreeing with RobotBeat. At least I think I am :)

Offline baldusi

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #35 on: 01/06/2012 02:30 pm »
I suppose, thinking about it, there is no reason why fast moving charged particles could not be efficiently converted to electrical power.

What about passing the charged particles through spirals of wire?
That's what the Focus Fusion guys are trying to do. Their basic idea is to create instabilities in B11-H plasma, that generates He nucleus. Since those are ions, they pass them through a coil and generate electricity directly. I don't think they are getting net energy. But apparently they've made a very nice source of X-Rays.

Offline Robotbeat

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Re: Fusion Thruster is 40 Times More Efficient Than Ion Engines
« Reply #36 on: 01/06/2012 04:11 pm »
I suppose, thinking about it, there is no reason why fast moving charged particles could not be efficiently converted to electrical power.

What about passing the charged particles through spirals of wire?
That's what the Focus Fusion guys are trying to do. Their basic idea is to create instabilities in B11-H plasma, that generates He nucleus. Since those are ions, they pass them through a coil and generate electricity directly. I don't think they are getting net energy. But apparently they've made a very nice source of X-Rays.
The first couple times I read your first sentence, I saw "Ford Focus." I knew they were working on an electric car, but I didn't know the power supply would be fusion-based! It doesn't help that there's also a "Ford Fusion."

Anyway, yeah. The Focus Fusion guys are the ones I was thinking of. (BTW, does General Fusion's scheme sound ridiculously complex to anyone? Not that that means it won't work, but it's still sort of weird. Like the Rube Goldberg of fusion devices then again, they kind of all are, aren't they?

Also, I think Tri-Alpha (partially funded by our friend Paul Allen) also was to use a non-heat-engine method of converting fusion energy to electricity.
« Last Edit: 01/06/2012 04:25 pm by Robotbeat »
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