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PostPosted: Wed Jun 29, 2011 2:06 pm 
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A Fusion Thruster for Space Travel
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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 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.

The first layer is a 5- to 10-µm-thick sheet of conductive metal foil. It responds to the teravolt-per-meter electric field created by the laser pulse by "acting as a de facto proton accelerator," says Chapman. The electric field releases a shower of highly energetic electrons from the foil, leaving behind a tremendous net positive charge. The result is a massive self-repulsive force between the protons that causes the metal material to explode. The explosion accelerates protons in the direction of the target’s second layer, a film of boron-11.

There, a complicated nuclear dance begins. The protons (which carry energy on the order of roughly 163 kiloelectron volts) strike boron nuclei to form excited carbon nuclei. The carbons immediately decay, each into a helium-4 nucleus (an alpha particle) and a beryllium nucleus. Almost instantaneously, the beryllium nuclei decay, with each one breaking into two more alpha particles. So for each proton-boron pair that reacts, you get three alpha particles, each with a kinetic energy of 2.9 megaelectron volts.


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PostPosted: Wed Jun 29, 2011 3:54 pm 
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I wonder if he actually tested it before writing the article :roll:


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PostPosted: Wed Jun 29, 2011 4:39 pm 
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Giorgio wrote:
I wonder if he actually tested it before writing the article :roll:


Good question. It looks rather dubious to me.


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PostPosted: Wed Jun 29, 2011 4:41 pm 
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DeltaV wrote:
pulse frequencies up to 75 megahertz,

Good laser


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PostPosted: Wed Jun 29, 2011 6:59 pm 
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A Fusion Thruster for Space Travel wrote:
...a commercially available benchtop laser starts the reaction.
...
Even at 50 percent efficiency, burning off 40 milligrams of the boron fuel would deliver a gigajoule of energy. The amount of power depends on the laser pulse rate. The motor could generate 1 megawatt per second if the pulses are frequent enough to start reactions that consume that amount of boron in 1000 seconds.


Sounds simple enough. With obvious non-rocketry applications. So you set up a series of commercially available benchtop lasers firing multiple times a second at tiny spots on a moving metal foil tape coated with some cheap and abundant boron to produce cheap, low maintenance, megawatt class, high Q aneutronic fusion fusion power plants which can be built for a few hundred thousand US dollars each and thus will soon be built in every community. The entire oil and energy industries collapse, wars over oil and energy all end, all existing nuclear fission plants are decommissioned, poverty and hunger end as a new era of peace on earth and good will toward men begins, and humans soon begin to colonize the stars using our spiffy new fusion powered rockets. And (hat tip to Douglas Adams), just after Chapman is awarded the Nobel Prize for Extreme Cleverness, he will be lynched by a rampaging mob of physicists from the NIF who had realized that one thing they couldn't stand was a smart-ass.

If only it were that easy... I suspect there just might prove to be a few technical challenges to using commercial off the shelf lasers to initiate Q positive p-b11 fusion.


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PostPosted: Wed Jun 29, 2011 7:52 pm 
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kurt9 wrote:
Giorgio wrote:
I wonder if he actually tested it before writing the article :roll:


Good question. It looks rather dubious to me.


On second thought, this concept uses a picosecond chirped pulse amplifier laser like those being used to make table-top Terrawatt lasers. A picosecond laser at terrawatt peak power can probably can induce fusion reactions. However, it seems to me that such a short pulse would not induce enough reactions to get a significant energy production.

If these are commercially available, it ought to be not that expensive to make the experimental apparatus. Manufacturing the target should not be that expensive.

What about Bremstralung losses?


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PostPosted: Wed Jun 29, 2011 8:50 pm 
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The idea does sound simple enough. To simple if you ask me.


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PostPosted: Thu Jun 30, 2011 7:47 am 
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kurt9 wrote:
If these are commercially available, it ought to be not that expensive to make the experimental apparatus. Manufacturing the target should not be that expensive.

The main issue I really see right now from a technological point of view is the rate of fire. The need of 75Mhz is way beyond what we can even dream to achieve with our technological level.

Edited to fix spelling


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PostPosted: Thu Jun 30, 2011 9:58 am 
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"Coulomb explosions" from intense laser light exist and are used to investigate matter. But the energy available can be calculated simply from nuclear charge and separation. It is not high enough to initiate fusion in normal materials.

Of course ultra-dense deuterium as proposed by Badiei, Andersson,Holmlid would mean these explosions can release much larger energies. But this is very tendentious and not what is proposed here anyway.

163keV needed.

V ~ (1/4pie0)(q/r) ~ Q*1E10*1.6E-19/2E-10 ~8Q where Q is unshielded nuclear charge after electron displacement for a 2A lattice constant.

E ~ QV ~ 8Q^2 eV

Trying for high Q, take Pt, Q=+78, covalent radius = 1.3A => min nuclear distance = 2.6A

E ~ 1500*5 = 7.5E3V (there is actually another factor 1/2 due to two nuclei being accelerated).

Looks like even if all electrons could be displaced by the laser (is that possible?) for Pt we have Coulomb energy of only 25 X or 50 X lower than needed. I suppose that is not bad. Can anyone think of a way to increase it? we need to max q1*q2/separation for two nuclear charges in a lattice.

Another key issue is the efficiency of this mechanism as way to generate high energy protons.

The nuclear reaction gives (reaction prob)*20 energy gain. We would need:
(proton acceleration efficiency)*(reaction prob)*20 > 1.

What is the cross-section for 160kev protons hitting boron and going down this pathway?

ALSO - how do we get from nuclei to protons.

What about a foil of (disputed) Rydberg hydrogen? I still think not nearly enough energy...

What efficiency do we get from a normal linear proton accelerator? The only claim might be that this laser mthod was more efficienct than other methods at making high energy protons.

I am sure if chucking high energy protons at boron is enough for Q>1 fusion we would know...


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PostPosted: Thu Jun 30, 2011 2:10 pm 
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Is Q>1 really necessary for this idea to work as a thruster?

Tom - you need to extend your equations to calculate the reaction mass used per unit thrust.

Given a solar powered satellite where surplus solar energy is available to drive the thruster, how much mass must be carried into orbit to provide satellite maneuvering thrust?

The real question here is total thrust energy vs mass to orbit cost for the various methods of spacecraft propulsion.

Thrust power also enters into the equations is some manner that is mission dependent.

Now if Q >> 1 the thrust to mass ratio gets very interesting!


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PostPosted: Thu Jun 30, 2011 4:02 pm 
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Giorgio wrote:
kurt9 wrote:
If these are commercially available, it ought to be not that expensive to make the experimental apparatus. Manufacturing the target should not be that expensive.

The main issue I really see right now from a technological point of view is the rate of fire. The need of 75Mhz is way beyond what we can even dream to achieve with our technological level.

Edited to fix spelling


I noticed that as well. The chirped pulsed amplifier lasers that I could find on the internet all had pulse rates in the kilohertz range (5-10 kHz). The pulse widths were 0.5 picoseconds as well. Some development work is yet to be done to make the laser that meets the specification in the article.


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PostPosted: Thu Jun 30, 2011 4:26 pm 
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PNeilson10 wrote:
Is Q>1 really necessary for this idea to work as a thruster?

Tom - you need to extend your equations to calculate the reaction mass used per unit thrust.

Given a solar powered satellite where surplus solar energy is available to drive the thruster, how much mass must be carried into orbit to provide satellite maneuvering thrust?

The real question here is total thrust energy vs mass to orbit cost for the various methods of spacecraft propulsion.

Thrust power also enters into the equations is some manner that is mission dependent.

Now if Q >> 1 the thrust to mass ratio gets very interesting!


If Q >> 1 it is interesting for many other reasons, like a viable fusion source.

If Q ~ 1 then maybe this is an efficient way to run what is effectively an ion drive.

But it does not look like it works at all.


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PostPosted: Thu Jul 07, 2011 11:25 pm 
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I've done a little digging about to see if I could find more background material on this concept.

First, there's at least one company selling 75MHz+ pulse rate chirped pulse amplification lasers. Calmar Laser's Mendocino product has a 10-100MHz repetition rate. It's a low-power laser but seems to at least prove that it's possible to fire a CPA laser that fast.

Recently I found a link to a 2009 paper titled "Fusion energy without radioactivity: laser ignition of solid hydrogen–boron (11) fuel".

PDF here: http://www.phys.unsw.edu.au/STAFF/VISIT ... yEnvir.pdf

The paper is theoretical in nature - it is describing simulations of fusion conditions, not actual experiments. However, it suggests that side-on ignition (vs the spherical compression method used by the NIF) has considerable potential given the new high contrast, high pulse energy lasers. Interestingly, under this approach they find that p-B11 fusion is not much more difficult than D-T fusion.

I don't have enough of a background in fusion physics (despite reading Talk-Polywell for some time now) to judge it, or to compare the numbers in the paper with Chapman's design. Still, it appears there is something there.


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PostPosted: Fri Jul 08, 2011 7:02 am 
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Good find for the laser, unfortunately is still several orders of magnitude out of the needed characteristics for the application envisioned by Chapman.
Engineering wise is not going to be an easy task to reach them.

Very nice also the paper. In section 6 they listed all the areas that need to be investigated to understand if this theoretical model has a potential.

It will be interesting to see how research will evolve.


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PostPosted: Fri Jul 08, 2011 7:04 am 
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dkfenger wrote:
Recently I found a link to a 2009 paper titled "Fusion energy without radioactivity: laser ignition of solid hydrogen–boron (11) fuel".

PDF here: http://www.phys.unsw.edu.au/STAFF/VISIT ... yEnvir.pdf

You have been blogged at Nextbigfuture:
http://nextbigfuture.com/2011/07/fusion ... ivity.html


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