[jamesr]

nemmart wrote:
Something is fishy though. According to http://www.howstuffworks.com/question192.htm, a 747 burns 36,000 gal over a 10 hr flight, or about a gallon a second.
A gallon of Jet A has ~121 MJ. 121 MJ delivered in 1 second is 121 MW. Therefore, averaged over a 10 hour flight, you need 120 MW, or 24 FF’s. The difference
might be explained as a peak vs. average output.

Sounds about right… I doubt they would operate at anything near full power while cruising. I’m surprised these back-of-the-envelope type calculations are in that close agreement.

[jamesr]

Breakable wrote: Is it silly me or is the 0.282 kN = 0.282 MW?
Edit:
Boeng output is probably per second
Where FF is per hour, in that case we need ~203 modules with electrical output
or 102 with full output in case efficiency is similar.

1W = 1Nm/s so 0.282MN/s is equivalent to 0.282MW

another way of looking at it is: Power = (force *distance) / time = force *speed

So if we take the maximum speed (rather than the cruising speed) then this should be where the maximum force the engines can produce is just maintaining the speed and not accelerating it.

We have max speed (from the same wiki link above) = mach 0.92 @35000ft = 988km/h = 274m/s
Force = 4*262kN = 1048kN

Therefore Power of 747 at full thrust = 1048000*274 = 287152000W ~300MW

or 60 5MW FF generators!

[jamesr]

Microwaving air is a really bad idea – you end up with lots of nasty NOx compounds

[jamesr]

Dr_Barnowl wrote: I think the comment from the ITER communications chief that you can’t use fusion for proliferation is bunk though – it’s true for these small devices, but part of the planning for tokamak reactors is to generate tritium using a breeder blanket … last time I looked, tritium was a key component of all hydrogen bombs, and for D-T fusion to work as a practical energy source you need to generate unprecedented quantities of the stuff. Which would seem to make it easier to make fusion warheads. Just sayin’.

Not really – H-bombs use Lithium Deuteride as the main fuel. The lithium breeds tritium from the neutrons produced in the fission primary. The only tritium may be in a small trigger, and a little mixed in the outer lay of the lithium deuteride to control the exact timing of the reaction. But tritium is not needed, it just makes the yield better & more predictable.

The Tritium produced at ITER & future D-T fusion reactors would be all used on-site as it’s made and only a few grams would ever be stored at a time.

Here is an old article from 1979 that explains it all: The Progressive

I think some proliferation worries are a bit misguided. Anyone that wants to know how to build a bomb can find out easily enough. The raw materials are fairly easily obtained. The only difficultly is in the technology to refine those materials and the precision manufacturing processes. Of which the most difficult part to hide is the energy intensive separation of the U-235 & Pu-239 to build a conventional fission bomb. And you can’t build an H-bomb without a fission primary. Hence the efforts to restrict the parts & materials needed to build centrifuges.

If you can enrich Uranium from 0.7% U235 to 2-3% for civilian power station then given time you can take it upto the >70% needed for a bomb. Even if a country isn’t allowed/doesn’t have centrifuges and instead is just sold manufactured fuel rods for a civilian plant, it can still make a Pu-239 bomb (& hence an H-bomb) from reprocessing the waste. Hence you need to control that side as well.

[jamesr]

Interesting idea…

From previous work I gather the optimum ratio for the p+B11 is nearer 5 parts hydrogen to one part Boron to keep the average Z value down, and so lessen the bremstralung losses.

If you had a mix of deuterium gas with natural decaborane (which has chemical formula B10H14) with the boron in it having the natural 20:80 ratio of B-10 to B-11. Then you don’t have to worry about separating out the boron isotopes. The B-10 in the system can react both with the deuterium (I’ve no idea what the cross-section for this is), but also mop up some of the unwanted neutrons from the D-D reactions in the reaction n + B-10 -> Li-7 + He-4

The overall number of neutrons escaping would still be larger than pure p+B11 but if it gets over the hurdle to achieve ignition then it may be worth it.

[jamesr]

vansig wrote:
what is the exit velocity?

I would think there would be quite a spread of speeds, partly from the thermal Maxwellian speed distribution of the ions, but also their orientation to the E & B fields. But the peak would still be around the peak of the thermal distribution. So at 100keV the speed is ~2000km/s or 0.007% of c. Rising to 5400km/s at 600keV

[jamesr]

nemmart wrote:

I think this is a really interesting question. I don’t understand why one should expect the alpha particles to all go in the right direction, down the axis of the anode. When the excited C12 fissions, I would expect the alpha particles to go shooting out in random directions, with piles of energy. More energy than is in the plasma. Is there a good reason to believe the plasma can capture this energy as opposed to the alpha particle punching through it?

When you run a DPF with deutrium what what percentage of the fusion energy gets captured by the plasma? Does the amount of neutron radiation change with direction?

The alphas for p+B11, or the neutrons in the case of D+D->He3 + n, will of course be emitted in random directions. But the very high magnetic field will cause any alphas to spiral round the field lines with a radius of:

r=mv/qB

where v is the velocity perpendicular to the magnetic field. For a 3MeV alpha emitted perpendicular to a 1GG field the radius is r=2.5E-6m. However the denisty of the plasma where the fusion occurs will mean it will have many collisions before it gets that far and slow quickly to the thermal ion temperature. At 100keV the radius drops to only 4.5E-7m, so the ions are confined easily by the magnetic field in the plasmoid.

Only when the magnetic field collapses creating a large electric field along the axis of the plasmoid are the ions accelerated out in the narrow beam

[jamesr]

vansig wrote:

As I understand it, one of the techniques used in electron tubes (aka valves) to reduce electrode erosion is to a maintain a negative potential on the “target” electrode. This decelerates the free electrons (that had been accelerated by the grid potential) so that most of the energy has been taken out of them and the electrons impact the plate at low energy. This is a bit like a lunar lander game played out on a very small scale.

Electrons travel through the vapourized decaborane, heating up the vapour to plasma on their way from the cathodes to the anode. The anode must be positively charged in order to attract the electrons, but the electrons give lots of their energy to the plasma. If 90% of the energy in a 45 keV electron were to go to the plasma, then the electron would be travelling slowly when it hits the anode (but i dont know the actual percentage). This process begins around the outside edge of the anode, where the cathodes are closest to it. The charge of the anode drops considerably as the electrons hit it and the plasma tendrils climb up.

The tendrils climb up and over the edge and become the plasmoid; so the parts of the electrodes most-exposed to heating vary through this pulse. All this happens on the order of nanoseconds, therefore skin effect will be important: the charges will be confined to the surfaces of the electrodes almost exclusively.

My understanding of the superiority of Beryllium is that it is much more transparent to x-rays, so it wont heat as much as copper. But otherwise its heat capacity and thermal conductivity counteract its higher melting temperature. It seems necessary to use a thin coating of a much higher melting temperature, thermally conductive material. (eg: graphite? single-walled nanotubes? )

But if, after the shot, the anode is turned slightly on its axis, then the next shot will contact a different, perhaps cooler, part of the surface.

You seem to be mixing a few different things.

At the start the high voltage in the capacitors is switched and the potential on the anode jumps to this 20-45kV. after the initial breakdown electrons in the plasma arc are accelerated by the E-field, but due to the relatively high pressure will undergo many collisions and quickly a drift velocity as the short accelerations between collisions average out. This movement of electrons forms the growing current (as more electrons are involved in it – not that they are going much faster) in the run-down and axial phase over microseconds, not nanoseconds. The energy (temperature) of the electrons reaching the anode in these phases is still fairly small. As you correctly say most of the heating of the bulk of the anode is due to the resistive heating in the thin skin where the MA size current flows. But this only would raise the surface temp by a degree or so each shot. So assuming in the milliseconds between shots, this heat has enough time to conduct down through the bulk of the anode where the cooling is provided, then over many shots the temperature of the surface will not get too high.

The plasmoid which forms at the focus lasts the tens of nanoseconds and heats the electrons and spits them out as a high energy beam of upto the 45keV quoted. It is this small population of very high energy electrons doing damage that we are talking about.

[jamesr]

Breakable wrote:

Have you read the analysis done by David MacKay in his book Sustainable Energy – without the hot air

He also mentions an organization Desertec which has been promoting the idea of large solar arrays in north Africa supplying most of Europe’s needs.

No I haven’t, but I am already cautioned by his claim of “without the hot air”.
Basically there are 2 camps in energy field – renewable camp and nuclear camp and they are trying to disprove each other all the time.

It is a good book, and goes through all the numbers for each type of renewable step by step (from a UK perspective). At one point when going through the numbers for nuclear waste he says

Please don’t get me wrong: I’m not trying to be pro-nuclear. I’m just pro-arithmetic.

.

But at the end when he comes up with 5 plans for the energy mix for the UK, the one where he also regards economic factors ends up with 44kWh per person per day of nuclear out of a total of 50kWh/p/d of electricity produced (see p211).

[jamesr]

playing devil’s-advocate a little here, but why not just leave it.

Nature will break it down eventually – sure it will take decades. Some animals will die, others may be better off – that’s life. As a punishment for humans we should cordon off the hole area from human activity and leave it as a nature reserve for a hundred years. If we can’t be trusted to manage the environment then we shouldn’t be allowed to use it.

Note I think everyone needs to take collective responsibility not just BP. Sure they take the brunt of the costs, but it affects us all in the long run. We wanted cheap oil/gas and so the safeguards were only as good as people and governments were prepared to pay & legislate for.

[jamesr]

Have you read the analysis done by David MacKay in his book Sustainable Energy – without the hot air

He also mentions an organization Desertec which has been promoting the idea of large solar arrays in north Africa supplying most of Europe’s needs.

[jamesr]

From the report linked to above:

From the total energy estimate and the electron energy, we can get an estimate of 4 mC for the total charge in the beam. If we then assume that the beam is spreading out linearly and covers a 1.4 cm radius circle at a distance of 4 cm

This is quite a big area – so I would think if the it were replaced by a fine mesh, rather than a simple hole, that let most of the electrons pass through to another small chamber behind the anode, they could be slowed down as you describe (extracting their energy in the process).

[jamesr]

As Aeronaut pointed out – the e-beam hits the anode and vaporizes a small are of a creating a pit ~18microns deep per pinch according to this report. Also it deposits its charge into the circuit, but this is not significant electrically.

Beryillium’s boiling point of 2742 K is not that different from copper’s at 2840 K, but since it is a much lighter element (& so each atom has less electrons) I would expect the electrons in the beam to penetrate further into it as they slow down, depositing their energy over a greater overall volume of material. This may mean the energy deposited in one spot is not enough for it to vaporize, or conversely it could mean an even larger pit is formed, only experimental testing (or very sophisticated modeling) will give us the answer.

[jamesr]

Aeronaut wrote: This is how the plasmoid and field are connected.

As far I was concerned the plasmoid is the field by definition. ie. when the magnetic field lines break and reconnect to form an closed structure, that is called a plasmoid.

This can only happen in the presence of a plasma with a temperature and pressure such that the collisional effects (electrical resistivity) enable transfer of energy from the ions & electrons to/from the field, and so change the topology of the magnetic field.

(PS. it is refreshing to have a thread that is slightly more on-topic than some of the other recent discussions)

[jamesr]

Taking the concept of a Thought Experiment from the scientific to the philosophical. I think there is a place for science fiction to probe areas of scientific research and technological developments to run through the what-if scenarios, and consequences (intended or not) of progress.

A sci-fi narrative can bring together many aspects of human interactions with technology and the world around them to investigate possibilities that would be difficult to comprehend as individual case studies.

So, for example, a sci-fi story based in a near-future world where focus fusion devices are a reality, can explore the shift in the power structure of society. So just as Asimov pondered one outcome of the 3-laws of robotics as revolution of the robots seeking to do what was perceived as in the interest of the greater good. The knock-on effects of what could be seen as a utopian goal of cheap clean energy (and so everything else) available to everyone, may ultimately backfire as different groups scrabble to retain power and control over society.

[Author]

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