[jamesr]

slane wrote:
The eyewalls of sunspots have huge amounts of the circular electric currents (1,000,000,000,000 Amperes for middle size circular sunspots), so plasmas in eyewalls of sunspots are in pinch state, that is, in high temperature high density state, so stable nuclear fusion reactions can happen in eyewalls of sunspots.
Why do you think that sunspots cannot have stable nuclear fusion reactions?

The current is irrelavent. It is quite easy on the sun to get large nett flows of quasi-neutral material, and hence large currents. The sun is after all quite big. But unless the individual ions are moving fast enough (ie. are hot enough) to overcome the coulomb repulsion you will get no fusion.

Now up in the corona is does for some reason get up to a million or two Kelvin, compared to the 4500K or so for a sunspot. so you could potentially have some fusion there – but even this is too cold to have any appriciable chance of fusion, never mind that it is so rareified that the chance of a collision of two ions is incredibly low.

Going back to your hurricane metaphor – the wind at the eyewall is fairly slow, the damge is done further out on the leading edge. But nowhere in a hurricance is there enough energy in the air to make is spontaniously combust or even the water in it boil.

James

[jamesr]

dash wrote:

Accelerator experiments under carefully controlled conditions, like you describe, are useful to determine the precise collision cross sections (reaction probabilities), but useless as a way of getting energy out.

I’m not necessarily convinced of this. First off, you want the target cold because you want to eliminate the thermal heat velocity noise. Maybe there is an exact energy the ions need to fuse.

Now as regards energy coming out, you’d charge up the target to a high or low voltage, whatever is correct to get the right speed ions hitting it. When an ion hits and fuses, does it change the net charge on the target? Maybe the charge stays the same.

Keeping it all cool instead of a general hot plasma seems easier to control. Hot plasma is difficult to contain, as everyone knows.

Just tossing out ideas. Thanks for your response.

-Dave

It’s good to toss ideas out there, but in this case its not going to work.

An accelerator of this energy would typically be only a few percent efficient at ionizing and accelerating the ions (this efficiency drops rapidly as the beam energy goes up). Even then, if an ion beam hit a solid target, the ions would interact (via coulomb force) with many atoms at once, leaving an ionized trail of atoms in their wake. Gradually loosing their energy and heating up the target material. The chance of hitting a nucleus head on and so getting close enough to fuse is tiny, so most will loose their energy and thermalise with the surroundings after a few dozen close-ish collisions.

Only a proportion of the fusion energy comes out as X-rays the bulk will be the kinetic energy of the Helium nuclei. These will come out at random angles – not related to the beam direction and quickly dissipate their energy in the target material as heat. If there were sufficient reactions to produce a meaningful amount of energy the target would vapourise and become a plasma anyway.

There is indeed some preferred energies due to resonances (excited states) in the resulting transient nucleus. There is a small spike in the absorption cross section at around 148keV which could be critical in igniting pB11 fusion plasmas, but the main broad peak is over 500keV. The difference between having a target at room temperature (300K = 0.025eV) and cooled to say 3K is insignificant compared to the width of the resonance peak which is around 10keV.

Nevins and Swain 2000 is a recent analysis of the pB11 reaction probabilites.

The reaction rate for thermal plasmas is a combination of the reaction cross section for each speed, the maxwell-boltzmann thermal distribution, and the losses due to radiation. This masks out the spike of the resonance, but it adds a significant contribution at low temperatures.

[jamesr]

dash wrote: I just had an idea in the shower, maybe this could be a workable approach?

Take a target of fuel and cool it to very low temperature so its thermal velocity is low. Keep it cold.

Then direct a stream of fuel ions of precise velocity at the target in a vacuum. Some will fuse, releasing heat and X rays.

The heat is waste, it heats up your target and must be eliminated. However the X-ray is your energy you capture, using Eric’s metal foil concentric spheres approach to get an electric current directly. This powers the cooling and ion gun and the pumps needed to recycle the gas that doesn’t fuse.

The whole thing is very stable and completely controllable. Nothing to wear out.

-Dave

Why bother cooling it? it’s not as if you need the collision speed to be an exact value so the sum of beam plus target adds up to a specific number. You want everything as hot as possible so the chance of the collision velocity of two fuel nuclei will be high enough to overcome the coulomb repulsion. If the collision is head on then each only needs to be going half the speed.

The energy required to accelerate ions in a beam will always be many orders of magnitude more than you can get out by having that ion release energy by undergoing fusion.

Accelerator experiments under carefully controlled conditions, like you describe, are useful to determine the precise collision cross sections (reaction probabilities), but useless as a way of getting energy out.

[jamesr]

slane wrote:

Somebody got cosmology wrong again.
Here is a quote from the infamous internet information source:
“The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core”
http://en.wikipedia.org/wiki/Sun

I certainly know it very well.
Astronomers think that the sun can maintain stable nuclear fusion reactions in its inner core just by a centripetal force (gravity), it is a good idea, but it cannot be verified by experiments, because nobody can go in inside of the sun. I must emphasis that physics is an experimental science.
Fusion scientists think that they can achieve stable nuclear fusion reactions just by electromagnetic forces, but they have not succeeded.
We do not know that heats can produce forces directly.
Sunspots kind stable nuclear fusion reactors are governed by a centripetal force and electromagnetic forces.

Fusion in stars is incredibly inefficient & slow, hence why they last so long. The temperature in the core of only 10million degrees is only just enough to get the hydrogen to fuse. Further out the temperature drops rapidly till at the surface it is only 5400C. Sunspots are dark because they are upto 1000 degrees cooler. There is no way any fusion can occur at these cold temperatures and low density.

In larger stars where the temperature & pressure is higher you can get other reactions such as the CNO cycle which make large stars burn much faster, hotter & brighter. Although there are certainly still large gaps in the knowledge of the internal workings of stars. The observations of millions of stars and their spectra, backed up by fusion probability cross section measurements on earth give us pretty good confidence that the fusion process in the core is the right model.

To get fusion on earth at any appriciable rate you need to satisfy the Lawson criteria. That is you need the product of temperature, particle density and confinement time to be above a critical value. Magnetic fields are useful in obtaining the confinement but if you don’t have the temperature to go with it nothing will happen.

For D-T reactions, which are by far the easiest, you need around 100million degrees (10 times hotter than the core of the sun) and for pB11 we will need an order of magnitude higher again. There are no shortcuts, or cunning ways to make it easier. To obtain breakeven (ie Lawson criteria) if you reduce one factor like temperature you need to make up for it with the others, eg by increasing the density.

For those that want to get into the hard core physics, these are good introduction: Nuclear Fusion Reactions and Plasma Physics. They may look a little scary but just skip over the equations if you just want an overview.

[jamesr]

We seem to be getting a little off-topic here but… I applaud your skepticism Dave, I too am skeptical of a lot of current scientific dogma. But I think that for something as well understood as radioactive heating it is worth towing the party line.

When we (focusfusion) are seen by the scientific community-at-large as being on the fringe and confrontational about everything I think they will not take us seriously on the subjects where we want them to listen, and take note of out paradigm shifting ideas.

It takes a lot to shift a well established paradigm, and I believe there are far too many career scientists out there these days not willing to take a step back and listen to what others are saying.

Theorys such as the standard model should be taught in school just as Newtonian gravity or any other theory is. The important thing to teach is scientific method. Theorys are useful tools to help us understand, predict and do calculations about the world. How ‘real’ something like neutrinos are is a question for philosophers, for me, they enable us to do experiments and calculations to good accuracy, and highlight (as in the solar neutrino problem) where our understanding needs improving. If someone comes up with a better theory it doesn’t mean previous calculations, such as the age of the earth, are necessarily wrong, they are just not as accurate as they could be.

[jamesr]

I don’t get your reasoning that radioactive decay heat is responsable. OK maybe it’s not the only factor butit has got to be by far the largest contribution to heat geteration. You only have to look at what the natural backround radioactivity all around us is to realise, and a quite back of the envelope calculation with the mass of the earth to realise that you can’t just toss it aside as irrelavent.

neutrinos are defintinely real – the spread in electron energy in beta decay means there must be a third particle shareing the energy – unless you want to throw out conepts like the conservation of momentum.

Most Uranium occurs in the crust as triuranium-octoxide which is pretty stable and has a density of around 8.3grams/cc. I suspect the proportion of Uranium dissolved into molten iron it would not sink to the core as the convective zone stirs it up to much to settle out. The central core of the earth is small compared to the whole volume – most of the heat would be produced in the rest of the mantle.

Fission is hard to acheive naturally – it needs a moderator, a light element such as hydrogen (as in water) to slow down the neutrons, in just the right proportion geometrically to acheive criticality. Uranium metal needs to be very pure with more than around 20% U-235, and no other neutron absorbing impurities to achive any appriciable level of fission on it’s own without a moderator.

The lack of active volcanism on venus now can be put down to the lack of water – plate technonics driven by water present in the crust cause most volcanism on earth. Subducted crust sinks into the mantle and dissolves making it less dense – it then rises and gases dissolved under pressure begin to come out of solution, driving their way through the crust. It is thought now that the major force on plates is being pulled down at the edges by the subduction, rather than being pushed as the mid ocean ridges or friction on the under side from the flow of mantle. The exception are volcanos like hawaii which form under hot convective plumes. It may be that on venus this mechanism is not strong enough now to cause volcanos of this type anymore.

The role of earths magnetic field is one aspect that is still little understood – how does the dynamo maintain itself?

[jamesr]

I’ve always loved the story of Rutherford giving his talk at the Royal Institution on the age of the earth in front of Lord Kelvin, who dozed off through most of it.

Lord Kelvin and the age of the earth – google books

One of the many things that inspired me to go into physics – how like a detective piecing together the clues from observations and discoveries we can learn the underlying truth of what is going on.

[jamesr]

I’m fully aware of what DPF & ITER are. As Brian said, I was trying to suggest that although some may regard the large scale & expensive tokamak projects as ‘the enemy’, there is a lot of valuable research going on in that field to do with materials properties and the interactions of plasmas with solid interfacing components.

Some of this may be applicable to the plasma/solid boundaries in a DPF. We need to take advantage of all the modelling and experimental results they get.

I should disclose my interest – I have just finished a Masters in nuclear physics, and will be starting a PhD in a few weeks modelling edge turbulence and instabilities in tokamaks and stellarators. Initially using data from the Mega Amp Spherical Tokamak (MAST) at Culham, UK and the Large Helical Device (LHD) in Japan.

Although my research may be from the mainstream side (I needed funding), I hope to be able to apply it different scenarios like focus fusion.

James

[jamesr]

I would think a lot will be learned from the ITER like wall experiments due to take place in JET soon

I know the plasma conditions are somewhat different in a tokamak, but the fundamentals of high energy ions colliding with the wall/electrode are similar.
They are looking at tungsten coated carbon fibre componsites for the divertor plates which take the brunt of the ion flux, and berylium on inconel (a nickel alloy) for the rest of the wall.

The goal in the tokamak is to stop high Z (atomic number) materials being sputtered or boiled off the surface and polluting the plasma. High Z ions in the plasma will result in higher bremstrahlung radiation losses and cool the plasma.

see Overview of the ITER-like wall project G F Matthews et al (2007)

[jamesr]

The dielectric must be an insulator to maintain bias voltage between foils, otherwise the whole thing will just short out. By an insulator I mean a material with a band gap energy of above a few eV. A semi-conductor for comparison has a band gap of the order of 1eV between the valence & conduction band, and a conductor has negligible or no gap at all. So at normal room temperatures eg 300K which corresponds to 0.025eV of energy for the average electron almost none are excited into the conduction band in an ‘insulator’. However if they are given a kick by the photo-electron from the x-ray absorption, an electron in the ‘insulator’ can gain the few eV needed to be excited into the ‘conduction’ band and so becomes mobile. The high energy photo-electron loosing a corresponding few eV in each interaction.

After a few thousand interactions the high energy photo-electron will have excited lots of secondary electrons and have slowly lost all its energy. This takes time (a few pico seconds) so the dielectric needs to be thick enough for all the energy to be deposited in the dielectric – I’m guessing a few to tens of microns.

If the photo-electron were to get to the next foil (or scatter back to the one it came from) then the atoms excited in the foil will not result in a movement of charge across the electic field created by the bias voltage (the foil being a conductor will all be at the same potential and hence no E-field within it and no motion of electron/hole pairs created, so they will just recombine locally with no nett current). So the proportion of all the original photo-electron’s energy deposited in the dielectric limits the the x-ray energy capture efficiency.

NB. all of this is just based on my understanding of X-ray absorption, and so how to get a measurable current from such an interaction, not on the specifics of the patent

[jamesr]

Collecting the energy of the X-Ray photons as as movement of electrons (ie. an electric current) involves a few processes.

First the X-Ray photon has to interact with an atom. The photoelectric absorption probability rises with the 4th power of atomic number of the material, so low atomic number materials (like carbon) do not interact well with X-rays. Hence the fleshy bits of people are dark on medical xray negatives & the calcium in bones is light.

So we want the x-ray photon to be absorbed in the high atomic number material (eg. tungsten). This results in an electron being emitted from the atom with pretty much the same energy as the photon had, ie 10-100keV. We want this electron to escape the foil and deposit its energy by exciting say 10000 secondary electrons of other atoms in the dielectic/insulator material.
Since we want the foils thin enough for the primary electron to esacpe the probability of an X-Ray being absorbed in any particular foil is still low – so we need lots of them.

On there own these secondary electron/hole pairs would just recombine not giving us any current. So by applying a bias voltage between the foils, the electrons & holes are drawn by the electric field between the foils creating a movement of charge and hence an electric current.

[jamesr]

Living in London I have had no need of a car at all. The underground and bus network is fine for most purposes, and when I need to travel somewhere not close to a station or late at night taxis are fine. Then on the weekends I can travel out of town to visit people by train or occasionally hire a car.

Mass transit can work but it needs to take priority over private cars. The growth in dedicated bus lanes and smart card ticketing systems has sped up bus travel in London so it is now faster than cars.

The trouble I see with most cities (especially in the US) is that due to cheap gas prices and available land they have grown so the suburban sprawl means the average commuter distance is tens of miles, rather than the few miles people used to walk or cycle to work in the past. This structure of city planning & suburban living cannot be got rid of quickly. It needs a complete change in city design and peoples expectations of wanting there own plot of land.

I am all for improving mass transit systems and encouraging people to live close to work/school etc. Having said all that though I am about to be moving out of London, so am finding that I can’t go without having a car as the rest of the UK doesn’t have adequate public transport in place.

James

[jamesr]

Lerner wrote: The anode, the inner electrode will absolutely have to have helium coolant tubes running through it

If the coolant is helium – I’m assuming then it enters as liquid helium at cyrogenic temperatures. With such a massive temperature gradient through the anode this I would think would have implications for the conductivity. The combination of lower conducivity at the hot surface and the relatively higher conducivity in the cool core of the anode, goes against the normal ‘skin effect’ of the effectively high frequency pulse of current flowing mostly near the surface.

Has any modeling been done to incorporate the temperature differentials on the electrical behaviour of the anode.

I have been playing around with a trial version of Comsol http://www.comsol.com recently and it would seem ideal for this type of modelling.

James

[jamesr]

Not sure about some of your figures there.

Decaborane density is 950 kg/m^3.
http://www.webelements.com/compounds/boron/decaborane_14.html

You quote here the density of solid decaborane – this has nothing to do with the the density of the plasmoid or the average gas density in the chamber.

From http://arxiv.org/ftp/arxiv/papers/0710/0710.3149.pdf
Total input energy in this example is 14.6 kJ, x-ray yield is 9.5 kJ and beam yield is 13.4
kJ, so total output energy exceeds input energy by a ratio of 1.57. Preliminary estimates
indicate that energy conversion of both the x-rays and the ion beam can reach 80% with proper
design, so that net energy production with close to 50% thermodynamic efficiency should be
possible, if other losses in the entire system can be reduced to levels small in comparison.

If total energy output ratio is 1.57 and you can extract 80% of that for electricity then the remainder will ultimately end up as heat in the system somewhere. Whether that’s in the device itself or the surrounding shielding.

If electricity output of a device is 5MW then there will be 5/0.8=1.25MW of losses – ie. heat to dispose of. Even if the conversion efficiency got up to 90% then there is still 10% not being converted which has to end up somewhere. I don’t reckon anyones wildest dreams would think the heat would only be 0.05%

A key challenge as I see it is that unlike in tokamaks where the plasma is kept away from the walls of the chamber, in a plasma focus the plasma will be touching the walls – heating them up and cooling itself down.

In response to Brian:

No, the whole plasma has to be kept quite hot

I guess its a question of how hot. I thought the overall plasma would cool between pulses to of the order of 10,000K and recombine partially. But the electrodes need to be kept under ~1000K.

James

[jamesr]

Brian H wrote:

Doesn’t make sense to me. The plasma must be maintained at a high temp to be useful, and that’s where the energy from the electron beam goes. Why would you want to drain power/heat from the plasma? Defeats the whole purpose.

Surely you can’t maintain the plasma at a high temperature. OK you want it to get very hot at the focus, but at the end of a pulse the small proportion of gas that was heated to a plasma is going to carry on bouncing round the chamber and heat up everything else. You need to keep the electrodes cool enough to keep their surface vapour pressure down so they can survive for a reasonable length of time, and the copper or beryllium vapour doesn’t poison the reaction.

I would suspect though that the heat extracted from the cooling mechanisms needed on the whole device will be low level thermal waste and not useful for generating any power. It could heat a few buildings or greenhouses though.

James

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