[jamesr]

Breakable,

I had a quick test of the new wiki, with a plan of fleshing it out with a few pages. However before I get started can the TeX option be turned on for rendering maths formulae.

According to http://meta.wikimedia.org/wiki/Help:Formula

To have math rendered, you have to set $wgUseTeX = true; in LocalSettings.php.

[jamesr]

Here’s a few notes to start with…

asymmetric_implosion wrote:
What is pulse power?

Any power source based on releasing the energy as series of short pulses. So for example the internal combustion engine is a pulsed power device, whereas the jet engine is a continuous power device.

Why use it?

Whether we are talking about chemical or nuclear reactions, often the easiest way to get an energy releasing reaction to proceed is to provide a limited amount of fuel, then an initial amount of energy to ignite it and let it burn in a (semi-)uncontrolled manner.

In order to design a system to burn continuously, you need to be able to get the balance of the reaction exactly right, and be able to remove the exhaust products.
Although potentially more efficient overall, it adds a lot of complexity.

In the case of Focus Fusion, the requirements needed to ignite the fuel (temperature & density) are substantially easier to achieve in a pulsed power device.

What are the components of a plasma focus?

– Energy storage – Capacitor bank
– Fuel supply – Decaborane compound of Hydrogen and Boron (enriched to be mostly Boron-11)
– Vacuum chamber – the main reaction vessel, containing:
– central cylindrical anode, with a hollowed out end
– array of surrounding rods at the cathodes.
– diagnostic sensors (pressure, temperature etc.)
– Rogowski coil for extracting energy from the ion beam
– “Onion” – layered shells of foil to extract energy from X-rays
– Vacuum pumps
– Switch gear & trigger mechanisms
– Cooling systems (to keep the anode & other parts within design limits)
– Coil to provide the small magnetic field, seeding the angular momentum of the filaments, improving performance
– Electromagnetic shielding (Faraday cage)
– Radiation shielding (neutron & gamma)
– Power conversion circuits & capacitors – to transform the pulsed output to AC feed suitable for the grid
etc..

What are typical specifications and limitations of these components?

Most of the components are standard off-the-shelf parts, apart from the electrodes themselves and the energy extraction coils & onion – for these its too soon to say.

Is there more than one way to build a plasma focus pulse power system?

The use of the hydrogen boron reaction and the requirement to get to Gigagauss field strengths, constrain the scaling of the device (such as defining the optimum anode length), effectively dictating most of the rest of the design. However there are many details that can vary, such as the number of cathodes, capacitor design etc. that can vary.

Can current technology support a working Fo-Fu power plant?

If & when the scientific feasibility is proven, then there will be a number of engineering challenges to overcome, for the anode cooling, onion design among others. However these, for example, are minor in comparison to the engineering challenges facing the mainstream magnetic and inertial confinement fusion programs at ITER & NIF.

In the bigger picture, considering a Fo-Fu power plant as a collection of 5MW black box generators, then it needs continued investment in the grid infrastructure to move away from the centralised big Gigawatt plants to a more decentralised model.

[jamesr]

ikanreed wrote: Every blow against fission is a blow against fusion in the short run too. It decreases demand for experts in nuclear reactions, it reinforces the misconception that scientists don’t understand nuclear power well enough, and hurts the money going into the blanket field of nuclear research. Nobody wins.

I couldn’t agree more.

Whether fusion becomes viable in 10years or 50, we need bulk power now. That means a new generation of tried & trusted fission plants. It also means we need a generation worth of children growing up with the necessary maths, physics & engineering to make it happen. And a generation worth of educating the people of the world about the true nature of risk.

[jamesr]

Steven,
Have you come across this. http://www.oup.co.uk/pdf/0-19-856264-0.pdf it is a useful excerpt that goes over the basics.

From this you will see on page 5 that the potential barrier is of the order of 1MeV the barrier is measured as a function of distance, r.

What you claim, if I am understanding you correctly, is that by lowering the potential at large r, that you shift the whole graph down and so lower the height of the barrier.

What I’m saying is that if you lower one, you lower both. It doesn’t matter what reference level you pick for what the potential is at large r, it is the same for both, the height they need to climb is the same.

If your theory was true, you could setup an experiment with the potential lower than 1MV with respect to your ‘observer’ and the barrier would be completely shifted out of the way, and make fusion trivial. This is clearly nonsense.

[jamesr]

I don’t mean to put a complete dampener on your plans, I just wanted to highlight the folly of your concept.

It really doesn’t matter where they were ionized. Once the ions have been oscillating back & forth in the well millions of times, scattering off all the others & mixing their energies & velocities, their origin is meaningless. Plasmas need to be understood by their statistical distributions and how they vary in space.

The potential is a macroscopic scalar quantity found by solving Poisson’s equation (ie integrating the charge density over the entire volume, with appropriate boundary conditions, eg the walls of the chamber are set to 0). It is a function of position, not of any particular particle.
The gradient of the potential at each point is then the local electric field.

In a plasma, by definition, macroscopic properties like this should only be used on scales larger than the Debye length: http://en.wikipedia.org/wiki/Debye_length

Below the Debye length the closest few dozen ions & electrons have to be accounted for individually rather than just considering the bulk, statistical properties like temperature. But this is still on the scale of nanometres to micrometers.

Fusion happens on a much smaller scale still. As I said before, as two ions become close enough to fuse, they will be essentially be at the same point in space. So they will be at the same potential. The only significant influence on one ion is the other ion.

[jamesr]

Further to my comment on Elsevier. The Guardian published this today:
Academic publishers have become the enemies of science

[jamesr]

Steven Sesselmann wrote: what I do believe is that ionizing particles below ground potential, lowers the relative Coulomb barrier.

In a word… No.

All the potential does is create a E-field to accelerate the ions & electrons. At the point of closest approach any two ions in question will be right on top of each other so the the potential difference between them will be near enough zero. The E-field creating the ‘Coulomb barrier’ from their own positive charge is many orders of magnitude larger.

Lets do a few quick sums.

The deuterium cross-section at 50keV is roughly 0.02barns = 2E-30 m^2 This probability of fusion reaction corresponds to the area swept out by one ion as it approaches the other
take the square root of this to get the rough distance (to an order of magnitude) = 10^-15m – We need them to get this close to fuse (NB this is smaller than what is sometimes thought as the geometric size of a nucleus which corresponds to its scattering cross-section)

The electric field created by one ion at the position of the other at this distance is E=1/(4pi*eps0) *q/r^2 ~10^20 V/m

So in any case a few hundred thousand V/m Electric field created externally is insignificant to the field the ions must work against as they approach each other.

[jamesr]

While I applaud your enthusiasm, I’m not sure you really get how fusion works. You mention half way through (08:00) that you don’t consider the ion collisions as the cause of fusion! It is all about this!

ions need to be travelling fast enough that, if they happen to be on a collision path, that their kinetic energy will be able to overcome the electrostatic repulsion of their positive charges until they are close enough for the strong force to take over (or at least close enough for them to have a quantum probability of tunnelling through the remaining potential barrier)

If you are to get an appreciable number of fusion reactions then you need the plasma dense enough. If it is this dense (at the bottom of the potential well) then since most of the collisions will be elastic scattering at small angles, the plasma will begin to ‘thermalise’.
Of all the scattering collisions, some ions will end up with more energy & some with less. The ones with more can escape the potential well, some will then hit the grid or walls of the chamber, leaving just the ‘cool’ ones trapped

You can keep it hot by supplying ‘even hotter’ ions at the 100keV or so your power supply can deliver but the energy needed to do this will always be huge in comparison to the fusion rate.

Do you have any estimated figures of density & velocity distribution vs radial position?

[jamesr]

A large part of that is from the Elsevier side of the company (28% revenue, but 44% of profits in 2006). http://en.wikipedia.org/wiki/Elsevier

They publish a huge proportion of all the high impact journals. Many universities and institutions are up in arms about the high subscription prices they charge, but can’t just stop getting them so there is little left in library budgets for anything else.

[jamesr]

Their ‘True Energy Cost’ bar chart is somewhat biased against nuclear. It always makes me laugh when I see so called scientific analysis like this with its ‘infinite risk’ poking out the top of the graph with the radioactive symbol next to it. Also giving nuclear its own bar is scary red to show the total cost. Why doesn’t it just have it in the green ‘external cost’ section.

The risks from nuclear are well known and quantifiable. There are way more deaths from coal, or hydro electric

Geothermal is hardly risk free either, adding to seismic risks and polluting water supplies

[jamesr]

opensource wrote: Thanks James, you always clear things up…

Glad to be of service…

opensource wrote: Can some gas be used to slow the x-rays to decrease reactivity and cooling?

X-rays always travel at the speed of light, c.

To have a chance for a significant proportion of the X-rays be re-absorbed by the plasma before leaving it you need something the size of a star (well not quite – but you get the idea)

[jamesr]

Its not about quantity. Quality is what matters. LPP, being a private commercial enterprise, are not going to just upload all their raw data. It has to be analysed and interpreted. There will be many series of shots testing and calibrating diagnostics, coming up with new combinations of timing, voltage, pressure and other adjustments before each set of actual ‘optimised’ experimental shots to gather proper data.

If you watch some of the videos posted previously it only takes a few minutes to setup and fire a shot, so I’d think they could do a couple an hour. If it has been opened up for cleaning or other adjustments though it would take a day or two to get back to operating conditions.

[jamesr]

opensource wrote: So if using x-rays forces us to work against the bremsstrahlung cooling effect in designing a DPF, then maybe we shouldn’t use x-rays? Have there been any clever proposals for dealing with bremsstrahlung cooling besides just scaling to work against it?

“Using x-rays” is the wrong way to look at it. In any hot plasma the ions & electrons are bouncing randomly around (influenced by the local E & B fields) – whenever an electron is scattered through an appreciable angle a photon is created with an energy corresponding to the change in momentum of the electron. This ‘breaking’ radiation is Bremsstrahlung, and if the electrons are at keV sized energies then the resultant photons are in the x-ray portion of the EM-spectrum.

X-rays cannot be avoided. If you want a hot, dense plasma it will radiate away a proportion of that energy – the hotter it gets the faster it radiates, making it hard to achieve the temperatures required for fusion. The recent x-ray measurements implying a rough 400keV electron temperature suggest FoFu-1 is achieving a sufficiently rapid pinch to heat the plasma faster than it can cool.

Since the proportion of fusion energy released and transferred to the ion & electron beams when the plasmoid collapses can never be enough to achieve nett energy gain, some of the energy lost from the plasma in x-ray radiation will need to be captured as well.

[jamesr]

It is not just the C-11 decay. The neutrons produced in the side reactions during operation will have been captured by various isotopes of all the materials that make up the device and surrounding structures. Such as Cu-63 +n -> Cu-64 which then decays to nickel or zinc with a half life of 12.7hrs. There will be hundreds of combinations of activation & decay reactions going on, each insignificant on its own but when modelled as a whole can add up to an appreciable contribution. Particularly the cumulative contribution from slightly longer lived isotopes after a few years of operation – hence why careful choice of materials down to every nut & bolt is needed.

Also care should be taken when saying the level is back to ‘background’. Background in New Jersey is different to Colorado. The contribution from the device needs to get down to ‘As Low As Reasonably Achievable’ (ALARA) http://www.nrc.gov/reading-rm/basic-ref/glossary/alara.html , which could be lower than the natural background.

[jamesr]

wolfram wrote: In most plasmas that I’m familiar with, electrons are consistently much hotter than ions.

That depends on the heating mechanism. For example, resistive heating is only significant for electrons, so they can get to a higher temperature this way. But this heating mechanism is only capable of getting plasmas to a few keV not >100keV (since resistivity falls with temperature).

Alternatively, as in tokamaks, ion cyclotron resonant heating (ICRH) can be used to specifically heat just the ions.

In the case of DPF, I see the 400keV electron temperature as a bad thing since it leads to excessive bremsstrahlung radiation losses. The key will be if the proportion of energy going to raise the electron temperature can be limited enough with the switch to p-B11 fuel.

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