[jamesr]

I have been to several talks & seminars by Steven Cowley. For those that don’t know he is the head of the Culham Centre for Fusion Energy, which hosts the European JET and UK MAST tokamak programs.

Although his role now may be slightly more political and managerial, he is a scientist through & through. A good entertaining speaker, that can tailor the level of a talk well to appeal to the audience in question.

Being very much in the mainstream of fusion research, and the one who is requesting large sums of public money to fund the programs, I think there is a careful campaign by the CCFE to portray the tokamak approach, and the whole program leading onto ITER & DEMO as a low risk, inevitable and necessary project, that just needs time (&money;) to provide the worlds energy needs in 50-100 years time.

There is a resistant to anything, that distracts from this message.

[jamesr]

I went to a talk about this about 6 months ago – very interesting

The main interest is in studying “warm dense matter”. What they mean by warm, and dense of course is different to everyday use. It is a strongly coupled plasma state such as you find in the interior of gas giants such as Jupiter

more info can be found at http://www.nature.com/nphys/journal/v5/n9/full/nphys1341.html

[jamesr]

More details are available at: http://www.frascati.enea.it/ignitor/

It is only a short duration device, designed to run for ~10s. Purely for experimental purposes, a real power producing device couldn’t be this size/design.
It only has normal copper coil magnets so cannot run for long before they heat up.

It will be good to have a tokamak that can reach ignition before ITER as there is still more information needed regarding the stability of plasmas with alpha particle heating.

[jamesr]

vansig wrote: Yes. It might look like this IR – UV PV cell, recently reported.
http://www.physorg.com/news188637189.html

I wonder if this works as well when x-rays scatter instead of being absorbed?

The x-rays will have a continuous spectrum of energies upto around 70keV (ie the electron Temperature). most of these will be photo-electrically absorbed creating an electron/ion pair (the electron gets pretty much all energy the photon had). At these energies a small proportion will be compton scattered one or more times before the photons will be absorbed. Note the level of PE absorption rises roughly with the 4th power of the matierial’s atomic number (& hence number of electrons as a possible target)

The primary high energy electron created by the absorption rips through the material ionizing thousands of atoms in its path, until it has slowed and given up all its energy. So if the ionization energy of the material is say 1.5eV, a 70keV primary electron will create 70000/1.5=~47000 secondary electrons. These would normally just recombine with the ions after a short time. To tap them off as an electric current you need the secondary electrons to be created in an insulating layer separated by two electrodes in which you apply a bias voltage to cause the electrons to drift to the anode & ions/holes to the cathode. If the insulator layer is too thick then the electrons will still have a chance of recombining & hence lower the efficiency

So you want thin foil electrodes made of a light metal, separated by insulator/semi-conductor layers with a high proportion of heavy elements to absorb the x-ray photons, and allow the drift of the secondary electrons to form a current.

[jamesr]

Salgado wrote: I am aware of the “cold fusion” claims. Very interesting stuff, as much as it is controversial.

My proposal was to have one part hot fuel (high speed protons) and one part cold fuel (solid crystalline boron).
If fusion yield is an issue, maybe the protons can be put in a circular path (like a circular accelerator), going round and round until it hits something (hopefully boron!).
Would the protons just scatter in the crystal lattice, eventually running out of the necessary energy for fusion?

You will get some fusion if the protons have enough energy for head on collisions to overcome the Coulomb repulsion barrier. Indeed this is the method Cockroft & Walton used to achieve the first fusion reactions in 1932 (they used deuterium ion beam electrostatically accelerated into a deuterated metal hydride target)

However this would be an incredibly small proportion, and the energy needed to accelerate the ion beam would be orders of magnitude higher than that released by the reaction.

The majority of small angle scatters will just deposit the energy of the protons in the crystal, ending up as heat. If the beam was intense enough to produce a significant level of fusion reactions the heat generated by all the ones that don’t fuse would instantly vaporise it and turn the target area into plasma anyway.

[jamesr]

To quote from my last year’s Masters notes on Risk Analysis and Nuclear Safety:

– Risk is defined as
Probability of a detriment occurring * Consequence of that detriment
– Risk can be reduced by engineered safeguards

– Hazard is an intrinsic property of an objective
– Hazard can be reduced by reducing the quantity of hazardous materials or modifying its form.

“While there is significant work on numerical calculations of risk. The question of developing a Safety Culture is one that involves whole questions of person to person interactions and organisational management. These may well have overtones which are not obvious” Rouchlin on “The Social Construction of Safety”

– Radiological protection principles require justification of any practice leading to exposure to radiation
– Nuclear Power generation is justified by the net benefit from the electricity produced
– After shut-down of a plant the benefit ceases, hence decommissioning policy is based on a systematic and progressive reduction of hazard

Risk is framed in three bands of ‘Tolerability of Risk’:
– Highest Risk: Intolerable region – unacceptable save in extraordinary cercumstance
– Intermediate: Risk must be managed as shown to be As Low As Reasonably Practicable (ALARP)
– Lowest Risk: Broadly acceptable region

[jamesr]

The bremsstrahlung is minimized by the specific level of the mag field, IIRC, not by keeping the thermal energy high. As far as the field lines of the plasma containing the heavier nuclei, I assume that they are all ionized, and hence tightly contained.

The quantum effect when the electron larmor radius due to a strong magnetic field is of the order of its de Broglie wavelength is a small (but key) factor, in that it may reduce bremsstrahlung from boron by a factor of 5 or so. Even with this effect the plasma would radiate its energy away and cool to well below the temperature needed for fusion in picoseconds rather than the nanoseconds needed to burn enough fuel to get nett energy gain. It is the recycling of energy from the helium to keep the hydrogen and boron hot enough for long enough that will make it viable.

At the magnetic field strengths involved, yes, the ions are well confined, and are bound to follow the field lines (gyrating around them). But no magnetic confinement is perfect, as collisions cause diffusion and transport of ions away from the dense core. Once an ion has scattered out to the last closed field line, or separatrix, of the plasmoid it will escape.

The question is – what is the probability for tritons or alphas that are created sufficiently close to the edge to have a chance of escaping before they have given up all their energy

[jamesr]

I think a large part of the cost is due to the nature of the deal for each country to get the skills to build every part of it, and so build up the knowledge and infrastructure to build other tokamaks in future.

So rather than have contractors bid on say the superconducting magnets then the consortium decide which one to go with. There ends up with several contractors making some each, which all have to be made to the same spec. So rather than one company making the tools and rigs needed to make lots identical parts. Each contractor in each country sets up its own factory to make a few. Which then have to be project managed and compared centrally to make sure they all meet the spec and will work together when fitted.

Repeat this for a million or so parts, coming from all over the world and you get an idea of the bureaucratic nightmare

[jamesr]

Brian,
As far as i knew the the a significant proportion of the He will be retained in the plasmoid (and needs to be if appreciable fusion is to occur). After >50ns (ie a long time), when hopefully most of the fuel as fused does the collapsing magnetic field create the strong axial electric field that accelerates the beam of ions and electrons.

If a triton or alpha is produced near the edge of the plasmoid then it might escape, but if it is generated somewhere such that the closed field lines of the plasmoid keep it trapped long enough to have the 20 or so collisions needed to deposit all its energy into the dense part of the plasma then that energy can make up for the huge losses due to bremsstrahlung radiation that is cooling the plasma.

By using all the information provided by these deuterium tests and understanding the processes going, I was suggesting that you could use the tritons to estimate how stable the plasmoid is to the production of fast ions, and so how much of their energy is transferred to the fuel ions to maintain the temperature high enough for them to fuse.

[jamesr]

Lerner wrote: Well, we, the experimental team don’t. We need more data. We need to either see clear evidence of neutrons from D reacting with tritium produced in the plasmoid or we need images to measure the radius of the plasmoid. We don’t have either yet, but we are working on it. Stay tuned!

So given 50% of the D-D reations produce a 2.45MeV neutron and the other 50% produce a triton which can then go on to produce a 14.1MeV neutron. Can you use the flight time data from both reactions to help recover the temperature of the plasmoid more accurately than with just the one peak.

Given D-T’s much higher cross section, would any confined tritium undergo enough scattering collisions to completely thermalise before undergoing fusion, or would a significant proportion fuse after only a few interactions and so the spread of the 14.1MeV peak would be too broad any not representative of the temperature.

If you assume tritium not only has time to thermalise, but also that any that does and remains confined will have a very high probability of fusing, can you use the ratio of D-D neutrons to D-T neutrons to gain some insight into the proportion of tritons (that would be produced with 1.01MeV) remained confined in the plasmoid. Could you then extrapolate the theory forward to get an estimate of how well confined the He produced in the pB11 reaction will be, and so how well the energy is recycled into the plasmoid to achieve ignition?

[jamesr]

QuantumDot wrote: On the Wikipedia page about DPF it says that, the ones that are larger in the MJ, MA range have pinch’s that last milliseconds, but i didn’t see what experiment that it was referring to and if they do exist does that mean that somewhere the right equipment exists, do you or anyone know? with the improvements that have already been made so far the millisecond confinement time with megawatts of input power look very good.

“These critical phases last typically tens of nanoseconds for a small (kJ, 100 kA) focus to around a microsecond for a large (MJ, several MA) focus”
http://en.wikipedia.org/wiki/Dense_Plasma_Focus

Large devices like the PF-1000 in Poland have been operating for years. The image at http://www.intimal.edu.my/school/fas/UFLF/ show the scale of the machine. They do not achieve a very dense focus though compared to smaller devices, and so the triple product of density*temperature*confinement time needed for appreciable levels of fusion is not as high.

More details of the scaling from kJ to MJ devices based on the Sing Lee model can be found at http://www.plasmafocus.net/

The size of FoFu-1 was calculated to be near optimal. When they switch to the pB11 from deuterium the electrodes will need to be swapped out for even smaller ones as I understand it.

[jamesr]

vansig wrote:
with .5 kJ in the electrons, and 1 kJ in the plasmoid, that’s 1.5 kJ;

The is total is 1kJ not 1.5kJ: The 0.5kJ in the electrons was estimated, as I understand, from the erosion from the electron beam on the anode. This means the ion beam also has 0.5kJ giving a total in the plasmoid of 1kJ before it collapsed and emitted the two beams. From the peak current they can tell how much energy was stored in the magnetic field, and so find the proportion that was focused into the plasmoid

The figure it needs to be compared against is the energy in the capacitors before the shot, which can be easily found from the voltage and capacitance Energy=0.5CV^2
I tried to see on the gallery pictures what the labels on the capacitors say they are but I can’t make it out.
But say they added up to 12uF, at 24kV charging voltage this gives 3.45kJ total. As they ramp up the charging voltage more energy will be able to be delivered into the pulse.

[jamesr]

Phil’s Dad wrote:
There was an experiment in the UK to get the public to run climate models on their home PC’s. The big picture was split into chucks they could cope with – probably just a handful of grid points/cells – which they ran for a week. They e-mailed their results to other members of the public to crunch and so on until it was boiled down to something a single machine could handle.

Actually that project used a different method. When you’re running on separate home PC’s you can’t exchange data very well as the bandwidth is too low and latency way too high. This is also true for all the other distributed computing projects like SETI or Folding@Home. Each machine needs a separate task that is not time sensitive or dependent on any other processors.

They gave each PC a whole earth to model, but with a fairly course grid. They are , after all interested in climate not local weather. Each PC was given slightly different starting conditions and response parameters. For example one scenario the oceans may absorb a little more CO2, another the rate of deforestation may be a little lower etc. By running the climate model forward from each of these scenario’s you learn how sensitive it is to variations in each parameter and how hard it can be pushed. So if you can estimate the errors on your starting assumptions you can extrapolate forward to give you the error on the prediction a hundred years from now.

[jamesr]

Aeronaut wrote: Cool deal. What would the outlook look like if you were to begin with each core thinking it was measuring and projecting the weather for 960 towns, villages, crossroads, etc? Iow, we have a much smaller plasma, so we can map it more precisely.

The smaller plasma does not make it simpler. The small scale, and high density just mean the grid size & time-step for the simulation are smaller – although not quite as bad as the situation in inertial confinement fusion.

Our little cluster is tiny really – I can only model 3D grids of a few hundred points on each side, maybe a billion gridpoints for a few tens of thousands of timesteps. To model the focus of a DPF device would take much more.

My code, as do most parallel codes of this sort, works by domain-decomposition – each processor handles a small local region, say 64x64x64 grid points then exchanges the field values, densities, velocities etc at the each edge with the relevant neighbouring processor each timestep. This should in theory scale well, that is it efficiently makes use of all the processors as you run it across more & more of them.
Then I may be able to run it on the UK’s larger academic supercomputer called Hector which has 22656 cpu cores, and is currently No.20 in the top500
The Met Office’s weather modeling computer is down at No.89 in the list. All these are still small in comparison to the DOE/NNSA/LANL or Oak Ridge’s computing facilities.

If FF-1 achieves its goals then it is this scale of computer that we will want to run detailed simulations on. However well the experiments go, these days in order to manufacture any nuclear device they must be simulated to prove the physics is well understood, and dosage/shielding calculations need to be done to design the required containment (however small the dose may be).

[jamesr]

As part of my PhD, I am writing a 3D model at the moment for simulating the turbulence at the edge of tokamaks to run on our 960-core parallel HPC cluster. All the time though, in the back of my mind, I keep thinking about how I can make it generic enough to cope with the conditions in a DPF.

Most simulations work either by simplifying the ion & electron motion by averaging out the fast gyro motion of the ions/electrons around the magnetic field lines, or just treating it as a single conducting fluid, with a resistivity that varies with density & temperature. These assume there is only one ion species (typically they just model a perfect hydrogen plasma), so the number of ions & electrons are the same.

Hopefully, when I get a bit further, my model will treat the electrons and different ion species as separate fluids, coupled together by the E&B fields and the odd collision. So in theory going from my edge plasma with some impurities to a hydrogen-boron plasma should not be too much extra.

One thing it won’t do though is handle the bremsstrahlung & other radiation effects key to a DPF begin viable. – That’s a whole new topic.

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