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People use optical methods for time of arrival of the current sheets. It is important to note that most plasma focus devices don’t rely on individual filaments but a uniform sheet of plasma. FoFu is somewhat unique in that respect. The methods are proven but they cannot measure the magnitude of the current or some relative measure of current in a meaningful way. B-dot probes, the most common magnetic probes for local B-field measurement, would be useful but they are shielded by plasma that is strongly dependent on local parameter that aren’t easy to diagnose so you cannot say if the measurement is “good” or not. Measurements using B-dots have been attempted with some success but the experiments are difficult and far from low cost if you think $50K is expensive. A fast framing camera can also be used but they are very expensive by the $50K standard. There are a number of paper in published literature that discuss these diagnostics since 2000. Papers by the NTU group on NX-2, PF-1000 papers published since 2009 and some work done by Moreno show these diagnostics in action. If you want a sort list of papers I can supply the information to download them.

The production of fusion reactions is a common thing in plasma focus devices. The applications for most groups is for producing neutrons for various applications so producing neutrons is not a problem. Most of the groups I mentioned above produce neutrons intentionally for those funding their work. Most of the groups are active based upon recent publications. If you aren’t going to use fusion fuel gases you need to pick a representative gas system. You can use H2 as a surrogate for D2 but you need twice the pressure which may change the way the filaments evolve. You can use heavier gases but the radiation emitted by the filaments changes which can impact the local environment by photoionization and secondary electron emission from the electrodes. Neon and Argon emit copious amounts of UV during their axial rundown.

The ion beam and electron beam do exit the pinch region in different directions. The electron beam hits the anodes so you have to make the anode hollow and put a beam diagnostic near the exit. The ion beam moves away from the anode and it is more straightforward to measure. People frequently use time of flight or ion spectrometers to measure the ion beam spectrum.

In my opinion, the best diagnostics package on a plasma focus right now is the PF-1000. They have multi-frame interferometry and neutron time of flight that allows them to reconstruct the ion distribution that generated the neutrons. The work is particularly relevant to FoFu research. I know Mr. Lerner does not put much faith in interferometry but it is an excellent diagnostic and it would address a number of the problems you are describing by measuring the electron density in its conventional form or the electron density gradient in a shearing form.

The problem with many of these experiments is the cost. As I said scientist are expensive and access to these facilities costs as well. There is a minimum contract value that most companies or big labs will accept because of admin costs will drain the contracts. I don’t know what you have in mind for low cost but I believe you mean a great deal of good will. General Electric has a rule about research. For every dollar spent on research, seven dollars are spent on development and $49 are spent on building the first working unit. A quick sum assuming ~$2M at the research base clear $100M pretty quickly. As with any fusion system, getting the plasma “right” is not the worst problem. The materials issues are the real problems. I suggest looking at materials issues if you want to do scaling down experiments because you can replicate the operating pressure of FoFu-1 and the current density at low cost. The erosion can be studied and the lifetime can be estimated.

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Tulse wrote:

It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason.

If the problem is gas circulation, then one presumably could just weld the cathode rods on the inside of a series of rings, which would make the assembly one piece while still allowing circulation.

I get the sense that there is a lot of interesting testing to be done on cathode parameters.

Welding is a poor alignment technique. If you are talking about alignments of 100 microns that is very difficult with welding. Machining can be done at the 25 micron level but it costs much more than welding.

I don’t know how much interesting physics will come from the cathode. People have studied it on and off for years. The largest gains in fusion yield or x-ray yield tend to come from anode changes. The anode is closest to the pinch so it seems to influence the final result much more than the cathode. Who knows though? I could be very surprised.

The real problem I see is the size of the parameter space. You have the diameter the cathode rods reside on, the number of rods, the diameter of the rods and the shape if you wish to explore blades, machined tubes, triangles, etc. Toss in materials and you can study for years burning up millions of dollars.

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markus7 wrote:
Solutions that could be investigated include:

Highest priority – New instrumentation applicable to both 1) the LPP fusor for continuously diagnosing asymmetry problems (sensors for the current and arrival time of each of the sixteen arc filaments and perhaps new sensors for the pinch region?) and 2) critical to the ‘dummy’ fusor test effort, sensors to serve as the ‘goodness’ indicators of testing in the absence of neutron production.

Secondary priorities –
Whatever people can think of
Big geometry variations

1) Wow! This is no small task.

The plasma sheet in most plasma focus devices, if monitored, uses either optical techniques or magnetic probes. The magnetic probes provide information current locally but they are subject to plasma shielding. If a moderate conductivity plasma is generated over the surface of the probes which commonly happens in even ~100 kA devices, the probes need to be corrected for magnetic shielding effects. The key is you have to know the conductivity of the plasma and its thickness. Neither is trivial. The optical emission techniques provide some data on plasma sheet asymmetry but the current or current density is not easy to derive. This is a significant problem that faces many high current plasma devices and people are working on the problem, but it is a very difficult problem.
You can forget about putting diagnostics in the pinch region. They typically screw up the pinch or get badly damaged on the first shot. Emission techniques or laser probing techniques seem to be the only option.

2) Smaller plasma focus devices of the ~300 kA level are reasonably common world wide. Nanyang Technical University in Singapore has two such devices. Kansas State University has a device. A couple companies in the US are using these devices as well. NSTech at the 2 MA level and Alameda Applied Sciences at the ~300 kA level. LLNL also has a ~200 kA device. So the test bed could be available if someone can spark and interest in any of these places. The common diagnostic choices for non-neutron producing reactions would be ion spectrometers which could detect alpha particles from p-11B reactions. They are simple devices in principle but they require alignment using an known ion source. Some papers exist on how to do it but you need a particle accelerator in most cases as the ion source. Other techniques would be nuclear activation using alpha particles as the source. I can imaging building a target that is activated by p-11B alpha particles. I would choose a beta emitter and use a scintillator to count the beta particles as they are produced. In small devices the yield is likely to be extremely low so counts could be a problem, but it might be interesting. My guess would be a money problem. People have specific funded program or internal goals and funding would be required to develop the diagnostic and complete the tests. I would guess ~$50K to develop a single unit activation system for alpha particles and calibrate it to yield. Scientists are expensive.

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Tulse wrote: Given that symmetry in the physical geometry of the electrodes appears to be important, is there any reason that the cathodes have to be separate rods? A solid piece, with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment. Is it necessary to have empty space between the individual cathode rods?

It’s not 100% clear why, but a plasma focus seems to work better with individual cathode rods. People speculate that some of the gas between the anode and the cathode needs to be pushed out while others cite debris as the reason. Groups have employed blades instead of rods that are mounted or welded to a single base piece that helps with alignment. The open area, rod diameter, etc seem to be able to cover a wide range of conditions and still achieve reasonable results. The cathode is not nearly as well studied in PF devices as the anode.

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5 shots with 2.6% variability; a nice start. 500 would be better but I know that takes time. I hope the small variability continues as the shot number increases.

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jamesr wrote: Thanks for the links. I haven’t read them all, but in R. Petr et al’s paper their ‘high power’ is 1.5kJ input yielding 0.08% of X-rays (ie 12J).

Using the figures from Eric’s 2007 Tech Talk (http://www.youtube.com/watch?v=O4w_dzSvVaM 36m30s in) he had:
Peak I 2.0MA
Gross input: 13.1kJ
X-Ray/Input: 81%
Beam/Input: 98%
(Beam+X-Ray)/Input: 1.79

so 0.81*13.1 = 10.6kJ ie. ~1000x as much X-ray energy as these lithography devices. One or two orders of magnitude more and maybe erosion resistant materials can cope with a reasonable lifetime, But we are talking about 3 orders of magnitude higher fluxes. The end of the anode has got to absorb only a tiny fraction of the X-rays passing through it if it is going to survive. Basically that means Beryllium is the only candidate that comes close.

I refer you to comments made in another thread (https://focusfusion.org/index.php/forums/viewthread/973/P15/#9513) about this. I did a little back of the envelope math and it turns out that W is not so bad when one considers a few things.

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jamesr wrote: I agree, if Be can be avoided that would be great.

However, for a device hoping for good fusion yields you need to be very careful that no high-Z ions can be eroded off and pollute the hottest part of the plasma as they will cause rapid cooling. But more importantly the pinch X-ray flux will be orders of magnitude higher than any ‘low power’ studies done before, and we need to be able to recover the X-rays in the onion. Which means the anode must be as transparent to them as possible, to avoid shadowing a large portion of the collecting area.

Having said that I wasn’t aware Molybdenum had been studied before – do you know the source paper(s)?

General materials survey

R. K. ROUT, A. SHYAM and V. CHITRA “EFFECT OF ELECTRODE MATERIALS ON THE NEUTRON EMISSION FROM A PLASMA FOCUS” Ann. nucl. Energy, Vol. 18, No. 6, pp. 357-358, 1991

Moly in a high rep rate source

R. Petr, A. Bykanov, J. Freshman, D. Reilly, and J. Mangano “Performance summary on high power dense plasma focus x-ray lithography point source producing 70nm line features in AlGaAs microcircuits” Rev. Sci. Instrum. Vol 75 No 8. pp 2551-2559 2004

To start, the paper by Rout et al shows that a very tiny fraction of the high Z atoms make it into the plasma but it doesn’t specify where the atoms are. Petr et al showed long lifetime operate of a Ne based plasma focus for ~5 million shots with moly and very little anode erosion.

R. Verma, P. Lee, S. Lee, S.V. Springham, T.L. Tan, R.S. Rawat and M. Krishnan, “Order of magnitude enhancement in neutron emission with Deuterium-Krypton admixture operation in miniature plasma focus device” App. Phy. Lett. 93 2008

B. L. Bures, M. Krishnan, R. Madden and F. Blobner “Enhancing Neutron Emission From a 500-J Plasma Focus by Altering the Anode Geometry and Gas Composition” IEEE Trans. Plasma Sci. Vol. 38 pp 667-671 2010

To further complicate the matter, high Z may not be a bad thing. People intentionally introduce high Z gases with the fuel gas to improve neutron yield. The high Z gases appear to improve compression of the pinch in small quantities (less than 10% by mass). Measurements with X-ray cameras do not show any contribution from a deuterium pinch. If the pinch was contaminated with high Z impurities they would by highly visible.

The final consideration is the x-ray contribution to the total yield. Even if the x-ray yield increases by orders of magnitude, the production of fusion charged particles will increase by orders of magnitude. The fusion contribution typically trumps the x-ray contribution by nearly 100x. If direct energy conversion is applied to the charged particles, something like 70% can be converted to electricity. For a 100 MJ per shot (fusion gain) system, that means nearly 69 MJ of energy derived from direct charged particle conversion. The x-rays are likely to be converted to electricity by heat then turbine. The x-rays make up ~1 MJ and can be converted at best 40%. Therefore, the x-rays provide ~400 kJ of electricity at best. One could claim some sort of exotic photovoltaic, but the conversion efficiency is not going to exceed 40%. By these arguments, the x-rays need to be shielded and not converted; a tiny fraction of the radiated power that is converted very inefficiently is not significant. Therefore, a refractory metal is the best choice for an anode.

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Why is everyone hung up on Be and C-based materials? Be is a mess of a material. It is toxic and can produce neutrons when hit by hard x-rays. The cost is a real problem too. Non diamond, C-based materials are susceptible to hydrogen etching and they generally have a poor electrical and thermal conductivity compared to metals. Based upon published studies about 20% of the bank power ends up on the anode when you have a highly electrically conductive metal and must be removed as heat. For a high repetition rate application like power generation, a modest resistivity material like graphite will only increase the heat load. As the electrodes get hot they tend to emit impurities or form other materials such as BC4 (insulator) in the presence of boron.

Studies were conducted nearly twenty years ago on electrode materials for plasma focus devices and the “best” materials were operated at up to 80 Hz (80 times a second) for 5 million shots. The winner was Molybdenum. It is a higher Z material which increases the x-ray dose a bit but the x-ray can generally be shielded with less than 0.5″ of lead or tungsten. The lead or tungsten shield is far less costly than a Be electrode.

It’s great folks are thinking outside the box from the typical copper electrode, but this was thoroughly studied in past experiments considering many of the key factors for fusion yield. Why reinvent the wheel when the wheel is well studied and it works?

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In principle, it should work for all fusion reactions that involve hydrogen in the tokamak or other long confinement schemes. Thank Tri-Alpha energy for this breakthrough. It was their work presented at APS-DPP last year that inspired this test. Lithium was the redheaded step child of fusion energy and now it is the king. Funny how one presentation can change the world. Go small science.

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Steven Sesselmann wrote: Some things we take for granted, but when we consider it carefully, it may not be so obvious after all.

Ground potential means absolutely nothing, because a bird sitting on a 10,000V power line feels no charge, in fact we can put a scientist inside a metal cage, charge it to -10,000 volts, and ask him to find the extra electrons, and of course he won’t find them, because inside his cage everything is just fine and normal.

My answer to this part anyway. All measurements are relative. This is known and accepted. In fact, those that work on extreme high voltage rely on it. We choose earth as a reference in most cases because it is large object that we all can agree on. It has a large quantity of electrons so adding or subtracting a tiny fraction doesn’t change the potential of the earth. It is a huge capacitor. Take a multimeter and stick the two probes in the earth. You will measure near zero voltage difference. Take your meter to your metal cage. Touch the cage and measure the voltage between the earth and the cage. (Special meter may be required at 10 kV). You measure it as negative relative to ground as an example. Discharge cage and place scientist inside. Charge cage. The scientist measures the potential of many points in the cage relative to each other. You should get a near zero voltage difference. This is were you stop. You need to complete the experiment. Take one probe and touch the earth outside the cage. The other probe touches the cage. If you have been consistent, the earth will measure the 10 kV with the opposite polarity. You can argue which object has lost electrons or gained electrons, but the number of electrons is so small relative to the total number of electrons in a solid that they are difficult to find. For the non-believer, try it for yourself. I would recommend a lower voltage than 10 kV.

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balsysr wrote:

I think the same kind of problem exists today, all the money is spent of research that backs the existing paradigmns, and only ridicule is spent on other ideas. A case in point is the work on focus fusion. The big money is all on gravitational fusion based sun models that have consumed billions of research dollars for little results. Approaches like focus fusion are relegated to “fringe” science and not funded as the money is all being spent elsewhere. A better approach in funding research would be to allocate a reasonable pool to non-mainstream ideas that have a reasonable scientific basis (like focus fusion).

Cheers,

Science has problems; no doubt. It isn’t a problem of conspiracy; it’s a problem of human nature. We have these feelings that screw with our analytic minds. Feelings connect us to people and concepts. One can love an idea as much as a person and this is the problem. People fall in love. You fight to protect what you love and some use their positions and knowledge to protect their beloved ideas. It’s not evil, but it is not proper science.

There seems to be a misconception about fringe science and mainstream science. As a researcher, I reserve ridicule for a select few; they usually earn it without anyone helping them or putting them down. I doubt a great deal. There is a significant difference. Doubt (skepticism) is a driving force behind science. One that doubts simply wants proof. The doubter is asking a question (occasionally rudely). All the skeptic wants is proof i.e. data. When the skeptic gets data they want to know how the data was collected in detail. So-called main stream folks know how to play by these rules. So called fringe science tends not to meet the burden of proof or the result cannot be reproduced. As I posted elsewhere, FoFu-1 just caught up to other plasma focus devices of the same current level in terms of fusion energy release. It’s not mean spirited or doubting, it’s a comparison between LPP’s Sept report and published literature. All the results to date fit into the existing framework. The new theory remains unproven while the old theory holds true. That may change in 2012. Time will tell.

DOE funded innovative confinement concepts for the last decade. Key problem with that program was none of the concepts showed any significant progress in that time. I remember going to meetings and seeing the same poster year after year by one group in that program. It was a waste. FoFu does not fall under the innovative concepts domain. The plasma focus was examined as a fusion system some forty years ago; it came up far short. A new theory exists claiming other people did it wrong. Perhaps they did; perhaps they didn’t. Don’t know. Way before my time. The LPP approach to fusion was tested with gov’t funding some time ago. What were the results? I don’t know but I take it those funding the experiments were not impressed. Perhaps the reasons are described somewhere with suggested improvements that were implemented in FoFu-1. I don’t know.

In the current funding framework, FoFu falls under the high energy density physics area. DOE is funding grants in this area. They have gone so far as to release a report calling high energy density physics the X-games of physics. The next solicitation comes out in summer or fall 2012 last thing I heard. These grants are on-line all the time, people just have to apply. The grants are reviewed and a few are selected for award. Some are conventional/mainstream while others are really out there on the fringe. The rules for the last year grants are likely still on-line (I’d post the file but it is too large even in PDF). This could be a reason fringe science doesn’t get funding; it doesn’t apply for the money. Check out a website called grants.gov for a list of potential funding sources. I’m pretty sure the next call of consequence will not be until the high energy density physics solicitation next year. If LPP wants gov’t funding at the $200K to $1M/year level all they need to do is apply and write a good proposal.

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Steven Sesselmann wrote:

I wouldn’t toss out the old knowledge yet, but I am up for looking at old things in a new light.

I agree, we need to retain our pre-existing knowledge, just suppose a different and hopefully simpler theory to explain the world.

My thoughts at the moment are leaning towards a world that is completely governed by electrostatic potential, where all four forces are explained by a single parameter. Say goodbye to gravity, strong force, weak force, and electromagnetic force. Further, the whole concept of force is wrong, no object is ever pushed or pulled by another object, stuff simply moves from the past to the future along the arrow of time.

Anyone who comes up with a new theory will face an uphill battle to get it accepted, established thought is thoroughly embedded. People who choose to work in science are often conservative introverts, and people who launch new ideas are often extrovert, which may explain why it takes so long between each new revolution in science. In a funny kind of a way, Einstein was a scientist and also a bit of an extrovert, and managed to sell his ideas to the world.

Step 1 is to find a problem that needs a new theory (easy)
Step 2 is to propose a theory that explains the problem (difficult)
Step 3 is to propose and experiment to prove it (usually very difficult)

Steven

Look for a book on a theory called expansion theory. It has an alternative explanation of the universe based upon electrons and the change of space itself. It’s a pretty easy read. The book points out many flaws in accepted science but it doesn’t really prove it’s own hypothesis. If you like it there are similar theories like it. The problem is expansion theory has proven wrong when tested against everyday life.

I also disagree; new theories are hard. Very, very hard to come up with. The burden of proof is that you must explain all that is known experimentally and things that are not known. Given all the time put into the standard model, a single human in a single lifetime cannot hope to produce such a theory. Einstein, a maverick and a genius, only scratched the potential of his theory. Right or wrong he never saw some of the most significant tests of his theory.

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Religion isn’t really the right world. Spirituality is a better word. Science requires leaps of faith as much as spiritual leaps of faith. Science claims to quantify things but you are relying on some framework as with any spiritual system. I agree that some people cling to these frameworks without cause. You can argue who’s clinging and who’s not but science does have sect-like groups. People live and die by their beliefs in science as much as spiritual beliefs. The arguments about ‘main stream’ and non-main stream science are proof that these sect-like groups exist. I’m not saying it’s a perfect correlation, but on I put this out there; the more I study science, the less impressed I am with its ability to answer the questions that really matter to me. Being 78% water with some C, N, O and a few other elements thrown in to give me shape isn’t very satisfying. That is what I’m made of but not who I am or how I relate to everyone else. I won’t claim every person is special or unique but I think we need to find answers for ourselves. Science can answer some of the puzzle but spirituality is just as valid an approach. That said, I have little time for people who force their answers on others.

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Most o-ring manufactures provide leak data under typical circumstances. Most vacuum applications use o-rings with a material diameter (minor diameter of toroid shaped o-rings) of 1/16″ to 1/4″ but they can be smaller and larger. The choice of minor diameter depends on mechanical tolerances and how well you adhered to standard o-ring groove rules. Again, manufacturers typically provide those rules for their o-rings in both circular and rectangular cross section. In most cases you can design o-ring systems that fully compress so they have zero space between the two pieces. Using an 1/8″ minor diameter o-ring with a 2″ major diameter (again toroidal geometry), I’ve completed experiments holding hydrogen at 5 Torr for 8-10 hours with a pressure change of less than 10 mTorr.

To make the seal last, I suggest a silicone vacuum grease applied extremely lightly. When done well, a viton o-ring will look like shiny and smooth. The hardness of the o-ring material is also important. I prefer hard o-rings for most applications. Soft o-rings tend not to do well in vacuum situations. Viton is the best choice in my opinion but some people like teflon or other common materials. If you are sealing metal to ceramic take great care to clean both pieces. O-ring sealing to rough ceramics is challenging.

Another thought that I haven’t tried yet but I intend to try in the next month or so is metal o-rings. Vendors can supply reusable steel o-rings that are completely sealed tubes with extremely thin walls. The o-rings can be coated in silver or tin to improve the seal. They should work wonderfully for situations when plasma or high temperature gas could corrode an o-ring but I have no experience. The ceramic could be coated in a thin ring of silver or tin where the o-ring mounts to improve the seal. Best of luck.

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Henning wrote: So graphene covered (highly conductive) carbon rods would be best? As the current only flows on the surface that’ll be enough…

See here: https://focusfusion.org/index.php/forums/viewthread/564/P15/#5633

You are likely to burn off 10-20 nm of graphene in a single shot. If it doesn’t burn off right away, it will be etched off by the hydrogen plasma very quicklyi. Hydrogen plasma etches graphene or graphite very efficiently. 20 nm is nothing. People etch microns of graphite per minute. The message is that the electrodes are consumed during operation. The mass loss is ~ 6 microgram per shot for a 60 kA device. If one assumes scaling based upon linear current density on the electrode, FoFu-1 will consume more like 3 MA/60 kA*6 micrograms per shot or 300 microgram per shot. You can calculate the material loss over the anode surface if you know the exact dimensions. At 2.5 g/cc in a 20 nm layer it won’t take but a few shots to erode the graphite and leave the underlying material. If material is Beryllium, it will be eroded and activated based upon x-ray flux. Let the ES&H problems begin.

From my experience, the extra x-rays from tungsten are far less of a problem than any other material problem. No material is perfect so one must compromise. Repetition rate operation, as will be needed for power production, has shown on several experiments to be best with tungsten at high current and moly at modest currents. No point in reinventing the wheel when the relevant data already exists.

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