[markus7]

asymmetric_implosion wrote: Sorry for the terribly slow reply. I wish I could say I meant the spelling but I R an engineer. 🙂

….

The FoFu approach….If one is to produce GigaGauss fields something has to happen that does not happen in a conventional plasma focus geometry. The filaments could be key to the FoFu approach. The measurements of magnetic fields in the pinch geometry is challenging but I do know a technique that is being developed base upon the deflection of a high energy proton beam passing through the pinch. I don’t know if the spatial resolution will be enough for the plasmoid but time will tell. I’ll be honest, I am very skeptical. Time will tell.

Glad to hear $50K is not too big in your mind. I think the materials testing could be taken a long way with $50K.

Thanks for the short list on diagnostics. It is valuable to have someone knowledgeable point one in the right direction.

I remember reading that a field the order of a Gigagauss is needed for LPP’s approach. I assume “measuring” such a field would have to be done indirectly and might be better called “estimating”.

[markus7]

Asymmertic (I assume that is the spelling you were intending), thanks for this detailed reply.

Yes, materials issues are low hanging fruit for low cost parallel experiments.

Also, $50K is not a lot of money. Such sums might be adequate for parallel efforts done in universities (where the ‘pay’ is mostly the student learning opportunities and for the professors a chance to publish results) or perhaps by advanced amateurs.

I would be grateful for a suggested short list of the most relevant papers regarding focus fusion diagnostics. Diagnostics of the ion beam from the pinch sounds particularly interesting.

Thanks for pointing out some of the many issues with doing, in effect, ‘model tests’ of focus fusion ideas to help sort through the design space. I’ll try to learn more about the subject and start with some of the leads you have suggested here.

To your knowledge, is LPP’s filament approach (rather than just a plasma sheet) required to generate the giga(?) gauss local fields in the pinch region needed to make the concept work? I had assumed it was. Perhaps the physics (and optimized geometries) for generating such high local fields in the pinch could be at least partially explored in less expensive ‘model tests’ at lower currents and so forth.

[markus7]

asymmertic_implosion 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.

Thanks very much for your well informed reply.

It is a safe bet that the majority of ideas about focus fusion devices that are easy and potentially productive have already been tried.

However, in my limited reading of the literature, it seems that focus fusion efforts have been, understandably, uniformly ‘focused’ on producing actual fusion, which can be discouragingly challenging and expensive.

I am wondering if sub-problems (such as uniformity of the arc filaments at the exit of the outer annulus) could be more cheaply addressed in units where no attempt is being made to produce actual fusion of any kind (no alpha particles or neutrons at all). More people might be interested in working in the area if they thought they had a realistic hope of solving significant sub-problems.

But someone (perhaps LPP) has to identify the sub-problems unique to focus fusion devices and suggest simple test setups adequate to explore potential solutions.

Using “uniformity of the arc filaments at the exit of the outer annulus” as an example, a lot of geometry configuration and initial magnetic field variations might be cheaply evaluated at perhaps lower currents than the 100 ka you mention as causing shielding effects. Also, note that the goal is symmetry measurements (at least time of arrival and current flow) not absolute values. So if the shielding effect was uniform, then we could still check for symmetry, perhaps at even higher currents. (Of course, if the shielding effect was not uniform, we could misread symmetric currents as asymmetric.)

Yes, the diagnostics in the pinch region must be non-intrusive. I was thinking of optical methods of visualizing the pinch symmetry, or perhaps measuring the current in the positively and negatively charged plasma jets that I assume (?) exit the pinch in opposite directions even when no fusion takes place.

If people thought they could do relatively simple, inexpensive experiments that might usefully sort through a lot of the design space for focus fusion devices without having to actually show fusion, they might be motivated to resurrect some of their old devices.

Costs might be pretty reasonable for simplified experiments.

[markus7]

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?

How about a wish list (appropriate for the Christmas season) of what people think might be useful variations to try? Perhaps this has been done previously on this forum. The design space is certainly large.

Tulse, I have started such a wish list with your suggestion over on my thread “Could simpler test setups running in parallel with LPP effort be useful?”. Feel free to comment further or add new wish list items.

Mark

[markus7]

Way cool. 2.6% variation in neutron production is very nice.

But since a big driver is asymmetry in the arc filaments, it seems to me some instrumentation to continuously monitor that asymmetry (in addition to the end product neutron production) could be very useful.

[markus7]

vansig wrote: Seems to me, that the only way to make all the alphas exit in the thrust direction is to have extremely strong magnetic fields. As far as i know, only DPF does this.

The difference of course is that these potentially gigagauss fields are DPF’s astonishing, stupendous, mind blowing, game changing trick. They are not a detail.

I don’t yet understand how DPF’s clever trick is being done. It is strange to think of the plasma as being inherently self organizing in its ability to convert an almost blind input of energy into the forces needed to compress a plasma sufficiently for fusion to occur.

That is really cool.

Maybe with a lot of work I will understand it one day.

But as of now, it is so mysterious that I wonder things like “What is special about the present general layout (besides it being the one that has been historically developed)? Maybe very different configurations would even better exploit these same natural plasma instabilities.”

[markus7]

vansig wrote:

Are you joking? If not, please explain how such a wonderous “nozzle” might work.

not joking, no; but neither have i thought it through, entirely.
these are all charged particles, all moving away from the focal point of the reaction.

so, some sort of magnetic field should do?

When I said “joking” that was regarding any nozzle similar to a normal rocket nozzle. The alpha particle’s would not elastically bounce off material walls as gases do, but would burrow in, so their momentum would not be redirected aft as in a normal rocket nozzle.

Yes, a very strong magnetic field with a ‘pinch’ at the upstream end would be able to generate thrust, but that is a huge change to the design. Also, I assume the ‘pinch’ would have to enclose the hardware shown which would result in the eventual vaporization of that hardware.

The diagram shows all the alpha particles exiting in the thrust direction, apparently by magic, without the presence of any such field.

[markus7]

TimS wrote:

Could you explain what you mean by “add a slight inductance at the base of the main electrodes”?

“Inductance is the property of an electrical circuit causing voltage to be generated proportional to the rate of change in current in a circuit.”

A current divided in N paths may not divide evenly if the paths are not symmetrical. However, by adding an equal impedance to each path, the current will divide more evenly. An series inductance is an impedance at high frequencies. This is a bad idea for the main cathodes for the reason stated by the previous poster- for a maximum current per voltage the cathode inductance should be reduced.

However, if it is desired to use the tungsten pins instead of the knife edge, it might help to add inductance to the pins. The main current is through the main cathodes, not the pins, so decreasing pin current would not be as bad. Inductance in the pin effectively filters out the higher frequences of current change through the pin, so that it would take time for a current path to build up from the first firing pin to the anode, and by then the current might have started from the other pins. The inductance of each pin could be increased by bending it into a spiral.

Oh yes. I see. Even if a fair number of turns had to be added, it sounds like an idea worth exploring. It is not difficult to imagine, say 16 pins whose bases were little free standing, open coils.

[markus7]

Henning wrote:

EDIT: I wonder if such an arrangement of 16 “spark plug plasma jets”, in addition to improving current sheath symmetry, might make it possible to eliminate using the main capacitor switches to trigger pulses. That is, the 16 outer cathode rods would, while running, be connected to the capacitors, and the “spark plug plasma jets” would take over the function of the vacuum switches, making them redundant. The function of the vacuum switches would be part of the fusor unit.

Generally a good idea, but I think the electrodes will shorten before the target voltage is reached, i.e. distance between cathode and anode are too short to hold back the sparks, as we try to reduce the diameter of the DPF and increase the amperage of the pulse (which goes along with the voltage). Anyone an idea how to reduce voltage and increase amperage? Well, by reducing inductance, which is somewhat one of the main goals…

Yes, after I posted, it occurred to me that glow or even spark discharges prior to reaching desired anode potential voltage could be a problem even for cold test hardware – single shot hardware as is now being tested. A real fusor running at a few hundred hertz would doubtless be so hot that glow and spark leakage would be entirely unacceptable. So I expect we would be stuck with the external vacuum switches even if it was found to be useful to install internal spark plug plasma generators to improve symmetry in the initial current sheath.

But that brings up a question. Could the cold shot tests of LPP’s fusor be providing misleading data? I invite you to comment on my other post “Would pre-heating LPP’s test chamber produce higher quality results?”

Of course, my interest in this second question is driven in part by the idea that the existing cold shots may be providing pessimistic results and neutron production will go up with a pre-heated fusor. (Maybe just heated by a tubular electric heater embedded in the cathode base plate and let copper’s conductance take care of heat distribution.)

Regarding your question about how we might increase shot amperage at a given voltage (mainly by reducing inductances?), I don’t know enough about the details of the existing setup to usefully comment. Is there any existing descriptive material (maybe even with present estimates of inductances) you could point me to?

[markus7]

vansig wrote:

This does not look like a detail to be worked out, but as an obvious, irrecoverable fatal flaw.

hardly a problem, at all. that’s what nozzles are for

Are you joking? If not, please explain how such a wonderous “nozzle” might work.

[markus7]

TimS wrote:

If the problem persists with the knife edge, I wonder if there would be some way to add a slight inductance at the base of the main electrodes so that the current sheath could be created to the knife edge (or pins) all around before lifting off to the electrodes? Perhaps returning to pins and adding a slight inductance in each would slow the current rise through the first pins and allow other pins to become active? I should go back to my old basic electronics class and work out the inductance and timing of the pins as they are…

Although the previous poster’s suggestion of using ‘laser spark plugs’, completely removing the need for high speed synchronized switches and the knife edge or pins, sounds pretty cool.

I had initially thought that the negative side of the capacitors was switched into the cathode electrodes separately, instead of having the positive side of the capacitors switched and then tied together to the anode. That way the shot energy would always be divided evenly between the electrodes. I had thought the circuit worked this way when I saw all those switches, specifically in order to avoid this problem of asymmetric current through the electrodes. I spent some time yesterday confusedly looking at the focus fusion pictures on flickr (aren’t they cool!) trying to figure out why the problem occurs before I realized I had it backwards. I had been wondering how all those separate circuits were synchronized so precisely… A schematic of the focus fusion device would be great.

It had also occurred to me that 16 capacitors, each firing its own cathode rod by individual, synchronized (highly synchronized!) vacuum switches would likely reduce the non-uniformity problem. There might also be an advantage in terms of reduced inductance in the separate circuits and higher peak currents, but I don’t know if that is possible. But that would be a totally new fusor design.

Could you explain what you mean by “add a slight inductance at the base of the main electrodes”?

“Inductance is the property of an electrical circuit causing voltage to be generated proportional to the rate of change in current in a circuit.”

[markus7]

TimS wrote: I see nothing about a source of magnetic force in this article, but the description starts by saying that a very strong electric force is created when the laser drives electrons off the foil, which is next to the boron film. The electric force would be in the correct direction to drive the alphas. Would this force be strong enough to reverse the path of high speed alphas heading toward it? Apparently it is strong enough to strip protons from metal nuclei… (is this really what is meant?)

Also, given the second to last paragraph of the article, it does not sound like Chapman has missed these problems:

The NASA engineer acknowledges that this collection of ideas is still a long way from being a practical device. For example, losses from the alpha particles striking the walls of the exhaust nozzle or each other lower the net power output. [em]Figuring out how to control the particles’ path is an important consideration.[/em]

The initial “very strong electric force .. created when the laser drives electrons off the foil” is sufficient for generating, it is claimed, 163Kev protons. The alpha particles are 2.5 to 3.8 Mev (see second figure). No, I don’t see any way that any electric field generated by the initial light pulse could be strong enough to deflect the alphas in the thrust direction.

Remember that almost half these alphas will be moving in the wrong direction. Electric fields totaling million of volts would be needed to get them to go the ‘right’ direction.

This does not look like a detail to be worked out, but as an obvious, irrecoverable fatal flaw.

[markus7]

jamesr wrote: Has anyone noticed the slashdoted article in IEEE Spectrum on NASA engineer John Chapman’s laser based pB11 thuser design.

http://science.slashdot.org/story/11/06/28/2229224/Fusion-Thrusters-For-Space-Travel
http://spectrum.ieee.org/aerospace/space-flight/a-fusion-thruster-for-space-travel/0

It seems he presented it at 8th IEEE International Conference on Plasma Science (ICOPS) and 24th Symposium on Fusion Engineering (SOFE) in Chicago this week

Is anyone there?? Has anyone got any more detailed information?

Chapman’s fusion rocket design puzzlingly ignores a momentum conservation issue.

See in particular the conceptual diagram of his fusion rocket in the link.
http://spectrum.ieee.org/aerospace/space-flight/a-fusion-thruster-for-space-travel/0

I can provisionally accept the author’s assertion that his picosecond pulsed laser will produce a “roughly 163 kiloelectron volts” shower of protons that then strike boron nuclei, fuse, and then eject three 2.9 MV alpha particles.

My difficulty is the ejected alpha particles will be moving in three random directions (while conserving momentum about the moving center of mass of the proton and stationary boron nuclei). The total momentum of the three alpha particles in the desired thrust direction must be the same as, or less than, the incoming 163 KV incoming proton’s momentum. The diagram shows nothing that redirects their 2.9 MV energy and momentum to the thrust direction. Of course, LPP’s fusor accomplishes this neat trick with GG magnetic fields, but I see nothing to fulfill that function in Chapman’s design.

The text does say: “’Electromagnetic forces’ push the target and the alpha particles in the opposite directions, and the particles exit the spacecraft through a nozzle, providing the vehicle’s thrust”. However, I have not been able to find out anything about the origins of Chapman’s mysterious, and immensely strong, ‘electromagnetic forces’.

Anyone have any ideas, or has Chapman just slipped a gear in his thinking?

[markus7]

Rezwan wrote: Your thoughts on the asymmetry of tungsten pins, the knife edge solution, and other factors of symmetry.

As I understand it, the source of the plasma sheath asymmetry problem is in the asymmetry of the initial current sheath on the surface of the insulator before it gets “kicked off” and driven down the annulus.

I am wondering if such asymmetries are inevitable with all designs that initiate the current sheath across an insulator as the present test model does. Even with a ‘perfect’ knife edge, I expect current filaments (even full up vacuum arcs?) will progressively pit the cathode, creating cathode ‘hot spots’, and deposit the vaporized metal on the insulator surface adjacent to the pits. The longer the device runs, the more asymmetric the initial current sheath is likely to be.

Have other methods of initiating the current sheet, perhaps “spark plug plasma jets” of some kind ever been tried? I am imagining one “spark plug plasma jet” for each cathode outer rod (16 total) aimed to fire across the annulus to the anode to initiate sheath current flow. Thus, the initial current sheet is created across the annulus, not across the surface of the insulator.

My preference, of course, would be for the simple knife edge cathode base to work.

EDIT: I wonder if such an arrangement of 16 “spark plug plasma jets”, in addition to improving current sheath symmetry, might make it possible to eliminate using the main capacitor switches to trigger pulses. That is, the 16 outer cathode rods would, while running, be connected to the capacitors, and the “spark plug plasma jets” would take over the function of the vacuum switches, making them redundant. The function of the vacuum switches would be part of the fusor unit.

EDIT 2: Mainly for people like me who are new to this forum, relevant previous threads include Spark Plugs? https://focusfusion.org/index.php/forums/viewthread/471/ and Laser spark Plugs https://focusfusion.org/index.php/forums/viewthread/855/. Both of these threads focus on initiating the external switches. But they might also be applicable to the idea of reducing the asymmetry problem in the initial plasma sheath in the main fusor, while, as a secondary effect, eliminating the need for high tech external switches.

[markus7]

jamesr wrote:

I think it would be better to quote yield as (Fusion energy released)/(Energy In), then later we can add figures for the recoverable portion of that energy, and finally the actually recovered portion.

I certainly agree from a technical standpoint. However, part of the “charm” of quoting neutron production, in addition to showing a higher production efficiency than the best known competitors, is that neutrons are definite proof of fusion for people who might otherwise be suspicious of LPP results.

[Author]

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