EDIT 11-30-2011 I have added a request for wish list items for ideas people want to suggest for testing. If some of those ideas look interesting enough, that might be motivation to start parallel efforts with simplified test setups to work on sub-problems. End EDIT-

The LPP effort is focusing, of necessity, on showing neutron production. This is technically challenging and expensive.

Could some of the important sub-problems be usefully ‘farmed out’ to be experimentally attacked in parallel efforts using cheaper, less challenging experimental setups? I am thinking cheaper because these ‘dummy’ fusors would use lower voltage, less finicky vacuum and gas requirements, and since they would be incapable of producing neutrons, there would be no need for neutron measurement hardware.

This idea is dependent on developing sensors to serve as ‘goodness’ indicators in the absence of neutron production. For example, coils might be able to detect the current and arrival time of each of the arc filaments (perhaps at the exit of the outer annulus where the filaments start their 180 degree turn) as an indicator of plasma symmetry. Perhaps symmetry sensors adjacent to the pinch region (UV, magnetic, or resulting plasma jet current?) could be useful.  If such sensors could be made, then it might be possible to cheaply investigate in parallel a lot of ideas that are now wildly impractical to test on the LPP neutron producing device.

Sub-problems that could be investigated include:

Arc filament symmetry and other characteristics in terms of minor hardware geometry or external field variations.
Metal erosion problems (wear such as pitting) in terms of effects on arc filament symmetry.
High repetition rate effects, including localized heating at erosion pits, on arc filament symmetry,

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

EDIT 11-30-2011 It has been a couple of days with no responses. I’ll change what I am asking.

If there were separate parallel experimental efforts using less challenging test setups as described above to work on sub-problems, what design space variations would you like to see? (Less challenging test setups at lower voltages, simpler geometries, and no possibility of neutron production would be within the capability of any university physics program and perhaps some advanced amateurs.)

Tulse on another thread suggested:

1) “A solid piece (cathode), with projections to guide the plasma filaments, would mean that one never had to worry about individual cathode alignment.”

I am interested in

2) the capability of heated surfaces (naturally heated in a fast repetition device) to reduce pitting and erosion typical of cold electrodes

3) the possibility of active control of each arc filament to maintain symmetry at the exit of the outer annulus.

Any other suggestions for a wish list?

markus7 - 27 November 2011 08:43 PM

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.

asymmertic_implosion - 30 November 2011 06:01 PM

markus7 - 27 November 2011 08:43 PM

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.

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.

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.

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

The papers are listed below.

Ion beam reconstruction from neutron time of flight:
P. Kubes, J. Kravarik, D. Klir, K. Rezac M. Bohata, M. Scholz, M. Paduch, K. Tomaszewski, I. Ivanova-Stanik, L. Karpinski and M. J. Sandowski. “Determination of Deuteron Energy Distribution From Neutron Diagnostics in a Plasma-Focus Device” IEEE Trans. Plasma Sci Vol 37. pp 83-87 2009

Interferometry

Pavel Kubes, Marian Paduch, Tadeusz Pisarczyk, Marek Scholz, Tomasz Chodukowski, Daniel Klir, Jozef Kravarik, Karel Rezac, Irena Ivanova-Stanik, Leslaw Karpinski, Krzysztof Tomaszewski, and Ewa Zieliñska. “Interferometric Study of Pinch Phase in Plasma-Focus Discharge at the Time of Neutron Production” IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 11, NOVEMBER 2009

Optical framing (This is one of my favorite papers with images)

Jos´e Moreno, Patricio Silva and Leopoldo Soto. “Optical observations of the plasma motion in a fast plasma focus operating at 50 J” Plasma Sources Sci. Technol. 12 (2003) 39–45

This short list has references to other diagnostics people have tried.  The list can be very extensive.  If you really want the best pulsed power diagnostics suite at the moment, you need to look at the Z-diagnostics.  The NIF diagnostics are coming along quickly.  They are focused on a more difficult problem but they are progressing by leaps and bounds every year. 

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.

asymmertic_implosion - 10 December 2011 07:40 PM

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

...

At the top of this screen, click “Your Control Panel”.  Then select “Username and Password” on the left margin.  Figs the spelunk as decired. 

BTW, is anyone else not seeing the smileys? I’m just getting text for whatever anyone has inserted.

In fact, I’m just seeing text on the selection pop-up, too.

Brian H - 10 December 2011 09:52 PM

asymmertic_implosion - 10 December 2011 07:40 PM

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

...

At the top of this screen, click “Your Control Panel”.  Then select “Username and Password” on the left margin.  Figs the spelunk as decired.

Thanks.  Now I R an engineer that can use spell checker.

asymmetric_implosion - 10 December 2011 07:40 PM

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”.

If FoFu is to work, it will require Gigagauss fields.  I don’t know if the filaments are the only way to do it. 

The proton measurement technique I referred to measures the fields by deflecting protons using the field.  It is a measurement that requires reconstruction but it can be done pretty accurately on paper.  The practical reconstruction will be done next year by a group at UC San Diego working with a group at UN-Reno at the ~1MA level.  I am optimistic the technique will work with reasonable accuracy.  I will see if I can find any papers on it and report back.