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Re: Dr. Park invites questions from the community

Posted: Fri Feb 12, 2016 3:57 pm
by mvanwink5
For instance, the key issue may be diagnostics, remember the rings inside that Dan was concerned about causing electron losses, they may still be needed, or it may be a desire to achieve break even to show that it can be done, or that the magnets are the big cost anyway, or with all the potential cusps that may require injectors. We don't even know if the device will be continuous, probably not, so will the magnets be superconducting. Why $30 million, is it the massive injectors or is it just the unknowns and potential equipment mods plus time to solve problems? Is he going for just potential well or scaling too? More than one machine?

It may all be moot if there is no money, been a long long time... Perhaps the next administration would be more amenable, start making contacts now?

Re: Dr. Park invites questions from the community

Posted: Fri Feb 12, 2016 4:27 pm
by JoeStrout
Well, I've sent the top-10 questions off to Dr. Park. Some of the questions are quite detailed, and possibly more than he bargained for. I think we should be grateful for whatever information he's able to provide.

I'll pass along his answers as soon as I get them!

Best,
- Joe

Re: Dr. Park invites questions from the community

Posted: Fri Feb 12, 2016 4:37 pm
by ladajo
I'll answer that the $30 million includes people and stuff. So that is $10 million per year for three years.
As far as people, a sufficient staff can certainly run $1 to 2 million per year in this context, and would also include periodic contracted external support.
So there goes, for argument's sake, $6 million.
That leaves about $24 million for stuff.
Stuff includes:
Facility and facility operating costs.
Test device (Chamber, core, neutral beam injectors, plasma guns, internal diagnostics support, interfacing, etc.)
Test device support equipment (power, vacuum, cooling, fuel system, controls, etc.)
Test equipment (sensors, processors, controls, data management, data processing, etc.)
Occasional contracted equipment to support build, maintenance, test, analysis, etc.
Expendables (fuel, test gas(s), cooling agent, maintenance supplies, special expendable materials - gaskets, seals, foils, films, etc.)

These are the bigger ticket items off the top of my head. So in context, say it is a 1 meter diameter machine... $$$

Re: Dr. Park invites questions from the community

Posted: Fri Feb 12, 2016 5:59 pm
by mvanwink5
Thanks for the speculation and guesses.

Re: Dr. Park invites questions from the community

Posted: Fri Feb 12, 2016 6:11 pm
by ladajo
You are welcome, it is what we do here on the internet. :)

Re: Dr. Park invites questions from the community

Posted: Fri Feb 12, 2016 7:50 pm
by mvanwink5
:D

Re: Dr. Park invites questions from the community

Posted: Thu Feb 18, 2016 6:28 pm
by JoeStrout
Dr. Park has kindly sent me very thoughtful answers to questions 1-6, and intends to work on the remaining questions over the weekend.

I know you will have follow-up questions, but let's try to be respectful of Dr. Park's time and glean as much as we can from just the answers provided for now.

Here they are: Dr. Park's answers to questions 1-6.


1. What are your remaining scientific concerns and which one concerns you the most?
Can we form a deep potential well in a high beta cusp using electron beam injection? In addition, we will be looking closely at the electron confinement scaling. Our current understanding is that the potential well and the electron confinement are closely related.


Why is this critical? A fusion reactor needs an efficient ion heating method (i.e. accelerating ions to ~20 keV or higher for D-T or around 200 keV for p-11B). We plan to achieve ion heating by forming an electrostatic potential well inside the cusp using electron beam injection. Our current understanding is that the formation of the well will require a majority of electrons in the cusp to be at high energies and their density needs to exceed the ion density. As such, good confinement of the electron beam is a necessary condition for the formation of the potential well. That’s why we were encouraged about the observed energetic electron confinement in a high beta cusp. By the way, the ion confinement is largely guaranteed due to the electrostatic confinement and concomitant cusp magnetic confinement.

The main goal of the next device is to achieve an ion energy in the range of 5 keV or higher (this 5 keV number is chosen after considering several factors including project cost, engineering complexity, and scientific clarity). If successful, the next machine would have demonstrated sufficient confinement of both ions and electrons and efficient ion heating, key requirements for a fusion reactor. In addition, we expect plasma stability to remain favorable due to the good magnetic field curvature. In our view, confinement, heating and plasma stability are the three most important elements for a fusion reactor.

Unknown:

In his 1985 patent, Bussard wanted to use the electron beam injection to form a potential well. From what I heard about Farnsworth, he also wanted to use a potential well to heat ions for fusion with the use of electron beam (for Farnsworth, I think this is more due to a virtual cathode aspect where the cathode made of electron clouds is not damaged from the ion bombardment). Both later changed their research direction when they could not heat ions from a potential well at a fusion-relevant ion density. Note that the fusion-relevant ion density is roughly on the order of 1x10^13/cc or higher. Otherwise, the fusion power output per reactor volume is too small for a practical device due to small fusion cross sections.

Our current understanding is that the failures to form a potential well are caused by the poor electron beam confinement. As a result, the gridded IEC was pursued for Farnsworth and Hirsch in the late 1960s and later adopted by others. For EMC2, Bussard started to pursue the magnetically insulated grid approach. Unfortunately, gridded IEC performance is still very poor after more than 60 gridded IEC devices. During our WB-8 campaign, we found out that the magnetically insulated grid does not work due to Debye shielding (I will provide you with more details later).
Instead, we now think it is in principle possible to form a deep electrostatic potential well using electron beam injection when there is sufficiently good electron beam confinement. This is because of the enhanced electron beam confinement in a high beta cusp, which greatly lowers the required beam input power. We have been in discussion with several plasma physicists on this particular issue and so far, nobody came up with a reason why we won’t see a deep potential well if 1) electron beam confinement is good as we theorize for a high beta cusp and 2) a sufficient electron beam current is provided. However, we are not aware of any previous work on this (both theory and experiment). As such, we remain cautious about the prospect of potential well formation until we successfully conduct experiments.


2. What are the prospects for operating Polywell in a steady-state vs. a pulsed mode? (And would the use of p-11B make steady-state operation easier or harder?)

We plan to operate a Polywell reactor in a steady state. There are a number of positive reactor attributes about Polywell;
At least macroscopically stable and will operate at high beta
Has natural divertors
No need to worry about helium ash
No need for current drive
As such, it makes sense to operate a Polywell reactor in a steady state (and not having to worry about mechanical stress from pulsed operation and energy recovery during afterglow). The use of p-11B does not make the steady-state operation any easier or harder at the fundamental level.


3. What scaling laws do you expect (if it is not too early to ask that)?

Electron beam confinement time: proportional to B^2 x R^3 x Ebeam^(-1.5).
This scaling comes from the diffusion model based on Grad’s high beta cusp theory. There is a major difference between the current Polywell scaling and previous cusp scaling. In Polywell, there is no need to consider ion cusp loss. This is because the Polywell can only work with a deep potential well due to the ion heating requirement. As such, the primary ion confinement comes from the potential well, while the ion cusp loss is expected to be small from a small gyro radius at the boundary. Plus, a loss of ions from the cusp does not yield a significant loss of energy since the ion kinetic energy is small at the boundary. This simplifies the power balance calculation for a Polywell reactor. As for the proportional factor, we plan to determine it from the next phase experiments.
Potential well depth: ~ 0.5 x Ebeam (estimate)
At present, we do not have a good theory on the potential well depth as a function of beam energy and current. In previous experiments (Krall, 1995 Physics of Plasma paper), we were able to produce a potential well on the order of 90% of beam energy. However, this was done only at low ion densities. At the moment, a well depth of 50% seems like a reasonable guess for the following reasons. The formation of a potential well will only occur when the electron confinement is good. In this case, both electrons and ions will be confined for a sufficiently long time and they will tend to equilibrate toward the same temperature. At 50% of well depth, the average kinetic energy of electrons and ions will be equal inside the potential well.
We have used these two scaling laws to estimate a reactor size and a fusion power output for D-T operation (e.g. PRX paper). It is noted that the same scaling law may yield a pathway to p-11B operation if 1) the electron beam confinement scaling is given as above with a proportional factor on the order of unity, and 2) if we can somehow achieve a potential well depth in the range of 85% of beam energy or higher. We have a number of possible mechanisms that may yield a deep potential well but they are speculations at this point. Thus, one of the major goals of the next phase experiments is to see how deep a potential well we can produce. By the way, we no longer consider electron recirculation to be an important factor. The recirculation may provide a factor of 2 or so enhancement in the confinement, if a lot of things go well. A factor of 2 is small and at this point, we do not consider it critical.


4. What is the primary mechanism impacting electron injection efficiency, and how does this scale in projections to a viable machine?

We do not consider electron injection efficiency to be of concern. From outside the cusp, the magnetic field lines converge toward the cusp. As long as the electron beams are well aligned along the field lines, most of electron beams should enter into the cusp. One possible exception would be the case of a very deep potential well that will electrostatically repel incoming beams. If that happens, it will be a wonderful problem to have and should not be difficult to address.


5. The machine designed to achieve and demonstrate high Beta effects had wide separations between magnets. Does this reflect improved cusp confinement compared to WB6 and imply a need to keep ExB losses from becoming dominate? For that matter, without a deep potential well, are ion cusp losses dominating the picture over electron cusp losses in this machine, at least as higher Beta is reached?

No for the first part and irrelevant for the second part. We chose the wide separation between the magnets as an attempt to generate a more spherical cusp B-field shape, compared to WB-6 or WB-7. At present, we do not view this as a critical factor in Polywell operation. As discussed and claimed in the patent application, we now view that the physics of high beta cusp confinement is related to the magnetic sheath from the diamagnetic current. This may lead to a future Polywell device based on linear cusp systems such as a spindle cusp or a picket fence.


6. How much did the internal magnetic field measuring coaxial cables limit achievable Beta?

Not relevant at all. We achieved a sufficiently high beta and observed the enhancement of electron beam confinement. As shown in the PRX paper, too high a beta may result in poorer confinement. Furthermore, the beta is a local quantity and it varies widely in the cusp system. The critical physics for the Polywell operation is the confinement of plasma when the nominal plasma beta is on the order of unity and not the exact beta value at one specific location.

Re: Dr. Park invites questions from the community

Posted: Sat Feb 20, 2016 12:00 am
by Skipjack
Thanks for these very informative answers, Dr Park and Joe!

Re: Dr. Park invites questions from the community

Posted: Sat Feb 20, 2016 4:27 pm
by bennmann
Thank you, Dr Park!

Re: Dr. Park invites questions from the community

Posted: Sun Feb 21, 2016 11:01 am
by rjaypeters
Thank you and we look forward to the remaining answers!

Re: Dr. Park invites questions from the community

Posted: Mon Feb 22, 2016 4:11 pm
by JoeStrout
Here are Dr. Park's answers for questions 7-10.



7. Is there a reconciliation between this machines low Beta cusp confinement numbers and those claimed for WB6? WB6 was ~ 60 passes , while this machine was ~7 passes. Given that simple biconic mirror confinement was quoted as ~ 5-8 passes (in the older patent application), the numbers for this machine ('Mini B' as it has been called here) seem small. Is the increased magnet spacing distance combined with the overall smaller size (relative differences magnified) significant, despite the increased B field strengths?

- First, I won’t be able to answer this question. This is because we do not know much about the WB-6 results. As you may know, an intense arcing event destroyed the WB6 device after only a handful of shots (3 shots were completed according to the staff who worked with Dr. Bussard on WB-6). In addition, I have no idea about where the 60 passes came from.

Instead, I will do my best to explain the plasma confinement in WB-7, WB-8 and Mini-B here. The understanding we gained from executing WB-8 and mini-B experiments is critical for us to move forward and I would like to share that part here.

We built the WB-7 as a replica of WB-6 in most aspects. We ran about 2,000 shots on WB-7. As for WB-8, we got another several thousands shots.

At a low beta cusp, the electron confinement time is proportional to E^(-0.875) – see page 16 of my presentation from 2014 US Japan IEC Workshop at U. Wisconsin (link) where the theoretical cusp-mirror loss rate is given.

So, for the same device size and the magnetic field strength, the cusp electron confinement time for ‘cold’ 10 eV electrons is 310 times better compared to the confinement time at ‘hot’ 7 keV electrons.

One example of seemingly good cusp confinement for cold electrons is the electron density decay time measurement (correlates with the electron confinement time) in page 10 of the same presentation for WB-8. It is ~180 microseconds for an estimated ‘cold’ electrons with a temperature of ~3 eV.

By the way, testing showed that there was no difference in the observed density decay time times for various grid bias voltages. This result is probably the most direct demonstration of why the magnetically insulated grid approach failed to produce a potential well. Grid bias had no impact on electron dynamics. If the potential well is formed with the grid bias, we would have seen the significant changes in electron dynamics from increasing bias voltages. In addition, no neutrons were observed when the grid was biased up to 20 kV in WB-8, while the plasma density is much higher for WB-8 compared to WB-7 (where we saw significant neutrons at 13-14 kV bias), providing another clear example of failed ion heating (from the lack of potential well formation) using magnetically insulated grid approach.

For a high beta cusp, the theoretical scaling law predicts electron confinement time is proportional to E^(-1.5.). In this case, the electron confinement time at 10 eV would be 18,000 better than at 7 keV.

“Mini-B” (in our nomenclature, we used the term, WBX) confinement time was given for 7 keV electron beam, not for the cold electrons at 3 or 10 eV. Poor confinement for a 7 keV electron beam in a smaller sized cusp machine like Mini-B is expected for a low beta case based on the cusp-mirror loss theory. In addition, there is no reason that the electron confinement in a hexahedral cusp is significantly better than for a biconic cusp in the case of low beta based on the cusp-mirror loss theory. Also note that the magnet spacing is not critical.

Back in 2008, we estimated electron confinement time for the WB-7 to be about 7 microseconds. The conditions are: 1 kG nominal cusp magnetic field, 8 cm plasma radius, grid bias at +13 kV, grid current at 42 A (electron current), and avg. electron density at 1.7x10^12/cc. The idea behind the electron confinement time estimate was that in steady-state, the grid current (which can be viewed as electron loss) would be equal to the total number of electrons divided by electron confinement time. Coincidentally, an estimated WB confinement time for 13 keV electrons in the WB-7 size device is about 4 microseconds.

This estimate had a number of shortcomings. The major ones are:

  1. We don’t know the energy of lost electrons to the grid. For example, were they at ‘cold’ 10 eV, ‘hot’ 13 keV, or something in between when they were in the bulk plasma? In addition, how their energy would have varied as they moved toward the metal surface since we do not know the electric field distribution near the grid surface and the metal joints (connecting “nubs”) between each grid coil.
  2. We don’t know where the loss of electrons occurred. The measured loss current is the electron current (assuming no secondary electron emission) to the entire coil surface (not intercepting the magnetic field lines) and to the metal joints connecting the coils (intercepting the field lines). In general, the plasma motion (in particular electron motion) is almost always along the field lines rather than across the field lines. So, how you would weight the surface area to analyze this grid current distribution was not a trivial question.
  3. We don’t know the loss rates along the other cusp openings that were not blocked by the metal joints. These losses would not show up as the grid current. So, we are ignoring significant part of electron loss by using only the grid current as the measurement data.

Despite these shortcomings, we nonetheless estimated the electron confinement time back in 2008 because that’s all the data we had at the time. They are poor man’s results (it is almost always a bad idea to analyze your data without detailed plasma diagnostics) and in retrospect it was a mistake that should not happen again.

One excuse was that I had only one year to build and operate the WB-7 device from scratch in a completely empty space, to produce neutrons, and to make physics measurements to understand the WB physics with the help of a ‘motley crew’ of physicists, engineers and technicians on a shoe string budget. We told the review panel exactly how we got this result with the related shortcomings and explained to them we needed to have a better measurement regime. They understood our situation and provided their findings to the Navy. Somehow, that was good enough for the Navy to take a chance on EMC2 (probably three reasons in retrospect: high energy price, technical competency, and honesty), in addition to the scientific merits of the Polywell concept, although I don’t think we fully understood how remarkable the Polywell approach was at that time. It was more of a seasoned physicists’ gut feeling based on perceived elegance. That’s why, to this date, I am grateful for the support from the Navy.

In comparison, WB-8 and WB-mini measurements did not have those shortcomings. In WB-8, the observed electron loss can be easily explained by the cusp mirror loss theory. Of note is that we never produced a high beta in WB-8 though we might have done so if we were given an opportunity to continue the project for another 6-12 months. In Mini-B, the electron confinement enhancement is clearly measured from low beta to high beta, which provided the breakthrough for the Polywell concept to move forward.



8. The reported neutron counts for the relatively large reaction space in WB-7 seem modest compared to simple amateur fusor reports with only modestly higher voltages, and this is ~ 3 orders of magnitude less than that claimed for WB6. Was deep potential wells obtained in WB7,and 8? You mention that deep potential wells were obtained in 1995, how about in WB7 and/ or 8?

We could not confirm if we had a potential well in WB-7. We definitely did not have the potential well in WB-8. The neutron rate of WB-7 was 1x10^6 neutrons/s compared to 1x10^9 neutrons/s from WB-6. Though those two devices operated at comparable grid bias voltages (12 kV for WB-6 and 13 kV for WB-7), the grid current was 4,000 A for WB-6 compared to about 40 A for WB-7. It was because in WB-6, the power system had no control whatsoever to protect the device from catastrophic arcing. The neutron data for WB-6 came when the arcing occurred and destroyed the device by dumping the entire amount of stored energy from the high voltage capacitors into one metal joint. On the other hand, the WB-7 was designed with the modern pulse power system with the protection circuit to suppress arcing and limit the current. This resulted in much lower grid current for WB-7 compared to the catastrophic arcing case of WB-6.

Assuming that both devices had similar ion energy and confinement time, WB-6 would have produced about 100 times more neutrons if the fusions were from the beam ion to target neutral reactions and 10,000 times more neutrons if fusions were from the beam ion to bean ion reactions. So, I don’t see any inconsistency between the WB-6 and WB-7 results though this comparison is only qualitative in nature.

Despite modest neutron yields per input power (e.g. the peak power of WB-6 was ~50 MW, 4,000 A at 12 kV: Thus it is no surprise why the WB-6 failed after just 3 shots). WB-6 and WB-7 are probably the only known gridded fusion devices that produced neutrons while operating with a positive bias voltage. The performance difference between a positively biased potential well machine (a virtual cathode) and a negatively biased gridded device (Fusor) can be attributed to the following simple reason: The electron, being a lighter species, moves ~60 times faster than deuterium ions for the same kinetic energy. This in turn results in a much larger loss current to the grid for a positively biased grid system compared to the negatively biased grid system. As a result, WB-6 and WB-7 would have greatly underperformed in neutron yields per given input power against comparable fusors unless the electron loss rate is greatly improved via WB (and assuming there was a potential well).

That’s why Mini-B results are so important. Now we know that a high beta cusp can provide a means for electron confinement good enough for a fusion reactor, which in turn can make the electrostatic ion acceleration and confinement a reality. Throw in the inherent plasma stability of cusp, we may have one of the most attractive fusion concepts.


9. What is the next proposed test machine going to look like, and what will be the biggest engineering challenge in its construction?

We plan to build a series of Polywell machines in the next phase. A spindle cusp, hexahedral cusp and the dodecahedron cusp. It is the right time to investigate this part of the physics now that we tackled the Wiffleball. This part is also related to investigating the possibility for p-11B. In addition, we absolutely plan to focus on the formation of a potential well with sufficient electron beam power and sufficiently good electron beam confinement. All previous WB devices (from WB-1 to WB-8) utilized a magnetically insulated gridded system, thus lacking the proper electron beams. That needs to be changed. We plan to have at least 10 MW of electron beam power to produce a potential well. Following in the tradition of WB-8 and Mini-B, we plan to make all the key measurements required to fully understand the physics of Polywell. If all goes well, we should have demonstrated the following in 3 years:

  1. A potential well depth of ~ 5 kV or higher (equivalently, average ion energy of 5 keV)
  2. Sufficiently good electron confinement in a high beta (without good enough electron confinement even 10 MWs is not going to be enough).
  3. Electron confinement time scaling data over 2-3 orders of magnitude. At present, the electron confinement from Mini-B is about 1-10 microseconds. In the next machine, we expect an electron confinement time between 1-10 milliseconds. If we can validate the scaling physics over 2-3 orders of magnitude, we can extend the scaling law to a power producing reactor.

There are some expected engineering challenges for the next phase (I won’t go into details). However, I don’t think it is going to be worse than the WB-8 where we had to come up with a design for 1) minimum 50 kV insulation, 2) cryogenic operation at liquid nitrogen temperatures, 3) UHV compatibility, 4) thermal loading over 1 MW/M^2, and 5) mechanical strength to handle over 30 metric tons of compressional and substantial shear stresses for each coil support at the same time in one package. What we did not discuss much is how simpler it is to design a Polywell device not utilizing magnetically insulated grid approach.

10. What are your thoughts on the proposed test plan and regimes for this new machine?

I can’t wait to build and test the new devices. If we can form a potential well at ~ 5 kV by extending the WB confinement to 1-10 millisecond range, the expected plasma parameters are on the order of: 5 keV ions at 6x10^13 cm-3 density using 10 MW of input power and 5 kG magnetic fields. At that point, the Polywell device can favorably compete with advanced tokamaks (e.g. EAST from China except for pulse duration) while being a much smaller size device and operating with lower magnetic fields. In my view, if one can come up with a device that can outperform the tokamak, that will be the beginning of the end for the decades old quest for practical fusion power (otherwise, we are back to square one and have to wait for tokamak to get significantly more economical).

Re: Dr. Park invites questions from the community

Posted: Mon Feb 22, 2016 5:50 pm
by hanelyp
Recirculation having not so great an effect on confinement is a surprise.

Re #7:
- Grid charge having no measurable effect on interior plasma dynamics fits my expectations. The point of grid charge is exterior dynamics: electron injection, recirculation, recovery of energy from lost electrons.
- I'd expect the energy of lost electrons to be biased towards the high end of the energy spectrum.
- The answer to this point suggests to me differences in conditions necessary for strong recirculation. Though details here are unclear.

Re: Dr. Park invites questions from the community

Posted: Mon Feb 22, 2016 8:22 pm
by ltgbrown
Well, with this quote, I am excited:

"That’s why Mini-B results are so important. Now we know that a high beta cusp can provide a means for electron confinement good enough for a fusion reactor, which in turn can make the electrostatic ion acceleration and confinement a reality. Throw in the inherent plasma stability of cusp, we may have one of the most attractive fusion concepts."

Re: Dr. Park invites questions from the community

Posted: Tue Feb 23, 2016 7:15 am
by choff
This part got me excited.

I can’t wait to build and test the new devices. If we can form a potential well at ~ 5 kV by extending the WB confinement to 1-10 millisecond range, the expected plasma parameters are on the order of: 5 keV ions at 6x10^13 cm-3 density using 10 MW of input power and 5 kG magnetic fields. At that point, the Polywell device can favorably compete with advanced tokamaks (e.g. EAST from China except for pulse duration) while being a much smaller size device and operating with lower magnetic fields. In my view, if one can come up with a device that can outperform the tokamak, that will be the beginning of the end for the decades old quest for practical fusion power (otherwise, we are back to square one and have to wait for tokamak to get significantly more economical).

Re: Dr. Park invites questions from the community

Posted: Tue Feb 23, 2016 7:30 pm
by ltgbrown
That quote got me excited also, but I had already copied the other one!