Dr. Park invites questions from the community

Consider specific people in the fusion research community, business, or politics who should be made aware of polywell research, and how we might reach them.

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Re: Dr. Park invites questions from the community

Postby mvanwink5 » Fri Feb 12, 2016 3:57 pm

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?
Near term, cheap, dark horse fusion hits the air waves, GF - TED, LM - Announcement. The race is on.

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Re: Dr. Park invites questions from the community

Postby JoeStrout » Fri Feb 12, 2016 4:27 pm

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
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Re: Dr. Park invites questions from the community

Postby ladajo » Fri Feb 12, 2016 4:37 pm

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... $$$
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Re: Dr. Park invites questions from the community

Postby mvanwink5 » Fri Feb 12, 2016 5:59 pm

Thanks for the speculation and guesses.
Near term, cheap, dark horse fusion hits the air waves, GF - TED, LM - Announcement. The race is on.

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Re: Dr. Park invites questions from the community

Postby ladajo » Fri Feb 12, 2016 6:11 pm

You are welcome, it is what we do here on the internet. :)
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)

What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)

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Re: Dr. Park invites questions from the community

Postby mvanwink5 » Fri Feb 12, 2016 7:50 pm

:D
Near term, cheap, dark horse fusion hits the air waves, GF - TED, LM - Announcement. The race is on.

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Re: Dr. Park invites questions from the community

Postby JoeStrout » Thu Feb 18, 2016 6:28 pm

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.
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Re: Dr. Park invites questions from the community

Postby Skipjack » Sat Feb 20, 2016 12:00 am

Thanks for these very informative answers, Dr Park and Joe!

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Re: Dr. Park invites questions from the community

Postby bennmann » Sat Feb 20, 2016 4:27 pm

Thank you, Dr Park!

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Re: Dr. Park invites questions from the community

Postby rjaypeters » Sun Feb 21, 2016 11:01 am

Thank you and we look forward to the remaining answers!
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Re: Dr. Park invites questions from the community

Postby JoeStrout » Mon Feb 22, 2016 4:11 pm

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Re: Dr. Park invites questions from the community

Postby hanelyp » Mon Feb 22, 2016 5:50 pm

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.
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Re: Dr. Park invites questions from the community

Postby ltgbrown » Mon Feb 22, 2016 8:22 pm

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."
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Re: Dr. Park invites questions from the community

Postby choff » Tue Feb 23, 2016 7:15 am

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).
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Re: Dr. Park invites questions from the community

Postby ltgbrown » Tue Feb 23, 2016 7:30 pm

That quote got me excited also, but I had already copied the other one!
Famous last words, "Hey, watch this!"


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