Generator testing concept

[annodomini2]

It is my understanding the primary generator point will be from the Helium ions produced in the pB11 reaction?

If so could a Helium ion drive be used to test generation concepts before building the full reactor?

I would guess the main issue would be energy levels and getting a supply of ions with the same energy level as expected from the reaction.

[Francisl]

Here is a list of particle accelerators. Some of them would be capable of performing those tests. The problem is finding the funding to conduct the tests.

[wolfram]

They’ve been building accelerators with KE’s of 8 MeV’s since the late 30’s, since we only need about 3 MeV’s, I actually think making one from scratch may not be too hard.

[asymmetric_implosion]

The charged particle beam to electricity component of the generator is very well tested already when you have a single charged particle species flying through it. I see two real concerns: can the x-rays be converted efficiently with the onion and does the ions produced by the plasma focus travel alone?

The x-ray issue is largely straightforward to test if the onion design exists on paper. The charged particle beam issue is more complex. Firing an accelerator alone isn’t enough. You need an intense ion current of >100 kA passing through something like 100 Torr of gas for tens of centimeters. There is some evidence in published literature that the ions are not traveling alone; they have electron partners. I don’t mean they are neutral in the sense of an atom but rather the heavy ions are pulling electrons along for the ride. If ions and electrons travel together, the net current is zero. With net zero current, you can’t use the transformer based technique proposed by LPP to convert the ion beam to electrical energy. Even if you get a current, what fraction is neutralized before you reach conversion location? If every He ion drags along one electron, you’ve cut the beam current in half. Breakeven suddenly got harder. A test with a conventional particle accelerator is hard because the currents are typically limited to less than 100A and passing ions into a gas cell for the test is challenging.

[Joeviocoe]

asymmetric_implosion wrote: The charged particle beam to electricity component of the generator is very well tested already when you have a single charged particle species flying through it. I see two real concerns: can the x-rays be converted efficiently with the onion and does the ions produced by the plasma focus travel alone?

The x-ray issue is largely straightforward to test if the onion design exists on paper. The charged particle beam issue is more complex. Firing an accelerator alone isn’t enough. You need an intense ion current of >100 kA passing through something like 100 Torr of gas for tens of centimeters. There is some evidence in published literature that the ions are not traveling alone; they have electron partners. I don’t mean they are neutral in the sense of an atom but rather the heavy ions are pulling electrons along for the ride. If ions and electrons travel together, the net current is zero. With net zero current, you can’t use the transformer based technique proposed by LPP to convert the ion beam to electrical energy. Even if you get a current, what fraction is neutralized before you reach conversion location? If every He ion drags along one electron, you’ve cut the beam current in half. Breakeven suddenly got harder. A test with a conventional particle accelerator is hard because the currents are typically limited to less than 100A and passing ions into a gas cell for the test is challenging.

It was my understanding the the intense magnetic field of the plasmoid, on the order of GigaGauss… will force electrons in the opposite direction (toward the cent of the anode) while sending the positive ions away from the anode. So before they leave the region of the plasmoid, they are already on opposite trajectories.

[asymmetric_implosion]

Joe,

That is a typical view of the plasma focus. It is also demonstrated to be incorrect. In the last couple years, plenty of measurements support that ions and electrons are traveling in both directions. Work by Roshan et al at Nanyang Technical University showed that ions are indeed traveling toward the anode when they should be moving away. It is about local potentials. The conventional view of a pinch creating fast ions is that magnetic field rapidly evacuates a small region of the pinch due to instabilities. With the magnetic field changing in time, you generate an electric field which accelerates electrons and ions. These ions and electrons move in opposite directions as one might expect so you get two opposite moving particle beams. The problem is the electric field has a radial profile with some electrons and ions accelerated in both directions. None of this requires Gigagauss fields. For the tech savvy, Malcom Haines wrote a review article on the subject of pinches which contains the full argument; just pony up $90 for it. Worth the read in my opinion as it gives the history of pinches back to the 1900’s. Anyway, even if you create pure beams of ions and electrons in opposite directions, you have the send these beam through a dense gas. As the ions fly by they pull electrons from neutral atoms. It is possible that some electrons chase the ions. More likely, the gas in the direction of the generator is partly ionized due to UV so the electrons are already hanging out. They hitch-hike with the ions leading to a reduced current. I know a number of measurements disagree on the impact of these electrons but it is another loss term to consider.

[Joeviocoe]

asymmetric_implosion wrote: Joe,

That is a typical view of the plasma focus. It is also demonstrated to be incorrect. In the last couple years, plenty of measurements support that ions and electrons are traveling in both directions. Work by Roshan et al at Nanyang Technical University showed that ions are indeed traveling toward the anode when they should be moving away. It is about local potentials. The conventional view of a pinch creating fast ions is that magnetic field rapidly evacuates a small region of the pinch due to instabilities. With the magnetic field changing in time, you generate an electric field which accelerates electrons and ions. These ions and electrons move in opposite directions as one might expect so you get two opposite moving particle beams. The problem is the electric field has a radial profile with some electrons and ions accelerated in both directions. None of this requires Gigagauss fields. For the tech savvy, Malcom Haines wrote a review article on the subject of pinches which contains the full argument; just pony up $90 for it. Worth the read in my opinion as it gives the history of pinches back to the 1900’s. Anyway, even if you create pure beams of ions and electrons in opposite directions, you have the send these beam through a dense gas. As the ions fly by they pull electrons from neutral atoms. It is possible that some electrons chase the ions. More likely, the gas in the direction of the generator is partly ionized due to UV so the electrons are already hanging out. They hitch-hike with the ions leading to a reduced current. I know a number of measurements disagree on the impact of these electrons but it is another loss term to consider.

Thank you… again 🙂

[Francisl]

Could this high temperature plasma be used to operate a MHD generator?

[asymmetric_implosion]

Maybe, but MHD generators tend to favor quasi-steady plasma flows and the plasmoid is far from quasi-steady unless you are firing at very high repetition rate. Can the plasmoid survive a modest transit in the dense fuel gas? If I’m understanding the long term goal of LPP, they wish to operate the focus at ~100 Torr. Even an alpha particle at 3 MeV has a range limited to 15-20 cm (using an analytical alpha particle slowing down model from Mayo, Intro to NE). The plasmoid is unlikely to travel that far so the MHD generator is right on top of the pinch which is probably bad for its lifetime. High energy pinches produces copious amounts of UV, X-rays and charged particles that can chew up materials.

[Francisl]

I went to the https://focusfusion.org homepage and use the search term “ion beam”. A number of articles were listed, such as: LPP Team Starts Looking at Ion-Beam Energy Extraction and New record beams bode well for applications.
I think I read an article a couple of years ago that LPP demonstrated to an investor that the beams had reasonable directionality. I hope there will be a good way to extract the energy.

[asymmetric_implosion]

The beams exist; I’m not saying they don’t. People have measured the beams using all sorts of techniques from nuclear activation to magnetic spectrometers. The beams have been an accepted part of plasma focus operation since the 1960’s. My comment regards extracting energy from them. I’m saying that the beams are composed of both ions and electrons. When the beam is composed of both ions and electrons, the net current is lower. If 100 kA of ions are moving toward the generator but 10 kA of electrons are moving with them it is a small issue. If 50 kA of electrons are along for the ride, you’ve lost half your generator current. Now you have a pretty serious problem. I don’t know the answer on magnitude of the electrons because it is strongly related to the experimental conditions. I’m simply saying it is observed and a potential problem to address. I recall a post earlier about the ion beam missing the generator region or the current seeming to be low. This could explain it. People have observed an off axis maximum in the ion beam emission. It is accepted that the ion beams have some angular profile from parallel to the pinch (0 deg) to perpendicular to the pinch (90 deg). The contribution of ions is suggested to peak +/- 5-10 deg off 0 deg for some gases using electric probes. Nuclear activation suggests the ions peak on axis (0 deg). The two measurements are reconciled when electrons are added to the picture. Electrons cannot activate the materials used in activation experiments but they can reduce the observed current on an electric probe. The reduction was ~20% so it is probably getting serious.

There is a world of literature out there that is not derived from LPP or their experiments. I know this message board is not familiar with it but relying solely on one group’s writing is not a good idea. It is easy for someone to lead you down a path when you rely on one source. From my perspective, LPP is doing two things different than the rest of the folks in the PF community; small electrode geometry and as they call it chewing the sheath. The small electrodes are interesting because it challenges some conventional wisdom about plasma focus devices. I think this is an interesting path which will benefit the Z-pinch community in general. Chewing the sheath is interesting but I don’t know if it is the reason for the results observed by LPP. We have observed filament breakdown leading to pinches as good as uniform breakdown. My opinion is filaments exist with or without the tungsten ring and there are other ways to create them that don’t rely on tungsten pins with poor current contact. Another PF company a few years back showed a ring with a triangular cross section produced very reproducible pinches operating at modest repetition rates (~1 Hz) for hundreds of shots. Rep rate operation changes matters as well as plasma is left behind between shots. We observe improvements in uniformity with increasing repetition rate. If chewing the sheath is essential, it will be hard to do at 200 Hz when a bunch of electrons are hanging out ready to breakdown uniformly.

[Lerner]

Just to clarify–we don’t think we have tested the new tungsten “teeth” and our published results pre-date using them. We do think our results are due to the small size of the electrodes and the axial field coil we are using. I agree that neutralization of the current is a potential problem but how much of one it is needs to be determined experimentally.

[asymmetric_implosion]

But you did use the tungsten pins? If so, that is very different than most PF devices I’ve run across in literature. Does that not qualify as chewing the sheath? I am curious if tests have been done without the tungsten pins and compared to tests with them.

I agree that an experiment is important to evaluate neutralization of the beam. The point was to say that a particle accelerator is not a good test bed for testing the generator without a gas box to serve as an electron source.

[Francisl]

asymmetric_implosion,
Please suggest some good websites we can use for information. It has been my experience that a lot of interesting articles are behind paywalls. Satisfying our curiosity can be expensive.

[asymmetric_implosion]

Here is one site with some of their papers put up for free. In the interest of disclosure, I am not a part of this group but I do know their work reasonably well.

http://www.intimal.edu.my/school/fas/UFLF/

I’m sad to say that experience is expensive. I pay $30 or more per article. I guess the question one must ask is the level of devotion to learning the subject. Just a warning, these are peer-reviewed publications. The authors expect some minimum understanding of plasmas and little is provided to help you in the journal articles other than references to other articles and more money spent. If you are seeking a basic appreciation of the subject, popular science type literature on the plasma focus exists and some of it is linked on this site. I look forward to the day when the pay walls fall down so everyone can see all the different interpretations of available data and decide from themselves what is the best interpretation.

[Author]

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