Viable design?

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Dawson Hubbard
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Viable design?

Post by Dawson Hubbard » Sun Feb 04, 2018 5:18 am

The title might be a little misleading, I am not trying anything completely new or something like that. I have decided to use a pulsed Z-pinch style machine to achieve attempt fusion. I have some experience in electronics and such beforehand, and this is the design I came up with.
WIN_20180204_00_08_27_Pro.jpg
It's a very rough rough draft I know. Gas thing is the gas valve, PWM is a pulse width modulator, so I can control the rate of electricity. I probably will add some form of electrode distance control, or some additional control elements. Also, a capacitor bank might be needed for a Z-pinch design
Anyways, I would like to know what is missing, what should be improved on, or if this whole design is just folly. I would like to do a Z-pinch for fusion, however.

Michael Bretti
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Re: Viable design?

Post by Michael Bretti » Sun Feb 04, 2018 6:11 pm

A pulsed power approach is certainly commendable and do-able for generating fusion neutrons, but is very different from a fusor or even beam-on-target system. Pulsed power is rather exciting for myself personally because it is both one of my main areas of focus and something that I work on regularly professionally. I find that pulsed power systems are some of the coolest systems in engineering. That being said, there is an incredible amount to consider when designing a pulsed power system. In regards to your immediate question whether or not your design is viable, you have not given nearly enough information for that to be properly answered. Without going into a massive engineering design lecture or review, below are some very basic and fundamental things to start to consider:

1.) Energy Storage - There are various forms of energy storage in pulsed power systems, each with their own applications. Capacitor banks, PFNs, transmission lines, and a wide combination of these basic topologies combined together. It also depends on your system parameters and end goal. What is the basis for using a PFN rather than a capacitor bank or transmission line? What is the difference between all of these types of energy storage? What peak power levels are you trying to achieve? Pulse shape, rise times, fall times, FWHM value, impedance, etc need to be considered. What current levels are you trying to achieve? A plasma pinch is largely dependent on the current delivered, which generates a self-induced magnetic field which induces the pinch itself. What critical levels of voltage and current, at what time scales are necessary for neutrons to be produced from fusion using your approach? It does not seem that any of this is addressed. A PFN may or may not be what you need depending on the system you try to build, as various types of pinch machines utilize different methods of energy storage.

2.) Switching - Switching in pulsed power applications are perhaps the most crucial and demanding part of the system. The switch will in part determine rise times, fall times, repetition rate, power level, delivery, etc. Peak power is dependent on switching time - certain switches cannot switch fast enough or high enough powers depending on you application. What was your design criteria for choosing a thyristor, as opposed to a thyratron, spark gap switch, etc? What power levels and repetition rates are you aiming for? Is this a long charging, high intensity single shot device, or high rep-rate device? Note that for pulsed power applications it is incredibly easy to generate huge peak power levels, but increasing the repetition rate while maintaining those levels is a very formidable challenge.

3.) Load - It seems like not much has gone into the design selection of your load. First off, what pressure levels is it operating at? Is this a vacuum based system, or at atmosphere or higher? Just generating an arc or pulsed breakdown between two tungsten electrodes in deuterium gas will not automatically result in a pinch or fusion, otherwise we would be seeing neutrons from the MW level deuterium thyratrons we use at our LINAC facility. Have you considered the the Paschen Curve and determining the necessary parameters relating breakdown voltage, pressure, and gas fill? Note also the safety considerations - arcing through deuterium can create explosive conditions if there is any air remaining in the chamber. Have you considered the electrical impedance of your load, and how you want this matched to your transmission line? Having a chamber with electrodes sticking from either end could result in huge mismatches depending on your overall system topology. In addition, why was the load selected the way it is? How does this compare to other pinch configurations and topologies?

4.) Overall Topology - What is your design criteria for selecting a PFN switched system vs something like a capacitor bank dumped system, a marx generator, or even a transmission line storage system (using an actual line rather than a discreet PFN)? Have you considered impedance matching for your system to account for energy reflections due to mismatched impedance between your transmission line and load?

5.) Instrumentation and Shielding - Any pulsed power devices is going to radiate a large pulse of EM energy. The shorter the pulse width, the higher the peak power, and the more intense and disruptive this burst is to nearby electronics. This is what would make detecting neutrons with such a device possibly more challenging than a standard fusor. A standard fusor generates enough interference just operating continuously, providing a steady stream of neutrons. Pulsed devices operate in very short bursts, so detection is not continuous. Activation and use of a bubble counter also may be problematic due to its very low repetition rate usually. An incredible amount of care and planning must be prepared for in shielding your electronics from the pulse interference (no, just wrapping something in tin foil does not equate to good shielding for devices such as these that can radiate upwards of MWs of peak instantaneous power across a broad range of frequencies.) This is critical and must be addressed if you are planning to prove neutron generation with such a device.

There are significantly more details and considerations to go over, but above are just some very basic questions to consider and address. If you cannot answer these immediately, then you may want to go back and do a lot of research on these types of systems. For example, if you are looking to do pinch based fusion, have you looked into and considered the Dense Plasma Focus? It is a rather simplistic device to construct (physically at least), and requires just a capacitor bank with sufficient energy and voltage - usually over 10KV, though fusion neutrons have been produced at very low Joule inputs down to 9J per shot, though most operate in the several thousands to tens of thousands of Joules per shot range. Small devices can be made with banks that can deliver hundreds of Joules, which is much easier and cheaper. It is also usually switched by a simple self-firing spark gap switch (though triggered gaps can just as easily be used as well), and requires quite low vacuum (as opposed to high vacuum) that is very easily attainable with a refrigeration roughing pump (although a very good system should be properly outgassed and prepped at high vacuum levels to achieve good fusion numbers every shot, especially for low power systems). This is one of the most widely studied pinch devices for neutron production, and the principles of the plasma are almost linearly scalable from tiny machines to huge devices. There are hundreds of papers on this device as well, including free design calculators for estimating the design criteria of the system. These can be operated single shot or low rep rate as well. Detection however, as mentioned above, is much more challenging than a fusor, with neutron pulses released in durations of several tens of nanoseconds, requiring fast and very good electronics and data acquisition, in addition to shielding. Neutron bursts are also generally on the order of 10^6 neutrons per shot, and relatively directional as well, so proper shielding is required. EM shielding is also extremely crucial since it is a spark gap switched device switching at high power levels and reasonably fast speeds.

If you are very serious about building a pulsed power neutron source, then you will want to do an exceptional amount of research. While the physical design can be made relatively cheaply (like in the case of a dense plasma focus), there are a lot more design considerations over a standard fusor, and such a device does inquire intensive calculation, whereas a working fusor can be slapped together and made operational with some good planning, experimentation, and no calculations. If you are still serious about it, you can feel free to message me if you want to discuss these types of systems, since I both work with pulsed power systems regularly and also have been planning on making a small fusion capable dense plasma focus with my personal system, though this will not happen for quite a while. However, I would also urge you to do a lot more research and demonstrate more comprehensive understanding of such systems before diving immediately into building such a device. It will save you lot of time, money, and frustration, and you will learn a lot over a wide multidisciplinary range of engineering topics.

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Dawson Hubbard
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Re: Viable design?

Post by Dawson Hubbard » Sun Feb 04, 2018 9:24 pm

I'd like to thank you very much for your reply. I did not expect to get such a lengthy and in depth reply, and I don't mean that as a bad thing. From what I understand from your reply, the gist of it is that I really need to put more thought into it than simply drawing up a simple circuit like that. I think I will put more thought into what I want from my fusion machine, then design everything backwards from it. I will look into the Dense Plasma Focus as a starting point for some more in depth research.

Michael Bretti
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Re: Viable design?

Post by Michael Bretti » Sun Feb 04, 2018 9:50 pm

No problem at all. Like I said, if you need any more info or want to discuss design, feel free to reach out to me whenever, I would be more than happy to help how I can. If you do decide to go this route as opposed to a fusor, it will certainly be a very demanding challenge, especially for neutron detection. However, a really cool thing about the dense plasma focus is that it is not just for producing fusion neutrons - it can be used with really any gas fill (though you can only get neutrons with D2 fill). During operation of the device, a plasmoid forms and collapses upon itself due to instabilities in the formation and collapse of the plasma filaments when the initial plasma sheath is accelerated down the outer electrode. As a result, two beams are generated in opposite directions from the spot of the plasmoid collapse at the end of the device: an ion beam, and an electron beam. When used with deuterium, you also get the additional byproduct of neutrons from the resulting pinch compression of the deuterium, as well as the electron beam and the deuteron beam. You can however use the device with argon, nitrogen, helium, etc, and you will get a beam of ions of the gas composition that is fueling it. If you use a Faraday Cup to collect the beam current, based on the pulse shape and content, you can actually determine if you generated a successful pinch. This means that you could initially test the device with other gases besides deuterium, then switch to deuterium once you confirm successful pinch. However, the electrodes and other circuit parameters may have to be adjusted when switching between gases. But it is a great multipurpose source that can be used for a wide range of experiments. For example, using a tungsten target in the way of the electron beam can be used to generate an intense pulsed x-ray source.

I think it is commendable to be looking into such a complex system as opposed to the typical fusor (not to knock the fusor itself at all, which is still a complex and challenging thing to get to fusion levels). Your initial diagram was also a good effort, especially properly drawing a basic simplified pulsed power PFN circuit - you will find that pulsed power is not a topic you will see discussed in any normal EE course description or degree, and is a very niche field in terms of electrical engineering. A dense plasma focus will be intensive, and will require a lot of research, but I think it is certainly do-able, and is probably your best bet for working with a pinch device at home potentially capable of producing neutrons. Even if you don't produce neutrons with it in the end, you can use it for a lot of other experiments in plasma pinch physics. Good luck with your efforts, and keep us posted on your progress!

Michael Bretti
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Re: Viable design?

Post by Michael Bretti » Sun Feb 04, 2018 10:08 pm

Also, here are some papers on the Dense Plasma Focus, with a focus on low powered and general designs (I have a lot of papers on the subject):


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