Plasma Sheath

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  • February 22, 2014 at 4:12 am #1541
    FrankOlson
    Participant

    Hello everyone, my name is Frank.
    I am a clinical laboratory science intern. My hobbies are jazz and electronics. I am fascinated by plasmas – everything about them. I have built a few high voltage power supplies to see their basic characteristics and behaviors. My favorite circuits are parallel RLC and ZVS circuits because of the electrical resonance phenomena unique to a capacitor and inductor in parallel. One of my projects this past year was to observe the effects of the electrolysis of water when the electrodes in solution were treated as reactive components in an RLC circuit, rather than being subjected to straight DC. Whenever I get stuck waiting for parts, or figuring out why a circuit wasn’t working I tended to develop tangent curiosities that sometimes help solve the problem I’m working on.

    This idea of ‘focus fusion’ is quite neat. The first time the whole idea ‘clicked’ in my head, I thought to myself how I had seen something similar to that plasma motion before, in a Jacob’s ladder. I know that a Jacob’s ladder can be run with an RLC or more commonly, a ZVS circuit. When utilizing these circuits, the plasma ascends up the electrodes and the resonant frequency of the tank in the ZVS circuit changes in relation the load of the coil that is coupled to the tank. The coil that is being coupled to the tank is essentially being shorted out when the plasma forms – this is the loss that requires more energy needing to be put into the ZVS tank to keep the oscillations going – the resonant frequency changes because of the work to form the plasma through inductor coupling and is ‘adjusted’ by the ever increasing plasma arc as it ascends up the ladder. This guy has a nice example of the ZVS topology.

    http://adammunich.com/zvs-driver/

    I guess one of my new projects is to look into and build such electrodes as seen in most focus fusion pictures, drawings and schematics. I’d like to hook it up as the “capacitor” in an RLC circuit or, maybe to the coupled coil in a Jacob’s ladder ZVS schematic where, instead of having the electrical short being from the two protruding ends of the coil’s wire, it could be into two ‘focus fusion’ style electrodes – in a partial vacuum. I’m just brainstorming as this would be a difficult build and I’m not out to get fusion as the result. I just want to see the plasma sheath, and have it pinch off at the top as is described, and as in the pictures I’ve seen. Thanks for reading my rant. I am a bit curious about something though.

    I not sure why or how the plasma is able to be a moving sheath instead of it choosing the shortest distance and simply ionizing to that one point like in a Jacob’s ladder (albeit vastly different electrode design). It must have something to due with whatever the current pressure in the vacuum would be at? Too high and it’s a streamer, too low and it’s just a cloud? I read the post about why there is a preference for the eight rods versus just another, bigger, tube cathode.
    Thanks! I look forward to keeping track of this DPF project and digesting all this forum has to offer.

    February 22, 2014 at 4:29 pm #13121
    Francisl
    Participant

    This article on plasma railgun will answer some of your questions.

    February 23, 2014 at 2:46 am #13124
    asymmetric_implosion
    Participant

    Frank,

    Some important differences between the Jacob’s ladder and the PF. The Jacob’s ladder climbs mainly due to convection; the arc is hot while the surrounding gas is cold. The travel speed is limited by typical convection speeds. In the PF, the plasma motion is driven by the magnetic pressure behind the sheath. Like a balloon, the sheath blows up as the pressure increase and pushes the plasma along the channel between the cathode and anode. Magnetic pressure can only exist where there is current flowing so the pressure is concentrated between the anode and cathode.

    The plasma focus circuit is a simple design on paper. You want a high voltage capacitor, a low inductance circuit and a low resistance circuit. The mechanical part of the system must withstand the current pulse which for most small plasma foci is easy enough. The optimum electrode design is typically a cathode diameter that is double the anode diameter. The anode radius is optimized when the Lee’s drive parameter is 70-90 kA/cm-sqrt(Torr). D=I_max/(a*sqrt(P), where D is the drive parameter, I_max is the peak current, a is the anode radius and P is the deuterium gas pressure in Torr. You cannot choose the pressure arbitrary. It must be between 1 and 20 Torr for most systems. The pinch effect is demonstrated from systems of a few amps to 26 MA. There a practical problems of small pinch devices. Pinch devices above 100 kA are easy enough to produce and literature is fully of devices of this scale.

    Good luck learning about PF device.s

    February 24, 2014 at 4:52 am #13132
    FrankOlson
    Participant

    I see. I read the article on the plasma rail gun. Thanks guys. Between wikipedia and lecture 11, this is making a lot more sense to me than it did last week.

    asymmetric_implosion,
    You mentioned that the plasma focus circuit is a simple design on paper. It’s a capacitor in parallel with a cathode and an anode that is being gapped by a gaseous medium, essentially. Then the plasma sheath comes into play – it’s a complicated thing. So how would you represent the cathode and anode, as a resistor, capacitor, or inductor? I know there would be stray elements of each, just like in real world components. I just don’t know which would be the prevailing for this case. My guess is that it’s a capacitor until it begins experiencing a dielectric breakdown, hence the plasma, then the resistive element dominates as the actual ‘capacitor’ bank keeps discharging into the cathode and anode. But, I know once the plasma forms the resistance would probably drop since, from what I remember, the plasma is a low resistance element once formed. This all happens so fast besides, it’s just mind blowing.

    http://en.wikipedia.org/wiki/Electromagnetic_forming

    Lewin, Walter. 8.02 Electricity and Magnetism, Spring 2002. (MIT OpenCourseWare: Massachusetts Institute of Technology), http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002 (Accessed 23 Feb, 2014). License: Creative Commons BY-NC-SA.

    February 24, 2014 at 2:59 pm #13134
    FrankOlson
    Participant

    Leopoldo, S. (2002). Braz. J. Phys. vol.32 no.1 São Paulo Mar. 2002. Brazilian Journal of Physics , 32(1), Retrieved from http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332002000100025

    Attached files

    February 25, 2014 at 4:41 am #13143
    asymmetric_implosion
    Participant

    The PF, when working properly, is a series RLC circuit. All components contribute to the R, L and C. The C is dominated by the capacitor so other capacitance can be neglected in most PF devices. There are a number of different R and L contributors to the system. A gas switch typically triggers the systems and it is a dynamic resistor and inductor. The resistance falls from >100MOhm to <0.01 Ohm in less than 50 ns. The inductance stabilizes based upon the switch geometry for the rest of the pulse. It is typically 50-100 nH. The feed plates contribute both R and L depending upon their geometry and conductor choice. The electrode region is most accurately modeling as a time varying and time varying R. There are a number of models that exist like the model by Sing Lee. I've posted the link to Lee's model a number of times on this forum. When I have the measured time derivative of the current and the voltage at the electrodes, I prefer to derive R and L as they vary with time. There are very complex numerical models that address the physics in detail but they are beyond the reach of the average person due to the processing power required to run them. Lee's model is probably the most accessable model for the average person (only need Excel) while maintaining some meaningful accuracy. Just a warning, garbage in and garbage out. You need to know a bit about PF devices before you just start throwing numbers in the model. It is not valid over a wide range of parameters.

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