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Few questions about plasma

  1. May 12, 2013 #1
    now even though I searched the forums for keywords and came up with none exactly matching my case excuse me if this has been asked.

    Firstly is it possible to have a high pressure but low temperature self sustaining plasma? i do understand that temperature corresponds to the plasma particle average kinetic energy which must be high enough to form a "burning plasma" but can a plasma like that be possible with low temperatures but high pressures? Even though I guess high pressure and low temperature don't go together that well as increasing pressure usually increases temperature.

    Secondly I wonder what would happen or should I say how the plasma would react if it would be between two electrical fields (static ones) like from a high voltage dc source.
    Like between two spheres one bigger (outside) and one smaller inside.
    Now if one would be + and the other - then the electrons would attract to one and the ions to other but if both would be at the same polarity like ++ or -- what would happen then ? Would the plasma be confined between the two spheres by the electrical field ?
    As long as I remember plasma is a ionized gas made of charged particles negative electrons stripped from positive nucleus or ions.The net charge is neutral but it is effected by electrical and magnetical fields.
  2. jcsd
  3. May 15, 2013 #2
    anyone? :)
  4. May 15, 2013 #3


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    Usually, laboratory plasmas are at very low pressure and high temperature. But there's a whole field studying atmospheric pressure plasmas, and these plasmas can exist even at gas temperature near room temperature. I don't know too much about them, but some electrons can still have high energy even if the temperature of the gas is fairly low. Maybe read this paper if you have the resolve. www.seas.ucla.edu/prosurf/Publications/paper58-IEEE.pdf‎ [Broken]

    Plasmas are very conductive, so if you have a difference in voltage, it will generate a large current through the plasma. If you put two positively charged spheres into a plasma, the plasma will quickly neutralize the spheres, since electrons will crash into the spheres. Actually, the spheres will gain a negative potential relative to the plasma because of a Langmuir sheath effect (the electrons move faster and crash into the sphere more than the positive ions). If you attach the spheres to some beefy power supply to hold them at some potential relative to the ground, then it depends on what is surrounding the plasma. If the plasma is close enough to the ground, a current could flow through the plasma. If the plasma is insulated from ground, then the bulk plasma potential will shift until the plasma stops flowing. (The plasma potential will be slightly higher than the spheres because of the sheath effect.)

    By the way, you can definitely have high pressures and low temperatures together, though it's hard to have a plasma in those conditions.

    By the way, if you have magnetic fields around, this complicates things...
    Last edited by a moderator: May 6, 2017
  5. May 25, 2013 #4
    Ok , one more thing , by confining plasma with magnetic fields you create a sort of pressure on the plasma thus keep it in a certain boundaries , now ofcourse for the pressure to be of a atleast " usable" extent you have to have a strong magnetic field,
    Now by having and electrical field don' t you create the pressure on the plasma just like with the magnetic field , because as long as i remember charged particles react with magnetic fields and change their trajectories just as much with electric fields , the only difference i suppose would be that in the case of electric field the ions and the electrons would want to directly interfere with the field thus distorting it right?
  6. May 26, 2013 #5
    Remember from electrostatics that the electric field inside of a conductor is zero and plasmas are extremely good conductors.

    An important concept in plasma physics is the Debye length. The Debye length is a measure of how far an DC electric field will penetrate a plasma. On distances longer than the Debye length the plasma will shield out the electric field. For a burring plasma the experiment Debye length is 0.1mm.
  7. May 27, 2013 #6
    a dumb question maybe but to confine a plasma , a burning one if you want isn't it enough to keep it in a certain boundaries , now even if the electric field penetration is so small , but if it is very strong on the sides it should reflect the charged particles.

    The only problem i guess is that one would need a unipolar electric field (not sure one exists ) to reflect both negatively and positively charged particles rather than just reflecting one kind and attracting the other. Which would disrupt the plasma I think.
  8. May 27, 2013 #7
    No its not enough. A burning plasma is going to have a high temperature high density core, and a low temperature low density edge. Thus the core of the plasma is going to be high pressure and the edge of the plasma is going to be at a low pressure. In order for the this system to exist you need an additional force that will balance this pressure gradient throughout the plasma.

    You can not support this pressure gradient with DC electric fields because they don't penetrate the plasma.

    There is no such thing.

    Nope this does not cause disruptions. A sheath would form. Sheaths naturally occur in all magnetically confined plasmas.
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