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Nominal voltage for plasma?

  1. Apr 5, 2012 #1
    What is the lowest voltage required in order for Helium to become ionized?
    Is that equal the ionization energy in electron volt per mole (kJ/mole)?
    If so it's only about 24 eV for Helium.

    Typical voltages are around a few thousand volts for most commercial plasma globes, sometimes around 10,000 volts for some homebrew ones.
    But ionization potential is typically 50 to 150 volts.

    So does this mean that the plasma is created at very low voltages but in the case for plasma globes are very high voltages?
    Last edited: Apr 5, 2012
  2. jcsd
  3. Apr 5, 2012 #2
    You have a misconception about ionization energy (sometimes colloquially referred to as potential) and breakdown potential.

    First of all, these two quantities have different dimensions, the first one energy (measured in electron-volts, a unit of energy), and the second one in units of electric voltage/potential (measured in volts). You are essentially comparing apples and oranges.
  4. Apr 5, 2012 #3
    My appreciation and gratitude for your answer is endless.
  5. Apr 5, 2012 #4
    There is a connention between ionisation potential and voltage. 24 electron volts is the energy gained by a electron in traveling through a potential difference of 24 Volts. So if you have a voltage gap of 24 volts and an electron travels through this gap then it will gain enough energy to ionise a He atom.

    If you have an electron source, like a hot filament and a voltage drop of 24 Volts at low pressure in helium a plasma can be formed and sustained.

    In a plasma ball or other cold electrode discharge there is no electron source so electrons are released from the surface by ion impact, also electrons are accelerated in the electric field in the gas. As some of the plasma have high pressures then collisions reduce the average energy of electrons. In this case the plasma needs a field E/N greater than a particular value. Where E in the field Voltage/Gap and N is the neutral gas density. The larger the gap the bigger the voltage needed. The higher the presuure the bigger the voltage needed. So with a cold electrode discharge we need enough voltage between the plasma and a surface to generate electrons from ion impact and a large enough E/N to sustain the plasma is the bulk.
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