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Breakdown mechanism in electrical discharages

  1. Feb 6, 2012 #1
    I've been reading up on methods of generating plasma via electrical discharges and I have some question. I'm learning this on my own here so please correct any faulty asumptions. It appears to me that a typical RF discharge across a dielectric cell operates below breakdown voltage and the plasma is generated via the Townsend avalanche. At some point (quickly) a sheath develops and the field in the plasma drops to near neutral although current still passes across the gap. Please correct me if I'm wrong.

    Question 1: What is the advantage of using a dielectric material? The only thing I can think of is that the electrodes need protected to avoid some type of sputtering.

    Question 2: I undestand the electon avalanche effect, but how does a current pass through the gap if the voltage is below breakdown. Does the applied field push the plasma electrons into the lower potential electrode and a higher energy electron busts through the sheath to take it's place? If so, wouldn't the electron temperature keep going up?

    On the other hand, I've seen short duration HV discharges that fire well above breakdown. In this case the high potential pushes electrons forward causing ionization/breakdown, no? A sheath layer forms soon after, dropping the field in the plasma.

    Question 3: Since the potential is higher, does that mean the sheath layer is much thicker? Also, how does/doesn't sheath formation coincide with breakdown in time? I'm guessing that most applied voltage waveforms much faster than sheath formation, so in this case breakdown would occur long before sheath formation? In either case, the field in the gap would drop, but I'm unclear on the two mechanisms.

    Question 4: Are there analytical solutions that account for both of these processes at once? Neglecting inelastic collisions by electrons for the moment, the diffusion processes could be modeled to show the evolution of breakdown and sheath formation? If so, have they been done in multiple dimensions (for something other than infinite planar electrodes).
  2. jcsd
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