You must be able to absorb the electron current to reverse the sheath fields in steady state.

You must be able to absorb the electron current

to reverse the sheath fields in steady state.

The electron saturation current is much larger than the positive ion Bohm current until

Ne/Ni ~1000.

The estimated current densities (in A/m2) for N+ (89 amu, SF3+), Ne, and N- (19 amu, F-). J(N+) is estimated using a modified Bohm velocity that accounts for the N- fraction at the sheath edge (from N+/Ne and Te/T(N-)). J(N-) & J(Ne) are calculated assuming T(N-)= 0.043 eV and Te = 2.5 eV

(for Te/T(N-) = 58) and 4.32 eV (Te/T(N-) = 100).

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Notes:

This table gives one two peices of information. First, what sort of current you would need to draw in order to reverse the sheath potential in a dc sense, but also, Second, what fraction of a biasing period would be required (at most) for electrons to electrically neutralize the positive charge collected during the remainder of the rf period.

Note: In high density chlorine discharges, it has been seen that the negative ion density during the active glow is about equal to the electron density (at least as I remember it.) Therefore, Ni/Ne = 10 is an _over_ estimate of the negative ion density there for CW glows even though it is the lowest value that I use.

U(Bohm) >= SQRT[{e*T_e*(1+alpha_s)}/{M(1+alpha_s*gamma)}]

alpha_s = the ratio of the negative ion density to electron density at the sheath edge.

gamma = the the ratio of the electron to negative ion temperatures at the sheath edge.