Re: Low voltage sparkgaps for ignition coil driver...
Tesla List wrote:
> Does this mean that the current goes up at small distances where the
> breakdown voltage increases? Is the electron transfer bypassing the small
> number of Air molecules in the gap?
The current won't go up unless you have significant electron emission,
which, with cold electrodes and low voltages (even though the field is
high in a V/m sense) is unlikely (unless you've coated your electrodes
with something like potassium and you are shining a bright light on
> What is the association to AC frequency? Is it reasonable to assume that
> voltage decreases at all distances with increasing frequency?
The breakdown vs frequency is roughly constant up to about 50 kHz, then
it starts to decrease, reaching a minimum (about .7-.8 that of dc) at
around 2 MHz, then it increases again, reaching a maximum at several
hundred MHz (around 1.2 that of dc), then falls rapidly (numbers and
frequencies for a 1 cm gap in air). The middle dip is due to ion
mobility. As the positive ions move across the gap, as a half cycle ends
and the next, of opposite polarity, starts, the ions get pushed back the
way they came. If the frequency increases enough, the ions don't have
time to clear the gap (depending on the ion mobility, the frequency, and
the length of the gap). A similar phenomenon occurs with electrons.
critical frequency for positive ions is k+ * V / (pi * d^2) were k+ is
the positive ion mobility. The electron mobility expression is similar
(substitute ke for k+). Finally, above a high frequency cutoff, the ions
are lost by diffusion, rather than mobility.
What is the
> Does it relate to molecular electron orbital polarizability? If so, this
> would predict a quantum drop in discharge voltage at the resonance frequency
> of the highest occupied and lowest unoccupied MO.
Jim Lux Jet Propulsion Laboratory
ofc: 818/354-2075 114-B16 Mail Stop 161-213
lab: 818/354-2954 161-110 4800 Oak Grove Drive
fax: 818/393-6875 Pasadena CA 91109