The Sparking Characteristics of Needle-To-Plane Coronas

The sparking voltage (V
s
) and maximum presparking corona current (I
max
) of needle-to-plane coronas have been measured as independent functions of polarity, tip radius (r), and needle-to-plane spacing (S). For a negative needle, V
s
and I
max
increase with S but are independent of r. For positive polarity, V
s
and I
max
increase with both S and r. Thus to increase the corona current while avoiding a spark, one should increase the spacing and voltage. At large spacings, I
max
is appreciably greater for negative polarity than for positive. Analysis of the data reveals that for negative polarity the maximum presparking current may be written in the approximate form I
max
∝V
s
(V
s
-V
O
)S
-2
where V
O
is the corona threshold voltage. This is an extrapolation of a relationship previously obtained for Trichel pulse corona. The analysis shows that the transition from negative corona to sparking occurs when the linear space charge density in the gap reaches a critical value Q
s
/S ≅10
-10
coul/mm, where Q
s
is the total space charge in the gap just before sparking occurs. It is shown that this condition amounts to spatial overlap of the multiple ion clouds in the gap, thereby creating a continuous ionized channel between the electrodes.