The Sparking Characteristics of Needle-To-Plane Coronas

The sparking voltage (Vs) and maximum presparking corona current (Imax) 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, Vsand Imaxincrease with S but are independent of r. For positive polarity, Vsand Imaxincrease 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, Imaxis 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 Imax∝Vs(Vs-VO)S-2where VOis 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 Qs/S ≅10-10coul/mm, where Qsis 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.