Multipoint to plane corona discharge I'm trying to make a multipoint to plane corona discharge, my cathode is a piece of metal with needles welded on it, and my anode is also a piece of metal. The experiment i was setting up is to treat water and so the anode is inside the water, as i turn on the electric, the corona discharge doesn't happen to all needle, only 1 to 3 needles have plasma discharge, and i'm doing 6 of them, can you help me with this problem? Do all needles have to be equally in length? Thanks!
 A: Once a corona develops on one of the needles, the voltage on the cathode plate drops and makes corona development on other needles less likely.
This is due to a limited power of your high voltage source. As a corona starts, its current causes a voltage drop on the internal resistance of the power source, which reduces the voltage between the cathode and the anode. So, you can possibly improve the performance of your system by using a more powerful source.
Another possible approach is to increase the distance between the cathode (needles) and the surface of the water. The idea is to increase the resistance to the corona current. As a result, the voltage drop associated with each corona won't be as significant and more coronas could develop. 
Once you have all coronas activated, you can possibly move the cathode closer to the water. As you do so, the current will be increasing and the voltage between the cathode and the anode will be decreasing, but it takes less voltage to maintain a corona than to start it, so it might work. 
Just equalizing the length of the needles is unlikely to make much difference.  
