The "EE" 12.5mA Limit Problem

By Miguel Olivo, July 18, 2003

When there is a spark between the "EE" electrode and the "Ground" electrode, the potential of the "EE" electrode shifts towards the ground potential (during the spark a low impedance path is created).

Since the capacitance between the plasma electrode and ground is large, the potential of the plasma electrode remains at the bias potential. Because of this, the potential difference between the plasma and the "EE" electrodes can become very large during the spark.

In order to protect the "EE" high voltage power supply (and HV coaxial cable) from being damaged we installed a "SPARK GAP" between these two electrodes. The size of the gap was chosen in such a way that a spark is triggered when the voltage difference is larger than about 8 kV. Now, if a spark in the gap is triggered, a continuous arc could be formed IF the "EE" power supply can deliver enough current. The "EE" power supply current limit was set at about 12.5mA (I don't know why it was set at this value). This value was large enough to keep the arc going. As a consequence of that, the "EE" voltage dropped to about:

12.5mA * 160kΩ = 2050V

 To "cure" this problem the "EE" voltage was lowered by the operators and then set again to the operational value. What actually happened was that by lowering the "EE" voltage the arc was extinguished and then one could set back the old "EE" value again.

I have set the limit to the current delivered by the "EE" high voltage power supply to about 3 mA. At this value there is not enough "juice" to sustain the arc. Sparks will occur but a continuous arc will not be formed.

[Miguel included a diagram which can be seen on the paper ION page for this note.]