Figure 4 Ion current and displacement current clearly shown.

A fast switching ionizer (10 msec. rise times and 100 msec. ON times) was placed under a CPM. The distance between the CPM and the ionizer was 30 cm. Float and Discharge Time tests were done with a CPM with a digital waveform display and are shown in Figure 5.  The upper part of the figure shows the FLOAT test and the lower part, the Discharge Time test.  Note that the upper figure shows ±Peak Voltages and “Plate Voltage (the last sample acquire).  The lower part of the figure shows Start Voltage, End Voltage Decay Time (from Start Voltage to End Voltage).  Again, the Plate Voltage is simply the last sample acquired.

The CPM waveform has a peak to peak voltage excursion of   ~100Volts and a discharge time of 0.71 sec.  Next a large (45 x 60 cm) insulating sheet was placed over the CPM plate, effectively blocking the ions from striking the plate. The same two tests were repeated and are shown in Figure 6.  The Float test waveform with the ion current blocked, is essentially identical to the waveform with the current allowed to strike the plate but the discharge time is increased by nearly 100 times. 

Figure 5.  CPM waveforms from a fast cycling ionizer.

Several conclusions can be drawn from this result.  First, the majority of the signal measured by the CPM in this case is from displacement current.  Second, the displacement current is indeed from a field created by the ionizer and will penetrate a dielectric easily.  A flat panel substrate is an example of a dielectric that cannot “see” this component of the swing.  MORE