Update: Plasma ignition source for microscope

Here is an update on how the series development of the plasma ignition source from the last article continued. If you haven’t read the last post yet, you’ll find it here.

Now it gets a little technical… ­čśë

It turned out to be absolutely correct that we had installed a current measurement with current display in the test carrier. This enabled us to determine the current consumption with plasma extinguished and ignited. As it turned out, even when the plasma was extinguished, 12.5 mA RMS already flowed even when only the high-voltage plug was inserted on the side of the plasma unit. This seemed to be quite a lot in terms of current consumption with ignited plasma, which was in the 25-30 mA RMS range. The plasma ignites when the process pressure in the FIB column is sufficiently low – the chamber is vacuumed – so that the mean free path length of the ionized process gas is large enough. We doubted our sanity for a moment. A small test then led us on the right track:
When we plugged the approx. 2.5 m long high-voltage coaxial cable into our plasma ignition source, but not the connector on the opposite side, a current of 0.5 mA RMS already flowed. Only when no cable was plugged in at all did no electricity flow.

We verified this and were sure that there was no display error, but that these currents were actually flowing.

The answer to that riddle:

In the case of non-ignited plasma, these were purely capacitive recharging currents, which already occurred at the 50 Hz alternating voltage frequency. The level of the current, in conjunction with the installed series resistors, suggested a distributed total capacity in the order of 80 nF, which resulted when the high-voltage socket was plugged into the system. Even the high-voltage coaxial cable, which was only plugged in at one end, already resulted in a low reloading current with its cable capacity. Due to the parasitic capacitive consumer, there was a corresponding phase shift between current and voltage and a low-pass filter with a cut-off frequency of about 250 Hz. With the consequence that at the 50 Hz no noticeable drop in the effective value of the output voltage could be measured, although the voltage drop at the series resistors is effectively more than 100V – the phase shift of the voltages makes it possible ­čśë So everything fitted together perfectly.

The tests on site at the customer with the test carrier were extremely important for the development of the status LED display on the series devices. We had to design an evaluation circuit so that the LED did not light up at the capacitive recharging currents, but only at higher currents that can be assigned to an ignited plasma in the system. This is what the customer wanted. The evaluation circuit also had to be based on the high voltage on the output side, which is not a trivial matter. The result is the following circuit board with a lightpipe to display the plasma status on the front panel:

Plasma-Z├╝ndquelle von innen

Circuit diagram and layout of the board were designed with the OpenSource “weapon” KiCad as always. To supply the operational amplifiers and comparators, we have generated a galvanically isolated auxiliary supply for the sake of simplicity –
by means of a 9V auxiliary winding on the transformer.

We have built the series devices of the plasma ignition source in a compact and robust desktop housing with the dimensions 250 mm x 174 mm x 100 mm. This makes them well suited for the intended mobile use. For the high voltage output, we have installed a special high voltage coaxial plug at the customer’s request. We also provide the customer with the appropriate connecting cables. The outer conductor of the plug is electrically insulated from the metallic housing by a POM insert.

Finally a few photos of the devices, still without covers and end pieces at the housing corners:

Plasma-Z├╝ndquelle Frontansicht ohne Abdeckung
Front view
Plasma-Z├╝ndquelle ohne Abdeckung R├╝ckansicht
Rear view

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