We are working on electric field-assisted chemical reactions and we want to make sure plasma is not forming in the system. I was told oscilloscopes can provide a straightforward way to detect plasma formation; however, I am not familiar with this instrument.
With oscilloscope you can detect the current flowing through the gap if plasma exists in it. For this you have to measure the voltage drop across the resistor connected in serios with the gap and power supply. Or you can detect the plasma radiaton if you connect the photodiode or photomultiplyer to the oscilloscope input.
If plasma glow is not visible you can measure current to find out if plasma (ionization) taking place or not. If you measure the current flowing through your load (your system) you will observe tiny spikes (of very short duration) in the current profile if plasma forms. If not u may notice at least a slight change in the current profile (it will be repeatable). If there is no plasma, it will be a smooth profile as supplied by the power supply.
You can get the current profile using a current probe or measuring the voltage drop (using oscilloscope probe 1:1 or 10:1) across a series resistor by connecting in series with the return wire.
Particular implementation strongly depends on setup details. AC or DC electric field, pulsed or not, is the electric field applied between a conductive electrodes or the electrodes are covered by dielectric, whether the chemical reactions can lead to the ionization or not. Note that a flame is a plasma without electrical discharge. Even a pressure range is an important factor. Gas or liquide media - important as well. RF or MW electric fields should be commented separately.
A gas discharge starts abruptly, when the voltage reached a critical value, that can be monitored by measurements of electrical current or light emission (see Yuriy Z Ionikh answer). The answer from Veda Prakash Gajula can give a good hint that parameters of the required equipment strongly depends on peculiarities of the setup. But it does not describes full range of opportunities or a general principles.
Why an electric field would assist a chemical reaction if there is no plasma? Presents of plasma, for me means, is just a presence of charged particles. So a resistivity of the media can be measured with additional electrodes by a multimeter...
Thanks Yuriy, Veda and Andrei for your reply. We want to prevent formation of plasma because we want to isolate the effect of an electric field on gas reaction systems. We are using a pulsed electric field generator capable of producing 3.5 KV with a pulse duration of 240 ns and a frequency of 50 KHz. We expect that a oscilloscope suitable to read this voltage signal may detect a variation in the signal in presence of plasma. We were just wondering what kind of change we should observe.
It seems that the only way to see the plasma is to detect its irradiation. The problem is that in such a way you will be able to prove its existence, but non-detecting the light signal does not guarantee its absense if the signal level is below your photodetector sensibility.
Without a conductive media (discharge/plasma) the electrode system should behave as a capacitor. I would suggest to measure current and voltage with oscilloscope and then compare voltage derivative with current. Current integral vs voltage is also possible. See publication attached for details. Pay attention to the current monitor used, a resistor is much more difficult to handle for so short pulses. Also you can try to use a capacitor (polymer, do not use ceramic ones) for charge measurements.
Short pulses with a long duration between them makes the detection of the light emission very hard. At least you would need to know the place where the emission takes place, but the location can change from pulse to pulse. However the difference between discharge and plasma here can be important as well. You might have a plasma without any electric field or light emission.
Andrei V. Pipa Andrei, can you explain please why do you recommend to use polymer capacitors instead of ceramic ones? In most publications that I saw authors used exactly ceramic capacitors.
Ceramic capacitors have a large dissipaton factor and thier capacitance value depends on applied voltage... Best properties have glass capasitances, but it hard to get glass capasitance with sufficiently high value. So, I would recommend a polymer capacitance, which properties are between glass (mica) and ceramics. May be a castom made caramic capasitors would have a good propreties, but I do not know how to get them.
Andrei, is quartz a good insulator for capacitors at high temperatures, say, 500 degrees C and higher?
Conductivity increases exponentially with temperature, 500 C is sufficiently high temperature, and I have no information about quartz properties at this temperature.
The measurement capacitance can be situated outside of the hot region. Connection of the current/voltage probes in a hot region can be problematic as well. The capacitance value should be significantly high.
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