I have used a dual probe to characterize the atmospheric pressure plasma jet. When the probe is placed inside the plasma, even without bias, it ignites and disrupts the plasma. I put a resistor and also inductance impedance in the probe circuit, but it still drains through the plasma. Is such a thing common? How can probes be used in plasma jets? How can this disorder be eliminated?
I would like the measure the plasma jet parameters using a single or double Langmuir probe. When I put the probe into the jet the flame is perturbed and an arc is generated between the flame and the probe. Could you please let me know that how you could overcome this problem in your similar measurements?
For the plasma jet formation, the potential is applied to an electrode wrt to the ground. After placing the probe in the plasma volume, you see the secondary plasma or glow around the probe which is due to the secondary ionization near the probe tip. In such plasma, probe potential is slightly negative which is close to the zero potential (grounded). Therefore, current will also flow through the probe due to the potential difference between the HV electrode and probe (metal electrode). This additional current path (HV electrode to probe) is creating a problem. Due to higher collisions, a small current can (flow of electrons) be responsible for creating the plasma near the probe. It is difficult to use the probe in such collisional plasma. Even though you may get an I-V curve but there is a lack of theory (collisional probe theory) to fit your data. Spectroscopy is the best tool to characterize such collisional plasma. I hope you will get my points.
Using Langmuir probes at atmospheric pressure is tricky as these plasmas usually have high plasma densities and are easily perturbed. However, there has work bin done on this (for example: https://www.frvt.utn.edu.ar/pdfs/Grupo_Descargas/2012_b.pdf).
In any case you should be prepared to collect very high currents from your probe, so be carefull with the measurement circuits!
Mangilal Choudhary
Thanks for your answer. Many articles have used a double probe and reported electron temperature I-V curve, for example:
1. Langmuir Probe Diagnostics of an Atmospheric-Pressure Microplasma, L. Blair, Kunning G. Xu, AIAA 2015-2805
2. Measurement of atmospheric pressure microplasma jet with Langmuir probes, Xu, Kunning G.; Doyle, Steven J., Journal of Vacuum Science & Technology A Vacuum Surfaces and Films volume 34, issue 5 (2016), ....
They must have seen the effect of probes on the plasma plume. Why did they not mention this disorder? How can these disturbances be minimized and the correct characteristic curve obtained?
I also used a dc motor to quickly insert the probe into and out of the plasma, but it still ignites.
Maybe they used smaller sweeping voltages than you do? what voltage did you apply to the probe?
Johannes Gruenwald I applied different bias voltage,
1. floated probe
2. positively biased probe (up to 200V)
3. negatively biased probe (up to -200V)
in all of them, the spark appeared on the probe.
I insert a Mega-Ohm resistor in the probe circuit but I could not stop the sparking.
I even isolated the circuit from the ground. ....
+/- 200 V is way too high - try smaller voltages, some ten volts should be more than enough.
Theoretically, the dual probe should float and not affect the plasma plume, but in practice, this is not the case. There is also a spark when no voltage is applied to the probe. I used Mega-Ohm resistors in the probe circuit to control the current, but there were still sparking. I also increased the bias to +/- 200 volts, but there was still an ignition.
My structure is coaxial. That is, a needle electrode on the axis and a cylinder with a conical grounded metal nozzle around it. I thought that the presence of a probe in the plasma plume would cause the conduction path from the needle electrode to the probe to be ignited. Therefore, I used a two-ring plasma jet on the glass tube to completely disconnect the electrodes from the probe. But the spark from the plasma to probe was still there.
ah, but if your cylinder is grounded than not the whole probe is floating, which means that you ground your jet plasma through the probe electrode. This might also cause the sparking.
Normally, you bias the tips of a double probe relatively to each other, not to ground.
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