I have made my own reactor system that i am operating at around 600e-3 mbar. I also made my own single Langmuir probe using tungsten wire of diameter 0.2mm. I have a supply that can give potential from -60 to 60 Dc source for plasma potential and the gap between the electrodes is about 50mm. Whenever i supply plasma potential i cannot observe probe current(tried both in mA and micro ampere range).
I have used the attached image from M. Konuma for my circuit as reference.
EDIT: Actually i did try to measure current using a resistor of about 10 ohm resistance in between probe and the DC supply of -60 to +60. I used galvanometer of micrometer range and digital multi meter in Milli ampere range. Probe voltage is applied between probe tip and ground. One of the possible problem might be due to low density of plasma as Johannes said since the plasma intensity is low. Also there might be other issues yet to be found.
If there exists an monoenergetic beam of plasma ions along with a direct correlation between the probe current and the area of the collector, in the case the probe current can be seen with a Langmuir probe using eq. (3-9), detailed in
"Q MACHINES" by R. W. Motley.
This can have a lot of reasons, maybe you electrical probe circuit is not entirely closed or the plasma density is too low to be detected. Maybe you could give some more information about the problems you have, and describe your setup in more detail.
What do you use for measuring the probe current, for example and between what electrodes do you apply the probe voltage? - between probe tip and ground?
Here you have not mentioned the apparatus using to measure the probe current. I can suggest that you bias the Langmuir probe through a resistance and measure the voltage drop across the resistance. This can give you the information about the probe current. However the current amplitude will be decided by the plasma density present in the discharge tube as you have shown in the diagram. The detail about the Langmuir probe , its circuit and other information can be obtained from any book on Plasma Diagnostics.
If you want to measure plasma current using Langmuir probe then you can use two methods:
1. Apply a DC bias from -80 to +30 on your probe through resistance and measure voltage drop across it as suggested by Mr. Rai.
2. You can build a Langmuir probe power supply that can generate a ramp voltage, further you can apply ramp on your probe and can simultaneously measure ion/ electron current.
Hope this will help.
First, off the bat, I do not think you will see anything on a 10 Ohm resistor. Langmuir probe currents are in the range of micro-amperes, so a 10 Ohm resistor will give you at best 1E-4 volts, say for a 10 uA current. Other thing to check is that you have a meaninngful discharge. You say if I understood correctly that the anode to cathode distance is 5 cm- this looks awfully short to have a meaningful positive column of plasma (i.e. a stable stationary plasma).
I think voltage applied to the probe biasing is less than the cathode fall voltage which is almost the discharge voltage in dc glow. Therefore, biasing voltage is not enough to counteract the discharge voltage and biasing is an-active for collection of probe current. You have to apply voltage to probe larger than the discharge voltage existing there to ensure the collection of probe current.
Dear Rakshya Thapa,
Where you connect the resistance of 10 ohm?
Is this the resistance as, shown in the fig?
Please.
Thanks
N Das
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