In single langmuir probe ,when we provide +ve voltage to probe, electrons are attracted towards probe and ,on applying negative voltage, ions will be attracted towards the probe.
But electron current is much greater than ion current at same voltage but of opposite polarity. It ue due to larger mass of ions. As ions have larger mass ,so less number of ions will be reached at probe.
But using double langmuir probe, we get same ion current as that of electron current at same voltage. How is it possible that ions having larger size can have same current as that of electrons??
It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between the various electrodes or between them and the surrounding vessel.
Hadi Jabbar Alagealy what will be effect of inserting one more electrode?
How ion current will be equal to electron curre
nt at same potential by inserting one more electrode??@
because you bias your double probe pins relative to each other - if the surface area is then equal they must draw the same current due to symmetry reasons and current closure.
@Johannes Gruenwald
How is it possible? Ions have 10000 times larger size than electrons.
Yes, but you have a retarding potential on one probe pin and an attractive on the other, decelerating electrons and accelerating ions. I am sure you are aware of the fact that you have also a floating potential on single probes, where the ion current equals the electron current despite the mass differences.
@Johannes Gruenwald I agree with you. It is possible when voltages are different but how is it possible at same voltages??
That's my question.
what do you mean by same voltages? If you have a single probe there are also no different voltages, it is one specific voltage point where the floating potential lies.
@Johannes Gruenwald ,same voltage mean, same +ve and -ve voltage to both probes, I mean one probe will get 20 Volt and other will get -20 volt.
yes, then you have a potential difference of 40 volts between the probe tips, which will accelerate the electrons towards the anode tip and the ions to the cathode tip. Both will have the same kinetic energy and the current loop is closed through the plasma giving the same current at each tip, just with different sign.
@Johannes Gruenwald , how is it possible that electron current and ion current will be same at same voltage?? While ion have 10000 times larger mass than electron??
How eq no of electrons and ions can be attracted towards probe tip?
Muhammad Arslan: if the current loop is closed you will have a space charge building up around the probe pin that initially draws too much current (usually the anode pin), which leads to a retarding field on this end and to an accelerating field on the other pin. This net E-field compensates the current discrepancies.
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